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

thermodynamic variability within the convective boundary layer due to horizontal convective rolls abstract data from the convection and precipitation electrification cape experiment conducted during the summer of 1991 are used to examine and quantify the horizontal variability of temperature and moisture within the convective boundary layer cbl potential temperature variations were only about 0 5 k while variations in water vapor mixing ratio values of 1 5 2 5 g kg 1 were observed throughout the cbl using radar aircraft and sounding data it is shown that horizontal convective rolls are the likely cause of these variabilities the enhanced moisture occurred within the roll updraft regions thus rolls were transporting moist air from the surface upward the observed cloud base heights obtained from cloud photogrammetry were produced from the highest moisture values within the roll updraft regions since the roll ascending branches contained moisture values that were most representative of the observed cloud base heights it is likely that measurements from within the roll updrafts would prov

observations of the sea breeze front during cape part i single doppler satellite and cloud photogrammetry analysis abstract an analysis of two sea breeze events on 6 august an onshore flow event and 12 august an offshore flow event 1991 is presented using single doppler observations satellite images and cloud pictures collected during the convection and precipitation electrification cape experiment documentation of the alongfrontal variability at the leading edge of the sea breeze circulation is presented for the first time the horizontal structure of the front was strongly modulated by the near perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 august these intersection points also appeared to be preferred locations for cloud development along the front horizontal convective rolls were also documented on 6 august however their orientation was nearly parallel to the sea breeze front as a result extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection po

three dimensional numerical modeling of convection produced by interacting thunderstorm outflows part ii variations in vertical wind shear abstract in this second paper in a series on outflow interactions we use the three dimensional model described in part i to examine the effects of vertical wind shear variations on cloud development along intersecting thunderstorm outflow boundaries three wind shear profiles are used in this study shear only above cloud base shear only below cloud base and shear both above and below cloud base as in part i the shear is unidirectional and is oriented perpendicular to the line containing the two initial outflow producing clouds which are spaced 16 km apart using the environmental thermodynamic structure from the control simulation in part i we vary the shear magnitude in each profile and examine the properties of cloud development in the region where the two outflows collide the outflow collision line or cl the model results show that the intensity and the time interval between successive cell updraft maxima of the first two clouds along the cl both of which are triggered by the outflow colli

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

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

understanding hector the dynamics of island thunderstorms abstract linear and nonlinear models are used to examine the development of island thunderstorms in particular the hector convective system that forms over the tiwi islands just north of australia the linear model is used to examine the flow response to an isolated elliptical heat source it is found that the low level convergence is maximized when the flow is weak and along the major axis of the heat source a dry version of the nonlinear model verifies the trends predicted by the linear model except at very low flow speeds where the convergence is bounded in the nonlinear model but increases indefinitely in the linear model deep convection develops over the heat source when a moisture profile with positive convective available potential energy cape is added to the nonlinear model the sensitivity of the convective strength defined by the accumulated rainfall and total condensate to wind speed and direction surface fluxes and low level moisture is then examined it is shown that the strength i

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

the role of boundary layer convergence zones and horizontal rolls in the initiation of thunderstorms a case study abstract the initiation of thunderstorms is examined through a combined observational and modeling case study the study is based on doppler radar aircraft mesonet balloon sounding and profiler and photographic data from the convection initiation and downburst experiment cinde conducted near denver colorado the study examines the initiation of a line of thunderstorms that developed along a preexisting quasi stationary boundary layer convergence line on 17 july 1987 the storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present the primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection the critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line the effect was to cause the updrafts in the convergence line

delineation of subtrappean mesozoic sediments in deccan syneclise india using travel time inversion of seismic refraction and wide angle reflection data 2 d shallow velocity structure is derived by travel time inversion of the first arrival seismic refraction and wide angle reflection data along the e w trending narayanpur nandurbar and n s kothar sakri profiles located in the narmada tapti region of the deccan syneclise deccan volcanic trap rocks are exposed along the two profiles inversion of seismic data reveals two layered velocity structures above the basement along the two profiles the first layer with a p wave velocity of 5 15 5 25 km s 1 and thickness varying from 0 7 1 5 km represents the deccan trap formation along the narayanpur nandurbar profile the trap layer velocity ranges from 4 5 to 5 20 km s 1 and the thickness varies from 0 95 to 1 5 km along the kothar sakri profile the second layer represents the low velocity mesozoic sediments with a p wave velocity of 3 5 km s 1 and thickness ranging from about 0 70 to 1 6 km and 0 55 to 1 1 km along the e w and n s profiles respectively presence of a low velocity zone lvz below the volcanic rocks in the study area is inferred from the travel time skip and amplitude decay of the first arrival refraction data together with the prominent wide angle reflection phase immediately after the first arrivals from the deccan trap formation the basement with a p wave velocity of 5 8 6 05 km s 1 lies at a depth ranging from 1 5 to 2 45 km along the profiles the velocity models of the profiles are similar to each other at the intersection point the results indicate the existence of a mesozoic basin in the narmada tapti region of the deccan syneclise

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

some characteristics of thunderstorms over bathurst and melville islands near darwin australia a study defining the occurrence structure and evolution of maritime continent island thunderstorms over bathurst and melville islands near darwin australia is presented satellite data for the november december pre monsoon transition season show unambiguous records of island thunderstorms on at least 65 of days over the period 1978 1986 storms develop most commonly between 1230lst and 1530lst and radar data indicate a mean lifetime of 2 5 h non occurrence was found on only 7 of the days examined and was associated with a return to dry flow typical of the early transition season with middle level subsidence or suppressed conditions in the wake of previous convective activity environmental sounding data indicated that these maritime continent thunderstorms develop in a regime with moderate cape low to moderate shear and high moisture the dynamic regime is characteristic of multicellular storms examination of the evolution of the storms indicates the existence of a typical lifecycle but with considerable variability in detail initially the lifecycle is consistent with the multicellular storm concept but the developing storms tend to aggregate to form one or more mesoscale systems which in turn may develop into propagating squall lines in the mature stage the developing storms have preferred geographic regions of genesis and intensification over the islands and while motion of the storms is variable it is generally consistent with the mean surface to 700mb pressure weighted flow

lifting by convergence lines abstract the lifting depth of a convergence line in an unstratified boundary layer beneath a stably stratified atmosphere is examined with both analytical and numerical models cases are considered with and without flow in the layer above the convergence line three different stability profiles above the boundary layer are also considered an inversion continuous stratification and a combination of the two for the case in which there is no flow above the convergence line analytical solutions are obtained for the lifting depth for the three different stability profiles simulations of the flow with a nonlinear nonhydrostatic model show good agreement with these analytical predictions the presence of flow in the upper layer increases the complexity of the problem due to the presence of gravity waves in the steady state solution for an atmosphere with just an inversion the analytical model predicts that for hydrostatic flow the depth of lifting is independent of the upper level flow while for nonh

tropical island convection in the absence of significant topography part ii nowcasting storm evolution abstract this paper examines influences on the short range prediction of organized convection under conditions of strong diurnal forcing the analyses are based on data provided by the maritime continent thunderstorm experiment which was conducted in 1995 over the tiwi islands 11 s 50 100 km north of the australian continent organized convection over the tiwi islands is often dubbed hector by residents and researchers alike the authors purpose is to utilize results from these analyses to improve convective storm nowcast systems and their associated forecast and warning products the environmental near surface wind direction is shown to be singularly influential in statistically predicting the location of convection and its time of occurrence over the tiwi islands this finding is robust despite the fact that mean wind speeds were merely 1 4 m s 1 the island boundary layer water vapor mixing ratio derived from a morning sounding and inland surface stations is shown to be correlated with measu

thunderstorm initiation organization and lifetime associated with florida boundary layer convergence lines abstract the initiation organization and longevity of thunderstorms associated with boundary layer convergence lines in the cape canaveral florida vicinity are examined using data from the convection and precipitation electrification cape experiment the project was conducted during july and august 1991 under low vertical wind shear situations this observational study is based on doppler radar mesonet balloon sounding and satellite data the primary convergence lines studied were the east coast sea breeze front ecsbf and a frequently occurring gust front from the west termed the west coast front wcf that originates with storms initiated by the west coast sea breeze front significantly fewer storms were associated with the ecsbf in comparison to the wcf this was because the convergence with the ecsbf was shallower and weaker and the updrafts were shallow and tilted the environmental winds were generally westerly near the top of the ecsbf and at storm steering level as a result the low le

observations of the sea breeze front during cape part ii dual doppler and aircraft analysis abstract the three dimensional kinematic structures of offshore and onshore flow sea breeze fronts observed during the cape experiment are shown using high resolution dual doppler and aircraft data the fronts interact with horizontal convective rolls hcrs that develop within the convective boundary layer nearly perpendicular intersections between the hcrs and sea breeze front were observed during the offshore flow case close to the front the hcr axes were tilted upward and lifted by the frontal updrafts consequently a deeper updraft was created at the intersection points providing additional impetus for cloud development furthermore clouds forming at periodic intervals along the ncrs intensified as they propagated over the front during the onshore flow case the hcr orientation was nearly parallel to the front extended sections of the front merged with the hcrs this process strengthened the front and is explained as the merger of like sign vortices associated with both the front and hcrs

numerical simulation of the diurnal evolution of tropical island convection over the maritime continent abstract numerical simulations of the diurnal evolution of tropical island convection observed during the maritime continent thunderstorm experiment mctex are performed using the meteorological research institute nonhydrostatic model mri nhm the mri nhm is double nested within a form of the australian bureau of meteorology research centre s limited area assimilation and prediction system specially operated for the mctex period excellent agreement is found between the simulation and observed evolution of the convective clouds over the tiwi islands on 27 november 1995 a transition from horizontal convection occurring during the morning to vertical convection in afternoon is evident in the morning the sea breeze appears along the coastlines with a clear contrast evident in structure between the windward and leeward sides at the windward coast the sea breeze intrudes inland more rapidly where the larger surface heat flux modifies the lowest air mass and makes the sea breeze front sbf indistinct

a further study of cumulus interactions and mergers three dimensional simulations with trajectory analyses abstract a total of nine three dimensional experiments are made to study cloud interaction and merging under the influence of different imposed conditions large scale lifting forcing environmental wind shear and cloud microphysical processes are the three parameters to be varied the basic design of the study is to generate several convective clouds randomly inside the model domain and then to observe and analyze the interactions and merging between the simulated clouds the locations as well as the intensities of simulated clouds while they interact with each other are not predetermined a two dimensional version of the model has been used to investigate the effects upon merging produced by varying large scale conditions with a gate dataset in this study we continue studying the cloud interactions and merging problems through using a fully three dimensional model and the same dataset ten merged systems involved precipitating clouds are identified in this numerical study eight mergers involve two

tropical island convection in the absence of significant topography part i life cycle of diurnally forced convection abstract diurnally forced convection was observed over the tiwi islands north of the australian continent as part of the maritime continent thunderstorm experiment immature peninsula scale 5 15 km sea breezes were observed to initiate moist convection early each day principally through convergence that results from the confluence or collision of peninsula breeze fronts convection initiated by peninsula scale breezes usually fails to organize beyond a small cluster of cells and dissipates as a local event mature island scale 100 km breezes develop by late morning and subsequently play a pivotal role in the forcing and evolution of organized convection the initiation of mesoscale convective systems mcss is observed to be a direct consequence of breeze front collisions for only 20 of the days on which organized convection develops this is referred to as type a forcing and it occurs when normal convective development is delayed or otherwise suppressed type a forcing is nature s backup mech

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

convection initiation associated with a sea breeze front a gust front and their collision abstract the initiation of convection associated with a sea breeze front a gust front and their collision is analyzed using data collected in east central florida during the convection and precipitation electrification project in conjunction with satellite surface and rawinsonde information dual doppler radar derived winds are used to determine the three dimensional kinematic factors critical to storm development the gust front which emanated from storms on the western half of the peninsula propagated more rapidly and was deeper than the sea breeze front which originated from the east coast and was characterized by a distinctly scalloped appearance convection associated with the sea breeze front appeared to develop preferentially at the vertices of this scalloped pattern where there were enhanced regions of convergence and upward motion on the gust front a helmholtz shearing instability produced an organized configuration of convergence and updraft maxima along its length however these were

the relationship between radar reflectivities and clouds associated with horizontal roll convection on 8 august 1982 abstract horizontal convective rolls hcrs and cloudstreets that occurred in northeastern colorado are studied using doppler radar data cloud photography and vertical soundings examination of the environment in which the hcrs form indicates a boundary layer wind speed that is much lower than those measured in previous studies evidence shows that the hcrs form in a well defined boundary layer yet the cloudstreets that develop seem to exist within a stable layer above a relationship between the radar reflectivity echoes and the cloudstreets is firmly established the echoes are aligned directly under the cloudstreets the echoes at lower levels are shown to be caused by convergence of particulate matter into updraft portions of the hcr circulation yet evidence indicates the cloud level echoes are due to backscatter from refractive index inhomogeneities at the surfaces of rising thermals examination of the echoes show that the strongest and tallest echoes are associated with cloud formation and tha

a numerical case study of convection initiation along colliding convergence boundaries in northeast colorado abstract a numerical cloud model has been used to simulate convective storm development on 17 july 1987 in northeast colorado the study involves the simulation of convergence along atmospheric boundaries and the subsequent development of convection the model was initialized using observed conditions for this case day from the convection initiation and downburst experiment cinde a two dimensional version of the clark ncar nested grid model is employed results indicate that convection in boundary line collision cases can be successfully simulated by using actual observed atmospheric data gradual deepening of the moisture layer in the convergence zone was shown to destabilize the local atmosphere leading to the initiation of deep convection on this day the modeled storm approximated the depth and intensity of the observed storms and displayed many of the features of the actual event sensitivity studies revealed that the timing and intensity of convection along boundaries is greatly affected by alter

thermodynamic and vertical velocity structure of two gust fronts observed with a wind profiler rass during mctex high time resolution wind profiler rass observations are used to describe the vertical velocity temperature and reflectivity fields of two gust fronts in detail the first was a freely propagating gust front and the second interacted with a rain cell near the profiler site the first of these shows a large updraft confined to the warm air ahead of the front this updraft coincided with the nonhydrostatic pressure jump the vertical motions within the gust front were an order of magnitude smaller the updraft impinging on the top of the boundary layer excited a clear gravity wave signature in the free troposphere the interaction of the vertical circulation and the weakly precipitating cloud in the second case coincided with explosive growth of the cell with reflectivities increasing from 30 dbz to 50 dbz in 6 min a descending reflectivity core was observed at this time precipitation loading played a significant role in a downdraft behind the gust front head leading to adiabatic warming as no evidence of evaporative cooling in the downdraft was seen a distinct clear air peak was visible in the profiler doppler spectra even during the heavy rain

trade wind rainfall near the windward coast of hawaii a dynamically based mesoscale climatology of rainfall and other data are examined for typical trade wind conditions near hawaii relative distributions of rainfall are deduced from radar reflectivity data for a 4000 km 2 region over the windward island and the upstream ocean the cumulative rainfall data suggest three zones of forcing that are characteristic of 1 the regional ocean 2 upstream island induced divergence and 3 flow reversal near and over the island it is confirmed that most intense rainband amplifications occur over a mesoscale convergence line that separates the easterly trade winds from an island induced westerly flow this flow separation line resembles a classical gravity current when positioned over the windward island and near shore the domain scale cumulative rainfall is statistically associated with the strength of island blocking as defined by a froude number fr when fr is in the ordinary range 0 3 the island maximum is five times greater than the oceanic average when fr is elevated 0 3 the rainfall maximum increases to 10 times the oceanic average overall the windward island maximum is seven times the upstream oceanic average increased rainfall is more strongly correlated with increased wind speed than with a reduction in dry static stability both component variables of fr unlike rainfall amount the position of the island maximum is relatively insensitive to fr except in the hours surrounding sunrise in the upstream divergence zone 20 45 km offshore rainfall is double the oceanic background it is speculated that the cause is blocking related there are very strong diurnal variations in rainfall amount and distribution both over the island and the upstream ocean windward island rainfall is at a maximum in the hours surrounding midnight when breeze and blocking forcings are fully cooperative in the coastal region seemingly unrelated oceanic rainfall exhibits a strong nocturnal maximum the temporal phase of which may be influenced by wind speed a deep minimum in domain scale rainfall occurs near noon