rapid destruction of a stratospheric potential vorticity anomaly by convectively induced inertial instability during the southern wisconsin extreme flooding event of 20 august 2018

early evolution of vertical vorticity in a numerically simulated idealized convective line the generation and organization of mesoscale convective vortices mcvs is a recurring theme in midlatitude and tropical meteorology during the warm season in this work a simulation of a finite length idealized convective line in a westerly shear environment is investigated in the absence of ambient vertical vorticity an asymmetry in average vertical vorticity forms rapidly at early times in the present simulation this study focuses on the formation and organization of vertical vorticity at these early simulation times previous simulations suggest that tilting of either ambient or storm generated horizontal vorticity is the primary mechanism responsible for the formation organization and maintenance of mcvs this study confirms recent work regarding the generation of vertical vorticity at early times in the simulation a lagrangian budget analysis of the vertical vorticity equation however shows that vorticity convergence becomes a comparable and at times dominant mechanism for the enhancement and long term organization of vertical vorticity early in the simulation despite differences in the initial ambient horizontal vorticity hodograph and convective available potential energy the lagrangian budget analysis in the present midlatitude case is consistent with the lagrangian budget results of a previous tropical squall line simulation the study of idealized convective lines in midlatitude environmental conditions therefore provide valuable insight into understanding vertical vorticity production in tropical squall lines and their potential relevance to tropical cyclogenesis

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

greenland glacial history and local geodynamic consequences summary space time reconstructions of the continental ice sheets that existed at last glacial maximum lgm have previously been produced using two entirely independent methodologies one based upon the use of theoretical models of ice sheet accumulation and flow and one based upon the geophysical inversion of relative sea level rsl histories from previously ice covered regions the analyses described in this paper demonstrate the significant advantages that derive from the simultaneous application of both methods to the particular case of greenland we thereby show that the ice 4g reconstruction of the glaciation history of this region from lgm to present which was based upon the geophysical inversion of rsl data alone was reasonably accurate in the peripheral regions where rsl data were available but inaccurate in the interior of the ice sheet which was unconstrained by such information we test the new model of greenland glacial history determined by the simultaneous application of the constraints that derive from ice sheet modelling and the geophysical inversion of rsl data by employing recently published geodetic inferences of mass balance over the entire interior region of the ice sheet and of gps measurements of vertical crustal motion these observations which were not employed to constrain the ice sheet reconstruction provide significant support for the new glacial history for greenland that our analyses have led us to infer

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

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

thermomechanical modelling of northern hemisphere ice sheets with a two level mass balance parameterization a 3 d time dependent thermomechanical ice sheet model was used together with a two level snow accumulation runoff mass balance model to investigatethe quaternary ice sheets of the northern hemisphere the model freely generates the ice sheet geometry in response to specified changes of surfacetemperature and mass balance and includes bedrock adjustment basal sliding and a full temperature calculation within the ice the mass balanceparameterisation makes a distinction between snowfall and melting yearly snow fall rates depend on the present precipitation distribution and are variedproportional to changes of surface temperature and the moisture content of the air the ablation model is based on the positive degree day method anddistinguishes between ice and snow melting this paper discusses steady state characteristics conditions for growth and retreat and response time scales ofice sheets as a function of a prescribed lowering of summer temperature most notably the modelled extents of the eurasian ice sheet for a summertemperature lowering of 6 7 k and of the laurentide ice sheet for a cooling of 9 10 k are in reasonable agreement with most reconstructions based ongeological evidence except for the presence of a large ice sheet stretching from alaska across the bering strait to most of eastern siberia in addition wetbasal conditions turned out to be always confined to the margin whereas central areas in these reconstructions remained always cold based this is ofrelevance for processes involving reduced basal traction

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 model computation of the temporal changes of surface gravity and geoidal signal induced by the evolving greenland ice sheet this paper deals with present day gravity changes in response to the evolving greenland ice sheet we present a detailed computation from a 3 d thermomechanical ice sheet model that is interactively coupled with a self gravitating spherical viscoelastic bedrock model the coupled model is run over the last two glacial cycles to yield the loading evolution over time based on both the ice sheet s long term history and its modern evolution averaged over the last 200 years results are presented of the absolute gravity trend that would arise from a ground survey and of the corresponding geoid rate of change a satellite would see from space the main results yield ground absolute gravity trends of the order of 1 gal yr 1 over the ice free areas and total geoid changes in the range between 0 1 and 0 3 mm yr 1 these estimates could help to design future measurement campaigns by revealing areas of strong signal and or specific patterns although there are uncertainties associated with the parameters adopted for the earth s rheology and aspects of the ice sheet model given the instrumental accuracy of a particular surveying method these theoretical trends could also be useful to assess the required duration of a measurement campaign according to our results the present day gravitational signal is dominated by the response to past loading changes rather than current mass changes of the greenland ice sheet we finally discuss the potential of inferring the present day evolution of the greenland ice sheet from the geoid rate of change measured by the future geodetic grace mission we find that despite the anticipated high quality data from satellites such a method is compromised by the uncertainties in the earth model the dominance of isostatic recovery on the current bedrock signal and other inaccuracies inherent to the method itself

positive degree day factors for ablation on the greenland ice sheet studied by energy balance modelling ice ablation is related to air temperature by the positive degree day factor variations of the positive degree day factor in west greenland are studied using an energy balance model to simulate ablation under different conditions degree day factors for simulated and measured ice ablation at nordbogletscher and qaman rssup sermia agree well with values around 8 mm d 1 c 1 degree day factors for snow are less than half those for ice energy balance modelling shows that degree day factors vary with summer mean temperature surface albedo and turbulence but there is only evidence of large positive degree day factors at lower temperatures and with low albedo 0 3 the greatest effect of albedo variations 0 3 0 7 is at lower temperatures while variations in turbulence have greater effect at higher temperatures current models may underestimate runoff from the greenland ice sheet by several tenths because they use a degree day factor for melting ice that is too small for the colder parts of the ice sheet i e the upper ablation area and the northerly margin

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

permanent fast flow versus cyclic surge behaviour numerical simulations of the austfonna ice cap svalbard a large part of the ice flux within ice caps occurs through spatially limited fast flowing units some of them permanently maintain fast flow whereas others operate in an oscillatory mode characterized by short lived active phases followed by long quiescent phases this surge type behaviour results from intrinsic rather than external factors thus complicating estimates of glacier response to climate change here we present numerical model results from austfonna an ice cap on svalbard that comprises several surge type basins previous studies have suggested a thermally controlled soft bed surge mechanism for svalbard we systematically change the parameters that govern the nature of basal motion and thereby control the transition between permanent and oscillatory fast flow surge type behaviour is realized by a relatively abrupt onset of basal sliding when basal temperatures approach the pressure melting point and enhanced sliding of marine grounded ice irrespective of the dynamic regime the absence of considerable volumes of temperate ice both in the observed and simulated ice cap indicates that fast flow is accomplished by basal motion over a temperate bed given an idealized present day climate the equilibrium ice cap size varies significantly depending on the chosen parameters

global water balance and atmospheric water vapour transport at last glacial maximum climate simulations with the canadian climate centre for modelling and analysis atmospheric general circulation model a series of new simulations of the climate state at last glacial maximum has been performed using the canadian second generation atmospheric general circulation model and are described herein the

sensitivity studies of northern hemisphere glaciation using an atmospheric general circulation model abstract the u k university global atmospheric modeling programme gcm is used to investigate whether the growth of northern hemisphere ice sheets could have been initiated by changes of orbital parameters and sea surface temperature two different orbital configurations corresponding to the present day and 115 kyr bp are used the reduced summer solar insulation in the northern hemisphere results in a decrease of the surface temperature by 4 to 10 c in the northern continents and to perennial snow in some high latitude regions therefore the model results support the hypothesis that a deficit of summer insulation can create conditions favorable for initiation of ice sheet growth in the northern hemisphere a decreased sea surface temperature northward of 65 n during the northern hemisphere summer may contribute to the maintenance of ice sheets a simple mixed layer ocean model coupled to the gcm indicates that the changes of sea surface temperature and extension of sea ice due to insulation changes pl

modeling north american freshwater runoff through the last glacial cycle the northern hemisphere ice sheets decayed rapidly during deglacial phases of the ice age cycle producing meltwater fluxes that may have been of sufficient magnitude to perturb oceanic circulation the continental record of ice sheet history is more obscured during the growth and advance of the last great ice sheets ca 120 000 20 000 yr b p but ice cores tell of high amplitude millennial scale climate fluctuations that prevailed throughout this period these climatic excursions would have provoked significant fluctuation of ice sheet margins and runoff variability whenever ice sheets extended to mid latitudes giving a complex pattern of freshwater delivery to the oceans a model of continental surface hydrology is coupled with an ice dynamics model simulating the last glacial cycle in north america meltwater discharged from ice sheets is either channeled down continental drainage pathways or stored temporarily in large systems of proglacial lakes that border the retreating ice sheet margin the coupled treatment provides quantitative estimates of the spatial and temporal patterns of freshwater flux to the continental margins results imply an intensified surface hydrological environment when ice sheets are present despite a net decrease in precipitation during glacial periods diminished continental evaporation and high levels of meltwater production combine to give mid latitude runoff values that are highly variable through the glacial cycle but are two to three times in excess of modern river fluxes drainage to the north atlantic via the st lawrence hudson and mississippi river catchments averages 0 356 sv for the period 60 000 10 000 yr b p compared to 0 122 sv for the past 10 000 yr high amplitude meltwater pulses to the gulf of mexico north atlantic and north pacific occur throughout the glacial period with ice sheet geometry controlling intricate patterns of freshwater routing variability runoff from north america is staged in the final deglaciation with a stepped sequence of pulses through the mississippi st lawrence arctic and hudson strait drainages

ice age ice sheet rheology constraints from the last glacial maximum form of the laurentide ice sheet state ot the art thermomechanical models of the modern greenland ice and the ancient laurenticle ice sheet that covered canada at the last glacial maximum lgm are not able to explain simultaneously the observed forms of these cryospheric structures when the same anisotropy enhanced version of the conventional glen flow law is employed to describe their rheology the lgm laurenticle ice sheet predicted to develop in response to orbital climate forcing is such that the ratio of its thickness to its horizontal extent is extremely large compared to the aspect ratio inferred on the basis of surface geomorphological and solid earth geophysical constraints we show that if the glen flow law representation of the rheology is replaced with a new rheology based upon very high quality laboratory measurements of the stress strain rate relation then the aspect ratios of both the modern greenland ice sheet and the laurenticle ice sheet that existed at the lgm are simultaneously explained with little or no retuning of the flow law

processes controlling the structure and longevity of two quasi linear convective systems crossing the southern new england coast abstractthis paper explores the structural evolution and physical processes that explain the modification of two quasi linear convective systems qlcss that encountered the densely populated new york city atlantic coastal region one qlcs on 31 may 2002 traversed the atlantic coastal boundary with little change in its intensity producing widespread severe wind damage across new york city and long island during this event warm air advection at 925 hpa helped destabilize the layer above this level over the coastal zone while the marine boundary layer deepened below this level the 0 3 km line perpendicular vertical wind shear was relatively strong which supported ascent along the leading edge of the diabatically generated cold pool the surface based convective system became slightly elevated as it moved over the marine waters in contrast the 23 july 2002 qlcs decayed upon encountering the atlantic coastline despite its coincidence with a surface cold front the most unstable cape values during thi

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

coupled 100 kyr cycles between 3 and 1 ma in terrestrial and marine paleoclimatic records earth s climate over the last one million years experienced several 100 kyr glacial cycles but no simple forcing mechanism has been identified numerous studies have tried to explain strong 100 kyr glacial cycles without recognizable forcing which has come to be known as the 100 kyr problem few studies have examined 100 kyr band paleoclimatic signals before 1 ma a recent study has demonstrated that benthic oxygen and carbon isotope records are phase locked and amplitude coupled at the 100 kyr band but that neither is phase locked to and amplitude coupled with the 100 kyr eccentricity signal between 3 and 1 ma this phasing and amplitude mismatch of the 100 kyr band between 3 and 1 ma between marine records and the eccentricity forcing signal has been called the late pliocene early pleistocene 100 kyr problem however it remains unknown whether terrestrial paleoclimate records are consistent with marine records at the 100 kyr band here i show that loess monsoon records from china are amplitude coupled with benthic oxygen and carbon isotope records at the 100 kyr band but not with the 100 kyr eccentricity forcing between 3 and 1 ma this observation provides further evidence in support of a free 100 kyr oscillation as the cause of the 100 kyr band amplitude variability in paleoclimatic records between 3 and 1 ma in contrast benthic oxygen isotope records and loess monsoon records at the 100 kyr band are not amplitude coupled with 100 kyr benthic carbon isotope records over the last 0 4 million years indicating that the late pleistocene 100 kyr climatic cycles may not result exclusively from a free oscillation

the johnstown flood of july 1977 a long lived convective system abstract the heavy rains responsible for the disastrous flash floods new johnstown pennsylvania on the night of 19 20 july 1977 are shown to be part of a large quasi circular mesoscale convective complex this complex can be traced back to an origin in south dakota nearly 96 h earlier the complex tends to become more compact and intense during the night with a radius of 150 km and to expand in response to new and peripheral convection during the day on the small synoptic scale the system is characterized by a pool of cool air at the surface and throughout the lower troposphere just north of the centroid of the rainstorm by a cyclonic circulation of substantial intensity in the lower and middle troposphere and by anticyclonic circulation in the upper troposphere airflow relative to the moving storm is southeasterly at the surface veering rapidly to southwesterly in the boundary layer and then to westerly in the upper troposphere substantial convective instability is present throughout the env

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

late pleistocene ice age scenarios based on observational evidence ice age scenarios for the last glacial interglacial cycle based on observations of boyle and keigwin concerning the north atlantic thermohaline circulation and of barnola et al concerning atmospheric co sub 2 variations derived from the vostok ice cores are herein analyzed northern hemisphere continental ice sheets are simulated with an energy balance model ebm that is asynchronously coupled to vertically integrated ice sheets models based on the glen flow law the ebm includes both a realistic land sea distribution and temperature albedo feedback and is driven with orbital variations of effective solar insolation with the addition of atmospheric co sub 2 and ocean heat flux variations but not in their absence a complete collapse is obtained for the eurasian ice sheet but not for the north american ice sheet we therefore suggest that further feedback mechanisms perhaps involving more accurate modeling of the dynamics of the mostly marine based laurentide complex appears necessary to explain termination i 96 refs 12 figs 2 tabs

ambient conditions associated with the maintenance and decay of quasi linear convective systems crossing the northeastern u s coast abstractquasi linear convective systems qlcss crossing the atlantic coastline over the northeastern united states were classified into three categories based on their evolution upon encountering the coast composite analyses show that convective lines that decay near the atlantic coast or slowly decay over the coastal waters are associated with 900 800 hpa frontogenesis with greater ambient 0 3 km vertical wind shear for the slowly decaying lines systems that maintain their intensity over the coastal ocean are associated with 900 hpa warm air advection but with little low level frontogenetical forcing neither sea surface temperature nor ambient instability was a clear delimiter between the three evolutions sustaining convective lines have the strongest environmental 0 3 km shear of the three types and this shear increases as these systems approach the coast in contrast the low level shear decreases as decaying and slowly decaying convective lines move toward the atlantic coastline there was als

late pliocene early pleistocene 100 ka problem 1 little is known about the 100 ka ice volume cycles immediately before 1 ma except that they are coherent with the 100 ka eccentricity cycles here we show that ice volume and ocean circulation change global carbon mass balance are phase locked and highly coupled regarding amplitude envelopes at the 100 ka band but neither is phase locked to and amplitude coupled to the 100 ka eccentricity orbital forcing signal between 3 1 ma we describe this phasing and amplitude mismatch at the 100 ka band between 3 1 ma between eccentricity forcing and paleo ice volume records as the late pliocene early pleistocene 100 ka problem a free 100 ka oscillation internal to our climate system especially those related to oscillations of atmospheric co2 concentration operating on an earth characterized by a high inertia deep thermohaline ocean that can store carbon and heat might hold the key for the late pliocene early pleistocene 100 ka problem

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

coupled energy balance ice sheet model simulations of the glacial cycle a possible connection between terminations and terrigenous dust we apply a coupled energy balance ice sheet climate model in an investigation of northern hemisphere ice sheet advance and retreat over the last glacial cycle when driven only by orbital insolation variations the model predicts ice sheet advances over the continents of north america and eurasia that are in good agreement with geological reconstructions in terms of the timescale of advance and the spatial positioning of the main ice masses the orbital forcing alone however is unable to induce the observed rapid ice sheet retreat and we conclude that additional climatic feedbacks not explicitly included in the basic model must be acting in the analyses presented here we have parameterized a number of potentially important effects in order to test their relative influence on the process of glacial termination these include marine instability thermohaline circulation effects carbon dioxide variations and snow albedo changes caused by dust loading during periods of high atmospheric aerosol concentration for the purpose of these analyses the temporal changes in the latter two variables were inferred from ice core records of these various influences our analyses suggest that the albedo variations in the ice sheet ablation zone caused by dust loading may represent an extremely important ablation mechanism using our parameterization of dirty snow in the ablation zone we find glacial retreat to be strongly accelerated such that complete collapse of the otherwise stable laurentide ice sheet ensues the last glacial maximum configurations of the laurentide and fennoscandian complexes are also brought into much closer accord with the ice 3g reconstruction of tushingham and peltier 1991 1992 and the ice 4g reconstruction of peltier 1994 when this effect is reasonably introduced

a 10 year radar based climatology of mesoscale convective system archetypes and derechos in poland abstractin this study a 10 year 2008 2017 radar based mesoscale convective system mcs and derecho climatology for poland is presented this is one of the first attempt for a european country to

modeling the marine extent of northern hemisphere ice sheets during the last glacial cycle mechanisms which determine time dependent changes of the marine ice margin in dynamic ice sheet models are important but poorly understood here we derive an empirical formulation for changes in the marine extent when modeling the northern hemisphere ice sheets over the last glacial cycle in a three dimensional thermo mechanically coupled ice sheet model we assume that the strongest control on changes in marine extent is ice calving and that the variable most crucial to calving is water depth the empirical marine extent relationship is tuned so that the major marine retreat history of the north american and eurasian ice sheets are modeled accurately in time and space we find that this empirical treatment based on water depth is sufficient to reproduce the observations and discuss the implications for the physics of marine margin changes and the dynamics of the northern hemisphere ice sheets since the last glacial maximum

the role of synoptic scale flow during tropical cyclogenesis over the north atlantic ocean abstract the synoptic scale flow during tropical cyclogenesis and cyclolysis over the north atlantic ocean is investigated using compositing methods genesis and lysis are defined using the national hurricane center nhc now known as the tropical prediction center best track data genesis lysis occurs when nhc first last identifies and tracks a tropical depression in the final best track dataset storm centered composites are created with the analysis of the tropical oceanic lower level atoll 900 hpa and 200 hpa winds for june november produced by nhc for the years 1975 93 results show that significant regional differences exist in 200 hpa flow during genesis across the atlantic basin composites of genesis in the western part of the basin show a 200 hpa trough ridge located to the west east of the atoll disturbance in the eastern half of the basin composites of genesis show a sprawling 200 hpa ridge centered northeast of the atoll disturbance the major axis of this elliptically shaped 20

effect of isostatic rebound on modelled ice volume variations during the last 200 kyr deformation of the lithosphere under an ice load can be approximated using the hypothesis of local damped isostasy this simple formulation has been systematically compared with a three dimensional model of the crust mantle system for simple ice load scenarios with a period in the range 20 100 kyr the comparison enables us to introduce the concepts of effective upper mantle density and effective relaxation time into the isostatic model for the response of the earth to the ice sheets these parameters depend on the earth model considered ice sheet size and the period of the load cycle the local damped isostasy model has been implemented in the louvain la neuve climate model to assess the impact of isostasy on continental ice volume variations for the last 200 kyr results suggest that isostasy acts as a negative feedback for ice volume during the glaciation process and acts as a positive feedback during the deglaciation moreover taking isostasy into account is necessary to simulate variations in northern hemisphere ice volume during isotopic stage 3 lastly the use of effective mantle density and effective relaxation time improves the model performance regarding specmap reconstructions c 2001 elsevier science b v all rights reserved

effect of isostasy on dynamical ice sheet modeling a case study for eurasia 1 isostatic adjustment is an important process when modeling dynamical ice sheet evolution a number of different types of earth model are used to model this process ranging from models with local or elastic lithospheres with a single relaxation time to account for temporal behavior to self gravitating spherically symmetric models of the crust mantle system we performed sensitivity tests to assess the dynamical response of ice sheets to isostatic adjustment the resulting ice sheet geometries depend on the response times of the system including the growth rate of ice sheets versus the relaxation time of the earth differences in lithospheric strength cause large differences in the modeled ice sheets with the largest differences in isostatic adjustment in the vicinity of the ice margins in addition we compared predictions based on an often used flexural model with an elastic plate and a single relaxation time to a spherically symmetric self gravitating viscoelastic approach no flexural model was able to reproduce the viscoelastic response completely due to the fact that the models have an intrinsically different spatial and temporal response when applied to a full transient glacial cycle in eurasia even the best fitting flexural model underestimated the ice volume at the lgm by 30 percent our results indicate that the commonly used value of 3000 years for the relaxation time in flexural models is too short for eurasia by at least 3000 years

the deglaciation of the northern hemisphere a global perspective orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming ice albedo sealevel and greenhouse gas feedbacks together with tropical warming from weakening winds in response to polar amplification of warming caused regionalto global near synchronization of deglaciation effects were larger at orbital rather than millennial frequencies because ice sheets and carbon dioxide vary slowly ice sheet linked changes in freshwater delivery to the north atlantic and possibly free oscillations in the climate system forced millennial climate oscillations associated with changes in north atlantic deep water nadw flow the north atlantic typically operates in one of three modes modern glacial and heinrich deglaciation occurred from a glacial mode ocean that in comparison to modern had shallower depth of penetration of nadw formed further south causing strong northern cooling and the widespread cold dry and windy conditions associated with the glacial maximum and the cold phases of the millennial dansgaard oeschger oscillations the glacial mode was punctuated by meltwater forced heinrich conditions that caused only small additional cooling at high northern latitudes but greatly reduced the formation of nadw and triggered an oceanic seesaw that warmed some high latitude southern regions centered in the south atlantic

terminating the 100 kyr ice age cycle we report a simulation of the most recent 100 000 year glaciationdeglaciation cycle of the late pleistocene ice age a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one level energy balance model ebm and vertically integrated ice sheet model ism the theory fully incorporates the influences of orbital insolation forcing glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases it correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21 000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently the latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model a limitation of all previous attempts to understand this global scale climate cycle our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of co2 is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence with the incorporation of this influence model predicted ice thickness distributions at last glacial maximum lgm are very similar to those of the recently described ice 4g reconstruction that was based upon the inversion of postglacial relative sea level histories

simple energy balance model resolving the seasons and the continents application to the astronomical theory of the ice ages a one level seasonal energy balance climate model which has explicit two dimensional land sea geography is introduced and its properties analyzed the model distinguishes land from sea surface strictly by the local thermal inertia employed the transport is governed by a smooth latitude dependent diffusion mechanism the model s seasonal cycle is solved for with and without ice feedback in each case good agreement with real data is accomplished with minimal tuning of the adjustable parameters the model sensitivity for small solar constant changes is in agreement with that of other accepted models however when the solar constant is lower a few percent the ice cap grows discontinuously from covering greenland and vicinity to a size three times larger and mostly into northern canada a similar discontinuous growth occurs when the orbital elements are changed to favor cool summers in the northern hemisphere this discontinuous sensitivity is discussed in the context of the milankovitch theory of the ice ages and the associated branch structure is shown to be analogous to the so called small ice cap instability known from simpler models although the discontinuity in sensitivity is somewhat conjectural both linear and nonlinear models have their largest sensitivity to orbital parameter changes in the northern portions of the large continents in the western hemisphere this also corresponds to the ice sheet boundary suggesting that the land sea configuration is crucial to large glacier growth irrespective of moisture budgets other experiments and implications are also discussed the remarkable paper by hays et al 1976 hereafter his presented compelling empirical evidence that continental ice sheet volume changes and sea surface temperature variations in the last 700 000 years are correlated with the changes in the earth s orbital elements theoretical arguments for such a connection date back to the last century but the most detailed and convincing arguments are due to milankovitch 1941 and the theory usually is associated with his name if the hypothesis is correct the theory and observations provide the cleanest experiment to determine the sensitivity of the earth s climate to truly external influences such a measure of climate sensitivity is of obvious importance considering the recent interest in estimating the change in climate due to other forcings such as the enhanced greenhouse effect due to increased levels of co2 in the atmosphere e g ramanathan 1981 hansen et al 1981 manabe and stouffer 1980 in effect climate models might be tested or calibrated by this natural experiment climate modeling has progressed dramatically in the last decade with the construction and testing of several general circulation models gcm s although some interesting ice age experiments have been done with these models gates 1976 manabe and hahn 1977 they still include only crude parameterizations of the slower components such as the oceans these numerical models are the ultimate tool that can be brought to bear upon the climate problem however they are almost as complicated as the climate itself hence experiments must be carefully designed and the interpretation is often difficult due to the expense of obtaining definitively long runs in parallel with gcm construction has been the development of x permanently at applied research corporation landover maryland 20785

degree day glacier mass balance modelling with applications to glaciers in iceland norway and greenland a degree day glacier mass balance model is applied to three glaciers in iceland norway and greenland for which detailed mass balance measurements are available over a period of several years model results are in good agreement with measured variations in the mass balance with elevation over the time periods considered for each glacier in addition the model explains 60 80 of the year to year variance in the elevation averaged summer season mass balance measurements on the glaciers using a single parameter set for each glacier the increase in ablation on the glaciers due to a warming of 2 c is predicted to range from about 1 m w e year 1 at the highest elevations to about 2 5 m w e year 1 at the lowest elevations predicted changes in the winter balance measured between fixed date are relatively small except at the lowest elevations on the icelandic and norwegian glaciers where the winter balance is significantly reduced equilibrium line altitudes are raised by 200 300 m on the icelandic and norwegian glaciers except at the highest elevations the winter balance of the icelandic and norwegian glaciers is predicted to decrease even if the warming is accompanied by a 10 increase in the precipitation no firm evidence of a climate related variation in the degree day factors or in the temperature lapse rate on the same glacier could be found the model furthermore reproduces large variations in the mass balance with elevation and from year to year on each glacier using the same parameter set we assume therefore that these parameters will not change significantly for the climate scenarios considered here

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

numerical reconstruction of a soft bedded laurentide ice sheet during the last glacial maximum we used a numerical ice sheet model to reconstruct the north american laurentide ice sheet during the last glacial maximum our model simulates ice sheet conditions that can be specified experimentally as either a rigid substrate hard bed or a wet deformable till soft bed basal sliding is excluded we use geologic records of former basal ice sheet processes to prescribe the distribution of hard and soft beds our reconstruction of the laurentide ice sheet is significantly lower in ice surface height and contains less ice volume than the climap maximum reconstruction in contrast our reconstruction agrees well with the ice 4g reconstruction both in height and volume because the ice 4g reconstruction is based on the inversion of relative sea level data whereas our reconstruction is based on glacial geology and ice mechanics this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the laurentide ice sheet that is most consistent with observations of relative sea level change and other geodynamic considerations

simulation of the last glacial cycle by a coupled sectorially averaged climate ice sheet model 2 response to insolation and co2 variations a two dimensional climate model which links the northern hemisphere atmosphere ocean mixed layer sea ice and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock the coupled model has been used to test the influence of several factors including snow surface albedo over the ice sheets in producing plausible ice age simulations using astronomically derived insolation and co2 data from the vostok ice core the impact of potentially important processes such as the water vapor transport clouds and deep sea circulation was not investigated in this study after several sensitivity experiments designed to identify the main mechanism governing surface temperature and ice accumulation the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr large variations of ice volume are simulated between 122 and 55 kyr b p with a rapid latitudinal extension of the north american and eurasian ice sheets starting at 120 kyr b p the simulated last glacial maximum is at 19 kyr b p the model is able to simulate deglaciation as well the simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions the major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations linked with an underestimation of the ice sheet extent and an excess in the prescribed co2 concentration sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations in the model a key mechanism in the deglaciation after the last glacial maximum appears to be the aging of snow which decreases its albedo the other factors which play an important role are in decreasing level of importance the ice sheet altitude insolation taiga cover and ice sheet extent a final set of experiments addresses the effects of co2 on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr this last experiment is made by forcing the model with both insolation and co2 variations this additional forcing improves the temperature and ice volume results despite the limitations inherent to the present modeling approach the sensitivity experiments performed can provide insight into the relative importance of possible mechanism responsible for the building and melting of huge ice sheets during the last glacial interglacial cycle

dynamics of the north american ice sheet complex during its inception and build up to the last glacial maximum the north american ice sheet complex played a major role in global sea level fluctuations during the late quaternary but our knowledge of its dynamics is based mostly on its demise from the last glacial maximum lgm a period characterised by non linear behaviour in the form of punctuated ice margin recession episodic ice streaming and major shifts in the location of ice divides in comparison knowledge of the pre lgm ice complex is poorly constrained largely because of the fragmentary nature of the evidence relating to ice sheet build up in this paper we explore the inception and growth of ice 120 20 ka using a glacial systems model which has been calibrated against a large and diverse set of data relating to the deglacial interval we make use of calibration data prior to the lgm but its scarcity introduces greater uncertainty which is partly alleviated by our large ensemble analysis results suggest that following the last interglaciation oxygen isotope stage ois 5e the ice complex initiated over the north eastern canadian arctic and in the cordillera within a few thousand years it then underwent rapid growth to an ois 5 maximum at 110 ka 5d and covered 70 of the area occupied by the lgm ice cover although only 30 by volume an ois 5 minimum is modelled at 80 ka 5a before a second phase of rapid growth at the start of ois 4 which culminated in a large ice complex at 65 ka almost as large as at the lgm subsequent deglaciation was rapid maximum modelled sea level contribution of 16 cm per century and resulted in an ois 3 minimum between ca 55 60 ka thereafter the ice complex grew towards its lgm configuration interrupted by several phases of successively less significant mass loss our results support and extend previous inferences based on geological evidence and reinforce the notion of a highly dynamic pre lgm ice complex e g with episodes of 10 s m of eustatic sea level equivalent in 50 but even the thin ois 5 ice sheets exhibit fast flow features several 1000 m a 1 in major topographic troughs notwithstanding the severe limitations imposed by the use of the shallow ice approximation we note that most fast flow features generated prior to the lgm correspond to the location of known ice streams during deglaciation i e in major topographic troughs and over soft sediments at the southern and western margins moreover the modelled flux of these ice streams sensu lato appears to be non linearly scaled to ice sheet volume i e there is no evidence that decay phases were associated with significantly increased ice stream activity this hypothesis requires testing using a model with higher order physics and future modelling would also benefit from additional pre lgm constraints e g dated ice free margin positions to help reduce and quantify uncertainties

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

results from the ice sheet model intercomparison project heinrich event intercomparison ismip heino results from the heinrich event intercomparison heino topic of the ice sheet model intercomparison project ismip are presented ismip heino was designed to explore internal large scale ice sheet instabilities in different contemporary ice sheet models these instabilities are of interest because they are a possible cause of heinrich events a simplified geometry experiment reproduces the main characteristics of the laurentide ice sheet including the sedimented region over hudson bay and hudson strait the model experiments include a standard run plus seven variations nine dynamic thermodynamic ice sheet models were investigated one of these models contains a combination of the shallow shelf ssa and shallow ice approximation sia while the remaining eight models are of sia type only seven models including the sia ssa model exhibit oscillatory surges with a period of 1000 years for a broad range of parameters while two models remain in a permanent state of streaming for most parameter settings in a number of models the oscillations disappear for high surface temperatures strong snowfall and small sediment sliding parameters in turn low surface temperatures and low snowfall are favourable for the ice surge cycles we conclude that further improvement of ice sheet models is crucial for adequate robust simulations of cyclic large scale instabilities

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

vostok ice core a continuous isotope temperature record over the last climatic cycle 160 000 years a continuous deuterium profile along the 160 000 year vostok ice core antarctica is interpreted in terms of atmospheric temperature changes this climatic record is the awaited terrestrial complement of the deep sea records supporting the existence of a relation between the pleistocene climate and orbital forcing

present day uplift patterns over greenland from a coupled ice sheet visco elastic bedrock model we present results from a fully coupled ice bedrock model calculation of the greenland ice sheet and the underlying earth during the last two glacial cycles the method treats the mutual interaction between the ice and the bedrock and yields the main glacial isostatic characteristics of the ice sheet evolution and the bedrock adjustment since the last glacial maximum by taking advantage of the splitting of the present day rate of bed uplift into a viscous and an elastic component these results allow one to distinguish between the bedrock response to the past and present ice sheet evolution respectively

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

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

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

basal temperature evolution of north american ice sheets and implications for the 100 kyr cycle 1 we simulate three dimensional ice temperature fields to examine spatial temporal history of the subglacial thermal environment during the last glacial cycle model results suggest that 60 80 of the laurentide ice sheet was cold based frozen to the bed at the lgm and therefore unable to undergo large scale basal flow the fraction of warm based ice increases significantly through the ensuing deglaciation with only 10 20 of the laurentide ice sheet frozen to the bed by 8 kyr bp this basal thermal evolution a function of both climatic and ice sheet history could enable a dynamical switch to widespread basal flow through the deglacial period because basal flow has the capacity to evacuate large amounts of ice from the interior of continental ice sheets creating thin and climatically vulnerable ice masses this switch in flow regime may have played a significant role in glacial terminations and the 100 kyr glacial cycle

climatology of summer midtropospheric perturbations in the us northern plains part ii large scale effects of the rocky mountains on genesis propagating convective storms across the us northern plains are often coupled with preexisting midtropospheric perturbations mps initiated over the rocky mountains a companion study part i notes that such mps occur most commonly at 12 utc early morning and 00 utc late afternoon using a regional reanalysis and a general circulation model gcm this study investigates how such a bimodal distribution of the mp frequency is formed the results point to two possible mechanisms working together while each has a different timing in terms of maximum effect the diurnal evolutions between the midtropospheric flows over the rockies and over the great plains are nearly out of phase due to inertial oscillation during the nighttime the westerly flows at 700 500 mb over the rockies intensify while flows at the same level over the great plains turn easterly these two flows converge over the eastern rockies and induce cyclonic vorticity through vortex stretching after sunrise the convergence dissipates and the cyclonic vorticity is redistributed by horizontal vorticity advection moving it downstream this process creates a climatological zonally propagating vorticity signal which in turn facilitates the early morning mp genesis at 12 utc the analysis also reveals marked dynamic instability conducive to subsynoptic scale disturbances in the midtroposphere over the rockies strong meridional temperature gradients appear over the north facing slopes of the rockies due to terrain heating to the south and the presence of cooler air to the north this feature along with persistent vertical shear creates a charney stern type of instability i e sign changes of the meridional potential vorticity gradient meanwhile the development of terrain boundary layer reduces the rossby deformation radius which subsequently enhances the likelihood for baroclinic short waves such effects are most pronounced in the late afternoon and therefore are supportive to the mp genesis around 00 utc examination of gcm experiments with and without orography further supports the critical role of the rocky mountains and its associated boundary layer impacts on the formation of mps

genesis of typhoon nari 2001 from a mesoscale convective system 1 in this study the origin and genesis of typhoon nari 2001 as well as its erratic looping track are examined using large scale analysis satellite observations and a 4 day nested cloud resolving simulation with the finest grid size of 1 33 km observational analysis reveals that nari could be traced 5 days back to a diurnally varying mesoscale convective system with growing cyclonic vorticity and relative humidity in the lower troposphere and that it evolved from a mesoscale convective vortex mcv as moving over a warm ocean under the influence of a subtropical high a weak westerly baroclinic disturbance an approaching and departing typhoon danas to the east and the kuroshio current results show that the model reproduces the genesis final intensity looping track and the general convective activity of nari during the 4 day period it also captures two deep subvortices at the eye eyewall interface that are similar to those previously observed a few spiral rainbands and a midget storm size associated with nari s relatively dry and stable environment we find that 1 continuous convective overturning within the mcv stretches the low level vorticity and moistens a deep mesoscale column that are both favorable for genesis 2 nari s genesis does not occur until after the passage of the baroclinic disturbance 3 convective asymmetry induces a smaller sized vortex circulation from the preexisting mcv 4 the vortex vortex interaction with danas leads to nari s looping track and temporal weakening and 5 midlevel convergence associated with the subtropical high and danas accounts for the generation of a nearly upright eyewall

a coupled climate ice sheet model applied to the quaternary ice ages a coupled climate ice sheet model is used in an attempt to simulate the northern hemispheric ice sheet volume variations implied by deep sea core records over the last several 105 years the climate model consists of a global one level diffusive energy balance equation with seasonal variation and land ocean contrast using a severely truncated series of associated legendre functions the ice sheet model uses a standard vertically averaged flow law and predicts ice thickness in a north south cross section bedrock deformation under the ice is included as linear flow confined to a thin asthenospheric channel the climate model is tuned to the modern climate and tested against results from atmospheric general circulation models for 18 000 and 10 000 yr b p it is suggested that the lack of sea ice variations causes the biggest departures from the gcm results the sensitivities to the orbital perturbations of the coupled model and of the ice sheet model alone with a prescribed snow budget distribution are examined some qualitative differences are found in the latitudinal forms of these sensitivities despite these differences however the two types of models produce similar ice age simulations over the last 700 thousand years in response to the orbital perturbations in particular fairly realistic simulations with strong 100 kyr cycles similar to those obtained by adding an ice calving mechanism to the ice sheet model in pollard 1982a are again produced in the coupled model

a high resolution model of the 100 ka ice age cycle significant improvements to the representation of climate forcing and mass balance response in a coupled two dimensional global energy balance climate model ebm and vertically integrated ice sheet model ism have led to the prediction of an ice volume chronology for the most recent ice age cycle of the northern hemisphere that is close to that inferred from the geological record most significant is that full glacial termination is delivered by the model without the need for new physical ingredients in addition a relatively close match is achieved between the last glacial maximum lgm model ice topography and that of the recently described ice 4g reconstruction these results suggest that large scale climate system reorganization is not required to explain the main variations of the north american na ice sheets over the last glacial cycle lack of sea ice and marine ice dynamics in the model leaves the situation over the eurasian ea sector much more uncertain the incorporation of a gravitationally self consistent description of the glacial isostatic adjustment process demonstrates that the na and ea bedrock responses can be adequately represented by simpler damped relaxation models with characteristic time scales of 3 5ka and 5 ka respectively these relaxation times agree with those independently inferred on the basis of postglacial relative sea level histories

warm core formation in tropical storm humberto 2001 at 0600 utc 22 september 2001 humberto was a tropical depression with a minimum central pressure of 1010 hpa twelve hours later when the first global positioning system dropwindsondes gps sondes were jettisoned humberto s minimum central pressure was 1000 hpa and it had attained tropical storm strength thirty gps sondes radar from the wp 3d and in situ aircraft measurements are utilized to observe thermodynamic structures in humberto and their relationship to stratiform and convective elements during the early stage of the formation of an eye the analysis of tropical storm humberto offers a new view of the pre wind induced surface heat exchange pre wishe stageoftropicalcycloneevolution humbertocontainedamesoscaleconvectivevortex mcv similar to observations of other developing tropical systems the mcv advects the exhaust from deep convection in the form of an anvil cyclonically over the low level circulation center on the trailing edge of the anvilanareaofmesoscaledescentinducesdryadiabaticwarmingin thelowertroposphere thenascentwarm core at low levels causes the initial drop in pressure at the surface and acts to cap the boundary layer bl as bl air flows into the nascent eye the energy content increases until the energy is released from under the cap on the down shear side of the warm core in the form of vigorous cumulonimbi which become the nascent eyewall this series of events show one possible path in which a mesoscale convective system may evolve into a warm cored structure and intensify into a hurricane

gcm response to late precambrian 590 ma ice covered continents recent coupled energy balance ice sheet modeling studies indicate that ice covered continents can be simulated for the late precambrian with 6 solar constant reduction we examine the ocean mixed layer response to such an ice sheet with the genesis 2 general circulation model and co2 levels varying from 0 5 2 5 times present the ocean ices over completely at 0 5 1 0x present levels with the final phase of sea ice growth occurring within a single model year a qualitatively different 2 5x co2 solution is close to equilibrium and yields open water between 25 n and 25 s paleolatitude the prevailing wind patterns suggest that an equatorial pacific type circulation may have developed in part of the neoproterozoic ocean