the life cycle of thunderstorm gust fronts as viewed with doppler radar and rawinsonde data abstract this paper presents the time dependent analysis of the thunderstorm gust front with the use of project nimrod data rhi cross sections of reflectivity and doppler velocity are constructed to determine the entire vertical structure the life cycle of the gust front is divided into four stages 1 the formative stage 2 the early mature stage 3 the late mature stage and 4 the dissipation stage a new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground while carrying some of the smaller precipitation ahead of the main echo core of the squall line this feature is called a precipitation roll as determined from rawinsonde data the cold air behind the gust front accounts for the observed surface pressure rise calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow the equation governing the propagation speed of a density current accurately predicts t

analysis of a nontornadic storm during vortex 95 abstract analysis of a supercell storm that did not produce a tornado near hays kansas is presented a well defined midlevel mesocyclone was apparent throughout most of the storm s life cycle numerous shallow circulations were observed along the rear flank gust front during the data collection period six of these circulations strengthened into intense low level mesocyclones each of these mesocyclones failed to produce a tornado the strongest low level mesocyclone referred to as vortex 4 underwent a life cycle that was consistent with other tornadic mesocyclones documented in the literature these results illustrate that the presence of a long lived mesocyclone at low levels is not sufficient for tornadogenesis to occur the kinematic structure of the low level mesocyclone that did not produce a tornado is compared with a tornadic mesocyclone from another storm in order to understand the characteristic differences between these circulations the results lead to the conclusion that the presence of

surface elevation change and susceptibility of coastal wetlands to sea level rise in liaohe delta china abstract the liaohe delta in china is an ecologically and commercially important wetland system under threat from sea level rise and marsh subsidence sediments deposited in coastal marshes could offer wetlands a potentially important means for adjusting surface elevation with rising sea level yet coastal wetland stability in liaohe delta is not well understood due to limited data from long term experiments in this study wetland surface elevation and vertical accretion were measured from 2011 to 2015 using a surface elevation table set and feldspar marker horizons in two phragmites and two suaeda marshes receiving liaohe river water the analysis shows that the phragmites marshes exhibited higher rates of marsh accretion and elevation change than the suaeda marshes the two phragmites marsh sites had average surface elevation change rates at 8 8 and 9 3 mm yr 1 vertical accretion at 17 4 and 17 6 mm yr 1 and shallow subsidence at 8 6 and 8 3 mm yr 1 the average rates of elevation change vertical accretion and shallow subsidence at two suaeda marsh sites were 5 8 and 6 3 mm yr 1 13 6 and 14 8 mm yr 1 and 7 8 and 8 5 mm yr 1 respectively the trends suggest that coastal marshes in liaohe delta are experiencing changes in average soil elevation that range from a net increase of 0 3 mm y 1 to 6 9 mm y 1 relative to averaged sea level rise in bohai sea reported by the 2016 state oceanic administration people s republic of china projection 2 4 5 5 mm y 1 which indicated that the four wetland sites would adjust to the sea level rise and even continue to gain elevation especially for the phragmites sites nevertheless the vulnerability of coastal wetlands in liaohe delta need further assessment considering the accelerated sea level rise the high rate of subsidence and the declining sediment delivery owing to anthropogenic activities such as dam constructions in the river basin

mesoscale to microscale simulations over complex terrain with the immersed boundary method in the weather research and forecasting model abstractimprovements to the weather research and forecasting wrf model are made to enable multiscale simulations over highly complex terrain with dynamically downscaled boundary conditions from t

kinematic constraints on glacier contributions to 21st century sea level rise on the basis of climate modeling and analogies with past conditions the potential for multimeter increases in sea level by the end of the 21st century has been proposed we consider glaciological conditions required for large sea level rise to occur by 2100 and conclude that increases in excess of 2 meters are physically untenable we find that a total sea level rise of about 2 meters by 2100 could occur under physically possible glaciological conditions but only if all variables are quickly accelerated to extremely high limits more plausible but still accelerated conditions lead to total sea level rise by 2100 of about 0 8 meter these roughly constrained scenarios provide a most likely starting point for refinements in sea level forecasts that include ice flow dynamics

the numerical simulation of non supercell tornadogenesis part i initiation and evolution of pretornadic misocyclone circulations along a dry outflow boundary abstract high resolution three dimensional simulations are used to study misocyclone initiation and development along the leading edge of an outflow boundary model conditions were designed such that this development could be simulated independent of moist processes the outflow boundary is allowed to propagate into a region of southerly low level flow which results in a vertical vortex sheet along the outflow s leading edge lobe and cleft instabilities follow and provide perturbations for the subsequent development of horizontal shearing instabilities these growing instabilities are the inaugural circulations of leading edge misocyclones with wavelengths ranging from 1 6 to 3 2 km the structure of these modeled misocyclones compares favorably to observed pretornadic misocyclones along outflow boundaries in northeast colorado vortex sheet dynamics are observed to exert substantial control over the structure of the evolving outflow leading edge the vertical vortex sheet passes through discrete evoluti

a study of the tornadic region within a supercell thunderstorm abstract the transition of a supercell thunderstorm into its tornadic phase is investigated through high resolution numerical cloud model simulations initiated within the interior portion of a previously simulated mature supercell storm with the enhanced grid resolution the low level cyclonic vorticity increases dramatically and the gust front rapidly occludes as small scale downdrafts develop in the vicinity of the low level center of circulation as the occlusion progresses a ring of high vorticity air surrounds the circulation center and could be conducive to multiple vortex tornado formation numerous features of the simulated transition bear resemblance to those observed in tornadic storms in the model simulation the large low level vorticity is generated through the tilting and intense stretching of air from the inflow side of the storm this vertical vorticity is derived from the horizontal vorticity of the environmental shear and also from horizontal vorticity generated solenoidally as low l

a numerical simulation of cyclic mesocyclogenesis abstract a three dimensional nonhydrostatic numerical model the advanced regional prediction system is used to study the process of cyclic mesocyclogenesis in a classic supercell thunderstorm during the 4 h simulation the storm s mesocyclone undergoes two distinct occlusions with the beginning of a third indicated at the end of the simulation the occlusion process exhibits a period of approximately 60 min and is qualitatively similar in each case initial midlevel 3 7 km mesocyclogenesis proceeds according to the classic picture that is via tilting of streamwise environmental vorticity the development of an evaporatively driven rear flank downdraft rfd signals the beginning of the occlusion process the developing rfd wraps cyclonically around the mesocyclone causing the gust front to surge outward simultaneously the occluding mesocyclone rapidly intensifies near the surface trajectory analyses demonstrate that this intensification follows from the tilting and stretching of near ground

simulation and analysis of tornado development and decay within a three dimensional supercell thunderstorm abstract a three dimensional numerical simulation using a two way interactive nested grid is to study tornado genesis within a supercell during a 40 minute period two tornadoes grow and decay within the storm s mesocyclone the tornadoes have life spans of approximately 10 minutes maximum ground relative surface wind speeds exceed 60 m s 1 during both tornadoes and horizontal pressure gradients reach 18 hpa km 1 during the second tornado comparison of the simulated storm evolution with doppler and field observations of supercells and tornadoes shows many similar features vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm tornadogenesis is initiated when mesocyclone rotation increase above cloud base the increased rotation generates lower pressure in the mesocyclone increasing the upward pressure gradient forces the upwar

stratospheric gravity wave fluxes and scales during deepwave abstractduring the deep propagating gravity wave experiment deepwave project in june and july 2014 the gulfstream v research aircraft flew 97 legs over the southern alps of new zealand and 150 legs over the tasman sea and southern ocean mostly in the low stratosphere at 12 1 km altitude improved instrument calibration redundant sensors longer flight legs energy flux estimation and scale analysis revealed several new gravity wave properties over the sea flight level wave fluxes mostly fell below the detection threshold over terrain disturbances had characteristic mountain wave attributes of positive vertical energy flux efz negative zonal momentum flux and upwind horizontal energy flux in some cases the fluxes changed rapidly within an 8 h flight even though environmental conditions were nearly unchanged the largest observed zonal momentum and vertical energy fluxes were mfx 550 mpa and efz 22 w m 2 respectively a wide variety of disturbance scales were found at flight level over

linear and nonlinear propagation of supercell storms abstract a nonlinear formula for updraft motion in supercell storms is derived from petterssen s formula for the motion of systems and the vertical equation of motion and tested on form preserving disturbances at each level continuous propagation of an updraft maximum is determined largely by the horizontal gradient of the nonhydrostatic vertical pressure gradient force nhvpgf at the updraft center the nhvpgf is deduced from the formal solution of the poisson equation for nonhydrostatic pressure in anelastic flow subject to homogeneous neumann boundary conditions at the ground and top boundary recourse also is made to published fields of partitioned vertical pressure gradient force updraft motion is partitioned into parts forced by horizontal gradients of hydrostatic pressure linear interaction between the environmental shear and updraft and nonlinear dynamical effects the dynamics of supercell storms for nearly straight and highly curved hodographs are found to be different nonlinear rotation

the ocean land atmosphere model olam part ii formulation and tests of the nonhydrostatic dynamic core abstract the dynamic core of the ocean land atmosphere model olam which is a new global model that is partly based on the regional atmospheric modeling system rams is described and tested olam adopts many features of its predecessor but its dynamic core is new and incorporates a global geodesic grid with triangular mesh cells and a finite volume discretization of the nonhydrostatic compressible navier stokes equations the spatial discretization of horizontal momentum is based on a c staggered grid and uses a method that has not been previously applied in atmospheric modeling the temporal discretization uses a unique form of time splitting that enforces consistency of advecting mass flux among all conservation equations olam grid levels are horizontal and topography is represented by the shaved cell method aspects of the shaved cell method that pertain to the olam discretization on the triangular mesh are described and a method of conserving momentum in shaved cells on a c staggered grid is p

drowning of the mississippi delta due to insufficient sediment supply and global sea level rise global sea level rise reduced sediment supply and subsidence threaten the stability of the mississippi delta calculations of riverine sediment load and storage indicate that 10 000 13 500 km2 of the delta could be submerged by ad 2100

the dependence of numerically simulated convective storms on vertical wind shear and buoyancy abstract the effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three dimensional numerical cloud model by varying the magnitude of buoyant energy and one directional vertical shear over a wide range of environmental conditions associated with severe storms the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature these include short lived single cells certain types of multicells and rotating supercells the relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and in particular suggests optimal conditions for the development of supercell type storms applications of this parameter to well documented severe storm cases agree favorably with the model results suggesting both the value of the model in studying these modes of convection as well as the value of this representation i

vegetation change and surface elevation dynamics in estuarine wetlands of southeast australia vertical accretion and surface elevation trends were studied in mangrove and saltmarsh wetlands in southeast australia a total of 69 surface elevation tables each associated with three feldspar marker horizons was deployed in 10 wetlands across 7 estuaries and monitored for three years saltmarsh and mangrove vegetation distributions were mapped for the same estuaries and elevation characteristics of the wetlands were modelled rates of vertical accretion were found to correlate with tidal range no relationship was found between rates of vertical accretion and surface elevation increase a positive relationship was demonstrated between contemporary rates of saltmarsh surface elevation change and longer term rates of mangrove encroachment into saltmarsh we conclude that landward mangrove encroachment may be facilitated by local factors contributing to saltmarsh compaction during drought conditions

large eddy simulation over complex terrain using an improved immersed boundary method in the weather research and forecasting model abstractthe weather research and forecasting wrf model is increasingly being used for higher resolution atmospheric simulations over complex terrain with increased resolution resolved terrain s

high precision measurements of wetland sediment elevation ii the rod surface elevation table abstract a new high precision device for measuring sediment elevation in emergent and shallow water wetland systems is described the rod surface elevation table rset is a balanced lightweight mechanical leveling device that attaches to both shallow 1 m in order to be stable the pipe is driven to refusal but typically to a depth shallower than the rod bench mark because of greater surface resistance of the pipe thus the rset makes it possible to partition change in sediment elevation over shallower e g the root zone and deeper depths of the sediment profile than is possible with the set the confidence intervals for the height of an individual pin measured by two different operators with the rset under laboratory conditions were 1 0 and 1 5 mm under field conditions confidence intervals for the measured height of an individual pin ranged from 1 3 mm in a mangrove forest up to 4 3 mm in a salt marsh

a three dimensional numerical simulation of splitting severe storms on 3 april 1964 abstract a three dimensional numerical storm model is used to investigate the observed splitting of several reflectivity echoes on 3 april 1964 in oklahoma representative soundings from this day exhibit a nearly one directional environmental wind shear vector and the presence of strong low level wind shear in the numerical simulation an initial cloud splits into two long lived rotating storms one that moves to the left of the mean winds and the other to the right the left moving storm develops more slowly than the right moving one due to the deviation of the environmental wind hodograph from a straight line below 1 km further the left mover eventually splits convergence induced by the cold low level storm outflow plays a major role in the development of both the first and second splits however the second split appears to be dynamically different than the first as the left moving updraft remains essentially unchanged while a new updraft forms immediately adjacent to it because of the different p

an immersed boundary method for the weather research and forecasting model this paper describes an immersed boundary method that facilitates the explicit resolution of complex terrain within the weather research and forecasting wrf model mesoscale models such as wrf are increasingly used for high resolution simulations particularly in complex terrain but errors associated with terrain following coordinates degrade the accuracy of the solution the use of an alternative gridding technique known as an immersed boundary method alleviates coordinate transformation errors and eliminates restrictions on terrain slope that currently limit mesoscale models to slowly varying terrain simulations are presented for canonical cases with shallow terrain slopes and comparisons between simulations with the native terrain following coordinates and those using the immersed boundary method show excellent agreement validation cases demonstrate the ability of the immersed boundary method to handle both dirichlet and neumann boundary conditions additionally realistic surface forcing can be provided at the immersed boundary by atmospheric physics parameterizations which are modified to include the effects of the immersed terrain using the immersed boundary method the wrf model is capable of simulating highly complex terrain as demonstrated by a simulation of flow over an urban skyline

the association of significant tornadoes with a baroclinic boundary on 2 june 1995 abstract on 2 june 1995 the large scale environment of eastern new mexico and western texas was generally favorable for the occurrence of supercells because of the presence of strong deep shear and storm relative helicity as well as sufficient convective available potential energy cape indeed many supercells occurred but the only storms to produce tornadoes were those supercells that crossed or developed and persisted on the immediate cool side of a particular outflow boundary generated by earlier convection surface conditions vertical vorticity and horizontal vorticity near this boundary are documented using conventional and special observations from the vortex field program it is shown that the boundary was locally rich in horizontal vorticity had somewhat enhanced vertical vorticity and enhanced cape theoretical arguments indicate that the observed horizontal vorticity around 1 10 2 s 1 largely parallel to the boundary can be readily produced with the type of buoyancy contrast obse

the garden city kansas storm during vortex 95 part ii the wall cloud and tornado abstract on 16 may 1995 a supercell storm produced an f1 tornado near garden city kansas during vortex verification of the origins of rotation in tornadoes experiment this event provided the first opportunity to synthesize data collected by a new airborne radar platform called eldora electra doppler radar developed by the national center for atmospheric research the evolution of the low and midlevel mesocyclone was presented in a companion paper in this paper the sequence of events that triggered tornadogenesis within the mesocyclone circulation is shown the occlusion downdraft documented in part i appears to promote vortex breakdown resulting in the generation of multiple vorticity centers it is one of these centers that intensifies into the garden city tornado the structural relationship between the observed wall cloud and the radar reflectivity associated with the developing hook echo and the kinematic wind field is also presented although the wall cloud appears to be uniform there

improved cryogenic coring device for sampling wetland soils this paper is the third in a series on the design and construction knaus 1986 and improvements knaus and cahoon 1990 of a cryogenic soil coring device cryocorer freezing wetland soils in place during sampling eliminates compaction dewatering and loss of flocculent material at the water sediment interface the cryocorer is suitable for sampling soils of emergent marsh and mangrove forests as well as shallow water bottoms although it has been used primarily for the former a small diameter frozen soil core minimizes disruption of the surface can be evaluated immediately for overall quality and can be used to measure soil profiles and subsample for further analysis the cryocorer continues to be used in studies of wetland accretion and soil bulk density throughout the us concomitant with the increased use of the device improvements in cryocorer design and application have occurred reported here are improvements in design that have been made since 1992 with references to wetland research in which the cryocorer has been used extensively

low level mesocyclonic concentration by nonaxisymmetric transport part i supercell and mesocyclone evolution an idealized simulation of a supercell using the regional atmospheric modeling system rams was able to produce a low level mesocyclone near the intersection of the forward and rear flank downdrafts the creation of the low level mesocyclone is similar to previous studies after 3600 s the low level mesocyclone underwent a period of rapid intensification during which its form changed from an elongated patch to a compact center this transition was also accompanied by a sudden decrease in pressure to 12 mb below that of the neighboring flow and was found to occur even in the absence of nested grids it is shown that the stage of strong intensification does not begin aloft as in the dynamic pipe effect and then descend to the surface rather the vortex is initiated near the surface and then builds upward the process is completed in 5 min and the final vortex can be clearly distinguished from the larger scale mesocyclone at the cloud base the reduction of pressure can be explained as a consequence of the evacuation of mass in the horizontal convergence equation this is in contrast to axisymmetric models of vortex intensification which generally rely on the evacuation of mass in the vertical divergence equation in the latter cases a positive horizontal convergence tendency is what initiates the concentrated vortex however nondivergent models prove that vorticity concentration can occur in the absence of any horizontal convergence here the concentration is associated with a negative horizontal convergence tendency

mississippi delta subsidence primarily caused by compaction of holocene strata coastal subsidence causes sea level rise shoreline erosion and wetland loss which poses a threat to coastal populations this is especially evident in the mississippi delta in the southern united states which was devastated by hurricane katrina in 2005 the loss of protective wetlands is considered a critical factor in the extensive flood damage the causes of subsidence in coastal louisiana attributed to factors as diverse as shallow compaction and deep crustal processes remain controversial current estimates of subsidence rates vary by several orders of magnitude here we use a series of radiocarbon dated sediment cores from the mississippi delta to analyse late holocene deposits and assess compaction rates we find that millennial scale compaction rates primarily associated with peat can reach 5mm per year values that exceed recent model predictions locally and on timescales of decades to centuries rates are likely to be 10 mm or more per year we conclude that compaction of holocene strata contributes significantly to the exceptionally high rates of relative sea level rise and coastal wetland loss in the mississippi delta and is likely to cause subsidence in other organic rich and often densely populated coastal plains

modern day tectonic subsidence in coastal louisiana subsidence is leading to the slow inundation of communities and wetlands of louisiana mississippi texas and alabama united states by the gulf of mexico the prevailing paradigm considers subsidence to be the result of young sediment compaction and or consolidation and human activities this paper describes the results of a test of this theory based on an examination of historic motions of benchmark in the michoud area of orleans parish louisiana this methodology allowed for an assessment of vertical change at different levels over time relative to a precise vertical datum north american vertical datum of 1988 navd88 data do not support the current theory on the origins of subsidence they demonstrate that tectonic causes dominate in the study area during 1969 1971 and 1971 1977 tectonism was responsible for 16 9 mm yr and 7 1 mm yr of subsidence respectively these contributions account for 73 and 50 of the total subsidence during these intervals the change in deep subsidence is attributed to renewed motion along a large normal fault michoud fault over the same time intervals intermediate depth subsidence due to compaction of pleistocene to middle miocene strata was constant 4 6 mm yr similarly subsidence due to shallow processes i e sediment compaction and groundwater offtake was 1 5 mm yr and 2 5 mm yr subsidence associated with petroleum extraction was not a factor due to the lack of local production

observations of nontornadic low level mesocyclones and attendant tornadogenesis failure during vortex abstract three storms intercepted during the verification of the origins of rotation in tornadoes experiment generated a moderate to strong mesocyclone within the lowest several hundred meters above the ground and qualitatively appeared capable of tornadogenesis yet did not produce a tornado such novel observations of what is considered tornadogenesis failure are documented and used to show the insufficiency of a low level mesocyclone for tornadogenesis possible modes of failure are discussed

demonstration of a cut cell representation of 3d orography for studies of atmospheric flows over very steep hills abstractadvances in computing are enabling atmospheric models to operate at increasingly fine resolution giving rise to more variations in the underlying orography being captured by the model grid consequently high resolution models must overcome the problems associated with traditional terrain following approaches of spurious winds and instabilities generated in the vicinity of steep and complex terrain cut cell representations of orography present atmospheric models with an alternative to terrain following vertical coordinates this work explores the capabilities of a cut cell representation of orography for idealized orographically forced flows the orographic surface is represented within the model by continuous piecewise bilinear surfaces that intersect the regular cartesian grid creating cut cells an approximate finite volume method for use with advection form governing equations is implemented to solve flows through the resulting irregularly shaped grid boxes comparison with a benchmark orograp

the relationship between overshooting tops in a tornadic supercell and its radar observed evolution abstractthis is a study of a tornadic supercell in kansas on 14 may 2018 in which data of relatively high spatiotemporal resolution from a mobile polarimetric x band doppler radar were integrate

the numerical simulation of nonsupercell tornadogenesis part ii evolution of a family of tornadoes along a weak outflow boundary abstract nonsupercell tornadogenesis along a weak outflow boundary has been simulated using a three dimensional moist convective cloud model thermodynamic conditions similar to those observed for nonsupercell tornado nst events of the high plains were utilized in the model initialization as the ensemble system of storm outflow boundary and leading edge vortices evolve six distinct life cycle stages for the development and decay of nsts are documented that span a period of about 35 min consistent with the results of part i of this numerical study vortex sheet dynamics exert considerable control over the outflow leading edge the progression of pretornadic life cycle stages serves to concentrate vertical vorticity effectively along the outflow boundary in discrete misocyclone circulations aligned in a 3 km wavelength pattern the organization of larger scale misocyclones and ultimate intensification to initial tornadic intensity occurs coincident with the rapid development of deep convection overhe

long term surface elevation change in salt marshes a prediction of marsh response to future sea level rise accretion rates and surface elevation changes were measured in three natural salt marshes in the wadden sea derived from these measurements a simple predictive model was made which describes changes in surface elevation during more than 100 years of salt marsh development at several sea level rise scenarios the model was tested with data on long term surface elevation changes at several successional stages in the salt marsh of schiermonnikoog the netherlands where sites differ in age from a few years to more than 100 years the model predicts in all scenarios a fast increase in surface elevation during the first 100 years of marsh development results showed a very high similarity between the prediction of the model and field data a scenario without sea level rise revealed a stabilization of surface elevation and inundation frequency after approximately 100 years in a scenario with a rise in mean sea level however marsh surface elevation continues to increase after 100 years although at a slower pace previous studies expected marsh surface elevation on coastal barrier marshes to keep pace with sea level rise if sea level rise was lower than 5 mm yr 1 we found that inundation frequency starts to increase after 100 years of marsh development even if sea level rise is lower than 5 mm yr 1 shrinkage of the clay layer during summer probably caused an elevation deficit in older marshes this means that in the long run more than 500 years marshes will probably degenerate on these islands especially at a low elevation new marshes may develop at sites with a higher elevation if these sites are available

the influence of the shear induced pressure gradient on thunderstorm motion abstract in the present investigation we propose a simple theory to explain how a veering environmental wind shear vector can cause an initially symmetric updraft to grow preferentially to the right of the shear vector and acquire cyclonic rotation the explanation offered is based on linear theory which predicts that interaction of the mean shear with the updraft produces favorable vertical pressure gradients along its right flank to asses the validity of linear theory for large amplitude updrafts the three dimensional shallow anelastic equations are numerically integrated using a simple parameterization for latent heating within a cloud and the linear and nonlinear forcing terms are separately analyzed these results suggest that although the nonlinear effects strongly promote splitting of the updraft the linear forcing remains the dominant factor in preferentially enhancing updraft growth on the right flank we believe this differential forcing is a major contributor to the observed predominance o

hook echoes and rear flank downdrafts a review abstract nearly 50 years of observations of hook echoes and their associated rear flank downdrafts rfds are reviewed relevant theoretical and numerical simulation results also are discussed for over 20 years the hook echo and rfd have been hypothesized to be critical in the tornadogenesis process yet direct observations within hook echoes and rfds have been relatively scarce furthermore the role of the hook echo and rfd in tornadogenesis remains poorly understood despite many strong similarities between simulated and observed storms some possibly important observations within hook echoes and rfds have not been reproduced in three dimensional numerical models

influences of the local environment on supercell cloud to ground lightning radar characteristics and severe weather on 2 june 1995 abstract radar cloud to ground cg lightning characteristics and storm reports were documented for 20 long lived supercell thunderstorms that occurred during a 6 h period in the west texas panhandle on 2 3 june 1995 these thunderstorms occurred in proximity to a preexisting mesoscale outflow boundary storms that remained on the warm side of the mesoscale outflow boundary and storms that formed directly on the boundary tended to produce weaker low level rotation lower maximum heights for the 40 dbz echo top and had the largest negative cg flash rates the largest negative flash rate was produced as each storm was gradually weakening in contrast out of 11 boundary crossing storms several important radar based measurands increased unambiguously after storms crossed the boundary 40 dbz echo top height in 5 cases radar reflectivity above the environmental freezing level in 6 cases and low level mesocyclone strength in 9 cases trends of the first two measurands were ambiguous for 4 of 11 cases aff

an improved method for computing horizontal diffusion in a sigma coordinate model and its application to simulations over mountainous topography abstract a set of modifications is presented to reduce the unphysical impact of horizontal diffusion in numerical models with a terrain following sigma coordinate system at model levels sufficiently far away from the ground vertical interpolation is used to compute diffusion truly horizontally when the coordinate surfaces are sloping close to the ground where truly horizontal computation of diffusion is not everywhere possible without intersecting the topography a combination of one sided truly horizontal diffusion and orography adjusted diffusion along the sigma surfaces is used for most of the variables the latter means that the diffusion coefficient is reduced strongly when the grid points involved in the computation of horizontal diffusion are located at greatly different heights for temperature one sided horizontal diffusion is not used because it damps the slope wind circulation in an unphysical way however a temperature gradient correction is applied to the terrain following part of the t

topographic effects on radiation in the wrf model with the immersed boundary method implementation validation and application to complex terrain abstracttopographic effects on radiation including both topographic shading and slope effects are included in the weather research and forecasting wrf model and here they are made compatible w

on the interaction of tropical cyclone scale vortices ii discrete vortex patches the interaction of vortex patches in relation to the observed scales and features reported in part i of this paper is investigated it is found that the initial approach of compound vortices such as tropical cyclones arises from distortion of their weaker outer vorticity fields care needs to be taken in applying simple concepts such as the propagation of each vortex on the gradient of its neighbour for similar vortices the initial interaction consists of mutual orbit and approach with only minor fluctuations in shape once the vortices approach to within a critical separation distance rapid merger occurs a simplified expression for this critical distance is derived and it indicates that binary tropical cyclones must approach to within 150 300 km before merger of the cores can occur when vortices have substantial differences in size or intensity the smaller weaker system shears into the outer circulation of the other and no core merger occurs it is suggested that this is typical of the interaction between tropical cyclones and both the monsoon trough and embedded mesoscale convective complexes the motion of interacting vortices consists largely of a mutual orbit which scales according to their relative sizes and intensities an important finding however is that higher frequency meanders may develop this indicates that a tropical cyclone interacting with a monsoonal shear zone may develop internal asymmetries and a trochoidal oscillations as a direct result of such interaction further weak mesoscale vortices within the cyclone circulation may continue to cause a track meander long after they have been sheared beyond the resolving capacity of atmospheric observations

the influence of preexisting boundaries on supercell evolution abstract a three dimensional nonhydrostatic cloud model is used to study the evolution of supercell thunderstorms with emphasis on the low level mesocyclone interacting with preexisting boundaries the impacts of low level environmental shear storm motion relative to boundary orientation and boundary strength are assessed in the low level shear experiments significant low level rotation is consistently observed earlier tends to be stronger and is longer lived in storms interacting with a boundary than in storms initiated in a homogeneous environment low level rotation is weaker in storms crossing the boundary and moving into the colder air in contrast all storms moving along or into the warm air ahead of the boundary develop significant low level rotation increasing the temperature gradient and shear across the boundary has little impact on the low level mesocyclone evolution storms interacting with a boundary characterized by only horizontal shear produce weaker mesocyclones than those creat

an immersed boundary method enabling large eddy simulations of flow over complex terrain in the wrf model abstractthis paper describes a three dimensional immersed boundary method ibm that facilitates the explicit resolution of complex terrain within the weather research and forecasting wrf model two interpolation methods trilinear and inverse distance weighting idw are used at the core of the ibm algorithm this work expands on the previous two dimensional ibm algorithm of lundquist et al which uses bilinear interpolation simulations of flow over a three dimensional hill are performed with wrf s native terrain following coordinate and with both ib methods comparisons of flow fields from the three simulations show excellent agreement indicating that both ib methods produce accurate results idw proves more adept at handling highly complex urban terrain where the trilinear interpolation algorithm fails this capability is demonstrated by using the idw core to model flow in oklahoma city oklahoma from intensive observation period 3 iop3 of the joint urban 2003 field campaign flow in oklahoma c

direct surface thermodynamic observations within the rear flank downdrafts of nontornadic and tornadic supercells abstract despite the long surmised importance of the hook echo and rear flank downdraft rfd in tornadogenesis only a paucity of direct observations have been obtained at the surface within hook echoes and rfds in this paper in situ surface observations within hook echoes and rfds are analyzed these mobile mesonet data have unprecedented horizontal spatial resolution and were obtained from the verifications of the origins of rotation in tornadoes experiment vortex and additional field experiments conducted since the conclusion of vortex the surface thermodynamic characteristics of hook echoes and rfds associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and rfds are favorable or unfavorable for tornadogenesis tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy potential buoyancy as measured by the convective available potential energy and equivalent potential temperature in the r

the dissipation of a left moving cell in a severe storm environment abstract observations have shown that thunderstorms sometimes undergo updraft splitting where one updraft moves to the right of the mean tropospheric wind and the other to the left observations also show that the left moving updraft tends to dissipate approximately 15 min after the splitting process the right moving cell however may exist for up to a few hours idealized modeling studies suggest that this behavior is related to the clockwise turning of the environmental shear vectors with height the interaction between the environmental shear and the storms updraft produces a high low pressure couplet oriented downshear this pressure pattern produces favorable vertical accelerations for the right mover this same process inhibits upward motion for the left mover in this paper an idealized simulation is presented that suggests an additional process that contributed to the decay of the left moving updraft analysis of low level storm relative winds for the left moving cell indicated that the inflow

evaluating implementations of the immersed boundary method in the weather research and forecasting model abstractthe terrain following coordinate system used by many atmospheric models can cause numerical instabilities due to discretization errors as resolved terrain slopes increase and the grid becom

severe downslope windstorm calculations in two and three spatial dimensions using anelastic interactive grid nesting a possible mechanism for gustiness abstract the clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model application of the conservation condition of kurihara and others is shown to result in significant improvements in the treatment of interactive nesting a significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow this result for the anelastic system is in agreement with the earlier results of phillips and shukla who considered the hydrostatic shallow water system of equations the interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 january 1972 in boulder using both two and three spatial dimensions the three dimensional simulation results in a gustiness signature in the

the garden city kansas storm during vortex 95 part i overview of the storm s life cycle and mesocyclogenesis abstract analysis of a supercell storm that produced an f1 tornado near garden city kansas is presented this event provided one of the first opportunities to synthesize data collected by a new airborne radar platform called eldora electra doppler radar developed by the national center for atmospheric research the early stages of development of the midlevel mesocyclone and the entire evolution of the low level mesocyclone are captured over a 70 min period the low level mesocyclone began as an incipient shallow circulation along a synoptic scale trough the circulation intensified and grew in depth via vortex stretching under the influence of a strong updraft as this rotation built up from the boundary layer it initially remained separate and distinct from the midlevel mesocyclone subsequently the two mesocyclones merge to produce a single column of rotation 4 5 km in diameter an occlusion downdraft develops within the mesocyclone circulation during the last passes by the storm signaling the beg

the step mountain coordinate model description and performance for cases of alpine lee cyclogenesis and for a case of an appalachian redevelopment abstract the problem of the pressure gradient force error in the case of the terrain following sigma coordinate does not appear to have a solution the problem is not one of truncation error in the calculation of space derivatives involved thus with temperature profiles resulting in large errors an increase in vertical resolution may not reduce and is even likely to increase the error therefore an approach abandoning the sigma system has been proposed it involves the use of step mountains with coordinate surfaces prescribed to remain at fixed elevations at places where they touch and define or intersect the ground surface thus the coordinate surfaces are quasi horizontal and the sigma system problem is not present at the same time the simplicity of the sigma system is maintained in this paper design of the model silhouette averaged mountains properties of the wall boundary condition and the scheme for calculation of the potential to kinetic energy conversion are presented for an

parameterizing surface wind speed over complex topography subgrid parameterizations are used in coarse scale meteorological and land surface models to account for the impact of unresolved topography on wind speed while various parameterizations have been suggested these were generally validated on a limited number of measurements in specific geographical areas we used high resolution wind fields to investigate which terrain parameters most affect near surface wind speed over complex topography under neutral conditions wind fields were simulated using the advanced regional prediction system arps on gaussian random fields as model topographies to cover a wide range of terrain characteristics we computed coarse scale wind speed i e a spatial average over the large grid cell accounting for influence of unresolved topography using a previously suggested subgrid parameterization for the sky view factor we only require correlation length of subgrid topographic features and mean square slope in the coarse grid cell computed coarse scale wind speed compared well with domain averaged arps wind speed to further statistically downscale coarse scale wind speed we use local fine scale topographic parameters namely the laplacian of terrain elevations and mean square slope both parameters showed large correlations with fine scale arps wind speed comparing downscaled numerical weather prediction wind speed with measurements from a large number of stations throughout switzerland resulted in overall improved correlations and distribution statistics since we used a large number of model topographies to derive the subgrid parameterization and the downscaling framework both are not scale dependent nor bound to a specific geographic region both can readily be implemented since they are based on easy to derive terrain parameters

impact of mississippi river freshwater reintroduction on enhancing marsh accretionary processes in a louisiana estuary abstract to counteract extensive wetland loss a series of diversion projects have been implemented to introduce freshwater and sediment from the mississippi river into louisiana coastal wetlands to keep pace with increases in water level due to subsidence louisiana coastal marshes must vertically accrete through the accumulation of both organic matter and mineral sediment the impact of mississippi river freshwater diversion on enhancing vertical marsh accretion mineral and organic matter accumulation was examined in breton sound estuary a coastal wetland experiencing marsh deterioration as result of subsidence and salt water intrusion using 137 cs dating and artificial marker horizons increases in the rate of vertical marsh accretion were measured at marsh sites along a spatial gradient which has been receiving diverted water from the mississippi river caernarvon diversion since 1991 vertical accretion and accumulation of mineral sediment organic matter and nutrients in the marsh soil profile increased at marsh sites receiving freshwater and sediment input iron and manganese content of the marsh surface sediment were shown to be an excellent signature of riverine sediment deposition soil extractable phosphorus was higher and extractable sodium was lower at sites nearest freshwater and sediment input results demonstrated that freshwater diversion through sediment input and lowering of salinity will enhance marsh accretion and stability slowing or reversing the rate of wetland loss

hurricane spiral bands abstract the spiral bands that occur in tropical cyclones can be conveniently divided into two classes outer bands and inner bands evidence is presented here that the outer bands form as the result of nonlinear effects during the breakdown of the intertropical convergence zone itcz through barotropic instability in this process a zonal strip of high potential vorticity the itcz shear zone or monsoon trough begins to distort in a varicose fashion with the potential vorticity pv becoming pooled in local regions that are connected by filaments of high pv as the pooled regions become more axisymmetric the filaments become thinner and begin to wrap around the pv centers it is argued that inner bands form in a different manner as a tropical cyclone intensifies due to latent heat release the pv field becomes nearly circular with the highest values of pv in the cyclone center the radial gradient of pv provides a state on which pv waves the generalization of rossby waves can propagate the nonline