the two dimensional dynamics of west african squall lines the two dimensional dynamics of multicell and unicell squall lines are examined using a numerical model both types feature a convective jump updraught with extensive deep rearward extending anvils the multicell case contains travelling convection cells and is unsteady but persistent whereas the unicell case is quasi steady in the multicell case an extensive density current is produced by the evaporation and drag of rain falling from a succession of convection cells which travel rearwards relative to the density current head the circulation within the density current is complex and consists of three mutually interactive branches a rotor a cold downdraught and a subsidence region in the unicell case a density current is not produced and the updraught is forced by a propagating wave of elevation generated as an interaction between the updraught water loading and evaporation in a very localized area the squall line structure is compared with other models and it is shown that certain propagating two dimensional tropical and mid latitude squall lines are dynamically similar their detailed structure being controlled by the inflow wind profile the multicell case is compared with a quasi two dimensional west african squall line which occurred on 22 june 1981 during the convection profonde tropicale copt 81 experiment performed in the northern part of the ivory coast good agreement is found between the model and the observations and hence the dynamics of the observed squall line is quantified the unicell case in which a density current and conventional downdraughts are absent has yet to be identified in atmospheric observations

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

a long lived mesoscale convective complex part i the mountain generated component abstract using data collected during colorado state university s south park area cumulus experiment in 1977 a sequence of multi scale convective events leading to the formation of a mesoscale convective complex is described in the first phase surface based cool advection in the elevated mountain basin delayed the full transition of the morning boundary layer into a deep mixed layer until well after convective instability was reached over the adjacent ridges the second phase was earmarked by the formation of convective precipitation echoes at hot spots over the high mountain terrain two groups of cells then propagated eastward across the mountain basin forming a line of discrete cells which moved across the foothills toward the high plains the cells further intensified at the foothills high plains interface and formed a still larger north south line of thunderstorms in the third phase this north south line of thunderstorms evolved into an expanding meso scale convective cluster as it contin

observations of dual slantwise circulations above a cool undercurrent in a mesoscale convective system a series of mesoscale convective systems mcss was observed in the uk during one of the intensive observation periods iop 3 in the convective storm initiation project csip a detailed case study of one of the mcss involving some novel interpretation of single doppler radar data reveals the evolution of stacked slantwise circulations associated with elevated upright convection a feature of this occasion was the existence near the surface of a strong undercurrent of cool air flowing against the direction of travel of the storm the air feeding both the upright and slantwise ascent originated not from near the surface but rather from layers between 1 and 3 km the upright convection was intense at first and was accompanied by a region of dry subsidence surrounding it together with the alternating layers of slantwise ascent and descent within the storm s precipitation area when the elevated upright convection weakened vigorous slantwise motions continued for some time but their slope decreased from the initial 1 in 4 to 1 in 9 the lowermost layer of slantwise descent corresponded to a moderately intense rear inflow jet the rear inflow jet did not penetrate the cool undercurrent and reach the surface instead beneath where it impacted the warm air capping the undercurrent the undercurrent took on the structure of a gravity wave without stagnation the raised head of the undercurrent lifted overlying air of high wet bulb potential temperature sufficiently to overcome the convective inhibition copyright c 2010 royal meteorological society

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

a model for rockfill compressibility the paper presents a macroscopic constitutive model for rockfill that includes the effect of water on compressibility and collapse phenomena breakage of rock particles and fracture propagation are basic underlying mechanisms controlled by the relative humidity of the air filling the rockfill voids a conceptual deformation model based on these mechanisms is first proposed and discussed the results of oedometer tests on a quartzitic slate rockfill in which the air relative humidity was controlled are then presented a significant finding is that bringing the relative humidity within the specimen to its maximum 100 rh leads to a collapse strain equal to that observed in flooded specimens an elastoplastic constitutive model consistent with the basic deformation framework is developed its parameters have a clear physical meaning guidelines for parameter determination are given model performance is finally compared with the results of the experimental programme

thunderstorms above frontal surfaces in environments without positive cape part i a climatology abstract the first of two papers describing thunderstorms that occur above frontal surfaces frequently in environments without positive convective available potential energy cape focuses on the climatology of such storms for the conterminous united states the dataset used consists of 1093 observations made over a 4 year period the events were selected using conventional network data and a set of criteria that eliminated thunderstorms rooted in the boundary layer a composite of the dataset shows that the typical elevated thunderstorm occurs northeast of an associated surface low pressure center and north of a surface warm front in a region with northeasterly surface winds the planetary boundary layer is generally very stable as determined by comparisons with both the 50 kpa and 85 kpa air the thunderstorms are usually found in the left exit region of a low level wind maximum an area of horizontal deformation the large scale environment is strongly baroclinic with large vertical wind shear an

a back analysis of beliche dam a back analysis of the beliche dam in southern portugal is described this dam was the subject of a class a prediction which grossly under predicted the actual settlement at the end of construction the analysis described here which incorporated an elasto plastic critical state model applied in terms of effective stress simulated the effect of collapse settlement and by so doing reproduced a maximum settlement which was close to that measured the earlier underprediction is attributed mainly to the fact that that analysis did not include the collapse settlement due to the partial impounding of the reservoir which occurred before construction was complete the back analysis described here covers the period from the start of construction in 1984 to approximately four years after the first complete impounding in january 1988 comparisons are made between the calculated and measured movements and stresses apart from the good agreement with the maximum recorded settlement the measured settlements general

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

mechanisms for quasi stationary behavior in simulated heavy rain producing convective systems abstract in this study idealized numerical simulations are used to identify the processes responsible for initiating organizing and maintaining quasi stationary convective systems that produce locally extreme rainfall amounts of particular interest are those convective systems that have been observed to occur near mesoscale convective vortices mcvs and other midlevel circulations to simulate the lifting associated with such circulations a low level momentum forcing is applied to an initial state that is representative of observed extreme rain events the initial vertical wind profile includes a sharp reversal of the vertical wind shear with height indicative of observed low level jets deep moist convection initiates within the region of mesoscale lifting and the resulting convective system replicates many of the features of observed systems the low level thermodynamic environment is nearly saturated which is not conducive to the production of a strong surface cold pool yet the convection qui

the response of simulated nocturnal convective systems to a developing low level jet some recent numerical experiments have examined the dynamics of initially surface based squall lines that encounter an increasingly stable boundary layer akin to what occurs with the onset of nocturnal cooling the present study builds on that work by investigating the added effect of a developing nocturnal low level jet llj on the convective scale dynamics of a simulated squall line the characteristics of the simulated llj atop a simulated stable boundary layer are based on past climatological studies of the llj in the central united states a variety of jet orientations are tested and sensitivities to jet height and the presence of lowlevel cooling are explored the primary impacts of adding the llj are that it alters the wind shear in the layers just above and below the jet and that it alters the magnitude of the storm relative inflow in the jet layer the changes to wind shear have an attendant impact on low level lifting in keeping with current theories for gust front lifting in squall lines the changes to the system relative inflow in turn impact total upward mass flux and precipitation output both are sensitive to the squall line relative orientation of the llj the variations in updraft intensity and system relative inflow are modulated by the progression of the lowlevel cooling which mimics the development of a nocturnal boundary layer while the system remains surfacebased the below jet shear has the largest impact on lifting whereas the above jet shear begins to play a larger role as the system becomes elevated similarly as the system becomes elevated larger changes to systemrelative inflow are observed because of the layer of potentially buoyant inflowing parcels becoming confined to the layer of the llj

numerical simulations of the postsunrise reorganization of a nocturnal mesoscale convective system during 13 june ihop_2002 abstracta convection permitting numerical model is used to simulate the postsunrise reorganization of a nocturnal mesoscale convective system mcs observed over western and central oklahoma on 13 june 2002 during the international h2o ihop 2002 field experiment the mcs reorganization consists of a transition from northwest southeast oriented convective rainbands near sunrise to a single northeast southwest ne sw oriented convective rainband with trailing stratiform precipitation later in the morning results indicate the importance of environmental preconditioning on mcs reorganization in particular the development of the ne sw rainband that redefines the mcs organization is facilitated by a similarly oriented zone of antecedent mesoscale upward motion which increases the depth of large water vapor mixing ratios this allows convective updrafts to be fed primarily by moist and conditionally unstable air from 1 to 2 5 km agl in the ne sw oriented rainband which lacks a surface cold pool during its

the evolution of an mcs over southern england part 1 observations observations are described of a mesoscale convective system mcs with a cold pool which propagated across southern england on 25 august 2005 the observations were made as part of the convective storm initiation project csip the observed mcs structure broadly followed well established theories including the presence of a weak rear inflow jet in detail however unsteady transitions occurred involving the formation of two distinct lines of showers ahead of the initial linear system in each case the cold pool merged with cold downdraughts from the new showers leading to a discontinuous propagation of the system one of these lines formed independently of the mcs very probably on a sea breeze convergence line the mechanism for formation of the other is unknown but it is possible that it was triggered by ascent associated with gravity waves generated by the mcs the merged cold pool was deeper and colder and propagated faster than the original system eventually forming a bow echo and arc cloud as it propagated across the english channel until completion of the merger the propagation velocity of the overall system had been controlled by a combination of the above mechanisms rather than simply by cold pool dynamics

observations of elevated convection initiation leading to a surface based squall line during 13 june ihop_2002 abstract the evolution of a mesoscale convective system mcs observed during the international h2o project that took place on the great plains of the united states is described the mcs formed at night in a frontal zone with four initiation episodes occurring between approximately 0000 and 0400 local time radar radiosonde and surface data together show that at least three of the initiation episodes were elevated occurring from moist conditionally unstable layers located above the boundary layer which had been stabilized by previous mcss initiation occurred in northwest southeast oriented lines where a southerly nocturnal low level jet terminated generating elevated convergence one initiation episode was observed using the s band dual polarization doppler radar s pol and occurred at the intersection of this convergence zone with a propagating wave calculations of the scorer parameter were consistent with wave trapping downdrafts from the developing convection generated both waves and bores w

the evolution of an mcs over southern england part 2 model simulations and sensitivity to microphysics simulations using the met office unified model at 1 km horizontal grid spacing of a mesoscale convective system mcs with a cold pool which propagated across southern england on 25 august 2005 are validated using detailed observations from the convective storm initiation project csip early organisation of the system is not especially well treated but the model goes on to form a system which developed qualitative and quantitative features remarkably similar to the observations a sensitivity study suggests that the initial linear system is driven by the position of a low level lid and upper level instability the linear organisation being promoted by a weak rear inflow jet forced by the upper level warm anomaly in the cloud anvil a weak cold pool develops in the absence of ice phase processes but this does not promote system propagation strengthening and descent of the rear inflow jet and acceleration of the system is promoted by the additional heating through glaciation and cooling through snow evaporation the surface cold pool and gust front are further strengthened by snow melting and rainfall evaporation with ice phase processes present the cold pool strengthens as a result of the system development and its strength is broadly correlated with system propagation speed during the middle phase of the system s lifetime propagation enables the convective band to sweep up any convective cells which trigger ahead of the system the differing scales of microphysical processes means that it is difficult to form a steady state system the observed transition phase corresponds to an increase in the slantwise nature of the flow in the storm and in the low level cooling by rainfall evaporation near the gust front combined with a change in ambient conditions advection over the sea which eventually enable the cold pool to propagate ahead of the system

numerical simulation of an intense squall line during 10 11 june 1985 pre storm part ii rear inflow surface pressure perturbations and stratiform precipitation abstract an intense rear inflow jet surface pressure perturbations and stratiform precipitation associated with a squall line during 10 11 june 1985 are examined using a three dimensional mesoscale nested grid model it is found that the large scale baroclinity provides favorable and deep rear to front flow within the upper half of the troposphere and the mesoscale response to convective forcing helps enhance the trailing extensive rear inflow however latent cooling and water loading are directly responsible for the generation of the descending portion of the rear inflow the role of the rear inflow is generally to produce convergence ahead and divergence behind the system and thus assist the rapid acceleration of the leading convection when the prestorm environment is convectively favorable and the rapid dissipation of the convection when encountering unfavorable conditions in this case study the rear inflow jet appears to have caused the splitting of the surface wake low as well as the organized

the evolution of the 10 11 june 1985 pre storm squall line initiation development of rear inflow and dissipation abstract mesoscale analysis of surface observations and mesoscale modeling results show that the 10 11 june squall line contrary to prior studies did not form entirely ahead of a cold front the primary environmental features leading to the initiation and organization of the squall line were a low level trough in the lee of the rocky mountains and a midlevel short wave trough three additional mechanisms were active a southeastward moving cold front formed the northern part of the line convection along the edge of cold air from prior convection over oklahoma and kansas formed the central part of the line and convection forced by convective outflow near the lee trough axis formed the southern portion of the line mesoscale model results show that the large scale environment significantly influenced the mesoscale circulations associated with the squall line the qualitative distribution of along line velocities within the squall line is attributed to the larger scale circulations associated with the le

mechanisms for the generation of mesoscale vorticity features in tropical cyclone rainbands a high resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands analysis of the vorticity interactions that occur within the simulated rainbands demonstrates that couplets of cyclonic anticyclonic mesovortices can be produced in outer bands the primary source of this vorticity is the upward tilting of system generated horizontal vorticity by diabatic heating gradients the vertical heating gradient in the stratiform cloud also creates a potential vorticity pv dipole that accelerates the tangential flow and develops a midlevel jet the strength of the jet is enhanced by the vortex pair that is oriented radially across the rainband the fourier decomposition of the absolute vorticity field shows two counterpropagating vortex rossby waves associated with the rainband the wave located on the inner side of the band transports energy toward the vortex center the outer wave is made up of high wavenumbers and uses the vorticity gradients generated by the rainband the results support the hypothesis that the heating profile in the stratiform regions of rainbands generates cyclonic pv across the freezing level which develops a midlevel jet this mechanism creates a vorticity gradient that enables the propagation of vortex rossby waves that could allow the rainbands to interact with the mean flow and potentially influence the evolution of the storm by contributing to the symmetric component of vorticity and the development of secondary eyewalls

a numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines abstract a set of 13 two dimensional numerical simulations based on the 22 and 23 june soundings from the convectionprofonde tropicale in 1981 copt81 experiment in west africa is used to study the organization and interactionof the convective and stratiform regions of squall line type convective systems the initial wind profiles arecharacterized by the african easterly jet aej and the tropical easterly jet tej located at about 3 5 km and14 km respectively the physical processes that generate and maintain the mesoscale inflow at the rear of squall line type mesoscaleconvective systems are thereby examined horizontal potential temperature gradients generated by a combinationof latent heat release in the convective region and unsaturated mesoscale descent both modulated by evaporation cause a horizontal pressure gradient and generate horizontal line parallel vorticity the rear inflow is a consequenceof these processes the convective activity induces a significant upscale influence ahead of the

deformation characteristics and control techniques at the shiziping earth core rockfill dam abstractthis paper presents a representative engineering case the shiziping dam to illustrate deformation controls of highrise earth core rockfill dams in seismic areas techniques for controlling dam deformation including structure design foundation treatment material properties and construction are discussed in detail field monitoring data and deformation characteristics were reviewed to verify control effectiveness the deformation during the construction and initial impoundment period accounted for 70 85 of the overall deformation the deformation was primarily influenced by weight wetting behavior and creep creep stabilized 7 or 8 months after the completion of construction the total deformation is caused by fill and foundation materials uneven deformation is mainly caused by foundation shape foundation compression symmetry and uniformity of fill materials uneven deformation may be improved by using reasonable dam zone and fill material designs and appropriately increasing core wall c

organizational modes of midlatitude mesoscale convective systems this paper discusses common modes of mesoscale convective organization using 2 km national composite reflectivity data the authors investigated linear mesoscale convective systems mcss that occurred in the central united states during may 1996 and may 1997 based upon the radar observed characteristics of 88 linear mcss the authors propose a new taxonomy comprising convective lines with trailing ts leading ls and parallel ps stratiform precipitation while the ts archetype was found to be the dominant mode of linear mcs organization the ls and ps archetypes composed nearly 40 of the studied population in this paper the authors document the characteristics of each linear mcs class and use operational surface and upper air data to describe their different environments in particular wind profiler data reveal that the stratiform precipitation arrangement associated with each class was roughly consistent with the advection of hydrometeors implied by the mean middle and upper tropospheric storm relative winds which were significantly different among the three mcs modes case study examples are presented for both the ls and ps classes which have received relatively little attention to this point as well the authors give a general overview of the synoptic scale meteorology accompanying linear mcss in this study which was generally similar to that observed by previous investigators

simulations of an observed elevated mesoscale convective system over southern england during csip iop 3 simulations of an elevated mesoscale convective system mcs observed over southern england during the convective storm initiation project csip provide the first detailed modelling study of a case of elevated convection occurring in the uk the study shows that many factors can influence the maintenance of elevated deep convection from large scale flow through to surface heating processes and diabatic cooling within the convective system it is also shown that interactions and feedback mechanisms between a stable layer and the storm can act to maintain deep convection the simulation successfully reproduced an elevated mcs above a low level stable undercurrent with a wave in the undercurrent linked to a rear inflow jet rij convection was fed from an elevated 840 hpa source layer with cape of about 350 j kg 1 the undercurrent in the simulation was approximately 1 km deep about half that observed unlike the observed mcs a transition from elevated to surface based convection occurred in the simulation due to the combined effects of a pre existing large scale e gradient advection and surface heating causing the system to encounter increasingly unstable low level air and a shallower stable layer that was more susceptible to penetration by downdraughts the transition to surface based convection was accompanied by the development of cold pool outflow and an increase in system velocity from about 6 to 10 m s 1 diabatic cooling from microphysical processes in the simulation enhanced the undercurrent and strengthened the rij this strengthened the wave in the undercurrent and led to more extensive convection the existence of a positive feedback process between the convection rij and stable layer is discussed uncertainty in the synoptic scale generating errors in the undercurrent is shown to be a major source of error for convective scale forecasts

discrete propagation in numerically simulated nocturnal squall lines abstract simulations of a typical midlatitude squall line were used to investigate a mechanism for discrete propagation defined as convective initiation ahead of an existing squall line leading to a faster propagation speed for the storm complex radar imagery often shows new cells appearing in advance of squall lines suggesting a causal relationship and prompting the search for an action at a distance mechanism to explain the phenomenon in the simulations presented the identified mechanism involves gravity waves of both low and high frequency generated in response to the latent heating which subsequently propagate out ahead of the storm the net result of the low frequency response combined with surface fluxes and radiative processes was a cooler and more moist lower troposphere establishing a shallow cloud deck extending ahead of the storm high frequency gravity waves excited in response to fluctuations in convective activity in the main storm were subsequently ducted by the storm s own upp

a long lived mesoscale convective complex part ii evolution and structure of the mature complex abstract an eight day episode in august 1977 is described wherein 14 mesoscale convective complexes mccs developed in the central united states including one to the immediate ice of the rocky mountains on each day of the episode in part i of this article the daytime genesis of one of these systems was traced from its pre convective roots in the mountains of central colorado to its incipient mcc stage on the plains of eastern colorado in this paper its continued nocturnal development into a large mcc over kansas is followed satellite imagery shows that this system remained coherent for at least three days as it passed off the east coast and across the western atlantic ocean analysis is focused on the mature stage of this and a second mcc in the episode in order to compare their major dynamic features to those of similar midlatitude systems reported in the literature and also to previous studies of tropical mesoscale convective systems many of the important characteristics of midlatitude mccs fo

summary of convective storm initiation and evolution during ihop observational and modeling perspective abstract the data rich international h2o project ihop 2002 experiment is used to study convective storm initiation and subsequent evolution for all days of the experiment initiation episodes were almost evenly divided between those triggered along surface based convergence lines and elevated initiation episodes that showed no associated surface convergence the elevated episodes occurred mostly at night and the surface based episodes occurred during the afternoon and evening surface based initiations were mostly associated with synoptic fronts and gust fronts and less so with drylines and bores elevated initiations were frequently associated with observable convergent or confluent features in the rapid update cycle ruc wind analysis fields between 900 and 600 hpa the ruc10 3 h forecast of the precipitation initiation episodes were correct 44 of the time allowing a tolerance of 250 km in space and for the forecast being early by one period however the accuracy was closely tied to the scale of

development and forcing of the rear inflow jet in a rapidly developing and decaying squall line during bamex abstract this study examines the development structure and forcing of the rear inflow jet rij through the life cycle of a small short lived squall line over north central kansas on 29 june 2003 the analyses were developed from airborne quad doppler tail radar data from the noaa and nrl p 3 aircraft obtained over a 2 h period encompassing the formation development and decay of the squall line during the bow echo and mesoscale convective vortex experiment bamex the strengthening of the system relative rear inflow to 17 m s 1 was concurrent with the formation of a bow echo an increased dynamic pressure gradient beneath the rearward tilted updraft and two counterrotating vortices at either end of the bow the later weakening of the rij to 8 m s 1 was concurrent with the weakening of the bow a decreased dynamic pressure gradient at midlevels behind the bow and the weakening and spreading of the vortices in a modeling study weisman quantified the forcing mechanisms responsible for the developm

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

the oklahoma kansas mesoscale convective system of 10 11 june 1985 precipitation structure and single doppler radar analysis abstract the 10 11 june mesoscale convective system observed in kansas during pre storm is studied using a variety of observations including conventional radar satellite and single doppler radar this storm at maturity consisted of a strong line of convection trailed by a broad region of stratiform rain the pre storm doppler radar observations show that the general airflow pattern is similar to that seen in previously analyzed cases however since the doppler observations were quite extensive in time and space they permit several details of the airflow to be revealed for the first time a rear inflow jet front to rear flow aloft and a mesoscale updraft and downdraft were all present the mesoscale downdraft commenced at the top of the slanted rear inflow jet sublimation and evaporation of hydrometeors in this flow apparently generated the necessary cooling to drive the mesoscale downdraft circulation the intensity and slope of the rear inflow jet varied with location in the storm which apparen

multiscale evolution of a derecho producing mesoscale convective system abstract in this paper the authors address one type of severe weather strong straight line winds the case of a mesoscale convective system that developed in eastern colorado on 12 13 may 1985 was studied the system formed in the afternoon was active until early morning and caused strong winds during the night a multiscale nonhydrostatic full physics simulation was performed to formulate a conceptual model of the main airflow branches of the system and to gain understanding of the physical processes involved in the strong wind generation in this storm four telescopically nested grids covering from the synoptic scale down to cloud scale circulations were used a lagrangian model was employed to follow trajectories of parcels that took part in the updraft and downdraft and balances of forces were computed along the trajectories the strong nocturnal winds were caused by downdrafts reaching the surface and by a dynamically forced horizontal pressure gradient force the most important branch of the do

initiation of convective storms at radar observed boundary layer convergence lines abstract the origin of 653 convective storms occurring over a 5000 km2 area immediately east of the colorado rocky mountains from 18 may to 15 august 1984 was examined seventy nine percent of the 418 storms that initiated within the study area occurred in close proximity to radar observed boundary layer convergence lines this percentage increased to 95 when only the more intense storms 60 dbze were considered colliding convergence lines initiated new storms or intensified existing storms in 71 of the cases a new storm took a median time of 24 min to grow to 30 dbz following line collision the convergence lines ranged in length between ten and several hundred kilometers both radar and mesonet stations indicated that the primary convergence was concentrated in a zone 0 5 to 5 km in width these lines were characterized on doppler radar as thin lines of enhanced reflectivity between 0 and 20 dbze and as a line of strong radial or azimuthal gradient in doppler velocity these lines were observed ev

the influence of convectively generated thermal forcing on the mesoscale circulation around squall lines abstract the dynamical processes that determine the kinematic and thermodynamic structure of the mesoscale region around 2d squall lines are examined using a series of numerical simulations the features that develop in a realistic reference simulation of a squall line with trailing stratiform precipitation are compared to the features generated by a steady thermal forcing in a dry simulation with no microphysical parameterization the thermal forcing in the dry simulation is a scaled and smoothed time average of the latent heat released and absorbed in and near the leading convective line in the reference simulation the mesoscale circulation in the dry simulation resembles the mesoscale circulation in the reference simulation and around real squall lines it includes an ascending front to rear flow a midlevel rear inflow a mesoscale up and downdraft an upper level rear to front flow ahead of the thermal forcing and an upper level cold anomaly to the rear of the thermal forcing it is also shown t

response of simulated squall lines to low level cooling abstract organized convection has long been recognized to have a nocturnal maximum over the central united states the present study uses idealized numerical simulations to investigate the mechanisms for the maintenance propagation and evolution of nocturnal like convective systems as a litmus test for the basic governing dynamics the experiments use horizontally homogeneous initial conditions i e they include neither fronts nor low level jet streams the simulated storms are allowed to mature as surface based convective systems before the boundary layer is cooled in this case it is then surprisingly difficult to cut the mature convective systems off from their source of near surface inflow parcels even when 10 k of the low level cooling has been applied the preexisting system cold pool is sufficient to lift boundary layer parcels to their levels of free convection the present results suggest that many of the nocturnal convective systems that were previously thought to be elevated may actually

a high plains squall line associated with severe surface winds abstract the characteristics of a severe squall line are examined using data acquired from the 1981 cooperative convective precipitation experiment ccope the case is unusual in that the squall line was decoupled from an immediate surface based inflow source due to a meso scale 20 200 km outflow pool produced by a separate mesoscale convective system both systems participated in the development of a mesoscale convective complex which subsequently produced sustained severe surface winds throughout its entire life cycle despite the absolutely stable presquall atmospheric boundary layer the squall line produced radar reflectivity values of 70 dbz and storm induced outflow of 30 m s 1 aircraft soundings in the presquall environment suggest the storm was sustained by an elevated layer of high valued c air overriding the cold dome produced by the developing mcc the strongest surface winds were located upshear from the convective line and consisted of a northerly alongline component because the

the global population of mesoscale convective complexes a global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective the data set includes date of occurrence time of first storms initiation maximum extent termination duration cold cloud shield areas and tracks from initiation to termination it is found that the typical convective complex is nocturnal generates a cold cloud shield area of approximately 350 000 km2 and persists for about 10 h the largest systems and most persistent systems tend to occur near the summer solstices for the globe about 400 systems occur each year primarily over land areas most systems develop in favoured zones although some activity occurs over every continent except antarctica and all major oceans the concentration of activity into favoured zones indicates that there must be special dynamic and or thermodynamic conditions necessary for convection to organize into convective complexes activity is strongly tied to the solar day and shifts from 35 s in early january to about 50 n during the boreal summer and back to 35 s by december within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward relatively little migration occurs in the ocean dominated southern hemisphere where the subtropical jet remains quasi stationary over the convective complex regions the nocturnal life cycles copious rainfall large cloud shields and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth system energy budget

large scale meteorological conditions associated with midlatitude mesoscale convective complexes abstract objective analyses of composited meteorological conditions attending ten mesoscale couvective complexes mcc reveal a number of distinctive characteristics and important interactions with the large scale environment the systems appear to be linked to eastward progression of a weak middle tropospheric short wave trough initial thunderstorms develop within a region of mesoscale convergence and lifting that is apparently forced primarily by low level warm advection the mcc system acquires mesoscale organization while it moves eastward ahead of the short wave trough diabatic heating eventually produces a system that is warm core in the middle troposphere and cold core in upper levels the mature mcc exhibits many similarities to tropical convective systems although it occurs within a considerably different large scale setting inflow within the lower half of the troposphere feeds convection within a region characterized by significant net upward mass flux and widespread precipitation thickne

multi sensor observations of a wave beneath an impacting rear inflow jet in an elevated mesoscale convective system observations of a wave generated below and propagating with an elevated mesoscale convective system mcs are presented the mcs and the wave tracked directly across the 90 km extent of the observational network deployed for the convective storm initiation project csip and maintained a coherent structure and velocity during this 1 5 hour period the rear inflow jet of the mcs was observed to impact upon but not penetrate the stable undercurrent that was located beneath the mcs apparently constricting the depth of the undercurrent which did not participate in the main convective updraught the wave crest was located approximately 15 km ahead of this constriction the overall wave led to between 200 and 1000 m of ascent and approximately 1500 m of descent however undulations were superimposed on the overall wave with a wavelength of approximately 7 km and a crest totrough amplitude of approximately 1000 m these undulations were associated with double maxima in surface pressure and the elevation of the observed low level wind anomaly where low level winds were backed by more than 90 t hese winds were never at rest relative to the system and the wave was therefore a gravity wave without stagnation the hydrostatic effect expected from the observed lifting in the wave was broadly consistent with the observed surface pressure perturbation of 2 hpa no cold pool outflow was ever observed instead the observed surface cooling of approximately 4 c was shown to be consistent with reduced solar insolation the generation of the observed wave structure and its implications for the maintenance of the mcs and numerical modelling of the system are discussed copyright c

a midlatitude squall line with a trailing region of stratiform rain radar and satellite observations abstract a squall line exhibiting an extensive trailing region of stratiform precipitation passed over the observational network of the national severe storms laboratory on 22 may 1976 satellite imagery and conventional radar observations document its evolution from a broken line of thunderstorms to a system of mesoscale proportions and single doppler radar observations describe aspects of its mature structure satellite measurements of cloud top temperature showed the system to be a mesoscale convective complex mcc the life cycle of the system exhibited the stages of development seen in tropical cloud clusters at maturity two prominent mesoscale flow regimes were identified at midlevels one marked by inflow into the system s front and continuing toward its rear and another associated with inflow entering the extreme rear of the system the rear inflow was associated with a cyclonic midlevel vortex in the stratiform precipitation region it produced a concavity or notch in the back edge of th

multiscale structure and evolution of an oklahoma winter precipitation event abstract a significant winter precipitation event occurred on 8 9 march 1994 in oklahoma snow accumulations greater than 30 cm 12 in were measured within a narrow corridor in northern oklahoma on the synoptic scale and mesoscale a correspondence between large snow accumulations and 600 hpa frontogenesis was revealed the precipitation was formed above the cold frontal surface owing to midtropospheric ascent associated with the cross frontal circulation in a region of elevated conditional instability the location of such a narrow corridor of large accumulations was not however disclosed by any patterns in the radar reflectivity data indeed during this event an elongated maximum of snow accumulation was not associated with a persistent band of enhanced reflectivity and vice versa dual polarization and dual doppler radar data allowed for a novel analysis of winter precipitation processes and structures within the context of the larger scale diagnosis it was possible to identify in order of

a composite model of mesoscale convective complexes abstract a composite analysis technique is used to investigate the evolution of mesoscale features of mesoscale convective complexes mccs the early stage of the mcc lifecycle is characterized by convergence vertical motion and heating being centered in the lower troposphere as the mcc matures the level of peak upward motion and heating shifts to the upper troposphere the system achieves and maintains its maximum divergence upward motion and anticyclonic vorticity in the upper troposphere during the latter half of the life cycle this is in contrast to gate tropical clusters where the maximum divergence upward motion and anticyclonic vorticity occurred at the mature stage of the cluster and then weakened this difference might be explained by an mcc being an inertially stable form of mesoscale convective system whose radius exceeds the rossby radius of deformation the mcc is shown to be an efficient rain producer exhibiting a precipitation efficiency exceeding 100 at the mature stage due to th

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

structure and evolution of a mesoscale convective system near the british isles frequent digital cloud imagery from meteosat and precipitation patterns from radar have been analysed together with surface observations and conventional and satellite soundings to develop a conceptual model of a small mesoscale convective system the system observed to the south west of england had many of the features previously found in the united states where the term mesoscale convective system or complex is applied to a cluster of thunderstorms that generates a common cirrus shield of mesoscale dimensions a mesoscale region of fairly uniform essentially stratiform rain developed beneath the cirrus shield downwind of the active convection thunder was widespread throughout the precipitation area which was also characterized by a surface mesohigh the mesoscale convective system and another similar one before it lasted for about half a day each during which time they produced large falls of rain over an extensive area the mesoscale convective systems developed in a region of locally strong baroclinicity on the eastern flank of a cut off low they happened to form over the sea and were decoupled from the surface by a cool easterly boundary layer flow their development could be traced to the onset of advection from france of warm moist air with a wet bulb potential temperature in excess of 18 c at 850 mb the arrival of this air coincided with the intensification of the se ly flow around the approaching cut off low

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

rear inflow in squall lines with trailing stratiform precipitation abstract the relative airflow and accompanying precipitation structure of squall lines trailed by mesoscale regions of stratiform rain are examined with emphasis on the occurrence of rear inflow i e the intrusion of environmental air into these storms across the trailing precipitation boundary three cases from kansas and oklahoma provide examples of strong rear inflow which crosses the back edge of the stratiform precipitation area at relative speeds exceeding 10 m s 1 the vertical profiles of relative flow at the trailing precipitation boundary of these three systems were remarkably similar with the rear inflow confined to a jet like layer centered at about 550 mb doppler radar data for two of these cases showed that the rear inflow jet occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels of the leading convective region where it merged with outflow from convective downdrafts to bolster the leading gust front strong front to rea

dendroindications of recent volcanic eruptions in kamchatka russia abstract precisely dated tree ring sequences of larch larix cajanderi mayr extending from 1801 1980 ad from kamchatka lying in the far east of asian continent were studied to see the influence of volcanic eruptions on tree growth and to assess the potential of tree rings for dating ancient volcanic events nearby volcanic events are clearly reflected in ring width patterns distinctly narrow rings are formed in the year following the year of eruption weaker eruptions however are marked by narrow rings in the year of eruption only after strong eruptions trees usually take 2 to 3 yr to return to normal growth the results indicate that tree rings are useful for dating ancient volcanic events

a review of beliche dam beliche dam a zoned earthdam with rock ll shoulders and a central clay core experienced large collapse settlements due to reservoir impounding and direct action of rainfall long term field records of vertical and horizontal displacements are available as well as a set of large scale laboratory tests on rockfill specimens it has been the subject of several numerical analyses that failed to capture the relevant effect of weather conditions on the behaviour of the dam recent developments in the constitutive modelling of rockfill allow a substantial improvement of modelling capabilities and this is illustrated in the paper laboratory test results under dry and flooded conditions were interpreted and material parameters were identified the complete history of dam construction impoundment and rainfall was then simulated by means of a coupled flow deformation model deformations during construction and impoundment have been reproduced long term deformations have consistently been related to rainfall r

interactions between upper and lower tropospheric gravity waves on squall line structure and maintenance abstract using a simplified thermodynamic sounding and variable vertical wind shear we investigate the role of gravity waves on the structure and propagation of a simulated two dimensional squall line based on an observed squall line environment the modeled troposphere has been divided into three distinct thermodynamic layers these consist of an absolutely stable atmospheric boundary layer abl an elevated well mixed layer and an upper tropospheric layer of intermediate stability we find the mixed layer to have a dual role it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts and it provides an ideal layer to trap meso scale 20 200 km wave energy generated in the stable layers the generated waves thus have a significant and lasting impact on the simulation we also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long lived squall lines experiments that vary the vertical wind shear profile demonstrate

a case study of a large patch of billows surmounted by elevated convection a region of large amplitude kelvin helmholtz billows of area 2000 km2 was observed in a region of moderate rain at the rear of a mesoscale convective system on a day during the convective storm initiation project in southern england the mesoscale convective system mcs was characterized by elevated convection above a layer of cool air separated by a sloping statically stable layer characterized by strong wind shear doppler radar observations showed that the large patch of billows was situated within this shear layer and that it persisted over the 2 h period of observation maintaining its position with respect to the mcs and producing surface pressure perturbations of 0 3 hpa potentially unstable air above the sloping shear layer reached its level of free convection while ascending above this layer the resulting elevated convection was in the form of cells whose spacing appeared to be influenced by the underlying billows results from large eddy model simulations are shown to support the hypothesis that although the large scale ascent alone would have triggered the elevated convection the billows exerted a secondary forcing effect on the convection copyright 2012 royal meteorological society

secondary initiation of multiple bands of cumulonimbus over southern britain i an observational case study summary special observing facilities have been assembled in southern england as part of the convective storm initiation project csip to study the mesoscale and convective scale processes that determine precisely where warm season convective showers will break out this paper reports the results of a case study during the pilot field campaign of csip in july 2004 one purpose of the pilot project was to demonstrate the value of various observational facilities and to evaluate the usefulness of a variety of analysis and synthesis techniques amongst other things the case study demonstrates the utility of high resolution imagery from the meteosat second generation satellite for tracking the early stages of the convective clouds and of a new clear air scanning radar at chilbolton for mapping both the top of the boundary layer and the initialgrowth of the convective cells that penetrate it the particular event studied involved the triggering of convection that developed into three parallel arcs of showers and thunderstorms the first arc was triggered along the leading edge of the outflow density current from an earlier cluster of showers but the convection in the second and third arcs was triggered by a different mechanism the paper describes in detail the way in which this convection broke through the stable layer or lid at the top of a boundary layer of variable depth the strength of the lid decreased and the depth of the boundary layer increased with timeas a result of diurnal heating but the precise locations where convection finally broke through were determined by the spatial variability in boundary layer depth the analysis suggests that a wave like modulation of the boundary layer depth of amplitude 150 m perhaps due to a gravity wave disturbance from the earlier cluster of showers had a greater influence on where the convection was triggered than the modest hills typically 200 m high in southern england

a case study of an unusually intense atmospheric gravity wave abstract a remarkable long lived large amplitude gravity wave in the carolinas and virginia on 27 february 1984 is investigated by means of a subsynoptic scale case study the wave was characterized by a minor wave of elevation followed by a sharp wave of depression with a period of 2 3 h a horizontal wavelength of 100 150 km and surface pressure perturbation amplitudes of 3 14 mb the wave propagated toward the east southeast at 1 5 m s 1 accelerating to more than 20 m s 1 after it crossed the atlantic coast wave passage was accompanied by gusty surface easterly winds reaching 30 m s 1 and an abrupt termination of precipitation with the rapid surface pressure fall the synoptic criteria identified by uccellini and koch as common to many cases of large amplitude gravity waves were present in this case gravity waves were first detected across western tennessee and northern mississippi in a wake depression region to the rear of an advancing squall line an amplifying wave emerged out of the wave packet

mechanisms supporting long lived episodes of propagating nocturnal convection within a 7 day wrf model simulation abstract a large domain explicit convection simulation is used to investigate the life cycle of nocturnal convection for a one week period of successive zonally propagating heavy precipitation episodes occurring over the central united states similar to climatological studies of phase coherent warm season convection the longest lived precipitation episodes initiate during the late afternoon over the western great plains 105 100 w reach their greatest intensity at night over the central great plains 100 95 w and typically weaken around or slightly after sunrise over the midwest 95 85 w the longest lived episodes exhibit average zonal phase speeds of 20 m s 1 consistent with radar observations during the period composite analysis of the life cycle of five long lived nocturnal precipitation episodes indicates that convection both develops and then propagates eastward along an east west oriented lower tropospheric frontal zone an elevated 2 km deep layer of high e air helps sustain convect

numerical study of convection observed during the winter monsoon experiment using a mesoscale two dimensional model abstract a two dimensional version of the pennsylvania state university mesoscale model has been applied to winter monsoon experiment data in order to simulate the diurnally occurring convection observed over the south china sea the domain includes a representation of part of borneo as well as the sea so that the model can simulate the initiation of convection also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle this allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck which typically occupies several hundred kilometers of the domain frank and cohen s cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area the major conclusions are ice phase processes are important in determining the level of maximum large scale heating and vertical motion because there is a strong anvil componen