a theoretical and numerical study of density currents in nonconstant shear flows the previous idealized two fluid model of a density current in constant shear is extended to the case where the inflow shear is confined to the low levels the analytical solution is determined by the conservation of mass momentum vorticity and energy it is found that a low level shear acts in a similar manner to a uniform vertical shear in controlling the depth of a steady state density current when the shear enhances the low level flow against the density current propagation the current is deeper than half of the domain depth time dependent numerical experiments are conducted for a variety of parameter settings including various depths and strengths of the shear layer the numerical results agree closely with the theoretical analyses numerical experiments are also performed for a case where the initial depth of the density current is set to be comparable to the low level shear which is much shallower than that given by the steady state solution the circulation at the density current head remains shallow and is nonsteady in this case whereas the time averaged flow still exhibits a deep jump updraft pattern that is close to the theoretical solution suggesting the applicability of the theoretical results to even more transient flows the simulated flow features are discussed in terms of balanced and unbalanced dynamics and in the context of forcing and uplifting at the frontal zone in long lived convective systems here the term balance refers to a flow configuration that satisfies the steady state solution of the idealized theoretical model

summertime cold fronts in southeast australia behavior and low level structure of main frontal types abstract the cold fronts research programme cfrp concentrated upon cold frontal systems synoptic scale fronts occurring in southeast australia during late spring and early summer november and december many of these tend to the complex with change lines associated with sea breezes prefrontal squall lines and related cold outflows garratt et al the present paper extends the earlier work to include additional observations of frontal events from field phase 3 of the cfrp held in late 1984 in addition we distinguish between this frontal system type 1 and a frontal system type 2 occurring throughout summer but predominantly late in the season which is often dry with a single major change line identified as a mesoscale coastal front 0bservations from the cfrp and other sources are described covering a wide range of frontal intensities with frontal speeds varying between 5 and 25 m s 1 the nature of the low level flow behind and in the vicinity of the active surface cold front scf is

research aircraft measurements of jet stream geostrophic and ageostrophic winds abstract radar altimeter and inertial navigation wind velocity measurements from research aircraft are shown to give the ageostrophic wind velocity distribution within a jet stream system near the tropopause results from one case study reveal cross height contour ageostrophic winds exceeding 20 m s 1 and centripetal acceleration induced ageostrophic winds approaching 100 m s 1 in the exit region of a jet streak which has entered into the trough region of a synoptic wave diagnostic calculations are made of the horizontal velocity divergence and the geostrophic and ageostrophic components of the observed wind velocity acceleration the results suggest caution in the application of near straight jet streak dynamics and the geostrophic momentum approximation to jet streak systems embedded within large amplitude synoptic wave regimes

the structure and evolution of cold surges east of the rocky mountains abstract northerly surges of cold air often move southward along the eastern side of the rockies from southern canada into mexico the strongest surges which generally develop in midwinter are associated with temperature decreases and pressure rises of 20 30 c and 15 30 mb respectively within 24 h surges are usually accompanied by a meridionally elongated pressure ridge and strong low level ageostrophic winds that parallel the terrain the width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters mexico this paper provides a detailed description of a northerly surge to the east of the rocky mountains that occurred on 12 14 november 1986 using both observational and model data the structural evolution of the surge is analyzed in addition the dynamics of the event is explored by diagnosing the momentum thermodynamic energy and vorticity equations to determine the typical synoptic scale evolution of these

rear flank outflow dynamics and thermodynamics in the 10 june 2010 last chance colorado supercell abstracton 10 june 2010 the second verification of the origins of tornadoes experiment vortex2 armada observed a supercell thunderstorm near last chance colorado tempest unmanned aircraft system uas data collected in the rear flank outflow revealed what appeared to be an elevated outflow head turbulent wake and a cold rear flank internal surge rfis surface thermodynamic and kinematic data collected by the sticknet and mobile mesonet indicated that the outflow wake may have extended to or very near the surface perhaps modifying or outright replacing the leading edge of the outflow at times single doppler data collected by the noaa x pol mobile polarimetric doppler radar noxp were supportive of the possibility of a downdraft in the outflow wake associated with low level divergence a conceptual model of the hypothesized rear flank outflow structure in the nontornadic phase of the last chance supercell is presented the observed turbulent wake is consistent with mixing associated with the rel

the life cycle of thunderstorm gust fronts as viewed with doppler radar and rawinsonde data abstract this paper presents the time dependent analysis of the thunderstorm gust front with the use of project nimrod data rhi cross sections of reflectivity and doppler velocity are constructed to determine the entire vertical structure the life cycle of the gust front is divided into four stages 1 the formative stage 2 the early mature stage 3 the late mature stage and 4 the dissipation stage a new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground while carrying some of the smaller precipitation ahead of the main echo core of the squall line this feature is called a precipitation roll as determined from rawinsonde data the cold air behind the gust front accounts for the observed surface pressure rise calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow the equation governing the propagation speed of a density current accurately predicts t

climatology of barrier jets along the alaskan coast part i spatial and temporal distributions abstract this paper investigates the temporal and spatial climatology of coastal barrier jets in the gulf of alaska the jets are divided into two categories based upon the origin of the air involved classic barrier jets fed primarily by onshore flow and hybrid jets fed primarily by gap flow from the continental interior the analyses are compiled from five years 1998 2003 of synthetic aperture radar images from the radarsat 1 satellite totaling 3000 images thermodynamic and kinematic data from the ncep reanalysis is used in the interpretation of the results the majority of coastal barrier jets occur during the cool season with the coastline near mount fairweather and the valdez cordova mountains experiencing the greatest number of barrier jets hybrid jets are even more strongly restricted to the cool season and are commonly found to the west of cross sound yakutat bay and icy bay some interannual variability in the total number of jets is observed coastal barrier jet formation is associate

effects of melting on frontogenesis abstract frontal precipitation systems are simulated with a 2d cloud model including ice phase microphysics despite the use of idealized frontogenetic forcing in the simulations some observed characteristics of frontal zones and their associated cloud and precipitation fields are reproduced in the simulations the effects of melting snow on surface frontogenesis is investigated it is found that the cooling effects of melting snow significantly accelerate surface frontogenesis in winter storm environments especially when the melting layer is close to the surface however the steady state surface frontal strength in the model is not sensitive to the melting effects finescale thermal and kinematic perturbations inside the frontal zone near the melting level quite similar to those recently reported in the literature are evident in the model results analysis of the model results suggests that cooling from melting snow may induce these thermal and kinematic perturbations and may enhance baroclinicity

central australian cold fronts abstract this paper presents an observational study of the structure and behavior of cold fronts over central australia during the late dry season a time of year when the prefrontal convectively well mixed layer is particularly deep the study is based on the results of the central australian fronts experiments cafe held in 1991 three fronts were documented in unprecedented detail for the australian region using a greatly enhanced surface observing network and a boundary layer wind profiler as well as serial upper air soundings data on the surface energy balance were obtained also a common feature of the fronts observed during cafe was that they were dry shallow 1 km deep and moved into a deep 4 km convectively well mixed boundary layer one of them initiated major dust storms across central australia a prominent feature of the fronts was the marked diurnal variation of their surface signature as they moved through the network noteworthy was the tendency during the night for the initiatio

a multiwinter analysis of channeled flow through a prominent gap along the northern california coast during caljet and pacjet abstract experimental observations from coastal and island wind profilers aircraft and other sensors deployed during the california land falling jets experiment of 1997 98 and the pacific land falling jets experiment of 2000 01 2003 04 were combined with observations from operational networks to document the regular occurrence and characteristic structure of shallow 400 500 m deep cold airstreams flowing westward through california s petaluma gap from the central valley to the coast during the winter months the petaluma gap which is the only major air shed outlet from the central valley is 35 50 km wide and has walls extending at most a modest 600 900 m above the valley floor based on this geometry together with winter meteorological conditions typical of the region e g cold air pooled in the central valley and approaching extratropical cyclones this gap is predisposed to generating westward directed ageostrophic flows driven by along gap pressure differences two case studies and a fiv

finescale vertical structure and dynamics of some dryline boundaries observed in ihop abstract several radar fine lines all with a humidity contrast were sampled in the central great plains during the 2002 international h2o project ihop this study primarily uses aircraft and airborne millimeter wave radar observations to dynamically interpret the presence and vertical structure of these fine lines as they formed within the well developed convective boundary layer in all cases the fine line represents a boundary layer convergence zone this convergence sustains a sharp contrast in humidity and usually in potential temperature across the fine line the key question addressed herein is whether at the scale examined here 10 km the airmass contrast itself in particular the horizontal density virtual potential temperature difference and resulting solenoidal circulation is responsible for the sustained convergence and the radar fine line for the 10 cases examined herein the answer is affirmative

density currents in shear flows effects of rigid lid and cold pool internal circulation and application to squall line dynamics idealized density current models that contain a rigid lid developed in previous studies are evaluated it is shown through numerical experiments that a realistic stable layer placed at the level of the rigid lid acts very much like a rigid lid in confining the environmental flow to the levels below and in controlling the behaviour of density currents that propagate in the flow when this stable layer is replaced by a neutrally stable layer the behaviour of the density current is very different these results support the hypothesis that in the atmosphere the tropopause or a tropospheric inversion layer acts much like a rigid lid in the idealized density current models and the solutions of these models are applicable to organized quasi two dimensional convective systems such as squall lines in the atmosphere the effects of cold pool internal circulations on density current behaviour are also examined when a single circulation is present initially inside the cold pool the direction of circulation that gives rise to a smaller shear across the density interface leads to a smoother interface and a much more steady density current head large amplitude eddies develop along the interface when the direction of circulation is reversed when two layers of shear are initially present inside the cold pool the flow pattern inside the cold pool tends to be dominated by the circulation of the lower layer in this case the behaviour of the density interface and density current head depends on more than just the initial cross interface shear the overall flow pattern in the density current in which the cold pool circulation contains rearward flows at the ground level bears a close similarity to that found in mature squall line systems as shown by comparisons with a simulated squall line when the cold pool in a squall line is defined in terms of the equivalent potential temperature the solutions of idealized density currents in sheared flows become more readily applicable to squall lines copyright 2002 royal meteorological society

impact of soil moisture and convectively generated waves on the initiation of a west african mesoscale convective system a mesoscale convective system mcs case study was observed over northeast mali as part of the african monsoon multidisciplinary analysis amma on 31 july 2006 observations of this case suggest that the soil moisture heterogeneity and atmospheric gravity waves emitted from a parent mcs were important trigger mechanisms for this system this study uses high resolution met office unified model metum simulations to assess the importance of the synoptic circulation land surface and gravity waves in the initiation and development of the mcs during the early afternoon shallow convection developed over a region of dry soil within a synoptic scale convergence zone which was caused by the confluence of the southerly monsoon flow with winds associated with the circulation around the saharan heat low two pronounced waves were emitted from a nearby parent storm and propagated towards the convergence zone when the second wave reached the location of the shallow convection deep convection was immediately initiated further convective cells developed later in the afternoon over dry soil many adjacent to strong soil moisture gradients these aggregated with the main storm which later developed into the case study mcs a comparison of model simulations with without the soil moisture heterogeneity and gravity waves shows that the synoptic scale circulation and convergence zones specified by the atmospheric analysis were the most important factors for the successful simulation of the mcs if the location of the initiation of the system is to be forecast accurately the land surface that is the soil moisture must be represented adequately in order to reproduce the timing of the secondary initiation of convection correctly the model must be able to capture gravity waves that are emitted by existing systems

climatology of strong intermountain cold fronts abstract motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern utah this paper presents a 25 yr climatology of strong cold frontal passages over the intermountain west and adjoining western united states using conventional surface observations and the north american regional reanalysis strong cold frontal passages are identified based on a temperature fall of 7 c or greater in a 2 3 h period a concurrent pressure rise of 3 hpa or greater and the presence of a large scale 700 hpa temperature gradient of at least 6 c 500 km 1 the number of strong cold frontal passages exhibits a strong continental signature with very few events 10 along the pacific coast and more than 200 events east of the continental divide the number of events increases dramatically from the cascade mountains and sierra nevada to northern utah indicating that the intermountain west is a frequent cold front breeding ground a composite of th

importance of diabatic processes on ageostrophic circulations observed during the fronts 87 experiment the impact of condensation and evaporation on real cross frontal circulations is studied using data from the fronts 87 field experiment the sawyer eliassen equation is solved numerically in its primitive equation form for one intensive observing period of fronts 87 three different schemes of parametrization of condensation heating are used comparison between diagnosed and observed ageostrophic circulations shows that latent heat release plays an important role in enhancing the ascending branch of the ageostrophic circulation ahead of the frontal zone in particular when a parametrization scheme for the low level moisture convergence is applied a cell of strong ascending motion is formed ahead of the frontal zone in good agreement with the observations further a simple parametrization of evaporative cooling below the frontal discontinuity leads to a reinforcement of downdraughts in the cold sector of the front as observed in regions where precipitation originating from the strong line convection evaporates in comparison with their adiabatic counterparts the current solutions show strong similarities with the observed circulation and indicate the predominance of diabatic effects in the frontal zone

the interaction of a pacific cold front with shallow air masses east of the rocky mountains abstract the study presented here describes the interactions that occurred between an advancing pacific cold front and shallow gulf of mexico and arctic air masses situated east of the rocky mountains during the verification of the origins of rotation in tornadoes experiment vortex field campaign on 17 18 april over oklahoma and adjacent states these interactions were driven largely by the complex topography of the region four air masses of distinctly different origin i e pacific polar high altitude continental gulf of mexico and arctic and the boundaries that separated them i e pacific cold front dryline and arctic front were observed within the experimental domain this event produced more than 1 million worth of damage in the experimental domain due to severe weather a dense network of ground based in situ and remote observing systems and two research aircraft equipped with in situ sensors and doppler radars gathered data that allowed the authors to document the passage of a vigoro

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

using the translation speed and vertical structure of gust fronts to infer buoyancy deficits within thunderstorm outflow abstractthis study investigates whether the thermodynamics of supercell rear flank outflow can be inferred from the propagation speed and vertical structure of the rear flank gust front to quantif

density currents in two layer shear flows an earlier two fluid model of an idealized densitycurrent in low level shear is extended to include variable upper level shear far field solutions are determined based on the conservation of mass momentum and vorticity and the conservation of bernoulli function energy along streamlines for inviscid flows it is found that the upper level and low level shears play similar roles in controlling the depth of steady state density currents in most cases large positive upper level shear supports a deeper density current and steeper front and therefore a stronger updraught it is also found that when the low level shear is weak and upper level shear occupies about half the domain depth larger positive shear can result in a shallower rather than a deeper density current this behaviour was not found for either constant shear flow or flows with only low level shear the behaviour is understood by examining the flow structure and flow force components as a function of the upper level shear furthermore by allowing the upper level shear to vary an overturning flow is permitted ahead of the density current this was not possible in the earlier model in which the upper level flow was assumed to be constant the present extension allows us to draw closer analogues between the model solutions and the circulation patterns found in typical squalllines in sheared environments time dependent numerical experiments are conducted for a range of upper and low level shears the depth and the propagation speed of simulated density currents are found to agree very well with predictions by the idealized theoretical model this verifies the validity of the theoretical model in addition numerical experiments with identically zero low level shear but differing upper level shears suggest that the deeper shear is as important as the low level shear in determining the uprightness of the upward branch of inflow in fact the presence of positive low level inflow shear may not be essential such results are also supported by the theoretical model and may have important implications for our understanding of squall line dynamics

planetary and synoptic scale signatures associated with central american cold surges abstract motivated by outstanding issues from a previous case study of a midlatitude cold surge that affected mexico and central america the climatology of central american cold surges is examined in this paper an independently derived listing of 177 cold surge events is employed for which the following properties are tabulated onset date duration time between cold surge events latitude of maximum equatorward penetration min and 48 h maximum surface temperature change at merida mexico t these data show that 75 of the cold surges have durations of 2 6 days the same timescale as mobile disturbances in the westerlies also there does not appear to be any relationship between t and the duration of the event although cold surges that penetrate to low latitudes min 7 10 n have a weak tendency to persist longer than those that do not penetrate to low latitudes min 15 20 n in addition the reding data indicate that the cold surges tend to reach their most equatorward extent where

borings in phosphatized cambrian siltstone pebbles estonia baltica the earliest known macroborings trypanites from baltica occur in early cambrian phosphatized siltstone pebbles from kopli quarry in tallinn estonia trypanites borings also occur in furongian phosphatized siltstone pebbles in northern estonia the intensity of bioerosion on these cambrian pebbles is low compared to analogue substrates from ordovician deposits of baltica these bored phosphatized siltstone pebbles show that bioerosion of hard substrates occurred in relatively cold climate epicontinental seas during cambrian time

the front range blizzard of 1990 part i synoptic and mesoscale structure abstract over 1 m of snow fell in the foothills of southeast wyoming and northeast colorado during the storm of 6 7 march 1990 the heavy snowfall combined with strong winds to product blizzard conditions resulting in major highways being closed for several days the heaviest snow fell in the vicinity of a narrow northerly barrier jet that developed in place along the front range of the colorado rockies strong warm air advection from the southeast was observed during the development of the barrier jet there was no cold air advection from the north rapid intensification of the barrier jet took place only after precipitation started and was concurrent with the development of heavy precipitation a mesoscale front marked the transition from southeasterly to northerly flow this front remained very close to the rain snow line and progressed toward the east at 1 2 m s 1 for about 15 h melting precipitation appears to be the dominant process contributing to the development of the barrier jet and mesoscale f

a dryline in southeast wyoming part ii airborne in situ and raman lidar observations part i of this study describes the mesoscale structure of a dryline over southeastern wyoming this dryline formed just east of the western rim of the high plains on 22 june 2010 and became more defined as it progressedeastwardduringtheafternoon partialsodescribesthenumericallysimulatedstructureandevolution of this dryline and the observed initiation of deep convection in the vicinity of the dryline an instrumented aircraft the university of wyoming king air repeatedly flew across this dryline mostly low enough to penetrate the moist air wedge east of the dryline flight level in situ data along these low level penetrations indicate relatively high values of convective available potential energy cape 1500jkg 21 yet low convective inhibition within a few kilometers of the dryline water vapor transects obtained from a compact nadir pointing raman lidar aboard the aircraft reveal an extremely sharp humidity gradient below flight level along the dryline coinciding with the fineline seen in operational weather radar base reflectivity imagery theyalsorevealseveralplumesofhigher specifichumiditywithinthedryelevatedmixedlayerabovethemoistair wedge possibly precursors of cumulus clouds the vertical structure of the dryline revealed by raman lidar and the flight level data correspond well to that in the high resolution numerical simulation

an observational and numerical study of an intense landfalling cold front along the northwest coast of the united states during coast iop 2 abstract this paper presents an observational and numerical study of an intense wintertime cold front that made landfall along the northwest coast of the united states during iop 2 3 4 dec 1993 of the coastal observation and simulation with topography experiment observations suggest that the offshore frontal zone was associated with two transitions the first characterized by a substantial temperature gradient a nearly vertically oriented wind shift from southwesterly ahead to westerly behind and a convective rainband while the second transition had a slantwise wind shift zone from westerly ahead to northwesterly behind the frontal zone was quite narrow 5 km wide and nearly vertical below 850 mb and its width increased by 1 2 orders of magnitude above 850 mb well before the frontal landfall low level flow to the west of the olympics was associated with geostrophic balance in the cross shore direction and downgradient acceleration in the alongshore direction which contributed to the formation

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

finescale vertical structure of a cold front as revealed by an airborne doppler radar abstract in the afternoon of 24 may 2002 a well defined and frontogenetic cold front moved through the texas panhandle detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary this paper primarily uses reflectivity and doppler velocity data from an airborne 95 ghz radar as well as flight level thermodynamic data to describe the vertical structure of the cold front as it intersected with the dryline the prefrontal convective boundary layer was weakly capped weakly sheared and about 2 5 times deeper than the cold frontal density current the radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail 40 m the echo structure and dual doppler inferred airflow in the vertical plane reveal typical features such as a nose a head a rear inflow current and a broad current of rising prefrontal air that feeds the accelerating front to rear current over the head the 2d cross fro

formation of mesoscale lines of pirecipitation severe squall lines in oklahoma during the spring abstract four distinct kinds of severe mesoscale convective line development are identified in oklahoma during the spring based on the analysis of an 11 year period of reflectivity data from the national severe storms laboratory s 10 cm radar in norman oklahoma the primary classes of fine formation are broken line back building broken areal and embedded areal each is described in detail along with illustrative examples comparisons are made with other observations and with numerical model simulations the former two classes of line formation have been previously documented while the latter two have not only the broken areal squall line has been realistically simulated numerically the environment for each of the types of line development was determined from data from the standard national weather service surface and upper air networks and from special rawinsonde launches it was found that broken line formation tends to occur along cold fronts in a multicell environment while back building occur

simulated density currents in idealized stratified environments abstract the effects of three distinct stratifications on density current dynamics are investigated using a nonhydrostatic numerical model i a stably stratified layer underneath a deep neutrally stratified flow representing a nocturnal boundary layer over land ii a neutrally stratified layer underlying a deep stably stratified flow representing a daytime boundary layer and iii a continuously stratified atmosphere in the first case a weak or intermediate stratification decreases the height of density currents and increases the propagation speed the same result holds in strongly stratified situations as long as the generated disturbances in the neighborhood of the head do not propagate away classical density currents occur in weak stratification multiheaded density currents in intermediate stratification and multiheaded density currents with solitary wave like or borelike disturbances propagating ahead of the current in strong stratification in the second case the upper layer stratificati

discrete frontal propagation over the sierra cascade mountains and intermountain west abstract on 25 march 2006 a complex frontal system moved across the sierra cascade mountains and intensified rapidly over the intermountain west where it produced one of the strongest cold frontal passages observed in salt lake city utah during the past 25 yr observational analyses and numerical simulations by the weather research and forecast wrf model illustrate that the frontal system propagated discretely across the sierra cascade mountains and western nevada this discrete propagation occurs in a synoptic environment that features a mobile upper level cyclonic potential vorticity pv anomaly that is coupled initially with a landfalling pacific cyclone and attendant occluded front the eastward migration of the upper level cyclonic pv anomaly ultimately encourages the development of a new surface based cold front ahead of the landfalling occlusion as troughing confluence and convergence downstream of the sierra nevada intensify preexisting baroclinity over nevada trajectories show that the n

terrain influences on synoptic storm structure and mesoscale precipitation distribution during ipex iop3 abstract the influence of topography on the evolution of a winter storm over the western united states and distribution of precipitation over northern utah are examined using data collected during the third intensive observing period iop3 of the intermountain precipitation experiment ipex the analysis is based on high density surface observations collected by the mesowest cooperative networks special radiosonde observations wind profiler observations next generation weather radar nexrad data and conventional data a complex storm evolution was observed beginning with frontal distortion and low level frontolysis as a surface occluded front approached the sierra nevada as the low level occluded front weakened the associated upper level trough moved over the sierra nevada and overtook a lee trough the upper level trough which was forward sloping and featured more dramatic moisture than temperature gradients then moved across nevada with a weak surface reflection as a pressure trough over no

convection initiation by density currents role of convergence shear and dynamical organization abstract steady state analytic models establish two key points concerning the impact of vertical shear on density currents and the implication for convection initiation first shear decreases the horizontal convergence and therefore the mean ascent associated with downshear propagating currents second shear has a basic effect on the dynamical organization if the downshear current travels at the speed of the ambient flow at a critical steering level an overturning circulation provides deep lifting although mean ascent is increased by shear in the case of upshear propagating currents the lifting is comparatively shallow because jumplike ascent occurs rather than deep overturning the convection initiation mechanism involving the downshear current is therefore very different from the upshear case these basic principles are borne out in two dimensional numerical simulations density currents generated by a stationary cold source imposed on an initially horizontally homogeneous sheared and neutra

earliest cavity dwelling organisms coelobionts lower cambrian poleta formation nevada the record of cavity dwelling organisms coelobionts is long ranging and is known to extend as far back as the upper lower cambrian older coelobionts from the middle lower cambrian poleta forma

cold fronts with and without prefrontal wind shifts in the central united states time series of cold fronts from stations in the central united states possess incredible variety for example time series of some cold fronts exhibit a sharp temperature decrease coincident with a pressure trough and a distinct wind shift other time series exhibit a prefrontal trough and wind shift that precedes the temperature decrease associated with the front by several hours in early march 2003 two cold fronts passed through oklahoma city oklahoma okc representing each of the above scenarios the cold front on 4 march was characterized by a coincident sharp wind shift pressure trough and a strong temperature decrease of 108c in 2 min on the other hand the cold frontal passage on 8 march was characterized by a prefrontal wind shift occurring over a 7 h period before the temperature decrease of 108c in 2 h twelve hours before frontal passage at okc both fronts had the same magnitude of the horizontal potential temperature gradient and petterssen frontogenesis by the time of frontal passage at okc the magnitude of the horizontal potential temperature gradient for the 4 march front was double that of the 8 march front and the frontogenesis was nearly four times as great the simultaneity of the surface horizontal potential temperature gradient and deformation and convergence maxima coincident with the wind shift was primarily responsible for the greater strength of the cold front in okc on 4 march compared to that on 8 march whether a prefrontal wind shift occurred was determined by the timing and location of cyclogenesis in the central united states on 4 march a cyclone was adjacent to the slope of the rocky mountains and developed on the cold front as it moved through oklahoma permitting greater frontogenesis and resulting in a cold frontal passage at okc with a simultaneous temperature decrease and wind shift on 8 march the cyclone moved eastward through oklahoma before the arrival of the cold front resulting in a prefrontal wind shift associated with the northerlies behind the cyclone followed by the frontal passage a 2 yr climatology of cold frontal passages at okc supports the results from the two cases above indicating that the timing and location of cyclogenesis was responsible for these two different cold frontal structures these results imply that for situations resembling those of this study the prefrontal trough is not directly associated with the cold front but is caused by external processes related to the lee troughing

an observational study of fronts and frontal mergers over the continental united states abstract the stormscale operational and research meteorology fronts experiment systems tests storm fest field program was carried out over the central united states to investigate the structure and dynamics of fronts and their attendant precipitation systems adjacent to complex topography and to evaluate new observing networks and analysis techniques for mesoscale research and weather forecasting this study presents the analysis of data taken between 8 and 10 march 1992 which was the most active period of storm fest this three day observing period was characterized by the passage of an extratropical cyclone and related fronts the synoptic scale conditions related to the cyclone and frontal evolutions are described multiple frontal zones i e pacific and arctic cold fronts quasi stationary polar front dryline maritime warm front and associated regions of precipitation are documented and their origins and interactions are explored these features both well defined and subtle were observed wi

dynamics of a thunderstorm outflow abstract the kinematic and thermodynamic structures of a thunderstorm outflow are examined by means of dual doppler radar analysis mesonet lower and sounding data the data were collected in the denver colorado area during june 1984 the dual doppler analysis shows that the cold outflow is ducted beneath the pbl inversion along the gust front there is a narrow quasi two dimensional updraft kelvin helmholtz instability khi developed along the top of the gust front head near the surface front and propagated backwards dissipating in the wake of the head region an isothermal layer aloft appears to have limited billow amplification to the quasi neutral layer below the gust front s leading edge had numerous inflections which are believed to result from barotrophic instabilities small vortices develop at some of the inflection points detailed analysis of one such circulation shows evidence of the formation of two enhanced updrafts separated by an occlusion downdraft these observations are the firs

numerical simulation of thunderstorm outflow dynamics part i outflow sensitivity experiments and turbulence dynamics abstract in this first paper of a two part series a two dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows by focusing only on the outflow and using essentially inviscid equations and high spatial resolution we are able to explicitly represent important physical processes such as turbulent mixing to simplify interpretation of the results the model atmosphere used in all experiments is calm and dry adiabatic this approach allows us to establish basic characteristics of modeled outflows in simple physical settings and provides a foundation for future studies using more realistic environments all simulated outflows are initialized by prescribing a controlled horizontal flux of cold air into the model domain through a lateral boundary in a series of sensitivity tests we examine three parameters of the cold air source region 1 the vertical temperature deficit profile 2 the magnitude of the temperature deficit and 3 the cold air depth by hol

airborne observations of a front near a col during fastex abstract an analysis of an oceanic front situated near a col defined by the surface pressure field is presented there have been few observational examples of this type of front presented in the literature the primary source of information for this study was data recorded by an aircraft equipped with a doppler radar the front was approximately two dimensional and the cross frontal scale at low levels was 30 40 km a prefrontal low level jet was identified in the high resolution analyses and was shown to be supergeostrophic surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance twi in the alongfront direction large negative values of twi the vertical shear is less than predicted for the given horizontal temperature gradient were located near a region of frontogenesis the strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in g

the 1993 superstorm cold surge frontal structure gap flow and tropical impact in the wake of the eastern united states cyclone of 12 14 march 1993 a cold surge originating over alaska and western canada brought northerlies exceeding 20 m s21 and temperature decreases up to 158c over 24 h into mexico and central america this paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale focusing on 1 the structure and evolution of the leading edge of the cold surge 2 the reasons for its extraordinary intensity and equatorward extent and 3 the impact of the surge on the tropics specifically on the strength of the trade winds and on the sea surface temperature in the eastern pacific the cold surge was initiated as a developing cyclone over the gulf of mexico and an upstream anticyclone east of the rockies caused an along barrier pressure gradient to form forcing topographically channeled northerlies along the rocky and sierra madre mountains to transport cold air equatorward on the mesoscale the leading edge of the cold surge possessed nonclassical frontal structure for example as the cold surge entered mexico the coldest air and the strongest wind arrived at about 900 hpa before affecting the surface suggestive of a tipped forward leading edge to the surge also satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air the cold surge had characteristics reminiscent of a kelvin wave a tipped forward cold front a pressure jump line a bore and a gravity current but none of these conceptual dynamical models was fully applicable associated with the cold surge gap winds up to 25 m s21 were observed in the gulfs of tehuantepec a tehuantepecer fonseca papagayo and panama owing to the strong cross mountain pressure gradient in the case of the tehuantepecer a rope cloud emanated from the isthmus of tehuantepec and turned anticyclonically consistent with an inertial oscillation on the synoptic and planetary scales the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold air damming by a low latitude upper tropospheric trough and by the lower branch of the secondary circulation associated with a confluent jet entrance region aloft the cold surge also impacted the tropical atmosphere and ocean by contributing to the strengthening of the northeast trade winds over the eastern pacific ocean and by inducing local cooling of the sea surface temperature in the gulfs of tehuantepec and papagayo by about 48 88c

regional variability in dust on snow processes and impacts in the upper colorado river basin dust deposition onto mountain snow cover in the upper colorado river basin frequently occurs in the spring when wind speeds and dust emission peaks on the nearby colorado plateau dust loading has increased since the intensive settlement in the western usa in the mid 1880s the effects of dust on snow have been well studied at senator beck basin study area sbbsa in the san juan mountains co the first high altitude area of contact for predominantly southwesterly winds transporting dust from the southern colorado plateau to capture variability in dust transport from the broader colorado plateau and dust deposition across a larger area of the colorado river water sources an additional study plot was established in 2009 on grand mesa 150 km to the north of sbbsa in west central co here we compare the 4 year 2010 2013 dust source deposition and radiative forcing records at grand mesa study plot gmsp and swamp angel study plot sasp sbbsa s subalpine study plot the study plots have similar site elevations environments and differ mainly in the amount of dust deposited and ensuing impacts at sasp end of year dust concentrations ranged from 0 83 mg g 1 to 4 80 mg g 1 and daily mean spring dust radiative forcing ranged from 50 65 w m 2 advancing melt by 24 49 days at gmsp which received 1 0 mg g 1 less dust per season on average spring radiative forcings of 32 50 w m 2 advanced melt by 15 30 days remote sensing imagery showed that observed dust events were frequently associated with dust emission from the southern colorado plateau dust from these sources generally passed south of gmsp and back trajectory footprints modelled for observed dust events were commonly more westerly and northerly for gmsp relative to sasp these factors suggest that although the southern colorado plateau contains important dust sources dust contributions from other dust sources contribute to dust loading in this region and likely account for the majority of dust loading at gmsp copyright 2015 john wiley sons ltd

effects of upper level shear on the structure and maintenance of strong quasi linear mesoscale convective systems recent observational studies have shown that strong midlatitude mesoscale convective systems mcss tend to decay as they move into environments with less instability and smaller deep layer vertical wind shear these observed shear profiles that contain significant upper level shear are often different from the shear profiles considered to be the most favorable for the maintenance of strong long lived convective systems in some past idealized simulations thus to explore the role of upper level shear in strong mcs environments a set of two dimensional 2d simulations of density currents within a dry statically neutral environment is used to quantify the dependence of lifting along an idealized cold pool on the upper level shear a set of three dimensional 3d simulations of mcss is produced to gauge the effects of the upper level shear in a more realistic framework results from the 2d experiments show that the addition of upper level shear to a wind profile with weak to moderate low level shear increases the vertical displacement of parcels despite a decrease in the vertical velocity along the cold pool interface parcels that are elevated above the surface 1 2 km overturn and are responsible for the deep lifting in the deep shear environments while the surface based parcels typically are lifted through the cold pool region in a rearward sloping path this deep overturning helps to maintain the leading convection and greatly increases the size and total precipitation output of the convective systems in more complex 3d simulations even in the presence of 3d structures these results show that the shear profile throughout the entire troposphere must be considered to gain a more complete understanding of the structure and maintenance of strong midlatitude mcss

multiscale analysis of a meso β frontal passage in the complex terrain of the colorado front range abstract data from a mesoscale observing network are used to describe the evolution of a complex boundary between a dry air mass near the foothills of the rocky mountains and a shallow moist air mass over the eastern plains synoptic scale analyses revealed that the origin of the moist air mass was associated with lee cyclogenesis mesoscale analyses provided a detailed picture of a localized anticyclonic circulation that developed within the larger scale flow mixing ratio data from the mesoscale observing network indicated the position of the boundary between the air masses it is shown that the cooler moister air on the plains advanced toward and retreated away from the foothills during the evening eventually downslope winds that were opposing the motion of the mesoscale boundary decreased and the anticyclonic circulation on the plains became more organized on an even smaller scale doppler lidar measurements revealed characteristics of the wind flow associated with the mesofront and the interacti

the frontal hydraulic head a micro α scale 1 km triggering mechanism for mesoconvective weather systems abstract measurements from the noaa boulder atmospheric observatory bao 300 m tower the national center for atmospheric research ncar sabreliner aircraft and the noaa goes 5 satellite give evidence for the cross front scale collapse of nonprecipitating surface cold frontal zones to horizontal distances of 1 km or less the leading edges of these frosts possess the characteristic structure of density current flows an elevated hydraulic head followed by a turbulent wake vertical motions at the frontal heads exceed 5 m s 1 at 300 m agl the ascent at the frontal head may act as a 1 km scale triggering mechanism for the release of potential instability and the formation of intense squall line mesoconvection

palaeoecology and evolution of marine hard substrate communities abstract marine organisms have occupied hard substrates since the archaean shells rocks wood and sedimentary hardgrounds offer relatively stable habitats compared to unconsolidated sediments but the plants and animals which inhabit them must develop means to gain and defend this premium attachment space hard substrate communities are formed by organisms with a variety of strategies for adhering to and or excavating the substrates they inhabit while mobile grazers organically attached and even soft bodied organisms may leave evidence of their former presence in ancient hard substrate communities a superior fossil record is left by sessile encrusters with mineralised skeletons and by borers which leave trace fossils furthermore encrusters and borers are preserved in situ retaining their spatial relationships to one another and to the substrate spatial competition ecological succession oriented growth and differential utilisation of exposed vs hidden substrate surfaces can all be observed or inferred hard substrate communities are thus excellent systems with which to study community evolution over hundreds of millions of years here we review the research on modern and ancient hard substrate communities and point to some changes that have affected them over geological time scales such changes include a general increase in bioerosion of hard substrates particularly carbonate surfaces through the phanerozoic this is at least in part analogous to the infaunalisation trends seen in soft substrate communities encrusting forms show an increase in skeletalisation from the palaeozoic into the mesozoic and cenozoic which may be a response to increasing levels of predation hard substrate communities considering borers and encrusters together show a rough increase in tiering through the phanerozoic which again parallels trends seen in soft substrate communities this extensive review of the literature on living and fossil hard substrate organisms shows that many opportunities remain for large scale studies of trends through time at the community and clade levels palaeontologists will especially benefit by closer integration of their work with that of neontologists particularly in aspects of ecology such as larval recruitment competition and succession

early cambrian reefs reef complexes and associated lithofacies of the shackleton limestone transantarctic mountains the shackleton limestone formed a carbonate platform that bordered part of the greater antarctic craton in middle and late early cambrian time in the holyoake range of the central transantarctic mountains this unit records deposition on a stable shelf on which flourished ecological reefs composed of microorganisms and archaeocyathans burrow mottled lime mudstone wackestone and packstone with patch reefs represent accumulation in shelf areas of relatively low to moderate energy thick ooidal grainstone units reflect deposition in higher energy shoals and as sand sheets that were associated with extensive reef complexes the framework of these reefs was principally the product of micro organisms by inference mostly cyanobacteria archaeocyathans constitute as much as 30 of some reefs but commonly they form less than 10 and are absent from some on the basis of microbial composition three reef types are recognized the first type is a renalcis boundstone that lacks archaeocyathans within these abundant upward directed thalii of renalcis formed a framework that trapped fine grained sediment the second type which forms the core of some larger reefs is composed of stromatactis bearing microbial boundstone the third yet most common reef type is variable in composition it is characterized by the presence of abundant epiphyton but may include archaeocyathans and the microbial microfossils girvanella and renalcis as well as cryptomicrobial clotted micrite in this type of reef frame building organisms typically constructed highly porous structures that had small interparticle and fenestral pores and large growth framework cavities as well as rare metre sized caverns within these spaces epiphyton and less commonly renalcis encrusted framework elements fine grained sediments accumulated and pervasive sea floor cements were precipitated boundstone fabrics in the shackleton limestone are highly complex with fabrics analogous to younger more metazoan rich reefs as well as deep water stromatactis bearing mud mounds the epiphyton girvanella archaeocyathan frameworks and stromatactis bearing boundstones both of which seemingly first appeared in the middle early cambrian are regarded as the precursors in structure composition and preferred hydrologic setting of the more extensive reefs and complex framework styles of later phanerozoic time

the dependence of storm longevity on the pattern of deep convection initiation in a low shear environment the sensitivity of storm longevity to the pattern of deep convection initiation e g multiple quasi linearly arranged initial deep convective cells versus an isolated deep convective cell is examined using idealized cloud resolving simulations conducted with a low shear initial environment when multiple deep convective cells are initialized in close proximity to one another using either a line of thermals or a shallow airmass boundary long lived storms are produced however when isolated deep convection is initiated the resultant storm steadily decays following initiation these results illustrate that a quasi linear mechanism such as a preexisting airmass boundary that initiates multiple deep convective cells in close proximity can lead to longer lived storms than a mechanism that initiates isolated deep convection the essential difference between the experiments conducted is that an isolated initial storm produces a shallower cold pool than when a quasi linear initiation is used it is argued that the deep cold pools promote deep forced ascent systematic convective cell redevelopment and thus long lived storms even in environments with small values of vertical shear the difference in cold pool depth between the simulations is attributed to differences in the horizontal flux of cold air to the gust front with a single initial storm the few convective cells that subsequently form provide only a limited source of cold air leading to a cold pool that is shallow and incapable of fostering continued updraft redevelopment

an observational and numerical study of a cold front interacting with the olympic mountains during coast iop5 abstract this paper documents the three dimensional flow and precipitation structures associated with a weak cold front interacting with the olympic mountains and the subsequent development of a puget sound convergence zone this study utilizes data collected during coast iop5 the fifth intensive observing period of the coastal observation and simulation with topography field experiment that took place on 11 12 december 1993 one of the most important data sources was a noaa p 3 aircraft which provided flight level data radar reflectivity and doppler winds as it circumnavigated the olympics initially frontal passage along the western foothills of the olympics was accompanied by a 2 3 c temperature drop a rapid wind shift to northwesterlies and an intense line of precipitation 35 45 dbz however the wind shift and associated precipitation structures attenuated when the front began to ascend the windward slopes of the olympics surface and p 3 observations document the deformation of the front

cold pools in shear gravity currents exist in many forms and develop within many types of flow the dynamics of long lived cumulonimbus are thought to be strongly influenced by the interaction of a spreading pool of cold low level air which may act as a gravity current and the ambient sheared flow in the boundary layer in interpreting two and three dimensional numerical models of such phenomena the behaviour of the vorticity dynamics in the ambient flow and the spreading cold air is commonly discussed in order to investigate these vorticity ideas from a gravitycurrent perspective three dimensional numerical simulations have been performed in which a cold pool spreads from an isolated source in the presence of a horizontal flow that is sheared with height it has been found that the shear of the incoming ambient flow does not qualitatively alter the horizontal spread of the cold pool and that the location of the region of maximum horizontal convergence remains on the along wind axis the actual strength of the ambient wind which is shown analytically to be measured by the wind strength at the head height has a greater bearing on the cold pool shape than the shear consequently it must be the details of the moist convection which play a crucial role in determining the cold pool s development in sheared flows as such this model is a general indication of the robustness of gravity current theory even in the presence of mean shear the vortex structure on the flanks of the spreading cold air is found to be significantly modified with a reversal in the dominant sign of vorticity and an increase in its intensity when such mean shear exists

mesoscale modeling of a cold front and its interaction with a diurnally heated land mass abstract a mesoscale numerical model incorporating a detailed planetary boundary layer scheme including momentum heat and moisture exchange with the lower boundary is used to study the change in structure of a dry summertime front as it moves towards a coastline two dimensional experiments show that the diurnal pressure fall over land causes a trough to form in the coastal region ahead of the approaching front frontogenesis at this mesoscale trough leads to the formation of a new front near the coast giving the appearance of strong acceleration of the original frontal system in the offshore region this process can occur over a wide range of times from early afternoon to late evening but no such acceleration occurs for fronts crossing the coast in the early morning hours these results indicate a tendency for more fronts to cross the coastline in the afternoon early evening period than at other times a statistic which is observed in southeast australia and oregon usa they also imply fronts sho

response of atmospheric convection to vertical wind shear cloud system resolving simulations with parameterized large scale circulation part i specified radiative cooling abstractit is well known that vertical wind shear can organize deep convective systems and greatly extend their lifetimes much less is known about the influence of shear on the bulk properties of tropical convection in statistical equilibrium to address the latter question the authors present a series of cloud resolving simulations on a doubly periodic domain with parameterized large scale dynamics based on the weak temperature gradient wtg approximation the horizontal mean horizontal wind is relaxed strongly in these simulations toward a simple unidirectional linear vertical shear profile in the troposphere the strength and depth of the shear layer are varied as control parameters surface enthalpy fluxes are prescribed the results fall in two distinct regimes for weak wind shear time averaged rainfall decreases with shear and convection remains disorganized for larger wind shear rainfall increases with shear as convection becomes organized into linear mesoscale systems this nonmonotonic dep

coherence between the great salt lake level and the pacific quasi decadal oscillation the lake level elevation of the great salt lake gsl a large closed basin lake in the arid western united states is characterized by a pronounced quasi decadal oscillation qdo the variation of the gsl elevation is very coherent with the qdo of sea surface temperature anomalies in the tropical central pacific also known as the pacific qdo however such coherence denies any direct association between the precipitation in the gsl watershed and the pacific qdo because in a given frequency the precipitation variation always leads the gsl elevation variation therefore the precipitation variation is phase shifted from the pacific qdo this study investigates the physical mechanism forming the coherence between the gsl elevation and the pacific qdo pronounced and coherent quasi decadal signals in precipitation streamflow water vapor flux and drought conditions are found throughout the great basin recurrent atmospheric circulation patterns develop over the gulf of alaska during the warm to cool and cool to warm transition phases of the pacific qdo these circulation patterns modulate the water vapor flux associated with synoptic transient activities over the western united states and in turn lead to the qdo in the hydrological cycle of the great basin as the gsl integrates the hydrological responses in the great basin the hydrological qdo is then transferred to the gsl elevation because the gsl elevation consistently lags the precipitation by a quarter phase about 3 yr in the quasidecadal time scale these processes take an average of 6 yr for the gsl elevation to eventually respond to the pacific qdo this creates a half phase delay of the gsl elevation from the pacific qdo thereby forming the inverse yet coherent relationship between them tree ring reconstructed precipitation records confirm that the quasi decadal signal in precipitation is a prominent feature in this region

investigations of a winter mountain storm in utah part i synoptic analyses mesoscale kinematics and water release rates abstract a winter storm passing across the north south orientated tushar mountains in southwest utah is investigated in this multipart paper this part i describes the evolving synoptic pattern mesoscale kinematics and calculated water release rates condensation or deposition in clouds over the western upstope part of the mountains horizontal mesoscale kinematic variables come from direct application of volume velocity processing to single c band doppler radar data water release rates are computed from updrafts derived from the radar data and from the vertical gradient of saturation mixing ratio obtained from soundings in stage i of the storm altostratus was present on the leading side of a long wave trough weak updrafts occurred only at the higher altitudes within the clouds where there was convergence and large scale synoptically forced lift downdrafts as great as 0 6 m s 1 occurred in the lower parts of the cloud where there was divergence the downdrafts were induced in part by sublimation c

the kinematic structure of a wasatch mountain winter storm during ipex iop3 the influence of orographic circulations on the precipitation structure of a wasatch mountain winter storm is examined using observations collected during the third intensive observing period iop3 of the intermountain precipitation experiment ipex the event featured the passage of a midlevel 700 550 hpa trough followed 3 h later by a surface trough prior to and during the midlevel trough passage large scale southwesterly flow impinged on the wasatch mountains low level confluence was observed between this southwesterly flow and along barrier southerly flow within 20 40 km of the wasatch mountains this confluence zone which moved toward the wasatch mountains during and following the passage of the midlevel trough was accompanied by low level convergence and precipitation enhancement over the upstream lowlands dual doppler analysis revealed the presence of a shallow along barrier jet near the base of the wasatch mountains that was surmounted by southwesterly cross barrier flow at mid and uppermountain levels this cross barrier flow produced strong 1 2 m s 1 ascent as it interacted with the steep windward slopes of the wasatch mountains where precipitation was roughly double that observed in the lowlands upstream flow deflection and splitting were also observed near the highest terrain features a narrow region of strong subsidence which at times exceede d2ms 1 was found to the lee of the wasatch and based on radar imagery appeared to modulate hydrometeor spillover aloft processes contributing to the evolution of the near barrier flow field including topographic blocking diabatic effects and surface friction contrasts are discussed

observations of atmospheric density currents using a tethered balloon borne turbulence probe system a tethered kite balloon with multiple instrumented probes clamped to the tethering cable is used to investigate atmospheric density current fronts passing through three sites at varying distances from the coast the system allows high frequency measurements of the three components of wind temperature and humidity to be made simultaneously at up to ten levels as the front passes through this enables a two dimensional visualization of the current and its circulation to be constructed as a result the structure of some of the wide variety of atmospheric density currents is revealed by direct observation the internal circulations and the effects of varying environments on the currents and their movement are described together with turbulence statistics one current was associated with what may have been a coastal trapped disturbance and in one case two opposing sea breezes appear to have collided inland two currents penetrating into surface inversions were dissociated from the surface one of these was travelling downwind and formed a strong near surface jet associated wave and bore structures were found to be travelling in layers above the currents in two cases in one case environmental air was found to intrude into the head region and convect internally while in another air from the head region extruded into the environmental flow the measurement array enabled the different terms of the dynamic equations to be evaluated and their relative contribution assessed a remarkable correlation is found between thermal and dynamic variables a study of the internal flux balance of daytime density currents shows that the internal flow plays a significant part in maintaining the capping inversion of the current eddies were observed at interfacial regions in which richardson numbers were significantly greater than 0 25 confirming some modelling studies frontal slopes in most cases are in accord with theoretical values but in one downwind case the slope is considerably less than predicted the inland currents are found to be deeper than coastal currents which may explain the increase in speed observed later in the day water vapour frontal structures are generally less marked than thermal fronts

a proposed method of surface map analysis present surface frontal analyses suffer from the defect that frontal positions are typically not collocated with zones of intense temperature contrast further individuals typically do not agree as to the existence type and location of fronts the author argues that the lack of a surface temperature analysis is mainly responsible for these flaws and it is proposed that such analysis preferably of potential temperature in regions of variable terrain elevation become part of routine procedure such an analysis will reveal nonfrontal baroclinic zones of considerable intensity most cold fronts except the strongest ones are denoted as baroclinic troughs propagating eastward in the prevailing westerly flow it is argued that when a meridional cold front exists in the presence of even a small meridional temperature gradient the wind shift should propagate away from the intense surface temperature gradient which then weakens an explanation is provided based on quasigeostrophic theory it follows that fronts are short lived phenomena

two dimensional convection in non constant shear a model of mid latitude squall lines numerical simulations of two dimensional deep convection are analysed using analytical models extended to include shallow downdraughts and non constant shear the cumulonimbus are initiated by low level convergence created by a finite amplitude downdraught these experiments have constant low level shear and differ only in the profile of mid and upper level winds quasi steady convenction is produced if the mid and upper level flow has small shear and the low level shear is large the surface precipitation ismaximized for no intial relative relative flow aloft if stationary this storm p o can give prodigious locilized rainfall p o is the two dimentisonal equivalent of the supercell these results are placed in context with previous two dimensional simulations attention is drawn to the similiarity with previous two dimensional simulations attention is drawn to the similarity with squall lines in central and eastern u s a storm p o is analysed by construction of time averaged fields of streamfunction vorticity teperature and height deviation the smoothness of these fields suggests a conceptual model of the storm dynamics which involves cooperation between distinct charcteristic flows an overturning updraught a jump type updraught a shallow downdraught a low level rotor and a boundary layer an idealized analytical model is described by solution of the equations for steady convection these solutions for the remote flow are derived from energy conversation mass continuity and a momentum budget and they give relationships between the non dimensional parameteres of the problem it is apparent that the convection is a high froude or low richardson number flow demanding the existence of a cross storm pressure gradient inherent in this idealized model is a vortex sheet between updraught and down draught and it is considered that the dynamical instability of this sheet is related to complexities in the numerical simulation furthermore these results show that in two dimensions both non constant shear and a shallow downdraught are necessary to maintain steady convection

a theory for strong long lived squall lines revisited based on the analysis of idealized two and three dimensional cloud model simulations rotunno et al hereafter rkw and weisman et al hereafter wkr put forth a theory that squall line strength and longevity was most sensitive to the strength of the component of low level 0 3 km agl ambient vertical wind shear perpendicular to squall line orientation an optimal state was proposed by rkw based on the relative strength of the circulation associated with the storm generated cold pool and the circulation associated with the ambient shear whereby the deepest leading edge lifting and most effective convective retriggering occurred when these circulations were in near balance since this work subsequent studies have brought into question the basic validity of the proposed optimal state based on concerns as to the appropriate distribution of shear relative to the cold pool for optimal lifting as well as the relevance of such concepts to fully complex squall lines especially considering the potential role of deeper layer shears in promoting system strength and longevity in the following the basic interpretations of the rkw theory are reconfirmed and clarified through both the analysis of a simplified two dimensional vorticity streamfunction model that allows for a more direct interpretation of the role of the shear in controlling the circulation around the cold pool and through an analysis of an extensive set of 3d squall line simulations run at higher resolution and covering a larger range of environmental shear conditions than presented by wkr

the eastern united states side door cold front of 22 april 1987 a case study of an intense atmospheric density current abstract a case study is presented of an unusual cold front that affected the east coast of the united states on 22 april 1987 noteworthy aspects of this front are its genesis behind a preexisting back door cold front its propagation to the southwest the shallow nature of the cold air associated with it 600 m and the absence of precipitation this front has been termed a side door cold front because of the significant differences between it and typical back door cold fronts such as its direction of propagation and vertical structure weather conditions in the mid atlantic states changed abruptly from partly cloudy skies light winds and 25 30 c surface temperatures ahead of the front to a windy gust to 20 m s 1 low overcast regime with temperatures 10 15 c cooler behind the front analysis of satellite imagery surface data and sounding data revealed that the front possessed density current structure during a portion of its lifetime observed front relative flow as well as agreement betwe

airborne doppler observations of a landfalling cold front upstream of steep coastal orography abstract this study uses airborne doppler radar observations to describe the mesoscale structure and evolution of a cold frontal system as it made landfall on the mountainous coast of oregon and northern california on 1 december 1995 during the coastal observations and simulations with topography experiment this section of coastline constitutes a steep approximately two dimensional north south oriented orographic barrier the front exhibited a northeast southwest orientation and thus intersected the axis of high terrain at an acute angle the along barrier pressure gradient and low level winds increased with time along the coastal zone and reached a maximum as the front made landfall stably stratified prefrontal flow was strongly blocked by the orography resulting in a confluent transition from pervasive southwesterly winds offshore to a narrow zone of accelerated south southwesterly flow near the coast where wind speeds approached 30 m s 1 at a height of 750 m above mean sea level postfrontal flow

meteorological tower measurements of a surface cold front abstract measurements from the boulder atmospheric observatory meteorological research tower are used to describe the structure and physical processes of a strong surface cold front analysis reveals that the horizontal gradients in temperature and wind velocity at the front are concentrated within a 200 m distance and measured vertical velocities exceed 5 m s 1 at the leading edge of the front calculations with the miller frontogenesis equation show the magnitude and relative importance of cross frontal confluence versus differential vertical motion tilting in forcing frontogenesis

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones part v the substructure of narrow cold frontal rainbands abstract the organization and structure of a narrow cold frontal rainband ncfr on the small mesoscale and the microscale have been investigated through quantitative radar reflectivity doppler radar observations airborne observations and surface measurements the ncfr was composed of small mesoscale regions of heavy precipitation called precipitation cores pcs oriented at an angle to the synoptic scale cold front in horizontal cross section the pcs were roughly elliptical in shape areas of lighter precipitation called gap regions grs separated the pcs the pcs were so oriented that their loading edges were regions of strong low level convergence the weather associated with the passage of a pc resembled that of a squall line gust front with concurrent windshifts and pressure checks occurring 5 min before heavy precipitation and a fall in temperature the changes in surface weather that accompanied the passage of a gr were more variable but tended to be less marked than for pcs thus the se

genesis and evolution of hierarchical cloud clusters in a two dimensional cumulus resolving model abstract a two dimensional cloud ensemble model is integrated over a basin scale domain with prescribed sea surface temperature sst to study the formation and evolution of cloud clusters over a large scale warm pool neither a basic zonal flow is prescribed nor is a single perturbation initially given the results show that deep convective clouds appear in hierarchical clustered patterns and are limited to the area of warm sst above 28 c the most fundamental cloud cluster in the model has a horizontal scale of a few hundred kilometers in which new cumulus clouds are generated at the leading edge of a propagating surface cold air pool the gust front it may last for days and propagate for a long distance if the background flow is broad and persistent as is the case in the low level convergence zone of the sst induced background flow the largest hierarchical propagating cloud systems in the model have horizontal scales up to 3000 km and consist of up to four cloud clusters that are generally of gust

a mesoscale numerical model using the nonhydrostatic pressure based sigma coordinate equations model experiments with dry mountain flows abstract a nonhydrostatic numerical model suitable for simulating mesoscale meteorological phenomena is developed and described here the model is the first to exploit the nonhydrostatic equation system in normalized pressure coordinates in addition to the commonly recognized advantages of coordinate models this model is potentially advantageous in nesting with large scale coordinate models the equation system does not support sound waves but it presents the internal gravity waves accurately external gravity waves are the fastest wave modes in the system that limit the integration time step however since short nonhydrostatic external waves are much slower than the speed of shallow water waves and because fast hydrostatic long waves imposes less severe restriction on the time step when they are resolved by many grid points a large time step compared to that determined by the speed of hydrostatic shallow water waves can be used when horizontal grid spacing is on the order of 1 km the syste

observations of nonclassical frontal propagation and frontally forced gravity waves adjacent to steep topography through the integrated analysis of remote sensing and in situ data taken along the front range of colorado this study describes the interactions that occurred between a leeside arctic front and topographically modulated flows these interactions resulted in nonclassical frontal behavior and structure across northeastern colorado the shallow arctic front initially advanced southwestward toward the front range foothills before retreating eastward then a secondary surge of arctic air migrated westward into the foothills during its initial southwestward advance the front exhibited obstacle like density current characteristics its initial advance was interrupted by strong downslope northwesterly flow associated with a high amplitude mountain wave downstream of the continental divide and by a temporal decrease in the density contrast across the front due to diurnal heating in the cold air and weak cold advection in the warm air the direction and depth of flow within the arctic air also influenced the frontal propagation the shallow obstacle like front actively generated both vertically propagating and vertically trapped gravity waves as it advanced into the downslope northwesterly flow resulting in midtropospheric lenticular wave clouds aloft that tracked with the front because the front entered a region where strong downslope winds and mountain waves extended downstream over the high plains the wave field in northeastern colorado included both frontally forced and true mountain forced gravity waves a sequence of scorer parameter profiles calculated from hourly observations reveals a sharp contrast between the prefrontal and postfrontal wave environments consequently analytic resonant wave mode calculations based on the scorer parameter profiles reveal that the waves supported in the postfrontal regime differed markedly from those supported in the prefrontal environment this result is consistent with wind profiler observations that showed the amplitude of vertical motions decreasing substantially through 16 km above mean sea level msl after the shallow frontal passage

airborne dual doppler observations of an intense frontal system approaching the pacific northwest coast airborne doppler radar data collected off the pacific northwest coast by a noaa wp 3d orion aircraft over an 8 h period on 8 december 1993 during the coastal observations and simulations with topography experiment reveal the mesoscale structure of an intense frontal system while it was well offshore and as it approached within 20 km of the oregon coastline during the offshore stage a portion of the narrow coldfrontal rainband was characterized by deep convective cores pseudo dual doppler analyses characterize the kinematic and precipitation structure of the deep convection pseudo dual doppler analyses describe the subsequent evolution of the narrow cold frontal rainband as it approached to within 20 km of the oregon coast deformation of the frontal zone appeared to cause the dissipation of one of three precipitation cores contained within the dual doppler area the precipitation cores and the strong convergence zone associated with the front conformed to some degree to the shape of the coastline near cape blanco oregon as the front neared the coast changes in the prefrontal flow that occurred as the front approached the coast were qualitatively consistent with theoretical and numerical studies of upstream orographic influence comparison of pseudo dual doppler derived velocity profiles with idealized numerical model calculations suggests that the nearshore evolution of the frontal rainband was significantly affected by an upstream influence of the coastal orography

finescale vertical structure and evolution of a preconvective dryline on 19 june 2002 abstract high resolution airborne cloud radar data and other international h2o project datasets are used to describe the vertical structure of an unusual prefrontal dryline this dryline observed in northwestern kansas on 19 june 2002 first progressed eastward and tilted toward the west and later became more stationary and reversed its tilt toward the moist side the convective boundary layer cbl depth difference also reversed only in the later phase did the dry side cbl become deeper than on the moist side echo and single dual doppler velocity data in a vertical transect across the dryline suggest a solenoidal circulation dynamically consistent with the observed horizontal buoyancy gradient both this gradient and the solenoidal circulation reversed in the later phase simultaneously confluence toward the dryline increased resulting in an increasing moisture gradient as well as a deepening cbl in the dryline convergence zone it is speculated that the baroclinically generated horizontal vortici

the effect of the continental boundary layer on the dynamics of fronts in a 2d model of baroclinic instability ii surface heating and cooling the effect of surface heating and cooling on the structure and evolution of the surface front that develops in the finite amplitude eady wave is investigated spatially uniform short wave radiative heating of the surface is weakly frontolytic although it appears this result may be sensitive to the way in which the boundary layer turbulence is parametrized the cross front potential temperature gradient is weakened principally by the differential heating produced by horizontal variations in the depth of the mixed layer and to a smaller extent by horizontal variations in the surface sensible heat flux the direct effect of enhanced daytime turbulent stresses is to retard the winds in the boundary layer reducing the convergence in the cross front plane while the indirect effect is to produce a strong cross isobaric flow towards the trough thereby increasing the convergence the net effect is to reduce the rate of frontogenesis near the surface front during the day the front is oriented vertically in the boundary layer because of the strong mixing nocturnal cooling through long wave radiative emission from the surface is strongly frontogenetic at night as the turbulence decays in the trough ahead of the surface front air parcels accelerate down the pressure gradient towards the centre of the trough producing low level cross front surges on both sides of the trough axis this enhanced nocturnal convergence rapidly strengthens the cross front potential temperature gradient and tilts the front westward with height such nocturnal frontogenesis is especially intense when it follows strong daytime turbulent mixing copyright 2005 royal meteorological society

non normal growth of kelvin helmholtz eddies in a sea breeze the generalized stability of a sea breeze front is analyzed using a two dimensional model the objective is to understand the mechanisms leading to the shedding of eddies behind the sea breeze front as seen in observations laboratory experiments and numerical models regions with ri 1 4 the energy source for optimal growth is the vertical shear of the mean horizontal wind the vertical shear in the upper part of the front and the horizontal shear in the lower part the growth begins with vertical advection by the perturbation velocity of the mean flow momentum located in the upper part of the front perturbations eventually propagate away from the localized shear area and a feedback mechanism is needed for this growth to be sustained this feedback occurs through temperature anomalies in the upper part of the front inducing pressure gradient anomalies in the lower part these gradients lead to a growing vertical wind component and this vertical wind component then enters the upper part of the front which reinforces the extraction of energy thereby closing the feedback loop and leading to both normal mode instability and in the stable regime large non normal growth we find that both the instability and the non normal growth are vulnerable to parameter changes that weaken this feedback loop

observation of a cold front with rope cloud abstract observations are presented of a cold front which passed over the florida peninsula and produced a rope cloud visible on satellite imagery the low level structure of the leading edge of the front is revealed with data from the 150 m tower at cape canaveral air force station it is shown that the leading edge of the front is very similar to thunderstorm gust fronts and the speed is predicted well by the gust front speed equation this indicates that the rope cloud is directly analogous to the arc cloud sometimes observed along thunderstorm gust fronts further the rope cloud appears to be a less intense manifestation of the forced convection that produces the narrow cold frontal rainbands observed in some other cold front studies

a numerical and analytical study of atmospheric undular bores summary in this paper the formation and structure of an atmospheric undular bore is examined with the aid of a numerical model and theoretical analysis the numerical model is first used to simulate a density current propagating into an unstratified environment a detailed comparison of the model results with existing theory shows that for a given pressure head the current can travel at a greater speed than that predicted by irrotational theory when the density current is allowed to propagate into a low level stable layer it is found that for certain parameter regimes an undular bore forms ahead of the current the results for two different types of stable layers are examined for a layer with a step profile of potential temperature comparison is made with both classical bore theory and the theory of density currents in two layer fluids and encouraging agreement is found when a linear profile is specified in the stable layer it is found that energy conserving solutions can be obtained the numerical fields are then compared with measurements of the morning glory an undular bore observed in north eastern australia and again agreement is good finally preliminary results of model simulations with stratification above the stable layer are discussed