a numerical simulation of quasi stationary tropical convective bands a method of imposing large scale ascent on a three dimensional numerical model is introduced and this is shown to be crucial in maintaining the quasi stationary convective bands observed in gate the effects of ascent on the overall convective development are to enhance updraughts and particularly to maintain the development of new convection cells the magnitutude of the forcing is varied to show the sensitivity of growth on convergence a quasi stationary convective band aligned approximately parallel to the low level wind shear is simulated in an initial environment defined from soundings obtained from the gate ships researcher and oceanographer on 3 september 1974 the organization is therefore quite distinct from that of fast moving squall lines aligned at right angles to the shear the simulated convective system has marked dynamical similarity to the observed bands these consisted of a number of cumulonimbus cells with new cells developing on the eastern end of the hand with deeper cells to the west the cells generally travel along the line the large shear due to the strong low level jet is crucial in influencing the band development and orientation the dynamical structure of the convective band is investigated by a trajectory analysis which indicates an organized three dimensional structure the transports of heat moisture and momentum are calculated and a quasi balance between large scale ascent and cloud induced descent on a time scale of a few hours is shown to exist apparent heat sources and moisture sinks are calculated and these have a bi modal vertical structure in general agreement with those in observed tropical cloud clusters

basal ice facies and supraglacial melt out till of the laurentide ice sheet tuktoyaktuk coastlands western arctic canada glacially deformed massive ice and icy sediments mi is in the eskimo lakes fingerlands and summer island area of the tuktoyaktuk coastlands western arctic canada show in the same stratigraphic sequences features characteristic of both basal glacier ice and intrasedimental ice basal ice features comprise 1 ice facies and facies groupings similar to those from the basal ice layers of contemporary glaciers and ice sheets in alaska greenland and iceland 2 ice crystal fabrics similar to those from basal ice in antarctica and ice cored moraines on axel heiberg island canada and 3 a thaw or erosional unconformity along the top of the mi is buried by glacigenic or aeolian sediments intrasedimental ice consists of pore ice and segregated ice formed within pleistocene sands deposited before glacial overriding the co existence of basal and intrasedimental ice within the mi is records their occurrence within the basal ice layer of the laurentide ice sheet stagnation of the ice sheet and melt out of till from the ice surface allowed burial and preservation of the basal ice layer on a regional scale the widespread occurrence of supraglacial melt out till with clast fabrics similar to those in the underlying ice suggests that such till can be well preserved during partial thaw of a continental ice sheet in lowlands underlain by continuous permafrost

momentum transport by a line of cumulonimbus abstract the vertical transport of horizontal momentum normal to a line of cumulonimbus observed during gate on 14 september 1974 is against the vertical momentum gradient contrary to the predictions of mixing length theory data from repeated aircraft passes normal to the line s axis at heights from 0 15 to 5 5 km are used to document the flux and determine its source the flux is concentrated in roughly a 25 km wide active zone just behind the leading edge of the line in kilometer scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces the fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical with the clouds at the surface well ahead of those aloft evaluation of the terms in the momentum flux generation equation confirms that the above process reflected by the velocity buoyan

a numerical study of a mesoscale convective system during toga coare part ii organization in part i the authors presented a successful numerical simulation of the life cycle of a warm pool mesoscale convective system mcs that occurred on 15 december 1992 during the tropical ocean global atmosphere coupled ocean atmosphere response experiment in this study the simulation results of part i are diagnosed to investigate the organization of the mcs and the convective onsets that occurred during the growing and mature stages of the mcs during the life cycle of the mcs four convective onsets occur in the presence of large scale ascent convective available potential energy cape and surface potential temperature drop off sptd it is found that the first convective onset is caused by the existence of upward motion cape and sptd in the model initial conditions the second convective onset is regulated by the favorable occurrence of sptd the third and fourth convective onsets arise from the development of upward motion associated with the westward propagation of the quasi 2day wave the four mesoscale precipitation features clustered together to form the mcs in response to the evolution of the vertical motion field the organization of the mcs is characterized by the presence of a midtropospheric mesovortex situated near the position of the first convective onset analysis of the relative vorticity rv budget indicates that the mesovortex originates and intensifies largely from vortex stretching induced by deep convective heating a decrease in rv above below the mesovortex arises because of the combined effects of the tilting and horizontal advection terms the tilting stretching and solenoidal terms our results suggest that the mesovortex played little role in the subsequent onsets i e second third and fourth and that other warm pool mcss occurring near the transequatorial flow are likely to be associated with mesovortices

characteristics of a subtropical squall line determined from tamex dual doppler data part ii dynamic and thermodynamic structures and momentum budgets abstract a thermodynamic retrieval method was used to study the dynamical and thermodynamical structure of a subtropical squall line which occurred on 17 may 1987 over the taiwan straits three dimensional wind fields were derived from the dual doppler data based on the methodology presented in part i of this paper subsequently fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations results show that the overall structural features of this subtropical squall line are similar to those of a tropical squall line the orientation of the squall line is almost in a north south direction in the lowest layer the gust front is located to the immediate east of the main convective updrafts high pressure occurs behind the gust front with low pressure to its east a buoyancy induced low pressure area lies beneath the convective updraft corresponding to the ascending warm environmental air in the middle and upper layers high pressure forms

the transition zone and secondary maximum of radar reflectivity behind a midlatitude squall line results retrieved from doppler radar data abstract thermodynamic and microphysical retrieval techniques are applied to dual doppler synthesized air motion fields for a midlatitude squall line which passed through the oklahoma kansas preliminary regional experiment for the stormscale operational and research meteorology program pre storm observational array in kansas and oklahoma on 10 11 june 1985 the retrieved pressure and potential temperature fields are consistent with surface network and sounding data while the retrieved microphysical fields show the characteristic secondary maximum of radar reflectivity in the stratiform region and the band of low reflectivity or transition zone lying between the leading convective line and the secondary maximum the retrieved fields indicate the processes producing the secondary maximum and transition zone minimum of radar reflectivity more quantitatively than has been possible in previous studies the primary processes accounting for these features of the radar reflectivity pattern were 1 the subst

analysis of a heavy rainfall event during tamex abstract a heavy rainfall event during the taiwan area mesoscale experiment intensive observing period 13 has been studied using upper air surface mesonet and dual doppler radar data the heavy rainfall 231 mm day 1 occurred over northwestern taiwan with the maximum rainfall along the northwestern coast and was caused by a long lived convective rainband in the prefrontal atmosphere it occurred in an upper level divergence region and along the axis of the maximum equivalent potential temperature at the 850 hpa level as a mei yu front advanced southeastward the postfrontal cold air in the lowest levels was retarded by the hilly terrain along the southeastern china coast as a result a low level wind shift line associated with a pressure trough at the 850 hpa level moved over the taiwan strait before the arrival of the surface front the westerly flow behind the trough interacted with a barrier jet along the northwestern coast of taiwan the barrier jet is caused by the interaction between the pref

shear parallel moist convection over the tropical ocean a case study from 18 february 1993 toga coare abstract this paper describes a newly observed form of shear parallel rainbands that were observed on 18 february 1993 in tropical ocean global atmosphere coupled ocean atmosphere response experiment using the ncar eldora airborne doppler radar interesting features of these rainbands included their rapidly changing and highly cellular nature their complicated and layered kinematic structure and the effects of the rainbands on the tropical environment in their lower levels the rainbands consisted of east west elongated clusters of small convective cells aligned with the z 0 5 km shear vector strong low level convergence inflow from the north and enhanced westerly winds extended from the surface up to 5 km much of this air was vented out of the storm at midlevels exiting to the north in the 0 c stable layer at z 5 7 km this midlevel outflow fed an area of widespread stratiform precipitation lying to the north of the rainband the cold pool outflow emanating from this area of precipitation t

a tropical squall line observed during the copt 81 experiment in west africa part 1 kinematic structure inferred from dual doppler radar data abstract this paper deals with the analysis of a tropical squall line observed on 22 june 1981 during the copt 81 convection profonde tropicale experiment the present part i is restricted to the study of the kinematic structure of the system which moved in a moderately unstable atmosphere faster than the environmental air at all levels at the observation time the squall line was in a mature stage the explicit description of the airflow within the system was inferred from dual and single doppler radar data the general characteristics of the squall line are found to be very similar to those of tropical squall lines observed during previous experiments such as vhimex or gate a large cloud system composed of an organized convective line ahead of an extensive trailing anvil cloud stratiform rain fast motion and long lasting structure and a well marked gust front signature at ground level the overall airflow presents a three dimensional structure at the leading edge convective scale updrafts a

the relationship between emex mesoscale precipitation feature properties and their environmental characteristics abstract characteristics of the prestorm environment of equatorial mesoscale experiment emex mesoscale precipitation features mpfs are related to properties of them systems using regression analysis although environmental thermodynamic parameters are poorly correlated with emex mpf properties mainly because environmental thermodynamic conditions varied little among mpfs kinematic parameters are well correlated to these properties lines whose environments have low level shear the shear between about 950 and 750 mb exceeding 5 m s 1 are oriented normal to the direction of this shear lines where the low level shear is under 5 m s 1 are oriented along the direction of the midlevel shear the shear between about 800 and 400 mb convective line speeds correlate well with the maximum speed of the rear to front flow in the troposphere below 300 mb direction of mpf motion is nearly coincident with the tropospheric mean wind direction convective line length is proportional to the magnitude of the men

structure of a midlatitude squall line formed in strong unidirectional shear abstract data from five doppler radars the surface mesonet aircraft and rawinsondes from the cooperative convective precipitation experiment ccope are used to document the structure and evolution of a squall line with unusually persistent cells and an anvil that spreads downwind in strong upper level westerlies the environmental sounding showed linear shear of 4 m s 1 km 1 through the troposphere a convective available potential energy of 600 m2 s 2 and a convective richardson number of 10 based on the wind in the lowest 6 km the orientation of the squall line comprised of high reflectivity centers spaced 20 40 km apart changed with time initially the squall line axis was normal to the environmental shear but with time it became parallel to the shear vector as the northeastern portion of the subcloud cold dome merged with cold air generated by individual storms that had formed ahead of the line the intensity of the cells within the squads line diminished as its axis became more parallel

thermodynamic analytic fields from doppler radar data by means of the mandop analysis the paper is devoted to scale interaction studies it presents a new approach for retrieving mesoscale and convective scale thermodynamic fields from doppler radar data in order to diagnose processes involved in these interactions thermodynamic fields are retrieved analytically using the analytic form of the three dimensional wind field obtained by the mandop analysis in the non dissipative and stationary first order anelastic approximation of the equation of motion applications on simulated data and experimental data from the toga coare show the capability of this new method to retrieve the pressure and temperature fields at various scales of motion and in particular at mesoscale this new analysis will be used intensively on doppler radar data collected during the fronts and atlantic storm tracks experiment in order to diagnose the multiscale interaction processes associated with frontal wave and or secondary cyclogenesis

a numerical study of the stratiform region of a fast moving squall line part ii relationship between mass pressure and momentum fields abstract in a companion paper a two dimensional simulation of a fast moving tropical squall line was successfully compared to observations performed during the copt81 experiment over west africa the full ice phase parameterization is shown to be crucial in the simulation of trailing anvil precipitation different diagnostic tools are applied to the simulated fields to further our understanding of the scale interactions within a squall line type mesoscale convective system the pressure organization is characterized by two marked features important for explaining the inner circulation first a front to rear midlevel pressure gradient and second the surface pressure mesohigh extending from the gust front to the rear of the most active part of the trailing stratiform region based on the hydrostatic approximation an original method of decomposition of the pressure field is proposed whereby dynamical and buoyant contributions depend only on the horizontal and vertical respectively the mean pressure i

momentum flux by lines of cumulonimbus over the tropical oceans abstract examination of aircraft and rawinsonde data gathered in nine tropical mesoscale convective line cases indicates that all but two lines systematically increased front to rear momentum at heights greater than about 4 km and rear to front momentum at lower levels where front is defined as the direction toward which the line is moving the convective lines were characterized by a leading 10 20 km wide band of convective clouds and a trailing region of stratiform cloudiness most wore propagating lines moving into the wind at all levels consistent with mixing length theory the vertical transport of the horizontal wind component parallel to the lines was down the vertical gradient of the component resulting in a decrease of its vertical shear smaller more random cloud groups and the upper portions of a convective line with isolated towers transported both components of horizontal momentum downgradient normalization of the vertical flux of horizontal momentum normal to the line u w su

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

momentum and mass transport by convective bands comparisons of highly idealized dynamical models to observations the importance of the vertical transport of horizontal momentum by convective clouds has been documented in studies of momentum budgets e g shapiro and stevens 1980 sui and yanai 1986 furthermore it is obvious that the boundary layer wind itself and hence surface heat moisture and momentum transports is affected by deep convection most of the airsea transfer in tropics is concentrated in regions with deep convection

distribution and growth of thaw slumps in the richardson mountains peel plateau region northwestern canada abstract retrogressive thaw slumps are one of the most active geomorphic features in permafrost terrain this study investigated the distribution and growth of thaw slumps in the richardson mountains and peel plateau region northwestern canada using tasseled cap tc trend analysis of a landsat image stack based on the tc linear trend image more than 212 thaw slumps were identified in the study area of which 189 have been active since at least 1985 the surface area of the slumps ranges from 0 4 to 52 ha with 10 slumps exceeding 20 ha the thaw slumps in the region are all situated within the maximum westward extent of the laurentide ice sheet based on relations between frequency distribution of slumps and that of terrain factors in the landscape the slumps are more likely to occur on the ice rich hummocky rolling moraines at elevations of 300 350 m and 450 500 m and along east facing slopes slope aspects of 15 to 180 with gradients of 8 to 12 pixel level trend analysis of the tc greenness transformation in the landsat stack allowed calculating headwall retreat rates for 19 thaw slumps the 20 year average retreat rates 1990 2010 period for 19 slumps ranged from 7 2 to 26 7 m yr 1 with the largest slumps having higher retreat rates at the regional scale the 20 yr headwall retreat rates are mainly related to slope aspect with south and west facing slopes exhibiting higher retreat rates and large slumps appear to be generating feedbacks that allow them to maintain growth rates well above those of smaller slumps overall the findings presented in this study allow highlighting of key sensitive landscapes and ecosystems that may be impacted by the presence and growth of thaw slumps in one of the most rapidly warming region in the arctic

upstream effects of mesoscale mountains abstract the alpine experiment alpex has revealed that low level air is typically diverted around the alps without reaching the mountaintop in pursuit of an understanding of the physical basis of this phenomenon and of its generality we have explored the characteristics of orographic blocking of a rotating continuously stratified quid as revealed in a simple model problem retaining full nonlinear and transient effects hydrostatic dynamics is assumed and the obstacle is taken to be an infinitely long ridge with height h x the key questions treated are the strength of the upstream deceleration of cross mountain flow and the length scale over which the decelerated region extends by means of scale analysis the controlling parameters are found to be the rossby number ro u fl and the froude number fr nhm u where u is the speed of the oncoming flow f is the coriolis parameter l the mountain half width n the brunt vaisala frequency and hm is the maximum mountain height the scale analysis also

evolution and dynamics of a late stage squall line that occurred on 20 february 1993 during toga coare abstract airborne doppler and flight level data are used to document the structure and evolution of portions of a late stage horseshoe shaped squall line system and its effect on vertical momentum and mass transports this system which occurred on 20 february 1993 during the tropical ocean global atmosphere coupled ocean atmosphere response experiment was similar to many previously studied but had some unique features first a slow moving transverse band which formed the southern leg of the horseshoe drew most of its low level updraft air from the squall line stratiform region on its north side rather than the environment to the south second a long lived cell with many properties similar to a midlatitude supercell formed 150 km to the rear of the squall line this cell was tracked for 4 h as it propagated into and then through the cold pool and finally dissipated as it encountered the convection forming the northern edge of the horseshoe finally as the squall line was dissipating a new con

the oceanic mesoscale convective system observed with airborne doppler radars on 9 february 1993 during toga coare structure evolution and budgets data collected on 9 february 1993 during the tropical ocean and global atmosphere programme coupled ocean atmosphere response experiment toga coare with the doppler radars on board the two national oceanic and atmospheric administration wp 3d and the national center for atmospheric research electra aircraft are analysed to study the structure and evolution of an equatorial oceanic mesoscale convective system mcs observations lasted more than five hours a period during which the cloud cluster characteristics changed from convective to stratiform this mesoscale system was approximately aligned along the tropospheric wind shear but it was nearly perpendicular to the low level shear series of three dimensional fields of reflectivity air velocity components retrieved pressure and temperature perturbations show similarities with previously observed convective systems over the tropical oceans in particular although the cloud top reached 18 km altitude relatively weak vertical motions and temperature perturbations resulted from a moderate convective instability in the low levels a cold front to rear flow intensified and deepened as the overall structure evolved from convective to stratiform the vertical profiles of mean horizontal divergence evolved from low level convergence in the cross line direction for the earliest analyses to mid level convergence from all directions during the later stratiform phase vertical momentum fluxes were important and led to an increase in the cross line wind shear and a decrease in the along line one analysis of radiosounding data from the nearest stations of the toga coare array shows that the period during which airborne doppler observations were conducted was characterized by relatively strong large scale ascent and apparent sources of heat moisture and momentum these diagnosed values and the mean radarderived ones agree fairly well this shows that the early convective and late stratiform phases of the mcs are equally important for the apparent sources of heat and momentum

kinematics and microphysics of the transition zone of the 10 11 june 1985 squall line abstract a high resolution composite analysis covering the entire breadth of the northern portion of a mature leading line trailing stratiform squall line system reveals that mean subsidence observed in the transition zone consisted of two different types of average downdraft one at upper levels that was mechanically forced and one at lower levels that was microphysically forced both the upper level and lower level mean downdrafts in the transition zone appeared to be the average effect of convective scale vertical drafts associated with convective structures that moved relative to the front edge of the convective line the structure of individual upper level convective scale downdrafts suggested that they may have been partially composed of gravity waves excited by the interaction of the penetrative convective updrafts of the mature and dissipating convective cells with the stable ambient flow the lower level mean downdraft extended from midlevels to near the surface but was maximum near the melting

fluvial clastic sediment yield in canada scaled analysis this report presents a set of maps of regional fluvial sediment yield in canada based mainly on the water survey of canada archive of riverine suspended sediment observations regional scaling rel

aerobic release and biodegradation of dissolved organic matter from frozen peat effects of temperature and heterotrophic bacteria abstract understanding the conditions of dissolved organic matter dom release from thawing peat in the arctic regions and identifying the pathways of processing dom by soil and aquatic heterotrophic bacteria are critical in the context of rapid climate change until now experimental approaches did not allow quantitative predictions of temperature and biota effects on carbon release from peat in permafrost affected aquatic environments in this study we incubated frozen peat and its aqueous leachate at various temperatures 4 25 or 45 c with and without culturable heterotrophic bacteria iodobacter sp extracted from thermokarst lakes to quantify the release and the removal rate of organic carbon oc with time the metabolic diversity of the native microbial community associated with the substrates involved in oc processing was also characterized transmission electron microscopy revealed that after degradation the associated bacteria are mostly located in the inner parts of plant cells and that the degradation of organic matter around bacteria is more pronounced at 4 and 25 c compared to 45 c the metabolic diversity of heterotrophic bacteria was equally high at 4 and 25 c but lower at 45 c regardless of the microbial consortium native community alone or with added culturable heterotrophs both the oc release from peat and the oc removal from peat leachate by bacteria were similar at 4 and 25 c very low apparent activation energies of dom biodegradation between 4 and 25 c 4 23 12 3 kj mol 1 suggest that the short period of surface water warming in summer would have an insignificant effect on dom microbial processing such duration 1 3 weeks is comparable with the water residence time in peat depressions and permafrost subsidences where peat degradation and dom microbial processing occur this questions the current paradigm of a drastic effect of temperature rise on organic carbon release from frozen peatlands and should be considered for modelling short term climate impacts in these regions

observations and scale analysis of coastal wind jets abstract blocking of onshore flow by coastal mountains was observed south of vancouver island british columbia by the noaa p 3 aircraft on 1 december 1993 winds increased from 10 m s 1 offshore to 15 m s 1 nearshore and became more parallel to shore in the blocked region which had a vertical scale of 500 m and an offshore scale of 40 50 km these length scale and velocity increases are comparable to theory the flow was semigeostrophic with the coast being hydrodynamically steep that is the coast acts like a wall and the alongshore momentum balance is ageostrophic this is shown by the nondimensional slope parameter the burger number b hmn flm being greater than 1 where hm and lm are the height and half width of the mountain n is the stability frequency and f is the coriolis parameter the height scale is given by setting the local froude number equal to 1 that is hl u n 500 m where u is the onshore component of velocity this scale is appropriate when hl is less than the mountain height

comparison between a three dimensional simulation and doppler radar data of a tropical squall line transports of mass momentum heat and moisture abstract results from a detailed three dimensional cloud model are extensively compared with doppler radar data in the case of a fast moving tropical squall line observed during the copt81 experiment the comparisons use a two dimensional statistical analysis in which we consider each four dimensional field as the sum of a two dimensional averaged vertical field and a four dimensional residual field representative of along line and temporal fluctuations these fluctuations are analyzed by computing the time average of spatial variances and the temporal variance of spatial averages the results indicate good agreement in both quality and quantity particularly for the vertical velocity and line normal wind the consequences of this agreement are two fold first a physical and comprehensive model of the convective part of a tropical squall line emerges that is coherent with both observations and simulations this conceptual model emphasizes the importance of along line fluctuations second it is possib

a preliminary morphology of precipitation systems in tropical northern australia a preliminary morphology of convective systems observed in the vicinity of darwin australia is presented several types of tropical convection during monsoonal and break season flow are identified using specific examples and compared with a range of systems observed worldwide in the general context of the bulk richardson number a significant spectrum of convective activity ranging from low shear and low convective available potential energy cape to high shear and moderate to high cape typical of mid latitude severe storms is identified significant differences between monsoonal flow and break season systems are apparent the formation of convective lines perpendicular to the low level shear and the apparent balance between advancing shallow cold pools and the shear are ubiquitous features convective scale downdraughts and mesoscale descent appear to be responsible for redistribution of 700 mb momentum to the planetary boundary layer pel the motion of the break season squall lines appears to result from an equilibrium between the cold pool and the undisturbed environmental pel flow deviations from this balance are hypothesized to result in the observed propagating and non propagating or slow moving modes examples of these modes are given showing the classical continuous development of new cells in a balanced state along the leading edge of long lived squalls and in the propagating case the discontinuous development of new cells on the cold pool ahead of the squall

numerical simulation of a midlatitude squall line in two dimensions abstract a two dimensional anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line initial conditions were adapted from observations of the 22 may 1976 oklahoma line model simulations were made with and without considering the ice phase of water these model storms have within the constraints of the model s geometry replicated the basic multicellular character and general line normal airflow typical of these lines in addition the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable further it was found that the initial conditions chosen resulted in model storms which were not only long lasting but also essentially repetitive indicating that a state of quasi equilibrium had been attained the storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations the inclusio

structure and evolution of numerically simulated squall lines abstract using a three dimensional numerical cloud model we investigate the effects of vertical wind shear on squall line structure and evolution over a wide range of shear magnitudes depths and orientations relative to the line we find that the simulated squall lines are most sensitive to the magnitude of the component of shear perpendicular to the line and that we may reproduce much of the range of observed structures by varying this single parameter for weak shear a line of initially upright to downshear tilted short lived cells quickly tilts upshear producing a wide band of weaker cells extending behind the surface outflow boundary for moderate to strong shear the circulation remains upright to downshear tilted for longer periods of time with vigorous short lived cells confined to a relatively narrow band along the system s leading edge at later times however these systems may also weaken as the circulation tilts upshear for strong deep shears oriented obliquely to the line the squal

fluvial mixing during floods 1 mixing in fluvial systems as characterised by longitudinal dispersion may be thought intuitively to scale with the river discharge or flow rate here this tenet is demonstrated theoretically to be well founded during normal flows but to be perfidious during flood conditions the results suggest that during floods longitudinal mixing rates in rivers can be several orders of magnitude larger than during normal flows and can actually decrease with further increases in flow magnitude the very large longitudinal dispersion coefficients found during extreme flows cannot be predicted using existing empirical equations neither can the rapid decrease with increasing flow rate this is not surprising because these equations do not cater for the extremely strong transverse velocity shear that exists in these flows that contain zones of quite different velocity and depth the conclusions are important because it appears that the great majority of observations of fluvial mixing have been made during normal flows when a scaling with flow rate occurs during floods however this behaviour switches and longitudinal dispersion scales inversely and non linearly with flow rate

scale interactions involved in the initiation structure and evolution of the 15 december 1992 mcs observed during toga coare part ii mesoscale and convective scale processes abstract this paper the second of a series documents the precipitation and kinematic and thermodynamic structure of a tropical mesoscale convective system observed by instrumented aircraft on 15 december 1992 during toga coare radar derived precipitation fields indicate that the studied system consists of two subsystems s1 and s2 characterized by distinct internal dynamics and morphological structures the retrieved kinematic and thermodynamic structures are compared in detail with the synoptic scale characteristics described in part i of this paper so as to evaluate the scale interactions involved in the internal organization of this convective system it is shown essentially that the synoptic scale circulation governs the mesoscale and convective scale motions and therefore determines the internal organization of the selected system in particular this study highlights the major role played by the synoptic scale vertical wind shear in the internal structure of this tropical mesoscale system m

dispersion model for varying vertical shear in vegetated channels a dispersion model for a wide range of depthwise vertical shear is derived by using perturbation analysis and power m law velocity profile for m 1 the velocity profile provides linear shear whereas m 1 provides nonlinear shear and for m 20 the velocity profile resembles the flow through emergent vegetation the power law represented parametrically simulates well the complex shear profiles involved in emergent and submerged vegetated flows the proposed model shows reasonable agreement with past data on vegetated flows for a wide range of nonlinear shear velocities

the role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during toga coare abstract a collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare the soundings were constructed primarily from aircraft data below 5 6 km and primarily from radiosonde data at higher altitudes the well documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended as noted by earlier investigators nearly all convective bands occurring in environments with appreciable shear below a low level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low level shear at a speed close to the wind maximum when there is appreciable shear at middle levels 800 400 mb convective bands form parallel to the shear with

barrier jets during tamex abstract a barrier jet is frequently found along the northwestern coast of taiwan in the prefrontal southwesterly flow regime during the taiwan area mesoscale experiment tamex it has a maximum wind speed of 14 m s 1 at approximately 1 km above the surface with a vertical wind shear approximately 10 10 3 s 1 below and 4 10 3 s 1 above that altitude during tamex the southwesterly monsoon flow strengthens over taiwan when the low level pressure trough surface front moves toward the southeastern china coast the barrier jet is a result of the stably stratified airflow past an island obstacle under a small froude number o 1 flow regime during the occurrence of a barrier jet a windward pressure ridge is observed along the southwestern coast due to island blocking in low levels the incoming southwesterly flow decelerates off the southwestern coast and moves around the island along the western coast the deflected airflow accelerates northward with a large cross contour wind component down the p

cryostratigraphy paleogeography and climate change during the early holocene warm interval western arctic coast canada botanical and cryostratigraphic records from northwest canada indicate that the climate of the early holocene was considerably warmer than today tree line was over 100 km farther north and a thaw unconformity dating from 8000 14c years bp formed at the base of an active layer 2 5 times thicker than at present numerous thermokarst lake basins formed in the preceding millennia both the botanical and cryostratigraphic indices described are products of summer conditions previous reconstructions of early holocene climate have not assessed the significance of paleocoastal location on the seasonality and extent of apparent climate warming at present there is a steep gradient in growing season conditions between cooler sites on the beaufort sea coast and warmer inland locations winter conditions are more uniform because both sea and land are snow covered coastal retreat in the region has been rapid due to sea level rising over a gently sloping shelf containing readily erodible sediments the coastlin

disequilibrium of holocene sediment yield in glaciated british columbia it is generally supposed that specific sediment yield the quantity of sediment passing a monitored river cross section per unit area drained upstream of that section declines as the area drained increases1 3 part of the sediment mobilized from the land surface is supposed to go back into storage at field edges and on footslopes and floodplains in contrast we show here that data from british columbian rivers reveal a pattern of increasing specific sediment yield at all spatial scales up to 3 104 km2 this results from the dominance of secondary remobilization of quaternary sediments along river valleys over primary denudation of the land surface the result controverts the conventional model which has been derived from studies of small highly disturbed agricultural catch ments the rivers are still responding to the last glaciation giving a landscape relaxation time greater than 10 kyr this holds pro found implications for geomorphological theory and for studies of erosion

a numerical investigation of squall lines part ii the mechanics of evolution abstract the physical processes involved in the evolution of the model squall line presented in part i of this study are examined it is found that both the thermal and dynamic effects are important in the development of the midlevel meso scale low pressure zone located just to the rear of the convective core based on this observation a positive feedback mechanism is proposed to explain the abrupt transformation of the more or less upright convection line into a quasi steady meso scale convective system possessing an extensive trailing stratiform region in order to set the stage for this transformation to take place the convective updraft is required to possess an initial upshear tilt qualitative arguments are given to show that this initial tilt might be the result of a moderate wind shear at low levels and none or reverse shear in the middle to high levels results from model sensitivity experiments are presented to support the theory in addition to the dynamic effects of the low level wind sh

the ice hill an example of retarded deglaciation in siberia abstract the geological structure of one of the southernmost sites of thick ice beds in the yenisei valley near the arctic circle is discussed the erosion contacts of the foliated and contorted ice abundance of glacial debris and the position between the basal till and ablation cover indicate a glacial origin for the ice layer the ice occurs in the cores of arcuate accretion ridges built of ablation materials the overlying sequence consists of a series of diamicton flows alternating with lacustrine and outwash sediments laterally the ice rock complex changes into thick lacustrine rhythmites underlain by basal tills the ice containing hummocky terrain is often lower than the adjacent lacustrine plain therefore inversion of ice controlled topography is suggested several stages of glaciokarst development are supposed to have occurred after an eastward moving ice lobe stagnated more than 50 ka bp the main depositional events are associated with the middle weichselian draining of ice dammed lake igarka and with early holocene topographic inversion the holocene warming apparently caused diapiric deformation of the buried ice probably because of the density and thickness differences between the ice and the overburden the long history of burial and preservation of the remnants of basal glacier ice was controlled by pleistocene permafrost landscape evolution in northern siberia is governed by such retarded deglaciation in contrast with atlantic environments the thick ablation sequence may be described in terms of a spitsbergen model but the siberian basal tills because of their position within the permafrost must be regarded as exfoliated layers of debris rich ice according to shantzer lavrushin s model the occurrence of buried glacier ice and the mode of distribution of the permafrost shows the relict nature of the latter in areas of modern and former retarded deglaciation intermittent diamicton flows are supposed to mark episodes of climatic amelioration like the pleistocene and holocene diamictons with driftwood at the ice hill such warm diamictons may account for some early holocene and similar puzzling interglacial ice advances

mature thunderstorm cloud top structure and dynamics a three dimensional numerical simulation study abstract an anelastic three dimensional model is used to investigate the effects of vertical wind shear regime on cloud top structure and internal properties of mature isolated midlatitude thunderstorms four comparative experiments designated a through d are performed with varying shear profiles in otherwise identical initializations cases a c assume strong shear differing only in the veering of the low level hodograph moderate in a strong in b and none in c weak shear everywhere 40 as great as in c is assumed in case d the strong shear cases a c show moderately vigorous quasi steady mature updrafts with strong midlevel mesovortex couplets and marked anvil elongation along the net vertical shear vector differences are modest especially at cloud top though with low level hodograph curvature the updraft is enhanced and skewed toward the cyclonic right flank the weak shear case d shows a weaker and less persistent mature updraft than a c along with weaker midlevel rotation and a much more n

pressure and buoyancy fields derived from doppler radar data in a tornadic thunderstorm abstract a method for retrieval of pressure and buoyancy distributions in deep convection is applied to doppler radar data collected at two analysis times during the tornadic del city oklahoma thunderstorm of 20 may 1977 change of a previous version of the technique necessitated by application to real data include procedures for handling irregularly bounded volumes and missing data and new assumptions to include reflectivity data and turbulent effects in the equations internal consistency cheeks on the quality of retrieved pressure fields imply that the input data are generally of good quality and point out times and heights within the storm at which greater confidence can be placed in the derived fields in the pretornadic stage the pressure distribution includes at each level a high low couplet across the updraft with the maximum pressure gradient generally oriented along the environmental shear vector at that altitude these results are in agreement with predictions of linear theory locations of

evolution and structure of the mesoscale convection and its environment a case study during the early onset of the southeast asian summer monsoon abstract the evolution and structure of mesoscale convection in the south china sea scs region are documented for the first time mainly using the dual doppler radar dataset collected during the south china sea monsoon experiment scsmex in 1998 in particular this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast asian monsoon seam for the case of 15 may 1998 interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection in the prefrontal region the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast to southwest oriented convection in the postfrontal region the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east to west orientation in addition the convergence between the southerly monsoon flow and the northerly postfrontal flow gener

terrain controls on the occurrence of coastal retrogressive thaw slumps along the yukon coast canada retrogressive thaw slumps rtss are among the most active landforms in the arctic their number has increased significantly over the past decades while processes initiating discrete rtss are well identified the major terrain controls on the development of coastal rtss at a regional scale are not yet defined our research reveals the main geomorphic factors that determine the development of rtss along a 238 km segment of the yukon coast canada we 1 show the current extent of rtss 2 ascertain the factors controlling their activity and initiation and 3 explain the spatial differences in the density and areal coverage of rtss we mapped and classified 287 rtss using high resolution satellite images acquired in 2011 we highlighted the main terrain controls over their development using univariate regression trees model coastal geomorphology influenced both the activity and initiation of rtss active rtss and rtss initiated after 1972 occurred primarily on terrains with slope angles greater than 3 9 and 5 9 respectively the density and areal coverage of rtss were constrained by the volume and thickness of massive ice bodies differences in rates of coastal change along the coast did not affect the model we infer that rates of coastal change averaged over a 39 year period are unable to reflect the complex relationship between rtss and coastline dynamics we emphasize the need for large scale studies of rtss to evaluate their impact on the ecosystem and to measure their contribution to the global carbon budget

mesoscale motion fields associated with a slowly moving gate convective band abstract the structure of the convective band of 14 september in the dense gate observing array is determined using wind and thermodynamic data primarily from multiple aircraft penetrations which are well distributed in the vertical and in time the well defined mesoscale features in the line which are 10 40 km in scale quasi two dimensional and persist for several hours determine the distribution of the convective scale features which are 5 km or less in size three dimensional not generally detectable for more than one flight leg at the leading edge a 30 km zone of strong ascent is computed from two dimensional continuity here lifting of the ambient air creates a favorable environment not found elsewhere for deep cumulonimbus clouds to develop their updrafts are weak 2 4 m s 1 on the average behind the updraft zone below 3 4 km is a broad descent zone it corresponds to the stratiform rain area and has little convection and some drying at lower levels on the average the mass flux by

multi scale site evaluation of a relict active layer detachment in a high arctic landscape abstract investigations into the susceptibility of permafrost landscapes response to thermokarst can be performed using various approaches depending on the scale of investigation in many cases point based field measurements are extrapolated to larger scales and vice versa the integration of scales often requires some form of ground control in addition to remote sensing surveys which are at times exclusively conducted as upscaling from discrete field measurements can provide spatial coverage and landscape scale significance downscaling from remote sensing can offer insight into processes and serve as calibration or verification here we present a multiple scale evaluation of an area initially interpreted as a relict active layer detachment slide before 1950 on melville island in the high arctic where differential interferometric synthetic aperture radar dinsar showed subsidence between 2013 and 2015 ground based cryostratigraphy measurements were combined with ground penetrating radar gpr to investigate permafrost ice content the results indicate greater subsidence within the relict active layer detachment as detected by dinsar gpr surveys and permafrost coring indicated the presence of an ice rich or massive ice layer near the base of the active layer in this area in addition cryostratigraphic evidences of thaw unconformity and of massive ice depth helped validate the interpretations of the geomorphology in the active layer detachment this combination of methods indicated a localized and inherited landform subsidence association which brought further insight into the interpretation of dinsar subsidence data the framework presented in this study demonstrates the importance of site specific investigations of thermokarst signal in order to understand the processes behind the remote sensing results

orographic effects on a squall line system over taiwan abstract a north south oriented multicellular squall line during the taiwan area mesoscale experiment s tamex intensive observation period 2 was studied using dual doppler radar data as it moved over the island of taiwan over the open ocean it moved eastward at a constant speed 15 5 m s 1 as the squall line encountered taiwan island s complex mountainous areas the leading edge moved slower in the mountain ridge areas than over the valleys as a result its orientation became approximately parallel to the terrain contours in the upper levels the eastward movement of the squall line aloft was less affected by the terrain compared to the low levels as a result the westward tilt of the rising motion at the front became less significant part of the system relative rear to front flow entered the rising branch of the storm s circulation as a result of orographic lifting as illustrated by the trajectory analysis and was enhanced by latent heat release the system relative rear to front flow did

scale interactions involved in the initiation structure and evolution of the 15 december 1992 mcs observed during toga coare part i synoptic scale processes this paper the first of a series examines the synoptic scale mechanisms involved in the initiation structure and evolution of a mesoscale convective system observed during toga coare this study relies upon the use of the japanese geosynchronous meteorological satellite 4 imagery and ecmwf model outputs from which diagnostic parameters are derived and interpreted this mesoscale convective system consists initially of two groups of convective entities that progressively move toward each other and merge it is shown that the synoptic scale flow creates a favorable environment for the formation of this large convective system through the production of convective available potential energy cape by horizontal advection and the enhancement of low level convergence in the region where the convective system formed moreover the general evolution of the system is found to be governed by the synoptic scale circulation and more precisely by the temporal evolution of cape and low level convergence the mechanisms leading to the initiation and general evolution of the system are examined the easterly equatorial jet at 500 hpa triggered positive potential vorticity areas that propagated westward and generated an anticyclonic circulation this anticyclonic circulation was enhanced during the development phase of the convective system through vortex stretching and tilting which accelerated the low level westerlies corresponding to the southern branch of this circulation and enhanced the low level convergence associated with the studied convective system in a companion paper the mesoscale and convective scale processes involved in the internal organization of this mesoscale convective system are examined using the airborne doppler radar dataset collected within the system from 1700 to 2100 utc the downscale interactions i e from synoptic scale to mesoscale and convective scale are scrutinized using both the synoptic scale context described in this paper and the mesoscale and convective scale characteristics derived from the airborne radar observations

the role of convectively generated rear inflow jets in the evolution of long lived mesoconvective systems abstract in this study the structure of convectively generated rear inflow jets and their role in the evolution of long lived mesoconvective systems are investigated through an analysis of idealized three dimensional simulations using a nonhydrostatic cloud model rear inflow jets are generated within these systems in response to the upshear tilting of the convective circulation as the horizontal buoyancy gradients along the back edge of the expanding system create a circulation that draws midlevel air in from the rear within this framework a wide range of rear inflow strengths and structures are produced depending on the magnitude of the ambient convective available potential energy cape and the vertical wind shear in general for environments characterized by weak to moderate vertical wind shear and weak to moderate cape the rear inflow jet descends and spreads along the surface well behind the leading edge of the gust front and the subsequent convective activity becomes weaker however for e

precipitation and kinematic structure of an oceanic mesoscale convective system part i momentum transport and generation abstract this is the second paper of a two part series documenting the structure of and momentum transport by a subtropical mesoscale convective system near taiwan using doppler radar data and in situ data from the noaa p 3 part i defines the basic system structure and evolution in part ii the momentum transport by the system is estimated and related to system structure and the momentum budget for a portion of the embedded convective band is evaluated profiles of the vertical flux of horizontal momentum are constructed from in situ data doppler radar data and both combined in a coordinate system with u normal to the line and positive eastward since the low level air is feeding the line from the east differences in the fluxes from the two sources appear to be mainly due to an underestimation of the mean vertical velocity from the doppler radar data the discrepancy results partially from the concentration of convergence in the boundary layer precisely where the doppler cannot adequately sample t

pressure and temperature perturbations within a squall line thunderstorm derived from sesame dual doppler data abstract some dynamic and thermodynamic properties of a convective cell within a squall line that occurred on 6 june 1979 were studied based on dual doppler observations the domain under investigation had a horizontal dimension of 27 km 27 km with 12 levels in the vertical the grid spacing used was 1 km vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment fields of deviation perturbation pressure density and virtual temperature were recovered from a three dimensional wind field using the thermodynamic retrieval method these retrieved fields were then subjected to internal consistency checks to determine the level of confidence our findings demonstrate that thermodynamic retrieval is feasible when random errors inherent in the radial wind components are minimized by proper smoothing errors in the computation of vertical velocity can be substantially reduced when a variational approach is used with the anelastic continuity equ

structure and evolution of the 22 february 1993 toga coare squall line aircraft observations of precipitation circulation and surface energy fluxes abstract this study documents the precipitation and kinematic structure of a mature eastward propagating oceanic squall line system observed by instrumented aircraft during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare doppler radar and low level in situ observations are used to show the evolution of the convection from an initially linear nnw sse oriented convective line to a highly bow shaped structure with an embedded low to midlevel counterclockwise rotating vortex on its northern flank in addition to previously documented features of squall lines such as highly upshear tilted convection on its leading edge a channel of strong front to rear flow that ascended with height over a rear inflow that descended toward the convective line and a pronounced low level cold pool apparently fed from convective and mesoscale downdrafts from the convective line rearward the observations of this system showed distinct multiple maxima in updraft strength with

multiple thermo erosional episodes during the past six millennia implications for the response of arctic permafrost to climate change anthropogenic warming may promote rapid permafrost thaw in the arctic and alter the global carbon cycle although several studies suggest increased thermo erosion as a result of recent warming a long term context is necessary to assess the linkages of thermokarst processes with climate variability we analyzed sediment cores from two lakes on the alaskan north slope usa one with lake ne14 and one without perch lake watershed thermo erosion distinct geochemical and lithological characteristics provide evidence for sedimentary input from carbonate rich permafrost soils associated with past retrogressive thaw slumping at lake ne14 but not at perch lake these characteristics include increases in ca sr ca k carbonate feldspar clay minerals percent caco3 and 13c and decreases in 87sr 86sr at least ten episodes of thermo erosion occurred over the past 6000 yr at lake ne14 most of these episodes coincided with periods of elevated summer temperatures but moisture variation and geomorphic factors likely played a role in driving their occurrence our results suggest that positive feedbacks facilitate reactivation of thermo erosion in ice rich terrain adding to the growing body of evidence that these arctic landscapes are unstable in a changing climate

controlled moraines origins characteristics and palaeoglaciological implications abstract controlled moraines are supraglacial debris concentrations that become hummocky moraine upon de icing and possess clear linearity due to the inheritance of the former pattern of debris rich folia in the parent ice linearity is most striking wherever glacier ice cores still exist but it increasingly deteriorates with progressive melt out as a result moraine linearity has a low preservation potential in deglaciated terrains but hummocky moraine tracts previously interpreted as evidence of areal stagnation may instead record receding polythermal glacier margins in which debris rich ice was concentrated in frozen toe zones recent applications of modern glaciological analogues to palaeoglaciological reconstructions have implied that a controlled moraine development can be ascribed to a specific process e g englacial thrusting or supercooling and b controlled moraine preservation potential is good enough to imply the occurrence of the specific process in former glacier snouts e g ancient polythermal or supercooled snouts these assumptions are tested using case studies of controlled moraine construction in which a wide range of debris entrainment and debris rich ice thickening mechanisms are seen to produce the same geomorphic features polythermal conditions are crucial to the concentration of supraglacial debris and controlled moraines in glacier snouts via processes that are most effective at the glacier permafrost interface end moraines lie on a process form continuum constrained by basal thermal regime the morphological expression of englacial structures in controlled moraine ridges is most striking while the moraines retain ice cores but the final deposits landforms tend to consist of discontinuous transverse ridges with intervening hummocks preserving only a weak impression of the former englacial structure these are arranged in arcuate zones of hummocky moraine up to 2 km wide containing ice walled lake plains and lying down flow of streamlined landforms produced by warm based ice a variety of debris entrainment mechanisms can produce the same geomorphic signature spatial and temporal variability in process form relationships will lead to the sequential development of different types of end moraines during the recession of a glacier or ice sheet margin

the influence of the shear induced pressure gradient on thunderstorm motion abstract in the present investigation we propose a simple theory to explain how a veering environmental wind shear vector can cause an initially symmetric updraft to grow preferentially to the right of the shear vector and acquire cyclonic rotation the explanation offered is based on linear theory which predicts that interaction of the mean shear with the updraft produces favorable vertical pressure gradients along its right flank to asses the validity of linear theory for large amplitude updrafts the three dimensional shallow anelastic equations are numerically integrated using a simple parameterization for latent heating within a cloud and the linear and nonlinear forcing terms are separately analyzed these results suggest that although the nonlinear effects strongly promote splitting of the updraft the linear forcing remains the dominant factor in preferentially enhancing updraft growth on the right flank we believe this differential forcing is a major contributor to the observed predominance o

the west african squall line observed on 23 june 1981 during copt 81 kinematics and thermodynamics of the convective region abstract the squall line observed during the night of 23 24 june 1981 was one of the most intense events during the copt 81 convection profonde tropicale experiment conducted in may and june 1981 at korhogo in the northern ivory coast by french and ivorian research institutes the presquall environment possessed a very large convective instability with a stable layer in the lowest levels the presence of dry air in the midlevels which promoted the development of convective downdrafts and the unusually low altitude of the strong easterly winds which aided the development of a leading anvil above 6 km are two noteworthy characteristics of the environment of this squall line its mesoscale structure as deducted from radar data is similar to that previously observed for tropical squall lines convective updrafts and downdraft occur in the leading heavy precipitation region convective region and a mesoscale updraft and downdraft is found within and below the trailing mid to upper level anvil clouds

cryostratigraphical studies of ground ice formation and distribution in a high arctic polar desert landscape resolute bay nunavut ground ice distribution and abundance have wide ranging effects on periglacial environments and possible impacts on climate change scenarios in contrast very few studies measure ground ice in th

retrieved thermodynamic structure of a subtropical orographically influenced quasi stationary convective line airborne doppler radar and flight level measurements were used to document the three dimensional thermodynamic structure of a convective line that developed off the southeastern coast of taiwan on 16 june 1987 during the taiwan area mesoscale experiment during the period of aircraft observation the convective line appeared to be quasi stationary in a region 90 km away from the coast calculations of perturbation pressure and buoyancy retrieved from the doppler radar derived wind fields show that a high pressure perturbation associated with negative buoyancy existed in the region of heavy precipitation in the lowest level in low levels an elongated zone of low pressure was documented along the line which was closely related to the distributions of buoyancy these retrieved features are basically consistent with those calculated from in situ observations in mid to upper levels the high low pressure perturbation lay around the updraft cores on the upshear downshear of environmental shear the quantitative diagnosis for the perturbation pressure indicates that the positive buoyancy associated with the in cloud latent heat release as well as evaporative cooling occurring near cloud base and subcloud regions is crucial to determine the low level perturbation pressure distributions however dynamic forcing contributing to the pressure perturbation cannot be ignored in middle levels because of updraft shear interaction due to the presence of pronounced cross line wind shear and corresponding updrafts reaching maximum values near midtroposphere in distinct contrast to squall lines which are rapidly moving the near surface temperature gradient across this quasi stationary line was very weak and at low levels no gust front rear inflow or dynamically induced high pressure was evident near the leading edge of the line analyses presented here suggest that the persistent stationary convergence in association with the deceleration of low level southeasterly flow as it approached the nearshore blocked flow played an important role in maintaining and organizing the deep convection in this case

formative stage of a long lived mesoscale vortex observed by airborne doppler radar abstract the formative stage of a long lived mesoscale cyclonic vortex was captured by the noaa p 3 aircraft as it investigated a developing mesoscale convective system mcs near the southeastern coast of taiwan on 16 june 1987 during the taiwan area mesoscale experiment the supporting environment of the mesovortex was characterized by an exceptionally moist atmosphere and moderate ambient vertical shear through a deep layer from the near surface to 6 km with much weaker shear and winds aloft in addition a pronounced low level mesoscale shear convergence zone which resulted from the interaction of southeasterly flow with northeasterly flow confined to the near coast region existed in the vicinity of the observed mesovortex composite three dimensional wind fields derived via pseudo dual doppler synthesis show the vortex had a horizontal diameter expanding from 40 km to 70 km in the lower to midtroposphere respectively and exhibited considerable tilt through this layer contrary to previously d

west african squall line thermodynamic structure retrieved from dual doppler radar observations abstract pressure and temperature fields within a west african squall line retrieved from dual doppler radar data collected during the copt 81 convection profonde tropicale experiment are presented the method for derivation of thew results is approximately similar to that proposed by gal chen based on the anelastic equation of motion comparisons between pressure and temperature fields deduced from radar data at the lowest levels and surface network measurements show good agreement the inferred thermodynamic structure displays the influence of a low level frontward flow which is mainly due to a density current of cold air generated in the stratiform region of the squall line and resulting from a mesoscale downdraft this frontward flow contributes to initiate and maintain a frontal updraft through both nonhydrostatic pressure perturbation and temperature difference between entering air and colder frontward flow at higher altitudes mixing with the environment reduces buoyancy in the frontal updr

longitudinal dispersion coefficient in single channel streams using a new channel shape equation for straight channels and a more versatile channel shape or local flow depth equation for natural streams a method is developed for prediction of the longitudinal dispersion coefficient in single channel natural streams including straight and meandering ones the method involves derivation of a new triple integral expression for the longitudinal dispersion coefficient and development of an analytical method for prediction of this coefficient in natural streams the proposed method is verified using 70 sets of field data collected from 30 streams in the united states ranging from straight manmade canals to sinuous natural rivers the new method predicts the longitudinal dispersion coefficient where more than 90 calculated values range from 0 5 to 2 times the observed values the advantage of the new method is that it is capable of accurately predicting the longitudinal dispersion coefficient in single channel natural streams without using detailed dye concentration test data a comparison between the new method and the existing methods shows that the new method significantly improves the prediction of the longitudinal dispersion coefficient

dual doppler radar analysis of a midlatitude squall line with a trailing region of stratiform rain abstract the mesoscale structure of a squall line system that passed over oklahoma on 22 may 1976 is investigated by dual doppler radar analysis the mature storm consisted of a leading line of deep convection which exhibited organized multicellular structure trailed by an extensive region of stratiform precipitation marked by a radar bright band at the melting level these contrasting radar echo regimes were separated by a narrow band of weak reflectivity at lower levels which has been termed the transition zone while conventional and single doppler radar analyses documented the persistence of this precipitation structure and revealed the corresponding kinematic structure in one part of the mature storm the dual doppler analysis demonstrates the pervasiveness of these features over much of the squall line s length the structure of a midtropospheric maximum of rearward system relative flow crossing the system is particularly well described by the dual doppler data this mesoscale current originat

paleoglaciological insights from the age and morphology of the jesse moraine belt western canadian arctic abstract the jesse moraine belt 350 km long was deposited by the northwest laurentide ice sheet on banks and victoria islands in the western canadian arctic archipelago during the last deglaciation the moraine belt is composed of ice cored controlled moraines that contrast markedly with adjacent glacial sediments deposited during preceding ice sheet retreat more than 80 radiocarbon ages from fossil molluscs illustrate that deposition of the moraine belt was brief occurring between 13 75 and 12 75 cal ka bp the chronology demonstrates that moraine deposition was coeval with ice sheet drawdown and ice streaming on adjacent victoria island detailed geomorphic mapping demonstrates that the moraine belt is the product of a regional change in ice marginal processes this involved a predominantly cold based ice margin giving way to polythermal bed conditions which were conducive to widespread deposition of controlled moraines and ice stream bedforms the expansion of warm based thermal regimes in the northwest laurentide ice sheet followed widespread ice sheet withdrawal from m clure strait and western amundsen gulf this suggests a re equilibration of regional ice divides in response to rapidly changing ice sheet margins and surface gradients the reconstructed dynamical behavior of the northwest laurentide ice sheet provides new insights regarding ice sheet responses to late glacial climate and relative sea level change among other variables

thawing of massive ground ice in mega slumps drives increases in stream sediment and solute flux across a range of watershed scales 1 ice cored permafrost landscapes are highly sensitive to disturbance and have the potential to undergo dramatic geomorphic transformations in response to climate change the acceleration of thermokarst activity in the lower mackenzie and peel river watersheds of northwestern canada has led to the development of large permafrost thaw slumps and caused major impacts to fluvial systems individual mega slumps have thawed up to 106 m3of ice rich permafrost the widespread development of these large thaw slumps up to 40 ha area with headwalls of up to 25 m height and associated debris flows drive distinct patterns of stream sediment and solute flux that are evident across a range of watershed scales suspended sediment and solute concentrations in impacted streams were several orders of magnitude greater than in unaffected streams in summer slump impacted streams displayed diurnal fluctuations in water levels and solute and sediment flux driven entirely by ground ice thaw turbidity in these streams varied diurnally by up to an order of magnitude and followed the patterns of net radiation and ground ice ablation in mega slumps these diurnal patterns were discernible at the 103 km2 catchment scale and regional disturbance inventories indicate that hundreds of watersheds are already influenced by slumping the broad scale impacts of accelerated slumping are indicated by a significant increase in solute concentrations in the peel river 70 000 km2 these observations illustrate the nature and magnitude of hydrogeomorphic changes that can be expected as glaciogenic landscapes underlain by massive ice adjust to a rapidly changing climate

the role of acids in electrical conduction through ice 1 electrical conduction through meteoric polar ice is controlled by soluble impurities that originate mostly from sea salt biomass burning and volcanic eruptions the strongest conductivity response is to acids yet the mechanism causing this response has been unclear here we elucidate conduction mechanisms in ice using broadband dielectric spectroscopy of meteoric polar ice cores we find that conduction through polycrystalline polar ice is consistent with jaccard theory for migration of charged protonic point defects through single ice crystals except that bulk dc conduction is impeded by grain boundaries neither our observations nor modeling using archie s law support the hypothesis that grain boundary networks of unfrozen acids cause significant electrolytic conduction common electrical logs of ice cores by electrical conductivity measurement ecm or dielectric profiling dep and the attenuation of radio waves in ice sheets thus respond to protonic point defects only this response implies that joint interpretation of electrical and chemical logs can determine impurity partitioning between the lattice and grain boundaries or inclusions for example in the greenland ice core project grip ice core from central greenland on average more than half of the available lattice soluble impurities h cl nh4 create defects understanding this partitioning could help further resolve the nature of past changes in atmospheric chemistry

characteristics of a subtropical squall line determined from tamex dual doppler data part i kinematic structure abstract in this study structural features of a subtropical squall line that occurred on 17 may 1987 over the taiwan straits were investigated using dual doppler data collected during the taiwan area mesoscale experiment tamex fields of the storm relative wind and reflectivity were derived in a horizontal domain of 45 km 25 km using an objective analysis scheme with 1 km grid spacing in all three directions there were ten analysis levels in the vertical ranging from 0 3 to 8 8 km vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment results show that many structural features of a subtropical squall line are similar to those for a fast moving tropical squall line a low level jet llj associated with the frontal system provides the necessary strong shear at lower levels on the front side of the squall line front to rear flow prevails at all levels and is accompanied by shallow rear to front flow on the back of the line ther

climate driven thaw of permafrost preserved glacial landscapes northwestern canada ice marginal glaciated landscapes demarcate former boundaries of the continental ice sheets throughout circumpolar regions permafrost has preserved relict ground ice and glacigenic sediments delaying the sequence of postglacial landscape change that transformed temperate environments millennia earlier here we show that within 7 10 6 km 2 of glaciated permafrost terrain extensive landscapes remain poised for major climate driven change across northwestern canada 60 100 km wide concentric swaths of thaw slump affected terrain delineate the maximum and recessional positions of the laurentide ice sheet these landscapes comprise 17 of continuous permafrost terrain in a 1 27 10 6 km 2 study area indicating widespread preservation of late pleistocene ground ice these thaw slump relict ground ice and glacigenic terrain associations are also evident at the circumpolar scale recent intensification of thaw slumping across northwestern canada has mobilized primary glacial sediments triggering a cascade of fluvial lacustrine and coastal effects these geologically significant processes highlighted by the spatial distribution of thaw slumps and patterns of fluvial sediment mobilization signal the climate driven renewal of deglaciation and postglacial permafrost landscape evolution

dynamic aspects of a distinctly three dimensional mesoscale convective system abstract the dynamics of a mesoscale convective system that matured in central kansas on 3 4 june 1985 is investigated based upon data from ground based dual doppler radar and other sources the system was distinctly three dimensional evolving to a wavelike shape owing to the intersection of two convective bands the two bands one oriented north south and the other east north west southwest are compared and contrasted with respect to their velocity pressure and buoyancy structure and the consequent attributes of their momentum transports and budgets dynamic retrieval of pressure and buoyancy from the wind fields provides insight into system structure and allows for the calculation of pressure gradients needed for the horizontal momentum budget several independent checks are carried out to ensure the quality of the pressure solution the solution is found to be more accurate if the velocity time derivatives are included in the retrieval process dissimilar structure of the two bands is highlighted b