recent progress in snow and ice research a review of snow and ice research in 1987 1990 is presented focusing on the effects of layers in seasonal snow covers ice mechanics on fresh water and sea ice and remote sensig of polar ice sheets these topics provide useful examples of general needs in snow and ice research applicable to most areas such as better representation in models of detailed processes controlled laboratory experiments to quantify processes and field studies to provide the appropriate context for interpretation of processes from remote sensing

intense sub kilometer scale boundary layer rolls observed in hurricane fran high resolution observations obtained with the doppler on wheels dow mobile weather radar near the point of landfall of hurricane fran 1996 revealed the existence of intense sub kilometer scale boundary layer rolls that strongly modulated the near surface wind speed it is proposed that these structures are one cause of geographically varying surface damage patterns that have been observed after some landfalling hurricanes and that they cause much of the observed gustiness bringing high velocity air from aloft to the lowest observable levels high resolution dow radar observations are contrasted with lower resolution observations obtained with an operational weather radar which underestimated peak low level wind speeds

doppler profiler and radar observations of boundary layer variability during the landfall of tropical storm gabrielle abstract detailed observations of boundary layer structure were acquired on 14 september 2001 prior to and during the landfall of tropical storm gabrielle the mobile integrated profiling system mips and the shared mobile atmospheric research and teaching radar smart r were collocated at the western florida coastline near venice very close to the wind center at landfall prior to landfall the boundary layer was rendered weakly stable by a long period of evaporational cooling and mesoscale downdrafts within extensive stratiform precipitation that started 18 h before landfall the cool air mass was expansive with an area within the 23 c surface isotherm of about 50 000 km2 east northeasterly surface flow transported this cool air off the west coast of florida toward the convergent warm core of the gabrielle and promoted the development of shallow warm and cold fronts that were prominent during the landfall phase airflow properties of the boundary layer around the coastal zone are examined using

wind profiles in tropical cyclone stratiform rainbands over land abstractobservations of 14 stratiform periods in outer tropical cyclone rainbands are used to evaluate wind structure using a velocity azimuth display vad technique applied to kamx miami weather surveillance radar 1988 doppler wsr 88d data these 14 cases occurred over land in southern florida from tropical storm fay and hurricanes gustav and ike during 2008 profiles show a maximum horizontal wind speed between 1000 and 1500 m in height with occasional evidence of a secondary horizontal wind maximum near 3500 5000 m storm relative wind components are calculated and radial wind profiles show a mean transition from radial inflow at low levels to radial outflow around 2500 3000 m altitude the radial inflow maximum is around 500 m while maximum outflow is more variable these profile characteristics are consistent with previous wind observations in rainbands over land and water changes in wind structure within one 4 h period are examined with changes seen linked to the environmental influence on

asymmetric structures in a simulated landfalling hurricane abstract highly asymmetric structures in a landfalling hurricane can lead to the formation of heavy rains wind gusts and tornados at prefered locations relative to the center of the hurricane in this study the development of asymmetric structures in an explicitly simulated idealized hurricane during landfall was investigated it was found that the boundary layer friction and its associated convection produce a low level positive potential vorticity pv band ahead of the hurricane the interaction between the pv band and the eyewall pv ring leads to a temporary weakening and reintensifying cycle asymmetric structures arise from the near discontinuity of the surface friction and the latent heat flux the breaking of the eyewall in the rear quadrants is favorable for the intrusion of the low moist entropy air into the core consequently pv increases significantly in the core in and just above the boundary layer due to the stabilization after the hurricane makes landfall the diabatic heating in the

the effect of relative flow on the asymmetric structure in the interior of hurricanes abstract asymmetric structure of tropical cyclones simulated by the geophysical fluid dynamics laboratory high resolution triply nested movable mesh hurricane model was analyzed emphasis was placed on the quasi steady component of the asymmetric structure in the region of the eyewall it was found that the asymmetry was primarily caused by the relative wind that is the flow entering and leaving the storm region relative to the moving storm a set of idealized numerical experiments was first performed both with a constant and a variable coriolis parameter f and the addition of basic flows that were either constant or sheared with height analysis was then made for one case of hurricane gilbert 1988 to demonstrate that the quasi steady asymmetric structure analyzed in the idealized studies could be identified in this real data case vorticity analysis in the variable f experiment indicated that quasi steady asymmetries resulted in the eyewall region through the effect of vorticity advection due to

transition of the hurricane boundary layer during the landfall of hurricane irene 2011 abstractthe hurricane boundary layer hbl has been observed in great detail through aircraft investigations of tropical cyclones over the open ocean but the coastal transition of the hbl has been

the transition of the hurricane frederic boundary layer wind field from the open gulf of mexico to landfall abstract numerous aircraft ship buoy and land nation data were composited with respect to the center of hurricane frederic for two time periods a 24 h period corresponding to the storm s position in the open gulf of mexico on 12 september 1979 and an 8 h period corresponding to the landfall of frederic near 0400 gmt on 13 september comparison of wind analyses for the two periods indicated a rotation of maximum inflow angles from the southeast to northeast quadrants and a strong frictional decrease of wind speed over land them and other features of the landfall analysis were compared with a model landfall study by moss and jones 1978 the landfall composite wind field was compared with the fujita damage vector analysis to determine the damage time interval and mean wind speed range damage vector directions were found to be well correlated with the surface streamlines with the most severe damage being associated with frederic s northern eyewall ten meter level wind speed data over water vo and

surface wind fields of 1995 hurricanes erin opal luis marilyn and roxanne at landfall hurricanes erin opal luis marilyn and roxanne were the most destructive hurricanes of 1995 at landfall luis and marilyn contained maximum sustained winds marine exposure estimated at near 60 and 46 m s21 respectively the strongest landfalling storm of the 1995 season luis decreased in intensity from a category 4 to 3 on the saffir simpson scale shortly before the eyewall crossed the islands of antigua barbuda st kittsnevis st barthelemy st martin and anguilla hurricane marilyn strengthened as it approached the u s virgin islands with st thomas bearing the brunt of the north and south eyewall winds of 46 m s21 marine exposure and st croix being affected by the relatively weak western eyewall peak winds of 35 40 m s21 marine exposure for luis and marilyn only surface winds with marine exposures were analyzed because of unknown small scale interactions associated with complex island terrain with 500 1000 m elevations wind engineering studies suggest that wind acceleration over blunt ridges can increase or speed up winds by 20 80 topographic effects were evident in damage debris analyses and suggest that an operational method of assessing terrain induced wind gusts such as a scaled down mesoscale model is needed after landfall as a marginal hurricane over central florida hurricane erin regained strength over the gulf of mexico with a well defined radar reflectivity structure erin struck the florida panhandle near navarre beach with maximum sustained surface winds of 35 40 m s21 affecting the destin ft walton area hurricane opal made landfall in nearly the identical area as erin with maximum sustained surface winds of 40 45 m s21 having weakened from an intensity of nearly 60 m s21 only 10 h earlier opal was characterized by an asymmetric structure that was likely related to cold front interaction and an associated midlevel southwesterly jet roxanne struck cozumel mexico with sustained surface winds marine exposure of 46 s21 crossed the yucatan and meandered in the southwest gulf of mexico for several days while in the bay of campeche roxanne s large area of hurricane force winds disabled a vessel which lead to the drowning deaths of five oil industry workers high resolution wind records are critical to preserving an accurate extreme wind climatology required for assessment of realistic building code risks unfortunately power interruptions to automated surface observing stations on the u s virgin islands st croix st thomas and destin florida prevented complete wind records of the eyewall passages of marilyn and opal respectively

the anomalous seismic response of the ground at the tarzana hill site during the northridge 1994 southern california earthquake a resonant sliding block the northridge california earthquake of 17 january 1994 mw 6 7 caused widespread damage in the san fernando valley and adjacent areas in many places the earthquake motion was amplified or strongly modified by the local ground response but the most localized and extreme site effect occurred at tarzana a small hill of gentle topography at the northern edge of the santa monica mountains where strong amplification horizontal peak acceleration of 1 82g and other unusual site effects were recorded at a cdmg accelerograph the anomalous site effect was con fined to a small area 50 m in radius around the station beyond which the ground shaking was down to its theoretically expected level if strong ground motion can vary so much in such a small scale microzonation of urban areas may be an impos sible goal at worst or at best only reliable at those sites where ground response has previously been measured hence to clearly understand the dynamic response of the ground at the tarzana site has become an important problem in earthquake hazard assessment and microzonation this article reports results on the characteristics of the seismic response of the tarzana site based on the investigation of aftershock data recorded on a local array and recorded accelerograms preliminary results strongly suggest that the tarzana hill is formed by an old landslide of the modelo formation such a structure is consistent with the anomalous resonant behavior at coo the observed resonant fre quencies and the complex body wave recordings observed throughout the local array the results also suggest that the 1 82g peak acceleration at co0 could have been caused by a small mw 1 3 earthquake located some 60 m from the station generated by a downslope sliding of the hill with a maximum displacement of 4 cm over a circular area of nearly 50 m in radius the slide was triggered by the north ridge mainshock

boundary layer structure and dynamics in outer hurricane rainbands part i mesoscale rainfall and kinematic structure abstract results of hurricane boundary layer experiments conducted in outer rainbands of hurricanes josephine 1984 and earl 1986 are presented comparisons of precipitation and kinematic structures in these storms and in hurricane floyd 1981 indicate that principal rainbands have common characteristic mesoscale and convective scale features in the boundary layer the two dimensional mesoscale structure suggests that these rainbands are made up of a linear aggregate of cellular reflectivity elements on the inner upshear side of the band and stratiform rain on the outer downshear side the bands are oriented perpendicular to the shear above the boundary layer and cells move downband at about 85 of the density weighted mean wind speed of the 0 2 6 km layer the boundary layer wind field is strongly influenced by the rainband with alongband and crossband wind maxima located on the outer side of the band axis and minima 4 8 km to the inner side maximum crossband convergence and cyclonic shear vor

the dynamics of boundary layer jets within the tropical cyclone core part i linear theory abstract observations of wind profiles within the tropical cyclone boundary layer have until recently been quite rare the recent massive increase in observations due to the operational implementation of the global positioning system dropwindsonde has emphasised that a low level wind speed maximum is a common feature of the tropical cyclone boundary layer here is proposed a mechanism for producing such a maximum whereby strong inward advection of angular momentum generates the supergradient flow the processes that maintain the necessary inflow against the outward acceleration resulting from gradient wind imbalance are identified as being i vertical diffusion ii vertical advection and iii horizontal advection two complementary tools are used to diagnose the properties and dynamics of the jet the first presented here is a linear analytical model of the boundary layer flow in a translating tropical cyclone it is an extension of the classical ekman boundary layer model as well as of earlier w

numerical simulation of typhoon gladys 1994 and its interaction with taiwan terrain using the gfdl hurricane model abstract numerical integrations using the geophysical fluid dynamics laboratory gfdl hurricane model were performed to study the evolution of typhoon gladys 1994 and its interaction with the taiwan terrain consistent with most previous studies the taiwan topography results in the deceleration of gladys s translation speed and southward deviation as it approaches taiwan on the other hand gladys accelerates northwestward while passing taiwan which is likely to be related to the moist processes and differs from the track pattern in the dry model of lin et al although the gfdl hurricane model forecast underestimates gladys s intensity the model can capture the evolution of gladys s intensity especially its weakening during landfall which is primarily due to the cutoff of the water vapor supply in the boundary layer as gladys approached the taiwan terrain other mesoscale phenomena including the pattern of heavy precipitation and the formation of secondary lows are well simulated by the model

doppler profiler observations of hurricane georges at landfall the boundary layer bl within the western eyewall of hurricane georges during its landfall on the mississippi coastline is described using measurements from a mobile doppler profiling system the turbulent layer depth roughly proportional to the bl wind magnitude attained a height of about 2 km above ground level agl during maximum measured winds of 40 45 m s 1 near 0 5 km altitude the bl contains updrafts and downdrafts of 1 3 m s 1 magnitude that may be associated with the vertical branches of horizontal convective rolls or wind streaks recently identified within landfalling hurricanes these turbulent eddies and the large vertical shear of horizontal winds within the bl above and below the jet maximum within the eyewall appear to produce a uniform mass concentration of relatively small raindrops within the bl

mesoscale and convective structure of a hurricane rainband abstract the mesoscale thermodynamic kinematic and radar structure of a hurricane floyd rainband observed on 7 september 1981 is presented data are from 26 aircraft passes through the rainband from 150 to 6400 m a composite technique which presents rainband structure as a function of distance from the storm circulation center reveals inflow from the outer edge of the band and a partial barrier to this flow below 3 km in the direction parallel to rainband orientation radar reveals cellular reflectivity structure on the upwind and central portions of the rainband the frequency of cellular precipitation decreases in favor of stratiform precipitation further downwind as the band spirals gradually towards the eyewall in the radial direction a decrease of 12 k in e is observed across the rainband in the subcloud layer convective scale up and downdrafts that are associated with cellular reflectivity structure are hypothesized to be responsible for the thermodynamic modification of the cloud and subc

the evolution of hurricane danny 1997 at landfall doppler observed eyewall replacement vortex contraction intensification and low level wind maxima danny made landfall as a minimal hurricane on the alabama coast on 19 july 1997 after drifting over mobile bay for over 10 h danny s unusually close proximity to the doppler radar wsr 88d in mobile provided an unprecedented view of the storm s complex and dramatic evolution during a prolonged landfall event over a 1 day period base reflectivity and velocity products were combined with aircraft reconnaissance information to detail the formation of concentric eyewalls and complete evolution of an eyewall replacement cycle this highly symmetric hurricane then underwent a rapid asymmetric transition in mobile bay during which a small eyewall mesovortex developed adjacent to intense convection in the western eyewall radar estimated rainfall increased dramatically during the asymmetric phase rates exceeded 100 mm h21 for nine consecutive hours west of the center while precipitation nearly vanished to the east changes in the distribution of precipitation corresponded with changes in the low level wind velocity structure a 25 h temporal composite of wsr 88d base velocities displayed axisymmetric intensification and contraction of danny s core during the eyewall replacement cycle later the asymmetric phase was dominated by further contraction and intensification on the west side only in the western eyewall a persistent boundary layer wind maximum evolved and contracted to a radius of only 10 13 km from the center concurrently eastside boundary layer winds diminished as the maximum winds rose to 1 1 5 km altitude and the radius expanded danny reached maximum intensity during this asymmetric phase in mobile bay with base velocities 44 m s21 85 kt at 600 m elevation eyewall contraction in the meandering storm combined with climatologically elevated ssts in shallow mobile bay probably played significant roles in danny s continued intensification there discrepancies arose in determining danny s structural patterns intensity and evolution during landfall interpretation varied depending on the type of platform used to observe the storm the continual sampling of the storm by the nearby wsr 88d provided detail not available from aircraft data alone doppler base velocities in the intense westside convection were much stronger than measured at flight level yet the opposite was true in the storm s drier east side doppler radar showed that the westside base velocity maxima were confined to the boundary layer 600 700 m and represented a slightly conservative estimate of maximum surface gusts recorded at dauphin island during the passage of danny s convective eyewall thus winds probably were even stronger at boundary layer levels below the wsr 88d s lowest scan elevation the shallowness and persistence of danny s boundary layer velocity maxima stressed the need for accurate wind information in the lowest few hundred meters of a tropical cyclone s eyewall for a better indication of the storm s true intensity and near surface wind velocities

the layered character of snow covers snow studies have generally ignored the layered nature of snow covers having achieved a good understanding of the properties and processes in homogeneous snow snow scientists should develop more insight into the evolution of the layers and their effects on overall snow response many of the outstanding problems in snow studies can only be solved by dealing with snow as a layered medium the various mechanisms by which layers are generated their effects and some of the outstanding research problems are described here

helical circulations in the typhoon boundary layer 1 low level wind data from the wsr 88d in guam obtained in typhoon dale 1996 and typhoon keith 1997 are analyzed for coherent structures consistent with the results of previous studies of atlantic hurricanes velocity anomalies associated with coherent structures were found in the boundary layer of both storms a total of 99 cases of coherent structures also known as roll vortices were documented during a 6 h evaluation period for each storm storm relative roll location roll vorticity asymmetries in the upward and downward momentum fluxes and signatures of circulations transverse to the mean flow associated with roll circulations were explored the effects of terrain and convective precipitation systems such as rainbands on the occurrence of rolls were investigated the results support and extend prior findings of roll observations and can be used to help validate theoretical and numerical models of coherent structures within tropical cyclones moreover the wind variations documented in this study may have application for wave runup and wind damage potential in tropical cyclones

the effect of snow on vehicle generated seismic signatures vehicle generated seismograms recorded under summer and winter conditions at fort devens ma are analyzed and compared the appearance and frequency content of the recorded ground motion change dramatically from summer to winter when a 0 7 m thick snow layer is present the snow layer causes a 10 db reduction in the amplitude of the acoustically coupled signal relative to the signal generated where the vehicle contacted the ground a magnitude squared coherence analysis and a simple wiener prediction model of the acoustic transmission near the observation point reinforce this conclusion

changes in the low level kinematic and thermodynamic structure of hurricane alicia 1983 at landfall abstract aircraft land station and buoy data were composited with respect to the center of hurricane alicia 1983 for three 8 h periods corresponding to prelandfall in the open gulf of mexico landfall in the galveston area and postlandfall in the vicinity of houston comparison of the wind analyses before during and after landfall emphasizes the land sea frictional asymmetry at landfall in addition other asymmetries in the surface wind field and differences between the flight level and the surface wind fields are revealed the asymmetric structure of the surface wind field may be interpreted as having resulted from the combined effects of land sea roughness differences background environmental flow and storm translation the land sea frictional difference acted to oppose the mean vortex flow over land and reinforce it over water the southwest background environmental flow acted nearly parallel to the coastline producing surface inflow on the left side and outflow on the right side while the

a simulation of hurricane landfall with a numerical model featuring latent heating by the resolvable scales abstract a nested grid hurricane model is used to transport a strong vortex over a straight coastline at 4 m s 1 the track at landfall of the vortex is 20 km to the left of a control simulation without land just before landfall a 15 km amplitude trochoidal oscillation of the vortex track occurs this amplitude is nearly double that of similar oscillations of the control simulation about 10 h before landfall a spiral rainband nearly surrounds the vortex at radii of 135 km this rainband has a weak secondary maximum in the tangential wind and is the model analog of the secondary eyewalls observed by willoughby et al in several hurricanes the rainfall in spiral rainbands diminishes during the 7 h before landfall however rainfall in the inner core of the vortex is greater during landfall than in the control simulation the greatest rainfall accumulation is to the right of the vortex however compared with the control simulation rainfall is greater to the left and less to the right of the vorte

multiplatform observations of boundary layer structure in the outer rainbands of landfalling typhoons this paper analyzes data collected from a new set of observational platforms in the coastal area of china which consist of a mobile observation system meteorological tower automatic weather station and doppler radars to investigate the mean and turbulent boundary layer structure and evolution during the landfall of typhoons an example of these data is provided from typhoon morakot 2009 vertical profiles of wind velocities and thermodynamic parameters from the observed data allow us to identify different boundary layer structures during and after landfall these structures sampled in regions of the outer core are stratified into periods where convection is occurring termed convective and periods where convection has recently 2 h occurred termed postconvective data analyses show that the thermodynamic mixed layer depth and inflow layer depth are higher during the convective period than the postconvective period the mixed layer depth is found to be within the strong inflow layer but the height of the maximum tangential wind speed is above the inflow layer during both periods contrary to recent observational studies of the boundary layer structure of tropical cyclones over water high frequency wind data show that momentum flux turbulent kinetic energy tke and integral length scales of wind velocities are all much larger during the convective period than the postconvective period the results suggest that convective downdrafts may play an important role in modulating turbulent flux tke vertical mixing and boundary layer recovery processes

a multiscale numerical study of hurricane andrew 1992 part vi small scale inner core structures and wind streaks abstract the objectives of part vi of this series of papers are to a simulate the finescale features of hurricane andrew 1992 using a cloud resolving grid length of 2 km b diagnose the formation of small scale wind streaks and c perform sensitivity experiments of varying surface fluxes on changes in storm inner core structures and intensity as compared to observations and a previous 6 km model run the results show that a higher resolution explicit simulation could produce significant improvements in the structures and evolution of the inner core eyewall and spiral rainbands and in the organization of convection the eyewall becomes much more compact and symmetric with its width decreased by half and the radius of maximum wind is reduced by 10 to 20 km a zone of deep and intense potential vorticity pv is formed at the edge of the eye a ring of maximum pv is collocated in regions of maximum upward motion in the eyewall and interacts strongly with the eyewall convection the convective cor

the dynamics of boundary layer jets within the tropical cyclone core part ii nonlinear enhancement abstract observations of wind profiles within the tropical cyclone boundary layer until recently have been quite rare however the recent spate of observations from the gps dropsonde have confirmed that a low level wind speed maximum is a common feature of the tropical cyclone boundary layer in part i a mechanism for producing such a maximum was proposed whereby strong inward advection of angular momentum generates the supergradient flow the processes that maintain the necessary inflow against the outward acceleration due to gradient wind imbalance were identified as being i vertical diffusion ii vertical advection and iii horizontal advection and a linear analytical model of the boundary layer flow in a translating tropical cyclone was presented and used to diagnose the properties of the jet and the near surface flow a significant shortcoming was that the jet was too weak which was argued to be due to the neglect of vertical advection here a high resolution dry hydrostatic numerical

determining snow water equivalent by acoustic sounding the possibility of determining snow water equivalent swe by the use of an acoustic impulse was assessed at two field locations in saskatchewan and british columbia canada these sites represent cold windswept prairie and temperate deep mountain snowcovers a continuous frequency swept acoustic wave was sent into the snowpack and received signal processing was then subsequently used to estimate the depth and density of each snow layer by a recursive relationship involving frequency modulated continuous wave fmcw radar and seismological techniques from this method it is also shown that the tortuosity of snow can be estimated data collected by gravimetric sampling was used as comparison to the swe values determined by the use of acoustic sounding the results showed that for the saskatchewan sites the correlation between the measured and the modeled values of swe was 0 86 whereas at the british columbia sites the correlation was 0 78 the difference in the correlations was interpreted as being due to additional acoustic measurement error at the british columbia sites caused by higher liquid water contents and more layers in the snowpack the measured and the modeled swe for saskatchewan snowpacks with high liquid water contents were found to be weakly associated with correlations of 0 30 the acoustically determined values of tortuosity were close to unity 1 which is in agreement with the values characteristic of snow as a porous substance further research is necessary to determine whether this technique can be applied to snow in other environmental conditions copyright 2007 john wiley sons ltd

a high resolution prediction study of two typhoons at landfall abstract the landfall on the china coast of two typhoons has been studied using both observational diagnostics and a 30 km 19 level prediction system the typhoons vary in size and in the amount of rainfall they produced after landfall use is made of a synthetic typhoon insertion scheme and the relatively dense observational network around china to provide objective analyses the authors illustrate that in the forecast model as the typhoon s outer circulation impinges on the china coast an outer cloud band develops offshore parallel to the coast extending cyclonically north and east during this time the inner circulation weakens as the outer cloud band strengthens the center of the circulation approaches it from the east and there is a short term intensification of both the inner region vertical motion and vorticity following landfall rapid dissipation of the core occurs using detailed diagnostics from the model integration the changing thermal structure of the forecast vortex is illustrated

surface cold pools in the outer rainbands of tropical storm hanna 2008 near landfall surface mesonet observations were used to document the structure of prominent cold pools associated with twoconvectiveouterrainbandsoftropicalstormhannaon5september2008 thedevelopingrainbandswere located 400 kmnorth ofthestormcenterastheycrossedthecarolinacoastlineandpassedover themesonet the combination of moderate cape moderate low level cross band vertical shear and dry midlevel air provided an environment supportive of surface cold pool formation and long lived quasi linear convection both rainbands exhibited multiple outward tilting convective cells and discrete line segments centered within a narrow zone of nearly continuous stratiform precipitation the mesonet observations provided a unique along and cross band perspective of discrete cold pools situated beneath andbehind the convection portions of each rainband prominentcold pools extended 40 80 km behind the rainband s leading edge and exhibited maximum potential temperature mixing ratio and equivalentpotentialtemperaturedeficitsof2 4 k 1 2 g kg 21 and4 10 k respectively inthecross banddirection convergence of storm relative inflow along the cold pool leading edge was coincident with a modest high pressure anomaly while inflow divergence prevailed through the cold pool and rainfall maxima several cold pools expanded along band while being advected downband by the prevailing cyclonic flow cold pool wakes were observed more than 50 km behind the rainband leading edge and up to 20 km downband from intense convection variations in cold pool intensitywerenotwell correlated with convectiveintensity rainfallrate or the degree of midlevel dryness implications of prominent cold pools on tropical cyclone convection size and intensity are discussed

mesoscale aspects of the downshear reformation of a tropical cyclone the downshear reformation of tropical storm gabrielle 2001 was investigated using radar reflectivity and lightning data that were nearly continuous in time as well as frequent aircraft reconnaissance flights initially the storm was a marginal tropical storm in an environment with strong 850 200 hpa vertical wind shear of 12 13 m s 1 and an approaching upper tropospheric trough both the observed outflow and an adiabatic balance model calculation showed that the radial vertical circulation increased with time as the trough approached convection was highly asymmetric with almost all radar return located in one quadrant left of downshear in the storm reconnaissance data show that an intense mesovortex formed downshear of the original center this vortex was located just south of rather than within a strong downshear left lightning outbreak consistent with tilting of the horizontal vorticity associated with the vertical wind shear the downshear mesovortex contained a 972 hpa minimum central pressure 20 hpa lower than minimum pressure in the original vortex just 3 h earlier the mesovortex became the new center of the storm but weakened somewhat prior to landfall it is argued that dry air carried around the storm from the region of upshear subsidence as well as the direct effects of the shear prevented the reformed vortex from continuing to intensify despite the subsequent weakening of the reformed center it reached land with greater intensity than the original center it is argued that this intensification process was set into motion by the vertical wind shear in the presence of an environment with upward motion forced by the upper tropospheric trough in addition the new center formed much closer to the coast and made landfall much earlier than predicted such vertical shear induced intensity and track fluctuations are important to understand especially in storms approaching the coast

numerical simulations of the extratropical transition of floyd 1999 structural evolution and responsible mechanisms for the heavy rainfall over the northeast united states abstract this paper examines the extratropical transition et of hurricane floyd along the u s east coast on 16 17 september 1999 using the fifth generation pennsylvania state university ncar mesoscale model mm5 down to 1 33 km horizontal grid spacing the 36 h mm5 simulation reproduced the basic features of the et event such as the track of floyd the development of a deep and intense baroclinic zone along the coast and its associated precipitation evolution and the tendency for the heavy 30 cm precipitation to fall in a relatively narrow 30 40 km wide band just inland of the coast however the mm5 overpredicted the moderate 10 20 cm precipitation amounts near the coast by 40 50 as the horizontal grid spacing was reduced to 1 33 km the mm5 was used to diagnose the evolution of the enhanced baroclinic zone and associated heavy precipitation to the north of floyd a deep layer of deformation frontogenesis extended from the surface to 400 mb as a result of confluence between the southeasterl

evolution of the inflow boundary layer of hurricane gilbert 1988 abstract on 12 september 1988 the two noaa wp 3d aircraft conducted an experiment in and around an intense outer rainband located 175 km southeast of the center of hurricane gilbert radial height cross sections along a constant azimuth reveal a rapid and an exceptionally large increase of the equivalent potential temperature e of the inflow but in a region radially outward from the rainband kinematic analyses that incorporate both in situ and pseudo dual doppler data illustrate that the inflow is only 2 km deep and strongly divergent prior to reaching the convective core of the band the doppler derived wind fields which compare favorably with the in situ wind fields demonstrate that there is a radially outward or return flow directly above the inflow soundings show that this return flow is unusually moist despite being dominated by mesoscale descent which contrasts the dry conditions found under the anvil of virtually all tropical mesoscale convective systems a one dimensional general structure