the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones viii a model for the seeder feeder process in warm frontal rainbands abstract previous field studies have indicated that warm frontal rainbands form when ice particles from a seeder cloud grow as they fall through a lower level feeder cloud in this paper we present results from a parameterized numerical model of the growth processes that can lead to the enhancement of precipitation in a seeder feeder type situation the model is applied to two types of warm frontal rainbands in the first type 1 situation the vertical air motions are typical of those associated with slow widespread lifting in the vicinity of warm fronts in the second type 2 situation the vertical air motions are stronger and more characteristic of the mesoscale the model simulations show that in the type 1 situations the growth of the seed ice crystals within the feeder zone is due to vapor deposition the feeder zone in this case is slightly sub saturated with respect to water due to the presence of the seed crystals in regions where the feeder zone is not seeded from aloft snow cryst

classification of ice crystal shapes in midlatitude ice clouds from three years of lidar observations over the sirta observatory this paper presents a study of ice crystal shapes in midlatitude ice clouds inferred from a technique based on the comparison of ray tracing simulations with lidar depolarization ratio measured at 532 nm this technique is applied to three years of lidar depolarization ratio observations from the site instrumental de recherche par teledetection atmospherique sirta observatory in palaiseau france amounting to 322 different days of ice cloud observations particles in clouds are classified in three major groups plates columns and irregular shapes with aspect ratios close to unity retrieved shapes are correlated with radiosounding observations from a close by meteorological station temperature relative humidity wind speed and direction results show a strong dependence of the relative concentration of different crystal shapes to most atmospheric variables such as the temperature with a clear successive dominance by platelike temperature above 20 c irregular temperatures between 60 and 40 c and columnlike shapes temperatures below 60 c particle shapes are almost exclusively columnlike below 75 c this is in sharp contrast with previous results of the same classification applied to tropical clouds and highlights the high geographic variability of the ice clouds distribution of microphysical properties results also suggest that ice clouds created by jet streams identified by high wind speeds are strongly dominated by columnlike shapes while front created ice clouds identified by lower wind speeds show a much more variable mix of shapes with the dominant shapes depending on other factors results also suggest different microphysical properties according to the average direction source of air masses and winds following these results a possible parameterization of ice crystal shapes in midlatitude ice clouds as a function of temperature is provided

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

estimating trajectory of free overfall nappe the upper surface profile of a rectangular free overfall is modeled using the free fall parabolic equation empirical and semiempirical relationships for the unknown quantities in this equation are presented it was found that the resulting simple and straightforward method adequately predicts the position of the upper nappe surface a relationship for establishing the vertical thickness of the nappe due to another researcher is given thus enabling the entire nappe profile from the brink to be described the equations are of use in the design of spillways and diversionary structures utilizing a free overfall

the improve 1 storm of 1 2 february 2001 part i development of a forward tilted cold front and a warm occlusion abstract analysis of observations and the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 are used to study the development of a forward tilted cold front off the coast of washington state the vertical velocity associated with the cold front produced a wide cold frontal rainband in the early stage of development the midtropospheric baroclinic zone or upper cold front moved forward with time over the warm sector to produce a structure similar to a split front the movement of the upper cold front was due to horizontal transport and frontogenetical propagation the frontogenetical propagation was produced by a combination of tilting and diabatic frontogenesis which resulted in a negative positive couplet of frontogenesis straddling the baroclinic zone the lower tropospheric cold front eventually caught up with the warm front to form a classical warm occlusion in the initial occluding process the converging frontal zones tilted i

simulation of w band radar reflectivity for model validation and data assimilation this paper describes a reflectivity forward operator developed for the validation and assimilation of w band radar data into regional arome applications de la recherche a l operationnel a meso echelle class numerical weather prediction models the forward operator is consistent with the arome ice3 one moment microphysical scheme and is devised for vertically pointing radars a new neighbourhood validation method called the most resembling column mrc method is designed to disentangle spatial location model errors from errors in the forward operator this novel method is used to validate the forward operator using data collected in diverse conditions by the airborne cloud radar rasta radar airborne system tool for atmosphere during a two month period over a region of the mediterranean the mrc method is then applied to retrieve the optimal effective shapes i e the mean axis ratios of the predicted graupel snow and pristine ice by minimising the standard deviation between observations and simulations the optimal mean axis ratio is approximately 0 7 for snow and 0 8 for graupel it is shown that treating snow and graupel particles as oblate spheroids with axis ratios close to their optimal values leads to good agreement between the observations and simulations of the ice levels conversely there is a large bias if snow and graupel particles are considered to be either spherical or overly flattened the results also indicate that pristine ice can be approximated by a sphere but this conclusion should be taken cautiously since the amount of pristine ice particles is probably overestimated in the ice3 microphysical scheme

effects of dispersed particulates on the rheology of water ice at planetary conditions we have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice i at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system our samples were molded with particulate volume fractions of 0 001 to 0 56 and particle sizes of 1 to 150 m deformation experiments were conducted at constant shortening rates of 4 4 10 7 to 4 9 10 4 s 1 at pressures of 50 and 100 mpa and temperatures 77 to 223 k for the pure ice samples initial grain sizes were 0 2 0 6 mm 0 75 1 75 mm and 1 25 2 5 mm stress strain curves of pure ice i under these conditions display a strength maximum u at plastic strains e 0 01 after initial yield followed by strain softening and achievement of steady state levels of stress ss at e 0 1 to 0 2 finer starting grain size in pure ice generally raises the level of u petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to ss evidently corresponds to the development of a steady state grain texture effects of particulate concentrations 0 1 are slight at these concentrations a small but significant reduction in u with respect to that for pure water ice occurs mixed phase ice with 0 1 is significantly stronger than pure ice the strength of samples with 0 56 approaches that of dry confined sand the magnitude of the strengthening effect is far greater than expected from homogeneous strain rate enhancement in the ice fraction or from pinning of dislocations orowan hardening this result suggests viscous drag occurs in the ice as it flows around the hard particulates mixed phase ice is also tougher than pure ice extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates the high pressure phase ice ii formed in 0 56 mixed phase ice during deformation at high stresses bulk planetary compositions of ice rock 0 4 0 5 are roughly 2 orders of magnitude more viscous than pure ice promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces

crater relaxation as a probe of europa s interior viscous relaxation rates of craters are examined to discriminate among models of europa s interior it is shown that the presence of an insulating surface regolith is required for ice lithospheres that do not contain a liquid h2o layer without the presence of such an insulating regolith relaxation times for craters 100 km in diameter are excessively long 1 b y and thus inconsistent with the absence of such features in voyager images finite element calculations indicate that a thick insulating surface frost layer raising surface temperatures 40 k above the solar radiation ambient temperature of 92 k allows a 25 km thick surface viscous layer to relax 100 km diameter craters on the short timescales 100 m y required if the icy lithosphere is 100 km thick and without a liquid h2o layer but containing a subsolidus convecting region surface temperatures need to be raised 20 k by insulation to allow relaxation of craters in this size range to occur within the required time

elastoviscoplastic relaxation of impact crater topography with application to ganymede and callisto 1 we use an elastoviscoplastic rheological model to study the long term relaxation of impact crater topography on ganymede and callisto we employ a complete set of rheological parameters for ductile creep in ice ih and implement a new shallower initial shape model under high heat flow conditions stresses are relieved in deeper less viscous material and the remainder concentrates as flexural bending stresses in a viscously stiffer near surface layer essentially a mechanical lithosphere forms that focuses stresses but with uplifts and stress levels that are always greater than those obtained for a purely viscous half space we discuss how classic elastic flexure and purely viscous relaxation are special cases of this more generalized rheological model stress concentration in the flexing i e bending layer can exceed the plastic yield strength of ice for high enough heat flows weakening the layer somewhat and increasing the total amount of relaxation these plastic strains tend to be limited to small magnitudes 1 mm permits both retention and relaxation of topography at the lower and higher ends respectively of the anticipated heat flow range consideration of relaxed craters in marius regio and of the palimpsests and penepalimpsests suggests ancient heat flows in excess of 10 mw m 2 consistent with other estimates

viscous flow rates of icy topography on the north polar layered deposits of mars we investigate the importance of viscous flow in shaping topography at the north polar layered deposits npld of mars by using finite element modeling to calculate the distribution of stresses and flow velocities present day impact craters on thenpldare too small and cold for viscous relaxation tohavebeen an important mechanism in controlling their current dimensions this effect may be ignored when analyzing crater size frequency distributions scarps at the npldmargins where avalanches of dust and carbon dioxide frost occur are sufficiently steep high andwarm to experience significant viscous flow we find flow velocities at the base of these steep scarps on the order of tens to hundreds of cm yr which are fast enough to significantly affect their slopeover kiloyear timescales alternatively the scarps could be close to steady state in which observed block falls provide a competing effect to viscous flow

aircraft observations of ice crystal evolution in an altostratus cloud abstract observations from a lagrangian spiral descent within altostratus cloud associated with a cold front were used to study the evolution of ice particle spectra by following populations of ice crystals as they fell through the cloud the flight track was corrected for wind effects and was divided into distinct regions for spatial comparison of ice particle spectra analysis of size spectra for particles larger than 800 m revealed heterogeneity on horizontal scales of 5 km in average particle diameter and concentration in the temperature range 40 to 20 c the ice crystal evolution was dominated by diffusional growth although observations of the evolution of the bimodal size spectra suggested that aggregation was occurring between 20 and 10 c aggregation dominated evolution

vertical structures of precipitation in cyclones crossing the oregon cascades abstract the vertical structure of radar echoes in extratropical cyclones moving over the oregon cascade mountains from the pacific ocean indicates characteristic precipitation processes in three basic storm sectors in the early sector of a cyclone a leading edge echo lee appears aloft and descends toward the surface updraft cells inferred from the vertically pointing doppler radial velocity are often absent or weak in the middle sector the radar echo consists of a thick vertically continuous layer extending from the mountainside up to a height of approximately 5 6 km that lasts for several hours when the middle sector passes over the windward slope of the cascades the vertical structure of the precipitation exhibits a double maximum echo dme one maximum is associated with the radar reflectivity bright band the second reflectivity maximum is located approximately 1 2 5 km above the bright band the secondary reflectivity maximum aloft does not appear until the middle sector passes over the wi

ganymede and callisto complex crater formation and planetary crusts new depth diameter d d rim and peak height and width measurements of fresh craters on ganymede and callisto are utilized to explore not only crater mechanics on icy satellites but intersatellite crater scaling and crustal properties and composition significant systematic differences in crater morphology on icy and rocky planets are confirmed simple to complex transition diameters on icy satellites including ganymede and callisto are much lower than those on rocky planets also complex crater depths and rim heights on ganymede and callisto are inherently 60 70 shallower than lunar complex craters despite similar surface gravity hence viscous relaxation is not as important as is generally assumed central peaks on most icy satellites are 1 2 km higher and considerably wider with respect to diameter than for similar sized lunar craters terracing and rim slumping is also rare in these craters central peaks in craters on ganymede and callisto larger than 15 km however are relatively small in both width and height however correlating with the occurrence of rim slumping thus central peak dimensions are an indirect indicator of the degree of rim slumping transition diameters for the occurrence of central peaks and rim slumps and for d d curve inflections for icy satellites scale approximately with the inverse of surface gravity except possibly at very low g the transition diameters for central peaks and for rim slumping differ however by 5 15 km on ganymede and callisto but by 50 or more km on the smaller icy satellites these large differences suggest that d d curves on icy satellites are comprised of three segments of decreasing slope one for smaller simple craters a second for central peak craters and a third for larger true complex craters with both central peaks and rim slumping the shallow slopes of d d curves on ganymede and callisto resemble that of the moon consistent with the occurrence of both central peaks and rim failure the high slopes of d d curves on the middle sized icy satellites indicate that they are middle segments consistent with the dominance of central peaks in those craters complex crater depths on icy satellites in general scale with the inverse of surface gravity the depths of all the observed large basin scale craters on these small satellites when scaled to ganymede gravity are very similar to those of complex craters on ganymede and callisto these large craters including odysseus 425 km diameter on tethys are probably true complex craters that have not undergone significant viscous relaxation ithaca chasma on tethys probably formed during the prompt collapse of the transient odysseus crater crater morphology is clearly controlled by both gravity and by large variations in material properties i e composition and should be useful in constraining bulk crustal composition i e ice rock ratios the similarity of various morphological transition diameters and complex crater depths on ganymede and callisto two geophysically very similar but geologically divergent large icy satellites indicates that the crusts of both bodies are dominated by water ice with only a minor rocky component

impact cratering on titan ii global melt escaping ejecta and aqueous alteration of surface organics abstract new three dimensional hydrodynamic simulations of hypervelocity impacts into the crust of titan were undertaken to determine the fraction of liquid water generated on the surface of saturn s largest moon over its history and hence the potential for surface modification of hydrocarbons and nitriles by exposure to liquid water we model in detail an individual impact event in terms of ejecta produced and melt generated and use this to estimate melt production over titan s history taking into account the total flux of the impactors and its decay over time our estimates show that a global melt layer at any time after the very beginning of titan s history is improbable but transient melting local to newly formed craters has occurred over large parts of the surface local maxima of the melt are connected with the largest impact events we also calculate the amount of volatiles delivered at the impact with various impact velocities from 3 km s for possible hyperion fragments to 11 km s for jupiter family comets and their retention as a possible source of titan s atmosphere we find the probability of impact ejecta escaping titan with its modern dense and thick atmosphere is rather low and dispersal of titan organics throughout the rest of the solar system requires impactors tens of kilometers in diameter water ice melting and exposure of organics to liquid water has been widespread because of impacts but burial or obscuration of craters by organic deposits or cryovolcanism is aided by viscous relaxation the largest impactors may breach an ammonia water mantle layer creating a circular albedo contrast rather than a crater

properties of embedded convection in warm frontal mixed phase cloud from aircraft and polarimetric radar paper associated with the cwvc clouds water vapour and climate dataset held on the ceda archive

nonhydrostatic dam break flows ii one dimensional depth averaged modeling for movable bed flows abstractthe dam break flow over a movable bed is an important problem in fluvial flow processes these flows are usually predicted by a one dimensional 1d approach based on a hydrostatic pressure distribution recent three dimensional 3d nonhydrostatic simulations of dam break waves over movable beds based on the reynolds averaged navier stokes rans equations revealed that the saint venant theory is not accurate in predicting the flow dynamics within the scour hole developed in this work a generalized 1d nonhydrostatic model for flow over movable beds is proposed assuming a linear nonhydrostatic pressure distribution the new set of 1d equations accounts for the vertical acceleration which is important in dam break waves over movable beds given the instantaneous curved beds generated over the erodible terrain these equations account for both the bed and suspended load transport modes a high resolution finite volume numerical scheme with a semi implicit treatment of nonhydrostatic terms is dev

a composite analysis of winter season overrunning precipitation bands over the southern plains of the united states abstract this investigation presents an extensive analysis of precipitation bands associated with winter season over running events in the southern plains of the united states the primary motivation is an investigation of the relevance of conditional symmetric instability csi theory to overrunning precipitation events this is accomplished by means of a statistical composite study of events of differing degrees of bandedness using synoptic sounding and national weather service radar data the composite study shows that it is difficult to distinguish between the environments of banded and nonbanded cases on the basis of individually examined shear and buoyancy parameters some statistically significant differences do occur when shear and buoyancy effects are combined in an evaluation of csi with more strongly banded events showing greater instability

microphysical properties of convectively generated fall streaks within the stratiform comma head region of continental winter cyclones abstractthis paper presents analyses of the microphysical structure of comma head stratiform precipitation in 14 continental cyclones focusing on fall streaks of hydrometeors produced by cloud top convective generating cells data were obtained at temperatures between 4 and 45 c using in situ instrumentation and the w band university of wyoming cloud radar all operated aboard the national science foundation national center for atmospheric research c 130 analyses are presented first for a case study of one cyclone followed by statistical analyses of the full dataset using radar based objective classifications the statistical percentile number concentrations averaged 1 9 times larger within the fall streaks compared to the regions between them and the corresponding ice water content and median mass diameter values averaged 2 2 and 1 1 times larger ice phase conditions were predominant within the stratiform precipitation with deposition and aggregation the primary ice growth mechanisms no distinc

instability characteristics of radar derived mesoscale organization modes within cool season precipitation near portland oregon abstractthe instability characteristics associated with different radar derived mesoscale organization modes are examined using six cool seasons of operational scanning radar data near portland oregon and operational sounding data from salem oregon additionally several years of microwave rain radar ka band vertically pointing radar data from portland and merwin washington are used to characterize the nature and occurrence of generating cells and fall streaks the combination of a new metric convective stratiform intermittency with the classification of radar reflectivity maps into convective and stratiform precipitation types was applied to periods when the freezing level was 1 4 km altitude this method distinguishes periods with embedded convective within stratiform mesoscale organization from those that were mostly convective or mostly stratiform mesoscale organization occurs in a continuum of states with predominantly stratiform structure occurring most frequently generating cells in the s

sublimation as a landform shaping process on pluto fields of pits both large and small in tombaugh regio sputnik planitia and the pitted uplands to the east and along the scarp of piri rupes are examples of landscapes on pluto where we conclude that sublimation drives their formation and evolution our heuristic modeling closely mimics the form spacing and arrangement of a variety of tombaugh regio s pits pluto s sublimation modified landforms appear to require a significant role for diffusive mass wasting as suggested by our modeling in our models the temporal evolution of pitted surfaces is such that initially lots of time passes with little happening then eventually very rapid development of relief and rapid sublimation small pits on sputnik planitia are consistent with their formation in n2 dominated materials as n2 ice readily flows some other stiffer volatile ice may play a role in supporting the relief of sublimation degraded landforms that exhibit several hundred meters of relief a strong candidate is ch4 which is spectroscopically observed to be associated with these features but the current state of rheological knowledge for ch4 ice at pluto conditions is insufficient for a firm assessment

observed microphysical structure of midlevel mixed phase clouds abstract this paper analyzes airborne measurements of six midlevel clouds observed over the great plains of the united states in late 1999 and early 2000 during the fifth of the complex layered cloud experiments clex 5 data show that these innocuous looking clouds display complicated microphysical and thermodynamic structures five of the six cases exhibit mixed phase conditions in temperatures ranging from near 0 to 31 c at altitudes of 2400 to 7200 m msl four of the cases consist of a single cloud layer while the other two are multilayered systems in the thin mixed phase single layered clouds that the authors observed there is an increase of liquid water content with altitude whereas the ice water content maximizes in the mid to lower part of the clouds this contrasts the two multilayered systems the authors observed in which significant amounts of ice occur in the top as well as the bottom of the individual layers of each system a lack of significant temperature inversions or wind shea

synoptic and topographic variability of northern california precipitation characteristics in landfalling winter storms observed during caljet abstract observations from northern california during the california landfalling jets caljet experiment are used to examine the mean characteristics of precipitation and their variances as functions of synoptic and topographic regimes ten cases involving the landfall of extratropical cyclones are analyzed with radar and rain gauge data collected at two sites one in the coastal mountains north of san francisco czd and the other in the central valley just west of sacramento kdax aside from the melting layer bright band the most striking feature in the 10 case composite vertical profile of radar reflectivity at czd was a distinct change in slope about 2 5 km above the bright band this shoulder is thought to represent a change in the growth rate of hydrometeors although the bright band was quite distinct about one third of the profiles in the composite did not exhibit this feature these nonbrightband nbb profiles had a low level slope where reflectivity increased with decreasing altitude a

dynamics of cloud top generating cells in winter cyclones part iii shear and convective organization abstractcloud top generating cells gcs are a common feature atop stratiform clouds within the comma head of winter cyclones the dynamics of cloud top gcs are investigated using very high resolution idealized wrf model simulations to examine the role of shear in modulating the structure and intensity of gcs simulations were run for the same combinations of radiative forcing and instability as in part ii of this series but with six different shear profiles ranging from 0 to 10 m s 1 km 1 within the layer encompassing the gcs the primary role of shear was to modulate the organization of gcs which organized as closed convective cells in simulations with radiative forcing and no shear in simulations with shear and radiative forcing gcs organized in linear streets parallel to the wind no gcs developed in the initially stable simulations with no radiative forcing in the initially unstable and neutral simulations with no radiative forcing or shear gcs were exceptionally weak with no clear organization

hydrometeor evolution in rainbands over the california valley abstract hydrometeor distributions were measured in two rainbands that passed over the california valley the ground radar was used to vector the university of wyoming s instrumented king air aircraft to the top of the rainband at which time an onboard computer algorithm was used to make multiple penetrations of an ensemble of ice particles that were assumed to descend at 1 or 2 m s 1 while drifting downwind distribution parameters were calculated for each penetration and the changes in these parameters were used to infer the different modes of growth that the particles undergo it was found that there were five distinct regions of particle growth nucleation and depositional growth were the dominant processes near the top of the rainband the hydrometeors then fell through a region of aggregation from the 20 c level to the 10 c level ice crystal multiplication was the dominant process from the 10 c level to the 4 level from 4 to o c aggregation was again the dominant processes below the 0 c l

organization and structure of clouds and precipitation on the mid atlantic coast of the united states part ii the mesoscale and microscale structures of some frontal rainbands abstract the mesoscale and microscale structures of the clouds and precipitation associated with a frontal system on the mid atlantic coast of the united states are investigated using radar reflectivity and doppler velocity data surface mesonet conventional surface upper air and microphysical data the frontal structure showed similarities to a warm occlusion with code air advection aloft preceding cold air advection at lower levels six rainbands were observed in association with the frontal system all of these rainbands developed within the region of coverage of the nws wsr 57 radar at cape hatteras north carolina two were upper level features associated with a prefrontal surge of cold air and the main push of cold dry air aloft these rainbands were similar in structure to prefrontal surge and wide cold frontal rainbands respectively observed on the pacific northwest coast the microphysical and small mesoscale structures of the wide cold frontal rainband are examined three of the rainbands

formation of crater palimpsests on ganymede crater palimpsests on ganymede are circular features of high albedo apparently formed by impacts very early in the satellite s history some palimpsests have distinct outward facing slopes at their outer margins suggesting that they are distinct deposits of significant thickness this view is supported by observations of superimposed craters that excavate through palimpsest deposits into underlying materials of contrasting albedo we suggest that crater palimpsests may result from volcanic extrusions triggered by large impact events early in ganymede s history impact excavation that occurred to a sufficient depth would penetrate the satellite s thin primordial lithosphere allowing mobile buoyant material from a vigorously convecting mantle to rise to the surface upon reaching the surface the material would spread radially to produce a circular deposit of relatively high albedo as the lithosphere cooled and thickened palimpsest formation would have ceased consistent with the observed concentration of palimpsest formation in the earliest part of ganymede s recorded geologic history constraints on the nature of the extruded material may be derived from a simple model of emplacement we use an analytic model for axisymmetric viscous flow to constrain the flow viscosity in terms of the aspect height radius ratio of the palimpsest a measured value of 0 02 would imply a viscosity marginally consistent with glacial ice viscosities smaller values of which are likely would imply still lower viscosities more appropriate to partially liquid flow if such flows were described by a bingham rheology their yield strengths were of the order of 1 kpa this is lower than is typical for even very mobile silicate lavas but may be reasonable for a water ice slush ice extrusion could have been a slow process taking thousands of years while slush extension would have been essentially contemporaneous with the impact event for the case of either solid ice and slush extrusion our conclusions for the thermal structure of ganymede at the time of palimpsest formation are similar there was a thin 10 km conducting lithosphere underlain by a vigorously convecting mantle that was very close to the solidus temperature

arctic multilayered mixed phase cloud processes revealed in millimeter wave cloud radar doppler spectra 1 cloud radar doppler velocity spectra lidar backscattering coefficients and depolarization ratios and aircraft in situ measurements are used to investigate microphysical processes occurring in a case of multilayered mixed phase clouds over the north slope of alaska some liquid cloud layers were observed to exist in well mixed atmospheric layers but others were found in absolutely stable atmospheric layers the observations suggest that strong cloud top cooling was necessary to produce the well mixed cloud layers clouds shielded from radiative cooling by overlaying clouds more frequently existed in absolutely stable layers the in situ measurements revealed that most liquid layers contained drizzle the production process of which was shown from the radar and lidar measurements to have been interrupted only during heavier ice precipitation events different layers interacted with one another by changing the radiative heating profile and by precipitation which changed the growth paths available to cloud particles and even initiated new hydrometeor classes

modeling overfalls using vertically averaged and moment equations a new set of vertically averaged and moment equations which assumes a linear longitudinal velocity distribution and quadratic vertical velocity and pressure distributions is used for modeling flow in the vicinity of horizontal rectangular free overfall with smooth and rough beds and sharp crested weirs with sloping upstream faces these equations are modeled using a hybrid petrov galerkin and bubnov galerkin finite element scheme for the rectangular free overfalls the predicted water surface profiles upstream of the overfall and the free jet trajectory agree well with the measured data the computed vertical velocity and pressure distributions at the brink and upstream of the overfall are found to be in good agreement with the measured data while the computed longitudinal velocity distributions compare well with a two dimensional potential flow model the computed results for sharp crested weirs with sloping upstream faces agree well with the measured data for an upstream weir slope of up to 27 with the horizontal for an upstream slope of 45 and steeper and for a large weir height the predicted water surface upstream of a weir shows numerical instability

submerged and unsubmerged natural hydraulic jumps in a bedrock step pool mountain channel high resolution digital elevation models dems for natural submerged and unsubmerged jump regions were tested against the classical hydraulic jump chj and engineering analogues that have dominated previous geomorphic and engineering research dems were compared for two discharge conditions the data showed bed and water surface features of the natural jump regions differ significantly from chj conditions and engineering analogues with respect to boundary conditions roughness elements were highly irregular in frequency spacing size and orientation and were comprised of form obliquity and bed scour in boulder bed and pure bedrock conditions the data also showed bed and water surface features of natural jump regions have similarities and deviations from chj conditions and engineering analogues with respect to hydraulic characteristics several variations were observed in the hydraulic characteristics between the ballistic and sloping jets revised conceptual models based on idealized corollaries were developed for the natural jump regions based on upstream energy head relative to the downstream tailwater depth and step geometry the dems highlight the importance of recognizing the interrelationship between spatial transcritical flow structures and localized topographic heterogeneities in bedrock channels

crater formation and modification on the icy satellites of uranus and saturn depth diameter and central peak occurrence depth diameters d d for fresh craters on the intermediate sized icy satellites of uranus and saturn have been determined using photoclinometry and shadow lengths and are compared with similar measurements on the terrestrial planets simple bowl shaped craters on icy satellites including those on miranda for which the highest resolution data are available are systematically 20 40 shallower than on the terrestrial planets this pronounced difference between crater depths on icy and rocky surfaces indicates that differences in impact velocity or surface gravity are not as important as the differences in mechanical properties between ice and rock in controlling simple crater morphology experimental impact studies indicate that differences in material properties such as angle of internal friction can control crater depth alternatively breccia lenses may be thicker in icy satellite simple craters complex craters on the icy satellites become significantly deeper with increasing crater diameters unlike complex craters on terrestrial planets which are nearly constant in depth central peaks and hence floor rebound are also volumetrically and morphologically more prominent than wall slumping rim collapse on the icy satellites the magnitude of viscous relaxation of very large craters e g herschel and odysseus on the icy satellites can be estimated from extrapolation of the d d fits at smaller crater diameters the transition diameter from simple to complex morphology is inversely correlated with gravity as it is on the terrestrial planets but at significantly smaller diameters than would be expected from a simple extrapolation of the terrestrial trend thus crater modification especially floor rebound is easier to initiate on icy satellites estimated effective cohesions for shocked icy material are lower by a factor of 10 20 relative to rock although estimated effective viscosities are similar the distinct systematic differences between crater morphology on icy and rocky worlds indicate that gross material property differences between rock and ices play a key role in crater formation and modification but gravity is still the primary driving force

surface convergence induced by cold fronts aloft and prefrontal surges abstract the magnitude of surface convergence produced by the movement of cold fronts aloft and prefrontal surges is derived by applying the linear divergence equation to observed surface pressure traces for pacific northwest warm occlusions and from a mesoscale model simulation of a warm occlusion like structure in the central united states convergence values of approximately 10 4 s 1 are found to be generated locally for periods of about 1 h yielding vertical displacements of 10 50 hpa it is hypothesized that such convergences should noticeably enhance condensation rates in the widespread lower stratiform clouds associated with warm occlusions and could be a key mechanism for the triggering of squall lines by cold fronts aloft in cyclones in the central united states

three dimensional kinematic and microphysical evolution of florida cumulonimbus part ii frequency distributions of vertical velocity reflectivity and differential reflectivity abstract high resolution radar data collected in florida during the convection and precipitation electrification experiment are used to elucidate the microphysical and kinematic processes occurring during the transition of a multicellular storm from convective to stratiform stages a statistical technique is employed to examine the evolving properties of the ensemble small scale variability of radar reflectivity vertical velocity and differential reflectivity over the entire storm differential radar reflectivity data indicate that the precipitation at upper levels was nearly glaciated early in the storm s lifetime dual doppler radar data show that throughout the storm s lifetime both updrafts and down drafts were present at all altitudes and that most of the volume of the radar echo contained vertical velocities incapable of supporting precipitation size particles thus the ensemble microphysical properties of the storm were increasingly dominated by particles falling in an environment of weak vertic

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones iv vertical air motions and microphysical structures of prefrontal surge clouds and cold frontal clouds abstract the vertical air motions and microphysical structures of the clouds associated with two mesoscale precipitation systems in the pacific northwest are examined using rawinsonde aircraft and vertically pointing doppler radar data a rainband associated with a prefrontal surge of cold air aloft was found to consist of deep 3 4 km convective cells natural seeding by cirrostratus cloud spread ice crystals throughout the rainband all of the precipitation growth observed took place through ice phase processes much of the moisture necessary for precipitation growth entered the rainband at low levels in the form of vapor and condensate associated with widespread stratiform cloud a weakly organized cold frontal precipitation area was found to consist of snow trails originating from shallow 1 2 km convective cells in a seeder zone above 5 5 km altitude below this level the trails swept through a feeder zone which consisted of stratiform cloud downdrafts observed in one region of the feeder zon

the occurrence of irregular ice particles in stratiform clouds abstract recent studies that have classified ice particles from airborne imaging probe data have concluded that the vast majority of ice particles in stratiform precipitation systems are of an irregular shape this conclusion stands in contrast to the findings from microscope observations of snow particles at the ground during the improvement of microphysical parameterization through observational verification experiment from november to december 2001 in the oregon cascade mountains improve 2 which show that most snow crystals either single crystals or the component crystals of snow aggregates are readily identified as regular types within established crystal classification systems this apparent contradiction is rectified by examining the definition of the term irregular as applied to ice particles and by considering limitations of different methods for observing ice particles it is concluded that the finding of the airborne probe based studies is a consequence of both limitations of the observin

mapping radar reflectivity values of snowfall between frequency bands motivated by the use of a ku ka band radar in the global precipitation measurement core satellite due to launch in 2013 we have studied the use of techniques to simulate radar observations of snowfall at these bands from data at c w bands by using scattering simulations to derive the approximate relationships of radar observables at the c w bands and those at ku ka bands in this paper we form the cross band relationships by simulating radar reflectivity and attenuation in snowfall the relations can be used to simulate observations at given bands from measurements at other bands when measurements are available at multiple frequencies the consistency of the model and the measurements can be used as a measure of the validity of the underlying assumptions of snowflake shape and constitution which have been an important topic in recent snowfall remote sensing research we also present mapping functions that can be used to perform cross band analysis of snowfall observations and examine the use of these for practical cases of combined ground and space based cloudsat radar measurements as well as airborne radar data from the 2003 wakasa bay experiment

finite element models of non newtonian crater relaxation models of viscous crater relaxation proposed to date for the icy satellites of the outer solar system assume the behavior of a newtonian medium with viscosity independent of effective stress while this is reasonable if both effective stresses and temperatures are very low laboratory data on ice indicate that at effective shear stresses 0 1 mpa and temperatures 173 k likely to occur in regions underlying craters non newtonian behavior is probable such material would have a viscosity effective stress relation given by 1 n where n 4 it is likely due to the low activation energy of ice at these temperatures that stress differences may affect the profile of relaxing craters to a greater extent than temperature differences to investigate the effects of a non newtonian rheology on the profiles of relaxing craters two dimensional finite element simulations have been performed initially effective stresses are highest in the region below the crater center producing a zone of relatively low viscosity relaxation flow forces this region into a convex bulge as the crater relaxes at later stages the low viscosity region moves to beneath the crater rim leading to enhanced relaxation of crater rims compared to a newtonian rheology the reduction of effective stress as the crater relaxes increases viscosity in the region beneath the crater as a result relaxation rates are quite rapid compared to those in later stages this is in marked contrast to the exponential behavior of depth with time associated with newtonian relaxation the short relaxation times observed for the craters modeled indicate that a silicate component may be required in the crusts of icy satellites to account for the observed crater populations

crater relaxation on ganymede implications for ice rheology controversy has existed over whether or not viscous relaxation is an important process on the icy satellites previous models involved large extrapolations of newtonian flow laws for ice whereas ice is known to exhibit non newtonian behavior recently the flow law parameters for ice at the appropriate temperatures and stresses have been measured numerical modelling of the viscous relaxation of basins on ganymede using these parameters has given implausibly short relaxation times however this model treated ice as a purely viscous substance so that no elastic lithosphere could develop near the surface here we treat ice as a maxwell visco elastic material and numerically model the relaxation of basins on ganymede we find that realistic young s moduli lead to little relaxation occurring in basins even 4 0 ga after their formation further we demonstrate that within broad limits the near surface temperature gradient has little effect on this result finally through examination of the distribution of maxwell times in the vicinity of the crater we show that most viscous relaxation occurs early in the basin s history when stresses are high and thus maxwell time is short as stresses are relieved the maxwell time becomes long and relaxation essentially ceases

depth averaged model of open channel flows over an arbitrary 3d surface and its applications to analysis of water surface profile a new set of depth averaged equations is introduced to study the flow over an arbitrary three dimensional 3d surface these equations are derived based on a generalized curvilinear coordinate system attached to the 3d bed surface therefore it allows us to include the effect of centrifugal force due to the bottom curvature these general equations make it possible to analyze flows over complex terrain without the limitation of mild slope assumption used in conventional depth averaged models this new model is then applied to calculate the water surface profiles of 1 flow over a cylindrical surface 2 flow over a circular surface and 3 flow with an air core vortex at a vertical intake a simple hydraulic experiment is conducted in the laboratory to observe the water surface profile of flow over a circular surface the results obtained from the model are in good agreement with experimental measurements and calculation by an empirical formula consequently it demonstrates the applicability of the model in cases of flow over a highly curved bottom

vertical variability of cloud hydrometeors in the stratiform region of mesoscale convective systems and bow echoes abstract during the bow echo and mesoscale convective vortex experiment the noaa p 3 research aircraft executed 17 spiral descents to the rear of convective lines to document the vertical variability of hydrometeors above within and below the stratiform melting layer ten spirals were behind lines that exhibited bowing at some stage in their evolution although quick descents on some spirals forced sampling of different particle zones clear trends with respect to temperature were seen for 16 spirals the ambient relative humidity with respect to ice was in the range of 100 4 at temperatures between 10 c and the melting layer but exhibited steady decreases below the melting layer to an average relative humidity with respect to water of 77 15 at 9 c in contrast one spiral conducted on 29 june 2003 directly behind a developing bow echo had a relative humidity with respect to ice averaging 85 at heights above the 0 c level and relative humidity with respect to ice further decreased below the

mesoscale circulations initiated by melting snow a linearized model is used to examine the dynamical effects of a mesoscale temperature perturbation in the lower troposphere the perturbation is a result of nonuniform cooling of the environment by melting snow an organized mesoscale thermal circulation develops with strong vertical motions in the interior outflow at low levels and a return flow at upper levels this circulation is similar to a sea breeze circulation but is forced by an elevated horizontal temperature gradient a possible application to the problem of observed enhanced precipitation regions near a rain snow boundary is considered

vertical cloud structure in a midlatitude cyclone from a 94 ghz radar abstract the vertical structure of clouds associated with a developing midlatitude cyclone was studied using a 94 ghz cloud radar accompanied by a host of other surface based instruments and rawinsondes a thickening cirrostratus deck was observed as the storm approached as the storm drew near low level moisture advection increased and a drizzle producing stratus deck quickly developed a rather lengthy period of light to occasionally moderate rain accompanied the passage of the storm when the storm pulled away to the northeast and the rain ended the character of the stratus deck changed markedly with no drizzle production evident other cloud features observed included generating cells and their resultant fallstreaks and an eye that apparently accompanied the passage of a negatively tilted upper level trough as evidenced by measurements from a 50 mhz wind profiler located near the cloud radar rawinsonde measurements showed that the cloud radar also traced the descent of the melting layer satel

viscosity of rock ice mixtures and applications to the evolution of icy satellites theory and experiments are used to establish lower and upper bounds on the ratio of actual viscosity to pure ice viscosity for a suspension of rock particles in a water ice matrix for typical conditions encountered in icy satellites this ratio is of order ten or possibly larger depending on unknown factors such as the particle size distribution it is shown that even this modest increase in viscosity may be enough to have caused a failure of solid state convective self regulation early in the evolution of a homogeneous rock water ice satellite provided the satellite is large enough and sufficiently silicate rich the criteria for this failure are satisfied by ganymede and are marginal for callisto if the silicates are hydrated failure of self regulation means that the viscosity is too high for the interior to remain completely solid and eliminate the heat production of long lived radioisotopes by solid state convection partial melting of the ice then occurs it is further shown that satellites of this size may then undergo runaway differentiation into a rock core and almost pure ice mantle because the gravitational energy release is sufficient to melt nearly all the ice and the rayleigh taylor instability time scale is short although the high pressure phases of ice melt the resulting water quickly refreezes at a higher level it is conjectured that these results explain the striking surface dissimilarity of ganymede and callisto if these satellites accreted cold and undifferentiated ganymede may have gone supercritical melted and differentiated because of a failure of self regulation whereas callisto remained undifferentiated to the present day like all proposed explanations for the ganymede callisto dichotomy this conjecture cannot be quantified with confidence because of inadequate or incomplete observations theory and experimental data

flow measurement in sloping channels with rectangular free overfall this technical note describes an experimental study of the free overfall from a rectangular channel with differing slopes and bed roughnesses the relationship between the upstream critical depth and brink depth is explored and found to be influenced by both slope and channel bed roughness with roughness having a greater effect at steeper slopes two empirical equations are proposed for calculating this relationship the first requiring only the channel slope to be known and the second requiring a knowledge of channel slope and roughness the accuracy of the two equations when used in a method for flow measurement are compared the first relationship in which the effects of slope and roughness are aggregated provides a useful estimate of channel discharge if only the channel slope is known if both bed roughness and slope are known then the second relationship can be used to calculate channel discharge with greater accuracy

testing the viability of topographic relaxation as a mechanism for the formation of lunar floor fractured craters historically two modes of origin for lunar floor fractured craters have been proposed subcrater laccolith emplacement and topographic relaxation to test the viability of the topographic relaxation model we employ an elastoviscoplastic finite element model and ductile rheology measurements of a devolatilized terrestrial diabase an analog to lunar crustal materials trials were run under two situations 1 under extreme conditions e g high heat flow to determine the maximum degree of relaxation that the model can accommodate and 2 under more nominal conditions in order to resolve how much relaxation can be expected to occur under more plausible lunar conditions we find that in the extreme case topographic relaxation is a viable means only for explaining the observed shallowing of the largest floor fractured craters 100 km in diameter results from the less extreme case demonstrate that lunar materials are simply too rigid to experience degrees of relaxation as large as those observed in the floor fractured crater population these results indicate that topographic relaxation is not the primary mechanism for the formation of floor fractured craters because the topographic relaxation model does not apply for the vast majority of floor fractured craters i e those with diameters less than 60 km we conclude that the laccolith intrusion model is a more viable alternative

nonhydrostatic granular flow over 3 d terrain new boussinesq type gravity waves understanding granular mass flow is a basic step in the prediction and control of natural or man made disasters related to avalanches on the earth savage and hutter 1989 pioneered the mathematical modeling of these geophysical flows introducing saint venant type mass and momentum depth averaged hydrostatic equations using the continuum mechanics approach however denlinger and iverson 2004 found that vertical accelerations in granular mass flows are of the same order as the gravity acceleration requiring the consideration of nonhydrostatic modeling of granular mass flows although free surface water flow simulations based on nonhydrostatic depth averaged models are commonly used since the works of boussinesq 1872 1877 they have not yet been applied to the modeling of debris flow can granular mass flow be described by boussinesq type gravity waves this is a fundamental question to which an answer is required given the potential to expand the successful boussinesq type water theory to granular flow over 3 d terrain this issue is explored in this work by generalizing the basic boussinesq type theory used in civil and coastal engineering for more than a century to an arbitrary granular mass flow using the continuum mechanics approach using simple test cases it is demonstrated that the above question can be answered in the affirmative way thereby opening a new framework for the physical and mathematical modeling of granular mass flow in geophysics whereby the effect of vertical motion is mathematically included without the need of ad hoc assumptions

power law rheology of ice and the relaxation style and retention of craters on ganymede numerical models of viscous relaxation of craters in ice using rheological laws determined from laboratory data previously indicated excessively short relaxation times te the time required for crater depth to become 1 e of its initial value for craters on the icy satellites a typical value of te was 104 years for a 100 km diameter crater these time scales are totally incompatible with the large regions of heavily cratered terrain on the icy satellites we discuss the application of a numerical unite element model incorporating the most recent rheological data for ice at temperatures and pressures appropriate to the near surface regions of ganymede the model permits a full representation of a temperature and stress dependent power law viscosity for ice for temperature gradients in reasonable agreement with those obtained from thermal and structural models of ganymede we obtain relaxation times te 10 years for craters with diameters less than 100 km for all craters with diameters greater than 10 km the strong dependence of viscosity on stress was found to significantly shorten the relaxation time an attempt was also made to determine whether newtonian or non newtonian rheological laws dominated relaxation for craters of various sizes newtonian rheologies appear to dominate crater relaxation only for situations where the effective temperature is several tens of degrees warmer than the surface is observed to be perhaps because of the effect of an insulating regolith and where temperature gradients are small in this case viscous relaxation of craters with diameters smaller than 25 km is expected to be dominated by newtonian flow laws provided that the currently accepted parameters gross extrapolations from near melting temperature measurements are valid for the low temperatures of the icy satellites on the other hand larger diameter craters undergo at least initially viscous relaxation determined by a strongly non newtonian rheological law and have geologically short relaxation times te 107 years for a 300 km crater although these times may be extended by silicate suspensions and by dispersion hardening this presumably accounts for the absence of such craters in unmodified form on ganymede finally the large stress exponent n 4 7 of the current rheological data for ice indicates a strong role for non newtonian flow in determining the topography of viscously relaxing craterforms of this size in principle a threshold crater diameter and hence stress for the onset of non newtonian viscous behavior could be obtained from galileo images allowing estimates to be made of near surface temperatures and thermal gradients in ganymede s past in practice however any morphological variability is almost certainly too subtle to be detected from imagery the calculated relaxation times from this work are still much shorter than those indicated by the topographic relief of ganymede s surface this remains an unresolved problem possible ways in which the observations can be resolved with current laboratory data on ice deformation are discussed

long term retention of impact crater topography on ganymede previous analyses into viscous relaxation of impact craters on ganymede have predicted short characteristic relaxation times of the order of 100 myr or less for larger craters in disagreement with the apparent great ages of ganymede s terrains by applying improved understanding of rheologic parameters and initial crater shapes to a viscoelastic model we calculate upper limit zero heat flow relaxation times in excess of the age of the solar system eliminating this discrepancy these very long relaxation times are not due to elastic effects but are simply due to the effective viscosity of water ice at the appropriate temperatures and stresses

retrieval of effective ice crystal size in the infrared sensitivity study and global measurements from tiros n operational vertical sounder 1 the difference between cirrus emissivities at 8 and 11 m is sensitive to the mean effective ice crystal size of the cirrus cloud de by using single scattering properties of ice crystals shaped as planar polycrystals diameters of up to about 70 m can be retrieved instead of up to 45 m assuming spheres or hexagonal columns the method described in this article is used for a global determination of mean effective ice crystal sizes of cirrus clouds from tovs satellite observations a sensitivity study of the de retrieval to uncertainties in hypotheses on ice crystal shape size distributions and temperature profiles as well as in vertical and horizontal cloud heterogeneities shows that uncertainties can be as large as 30 however the tovs data set is one of few data sets which provides global and long term coverage having analyzed the years 1987 1991 it was found that measured effective ice crystal diameters de are stable from year to year for 1990 a global median de of 53 5 m was determined averages distinguishing ocean land season and latitude lie between 23 m in winter over northern hemisphere midlatitude land and 64 m in the tropics in general larger des are found in regions with higher atmospheric water vapor and for cirrus with a smaller effective emissivity

relaxation of impact basins on icy satellites the extent and style of viscous relaxation of the largest crater basins on the icy satellites of saturn can be used as a probe of the viscosity of the deep interiors of these bodies in particular some constraints on the size of a silicate core and the magnitude of the internal viscosity gradient can be determined if the relaxation process is sufficiently well understood we discuss the application of a unite element model incorporating spherical symmetry to the general problem of relaxation of large craters on spherical bodies we examine the influence of core size geothermal gradient and rheologic law on relaxation flow and consider the implications for the internal structures of mimas and tethys our model results show that relaxation of crater topography on a spherical satellite is qualitatively and quantitatively different from relaxation on a planar surface once the crater diameter becomes a significant fraction of the satellite radius for small to moderate sized basins relaxation flow is bidirectional with flow both from rim to bowl and rim to surrounds this is the same style of flow observed for craters on planar surfaces for large enough basins however the spherical geometry of the problem leads to flow that is unidirectional toward the crater bowl throughout the satellite the transition from bidirectional to unidirectional flow appears to be very sensitive to the basin diameter the critical basin diameter for transition increases with core size thermal gradient rim height and rheologie law stress exponent unidirectional and bidirectional flow produce fundamentally different stress fields in the regions surrounding the relaxing crater relaxation velocities for large craters on spherical bodies can be appreciably more rapid than those for equivalent craters on planar surfaces again because of the geometric effects of relaxation on a sphere the difference in degree of relaxation between the large basins odysseus on tethys which has undergone significant modification and herschel on mimas which appears essentially unmodified can be explained by different thermal histories for tethys and mimas by the effects of stress dependent rheology or by a combination of the two the influence of a core at sizes consistent with the observed densities of the satellites on relaxation time scales is unimportant if the ithaca chasma graben system on tethys is the result of stresses generated by relaxation of the odysseus basin then tethys s core must be small enough that it allows unidirectional relaxation flow for a basin of odysseus s size if this assumption it correct we estimate an upper limit of roughly 20 of tethys s radius for the radius of the satellite s core

nonhydrostatic dam break flows i physical equations and numerical schemes abstractmodeling of dam break flows is frequently required in civil and environmental engineering because of the risk associated with this catastrophic flow typically model predictions are conducted using the saint venant hydrostatic theory which can lead to unrealistic predictions the prediction of the amplitude of nonhydrostatic waves generated during dam break flows is an important engineering problem given the risk of overtopping of flow in manmade canals or the increasing of flooding areas in natural watercourses the weakly nondispersive and fully nonlinear serre equations are a suitable choice for modeling these flows but there is a lack of a systematic assessment of this system of equations for dam break flow modeling reported in the literature in this paper the serre equations are applied to dam break flows over horizontal rigid bottoms whereas in the second part of this research the nonhydrostatic dam break waves over erodible beds are considered here a high resolution finite volume m

aircraft observations of the origin and growth of very large snowflakes a new high volume precipitation probe capable of optically imaging particles up to about 5 cm was flown in a winter storm during the second canadian atlantic storms program casp ii dendritic crystals originating aloft in a region of weak convective instability were tracked and observed to aggregate into 4 5 cm snowflakes within an isothermal layer near 0 c the region containing the very large snowflakes was characterized by high radar reflectivities and rapid accumulation of snow at the surface

roundness and aspect ratio of particles in ice clouds abstract the frequency of occurrence of the aspect ratio and roundness of particles in ice clouds from aircraft observations have been examined images of cloud particles were measured by a cloud particle imager cpi at 2 3 m resolution installed on the national research council nrc of canada convair 580 data were collected in winter midlatitude and polar stratiform clouds associated with frontal systems during three field projects in the canadian and u s arctic and over the great lakes approximately 106 images of particles measured in ice clouds were included in the statistics the frequency of occurrence of the aspect ratio and roundness were calculated in eight 5 temperature intervals from 40 c to 0 c in each temperature interval the distributions were calculated for 12 size intervals in the range from 20 m to 1 mm it was found that the roundness is a function of particle size and within each size interval it does not depend significantly on temperature however the aspect ratio of parti

evolution of a cold front encountering steep quasi 2d terrain coordinated aircraft observations on 8 9 december 2001 during improve 2 abstract research aircraft observations from the 8 9 december 2001 case of the second phase of the improvement of microphysical parameterization through operational verification experiment improve 2 describe the evolution of a wide cold frontal rainband wcfr during its eastward advance from the pacific coastline to a point 200 km inland over the cascade mountains of oregon this analysis has two objectives first to illustrate the rapid weakening of the circulation associated with a landfalling wcfr and the relationship of these changes to terrain induced airflow modifications and second to quantify the degree to which this weakening impacted cloud microphysical properties such as liquid water content ice particle concentrations and precipitation rate the kinematic structure of the wcfr is detailed using doppler radar observations from a noaa p 3 aircraft while some concomitant cloud microphysical properties are documented using flight level measurements from the university of washington convai

frontogenesis and symmetric stability in a major new england snowstorm abstract synoptic and doppler radar data are used to study the roles of large scale frontogenetical forcing and of moist symmetric instability in the new england snowstorm of 5 6 december 1981 associated with an explosively intensifying cyclone offshore radar reflectivity patterns showed a tendency toward banded structure particularly near the leading northwestern edge of the storm only a minor portion of the snowfall however was associated with this pronounced bandedness from a set of constant pressure analyses the frontogenetical forcing was measured from the variation along the temperature gradient of the geostrophic wind component in the direction of this gradient over southeastern new england maximum forcing found near 500 mb at the outset of the storm descended to the layer between 850 and 700 mb 24 h later magnitudes were 3 7 10 10 deg m 1 s 1 observed rates of strengthening of temperature gradient were less than half this value implying relative adiabatic cooling in the rising

impact basin relaxation at iapetus abstract we investigate impact basin relaxation on iapetus by combining a 3d thermal evolution model robuchon g choblet g tobie g cadek o sotin c grasset o 2010 icarus 207 959 971 with a spherical axisymmetric viscoelastic relaxation code zhong s paulson a wahr j 2003 geophys j int 155 679 695 due to the progressive cooling of iapetus younger basins relax less than older basins for an ice reference viscosity of 10 14 pa s an 800 km diameter basin relaxes by 30 if it formed in the first 50 myr but by 10 if it formed at 1 2 gyr bigger basins relax more rapidly than smaller ones because the inferred thickness of the ice shell exceeds the diameter of all but the largest basins considered stereo topography shows that all basins 600 km in diameter or smaller are relaxed by 25 or less our model can match the relaxation of all the basins considered within error by assuming a single basin formation age 4 36 ga for our nominal viscosity this result is consistent with crater counts which show no detectable age variation between the basins examined

ice particle habits in stratiform clouds ice crystals in clouds in the atmosphere have shapes that relate to their density terminal fall velocity growth rate and radiative properties in calculations for climate change predictions forecasting of precipitation and remote sensing retrievals idealized crystal shapes such as columns needles plates and dendrites are often assumed the objective of this work is to study the frequency of occurrence of different habits of ice particles in natural clouds from aircraft observations images of cloud particles were measured by a pms optical array probe 2dc at 25 m resolution installed on the national research council convair 580 the processing of particle images was conducted with a newly developed algorithm for pattern recognition data were collected during four field projects in the canadian and us arctic over the north atlantic near newfoundland and over the great lakes approximately 5 106 images of cloud particles having a size larger than 125 m were analysed the cloud particles were classified into four categories spheres irregulars needles columns and dendrites the habit classification of particles was done for three different size ranges 125 m 250 m and 500 m the frequency of occurrence of different habits was found for each 5 degc temperature interval in the range 45 c t 0 c it was concluded that the majority of ice particles in natural clouds were of irregular shape the frequency of occurrence of irregular ice decreases with increasing particle size on average the concentration of particles larger than 125 m was approximately constant down to 35 c whereas the concentration of particles larger than 500 m decreased at temperatures below 15 c since the data were collected in different climatic zones within many cloud lypes and covered a significant cloud path length 3 6 104 km the conclusions are applicable to most stratiform clouds containing ice

implications of using broadband photometry for compositional remote sensing of icy objects water ice has such a low absorption coefficient at visual wavelengths 0 01 cm 1 that a very small amount of particulate material can significantly darken an icy surface a variety of ice plus particle mixtures were studied to show that particulate contaminations of 1 by weight even 0 1 or less in some cases in ice or frosts result in reflectance levels close to that of the contaminants in a very clear ice no bubbles it is plausible to have a reflectance 0 05 for particulate contaminations 10 7 by weight for submicron dark particles such as carbon lampblack scattering conditions compete for domination with contaminants for control of visual reflectance implying that the apparent reflectivity level and color of a surface is a poor indicator of ice content a dark surface e g albedo 0 05 does not necessarily imply that there us very little water ice present infrared jhk colors of water ice and other minerals including ice mineral mixtures show that some orthopyroxenes can have jhk colors very similar to fine grained water frosts in general it is possible that the jhk colors of an ice plus particulate mixture can fall anywhere in the classical j h versus h k diagram thus the diagram cannot be used to distinguish a predominately rock surface from one which is predominantely ice for one specific case an important exception is the case where both the j h and h k colors are 0 2 it appears that such colors indicate a relatively pure icy surface in some cases the diagram might be used as a statistical tool to distinguish between the compositions of surfaces within a class of objects but the validity of such comparisons decreases for different classes such as the main belt asteroids when compared to outer solar system satellites where water ice is more stable

the improve 1 storm of 1 2 february 2001 part ii cloud structures and the growth of precipitation abstract on 1 2 february 2001 a strong cyclonic storm system developed over the northeastern pacific ocean and moved onto the washington coast this storm was one of several that were documented during the first field phase of the improvement of microphysical parameterization through observational verification experiment improve in the 1 2 february case soundings and wind profiler measurements showed that a wide cold frontal rainband was coincident with the leading edge of an upper level cold front in a classical warm occlusion ground based radar observations revealed the presence of subbands within the wide cold frontal rainband and two layers of precipitation generating cells within this rainband one at 5 7 km msl and the other at 9 10 km msl the lower layer of generating cells produced fallstreaks that were traced from the cells down to the radar bright band at 2 km msl observations suggest a connection between the subbands and the lower layer of generating cells a research aircraft equipped

observed microphysical evolution for two east coast winter storms and the associated snow bands abstractthis paper presents the observed microphysical evolution of two coastal extratropical cyclones 19 20 december 2009 and 12 january 2011 and the associated passage of heavy snowbands in the cyclone comma head the observations were made approximately 93 km east of new york city at stony brook new york surface microphysical measurements of snow habit and degree of riming were taken every 15 30 min using a stereo microscope and camera and snow depth and snow density were also recorded a vertically pointing ku band radar observed the vertical evolution of reflectivity and doppler vertical velocities there were rapid variations in the snow habits and densities related to the changes in vertical motion and depth of saturation at any one time a mixture of different ice habits was observed certain ice habits were dominant at the surface when the maximum vertical motion aloft occurred at their favored temperature for depositional growth convective seeder cells above 4 km msl resulted in relativel

surface microphysical observations within east coast winter storms on long island new york abstractsurface observations of ice habit and degree of riming were measured for 12 cyclone events over 3 winter seasons at stony brook new york on the northeast coast of the united states a total of 205 6 cm of snow accumulated during these storms with an average degree of riming of 1 25 out of 5 and snow to liquid ratio ranging from 3 1 to 17 1 there were consistent spatial patterns of habit and riming intensity relative to the cyclone structure cold type habits side planes and bullets commonly occurred within the outer comma head to the north and northeast of the cyclone center in the middle of the comma head moderately rimed dendrites plates and needles were observed close to the cyclone center heavy riming was observed with needles and graupel the western quadrant of the comma head had primarily plates and dendrites with little to no riming periods of light riming and high snow liquid ratios 13 1 are dominated by cold type habits dendrites and plates and have similar vertical m

ice crystal habits and growth processes in stratiform clouds with embedded convection examined through aircraft observation in northern china abstractice crystal habits and growth processes in two cases of stratiform clouds with embedded convection are investigated using data observed simultaneously from three aircraft on 18 april 2009 and 1 may 2009 as part of the beijing cloud experiment bce the results show that the majority of ice crystal habits found in the two cases at temperatures between 0 and 16 c included platelike needle column capped column dendrite and irregular a mixture of several ice crystal habits was identified in all of the clouds studied however the ice crystals recorded in the embedded convection regions contained more dendrites and possessed heavier riming degrees and the ice crystals identified in the stratiform clouds contained more hexagonal plate crystals both riming and aggregation processes played central roles in the broadening of particle size distributions psds and these processes were more active in embedded convection regions than in stratiform regions however riming was more prevalent in the

viscous relaxation of craters within the martian south polar layered deposits abstract the south polar layered deposits pld are of fundamental importance to martian climatology as they may comprise the largest reservoir of near surface water on mars the south pld exhibit relatively young crater retention surface ages which are widely attributed to recent resurfacing however we show that both constructional and destructional resurfacing mechanisms such as dust deposition and water ice sublimation respectively are inconsistent with the size depth and spatial distributions of south pld craters we demonstrate that another process viscous creep relaxation of dusty water ice is more compatible with the observed cratering of south pld surfaces the results of our finite element relaxation simulations suggest that despite their apparent youthfulness the pld have been stable for at least several hundred million and perhaps even over a billion years consequently our modeling implies that the time scales for the formation and preservation of the layers characteristic of the south and possibly north pld are much longer than generally assumed

organization and structure of clouds and precipitation on the mid atlantic coast of theunited states part iii the evolution of a middle tropospheric cold front abstract the development of a complex middle tropospheric frontal structure the various weather associated with itsprogression across the united states and its role in the production of precipitation in the eastern third of theunited states are examined the frontal structure consisted of two features a middle tropospheric cold front associated with a strong 500mb short wave that moved eastward from the pacific ocean and a leeside warm front that formed in a northwardsloping zone of warm air advection associated with a trough in the lee of the rocky mountains the middletropospheric cold front overtook the leeside warm front to produce a warm occlusion like structure in the middletroposphere as this system progressed eastward across the united states precipitation from light rain to convectiveshowers occurred along the leading edge of the middle4ropospheric frontal zone well ahead of a decayingsurface trough this study highlights the importance of middle tropospheric frontal structures in the org

on the resurfacing of ganymede by liquid water volcanism abstract a long popular model for producing ganymede s bright terrain involves flooding of low lying graben with liquid water slush or warm soft ice the model suffers from major problems however including the absence of obvious near surface heat sources the negative buoyancy of liquid water and the lack of a mechanism for confining the flows to graben floors we present new models for cryovolcanic resurfacing to overcome these difficulties tidal heating within an ancient laplace like orbital resonance showman and malhotra 1997 icarus 127 93 showman et al 1997 icarus 129 367 provides a plausible heat source and could allow partial melting to occur as shallow as 5 10 km depth our favored mechanism for delivering this water to the surface invokes the fact that topography such as a global set of graben causes subsurface pressure gradients that can pump water or slush upward onto the floors of topographic lows graben despite the negative buoyancy of the liquid these eruptions can occur only within the topographic lows furthermore as the low areas become full the pressure gradients disappear and the resurfacing ceases this provides an explanation for the observed straight dark bright terrain boundaries water cannot overflow the graben so resurfacing rarely embays craters or other rough topography pure liquid water can be pumped to the surface from only 5 10 km depth but macroscopic bodies of slush ascending within fractures can reach the surface from much greater depths due to the smaller negative buoyancy of slush a challenge for these models is the short predicted gravitational relaxation timescale of topographic features at high heat flows the resurfacing must occur before the graben topography disappears we also evaluate alternate resurfacing mechanisms such as pumping of liquid water to the surface by thermal expansion stresses and buoyant rise of water through a silicate contaminated crust that is denser than liquid water and conclude that they are unlikely to explain ganymede s bright terrain

shallow normal fault slopes on saturnian icy satellites fault dips are a function of the coefficient of internal friction i of the lithospheric material laboratory deformation experiments of h2o ice at conditions applicable to icy bodies yield 0 i 0 55 such that normal faults dip between 45 and 59 we tested the hypothesis that normal faults on icy bodies reflect these values by using digital elevation models to examine geometries of large extensional systems on three saturnian satellites analyzed faults within ithaca chasma on tethys and avaiki chasmata on rhea all exhibit shallower than predicted topographic slopes across the fault scarp which we term fault slopes a scarp of padua chasmata within dione s wispy terrain also has a shallow fault slope although three others that make up palatine chasmata exhibit steeper slopes as predicted we infer that viscous relaxation is the most viable explanation for these shallow fault slopes and we model the potential role of viscous relaxation in creating shallow fault slopes our modeling results support formation of these normal faults with steep dips consistent with deformation experiments followed by their relaxation due to lithospheric heating events related to radionuclide decay the steepest fault slopes in this terrain yield 0 i 0 73 for dione s lithospheric ice which overlaps the dip range predicted from experiments results of this work suggest that viscous relaxation substantially affected fault slopes on tethys rhea and dione by implication these processes may have also affected fault geometries on other icy satellites

the shape and density evolution of snow aggregates abstractsnow aggregates evolve into a variety of observed shapes and densities despite this diversity models and observational studies employ fractal or euclidean geometric measures that are assu

viscous relaxation of impact craters on icy planetary surfaces determination of viscosity variation with depth spacecraft images show that the icy galilean satellites have surfaces with very low topographic relief impact craters on ganymede and callisto are anomalously shallow and are characterized by sharp well defined rims and domed floors these morphological characteristics can be explained by viscous relaxation of topography on an icy crust in which the viscosity is uniform or decreases with depth under these conditions large craters relax more rapidly than small craters therefore explaining a possible underabundance of large craters viscous relaxation on an icy crust that is thin compared to the crater diameter or on a thick icy crust in which viscosity increases with depth could not produce this crater morphology and would result in the more rapid relaxation of small craters rather than large craters the results of this study suggest that more detailed analysis of relaxing impact crater morphology may resolve the rate of viscosity decrease with depth and so provide evidence on the interior thermal evolution of icy planetary bodies

flank tectonics of martian volcanoes on the flanks of olympus mons is a series of terraces concentrically distributed around the caldera their morphology and location suggest that they could be thrust faults caused by compressional failure of the cone in an attempt to understand the mechanism of faulting and the possible influences of the interior structure of olympus mons we have constructed a numerical model for elastic stresses within a martian volcano in the absence of internal pressurization the middle slopes of the cone are subjected to compressional stress appropriate to the formation of thrust faults these stresses for olympus mons are 250 mpa if a vacant magma chamber is contained within the cone the region of maximum compressional stress is extended toward the base of the cone if the magma chamber is pressurized extensional stresses occur at the summit and on the upper slopes of the cone for a filled but unpressurized magma chamber the observed positions of the faults agree well with the calculated region of high compressional stress three other volcanoes on mars ascraeus mons arsia mons and pavonis mons possess similar terraces extending our analysis to other martian volcanoes we find that only these three and olympus mons have flank stresses that exceed the compressional failure strength of basalt lending support to the view that the terraces on all four are thrust faults

the thermal gradient of callisto constrained by asgard basin rheological and chemical implications using the width of concentric grabens formed during the relaxation of the asgard basin on callisto in combination with rheological flow laws of ice and laws concerning the variations in strain rate around a relaxing crater the surface thermal gradient of callisto at 4 b y is estimated to be near 0 5 k km 1 it is assumed that the graben width depends on the depth of the brittle ductile transition zone around the asgard basin the depth of this brittle ductile transition zone varies from 23 km at 200 km from the crater rim to 4 km at 800 km from the crater rim it is assumed that the thermal characteristics were constant over the extended zone and that the difference in the depth of the brittle ductile transition zone is only due to differences in the strain rate around the basin during the relaxation of the initial cavity calculations show that newtonian flow law for ice gives a strain rate of external relaxation at the crater rim of about 10 14 s 1 while a power law gives a strain rate of about 10 6 s 1 assuming a surface temperature between 100 and 150 k the order of magnitude of the calculated thermal gradient is independent of the chosen flow law this calculated gradient is however one order of magnitude lower than those calculated theoretically with a chondritic abundance of radioactive elements in the silicate part of callisto

volcanic constructs on ganymede and enceladus topographic evidence from stereo images and photoclinometry paul m schenklunar and planetary institute houston texasjeffrey m moorespace sciences division and the seti institute nasa ames research center moffett field californiaabstract the morphology of volcanic features on ganymede differs significantly from that onthe terrestrial planets few if any major volcanic landforms such as thick flows or shieldvolcanoes have been identified to date using new stereo voyager images we have searchedganymede for relief generating volcanic constructs we observed seven major types of volcanicstructures including several not previously recognized the oldest are broad flat topped domespartially filling many older craters in dark terrain similar domes occur on enceladus togetherwith smooth dark deposits these domes indicate that the volcanic history of the dark terrain iscomplex bright terrain covers vast areas although the style of emplacement remains unclear smooth bright materials embay and flood older terrains and may have been emplaced as low viscosity fluids associated with smooth bright material are a number of scalloped shaped semi enclosed scarps that cut into preexisting terrain in planform these structures resemble terrestrialcalderas the youngest volcanic materials identified are a series of small flows that may haveflooded the floor of the multiring impact structure gilgamesh forming a broad dome theidentification of volcanic constructs up to 1 km thick is the first evidence for extrusion ofmoderate to high viscosity material on ganymede viscosity and yield strength estimates for thesematerials span several orders of magnitude indicating that volcanic materials on ganymede have arange of compositions and or were extruded under a wide range of conditions and or eruptive styles introductionthe voyager survey of the outer solar system has revealedan astonishing variety of endogenic landforms on the surfacesof icy satellites despite extensive endogenic resurfacing ofsomc satellites for example ganymede enceladus andtriton volcanism on these surfaces has little topographicexpression we do not see massive volcanic edifices similarto olympus mons or mauna kea even the best putativeexamples of volcanic landforms seen on ariel miranda triton and enceladus exhibit rarely more than 1 2 km ofrelief jankowski and squyres 1988 pozio and kargel 1989 schenk 1991a croft et al 1995 this may berelated to the soft theology of ice see poirier 1982 durham et al 1983 and kirby et al 1987 or torheology and volcanic styles peculiar to icy petrologicsystems e g kargel 1991 kargel et al 1991 well over half of the surface of ganymede radius 2634 kin has been covered by endogenic effusions it is now consideredlikely that ganymede experienced one or more episodes ofchaotic orbital behavior malhotra 1991 possiblyresulting in tidal heating volcanic resurfacing constitutes arecord of ganymede s compositional and thermal evolution andmay record some of these putative tidal heating events yetthis satellite has been the subject of morphological study for15 years without the unambiguous detection of relief copyright 1995 by the american geophysical union paper number 95je01854 0148 0227 95 95je 01854505 00generating volcanic constructions on its landscape the styleof emplacement and composition of those volcanic depositsthat have been identified remains unclear on any planetary surface volcanic and other nonimpactconstructs can be difficult to recognize in spacecraft images particularly those obtained under high sun conditions e g malin 1978 topography is especially low on the galileansatellites and imaging resolution is at best 500 m pixel major local albedo variations on ganymede further complicategeologic and topographic mapping with these problems inmind we have undertaken the production of stereo pairs ofvoyager images where coverage permits to aid in the searchfor and characterization of volcanic landforms on ganymede ssurface in addition to an examination of previously identifiedvolcanic materials we report the discovery of severalpreviously unrecognized volcanic landforms and at least twotypes with substantial relief and describe their morphologyand geologic setting morphology of volcanic depositsindicates that ganymede extrusions probably had a range oftheologies and may be compositionally variable we concludewith speculations and predictions regarding their mode ofemplacement and the implications of volcanic landforms forthe geologic evolution of ganymede stereo imaging with voyagerthis study was motivated by the recognition that asignificant fraction of the surfaces of most of the outer planetsatellites were imaged by voyager in stereo e g croft andsoderblom 1991 schenk and moore 1993 we can usestereo imaging to detect subtle topographic features that were19 009

a comprehensive habit diagram for atmospheric ice crystals confirmation from the laboratory airs ii and other field studies abstract recent laboratory experiments and in situ observations have produced results in broad agreement with respect to ice crystal habits in the atmosphere these studies reveal that the ice crystal habit at 20 c is platelike extending to 40 c and not columnar as indicated in many habit diagrams found in atmospheric science journals and texts these diagrams were typically derived decades ago from laboratory studies some with inherent habit bias or from combinations of laboratory and in situ observations at the ground observations that often did not account for habit modification by precipitation from overlying clouds of varying temperatures habit predictions from these diagrams often disagreed with in situ observations at temperatures below 20 c more recent laboratory and in situ studies have achieved a consensus on atmospheric ice crystal habits that differs from the traditional habit diagrams these newer results can now be combined to give a comprehensive description of ice crystal habits

detection of faceted crystals in deep convective clouds via the antisolar peak 1 it is shown that an antisolar peak can clearly be detected in backscattered near infrared reflectances from deep convective cloud tops this peak should be caused only by regularly faceted ice crystals primarily columns and plates rather than rimed or irregular particles using 13 years of avhrr advanced very high resolution radiometer data the average peak stands 22 above the background for sampled clouds having brightness temperatures below 220 k the implied mean contribution of pristine crystals to total ice surface area based on a published model is 13 5 local estimates of this percentage agree roughly with available field data the distribution of peak strength is positively skewed suggesting that a few deep cumuli may be rich in regular particles while a majority contain even less than the 13 mean the antisolar peak varies geographically with largest mean values over the central atlantic and pacific itczs and smallest values over the caribbean american region during summer curiously there is no significant land ocean contrast in the antisolar peak even though modis moderate resolution imaging spectroradiometer data indicate a strong land ocean contrast in cumulus glaciation temperature so it does not appear that supercooled water amounts at lower levels regulate the habit distribution observed at cloud top in any straightforward way variations in the antisolar peak may be significant for backscatter lidar retrievals of cloud extinction

the geology of callisto the geology of callisto is not boring although cratered terrain dominates callisto a key end member of the jovian satellite system a number of more interesting features are apparent cratered terrain is broken into irregular mappable bright and dark subunits that vary in albedo by a factor of 2 and several relatively smooth units are depleted of small craters some of these areas may have been volcanically resurfaced lineaments including parallel and radial sets may be evidence for early global tectonism frost deposition occurs in cold traps and impact scars have formed from tidally disrupted comets geologic evidence suggests that callisto does have a chemically differentiated crust central pit and central dome craters and palimpsests are common the preferred interpretation is that a relatively ice rich material at depths of 5 km or more has been mobilized during impact and exposed as domes or palimpsests the close similarity in crater morphologies and dimensions indicates that the outermost 10 km or so of callisto may be as differentiated as on ganymede the geology of cratered terrain on callisto is simpler than that of cratered terrain on ganymede however orbital evolution and tidal heating may provide the answer to the riddle of why callisto and ganymede are so different malhotra 1991 we should expect a few surprises and begin to answer some fundamental questions when callisto is observed by galileo in late 1996