a scale discriminating vorticity budget for a mesoscale vortex in a midlatitude continental mesoscale convective system abstract the authors employ data from the noaa wind profiler network to present a scale discriminating vorticity budget for a mesoscale convective vortex mcv that was generated by a mesoscale convective system mcs in oklahoma and kansas on 1 august 1996 a spatial bandpass filter was used to divide observed wind into mesoscale and synoptic components then the authors sought sources and sinks of vorticity in those two components over 9 h of the mcv s lifetime the vorticity budget reveals that both the mesoscale and synoptic winds supplied significant vorticity to the mcv the vortex s origin could not be proved but data weakly suggest that tilting may have been mostly responsible convergence of absolute vorticity by the mesoscale wind was the reason the mcv grew deeper and stronger as the mcs weakened finally tilting of synoptic and mesoscale vorticity by gradients in mesoscale vertical motion was responsible for a secondary deepening of the mcv as the mcs dissipated the budget suggests that if

large scale patterns associated with tropical cyclogenesis in the western pacific abstract five characteristic low level large scale dynamical patterns associated with tropical cyclogenesis in the western north pacific basin are examined along with their capacity to generate the type of mesoscale convective systems that precede genesis an 8 yr analysis set for the region is used to identify and create composites for the five characteristic patterns of monsoon shear line monsoon confluence region monsoon gyre easterly waves and rossby energy dispersion this brings out the common processes that contribute to tropical cyclogenesis within that pattern which are described in detail a 3 yr set of satellite data is then used to analyze the mesoscale convective system activity for all cases of genesis in that period and to stratify based on the above large scale patterns it is found that mesoscale convective systems develop in all cases of genesis except one seventy percent of cases developed mesoscale convective systems at more than one time during the genesis period and 44 of

holocene paleoceanography of the northwest passage canadian arctic archipelago abstract a series of cores box and piston were collected at 2 key locations in lancaster sound cores 2004 804 009 bc and pc and barrow strait cores 2005 804 004 bc and pc in the canadian arctic archipelago to document the evolution of sea surface conditions in the main axis of the northwest passage during the holocene time period reconstruction of sea surface parameters summer temperature and salinity duration of sea ice cover were estimated based on transfer functions using dinoflagellate cyst assemblages as proxy indicators the chronology of these cores is based on calibrated ams 14c dates 210pb analyses and correlations between paleomagnetic secular variations of the geomagnetic field and a predicted spherical harmonic model of the geomagnetic field cals7 k 2 our age models for both cores indicate that 009 pc spans the last 11 100 cal bp while 004 pc encompasses the last 10 800 cal bp calculated sedimentation rates are in the range of 43 140 cm kyr for core 009 pc and 15 118 cm kyr for core 004 pc allowing for a millennial time scale resolution in each core our results indicate relatively harsh conditions in lancaster sound between 10 800 and 9000 cal bp summer temperatures 2 c cooler than at present which we associate with the presence of active ice streams in northernmost baffin bay and a weak west greenland current this is followed by a warming trend up to 3 c higher than present that took place between 8500 and 5500 cal bp which we associate with the holocene thermal maximum and to a large scale atmospheric pattern such as the arctic oscillation operating at the millennial time scale this is concomitant with an increase in the relative abundance of phototrophic dinoflagellate cyst taxa a gradual cooling of sea surface temperature and increased sea ice follow this from 5500 cal bp until the modern period in barrow strait harsh sea surface conditions prevailed from 11 100 to 5500 cal bp with summer temperatures as low as 4 c cooler than at present the warming trend occurred later in this region between 5500 and 1000 cal bp after which a gradual cooling is observed until the modern period the apparent shift or opposite warming trends between lancaster sound and barrow strait after 5500 cal bp could be the result of a change in atmospheric circulation patterns related to a possible shift in the ao mode from ao to ao comparison of ice core 18o record from devon island ice cap and the reconstructed sea surface temperature from core 009 pc suggests a strong atmospheric oceanic coupling throughout the holocene in this area

a record of holocene summer climate from a canadian high arctic ice core melt layers in ice cores are formed by melting on the snow pack surface which produces water that percolates down and refreezes in the colder snow layers below melt layers are distinguished by their bubble texture because more ice forms in the snow pack in warm summers than in cool summers the changing concentration with depth of melt layers in snow pits1 2 and ice cores3 5 is an indicator of past summer climate here we report on studies of melt layers in two canadian high arctic surface to bedrock ice cores although changes in elevation of the drill site owing to isostatic depression and uplift may contribute up to 40 of the total climatic signal recorded in the melt layers we interpret their varying concentrations in terms of changing summer temperatures over the past 10 000 years the warmest summers occurred 8 10 kyr ago and the coldest only 150 years ago the summers over the past 100 years have been the warmest for more than 1 000 years but are still not as warm as those of the early holocene the melt layer record is in broad agreement with geological records of glacier retreat during the holocene6 8 indicating that these data also contain information of regional significance

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

the multiple vortex nature of tropical cyclogenesis abstract this study explores the extent to which potential vorticity pv generation and superposition were relevant on a variety of scales during the genesis of tropical storm allison allison formed close to shore and the combination of continuous doppler radar satellite aircraft and surface observations allows for the examination of tropical cyclogenesis in great detail preceding allison s genesis pv superposition on the large scale created an environment where decreased vertical shear and increased instability surface fluxes and low level cyclonic vorticity coexisted this presented a favorable environment for meso scale pv production by widespread convection and led to the formation of surface based meso scale vortices termed convective burst vortices cbvs the cbvs seemed to form in association with intense bursts of convection and rotated around each other within the meso circulation field one cbv eventually superposed with a mesoscale convective vortex mcv resulting in a mor

genesis of tropical storm eugene 2005 from merging vortices associated with itcz breakdowns part i observational and modeling analyses abstract although tropical cyclogenesis occurs over all tropical warm ocean basins the eastern pacific appears to have the highest frequency of tropical cyclogenesis events per unit area in this study tropical cyclogenesis from merging mesoscale convective vortices mcvs associated with breakdowns of the intertropical convergence zone itcz is examined this is achieved through a case study of the processes leading to the genesis of tropical storm eugene 2005 over the eastern pacific using the national centers for environmental prediction reanalysis satellite data and 4 day multinested cloud resolving simulations with the weather research and forecast wrf model at the finest grid size of 1 33 km observational analyses reveal the initiations of two mcvs on the eastern ends of the itcz breakdowns that occurred more than 2 days and 1000 km apart the wrf model reproduces their different movements intensity and size changes and vortex vortex interaction at nearly the right timing and location at

holocene paleoclimatology of the queen elizabeth islands canadian high arctic abstract a wide variety of evidence reflecting in different ways the changing climate of the queen elizabeth islands during the holocene is reviewed all proxies pertain to summer conditions many sources of information provide weak or equivocal paleoclimatic signals but a general pattern of events can be discerned temperatures in the early to mid holocene were highest comparable with or higher than temperatures prevailing for much of this century although much evidence points to a mid holocene thermal maximum there is also considerable evidence that conditions were warmer in the early holocene possibly related to orbitally induced radiation anomalies the apparent mid holocene thermal maximum may reflect lags in the response of the environment and of some proxies in recording paleoclimatic conditions more direct indices point to warmest conditions in the early holocene before 7500 bp temperatures declined from 3000 bp culminating in exceptionally low temperatures from 100 400 bp this may have been the coldest period of the entire holocene resulting in glacier advances to post glacial maximum positions the period since 1925 has witnessed a pronounced increase in temperature leading to negative mass balances on glaciers and ice sheets throughout the archipelago this period is the warmest for at least 1000 years and perhaps for several thousand years modern climate in the queen elizabeth islands may thus be characteristic of the early to mid holocene and atypical of conditions which prevailed for much of the last few thousand years

a climatology of midlatitude mesoscale convective vortices in the rapid update cycle abstract climatological characteristics of mesoscale convective vortices mcvs occurring in the state of oklahoma during the late spring and summer of four years are investigated the mcv cases are selected based on vortex detection by an objective algorithm operating on analyses from the rapid update cycle ruc model consistent with a previous study true mcvs represent only about 20 of the mesoscale relative vorticity maxima detected by the algorithm the mcvs have a broad range of radii and intensities and their longevities range between 1 and 54 h their median radius is about 200 km and their median midlevel relative vorticity is 1 2 10 4 s 1 there appears to be no significant relationship between mcv longevity and intensity similar to past estimates approximately 40 of the mcvs generate secondary convection within their circulations the mean synoptic scale mcv environment is determined by the use of a ruc based composite analysis at four different stages in the mcv life cycle defined b

the response of simulated nocturnal convective systems to a developing low level jet some recent numerical experiments have examined the dynamics of initially surface based squall lines that encounter an increasingly stable boundary layer akin to what occurs with the onset of nocturnal cooling the present study builds on that work by investigating the added effect of a developing nocturnal low level jet llj on the convective scale dynamics of a simulated squall line the characteristics of the simulated llj atop a simulated stable boundary layer are based on past climatological studies of the llj in the central united states a variety of jet orientations are tested and sensitivities to jet height and the presence of lowlevel cooling are explored the primary impacts of adding the llj are that it alters the wind shear in the layers just above and below the jet and that it alters the magnitude of the storm relative inflow in the jet layer the changes to wind shear have an attendant impact on low level lifting in keeping with current theories for gust front lifting in squall lines the changes to the system relative inflow in turn impact total upward mass flux and precipitation output both are sensitive to the squall line relative orientation of the llj the variations in updraft intensity and system relative inflow are modulated by the progression of the lowlevel cooling which mimics the development of a nocturnal boundary layer while the system remains surfacebased the below jet shear has the largest impact on lifting whereas the above jet shear begins to play a larger role as the system becomes elevated similarly as the system becomes elevated larger changes to systemrelative inflow are observed because of the layer of potentially buoyant inflowing parcels becoming confined to the layer of the llj

mechanisms for quasi stationary behavior in simulated heavy rain producing convective systems abstract in this study idealized numerical simulations are used to identify the processes responsible for initiating organizing and maintaining quasi stationary convective systems that produce locally extreme rainfall amounts of particular interest are those convective systems that have been observed to occur near mesoscale convective vortices mcvs and other midlevel circulations to simulate the lifting associated with such circulations a low level momentum forcing is applied to an initial state that is representative of observed extreme rain events the initial vertical wind profile includes a sharp reversal of the vertical wind shear with height indicative of observed low level jets deep moist convection initiates within the region of mesoscale lifting and the resulting convective system replicates many of the features of observed systems the low level thermodynamic environment is nearly saturated which is not conducive to the production of a strong surface cold pool yet the convection qui

the oceanic mesoscale convective system and associated mesovortex observed 12 december 1992 during toga coare this study documents the precipitation and kinematic structure of a mature mature to decaying westward propagating near equatorial oceanic mesoscale convective system mcs observed by airborne doppler radars during toga coare the tropical ocean global atmosphere coupled ocean atmosphere response experiment this system occurred on 12 december 1992 during the convectively active phase of an intraseasonal oscillation and was followed at its dissipating stage by a redevelopment of convection that led to the mcs observed on 13 december 1992 these two successive mcss were associated with a two day atmospheric disturbance radar deduced airflows at two time periods reveal many similarities with other tropical oceanic cloud clusters in particular a marked rear inflow jet associated with a midlevel cyclonic mesovortex was observed within the rear of the stratiform region the mesovortex was better defined both in size and depth during the mature to decaying stage and was located more inside the system within a region of convergence due to the westward propagation of the rear inflow countergradient transports of momentum normal to the mean orientation of the system at midlevels and downgradient transports at lower and higher levels helped to intensify this rear inflow these transports were mostly accomplished by eddy structures cyclonic vorticity was concentrated at the rear of the mcs and peaked at the centre of the observed closed wind circulation as previously observed stretching of the pre existing vertical vorticity was the dominant dynamical mechanism which helped to amplify the mesovortex at mid to upper levels while tilting of the horizontal vorticity into the vertical was a lesser mechanism and had an opposite effect in most respects advective processes were negatively correlated with the stretching and tilting mechanisms but not sufficiently to be balanced the resulting tendency was an increase of cyclonic vorticity at low to mid levels and a net decrease above

warm core vortex amplification over land abstract a convectively generated mesoscale vortex that was instrumental in initiating and organizing five successive mesoscale convective systems over a period of three days is documented two of these convective systems were especially intense and resulted in widespread heavy rain with localized flooding based upon radar and satellite data the detectable size of the vortex became much larger following the strong convective developments nearly tripling its initial diameter over its three day life cycle during nighttime when convection typically intensified within the vortex movement of the system tended to slow following dissipation of the convection in the morning the daytime movement accelerated cross sections of potential vorticity taken through the vortex center clearly show a maximum at midlevels and a well defined minimum directly above the vortex and the potential vorticity maximum were essentially colocated and the system was nearly axisymmetric in the vertical over the three day life

a multi proxy lacustrine record of holocene climate change on northeastern baffin island arctic canada reconstructions of past environmental changes are critical for understanding the natural variability of earth s climate system and for providing a context for present and future global change radiocarbon dated lake sediments from lake cf3 northeastern baffin island arctic canada are used to reconstruct past environmental conditions over the last 11 200 years numerous proxies including chironomid inferred july air temperatures diatom inferred lakewater ph and sediment organic matter reveal a pronounced holocene thermal maximum as much as 5 c warmer than historic summer temperatures from 10 000 to 8500 cal yr b p following rapid cooling 8500 cal yr b p lake cf3 proxies indicate cooling through the late holocene at many sites in northeastern canada the holocene thermal maximum occurred later than at lake cf3 this late onset of holocene warmth is generally attributed to the impacts of the decaying laurentide ice sheet on early holocene temperatures in northeastern canada however the lacustrine proxies in lake cf3 apparently responded to insolation driven warmth despite the proximity of lake cf3 to the laurentide ice sheet and its meltwater the magnitude and timing of the holocene thermal maximum at lake cf3 indicate that temperatures and environmental conditions at this site are highly sensitive to changes in radiative forcing

patterns of precipitation and mesolow evolution in midlatitude mesoscale convective vortices abstract surface pressure manifestations of mesoscale convective vortices mcvs that traversed oklahoma during the periods may august 2002 05 are studied using the weather surveillance radar 1988 doppler wsr 88d the oklahoma mesonet and the noaa profiler network data forty five mcvs that developed from mesoscale convective systems mcss have been investigated 28 62 of which exhibit mesolows detectable at the surface within this group three distinct patterns of precipitation organization and associated mesolow evolution have been identified the remaining 17 38 of the cases do not contain a surface mesolow two repeating patterns of precipitation organization are identified for the latter group the three categories of mcvs possessing a surface mesolow are as follows nineteen are classified as rear inflow jet mcvs and tend to form within large and intense asymmetric mcss rear inflow into the mcs enhanced by the development of an mcv on the left hand side relative to system motion pro

a palaeolimnological record of diatom community dynamics and late holocene climatic changes from prescott island nunavut central canadian arctic palaeolimnological analyses from lake pw03 a small dilute lake on crystalline bedrock on prescott island nunavut canada 73 07 n 96 41 w 243 m a s l provide a sensitive record of the impacts of holocene climatic change on diatom communities a high resolution sequence from 3500 cal bp to present indicates that a significant change in diatom assemblages takes place at 2300 cal bp when alkaliphilous fragilarioids and planktonic taxa are replaced by benthic epiphytic and acidophilous diatoms these changes coincide with the onset of neoglacial cooling and likely reflect more persistent ice cover and associated acidification of lake water maximum diatom species richness occurs during this period of transition dense bryophyte communities in the littoral zone provide habitats to sustain diatom production even under the cooler conditions following 2300 cal bp the concentration of diatom valves declines at 500 cal bp coincident with cooling at the time of the little ice age and minor chang

convection in an african easterly wave over west africa and the eastern atlantic a model case study of helene 2006 convective systems embedded in an african easterly wave aew over west africa and the eastern atlantic in september 2006 are investigated the study is based on cosmo model runs with a horizontal resolution of 2 8 km the developing mature and decaying phases of a mesoscale convective system over land are identified over the ocean the convection consists of the development and decay of short lived systems there are significant differences between the structure of convection over land and water over land the updraught heating and vertical vorticity are tilted in the vertical over water the convective cores are upright and collocated with positive relative vorticity the atmosphere is drier at mid levels over land and stronger shear downdraughts and a distinct cold pool occur the influence of the convective systems on their environment is assessed with potential temperature and relative vorticity budgets for regions encompassing the convective systems during the convective system s life cycle the relative vorticity increased at the height of the african easterly jet over land due to the eddy flux term whereas the relative vorticity maximum over the ocean moved downwards with time due to stretching high values of low level relative vorticity were seen over the ocean before the aew trough crossed the west african coast the low level circulation that developed as a positive low level vorticity anomaly associated with the westward extension of the saharan heat low moved over the atlantic and southwestwards in addition positive vorticity associated with the low level monsoon flow contributed to this circulation the cyclogenesis of hurricane helene was initiated when the mid level vorticity anomaly of the aew moved over the low level circulation initiating convection and enhancing the low level vorticity through vortex stretching copyright 2010 royal meteorological society

climate modulates the acidity of arctic lakes on millennial time scales high resolution diatom stratigraphies from two lakes on eastern baffin island nunavut canadian arctic are used to reconstruct lake water ph over the past 5 k y despite contrasting geomorphic histories and markedly different diatom floras the inferred ph of both lakes declined in concert with neoglacial cooling this result confirms that climate exerts a first order influence on the acidity of these poorly buffered oligotrophic lakes through links between lake ice cover primary productivity and dissolved inorganic carbon dynamics diatom based ph inferences from dilute lakes can therefore assist paleoclimatic interpretations from lake sediments

diurnal evolution and structure of long lived mesoscale convective vortices along the mei yu front over the east china plains abstractthe structure and diurnal evolution of long lived eastward propagating mesoscale convective vortices mcvs along typical summertime mei yu fronts over the east china plains are investigat

the chinese loess in xifeng ii a study of anisotropy of magnetic susceptibility of loess from xifeng summary measurements of specimens from the loess profile of xifeng gansu province china all show original magnetic fabric for sediments i e nearly horizontal foliation and the commonly oblate magnitude ellipsoid the degree of anisotropy always shows a strong correlation with the foliation f rather than with the lineation l specimens from wind blown loess redeposited water lain loess and red clay show their own distribution areas respectively in a diagram of l versus f magnetic fabric provides evidence for the suggestion that the red clay like the overlying wind blown loess is aeolian in origin the data points from the red clay all falling within the distribution area of the wind blown loess data points the magnetic fabric measured from present day palaeosols may still represent the features of the parent loess although the orientation of fabric has been slightly weakened through pedogenesis as shown in the present study

the recession of the inland ice margin during the holocene climatic optimum in the jakobshavn isfjord area of west greenland recent subsurface mapping of parts of the greenland inland ice margin in the region of jakobshavn isbrae indicates that the fjord system in the period of at least 2700 4700 calendar yr b p was more ice free than at present and that the front of the glacier was at least 15 km behind the present position the 14c datings of subfossils brought to the present ice margin fit with the climatic records from ice cores and confirm the favourable conditions for greenland s first settlers the sarqaq people who arrived in the region about 4000 yr ago to find hunting grounds 10 20 larger than the present

mesoscale interactions in tropical cyclone genesis with the multitude of cloud clusters over tropical oceans it has been perplexing that so few develop into tropical cyclones the authors postulate that a major obstacle has been the complexity of scale interactions particularly those on the mesoscale which have only recently been observable while there are well known climatological requirements these are by no means sufficient a major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development observations exist for only a few forming cases in these the moist convection in the preformation environment is organized into mesoscale convective systems each of which have associated mesoscale potential vortices in the midlevels interactions between these systems may lead to merger growth to the surface and development of both the nascent eye and inner rainbands of a tropical cyclone the process is essentially stochastic but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences for example a monsoon trough provides a region of reduced deformation radius which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices further a strong monsoon trough provides a vertical wind shear that enables long lived midlevel mesoscale vortices that are able to maintain or even redevelop the associated convective system the authors develop this hypothesis by use of a detailed case study of the formation of tropical cyclone oliver observed during toga coare 1993 in this case two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband the eye developed on the edge of the major convective system and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere the importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western north pacific

heightened sensitivity of a poorly buffered high arctic lake to late holocene climatic change a diatom based paleolimnological investigation was conducted on late holocene sediments from a poorly buffered lake informally named rock basin lake on ellesmere island arctic canada the fossil diatom record is unlike any other obtained thus far from high arctic regions exhibiting dynamic assemblage shifts over the entire 3300 yr sedimentary record multiple proxies i e diatoms ph reconstructions biogenic silica c n ratios total organic carbon appear to sensitively track rapid limnological changes which are associated with distinct climate intervals as inferred from other regional proxy records the highly responsive nature of the diatom assemblages in rock basin lake relative to those recorded from nearby alkaline sites appears to be related to this lake s limited ability to buffer changes in ph the dynamic species responses suggest that the diatoms in rock basin lake are faithfully tracking climatic changes and that low alkalinity lakes may provide the most sensitive diatom based paleolimnological records from high arctic regions

the vertical structure of mesoscale convective vortices abstract simulations of two cases of developing mesoscale convective vortices mcvs are examined to determine the dynamics governing the origin and vertical structure of these features although one case evolves in strong vertical wind shear and the other evolves in modest shear the evolutions are remarkably similar in addition to the well known mesoscale convergence that spins up vorticity in the midtroposphere the generation of vorticity in the lower troposphere occurs along the convergent outflow boundary of the parent mesoscale convective system mcs lateral transport of this vorticity from the convective region back beneath the midtropospheric vorticity center is important for obtaining a deep column of cyclonic vorticity however this behavior would be only transient without a secondary phase of vorticity growth in the lower troposphere that results from a radical change in the divergence profile favoring lower tropospheric convergence following the decay of the nocturnal mcs subsequent con

the evolution of the 10 11 june 1985 pre storm squall line initiation development of rear inflow and dissipation abstract mesoscale analysis of surface observations and mesoscale modeling results show that the 10 11 june squall line contrary to prior studies did not form entirely ahead of a cold front the primary environmental features leading to the initiation and organization of the squall line were a low level trough in the lee of the rocky mountains and a midlevel short wave trough three additional mechanisms were active a southeastward moving cold front formed the northern part of the line convection along the edge of cold air from prior convection over oklahoma and kansas formed the central part of the line and convection forced by convective outflow near the lee trough axis formed the southern portion of the line mesoscale model results show that the large scale environment significantly influenced the mesoscale circulations associated with the squall line the qualitative distribution of along line velocities within the squall line is attributed to the larger scale circulations associated with the le

detection and prediction of warm season midtropospheric vortices by the rapid update cycle the authors perform a statistical and dynamical analysis of midtropospheric mesoscale vortices captured by analyses from the rapid update cycle version 2 ruc 2 during the period 1 may to 31 august 1999 a total of 203 vortices meeting conditions of an automated algorithm were found of these 86 were observed to form within organized convection and were termed mesoscale convective vortices mcvs mcvs were favored over a broad area from eastern colorado and western nebraska to the mississippi river valley essentially collocated with the loci of organized convection the remaining 117 vortices termed dry vortices clustered in the immediate lee of the rocky mountains and over the southeastern united states vortices arising from convection had considerably greater intensity and longevity than dry vortices they were roughly five times more likely to be involved with the triggering of new convection a relationship was found between intensity and longevity such that there appears to be a maximum vortex lifetime that can be estimated from its maximum intensity vortices arising from convection had markedly greater humidity and water vapor mixing ratio underneath their centers compared to dry vortices consistent with many dry vortices having a topographic origin and mcvs arising from organized convection parameters such as horizontal scale background vertical wind shear and horizontal deformation were not systematically related to intensity or longevity prediction of mesoscale vortices by the ruc 2 was examined for a subsample of all cases in general the ruc 2 was able to predict the evolution of vortices once analyzed but had virtually no skill at predicting 12 h in advance the formation of the vortices however forecasts of organized convection should still benefit from accurate predictions of long lived vortex tracks

organizational modes of midlatitude mesoscale convective systems this paper discusses common modes of mesoscale convective organization using 2 km national composite reflectivity data the authors investigated linear mesoscale convective systems mcss that occurred in the central united states during may 1996 and may 1997 based upon the radar observed characteristics of 88 linear mcss the authors propose a new taxonomy comprising convective lines with trailing ts leading ls and parallel ps stratiform precipitation while the ts archetype was found to be the dominant mode of linear mcs organization the ls and ps archetypes composed nearly 40 of the studied population in this paper the authors document the characteristics of each linear mcs class and use operational surface and upper air data to describe their different environments in particular wind profiler data reveal that the stratiform precipitation arrangement associated with each class was roughly consistent with the advection of hydrometeors implied by the mean middle and upper tropospheric storm relative winds which were significantly different among the three mcs modes case study examples are presented for both the ls and ps classes which have received relatively little attention to this point as well the authors give a general overview of the synoptic scale meteorology accompanying linear mcss in this study which was generally similar to that observed by previous investigators

arctic lake ontogeny across multiple interglaciations abstract few lakes in the arctic preserve sediments older than holocene age because of pervasive glacial scour during the last ice age here we present sediment diatom and geochemical records from a lake on east central baffin island cf8 nunavut canada that captures three successive interglacial periods within the last 200 000 years a portion of marine isotope stage mis 7 the last interglacial mis 5e and the holocene mis 1 an additional unit of diatom rich organic sediment occurs between the latter two intervals and is ascribed to mis 5a interstadial conditions the lake cf8 paleolimnological record reveals similar ontogenetic trends within each interglacial early postglacial environments in both the holocene and mis 5e were characterized by a dominance of colonial benthic fragilarioid diatoms that thrived in relatively alkaline waters these species shifts do not coincide with major changes in base cation delivery to the sediments suggesting that shifts in diatom assemblages are controlled mainly by climate driven ph dynamics diatom assemblages then transitioned into dominance by tychoplanktonic aulacoseira species likely in response to climate driven ph dynamics the highest sustained abundances of aulacoseira occur in sediments ascribed to mis 5e and this is the only interval in which thalassiosiroid centric taxa also occur e g discostella and puncticulata spp given that these planktonic taxa typically reach high abundances during extended periods of open water conditions we surmise that mis 5e was the warmest interval recorded the overall similarity of lake ontogenetic trajectories recorded within each interglacial period suggests that climatic and edaphic factors drive a complex succession of environmental changes through indirect effects on lake ice cover habitat availability and lake water ph in recent decades past 50 yrs an unprecedented increase in periphytic eunotia taxa is evident these diatoms were only present intermittently during previous glacial interglacial cycles the expansion of eunotia is attributed to increased habitat availability associated with declining ice cover during recent 20th century warming by integrating data from several distinct interglacials the cf8 diatom record considerably extends our understanding of arctic lake ontogeny and provides an unparalleled natural archive with which to compare recent changes associated with anthropogenic climate warming

evolution of paleo sea surface conditions over the last 600 years in the mackenzie trough beaufort sea canada abstract in order to document long term climate trends and predict future climate change for the arctic we need to look at the geological record to establish the link between historical and pre industrial sea surface parameters dinoflagellate cysts dinocysts are used as proxy indicators of sea surface parameters temperature salinity sea ice cover primary productivity jointly with transfer functions and a modern dinocyst reference database to reconstruct the evolution of sea surface conditions at decadal and multi decadal timescales here we present the fossil dinocyst assemblages established from three sediment cores collected along an inshore offshore transect in the mackenzie trough during the 2004 cases canadian arctic shelf exchange study cruise the chronology of each core was determined using 210 pb activity and ams 14 c measurements in core 912a sediment cores 912a 909b and 906b cover the last 600 200 and 100 years respectively palynomorph influxes increase from the bottom to the top of each core illustrating an increasing productivity over the last 600 years until 1850 ad when we observe a decrease of productivity until today we determined a succession of two assemblages over the last 600 years assemblage i at the base of each core is mostly composed of dinocysts from heterotrophic taxa the modern assemblage assemblage ii at the top of each core is mostly composed of dinocysts from autotrophic taxa quantitative reconstructions of sea surface parameters reveal a sharp increase in summer august temperature 2 to 5 c throughout the study area from 1400 ad until 1800 1850 ad after which the increase between 0 5 and 1 0 c is much slower until modern times

impacts of increasing low level shear on supercells during the early evening transition abstractthe dynamical response of simulated supercells to temporally increasing lower tropospheric vertical wind shear is investigated using idealized simulations these simulations are based upon observed soundings from two cases that underwent an early evening transition during the second verification of the origins of rotation in tornadoes experiment vortex2 mature supercells were simulated in observed afternoon environments with moderate vertical wind shear and then compared to simulated supercells experiencing observed evening increases in lower tropospheric shear the primary effect of the increase in low level shear is to establish larger values of vertical vorticity at lower altitudes in the storm s updraft in turn this leads to a nonlinear increase in the updraft strength due to the enhanced dynamic pressure minimum associated with larger vorticity in the storm s mesocyclone this is particularly important at low levels where it increases the storm s ability to lift cool surface air includ

diatom community responses to late holocene climatic variability baffin island canada a comparison of numerical approaches a high resolution diatom stratigraphy from the late holocene sediments of a small arctic lake on bafe n island nunavut canada has been analysed by several numerical methods and considered in relation to independent palaeoclimatic proxies to enable an objective assessment of the response of freshwater diatom communities to climatic change diatom relative frequencies and absolute abundances were subjected to several manipulations including application of a weighted averaging transfer function for summer water temperature conversion of valve concentrations to cell biovolumes biostratigraphic rate of change and taxonomic richness estimated by rarefaction analysis the major directions of variability within the derived diatom data were explored using principal components analysis diatom inferred lakewater temperatures and indices of diatom palaeoproductivity declined during late holocene cooling and they faithfully record post little ice age warm ing between 2000 cal bp and the little ice age which includes the coldest intervals of the neoglacial diatom assemblages diversie ed and destabilized ree ecting a state of intermediate disturbance together these results examine different methods of interpreting freshwater diatom stratigraphic data with the objective of gaining palaeoclimatic insights from fossil assemblages

diatom community response to multiple scales of holocene climate variability in a small lake on victoria island nwt canada abstract a lake sediment core spanning 9900 years collected from a small lake on western victoria island nwt canada provides a high resolution record of diatom community dynamics over the holocene ten radiocarbon dates and 210 pb dating provided the core chronology loss on ignition loi magnetic susceptibility biogenic silica content and diatom concentrations provided information on changes in the sedimentary environment loi gradually increased over the holocene whereas magnetic susceptibility showed an inverse trend biogenic silica content showed three distinct peaks spaced approximately 3000 years apart major shifts in diatom assemblages occurred at 8100 8000 5800 5700 and 3800 3500 cal yr bp there is evidence of diatom community response to centennial scale variations such as the medieval warm period 1000 700 cal yr bp little ice age 800 150 cal yr bp and recent warming although recent changes in diatom community composition productivity and species richness are apparent they were surpassed at other periods throughout the holocene variations of the taxa within the genera staurosira pseudostaurosira fragilaria and staurosirella usually combined into one genus in arctic lake sediment studies suggest these taxa provide useful insight into paleonvironmental questions

long lived mesoconvective vortices and their environment part ii induced thermodynamic destabilization in idealized simulations abstract idealized numerical simulations are used to quantify the effect of quasi balanced lifting arising from the interaction of the ambient vertical shear with midtropospheric cyclonic vortices mcvs generated by mesoscale convective systems on thermodynamic destabilization over a range of ambient vertical shear strengths and vortex characteristics observed in part i maximum upward displacements occur beneath the midtropospheric potential vorticity anomaly near the radius of maximum tangential vortex winds the location of the region of upward displacements relative to the ambient vertical shear vector depends on the relative strength of the vortex tangential flow and the ambient vertical shear and ranges from downshear for vortices of moderate strength in strong ambient vertical shear to 90 to the left of downshear for strong vortices in weak ambient vertical shear although significant upward displacements occur most rapidly with small vortices in strong ambient vertical shear maximum upward di

analysis of a small vigorous mesoscale convective system in a low shear environment part ii evolution of the stratiform precipitation and mesoscale flows abstract the evolution of the mesoscale flow and precipitation distribution are investigated for a small mesoscale convective system mcs that evolved in a nearly barotropic environment exhibiting moderate instability and weak wind shear observations primarily from a single doppler radar detail the growth of the mcs from the merger of several clusters and lines of vigorous convective cells into a mature state consisting of a weaker convective line trailed by an expanding stratiform precipitation region analysis of radar reflectivity reveals that this stratiform region formed in situ in the presence of weak mesoscale updraft as decaying convective cores coalesced rather than through rearward advection of ice particles directly from the convective region in the absence of sufficient low level shear the mcs collapsed rapidly as it assumed the structure of the archetypal convective line and trailing stratiform precipitation region velocity azimuth displays reveal mesoscale updrafts of about 70 cm s 1 d

simulated impacts of a mesoscale convective system on the track of typhoon robyn during tcm 93 abstract the contribution that a mesoscale convective vortex that developed within the circulation of typhoon robyn 1993 may have had to a significant tropical cyclone track change is simulated with a mesoscale model using initial conditions that approximate the circulations measured by aircraft during the tropical cyclone motion tcm 93 field experiment a dry version of the psu ncar mesoscale model is used to first investigate the dynamic aspects of the interaction a deceleration of about 2 m s 1 and then more northward movement similar to that observed for typhoon robyn could have been produced by an interaction with the mesoscale convective vortex of the type modeled in the control run sensitivity of the simulated track change is tested for various aspects of the mesoscale vortex and tropical cyclone it is found that track deflections between 67 and 130 km in 18 24 h could be produced under a variety of realistic scenarios as the mesoscale vortex is advected around the tropical cyclone by the

genesis of typhoon chanchu 2006 from a westerly wind burst associated with the mjo part ii roles of deep convection in tropical transition abstractin this study the life cycles of a series of four major mesoscale convective systems mcss during genesis and their roles in transforming a vertically tilted vortex associated with a westerly wind burst hereafter the wwb vortex into typhoon chanchu 2006 are examined using 11 day cloud resolving simulations presented in part i it is found that the tilted wwb vortex at early stages is characterized by an elevated cold core layer about 200 hpa thick below and a weak warm column above with large vertical wind shear across the layer which extends over a horizontal distance of about 450 km between the vortex s 400 and 900 hpa centers during the final two days of the genesis process the upper level warm column increases in depth and intensity as a result of the absorption of convectively generated vortices cgvs including a mesoscale convective vortex mcv causing more rapid amplification of cyclonic vorticity in the middle than the lower troposphere the commencement of sustained intens

mesoscale convective vortex that causes tornado like vortices over the sea a potential risk to maritime traffic abstractstrong gusty winds in a weak maritime extratropical cyclone ec over the tsushima strait in the southwestern sea of japan capsized several fishing boats on 1 september 2015 a c band doppl

climatically controlled chemical and biological development in arctic lakes 1 we investigated the factors controlling lake evolution in arctic ecosystems using a multiproxy paleolimnological approach on a small lake on baffin island arctic canada lakewater ph was inferred from fossil diatom assemblages whereas primary production was assessed from sediment concentrations of diatom valves and spectrally inferred chlorophyll a our reconstructed limnological variables registered synchronous changes and showed a close coupling to holocene climatic fluctuations as inferred by numerous independent paleoclimate proxies without exception our highest ph and production values occurred during warm intervals and vice versa a return towards paleolimnological conditions of the warm early holocene has occurred since the midtwentieth century corresponding to climate warming following the little ice age maximum recent values of our reconstructed parameters are either directly comparable to or in some cases exceed values attained during the holocene thermal maximum 8000 10 000 years ago our data suggest that climate has a first order influence on primary production and the regulation of in lake dic dynamics and hence on lakewater ph through its modulation of lake ice cover we conclude that direct forcing by climate is more important than catchment processes in controlling the chemical and biological development of ice dominated arctic lake ecosystems at the scale of the holocene

convective contribution to the genesis of hurricane ophelia 2005 abstract the convection occurring in the tropical depression that became hurricane ophelia 2005 was investigated just prior to tropical storm formation doppler radar showed a deep wide intense convective cell of a type that has been previously thought to occur in intensifying tropical depressions but has not heretofore been documented in detail the updraft of the cell was 10 km wide 17 km deep had updrafts of 10 20 m s 1 throughout its mid to upper levels and contained a cyclonic vorticity maximum the massive convective updraft was maintained by strong positive buoyancy which was maximum at about the 10 km level probably aided by latent heat of freezing evaporative cooling and precipitation drag occurred in the rain shower of the cell but were insufficient to produce a strong downdraft or gust front outflow to force the updraft the convective updraft was fed by a layer of strong inflow that was several kilometers deep wind induced turbulence just above the ocean surface enriched the equi

a satellite based climatology of central and southeastern u s mesoscale convective systems abstracta satellite based climatology is presented of 9607 mesoscale convective systems mcss that occurred over the central and southeastern united states from 1996 2017 this climatology is cons

a subtropical oceanic mesoscale convective vortex observed during sowmex timrex abstractthis study examines a subtropical oceanic mesoscale convective vortex mcv that occurred from 1800 utc 4 june to 1200 utc 6 june 2008 during intensive observing period iop 6 of the southwest monsoon experiment sowmex and the terrain influenced monsoon rainfall experiment timrex a dissipating mesoscale convective system reorganized within a nearly barotropic vorticity strip which formed as a southwesterly low level jet developed to the south of subsiding easterly flow over the southern taiwan strait a cyclonic circulation was revealed on the northern edge of the mesoscale rainband with a horizontal scale of 200 km an inner subvortex on a scale of 25 30 km with maximum shear vorticity of 3 10 3 s 1 was embedded in the stronger convection the vortex relative southerly flow helped create local potential instability favorable for downshear convection enhancement strong low level convergence suggests that stretching occurred within the mcv higher e air associated with significant pot

the propagation and diurnal cycles of deep convection in northern tropical africa the propagation and diurnal cycle of organized convection in northern tropical africa are examined using five years 1999 2003 of digital infrared imagery for may august reduced dimension techniques are used to document the properties of cold clouds proxies for deep convection and precipitation large scale environments are diagnosed from global analyses organized convection in africa consists of coherent sequences or episodes which span an average distance of about 1000 km and last about 25 h a substantial fraction of events exhibits systematic propagation at regional to continental scales while undergoing decay and regeneration episodes with 36 h duration and 1472 km span recur at a one per day interval most episodes have phase speed of 10 20 m s 1 which is faster than most african easterly waves convective episodes tend to initiate in the lee of high terrain consistent with thermal forcing from elevated heat sources average diurnal frequency maxima result from the superposition of local diurnal maximum with the delayed phase arrival of systems propagating from the east propagation occurs with moderate low to mid tropospheric shear which varies with the african easterly jet migration and west african monsoon phases frequent deep convection occurs with local shear maxima near high terrain for the peak monsoon period and for 10 w 10 e where easterly waves and convective systems are frequent 35 of cold cloud episodes occur east of the wave trough compared with about 24 to the west based on the coherent behaviour of organized propagating convection inferences may be made regarding the prediction of precipitation beyond one or two days copyright 2008 royal meteorological society

convective episodes in the east central united states nine years of composited radar data are investigated to assess the presence of organized convective episodes in the east central united states in the eastern united states the afternoon maximum in thunderstorms is ubiquitous over land however after removing this principal diurnal peak from the radar data the presence and motion of organized convective systems becomes apparent in both temporally averaged fields and in the statistics of convective episodes identified by an objective algorithm convective echoes are diurnally maximized over the appalachian chain and are repeatedly observed to move toward the east partly as a result of this the daily maximum in storms is delayed over the piedmont and coastal plain relative to the appalachian mountains and the atlantic coast during the 9 yr studied the objective algorithm identified 2128 total convective episodes 236 yr 1 with several recurring behaviors many systems developed over the elevated terrain during the afternoon and moved eastward often to the coastline and even offshore in addition numerous systems formed to the west of the appalachian mountains and moved into and across the eastern u s study domain in particular many nocturnal convective systems from the central united states entered the western side of the study domain frequently arriving at the eastern mountains around the next day s afternoon maximum in storm frequency a fraction of such well timed systems succeeded in crossing the appalachians and continuing across the piedmont and coastal plain convective episodes were most frequent during the high instability low shear months of summer which dominate the year round statistics even so an important result is that the episodes still occurred almost exclusively in above average vertical wind shear despite the overall dominance of the diurnal cycle the data show that adequate shear in the region frequently leads to long lived convective episodes with mesoscale organization

spatiotemporal characteristics and large scale environments of mesoscale convective systems east of the rocky mountains abstractthe spatiotemporal variability and three dimensional structures of mesoscale convective systems mcss east of the u s rocky mountains and their large scale environments are characterized

the formation of hurricane humberto 2001 the importance of extra tropical precursors the authors examine the formation of hurricane humberto 2001 in the atlantic basin precursor disturbances originating in the extratropics appear critical for inducing tropical cyclogenesis one precursor near the tropopause arose during the development of hurricane erin on 7 september and could be tracked continuously for 12 days until 1200 utc 19 september a fully explicit numerical simulation with the psu ncar mesoscale model produces a generally realistic evolution of the developing storm during a 4 day period beginning 0000 utc 19 september diagnostics of the simulations reveal that moderate vertical shear was crucial for organizing convection and this shear was enhanced on the periphery of an upper level vortex within a lower to mid tropospheric baroclinic zone downdraughts and evaporative cooling produced a rearward propagating bore that organized new convection in a notably moistened environment with small cape vorticity generation maximized near 1 km above mean sea level and the vortex built upward with time this evolution contrasts markedly with downward development conceptual models of tropical cyclogenesis and points to the crucial need for observations in the lower troposphere before depression formation convective scale dynamics were governed by intense convective cores which produced most of the diabatic heating and lower tropospheric vertical vorticity they were constrained by vertical shear to populate the downshear left quadrant of the vortex and therefore their collective behaviour was organized despite being quasi random within the favoured region ultimately the shear that helped organize the convection in the developing depression hampered humberto s subsequent intensification copyright 2006 royal meteorological society

subkilometer simulation of a torrential rain producing mesoscale convective system in east china part i model verification and convective organization abstracta nocturnal torrential rain producing mesoscale convective system mcs occurring during the mei yu season of july 2003 in east china is studied using conventional observations surface mesoanalysis satellite and radar data and a 24 h multinested model simulation with the finest grid spacing of 444 m observational analyses reveal the presence of several larger scale conditions that were favorable for the development of the mcs including mei yu frontal lifting moderate cold advection aloft and a moist monsoonal flow below and an elongated old cold dome left behind by a previously dissipated mcs results show that the model could reproduce the evolution of the dissipating mcs and its associated cold outflows the triggering of three separate convective storms over the remnant cold dome and the subsequent organization into a large mcs and the convective generation of an intense surface meso high and meso scale radar reflectivity morphologies in particular the model reproduces the passage of

surface cyclogenesis from convectively driven amplification of midlevel mesoscale convective vortices mesoscale convective vortices mcvs are midtropospheric warm core cyclonic circulations that often develop in the stratiform region of mesoscale convective systems typically divergent anticyclonically circulating mesoscale cold anomalies appear both above and below the mcv the upper level cold anomaly is usually found near the tropopause while the low level anomaly is surface based and exhibits locally higher pressure one aspect of mcvs that has received much attention recently is the role that they may play in tropical cyclogenesis of special interest is how an mcv amplifies when deep convection redevelops within the borders of its midlevel cyclonic circulation and how the amplified mcv transforms the divergent surface based cold pool with anomalously high surface pressure into a convergent cyclonic circulation with anomalously low pressure the pennsylvania state university national center for atmospheric research fifth generation mesoscale model is used to simulate an mcv that was instrumental in initiating within the borders of the midlevel vortex s circulation several successive cycles of convective development and decay over a 2 day period after each cycle of convection both the horizontal size of the cyclonic circulation and the magnitude of the potential vorticity associated with the vortex were observed to increase the simulation reproduces the development and evolution of the mcv and associated convective cycles mesoscale features responsible for the initiation of convection within the circulation of the vortex and the impact of this convection on the structure and evolution of the vortex are investigated a conceptual model is presented to explain how convective redevelopment within the mcv causes low level heights to fall and cyclonic vorticity to grow downward to the surface applying this conceptual model to a tropical marine environment is also considered

the vertical vorticity structure within a squall line observed during bamex banded vorticity features and the evolution of a bowing segment abstracta quasi linear convective line with a trailing stratiform region developed during the bow echo and mesoscale convective vortex experiment bamex while being sampled by two airborne doppler radars the finescale reflectivity and doppler velocities recorded by the radars documented the evolution of the convective line bands of positive and negative vertical vorticity oriented parallel to the convective line were resolved in the analysis this type of structure has rarely been reported in the literature and appears to be a result of the tilting and subsequent stretching of ambient horizontal vorticity produced by the low level wind shear vector with a significant along line component the radar analysis also documented the evolution of an embedded bow echo within the convective line embedded bow echoes have been documented for a number of years however a detailed analysis of their kinematic structure has not been previously reported in the literature the counterrotating circulation patterns tha

using paleolimnology to track holocene climate fluctuations and aquatic ontogeny in poorly buffered high arctic lakes fossil diatom assemblages and spectrally inferred dissolved organic carbon doc and sedimentary chlorophyll a sedchla were analysed on lake sediment cores from two poorly buffered lakes on pim

mesoscale convective vortices observed during bamex part ii influences on secondary deep convection abstract observations from the bow echo and mesoscale convective vortex mcv experiment are used to examine the role of the five mesoscale convective vortices described in part i on heavy precipitation during the daytime heating cycle persistent widespread stratiform rain without deep convection occurs for two strong mcvs in conditionally stable environments with strong vertical shear two other mcvs in moderate to strong vertical shear have localized redevelopment of deep convection termed secondary convection on their downshear side where conditional instability exists the strongest of the five mcvs occurs in weak vertical shear and has widespread secondary convection which is most intense on its conditionally unstable southeast periphery the two mcvs with only localized secondary convection have well defined mesoscale vertical motion couplets with downshear ascent and upshear descent above the planetary boundary layer pbl although the amplitude is significantly greater the kinematically der

the global population of mesoscale convective complexes a global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective the data set includes date of occurrence time of first storms initiation maximum extent termination duration cold cloud shield areas and tracks from initiation to termination it is found that the typical convective complex is nocturnal generates a cold cloud shield area of approximately 350 000 km2 and persists for about 10 h the largest systems and most persistent systems tend to occur near the summer solstices for the globe about 400 systems occur each year primarily over land areas most systems develop in favoured zones although some activity occurs over every continent except antarctica and all major oceans the concentration of activity into favoured zones indicates that there must be special dynamic and or thermodynamic conditions necessary for convection to organize into convective complexes activity is strongly tied to the solar day and shifts from 35 s in early january to about 50 n during the boreal summer and back to 35 s by december within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward relatively little migration occurs in the ocean dominated southern hemisphere where the subtropical jet remains quasi stationary over the convective complex regions the nocturnal life cycles copious rainfall large cloud shields and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth system energy budget

prediction and diagnosis of tropical cyclone formation in an nwp system part i the critical role of vortex enhancement in deep convection this is the first of a three part investigation into tropical cyclone tc genesis in the australian bureau of meteorology s tropical cyclone limited area prediction system tc laps an operational numerical weather prediction nwp forecast model the primary tc laps vortex enhancement mechanism is presented in part i the entire genesis process is illustrated in part ii using a single tc laps simulation and in part iii a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in tc laps the primary vortex enhancement mechanism in tc laps is found to be convergence stretching and vertical advection of absolute vorticity in deep intense updrafts which result in deep vortex cores of 60 100 km in diameter the minimum resolvable scale is limited by the 0 15 horizontal grid spacing on the basis of the results presented it is hypothesized that updrafts of this scale adequately represent mean vertical motions in real tc genesis convective regions and perhaps that explicitly resolving the individual convective processes may not be necessary for qualitative tc genesis forecasts although observations of sufficient spatial and temporal resolution do not currently exist to support or refute this proposition relatively large scale 30 km and greater lower to midlevel tropospheric convergent regions have been observed in tropical oceanic environments during the global atmospheric research programme garp atlantic tropical experiment gate the equatorial mesoscale experiment emex and the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare and regions of extreme convection of the order of 50 km are often remotely observed in tc genesis environments these vortex cores are fundamental for genesis in tc laps they interact to form larger cores and provide net heating that drives the system scale secondary circulation which enhances vorticity on the system scale akin to the classical eliassen problem of a balanced vortex driven by heat sources these secondary vortex enhancement mechanisms are documented in part ii in some recent tc genesis theories featured in the literature vortex enhancement in deep convective regions of mesoscale convective systems mcss has largely been ignored instead they focus on the stratiform regions while it is recognized that vortex enhancement through midlevel convergence into the stratiform precipitation deck can greatly enhance midtropospheric cyclonic vorticity it is suggested here that this mechanism only increases the potential for genesis whereas vortex enhancement through low to midlevel convergence into deep convective regions is necessary for genesis

comparison between two case studies of developing and nondeveloping african easterly waves during namma and amma sop 3 absolute vertical vorticity budget abstract two west african disturbances observed in august and september 2006 during the national aeronautics and space administration african monsoon multidisciplinary analysis namma and the special observing period 3 amma sop 3 have been simulated using the meso nh numerical model with explicit convection the first disturbance spawned hurricane helene 2006 off the west african coast and the second one referred to as perturbation d though relatively intense failed to develop over the continent each case was associated with a well defined african easterly wave aew trough with embedded growing and decaying convective activity of various size duration and intensity the aim of this work is to investigate the contribution of these convective systems in the generation and maintenance of cyclonic vorticity associated with the aew trough with respect to the synoptic scale processes the absolute vorticity budgets are analyzed during the continental and oceanic transition stages of these aew

interactions between a developing mesoscale convective system and its environment part ii numerical simulation the 19 july 1993 mesoscale convective system mcs discussed in part i was simulated using the regional atmospheric modeling system rams the model was initialized with variable physiographic and atmospheric data with the goal of reproducing the convective system and its four dimensional environment four telescopically nested moving grids allowed for horizontal grid spacings down to 1 67 km on the cloud resolving grid comparisons with the analysis show that the propagation evolution and structure of this mcs were well simulated the simulation is used to further investigate the interactions between this mcs and its surrounding environment in part i the doppler derived winds indicated that upshear westward propagating gravity waves left uppertropospheric front to rear and midtropospheric rear to front flow perturbations in their wake a similar flow structure developed in the simulated mcs and unlike the doppler results the low frequency waves that produced it were resolved in the data in the simulation much of the convectively generated temperature and momentum perturbations propagated westward with the waves leaving a warm wake in the clear air trailing the system although the gravity waves traveled rearward the perturbation flow in their wake was not strong enough to reverse the upper tropospheric storm relative winds thus most of the anvil condensate advected ahead of the convective line as the mcs encountered the low level jet the midtropospheric upward mass flux increased but gravity wave motions became less detectable the upper tropospheric anvil pushed westward into the strong flow as the system expanded into a characteristically oval shape temperature and momentum perturbations propagated rearward along with the anvil in a disturbance that resembled an advective outflow unlike the gravity waves this disturbance became almost stationary with respect to the ground and it retained its continuity through the rest of the simulation vertical cross sections indicate that a large slab of convectively processed air had detrained into the upper troposphere prior to this event much of the warm temperature anomalies generated within the convective towers either remained in the updrafts or propagated outward with the gravity waves early on individual updrafts were relatively erect and although condensate did detrain eastward in the forward anvil the temperature anomalies did not propagate with it in contrast convective updrafts associated with the expanding oval anvil disturbance were more continuous and they tilted strongly westward with height

initiation of a mesoscale convective complex over the ethiopian highlands preceding the genesis of hurricane alberto 2000 1 a tropical disturbance which would later become hurricane alberto 2000 was traced back in time as a mesoscale convective complex mcc to the ethiopian highlands eh where the mcc first developed meteosat 7 imagery indicates that the mcc develops during the late afternoon and evening of 28 july 2000 and that a mesovortex mv was evident on the morning of 29 july 2000 a preliminary mesoscale model simulation features the development of two areas of maximum relative vorticity in the middle troposphere with the maturation of one of the vorticity maxima into a significant mv the higher level vorticity center of the simulated mv tracks westward and is nearly collocated with the vortex signature in satellite imagery while the lower level center remains near the lee side of the eh the vortex signature in satellite imagery was traceable to the cyclogenesis stage over the eastern atlantic ocean

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

mesoscale convective systems 1 mesoscale convective systems mcss have regions of both convective and stratiform precipitation and they develop mesoscale circulations as they mature the upward motion takes the form of a deep layer ascent drawn into the mcs in response to the latent heating and cooling in the convective region the ascending layer overturns as it rises but overall retains a coherent layer structure a middle level layer of inflow enters the stratiform region of the mcs from a direction determined by the large scale flow and descends in response to diabatic cooling at middle to low levels a middle level mesoscale convective vortex mcv develops in the stratiform region prolongs the mcs and may contribute to tropical cyclone development the propagation of an mcs may have a discrete component but may further be influenced by waves and disturbances generated both in response to the mcs and external to the mcs waves of a larger scale may affect the propagation velocity by phase locking with the mcs in a cooperative mode the horizontal scale of an mcs may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the rossby radius of the mcv the vertical redistribution of momentum by an mcs depends on the size of the stratiform region while the net vertical profile of heating of the large scale environment depends on the amount of stratiform rain regional variability of the stratiform rain from mcss affects the large scale circulation s response to mcs heating

the genesis of hurricane guillermo texmex analyses and a modeling study abstract the transformation of a mesoscale convective system into hurricane guillermo was captured by aircraft and doppler wind data during the tropical experiment in mexico the early phase of the system evolves in a way very similar to previously documented mesoscale convective systems with a midlevel mesocyclone developing in the stratiform precipitation region more unusually the cyclone extends to low altitudes a weak cyclone is discernable even in the 300 m altitude wind field after another day of evolution a small surface based warm core cyclone is observed to develop within the relatively cold air associated with the mesocyclone aloft this mesocyclone develops into a hurricane over the subsequent day a nonhydrostatic axisymmetric numerical model is used to explore the evolution of the initially cold core midlevel vortex into a tropical cyclone a mesoscale midlevel showerhead is switched on in an initially quiescent tropical atmosphere overlying a warm ocean surface evaporation of t

the kinematics of a midlatitude continental mesoscale convective system and its mesoscale vortex the authors present a unique scale discriminating study of the environment relative circulations within a mesoscale convective system mcs and mesoscale convective vortex mcv the mcs a leading convective line and trailing stratiform region that became asymmetric passed through the national oceanic and atmospheric administration noaa profiler network npn in kansas and oklahoma on 1 august 1996 the mcv appeared in the mcs s stratiform region just prior to the system s mature stage and grew to a depth of over 12 km as the mcs dissipated the mcv did not apparently survive to the next day a spatial bandpass filter was used to divide observed wind into a component that was predominantly synoptic background wind and a component that was predominantly a mesoscale perturbation on that background wind a mesoscale updraft mesoscale downdraft and divergent outflows in the lower and upper troposphere were evident after the synoptic background wind was removed so these four circulations were internal and fundamental to the mcs the mesoscale perturbation in wind in the middle troposphere extended farther behind the mcs than ahead of it consistent with analytic studies and numerical simulations of gravity waves generated by heat sources characteristic of mcss with leading convective lines and trailing stratiform regions deepening of the mcv appeared to be reflected in the vertical wind shear at the vortex s center as the mcv strengthened the mesoscale shear through its lower part decreased perhaps as wind became more vortical at increasing altitudes mesoscale and synoptic vertical shears were of similar magnitude so an average of environmental soundings outside an mcs probably does not accurately represent the shear that affects an mcv this suggests the need to reevaluate how the kinematical settings of mcvs are diagnosed

coherent regeneration and the role of water vapor and shear in a long lived convective episode abstract in a recent radar based climatological study of warm season precipitation over the continental united states carbone et al found a high frequency of long lived coherent rainfall episodes many of the events were of longer duration than normally associated with mesoscale convective complexes and exhibited phase speeds 10 m s 1 in excess of the phase speed associated with synoptic systems the observations led to the speculation that cold pool dynamics and wavelike propagation mechanisms were responsible for the longevity of the systems one of the long lived episodes included in the statistics of the carbone et al study is described here occurring on 14 15 july 1998 the system lasted 50 h and traveled over 2800 km at its peak intensity the system was a bow echo producing damaging wind large hail and local flash flooding an interesting aspect of the event was an abrupt 90 turn in the storm s orientation and propagation vector midway through its life the environmental factors that led t

mesoscale convective vortices observed during bamex part i kinematic and thermodynamic structure abstract five cases of mesoscale convective vortices mcvs are described from observations collected during the bow echo and mcv experiment bamex over the central united states during the period from 20 may to 6 july 2003 in the present paper the kinematic and thermodynamic structure of each vortex and its environment are emphasized data consist of bamex dropsondes the national oceanic and atmospheric administration profiler network and national weather service soundings in addition weather surveillance radar 1988 doppler observations documented the signatures of nascent mcvs within nocturnal convection systems as well as the spatial pattern of convection within mcvs during the following day the vertical structure of each vortex was highly dependent on the vertical shear and the presence of upper tropospheric cyclonic vorticity anomalies in strong shear a pronounced downshear tilt of the vortex was evident but with the presence of an upper tropospheric trough the tilt was upshear in the upp

baroclinic transition of a long lived mesoscale convective vortex abstract the period 5 15 june 2003 during the field phase of the bow echo and mesoscale convective vortex mcv experiment bamex was noteworthy for the wide variety of mesoscale convective systems mcss that occurred of particular interest was a long lived mcv that formed in the trailing stratiform region of an mcs over west texas at 0600 utc 10 june this mcv was noteworthy for its i longevity as it can be tracked from 0600 utc 10 june to 1200 utc 14 june ii development of a surface cyclonic circulation and attendant 2 to 4 hpa sea level pressure perturbation iii ability to retrigger convection and produce widespread rains over several diurnal heating cycles and iv transition into a baroclinic surface cyclone with distinct frontal features baroclinic transition defined here as the acquisition of surface fronts occurred as the mcv interacted with a remnant cold front left behind by a predecessor extratropical cyclone over the great lakes region although the mcv developed well def

influence of balanced motions on heavy precipitation within a long lived convectively generated vortex abstract the forcing of heavy precipitation within a long lived convectively generated mesoscale vortex mcv is investigated with the aid of diagnoses from rapid update cycle gridded analyses organized convection within the mcv followed a distinct diurnal cycle which featured organized mesoscale convective systems mcss that matured overnight near the mcv center on successive days the mcv was typically most intense in the middle troposphere but intensified within the lower troposphere during the episodes of organized nocturnal convection the lower tropospheric vertical shear was an important organizing factor in mcs development and sustenance in the sense that its interaction with the cold temperature anomaly beneath the mcv center determined where balanced lower tropospheric ascent occurred from trajectory analyses we estimate that balanced ascent accounted for approximately half of the total vertical displacement of the thermodynamically unstable air that eventually composed elevated saturated

multi proxy record of postglacial environmental change south central melville island northwest territories canada abstract a sediment core from lake bc01 75 10 945 n 111 55 181 w 225 m asl on south central melville island nwt canada provides the first continuous postglacial environmental record for the region fossil pollen results indicate that the postglacial landscape was dominated by poaceae and salix typical of a high arctic plant community whereas the arctic herb oxyria underwent a gradual increase during the late holocene pollen based climate reconstructions suggests the presence of a cold and dry period 12 000 cal yr bp possibly representing the younger dryas followed by warmer and wetter conditions from 11 000 to 5000 cal yr bp likely reflective of the holocene thermal maximum the climate then underwent a gradual cooling and drying from 5000 cal yr bp to the present suggesting a late holocene neoglacial cooling diatom preservation was poor prior to 5000 cal yr bp when conditions were warmest suggesting that diatom dissolution may in part be climatically controlled diatom concentrations were highest 4500 cal yr bp but then decreased substantially by 3500 cal yr bp and remained low before recovering slightly in the 20th century an abrupt warming occurred during the past 70 yr at the site although the magnitude of this warming did not exceed that of the early holocene

the variable nature of convection in the tropics and subtropics a legacy of 16 years of the tropical rainfall measuring mission satellite for over 16 years the precipitation radar of the tropical rainfall measuring mission trmm satellite detected the three dimensional structure of significantly precipitating clouds in the tropics and subtropics this paper reviews and synthesizes studies using the trmm radar data to present a global picture of the variation of convection throughout low latitudes the multiyear data set shows convection varying not only in amount but also in its very nature across the oceans continents islands and mountain ranges of the tropics and subtropics shallow isolated raining clouds are overwhelmingly an oceanic phenomenon extremely deep and intense convective elements occur almost exclusively over land upscale growth of convection into mesoscale systems takes a variety of forms oceanic cloud systems generally have less intense embedded convection but can form very wide stratiform regions continental mesoscale systems often have more intense embedded convection some of the most intense convective cells and mesoscale systems occur near the great mountain ranges of low latitudes the maritime continent and amazonia exhibit convective clouds with maritime characteristics although they are partially or wholly land convective systems containing broad stratiform areas manifest most strongly over oceans the stratiform precipitation occurs in various forms often it occurs as quasi uniform precipitation with strong melting layers connected with intense convection in monsoons and the intertropical convergence zone it takes the form of closely packed weak convective elements where fronts extend into the subtropics broad stratiform regions are larger and have lower and sloping melting layers related to the baroclinic origin of the precipitation

holocene climatic records from agassiz ice cap ellesmere island nwt canada four ice cores from the top of the agassiz ice cap and down a flow line have been variously analysed for 8 18o ecm solid conductivity and ice melt layer stratigraphy stratigraphic correlation

structures and dynamics of quasi 2d mesoscale convective systems abstract recently three distinct archetypes for midlatitude linear mesoscale convective systems mcss have been identified this article focuses on the fundamentals of two of these archetypes convective lines with trailing stratiform ts precipitation and convective lines with leading stratiform ls precipitation both the ts and ls modes typically exhibit quasi 2d reflectivity patterns and quasi 2d environmental storm relative wind fields ongoing work has revealed that there are three common flow structures for these quasi 2d mcss front fed ts systems which are sustained by front to rear storm relative inflow as well as front fed ls and rear fed ls systems which are sustained by rear to front storm relative inflow this paper summarizes the observed structures of the front fed ts front fed ls and rear fed ls modes and then outlines an idealized numerical experiment in which these modes were simulated the authors analyze the basic simulated kinematic and microphysical structures and provid

balanced and unbalanced circulations in a primitive equation simulation of a midlatitude mcc part i the numerical simulation abstract a midlatitude mesoscale convective complex mcc which occurred over the central united states on 23 24 june 1985 was simulated using the regional atmospheric modeling system rams the multiply nested grid simulation agreed reasonably well with surface upper air and satellite observations and ground based radar plots the simulated mcc had a typical structure consisting of a leading line of vigorous convection and a trailing region of less intense stratiform rainfall several other characteristic mcc circulations were also simulated a divergent cold pool in the lower troposphere midlevel convergence coupled with a relatively cool descending rear inflow jet and relatively warm updraft structure and a cold divergent anticyclone in the tropopause region early in the mcc simulation a mesoscale convectively induced vortex mcv formed on the eastern edge of the convective line while frequently associated with mccs and other mesoscale convective systems mcss mcvs are more typically repo

scale interactions during the formation of typhoon irving the development of typhoon irving is investigated using a variety of data including special research aircraft data from the tropical cyclone motion tcm 92 experiment objective analyses satellite data and traditional surface and sounding data the development process is treated as a dry adiabatic vortex dynamics problem and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation synoptic scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development mesoscale interactions with the low level large scale circulations and with other midlevel mesoscale features result in development of vorticity in the midlevels and enhancement of the low level vorticity associated with the developing surface cyclone multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low level confluence and a tendency toward recurrent development of associated mesoscale convective vortices this is observed in both aircraft data and satellite imagery where subsequent interactions including mergers with the low level tropical depression vortex are observed a contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices results from a simple baroclinic experiment show that the midlevel vortices affect the large scale low level circulation in two ways 1 initially interactions between midlevel vortices produce a combined vortex of greater depth 2 interaction between midlevel vortices and the low level circulation produces a development downward of the midlevel vorticity this strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere

cloud resolving simulations of mesoscale vortex intensification and its effect on a serial mesoscale convective system abstract the authors performed numerical simulations of the 27 28 may 1998 mesoscale convective vortex mcv and mesoscale convective system mcs to understand how the mcv modulated the convection and how the convection intensified the vortex the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 was used initialized with the analysis from the rapid update cycle version 2 ruc 2 and nested to achieve 1 5 km grid spacing covering the mcs the model successfully simulated a north south convective band that formed near the center of the mcv in the early evening the simulation then correctly reoriented the convection along a roughly east west line in response to northward transport of warm conditionally unstable air within the nocturnal low level jet balanced vertical motion was found to contribute substantially to mesoscale lifting and thermodynamic destabilization which localized the convection horizontal transport of moist uns

the large scale environments of the global populations of mesoscale convective complexes abstract the mean genesis environment was constructed for each of five mesoscale convective complex mcc population centers around the world africa australia china south america and the united states it is found that the environments are very similar and exhibit many of the same dynamic and thermodynamic structures that are present with systems in the united states in particular mccs initiate within prominent baroclinic zones characterized by locally large values of lower tropospheric vertical wind shear and convective available potential energy cape typically a low level jet of air with low static stability high equivalent potential temperature oriented nearly perpendicular to the baroclinic zone intrudes into the genesis region and is forced to ascend over a relatively shallow surface based layer of relatively cool air pronounced warm advection accompanied by strong lower tropospheric veering overlays the surface based cool layer a local maximum in absolute humidity and a local minimu

mesoscale convective vortex formation in a weakly sheared moist neutral environment abstract idealized simulations of a diabatic rossby vortex drv in an initially moist neutral baroclinic environment are performed using the fifth generation national center for atmospheric research pennsylvania state university ncar psu mesoscale model mm5 the primary objective is to test the hypothesis that the formation and maintenance of midlatitude warm season mesoscale convective vortices mcvs are largely influenced by balanced flow dynamics associated with a vortex that interacts with weak vertical shear as a part of this objective the simulated drv is placed within the context of the bow echo and mesoscale convective vortex experiment bamex field campaign by comparing its tangential velocity radius of maximum winds cape and shear with the mcvs observed in bamex the simulations reveal two distinct scales of development at the larger scale the most rapidly growing moist baroclinic mode is excited and exponential growth of this mode occurs during the simulation embedded within the

the effect of the balcones escarpment on three cases of extreme precipitation in central texas abstractthe proximity to the gulf of mexico and local topography makes central texas particularly prone to heavy precipitation and deadly flood events specifically the balcones escarpment located in central texas creates extremely favorable hydrologic characteristics for damaging floods urban centers such as san antonio and austin texas are located along this terrain feature and have suffered at times even with mitigation strategies catastrophic flood damage while the hydrologic effects of the balcones escarpment are well known the meteorological impacts are uncertain the purpose of this study is to evaluate the effect of the balcones escarpment in three cases of extreme precipitation in which the rainfall was maximized near the escarpment numerical simulations for each event were run at convection allowing grid spacing using the weather research and forecasting wrf model and were used as control runs then the balcones escarpment was removed by moving the associated terrain gradient to th