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

warm core vortex amplification over land abstract a convectively generated mesoscale vortex that was instrumental in initiating and organizing five successive mesoscale convective systems over a period of three days is documented two of these convective systems were especially intense and resulted in widespread heavy rain with localized flooding based upon radar and satellite data the detectable size of the vortex became much larger following the strong convective developments nearly tripling its initial diameter over its three day life cycle during nighttime when convection typically intensified within the vortex movement of the system tended to slow following dissipation of the convection in the morning the daytime movement accelerated cross sections of potential vorticity taken through the vortex center clearly show a maximum at midlevels and a well defined minimum directly above the vortex and the potential vorticity maximum were essentially colocated and the system was nearly axisymmetric in the vertical over the three day life

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

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

mesoscale convective complexes and persistent elongated convective systems over the united states during 1992 and 1993 abstract large long lived convective systems over the united states in 1992 and 1993 have been classified according to physical characteristics observed in satellite imagery as quasi circular mesoscale convective complex mcc or elongated persistent elongated convective system pecs and cataloged the catalog includes the time of initiation maximum extent termination duration area of the 52 c cloud shield at the time of maximum extent significant weather associated with each occurrence and tracks of the 52 c cloud shield centroid both mcc and pecs favored nocturnal development and on average lasted about 12 h in both 1992 and 1993 pecs produced 52 c cloud shield areas of greater extent and occurred more frequently compared with mccs the mean position of initiation for pecs in 1992 and 1993 followed a seasonal shift similar to the climatological seasonal shift for mcc occurrences but was displaced eastward of the mean position of mcc initiation in 1992 and 1993 the spatial distribution

the role of vertical wind shear in modulating maximum supercell updraft velocities abstractobserved supercell updrafts consistently produce the fastest mid to upper tropospheric vertical velocities among all modes of convection two hypotheses for this feature are investigated

processes controlling the structure and longevity of two quasi linear convective systems crossing the southern new england coast abstractthis paper explores the structural evolution and physical processes that explain the modification of two quasi linear convective systems qlcss that encountered the densely populated new york city atlantic coastal region one qlcs on 31 may 2002 traversed the atlantic coastal boundary with little change in its intensity producing widespread severe wind damage across new york city and long island during this event warm air advection at 925 hpa helped destabilize the layer above this level over the coastal zone while the marine boundary layer deepened below this level the 0 3 km line perpendicular vertical wind shear was relatively strong which supported ascent along the leading edge of the diabatically generated cold pool the surface based convective system became slightly elevated as it moved over the marine waters in contrast the 23 july 2002 qlcs decayed upon encountering the atlantic coastline despite its coincidence with a surface cold front the most unstable cape values during thi

the generation and propagation of a nocturnal squall line part i observations and implications for mesoscale predictability abstract the initiation and forcing of a squall line on 26 27 may 1985 near wichita kansas is examined the squall line is secondary convection in the sense that it is preceded in the diurnal cycle by other mesoscale convective systems the squall fine develops in synoptic conditions frequently associated with convective weather in the central united states this includes moderate shear moderate to high convective available potential energy a quasi stationary dryline a low level southerly jet and an 85 kpa moist tongue the initiation of free convection is specifically attributed to a gust front that propagates into a low level jet with attendant horizontal vorticity convergence and moisture the gust front initially propagates as a gravity current and subsequently as an internal undular bore observations of gravity currents gravity waves and the collision of radar echo boundaries provide valuable clues for short period forecasting they establish the presence of potential triggering mechanism

organization and environmental properties of extreme rain producing mesoscale convective systems this study examines the radar indicated structures and other features of extreme rain events in the united states over a 3 yr period a rainfall event is defined as extreme when the 24 h precipitation total at one or more stations surpasses the 50 yr recurrence interval amount for that location this definition yields 116 such cases from 1999 to 2001 in the area east of the rocky mountains excluding florida two kilometer national composite radar reflectivity data are then used to examine the structure and evolution of each extreme rain event sixty five percent of the total number of events are associated with mesoscale convective systems mcss while a wide variety of organizational structures as indicated by radar reflectivity data are seen among the mcs cases two patterns of organization are observed most frequently the first type has a line often oriented east west with training convective elements it also has a region of adjoining stratiform rain that is displaced to the north of the line the second type has a back building or quasi stationary area of convection that produces a region of stratiform rain downstream surface observations and composite analysis of rapid update cycle version 2 ruc 2 model data reveal that training line adjoining stratiform tl as systems typically form in a very moist unstable environment on the cool side of a preexisting slow moving surface boundary on the other hand back building quasistationary bb mcss are more dependent on mesoscale and storm scale processes particularly lifting provided by storm generated cold pools than on preexisting synoptic boundaries

climatology of the simulated great plains low level jet and its contribution to the continental moisture budget of the united states abstract the great plains region of the united states is characterized by some of the most frequent and regular occurrences of a nocturnal low level jet llj while the llj is generally confined to the lowest kilometer of the atmosphere it may cover a substantial region of the great plains and typically reaches maximum amplitudes of more than 20 m s 1 a two month springtime simulation with the goddard earth observing system geos 1 atmospheric general circulation model agcm has produced a great plains llj with a vertical and temporal structure directionality and climatological distribution that compare favorably with observations the diurnal cycle of the low level flow is dramatic and coherent over a subcontinental area that includes much of the western united states and northern mexico this cycle can be interpreted as the nightly intrusion of the anticyclonic subtropical gyre associated with the bermuda high into the north american continent as surface friction decreases the agcm also sim

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

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

views on applying rkw theory an illustration using the 8 may 2009 derecho producing convective system abstractthis work presents an analysis of the vertical wind shear during the early stages of the remarkable 8 may 2009 central u s derecho producing convective system comments on applying rotunno klemp weisman rkw theory to mesoscale convective systems mcss of this type also are provided during the formative stages of the mcs the near surface based shear vectors ahead of the leading convective line varied with time location and depth but the line normal component of the shear in any layer below 3 km ahead of where the strong bow echo developed was relatively small 6 9 m s 1 concurrently the midlevel 3 6 km line normal shear component had magnitudes mostly 10 m s 1 throughout in a previous companion paper it was hypothesized that an unusually strong and expansive low level jet led to dramatic changes in instability shear and forced ascent over mesoscale areas these mesoscale effects may have overwhelmed the interactions between the cold pool and low level shear that modulate system st

environmental distinctions between cellular and slabular convective lines abstract the organizational mode of quasi linear convective systems often falls within a spectrum of modes described by a line of discrete cells on one end cellular and an unbroken two dimensional swath of ascent on the other slabular convective events exhibiting distinctly cellular or slabular characteristics over the continental united states were compiled and composite soundings of the respective inflow environments were constructed the most notable difference between the environments of slabs and cells occurred in the wind profiles lines organized as slabs existed in much stronger low level line relative inflow and stronger low level shear a compressible model with high resolution x 500 m was used to investigate the effects of varying environmental conditions on the nature of the convective overturning the numerical results show that highly cellular convective lines are favored when the environmental conditions and initiation procedure allow the convectively generated cold pools to

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

high resolution satellite and airborne thermal infrared imaging of precursory unrest and 2009 eruption at redoubt volcano alaska abstract a combination of satellite and airborne high resolution visible and thermal infrared tir image data detected and measured changes at redoubt volcano during the 2008 2009 unrest and eruption the tir sensors detected persistent elevated temperatures at summit ice melt holes as seismicity and gas emissions increased in late 2008 to march 2009 a phreatic explosion on 15 march was followed by more than 19 magmatic explosive events from 23 march to 4 april that produced high altitude ash clouds and large lahars two or three lava domes extruded and were destroyed between 23 march and 4 april after 4 april the eruption extruded a large lava dome that continued to grow until at least early july 2009

the simulated structure and evolution of a quasi idealized warm season convective system with a training convective line abstractthis study details the development and use of an idealized modeling framework to simulate a quasi stationary heavy rain producing mesoscale convective system mcs a 36 h composite progression of atmospheric fields computed from 26 observed warm season heavy rain producing training line adjoining stratiform tl as mcss was used as initial and lateral boundary conditions for a numerical simulation of this mcs archetype a realistic tl as mcs initiated and evolved within a simulated mesoscale environment that featured a low level jet terminus maximized low level warm air advection and an elevated maximum in convective available potential energy the first stage of mcs evolution featured an eastward moving trailing stratiform type mcs that generated a surface cold pool the initial system was followed by rearward off boundary development where a new line of convective cells simultaneously redeveloped north of the surface cold pool boundary backbuilding persisted on the western end of the new lin

a composite perspective on bore passages during the pecan campaign abstractatmospheric bores have been shown to have a role in the initiation and maintenance of elevated convection previous observational studies of bores have been case studies of more notable eve

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

environment and early evolution of the 8 may 2009 derecho producing convective system abstractthis study documents the complex environment and early evolution of the remarkable derecho that traversed portions of the central united states on 8 may 2009 central to this study is the comparison of the 8 may 2009 derecho environment to that of other mesoscale convective systems mcss that occurred in the central united states during a similar time of year synoptic scale forcing was weak and thermodynamic instability was limited during the development of the initial convection but several mesoscale features of the environment appeared to contribute to initiation and upscale growth including a mountain wave a midlevel jet streak a weak midlevel vorticity maximum a denver cyclone and a region of upper tropospheric inertial instability the subsequent mcs developed in an environment with an unusually strong and deep low level jet llj which transported exceptionally high amounts of low level moisture northward very rapidly destabilized the lower troposphere and enhanced frontogenetica

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

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

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

structures and dynamics of quasi 2d mesoscale convective systems abstract recently three distinct archetypes for midlatitude linear mesoscale convective systems mcss have been identified this article focuses on the fundamentals of two of these archetypes convective lines with trailing stratiform ts precipitation and convective lines with leading stratiform ls precipitation both the ts and ls modes typically exhibit quasi 2d reflectivity patterns and quasi 2d environmental storm relative wind fields ongoing work has revealed that there are three common flow structures for these quasi 2d mcss front fed ts systems which are sustained by front to rear storm relative inflow as well as front fed ls and rear fed ls systems which are sustained by rear to front storm relative inflow this paper summarizes the observed structures of the front fed ts front fed ls and rear fed ls modes and then outlines an idealized numerical experiment in which these modes were simulated the authors analyze the basic simulated kinematic and microphysical structures and provid

on the development of strong ridge episodes over the eastern north atlantic 1 the occurrence of strong and persistent mid latitude anticyclonic ridges over the eastern north atlantic is a major contributor to the occurrence of severe winter droughts over western iberia we analyze the development of strong and persistent ridge episodes within 40 50 n 40 w 5 e which are defined as 300 hpa geopotential height anomalies above 50 gpm that persist for at least 10 consecutive days results suggest that the generation and maintenance of these episodes with positive stratospheric geopotential anomalies over the north american continent and the adjacent north pacific are associated with an intensified polar jet such positive anomalies tend to detach from the main stratospheric anomaly and propagate eastwards and downwards as rossby tropospheric waves furthermore the eastern north atlantic ridge is generated and repeatedly reinforced until the stratospheric anomaly dissipates results also show evidence for waves breaking anticyclonically during the episodes which is dynamically coherent with their persistency and quasi stationarity

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

mesoscale ascent in nocturnal low level jets abstracta theory for gentle but persistent mesoscale ascent in the lower troposphere is developed in which the vertical motion arises as an inertia gravity wave response to the sudden decrease of t