structure and evolution of the 22 february 1993 toga coare squall line organization mechanisms inferred from numerical simulation abstract mechanisms responsible for meso and convective scale organization within a large tropical squall line that occurred on 22 february 1993 during the tropical ocean global atmosphere coupled ocean atmosphere response experiment are investigated using a three dimensional numerical cloud model the squall line occurred in an environment typical of fast moving tropical squall lines characterized by moderate convective available potential energy and moderate to strong vertical shear beneath a low level jet with weak reverse vertical shear above a well simulated aspect of the observed squall line is the evolution of a portion of its leading convective zone from a quasi linear to a three dimensional bow shaped structure over a 2 h period this transition is accompanied by the development of both a prominent mesoscale vortex along the northern edge of the 40 60 km long bow shaped feature and elongated bands of weaker reflectivity situated rearward and oriented transverse to the leading edge within enha

mesoscale characteristics of monsoonal convection and associated stratiform precipitation abstract observations undertaken on 12 january 1990 at darwin australia 12 s 130 e are used to document the structure of a monsoonal rainband in a low cape low shear tropical environment dual doppler radar analyses are employed to investigate the structure and kinematics of the convective and stratiform regions a system with the characteristics of a relatively short lived squall line in which warm rain processes play a significant role in the production of precipitation is evident planetary boundary layer cold pool production is important in the organization and motion of the system a trailing stratiform region is evident with a mean updraft downdraft circulation but is composed of in situ decaying convective cells a storm relative mesoscale cyclonic circulation is also observed within the stratiform cloud this vortex was maintained by thermodynamically induced midlevel convergence convectively generated storm scale circulations and their interaction with the background monsoon now

structure of a midtropospheric vortex induced by a mesoscale convective system on 9 june 1988 a mesoscale 200 km diameter convectively generated vortex mcv passed through the colorado wind profiling network the generating convective system which was too small to meet maddox s mesoscale convective complex criteria developed beneath a high amplitude mid and upper level ridge axis profiler winds obtained within the stratiform region of the convective system and near the center of the ensuing mcv form the basis of this study with the addition of satellite including rapid scan images and conventional and doppler radar data this vortex was well observed over its entire life cycle the slowly moving vortex was observed by the flagler 50 mhz profiler for more than 16 h for a portion of this period the circulation was quasi steady hence the spatial structure of the vortex was depicted by timeto space conversion of the profiler data the results show a strong coherent circulation with maximum tangential speeds greater than 16 m s21 at 6 km above mean sea level msl and at radii of 50 70 km corresponding relative vorticity was more than three times as large as the local coriolis parameter the long duration and the apparent dominance of vorticity over divergence suggest that the flow in the vortex was in a quasi balanced state hence the temperature perturbation necessary to support the tangential velocity distribution was computed assuming gradient balance in the radial direction the resulting estimated temperature structure showed a warm core with a magnitude of 2 38c at 7 3 km msl and a cold core below 6 3 km msl the associated potential vorticity pv structure at the vortex center consisted of a lens of high pv with a maximum at 6 3 km msl and relative minima above and below this height

internal waves in a small reservoir horizontally distributed temperature time series recorded in the thermocline of a small reservoir during autumn are used to identify both basin scale and higher frequency internal waves stationary and nonstationary spectral analyses are used to identify dominant frequencies the horizontal distribution of data allows a map of the basin scale wave energy to be determined three layer normal mode modeling incorporating damping emulates the basin scale behavior well this provides estimates of velocity shear and hence the internal stability the nonstationary frequency analysis identifies times of high frequency internal wave activity wave periods of 2 10 min this is definitely linked to the basin scale wave driven shear and hence to deceases in stability estimates of dissipation for the thermocline and hypolimnion are 3 10 8 and 6 10 9 m 2 s 3 respectively the inferred variability in the hypolimnetic value is demonstrated

dynamics of large polymictic lake i field observations the internal dynamics of clear lake california a large multibasin and polymictic lake under stratified conditions is described in this work particular attention is paid to one of the subbasins of clear lake oaks arm several field experiments were conducted in the lake to obtain data of wind currents and water temperature using a wide range of devices based on field observations and previously published results on basin responses to wind stress a conceptual model of the internal circulation of the oaks arm is proposed its internal dynamics are characterized by diurnal cycles of setup and relaxation of horizontal temperature gradients driven by the wind forcing the setup and relaxation processes are modulated by the influence of the earth s rotation and create a residual cyclonic circulation this circulation mechanism is here referred to as baroclinic pumping it is in all likelihood not exclusive to clear lake but probably exists in other large and relatively shallow lakes forced by strong and almost periodic wind events where basin scale internal waves do not play a major role in the dynamics of the system

the structure and dynamics of internal waves in baldeggersee1 analysis of long records of temperature depth distribution determined at lo or 20 min intervals at several stations and extending over three stratification seasons provides a detailed picture of the characteristics dynamics and seasonal variation in activity of internal waves in a small lake baldeggersee switzerland wind stress in general was low with occasional bursts of higher energy the ensuing slightly damped internal seiches govern the dynamics of the stratified lake the vertical structure displays a dominant response of the first vertical mode responses in the second vertical mode hitherto rarely recorded are also demonstrated with the help of spectral analysis and models the horizontal structure of the seiches motion displays dominance and persistence of the fundamental basin mode spectral analysis also discloses responses of the second and third basin modes multiplying of the spectra with the local buoyancy frequency leads to a collapse of all spectra into a narrow band pointing toward the possibility of a universal spectrum new light is shed on apparently common examples of waveform asymmetry and on their interpretation it is concluded that in this small basin low energy example internal waves provide motions at all levels but little vertical mixing

a numerical study of the genesis of extratropical convective mesovortices part i evolution and dynamics abstract the purpose of this study is to understand the genesis of extratropical convective mesovortices and the large scale environmental features that influence the vortex formation a hypothesis is proposed that mesovortices form in the stratiform rain regions of mesoscale convective systems mcss due to the reduction of static stability that reduces the effective local rossby radius in such regions a conceptual model of the mesoscale convective cyclogenesis is introduced which describes the three stages of the mesovortex formation a modified version of the pennsylvania state university national center for atmospheric research three dimensional hydrostatic mesoscale model is used to simulate mesovortex genesis in analytically generated pre mcs large scale environments the model simultaneously incorporates parameterized convection and a grid resolvable convective scheme containing the effects of hydrostatic water loading condensation evaporation freezing melting and sublimation a control si

doppler radar observations of an asymmetric mesoscale convective system and associated vortex couplet abstract a study of the 28 may 1985 asymmetric mesoscale convective system mcs observed during pre storm is presented dual doppler analysis revealed a well defined cyclonic mesovortex in the northern portion of the stratiform region in accord with previous studies on this type of circulation at maximum extent the closed cyclonic circulation had a diameter of approximately 80 km and a depth of 7 km a smaller anticyclonic circulation was present to the south of the cyclonic vortex counterrotating vortices in asymmetric mcss have been identified in recent modeling studies however prior to this study observational confirmation of the anticyclonic vortex has been elusive

interaction of a warm season frontal system with the coastal mountains of the western united states part i prefrontal onshore push coastal ridging and alongshore southerlies abstract observational analyses and numerical simulations are used to investigate the interaction between a warm season frontal system trough and the complex terrain of the western united states prior to frontal landfall synoptically driven offshore flow was associated with the northward development of a thermal trough and a coastal pressure ridge and associated southerly winds moved northward along the coasts of northern california oregon washington and southwestern british columbia this coastal pressure ridge coupled with the subsequent weakening of the offshore flow resulted in the onshore push of cool marine air approximately 24 h before frontal passage the onshore surge of cool marine air into warm dry continental air associated with the thermal trough produced a mesofront that moved over the coastal and cascade mountains diagnosis of a realistic simulation of the event shows that both solar heating and adiabatic warming from downslope flow over the rockies and the cascade mountains are i

tests of an extension to internal seiches of defant s procedure for determination of surface seiche characteristics in real lakes the success of a defant s method of calculating surface seiche periods and structures in real lakes is well known to test whether it can be adapted and applied with similar success in studies of internal seiches in stratified lakes in which density structure is simulated by two homogeneous layers we compare the predictions of a computerized version of mortimer s modified defant procedure against observations of internal seiche motion in eight lake basins and in three examples against the results of more elaborate models the acceptably close agreement inspires confidence that the modified method is useful for practical predictions which although approximate described the principal characteristics of the often large water mass displacements and oscillations through their influence on mixing and dispersal those motions profoundly affect the chemical and biological economies of many lakes to aid in microcomputer and even pocket calculator applications we set out steps in the calculation in tabular flow chart form a comparable table for surface seiche calculation is added the procedure is not applicable to large lakes in which effects of the earth s rotation are dominant but for the two largest basins tested here leman and ness a modification is introduced to take approximate account of rotation

dynamics of bottom currents in a small lake1 the dynamics of bottom currents in baldeggersee switzerland were investigated with a specially designed recording current meter sensitive to current velocities of 0 3 2 5 cm s l supporting information was obtained from thermistor chain temperature recordings and from records of atmospheric boundary layer parameters measured in the center of the lake during 1980 1981 bottom currents recorded for l week periods on the central bottom plateau of the lake 65 m water depth were highly variable the mean speed was slightly 1 cm sk with no seasonal trend speeds 2 5 cm s l occurred after periods of high wind quiescent periods were rare and seldom lasted more than 1 h spectral analysis shows that the primary mechanism generating bottom currents during stratification is internal gravity waves in the thermocline during winter mixing when chemical stratification is observed below 30 m the wind stress distribution is directly reflected in the bottom current pattern in both periods wind induced disturbances were felt in the bottom layer with little time delay indicating the event dominated structure of bottom current dynamics bursts of particle transport in the bottom layer are observed during periods of high wind suggesting episodic particle resuspension such events may be important for the control of mass exchange at the sediment water interface

turbulent mixing induced by nonlinear internal waves in mono lake california describing the spatial and temporal occurrence of turbulent mixing and the mechanisms of turbulence production are fundamental problems in physical limnology and oceanography here we present results from mono lake california that illustrate pronounced steepening of the pycnocline upon cessation of strong wind forcing and the enhanced turbulence resulting from this nonlinearity three events occurred with wind speed of 10 m s21 upwind the upper pycnocline upwelled but on relaxation of the wind it downwelled 6 10 m and compressed the similarity of wave amplitude during compression to mixed layer depth and lake numbers 4 were indicative of significant nonlinearity energy in the internal wave field increased by two orders of magnitude but 50 of the energy was lost within 24 h the largest energy losses occurred at low frequencies rather than via an energy flux from low to moderate and high frequencies eddy diffusivities computed over 6 d intervals using the heat budget method averaged 1026 m2 s21 in the energized pycnocline but were five to 10 times higher during the initial upwelling and downwelling periods microstructure casts taken 1 2 d after the first event revealed that the percentage of the pycnocline that was turbulent varied with bottom slope being 80 turbulent where bottom slopes were up to 10 6 0 where bottom slopes were 3 and less than 7 where bottom slopes were 0 1 the threshold for turbulent transport was exceeded only 10 of the time and only where bottom slopes were highest

mesoscale and convective scale characteristics of mature hurricanes part i general observations by research aircraft abstract the analysis of instrumented aircraft data from four recent mature hurricanes reveals common mesoscale and convective scale features the structure and organization of the convection precipitation and wind flow are studied by combining the airborne observations using basic instrumentation with quantitative radar measurements three predominant precipitation features seen are 1 the eyewall 2 a stratiform rain region adjacent to and outside the eyewall and 3 rainbands that contain both convective and stratiform regions the eyewall is so named because it surrounds the rain free eye but several additional features distinguish it from other convective bands in the hurricane in particular the updraft tangential wind maximum and highest rainfall have preferred locations relative to each other with the updraft being radially inward from the rain and wind maxima the 10 dbz radar reflectivity contour indicative of the inner edge slopes substantially outward with height and there is evide

lake morphometry sediment mixing and the selection of sites for fine resolution palaeoecological studies abstract palaeoecologists are increasingly using close interval or continuous sampling of lake sediments to construct fine resolution records of environmental change the degree of temporal resolution sought in these studies is typically decades or years the use of lake sediments for such fine resolution reconstructions assumes that temporal mixing of the sediments is negligible in this paper we examine the physical and biological processes that cause sediment mixing in small lakes we use physical models and empirical data to develop a set of heuristics relating lake depth and area to sediment mixing by slumping waves currents and bioturbation we also develop a heuristic that allows information on lake depth and area to be used to determine if the sediments of a lake will likely be massive or annually laminated in general the degree of sediment mixing likely to occur in a small lake is directly related to the depth and surface area of the lake however each of the agents of mixing has a different critical relationship with lake depth and surface area we present a series of equations and a summarizing figure that based on lake surface area and maximum depth allows the type of thermal stratification polymictic dimictic or meromictic and the degree of sediment mixing present in the lake to be inferred

barrier jets during tamex abstract a barrier jet is frequently found along the northwestern coast of taiwan in the prefrontal southwesterly flow regime during the taiwan area mesoscale experiment tamex it has a maximum wind speed of 14 m s 1 at approximately 1 km above the surface with a vertical wind shear approximately 10 10 3 s 1 below and 4 10 3 s 1 above that altitude during tamex the southwesterly monsoon flow strengthens over taiwan when the low level pressure trough surface front moves toward the southeastern china coast the barrier jet is a result of the stably stratified airflow past an island obstacle under a small froude number o 1 flow regime during the occurrence of a barrier jet a windward pressure ridge is observed along the southwestern coast due to island blocking in low levels the incoming southwesterly flow decelerates off the southwestern coast and moves around the island along the western coast the deflected airflow accelerates northward with a large cross contour wind component down the p

a mesoscale convective complex generated inertially stable warm core vortex abstract a long lived mesoscale convectively generated vortex mcv associated with a mesoscale convective complex mcc is documented the mcv with a rossby number of approximately 0 5 is investigated as a feature intrinsic to the organization of the mcc on 6 7 july 1982 a particularly large and intense mcc developed in a region of high convective available potential energy cape but weak vertical wind shear bulk richardson number 150 and weak advection of temperature and vorticity convection initially was organized in a narrow line with elements propagating relative to the mean environmental flow these elements subsequently developed a large semicircular area of stratiform precipitation and a surface mesolow to the rear heavy rain fell over a broad area amounts as great as 10 9 cm accompanied by flooding were reported in central oklahoma as the large semicircular rain area dissipated a three layered structure became evident a large upper tropospheric anticyclone a rain cooled mesoscale h

forced resonant oscillations as a response to periodic winds in a stratified reservoir the response of the sau reservoir to a wind field characterized by having marked periodicities of 12 h and 24 h has been studied measurements of temperature with a thermistor string and currents with an acoustic doppler current meter show that the reservoir also responds with the same water periodicities during certain times of the stratified period some of the natural oscillation modes of the reservoir are close to these forcing wind periods in particular in mid july the vertical mode v2 is close to 12 h and in mid to end of september the vertical mode v3 is close to 24 h in these situations these modes are selected out of the spectrum of possible internal waves and the reservoir behaves as a forced oscillator in resonance with the wind the structure and the period of these vertical modes have been elucidated by using the 3d model elcom both modes are affected by the earth s rotation at the widest part of the reservoir

numerical simulation of the meso β scale structure and evolution of the 1977 johnstown flood part ii inertially stable warm core vortex and the mesoscale convective complex abstract a mesoscale warm core vortex associated with the mesoscale convective complex mcc that produced the 1977 johnstown flood is examined using a three dimensional nested grid model simulation of the flood episode in the simulation the vortex plays a key role in determining the evolution of the mcc a squall line and the distribution of heavy precipitation the vortex has a space scale of 100 200 km in diameter and a time scale of more than 18 hours its low pressure center extends from the midtroposphere down to the surface and its maximum vorticity occurs between 850 and 700 mb a pool of cool moist downdraft air develops in the surface to the 850 mb layer beneath the warm core while a cold dome forms in the vicinity of the tropopause above the warm core following forcing from repeated deep convection prior to model initial time the vortex is initiated by mesoscale ascent associated with a traveling meso scale wave genesis takes place in a nearly saturated slightly conditionally unstabl

rates and uncertainties on seismic moment and deformation in eastern canada 1 on the basis of earthquake statistics from the canadian earthquake hazard model we estimate the rates of seismic moment and deformation in seismic zones of eastern canada we use a logic tree approach to derive median values and 66 confidence intervals for models based on earthquake cluster distributions and geological and tectonic structures typical moment rates are low 0 1 1 1017 n m yr 1 equivalent to a magnitude mw 7 earthquake every 350 3500 years in each seismic zone in a few zones such as baffin bay moment rates reach 2 10 1017 n m yr 1 expressed in terms of relative velocity between zone boundaries seismic deformation rates are typically 0 2 0 5 mm yr 1 for a few regions of high seismicity such as baffin bay charlevoix or grand banks the relative velocities reach 1 2 mm yr 1 strain rates are typical of the intraplate environment 10 13 10 10 yr 1 with values locally as high as 10 8 yr 1 these rates give some insight into geodynamic aspects of the northeastern north america plate assuming that the seismic deformation rates represent long term deformation of the lithosphere they provide some constraint to the amplitude and variability of intraplate strain on a shorter timescale seismic strain rates can be compared with postglacial rebound predictions and might help constrain some aspects of these models because they reflect the seismic energy released over the last few hundred years our estimates in combination with geodetic measurements can also help constrain models of earthquake dynamics in eastern north america

the formation of a cooling induced mesovortex in the trailing stratiform region of a midlatitude squall line abstract there have been some ambiguities in recent observational studies as to whether midlevel mesovortices are induced by latent heating or cooling and develop in the descending or ascending portion of mesoscale convective systems mcs s in this study a comprehensive examination of a cooling induced mesovortex in the trailing stratiform region of a midlatitude squall line that occurred on 10 11 june 1985 during the preliminary regional experiment for storm central pre storm is presented using a 20 h high resolution simulation of the squall system this cooling induced midlevel vortex originates from the preexisting cyclonic vorticity associated with a traveling meso scale short wave the vortex is intensified in the descending rear to front rtf inflow as a result of continued sublimative melting and evaporative cooling in the stratiform region it decouples from the front to rear ftr ascending and anticyclonic flow in the upper troposphere during the formative stage the vortex tilts northw

the temporal evolution of baroclinic basin scale waves in a rotating circular basin the temporal evolution of baroclinic basin scale waves in a rotating circular basin following an initial forcing event is investigated using a laboratory study experiments conducted in the circular domain containing a two layer fluid with a flat bottom and vertical sidewalls demonstrate that the response is essentially linear with frictional effects at the boundaries steadily dissipating wave energy experiments conducted in the same configuration but with the addition of simple topographic features either a radially protruding cape or a bathymetric ridge exhibit wave topography interactions that result in the formation of an eddy field and an offshore flow respectively the rate of wave decay as well as the amount of horizontal mixing occurring within the basin is significantly enhanced by such interactions the results of this study are then considered in terms of their implications for the baroclinic basin scale wave energy pathways in large stratified lakes influenced by the earth s rotation

precipitation and kinematic structure of an oceanic mesoscale convective system part i momentum transport and generation abstract this is the second paper of a two part series documenting the structure of and momentum transport by a subtropical mesoscale convective system near taiwan using doppler radar data and in situ data from the noaa p 3 part i defines the basic system structure and evolution in part ii the momentum transport by the system is estimated and related to system structure and the momentum budget for a portion of the embedded convective band is evaluated profiles of the vertical flux of horizontal momentum are constructed from in situ data doppler radar data and both combined in a coordinate system with u normal to the line and positive eastward since the low level air is feeding the line from the east differences in the fluxes from the two sources appear to be mainly due to an underestimation of the mean vertical velocity from the doppler radar data the discrepancy results partially from the concentration of convergence in the boundary layer precisely where the doppler cannot adequately sample t

circulations associated with a mature to decaying midlatitude mesoscale convective system part ii upper level features abstract the vertical structure of a midtropospheric mesovortex that developed during the decay of a midlatitude mesoscale convective system over kansas and oklahoma on 23 24 june 1985 is documented surface rawinsonde wind profiler and dual doppler data are used to define its structure as has been observed in other midlatitude and tropical cases the mesovortex occurred within the trailing stratiform precipitation region of a squall line system it was associated with a preexisting synoptic scale short wave trough and as the circulation developed it deformed the back edge of the stratiform precipitation region into a hooklike pattern the 100 200 km mesovortex was confined to the midtroposphere 3 8 km with a maximum amplitude just above the 0 c level the vortex axis sloped toward the northeast but its orientation changed hour by hour over the 2 h period of dual doppler coverage overall the mesovortex was warm core although its thermal structure was complex and apparently significantly infl

midlevel cyclonic vortices generated by mesoseale convective systems abstract despite the large number of convective systems that occur over the central united states every year there are typically only a few well defined midlevel vortices apparent in satellite imagery after the overlying anvil debris from some convective complexes has dissipated or advected away a climatology of mesoscale convectively generated vortex mcv events for 1981 1988 is presented and the synoptic setting in which the circulation becomes apparent is discussed proximity sounding data from numerous cases are used to examine features of the kinematic and thermodynamic setting of mcvs at various lifelycle stages defined by satellite imagery features ofthe large scale environment that appear conducive to the formation and longevity of mcvs include weak flow weak vertical shear weak background relative vorticity and intense horizontal and vertical moisture gradients the rapid mesovortex generation observed can be explained by the stretching term of the vorticity equation most mcvs emerge fro

structure and evolution of the 22 february 1993 toga coare squall line aircraft observations of precipitation circulation and surface energy fluxes abstract this study documents the precipitation and kinematic structure of a mature eastward propagating oceanic squall line system observed by instrumented aircraft during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare doppler radar and low level in situ observations are used to show the evolution of the convection from an initially linear nnw sse oriented convective line to a highly bow shaped structure with an embedded low to midlevel counterclockwise rotating vortex on its northern flank in addition to previously documented features of squall lines such as highly upshear tilted convection on its leading edge a channel of strong front to rear flow that ascended with height over a rear inflow that descended toward the convective line and a pronounced low level cold pool apparently fed from convective and mesoscale downdrafts from the convective line rearward the observations of this system showed distinct multiple maxima in updraft strength with

mesoscale downdraft influences on vertical vorticity in a mature mesoscale convective system abstract the vorticity dynamics of a lower tropospheric mesovortex within a mature mesoscale convective system mcs is investigated with observations from a special sounding network and with dual doppler radar measurements the data show that the vorticity distribution and tendency were dictated by the mesoscale downdraft which formed within the storm s trailing stratiform region and not by the mesoscale updraft at midlevels preexisting vertical vorticity was amplified by convergence as environmental air overtook the storm from the rear and funneled into the evaporatively cooled mesoscale downdraft vertical vorticity within the stratiform region also increased by the twisting of horizontal vorticity associated with a backing wind the horizontal vorticity vector pointed opposite the velocity vector and tipped into positive vertical vorticity when the row encountered the mesoscale downdraft at lower middle storm levels the twisted vorticity was subsequently amplified by convergence along with preexi

midlevel vorticity structure of the 10 11 june 1985 squall line abstract the comprehensive analysis of the kinematic structure of the mature phase of the 10 11 june 1985 squall line system was used to examine the midlevel vertical vorticity structure of the storm to show that relative vertical vorticity in the stratiform region at midlevels was organized into bands both cyclonic and anticyclonic oriented parallel to the convective line with anticyclonic vorticity between the rear of the convective line and the heaviest stratiform precipitation and cyclonic vorticity farther back since previous studies have not found anticyclonic vorticity over such a large portion of the stratiform region at midlevels and since the concentration of anticyclonic vorticity may have been detrimental to the longevity of the storm by limiting the development of an inertially stable cyclonic circulation the tilting and stretching terms of the vertical vorticity equation were computed to determine how the observed vertical vorticity pattern was maintained tilting of horizontal vorticit

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

formative stage of a long lived mesoscale vortex observed by airborne doppler radar abstract the formative stage of a long lived mesoscale cyclonic vortex was captured by the noaa p 3 aircraft as it investigated a developing mesoscale convective system mcs near the southeastern coast of taiwan on 16 june 1987 during the taiwan area mesoscale experiment the supporting environment of the mesovortex was characterized by an exceptionally moist atmosphere and moderate ambient vertical shear through a deep layer from the near surface to 6 km with much weaker shear and winds aloft in addition a pronounced low level mesoscale shear convergence zone which resulted from the interaction of southeasterly flow with northeasterly flow confined to the near coast region existed in the vicinity of the observed mesovortex composite three dimensional wind fields derived via pseudo dual doppler synthesis show the vortex had a horizontal diameter expanding from 40 km to 70 km in the lower to midtroposphere respectively and exhibited considerable tilt through this layer contrary to previously d

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 mesoscale vortex couplet observed in the trailing anvil of a multicellular convective complex abstract observations collected during the oklahoma kansas pre storm experiment are used to document the evolution and structure of a mesoscale vortex couplet that developed in the mesoscale convective system that occurred on 16 17 june 1985 the evolution of the circulations was captured by dual doppler radar observations for 1 4 hours this allowed an evaluation of the various terms of the vertical vorticity equation which give insight into the mechanisms that are important in the generation of the circulations the primary mechanism responsible for the formation of the observed vortices was the interaction of the larger scale flow with low level momentum transported to higher levels by multiple convective updrafts as a consequence vertical shear of the horizontal wind was important to initial vorticity production the vorticity generated in this manner was subsequently increased in strength due to middle level convergence when the convection weakened and dissipated the primary source of vorticity w

mesoscale organization of springtime rainstorms in oklahoma abstract radar reflectivity and raingage data obtained during six springtimes indicate the types of mesoscale organization that occur in association with major rain events in oklahoma at least 25 mm of rain in 24 h over an area exceeding 12 500 km2 in these storms the primary rain area is found to be a contiguous region of precipitation 10s to 100s of km in scale that consists partly of deep convection and partly of stratiform rain the patterns of rain formed by the convective and stratiform areas comprise a continuous spectrum of mesoscale structures about two thirds of the cases examined exhibited variations on the type of organization in which convective cells arranged in a moving line are followed by a region of stratiform rain storm organization was graded according to the degree to which it matched an idealized model of this leading line trailing stratiform structure the precipitation pattern was further graded according to whether its structure was relatively symmetric with respect to an a

seasonal evolution of the basin scale internal wave field in a large stratified lake the response of the water column to varying conditions of stratification and wind forcing was investigated in lake kinneret israel using data collected from thermistor chains and acoustic doppler current profilers during 1997 and 1998 the strong daily sea breeze was found to generate a vertical mode 1 internal kelvin wave and basin scale internal poincarewaves of vertical modes 1 2 and 3 the kelvin wave the dominant component of the internal wave field was responsible for alongshore velocities in the nearshore regions in the upwind nearshore regions velocities were dominated by the forced response to the wind and were cross shore in nature in the lake interior the kelvin wave effect on the horizontal velocity field was minor compared to the higher vertical mode poincarewaves the kelvin wave is shown to exist in resonant and nonforced states with the wind whereas the vertical mode 1 poincarewave energy remained relatively constant despite large variability in the forcing condi tions the energy in the higher mode poincarewaves varied greatly both on daily and seasonal timescales the results demonstrate that the wind energy forces multiple basin scale internal wave modes and that prior motion in the water column must be considered when determining the subsequent internal wave response in periodically forced systems lake kinneret israel has long been an important regional resource providing a substantial supply of drinking water as well as supporting a commercial fishery the monomictic lake is forced daily during the summer by a powerful sea breeze with stratification and wind patterns showing little variation from year to year these factors along with a fa vorable bathymetric pattern combine to make lake kinneret an ideal place for the field investigation of large scale inter nal waves and related phenomena such as the benthic bound ary layer horizontal transport and vertical mixing processes in addition to providing substantial insight into the local pro cesses and hence influencing local management study of the lake also provides fundamental knowledge of the dynam ics of enclosed basins on a geophysical scale during summer the internal rossby radius in lake kin neret is typically one half the basin width hence large scale motion in the lake will be affected by the earth s rotation and basin scale internal waves may be of topographic ross by kelvin or poincareform topographic rossby waves are sub inertial frequency oscillations that exist only in rotating fluids with variable bathymetry mysak et al 1985 stocker and hutter 1987 and these waves result from changes in depth affecting the relative vorticity of fluid elements in the absence of vertical boundaries wavelike solutions to the lin ear inviscid equations of motion for a flat bottomed region of homogeneous fluid with a constant rate of rotation are 1

the accuracy of vertical air velocities from doppler radar data abstract eight methods of calculating vertical air velocity in a column are compared each method requires some or all of the following data horizontal divergence vertical precipitation velocity hydrometeor terminal fall speed and vertical air velocity boundary conditions some or all of these quantities are commonly deduced or specified by a researcher during the analysis of doppler radar data the responses of the methods to different magnitudes and behaviors of errors in the input data are examined with a monte carlo method the experiments are conducted with both random and systematic errors two idealized kinds of systematic errors are considered bias error a constant error through the column and trend error an error of constant magnitude that changes sign at the midpoint of the mass of the column the performances of the methods are mapped over error space a researcher knowing approximately the characteristics of the errors of a particular set of doppler radar data can use the results of

the degeneration of internal waves in lakes with sloping topography in a laboratory study we quantified the temporal energy flux associated with the degeneration of basin scale internal waves in closed basins the system is two layer stratified and subjected to a single forcing event creating available potential energy at time zero a downscale energy transfer was observed from the wind forced basin scale motions to the turbulent motions where energy was lost due to high frequency internal wave breaking along sloping topography under moderate forcing conditions steepening of nonlinear basin scale wave components was found to produce a high frequency solitary wave packet that contained as much as 20 of the available potential energy introduced by the initial condition the characteristic lengthscale of a particular solitary wave was less than the characteristic slope length leading to wave breaking along the sloping boundary the ratio of the steepening timescale required for the evolution of the solitary waves to the travel time until the waves shoaled controlled their development and degeneration within the domain the energy loss along the slope the mixing efficiency and the breaker type were modeled using appropriate forms of an internal iribarren number defined as the ratio of the boundary slope to the wave slope amplitude wavelength this parameter allows generalization to the oceanographic context analysis of field data shows the portion of the internal wave spectrum for lakes between motions at the basin and buoyancy scales to be composed of progressive waves both weakly nonlinear waves sinusoidal profile with frequencies near 10 24 hz and strongly nonlinear waves hyperbolic secant squared profile with frequencies near 10 23 hz the results suggest that a periodically forced system may sustain a quasi steady flux of 20 of the potential energy introduced by the surface wind stress to the benthic boundary layer at the depth of the pycnocline

spatial structure of the dominant basin scale internal waves in lake kinneret field data and numerical simulations were used to investigate the effects of basin shape continuous stratification and rotation on the three dimensional structure of the dominant natural basin scale internal wave modes in lake kinneret a stratified lake large enough so that the earth s rotation influences the wave motion the structure of the modes was inferred from power spectral density of measured and simulated isotherm vertical displacements and from rotary power spectral density of isopycnal velocities obtained from numerical simulations the shape of the lake at the level of the thermocline in conjunction with the dispersion relationship determines the horizontal configuration of the natural modes the dominant response to wind forcing was an azimuthal and vertical mode 1 kelvin wave with a natural period of 22 6 h that propagated around the entire basin the second most dominant response was a vertical mode 1 wave with a natural period close to 10 5 h and composed of two counter rotating

three dimensional evolution of simulated long lived squall lines abstract simulations of squall lines using nonhydrostatic convection resolving models have been limited to two dimensions or three dimensions with the assumption of along line periodicity the authors present 3d nonhydrostatic convection resolving simulations produced using an adaptive grid model where the lines are finite in length and the restriction to along line periodicity is removed the base state for the simulations is characterized by weak shallow shear and high convective available potential energy cape an environment in which longlived midlatitude mesoscale convective systems mcss are observed the simulated systems bear strong resemblance to many observed systems suggesting that large scale forcing absent in the horizontally homogeneous environment is not needed to produce many of the distinguishing features of midlatitude mcss in simulations without coriolis forcing the presence of line ends leads to mature symmetric systems characterized by a central region of strong convectio

evolution and structure of the 6 7 may 1985 mesoscale convective system and associated vortex abstract observations collected during the oklahoma kansas pre storm experiment are used to document the evolution and structure of the mesoscale convective system mcs that occurred on 6 7 may 1985 the storm began when a short squall line developed in an area of preexisting thunderstorm activity thunderstorm updrafts along the squall line lifted warm moist air with its southerly momentum to the upper troposphere a broad region of convective outflow and a meso scale updraft region with a mean vertical velocity in excess of 15 10 3 mb s 1 were created a stratiform rain area with an embedded mesovortex formed behind the squall line the vortex resided beneath the deepest upper level outflow the mesovortex altered the wind field and consequently became the principal organizational feature within the mcs a descending current from the storm s rear that depending on location extended from 1 km to the upper troposphere was intensified and focused by the vortex the descending rear inflow had a peak

a theoretical study of cold air damming abstract the dynamics of cold air damming are examined analytically with a two layer steady state model the upper layer is a warm and saturated cross mountain easterly or southeasterly onshore flow the lower layer is a cold mountain parallel northerly jet trapped on the windward eastern side of the mountain the interface between the two layers represents a coastal front a sloping inversion layer coupling the trapped cold dome with the warm onshore flow above through pressure continuity an analytical expression is obtained for the inviscid upper layer flow with hydrostatic and moist adiabatic approximations blackadar s pbl parameterization of eddy viscosity is used in the lower layer equations solutions for the mountain parallel jet and its associated secondary transverse circulation are obtained by expanding asymptotically upon a small parameter proportional to the square root of the inertial aspect ratio the ratio between the mountain height and the radius of inertial oscillation the geometr

basin scale motion in stratified upper lake constance this paper describes experimental and modeled wind induced oscillations in upper lake constance with an emphasis on a coherent understanding of the basin scale internal dynamics in this example of a large and stratified lake data were collected with eight lake diagnostic systems ldss consisting of thermistor chains and wind anemometers the isotherm displacements as measured by the ldss were interpreted using the three dimensional hydrodynamic estuary and lake computer model elcom three types of basin scale waves were found to dominate the wave motion the vertical mode one kelvin wave that had an observed period around 90 h two vertical mode one poincare waves that had periods near 8 h and 12 h and a vertical mode two poincare wave that had a period near 14 h after strong westerly winds upwelling of cold bottom water was observed east of the sill of mainau where the lake s two subbasins connect the width and length ratios of the subbasins spatial variations of the wind field and rotational effects over the lake are shown to play critical roles in the details of the upwelling structure a sudden fall of the isotherms in lake uberlingen formed a surge the reflection of the surge from the northwestern boundary induced a vertical mode two response leading to an intrusion in the metalimnion that caused a three layer velocity structure in the smaller subbasin

internal seiche dynamics in lake geneva we analyzed season long water level records at 12 stations around the lake of geneva local name leman for evidence of internal seiches modified by coriolis force and compared the results with predictions from a two layer numerical model with real bottom topography for typical wind situations results are also compared with those obtained from current and temperature measurements in the lake agreement was satisfactory in all cases model predictions and measurements both indicated that only three internal seiche modes are excited the 1st mode and the 3rd mode which are kelvin seiche oscillations and the 12th mode which is a poincare seiche the model driven by winds from different directions demonstrates that the wind field constrained by the local topography determines which of the modes is generated

the genesis of severe long lived bow echoes abstract a series of idealized simulations using a nonhydrostatic cloud model is used to investigate the genesis of bow echoes a bow shaped system of convective cells that is especially noted for producing long swaths of damaging surface winds it is hypothesized that severe long lived bow echoes represent a dynamically unique form of mesoconvective organization being produced for a restricted range of environmental conditions including a convective available potential energy cape of at least 2000 m2 s2 and vertical wind shears of at least 20 m s 1 over the lowest 2 5 5 km agl the key structural features include a 40 100 km long bow shaped segment of convective cells with a strong rear inflow jet extending to the leading edge of the bow at 2 3 km agl and cyclonic and anticyclonic eddies referred to as bookend vortices on the northern and southern flanks of the bowed segment respectively this structure characteristically develops three to four hours into the lifetime of a convective system a