aquaplanet experiments using cam s variable resolution dynamical core abstracta variable resolution option has been added within the spectral element se dynamical core of the u s department of energy doe ncar community atmosphere model cam cam se allows for static refinement via conforming quadrilateral meshes on the cubed sphere this paper investigates the effect of mesh refinement in a climate model by running variable resolution var res simulations on an aquaplanet the variable resolution grid is a 2 222 km grid with a refined patch of 0 25 28 km resolution centered at the equator climatology statistics from these simulations are compared to globally uniform runs of 2 and 0 25 a significant resolution dependence exists when using the cam version 4 cam4 subgrid physical parameterization package across scales global cloud fraction decreases and equatorial precipitation increases with finer horizontal resolution resulting in drastically different climates between the uniform grid runs and a physics induced grid imprinting in the var res simulation

comprehensive simulation of the middle atmospheric climate some recent results this study discusses the results of comprehensive time dependent three dimensional numerical modelling of the circulation in the middle atmosphere obtained with the gfdl skyhi troposphere stratosphere mesosphere general circulation model gcm the climate in a long control simulation with an intermediate resolution version 3 in horizontal is briefly reviewed while many aspects of the simulation are quite realistic the focus in this study is on remaining first order problems with the modelled middle atmospheric general circulation notably the very cold high latitude temperatures in the southern hemisphere sh winter spring and the virtual absence of a quasi biennial oscillation qbo in the tropical stratosphere these problems are shared by other extant gcms it was noted that the sh cold pole problem is somewhat ameliorated with increasing horizontal resolution in the model this suggests that improved resolution increases the vertical momentum fluxes from the explicitly resolved gravity waves in the model a point confirmed by detailed analysis of the spectrum of vertical eddy momentum flux in the winter sh extratropics this result inspired a series of experiments with the 3 skyhi model modified by adding a prescribed zonally symmetric zonal drag on the sh winter westerlies the form of the imposed momentum source was based on the simple assumption that the mean flow drag produced by unresolved waves has a spatial distribution similar to that of the eliassen palm flux divergence associated with explicitly resolved gravity waves it was found that an appropriately chosen drag confined to the top six model levels above 0 35 mb can lead to quite realistic simulations of the sh winter flow including even the stationary wave fields through august but that problems still remain in the late winter springtime simulation while the imposed momentum source was largely confined to the extratropics it produced considerable improvement in the simulation of the equatorial semiannual oscillation with both the easterly and westerly phases being somewhat more intense than in the control simulation a separate experiment was conducted in which the skyhi model was simplified so that it had no topography and so that the seasonal cycle was frozen in perpetual equinox conditions these changes result in a model that has much reduced interhemispheric asymmetry this model spontaneously produces a long period mean flow oscillation of considerable amplitude in the tropical upper stratopause the implication of this result for the general issue of obtaining a qbo in comprehensive gcms is discussed

storm track dynamics this paper reviews the current state of observational theoretical and modeling knowledge of the midlatitude storm tracks of the northern hemisphere cool season observed storm track structures and variations form the first part of the review the climatological storm track structure is described and the seasonal interannual and interdecadal storm track variations are discussed in particular the observation that the pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales are highlighted as challenges that any comprehensive storm track theory or model has to be able to address physical processes important to storm track dynamics make up the second part of the review the roles played by baroclinic processes linear instability downstream development barotropic modulation and diabatic heating are discussed understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics and provides a context within which both observational and modeling results can be interpreted the eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks the final part of the review deals with the ability to model storm tracks the success as well as remaining problems in idealized storm track modeling which is based on a linearized dynamical system are discussed perhaps on a more pragmatic side it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure and to a certain extent the seasonal and enso related interannual variations of storm tracks in depth comparisons between observed and modeled storm track variations are still lacking

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 of equatorial convection to horizontal resolution in aquaplanet simulations with a variable resolution gcm abstract climate simulations using idealized zonally symmetric boundary conditions an aquaplanet are performed in order to highlight possible deviations from zonality introduced by the spatial variations in resolution caused by the variable resolution technique simulations at three different uniform resolutions with spectral truncations t21 t63 and t106 are used as control simulations and analyzed in order to describe the zonal climate simulated by this version of the arpege climat general circulation model and establish its sensitivity to varying spatial resolution the model simulates a single equatorial precipitation maximum in which the activity of the convective cells is modulated by westward and eastward propagating equatorial waves depending on the resolution a simulation with the variable resolution technique using a truncation t63 and a stretching factor of 3 with the maximum resolution pole placed at the equator is then compared to the uniform resolution simulations deviations from the

northern hemisphere storm tracks during the last glacial maximum in the pmip2 ocean atmosphere coupled models energetic study seasonal cycle precipitation mid latitude eddies are an important component of the climatic system due to their role in transporting heat moisture and momentum from the tropics to the poles and also for the precipitation associated with their fronts especially in winter we study northern hemisphere storm tracks at the last glacial maximum lgm and their influence on precipitation using ocean atmosphere general circulation model oagcm simulations from the second phase of the paleoclimate modelling intercomparison project pmip2 the difference with pmip1 results in terms of sea surface temperature forcing fundamental for storm track dynamics is large especially in the eastern north atlantic where sea ice extends less to the south in oagcms compared to atmospheric only gcms our analyses of the physics of the eddies are based on the equations of eddy energetics all models simulate a consistent southeastward shift of the north pacific storm track in winter related to a similar displacement of the jet stream partly forced by the eddies themselves precipitation anomalies are consistent with storm track changes with a southeastward displacement of the north pacific precipitation pattern the common features of north atlantic changes in the lgm simulations consist of a thinning of the storm track in its western part and an amplification of synoptic activity to the southeast in the region between the azores islands and the iberian peninsula which reflects on precipitation this southeastward extension is related to a similar displacement of the jet partly forced by the eddies in the western north atlantic the synoptic activity anomalies are at first order related to baroclinic generation term anomalies but the mean flow baroclinicity increase due to the presence of the laurentide ice sheet is partly balanced by a loss of eddy efficiency to convert energy from the mean flow moisture availability in this region is greatly reduced due to more advection of dry polar air by stationary waves leading to less synoptic scale latent heat release and hence less precipitation also in terms of seasonality the stormy season is shifted later in the year by a few days to a month depending on the season and the model considered this shift does not directly reflect on the first order seasonal cycle of precipitation which also depends on other mechanisms especially in summer

sensitivity of the eddy momentum flux to meridional resolution in atmospheric gcms abstract gcm experiments with zonally symmetric climates are used to demonstrate that the increase in the meridional eddy momentum fluxes and zonal surface winds that occurs when resolution is increased is primarily due to the increase in meridional rather than zonal resolution it is argued that the sensitivity to meridional resolution reflects the need to resolve the small scales generated in the rossby wave field as waves radiate from the midlatitude baroclinic eddy source region into regions with small mean winds some additional experiments highlight the sensitivity of surface winds and eddy momentum fluxes to the subgrid scale horizontal mixing formulation in low revolution models

how much subsidence is enough to change the morphology of river deltas understanding the effect of subsidence on fluviodeltaic morphology is important not only to maintain sustainable coastal cities and habitats but also to interpret the information contained in the stratigraphic record while tectonic steering in alluvial environments has been investigated similar studies in fluviodeltaic environments are limited to physical experiments and field observations we perform numerical experiments with a parcel based cellular model to analyze the deltaic surface and subsurface responses to regional subsidence we quantify model results using robust metrics and show that while sediment partitioning and shoreline pattern vary gradually with increasing subsidence rate channel mobility and stratigraphic connectivity of channel deposits show a threshold transition conditions for this transition are captured with a dimensionless filling index defined as the ratio between the rates of accommodation creation and sediment supply a channel locking mechanism activates when exceeds 0 6 and is responsible for the threshold transition

the impact of changing the horizontal diffusion scheme on the northern winter climatology of a general circulation model a general circulation model was used in perpetual january mode to study the wintertime climatological sensitivity to changes in the horizontal hyper laplacian diffusion scheme at both spectral t21 and t42 horizontal resolutions analysis of the global energy cycle revealed a large impact especially on the conversion of eddy to zonal kinetic energy less diffusion resulted in less zonal available potential energy weaker baroclinic conversions and caused the barotropic conversion of eddy to zonal kinetic energy to become significantly weaker it was found that by weakening the diffusion in the t21 model the eddy kinetic energy increased and became more realistic yet at the same time the conversion of eddy to zonal kinetic energy decreased and became less realistic weakening the horizontal diffusion caused a northern hemisphere tropospheric response similar to that obtained by increasing the upper level orographic gravity wave drag in the model the zonal mean zonal winds weakened poleward of the subtropical jets the ferrel cells weakened and the troposphere warmed up at high latitudes the weakening of the zonal wind and the ferrel cells was associated with a weakening of the upper level convergence of the poleward eddy momentum flux and the wave mean flow interaction was demonstrated using eliassen palm diagnostics a substantial part of the change in the poleward eddy momentum flux was due to large changes in the poleward momentum flux of the transient eddies in the mid latitude stormtracks and changes in their horizontal phase tilt a large northern hemisphere stationary wave change appeared in these regions and had a similar structure in the atlantic sector to that seen when strengthening upper level gravity wave drag

upper level atmospheric stationary waves in the twentieth century climate of the intergovernmental panel on climate change simulations 1 the upper level stationary waves are defined as the deviations from longitudinal symmetry of the 250 hpa climatological monthly mean stream function the climatological averaging period is over the years 1980 to 2000 the coupled model simulations are those of the climate of the 20th century experiment 20c3m prescribed by ipcc the model results are compared to the ncep ncar and era40 reanalyses the comparison shows the following 1 the amplitude of the modeled waves in the northern hemisphere is generally weaker than observed that is modeled flow is too zonal in the southern hemisphere there are phase as well as amplitude discrepancies but no clear biases 2 the correlation of the waves in the reanalyses and models in the northern hemisphere in winter averages about 0 9 but decreases sharply outside of winter 3 for many models the correlation of the waves in the reanalyses and models outside of the wintertime northern hemisphere is poor sometimes being less than 0 7 the most prominent systematic error occurring across all models is the underestimate of the trough ridge in the northern hemisphere winter over the north atlantic sector 60 w 0 w 4 the models having the coarsest horizontal resolution consistently underperform compared to the others for models of horizontal resolution finer than approximately 2 5 the relation to horizontal resolution and fidelity of the simulation is somewhat less clear 5 the interannual variability of the waves is consistently underestimated by almost all the models throughout the year and over the northern hemisphere the differences between the models and the reanalyses are surprisingly large given the large scale of the features diagnosed and the fairly long averaging period comparison to amip2 integrations indicates that differences in the ocean simulation in the coupled models are likely not an overwhelming influence on the agreement with reanalyses

impact of variable resolution meshes on midlatitude baroclinic eddies using cam mpas a abstractthe effects of a variable resolution mesh on simulated midlatitude baroclinic eddies in idealized settings are examined both aquaplanet and held suarez experiments are performed using the model for prediction across scales atmosphere mpas a hydrostatic dynamical core implemented within the national science foundation department of energy nsf doe community atmosphere model cam mpas a in the real world midlatitude eddy activity is organized by orography land sea contrasts and sea surface temperature anomalies in these zonally symmetric idealized settings transients should have an equal probability of occurring at any longitude however the use of a variable resolution mesh with a circular high resolution region centered at 30 n results in a maximum in eddy kinetic energy on the eastern side and downstream of this high resolution region in both aquaplanet and held suarez cam mpas a simulations the presence of a geographically confined maximum in both simulations suggests this response

importance of the resolution of surface topography in indian monsoon simulation abstractthe influence of surface topography resolution in indian summer monsoon simulation is investigated three sets of six member ensemble simulations with climatological sea surface temperature are conducted with the community atmospheric model version 5 1 cam5 1 coarse simulation at 1 9 2 5 latitude longitude resolution fine simulation at 0 47 0 63 resolution and hybrid simulation that is using coarse surface topography imposed on the fine configuration with regard to the representation of the surface topography substantial differences occur at the regional scales between the simulations especially over the foothills and steep flanks of the mountains in the coarse and hybrid simulations the orographic height of the foothills is overestimated whereas that of the steep flanks adjacent to the foothills is underestimated the biases are severe up to 1 km over the himalayas and tibet and have detrimental effects on regional climate through barrier effects on the low level flow and

alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization systematic westerly biases in the northern hemisphere wintertime flow of the meteorological office 15 layer operational model and 11 layer general circulation model are described evidence that the failure to parametrize subgrid scale orographic gravity wave drag may account for such biases is presented this evidence is taken from aircraft studies surface pressure drag measurements and studies of the zonally averaged momentum budget a parametrization scheme is described in which the surface stress is proportional to the near surface wind speed and static stability and to the variance of subgrid scale orography the stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear a richardson number dependent wave breaking formulation is devised and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability it is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere warms polar regions by adiabatic descent and decelerates the mean westerlies in the troposphere the influence of the parametrization scheme on integrations of the 11 layer model is described and found to be generally beneficial in a discussion of the reasons why this problem has only recently emerged it is suggested that the satisfactory northern hemisphere winter circulations of previous coarser general circulation models were due to a compensation implied by underestimating both the surface drag and the horizontal flux of momentum by explicitly resolved large scale eddies

comparison of the structure of the itcz in the west pacific during the boreal summers of 1989 93 using amip simulations and ecmwf reanalysis the dynamic structure and interannual variability of the itcz in the western north pacific and east asia during the boreal summers of 1989 93 are investigated european centre for medium range weather forecasts ecmwf reanalyses are used to characterize the dynamical structure of the itcz the itcz structure in the national center for atmospheric research ncar community climate model 3 ccm3 6 and the national aeronautics and space administration nasa goddard institute of space studies giss si2000 gcm is also explored the itcz axis can be partitioned into three sections in the western north pacific and east asia the continental monsoon trough cmt in india the bay of bengal and indo china the oceanic monsoon trough omt in the pacific warm pool and the trade wind trough twt in the central pacific the confluence point cp is defined as an area where the broad cyclonic flow in omt changes to the easterly trades of the twt east of the cp both gcms simulate a weaker easterly trade wind regime with a broader itcz due to the dislocation of the pacific high to the west of the cp both gcms simulate a weaker cross equatorial flow and omt both models show skill in simulating the interannual variability of the location of the cp however for the peak enso warm phase years 1992 93 the spread among ensemble members of the cp longitude is comparable to the shift of ensemble mean cp location relative to 1989 that is the internal variability of each gcm is on the same order as the simulated interannual variability in this region

consistent vertical and horizontal resolution abstract simple physical relations namely the rossby ratio between vertical and horizontal scales in quasi geostrophic flow and the dispersion relation for internal gravity waves are used to estimate the vertical resolution consistent with a given horizontal resolution using these relations we find that virtually all large scale models and observing systems have inadequate vertical resolutions in models the excess horizontal resolution can lead to increased model noise rather than improved accuracy in observing systems the finer horizontal scales can be severely misrepresented

sensitivity of simulated climate to model resolution abstract dynamical measures of the climate e g winds eddy fluxes simulated by a general circulation model are compared at different horizontal and vertical resolutions for the december january and february period the simulations of the troposphere are found to improve significantly as the horizontal resolution increases in the range of spectral truncations from t21 to t63 little sensitivity is found to changes in vertical resolution between about 2 8 km and 0 7 km vertical grid spacing the improvements in the southern hemisphere troposphere are greater than in the northern hemisphere as the horizontal resolution increases the eddy momentum fluxes and kinetic energies in both hemispheres increase monotonically with horizontal resolution at t63 the southern hemisphere winds eddy fluxes and eddy kinetic energies agree favorably with observations while serious discrepancies are present at lower resolutions in the northern hemisphere the eddy momentum flux at t63 is slightly larger than obser

true to milankovitch glacial inception in the new community climate system model the equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115 000 years ago the difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones inferred from reconstructions for the last glacial inception this is a major improvement over previous similar studies and the increased realism is attributed to the higher spatial resolution in the atmospheric model which allows for a more accurate representation of the orography of northern canada and siberia the analysis of the atmospheric heat budget reveals that as postulated by milankovitch s hypothesis the only necessary positive feedback is the snow albedo feedback which is initiated by reduced melting of snow and sea ice in the summer however this positive feedback is almost fully compensated by an increased meridionalheat transportin theatmosphereandareducedconcentrationoflow arcticclouds in contrasttosimilar previous studies the ocean heat transport remains largely unchanged this stability of the northern north atlanticcirculation is explainedby the regulatingeffect of thefreshwaterimportthroughthe naresstrait and northwest passage and the spiciness import by the north atlantic current

how well can we simulate past climates evaluating the models using global palaeoenvironmental datasets abstract global syntheses of palaeoenvironmental data are required to test climate models under conditions different from the present data sets for this purpose contain data from spatially extensive networks of sites the data are either directly comparable to model output or readily interpretable in terms of modelled climate variables data sets must contain sufficient documentation to distinguish between raw primary and interpreted secondary tertiary data to evaluate the assumptions involved in interpretation of the data to exercise quality control and to select data appropriate for specific goals four data bases for the late quaternary documenting changes in lake levels since 30 14 c kyr bp the global lake status data base vegetation distribution at 18 14 c kyr and 6 14 c kyr bp biome 6000 aeolian accumulation rates during the last glacial interglacial cycle dirtmap and tropical terrestrial climates at the last glacial maximum the lgm tropical terrestrial data synthesis are summarised each has been used to evaluate simulations of last glacial maximum lgm 21 calendar kyr bp and or mid holocene 6 cal kyr bp environments comparisons have demonstrated that changes in radiative forcing and orography due to orbital and ice sheet variations explain the first order broad scale in space and time features of global climate change since the lgm however atmospheric models forced by 6 cal kyr bp orbital changes with unchanged surface conditions fail to capture quantitative aspects of the observed climate including the greatly increased magnitude and northward shift of the african monsoon during the early to mid holocene similarly comparisons with palaeoenvironmental datasets show that atmospheric models have underestimated the magnitude of cooling and drying of much of the land surface at the lgm the inclusion of feedbacks due to changes in ocean and land surface conditions at both times and atmospheric dust loading at the lgm appears to be required in order to produce a better simulation of these past climates the development of earth system models incorporating the dynamic interactions among ocean atmosphere and vegetation is therefore mandated by quaternary science results as well as climatological principles for greatest scientific benefit this development must be paralleled by continued advances in palaeodata analysis and synthesis which in turn will help to define questions that call for new focused data collection efforts

the berlin troposphere stratosphere mesosphere gcm climatology and forcing mechanisms the climatological structure of the berlin troposphere stratosphere mesosphere general circulation model is presented and compared with observations the zonal mean zonal wind and temperature display some successes for the simulation in that all of the climatological features are captured but there are some systematic errors regarding their location and strength in particular the location and structure of the polar night jet in the northern winter is incorrect it is located too far polewards at high levels and is too weak in the middle and lower stratosphere on the positive side there is a separation between the polar night jet and the subtropical jet the temperature in the winter polar upper troposphere and lower stratosphere is too high presumably arising from the treatment of radiative processes consistent with hydrostatic balance the 30 hpa and other isobaric surface lies too high the polar vortex is both too asymmetric and too weak this leads to an underestimate of the modelled zonal velocity which is consistent with the poleward rather than equatorward propagation of the stationary planetary waves in the lower and middle stratosphere in the upper stratosphere the poleward heat and momentum fluxes due to these waves are too strong despite these systematic errors the seasonal evolution of both hemispheres is quite well simulated the large variability and the strong episodic forcing of the northern winter contrasts well with the relatively stable southern winter and the breakdown of the southern polar vortex in springtime the magnitude of the variability in the northern hemisphere is too small in the model and it is proportionally too large in the early winter

importance of the resolution of surface topography vis a vis atmospheric and surface processes in the simulation of the climate of himalaya tibet highland as himalaya tibet highland hth a spatially extensive and complex terrain plays influential roles in the regional and global climate its representation in climate models is a decisive factor in climate simulations it is established that higher spatial resolution improves the simulation of several seasonal mean features over this region but how much improvement comes from the better representation of surface topography and how much comes from the better modeling of the atmospheric and surface processes are still not clear to understand this three sets of 6 member ensemble simulations are conducted using the ncar community atmosphere model version 5 1 cam5 1 at 1 1 9 2 5 resolution coarse 2 0 47 0 63 resolution fine and 3 topography is prescribed at the coarse resolution and rest of the model processes are computed at the fine resolution hybrid the coarse resolution overlooks most of the intricate features of the topography with a severe bias of 1 2 km but fine resolution does a much better job in their representations surface air temperatures are found to be strongly dependent on the resolution of topography and therefore it is desirable to resolve topography in models for the realistic simulation and projection of temperatures rest of the variables viz seasonal mean seasonal cycle and probability distribution of precipitation tropospheric temperature and moisture tibetan anticyclone show remarkable improvements with the increase in resolution of the atmospheric and surface processes influence of the resolution of topography is found to be limited to the model levels adjacent to the surface and for higher model levels the resolution of the atmospheric and surface processes is noted to play a more crucial role

northern hemisphere storm tracks in present day and last glacial maximum climate simulations a comparison of the european pmip models abstract extratropical weather systems are an essential feature of the midlatitude climate and global circulation at the last glacial maximum lgm the formation of regions of high transient activity referred to as storm tracks is strongly affected by the presence of large ice sheets over northern america and scandinavia and by differences in sea surface temperature sst distributions in the framework of the palaeoclimate modelling intercomparison project simulations of the lgm climate have been run with a wide range of atmospheric general circulation models agcms using the same set of boundary conditions allowing a valuable comparison between simulations of a climate very different from the present one in this study the authors focus on the storm track representation in the models and its relationship with the surface temperatures the mean flow and the precipitation storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three dimensional

the effect of sea ice on the transient atmospheric eddies of the southern hemisphere two 10 y simulations with a full seasonal cycle and 96 72 19 resolution were carried out with a version of the lmd gcm to diagnose the role of sea ice on the extratropical climatology of the southern hemisphere the control integration used the usual observed sea ice distribution while the anomaly simulation imposed a scenario in which all sea ice was entirely replaced by open ocean the simulated control climate was compared with available observational based analyses relevant diagnostics of the time mean and indicators of the transient eddy activity have been evaluated for both integrations the impact was shown throughout the troposphere and was larger and more organised in winter we found reduced westerly flow and both falls and rises in sea level pressure in the region from which sea ice was removed the removal of ice in the southern ocean affects the baroclinic structure of the atmosphere changes in baroclinicity and eddy activity are consistent with changes in the mean climate in general the meridional wind variance the poleward transient temperature flux and the eddy flux convergence of westerly momentum were weaker over the southern ocean however a strengthening of the variance downstream of the subtropical jet was found the position of the main storm track tends to be slightly displaced equatorward in the anomaly case

climate model biases in the width of the tropical belt abstractearth s arid subtropics are situated at the edges of the tropical belt which encircles the planet along the equator and covers half of its surface area the climate of the tropical belt is strongly influenced by the hadley cells with their subsidence and easterly trade winds both sustaining the aridity at the belt s edges the understanding of earth s past present and future climates is contingent on understanding the dynamics influencing this region an important but unanswered question is how realistically climate models reproduce the mean state of the tropical belt this study augments the existing literature by examining the mean width and seasonality of the tropical belt in climate models from phase 5 of cmip cmip5 and experiments from the second phase of the chemistry climate model validation ccmval 2 activity of the stratospheric processes and their role in climate sparc project while the models overall reproduce the structure of the tropical belt width s seasonal cycle they und

the effect of horizontal resolution on indian monsoon depressions in the met office nwp model monsoon depressions are synoptic scale features that are responsible for a significant fraction of the rain over northern india during the summer monsoon season and as such it is important to quantify their structure and behaviour in numerical weather prediction models it is known that increasing model resolution is strongly correlated with improved forecasts in the short term and global circulation in the longer term as well as better representation of tropical cyclones here we explore the sensitivity of depressions to changes in resolution using the met office unified model seven nwp case studies of depressions from 2013 15 were run at eight resolutions corresponding to equatorial grid spacing of between 16 and 208 km and compared with data for the same events from trmm and era interim reanalysis we found that at the low resolution end of the spectrum increases in resolution led to improvements in the composite structure but with diminishing returns the model also persistently overestimated the depression intensity in particular the wind speed and the warm core aloft with the source appearing to originate in the mid troposphere the sensitivity of the diurnal cycle to resolution was also explored the stratiform component was found to be very well represented by the model whereas the convective component was described quite poorly improvement in most components of structure with increasing model resolution were marginal beyond n320 63 km and n512 39 km for dynamic and thermodynamic fields respectively

atmospheric circulation and its effect on arctic sea ice in ccsm3 simulations at medium and high resolution the simulation of arctic sea ice and surface winds changes significantly when community climate system model version 3 ccsm3 resolution is increased from t42 2 8 to t85 1 4 at t42 resolution arctic sea ice is too thick off the siberian coast and too thin along the canadian coast both of these biases are reduced at t85 resolution the most prominent surface wind difference is the erroneous north polar summer anticyclone present at t42 but absent at t85 an offline sea ice model is used to study the effect of the surface winds on sea ice thickness in this model the surface wind stress is prescribed alternately from reanalysis and the t42 and t85 simulations in the offline model ccsm3 surface wind biases have a dramatic effect on sea ice distribution with reanalysis surface winds annual mean ice thickness is greatest along the canadian coast but with ccsm3 winds thickness is greater on the siberian side a significant difference between the two ccsm3 forced offline simulations is the thickness of the ice along the canadian archipelago where the t85 winds produce thicker ice than their t42 counterparts seasonal forcing experiments with ccsm3 winds during spring and summer and reanalysis winds in fall and winter relate the canadian thickness difference to spring and summer surface wind differences these experiments also show that the ice buildup on the siberian coast at both resolutions is related to the fall and winter surface winds the arctic atmospheric circulation is examined further through comparisons of the winter sea level pressure slp and eddy geopotential height at both resolutions the simulated beaufort high is quite weak weaker at higher resolution eddy heights show that the wintertime beaufort high in reanalysis has a barotropic vertical structure in contrast high ccsm3 slp in arctic winter is found in association with cold lower tropospheric temperatures and a baroclinic vertical structure in reanalysis the summertime arctic surface circulation is dominated by a polar cyclone which is accompanied by surface inflow and a deep ferrel cell north of the traditional polar cell the arctic ferrel cell is accompanied by a northward flux of zonal momentum and a polar lobe of the zonal mean jet these features do not appear in the ccsm3 simulations at either resolution

tropical waves in a gcm with zonal symmetry abstract tropical wave phenomena have been examined in the last 520 days of two 15 year runs of a low resolution general circulation model ccmo the model boundary conditions were simplified to all land perpetual equinox and no topography the two runs were for fixed soil moisture at 75 and 0 the so called wet and dry models both models develop well defined itczs with low level convergence erratically concentrated along the equator highly organized eastward propagating waves are detectable in both models with different wave speeds depending on the presence of moisture the wave amplitudes in e g vertical velocity are many orders of magnitude stronger in the wet model the waves have a definite transverse nature as precipitation low level convergence patches tend to move systematically north and south across the equator in the wet model the waves are distinctly nondispersive and the transit time for passage around the earth is about 50 days consistent with the madden julian frequency