pressure perturbations and upslope flow over a heated isolated mountain abstract surface and upper air data collected as part of the cumulus photogrammetric in situ and doppler observations cupido experiment during the 2006 monsoon season around the santa catalina mountains in southeast arizona are used to study the diurnal variation of the mountain scale surface convergence and its thermal forcing the thermal forcing is examined in terms of a horizontal pressure gradient force which is derived assuming hydrostatic balance the mountain is 30 km in diameter 2 km high and relatively isolated the environment is characterized by weak winds a deep convective boundary layer in the afternoon and sufficient low level moisture for orographic cumulus convection on most days the katabatic divergent surface flow at night and anabatic convergent flow during the day are in phase with the diurnal variation of the horizontal pressure gradient force which points toward the mountain during the day and away from the mountain at night the daytime pressure deficit over the mo

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

numerical study of an observed orogenic mesoscale convective system part 2 analysis of governing dynamics abstract a detailed analysis of the dynamics and thermodynamics responsible for the structure growth and propagation of an orogenic mesoscale convective system simulated in two dimensions is made the process of scale interaction is addressed through fourier analysis and reynolds averaging analysis of representative predicted variables diabatic forcing and momentum acceleration terms additional dynamical analysis is accomplished through sensitivity experiments in which coriolis diabatic heating and ambient airflow are varied the general conclusion is that the simulated orogenic development is a geostrophic adjustment process to convective heating which is itself modulated and maintained by orographically induced flow systems the heating scales range over a nearly continuous spectrum ranging from 10 250 km the heating occurs in response to primary advective gravity modes the larger scale gravity wave disturbances modulate the smaller scales by organizing mean upward vertical motion patterns the la

upscale evolution of mcss doppler radar analysis and analytical investigation abstract the development of two small mesoscale convective systems mcss in northeastern colorado is investigated via dual doppler radar analysis the first system developed from several initially isolated cumulonimbi which gradually coalesced into a minimal mcs with relatively little stratiform precipitation the second system developed more rapidly along an axis of convection and generated a more extensive and persistent stratiform echo and mcs cloud shield in both systems the volumetric precipitation rate exhibited an early meso scale convective cycle a maximum and subsequent minimum followed by reintensification into a modest mature stage this sequence is similar to that noted previously in the developing stage of larger mcss by mcanelly and cotton they speculated that the early meso convective cycle is a characteristic feature of development in many mcss that is dynamically linked to a rather abrupt transition toward mature stage structure this study presents kinematic evidence in suppo

severe convection and lightning in subtropical south america satellite radar and radiometer data show that subtropical south america has the world s deepest convective storms robust mesoscale convective systems and very frequent large hail we determine severe weather characteristics for the most intense precipitation features seen by satellite in this region in summer hail and lightning concentrate over the foothills of western argentina lightning has a nocturnal maximum associated with storms having deep and mesoscale convective echoes in spring lightning is maximum to the east in association with storms having mesoscale structure a tornado alley is over the pampas in central argentina distant from the maximum hail occurrence in association with extreme storms in summer flash floods occur over the andes foothills associated with storms having deep convective cores in spring slow rise floods occur over the plains with storms of mesoscale dimension this characterization of high impact weather in south america provides crucial information for socioeconomic implications and public safety

a forced gravity wave model of serf organizing convection abstract a two dimensional hydrostatic nonrotating numerical model with a cumulus paramelerizafion is developed to study the early stages of mesozcale convective systems amplifying forced gravity waves occur when peneirative downdrafts are present updraft heating by itself is unable to cause convective sysiems to intensify propagation speeds are in rough agreement with those observed in midlatitude mesoscale convective systems the conditionalityof the convection and the horizontal advection of precipitation by the relative wind produce las between lifting and convection that are not found in conventional wave cisk models these lags slow the growth and reduce the propasation speeds of forced gravity waves

the evolution of convective storms initiated by an isolated mountain ridge abstractthe evolution of convective storms over the black hills an isolated mountain ridge in south dakota and wyoming and a regional convection hotspot is investigated using a 10 yr observational climatology and quasi idealized numerical simulations radar observed diurnally forced mountain convection events are classified according to their maximum cell track length and duration which are quantified using an automated cell tracking algorithm environmental conditions during these events are obtained from operational radiosonde and model analysis data these data suggest that mountain forced convective cells generally struggle to survive in the convectively inhibited flow downwind of the black hills those cells that do survive downwind prefer environments with strong bulk vertical shear over the 0 6 km layer which favors organized multicellular or supercellular convection under slightly weaker shear the cells tend to dissipate rapidly as they propagate downwind relatively weak winds aloft when c

geomorphological significance of the palaeodrainage network on a karst plateau the una korana plateau dinaric karst croatia abstract karst plateaus often have a complex geological and geomorphological history it is widely accepted that their development requires a long period of karst denudation this study reconstructs the palaeodrainage network of a karst plateau analyses its properties and establishes its geomorphological significance the main purpose of this research was to deepen our understanding of a key stage in the evolution of karst plateaus the transition from a fluvial land surface to one dominated by karst surface processes the study was conducted on a large part of the una korana plateau the largest plateau in the dinaric karst the majority of the plateau is made of carbonate rocks of triassic jurassic and cretaceous age that set the conditions for the development of the karst we have reconstructed the palaeodrainage network based on 1 25 000 topographic maps with 10 m contours the transition of the surface drainage network to the underground karst drainage network is still in progress so depending on the degree of karstification the drainage network was divided into three categories active dry and relict it was found that 90 5 of the pre existing drainage network has undergone some degree of karstification the active surface drainage network gradually shifted to a dry network then to a relict network the surface drainage network is gradually replaced by a dense network of dolines today the flat and karstified inter fluvial area is drained underground towards the main watercourses and these drain the entire region over the surface towards the pannonian basin this is the largest known karst palaeodrainage network in the dinaric karst that has been reconstructed in this way

numerical simulation of tornadogenesis in a high precipitation supercell part i storm evolution and transition into a bow echo abstract a nested grid primitive equation model rams version 3b was used to simulate a high precipitation hp supercell which produced two weak tornadoes six telescoping nested grids allowed atmospheric flows ranging from the synoptic scale down to the tornadic scale to be represented in the simulation all convection in the simulation was initiated with resolved vertical motion and subsequent condensation latent heating from the model microphysics no warm bubbles or cumulus parameterizations were used part i of this study focuses on the simulated storm evolution and its transition into a bow echo the simulation initially produced a classic supercell that developed at the intersection between a stationary front and an outflow boundary as the simulation progressed additional storms developed and interacted with the main storm to produce a single supercell this storm had many characteristics of an hp supercell and eventually evolved into a bow echo with a rotating comma head structure an analysi

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

a case study of terrain influences on upscale convective growth of a supercell abstractsatellite and ground based radar observations have shown that the northern half of argentina south america is a region susceptible to rapid upscale growth of deep moist convection into l

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

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

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

serial upstream propagating mesoscale convective system events over southeastern south america abstract serial mesoscale convective system mcs events with lifetimes over 18 h and up to nearly 70 h are routinely observed over southeastern south america from infrared satellite imagery during the spring and summer these events begin over the southern la plata river basin with individual convective systems generally moving eastward with the cloud layer mean wind however an important and common subset of these serial mcs events shows individual mcss moving to the east or southeast yet the region of convective development as a whole shifts upstream to the north or northwest analyses of the composite mean environments from 10 of these upstream propagating serial mcs events using ncep ncar reanalysis data events indicates that the synoptic conditions resemble those found in mesoscale convective complex environments over the united states the serial mcs events form within an environment of strong low level warm advection and strong moisture advection between the surface and 700 hpa from the amazon

idealized numerical simulations of quasi stationary convective systems over the northwestern mediterranean complex terrain northwestern mediterranean coastal regions are frequently affected by torrential rainfall associated with quasi stationary mesoscale convective systems mcss the present work examines how the characteristics of a conditionally unstable flow impinging on the coastal complex terrain of the northwestern mediterranean can affect the location and intensity of quasi stationary mcss the study is based on idealized simulations but including the major ingredients of northwestern mediterranean heavy precipitation events a moist conditionally unstable marine flow of about 100 km width facing the true terrain composed of the massif central surrounded by the alps and the pyrenees we find that mcss are located upstream of the mountain range with a slow flow whereas with a rapid flow the heaviest precipitation is over the massif central slopes in a similar way when the lateral environment is drier the heaviest precipitation is located upstream whereas a humid environment favours precipitation over the slopes the dominant lifting mechanism is strongly related to the location of the system 1 direct orographic triggering for systems over mountain slopes and 2 cold pool triggering for upstream systems in addition the neighbouring mountains interplay through deflection of the flow and induced low level convergence favoured by slow or dry lateral environments and through cold pool blocking within valleys copyright 2012 royal meteorological society

numerical study of an observed orogenic mesoscale convective system part 1 simulated genesis and comparison with observations abstract the interaction of topographically induced thermally and mechanically driven diurnal flow regimes in the lee of the rockies is shown to lead to the growth of a mesoscale convective system mcs an organic mcs observed during the 1977 combined south park area cumulus experiment and high plains experiment is numerically simulated with a two dimensional nonhydrostatic cloud model covering spatial scales of 1000 km in this numerical investigation meso meso and meso scales of motion are represented simultaneously as a result interesting features of cloud mesoscale interaction are predicted that cannot be represented in cloud parameterization frameworks based on the results of this simulation a six stage conceptual model of orogenic development is given

sensitivity of high resolution idealized simulations of thunderstorms to horizontal resolution and turbulence parametrization idealized simulations of deep moist convection are performed with the meso nh model at kilometric scales to assess the impact of the horizontal grid spacing 4 km 2 km 1 km and 500 m and turbulence scheme one dimensional t1d versus three dimensional t3d turbulence the simulations generate one cell which splits into two convective systems a leftward moving multicellular system and a rightward moving supercell objective criteria based on statistical properties and bulk quantities are examined for both systems to characterize the convection they show that the accumulated rainfall and corresponding surface area increase with increasing resolution as does the area covered by the updraughts while the 90th quantile of the intensity of updraught cores decreases the 4 km horizontal grid spacing is set apart as it clearly under resolves the convective motions but the difference between 2 and 1 km horizontal resolutions is larger than between 1 km and 500 m suggesting the beginning of convergence at 500 m also 1 km appears to represent the deep convective structures more correctly than 2 km for a practical weather forecast of organized convective systems t3d induces more mixing and enhances the microphysical processes compared with t1d producing larger amounts of cloud cover and precipitation also the magnitude of the pressure anomaly on the southeastern flank of the supercell is stronger accentuating the path curvature the difference between t1d and t3d becomes perceptible at 2 km pointing out the necessity to deal with horizontal turbulent fluxes at kilometric resolutions although a stronger numerical diffusion added to t1d allows the necessary damping to be introduced at the spectral energy tail it removes a part of the physical mixing and still misses some variance at larger scales the ratio between resolved and total turbulent kinetic energy tke decreases with increasing resolution for both t1d and t3d which is unexpected the main explanation is insufficient turbulent mixing inside convective clouds more pronounced at coarser resolution which is also confirmed by the vertical velocity spectra at 500 m horizontal resolution the subgrid tke is mainly due to dynamical processes with maxima located at the upper level of the convective systems in areas of stronger potential temperatures associated with downdraughts however thermal production is mostly negative underlying the lack of entrainment at the cloud edges

gregarious tropical convection abstract a beat source with a vertical profile like that of observed tropical mesoscale convective systems mcss is shown to cause through inviscid gravity wave dynamics upward displacement at low levels in a mesoscale region surrounding the heating typical values are 10 30 area contraction at the surface everywhere within 270 km of the heating 6 h after it starts as a result conditions near an existing mcs but beyond the area of mcs outflow become more favorable for the development of additional convection this theory predicts that cloud clusters should be gregarious infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday supercluters

a numerical technique to incorporate frontal boundaries in two dimensional layer models of ocean dynamics abstract this paper describes a novel numerical technique that exploits the method of bicharacteristics to allow small fronts to he included in numerical layer models of the ocean the approach treats a front as a free boundary and solves the interior problem using a standard finite difference scheme it employs a method based on the bicharacteristics of the equations to locate the boundary and enforce the boundary conditions the bicharacteristics for the nonlinear shallow water wave equations are derived and their application to the problem is outlined the method is demonstrated and evaluated by compared calculated approximate solutions first to the exact solution of a simple problem and second to a numerical solution of the more complicated problem of the radially symmetric discharge of a buoyant fluid into a stationary deep ambient fluid obtained using a simple alternative method the scheme is then successfully applied to investigate the effect of a uniform crossflow in the ambient fluid on the

the optimal state for gravity currents in shear this study examines the lifting of sheared environmental air by gravity currents focusing primarily on the theoretical optimal state in which near surface flow is turned into a vertically oriented jet theoretical models are presented from multiple perspectives including the vorticity perspective that was first presented by rotunno klemp and weisman and a flow force balance perspective based on conservation of mass and momentum the latter approach reveals a constrainton the depth of the environmental shearlayer relative to the depth of the cold pool based on these control volume constraints a numerical solution for steady inviscid isentropic flow is obtained that shows how the cold pool interface has a slightly concave shape and is nearly although not strictly vertical then by initializing a time dependent numerical model with a stagnant cold pool in an environment with low level shear it is shown that a statistically steady flow can be maintained with all the important elements of the analytic solution most notably the front relative flow is negligible behind the surface gust front at all levels the interface of the cold pool maintains a predominantly vertical structure and the net generation of vorticity by buoyancy within a control volume closely matches the horizontal flux of environmental vorticity on the side of the control volume sensitivity simulations confirm that the constraints identified by the analytic study must be met for the optimal state to be realized and that lifting of near surface environmental air is optimized when a vertically oriented jet is created and maintained