observations of planetary boundary layer structure in the eastern equatorial pacific abstract atmospheric soundings were collected from a research ship in the eastern equatorial pacific to determine the kinematic and thermodynamic structure of the planetary boundary layer pbl in the vicinity of a strong front in sea surface temperature sst these observations have been used to test the hypothesis proposed by wallace et al and hayes et al that the low level wind shear was enhanced on the cold side of the front because of greater static stability the pbl structure was analyzed from a transect along 110 w from 10 n to 5 s with a focus on the 500 m deep atmospheric mixed layer as defined with respect to virtual potential temperature v significant mixed layer variability was found due to diurnal and other effects but there were systematic differences in mean structure as a function of latitude the mixed layer was neutrally stratified over warm water between 6 5 n and 1 5 n and was slightly stably stratified over the cooler water between 1 n and 2 s the mean total vertical wind

quasi lagrangian large eddy simulations of cross equatorial flow in the east pacific atmospheric boundary layer using a large eddy simulation les the atmospheric boundary layer abl is numerically modeled along 958w from 88 st o 48n during boreal autumn and compared to observations from the east pacific investigation of climate processes in the coupled ocean atmosphere system epic 2001 since the local abl winds are predominantly southerly in this season a quasi lagrangian forcing is used in which the abl air column is forced as if it were advecting northward with the mean september october 2001 meridional wind across the equatorial cold tongue and the rapidly warming ssts to the north pressure gradients and large scale zonal advective tendencies are prescribed as a function of latitude where possible observations from the epic 2001 experiment are used for forcing and for comparison with model results the abl s modeled vertical structure accords with the conceptual model of wallace et al and agrees well with observations surface stability accounts for the minimum in surface wind over the equatorial cold tongue and the maximum over the warm water to the north stability of the lower abl over the cold tongue allows a jet to accelerate at about 500 m height relatively uncoupled to the frictional surface layer vertical mixing over the warm water to the north distributes this momentum to the surface additional simulations were performed to explore the modeled abl s sensitivity to pressure gradients zonal advection free tropospheric humidity and initial conditions the model abl was robust changing the forcings resulted in little change in the modeled structure the strongest sensitivity was of stratocumulus clouds over the cold tongue to cloud top radiative cooling once formed at the southern edge of the cold tongue modeled stratocumulus clouds demonstrate a remarkable ability to maintain themselves over the cold tongue in the absence of surface fluxes by radiative cooling at their tops the persistence of thin stratocumulus clouds in this lagrangian model suggests that horizontal advection of condensate might be an important process in determining cloudiness over the cold tongue

large scale characteristics of the atmospheric boundary layer in the eastern pacific cold tongue itcz region abstract observations from the eastern pacific investigation of climate 2001 epic2001 field campaign and a simple mixed layer model are used to study the large scale structure and dynamics of the atmospheric boundary layer abl in the eastern pacific vertical and latitudinal distributions of the meridional pressure gradient winds and other variables are examined and the momentum balance of the mixed layer is explored for a latitudinal range from 0 to 10 n along 95 w including the equatorial cold tongue sea surface temperature sst front and convective region at the latitude of the intertropical convergence zone itcz the surface meridional pressure gradient is partitioned into contributions from the abl largely controlled by the gradient in sst and from the troposphere above the boundary layer mainly controlled by elevated heating and atmospheric wave activity the mean meridional gradient in surface pressure is dominated 90 by the abl contribution however its temporal variability prim

effect of orbital scale climate cycling and meltwater drainage on ice sheet grounding line migration 1 ice shelf response to climate cycling introduces hysteretic behavior in outlet glaciers with the evolution of meltwater drainage networks enhancing this effect the 2002 collapse of the larsen b ice shelf one of the floating extensions formed from glaciers flowing off the antarctic peninsula into the weddell sea followed the development of widespread poorly drained surface melt ponds during the atypical warmth of the 2001 2002 summer melt season yet for at least the past half century the floating termini of outlet glaciers in greenland did not collapse despite being exposed to summer mean temperatures and melt rates that far exceed those recorded in 2001 2002 on the antarctic peninsula we suggest that this paradox can be explained by the advection of well developed englacial drainage networks within greenland s extensive ablation zone the efficiency of this plumbing system limits meltwater ponding within crevasses and the subsequent ice shelf failure observed on the antarctic peninsula whether progressive or abrupt ice shelf removal leads to near synchronous glacier retreat conversely glaciation of fjords is delayed during ice shelf formation ultimately the relatively rapid loss and gradual establishment of ice shelf buttressing amid climate cycling leads to an asymmetric response in outlet glacier dynamics this asymmetry may have implications for the future of existing and recently calved ice shelves eustatic sea level and the hysteretic behavior of ice sheets

boundary layer cloud and drizzle variability in the southeast pacific stratocumulus regime abstract the southeast pacific stratocumulus regime is an important component of the earth s climate system because of its substantial impact on albedo observational studies of this cloud regime have been limited but during the past 5 yr a series of cruises with research vessels equipped with in situ and remote sensing systems have provided unprecedented observations of boundary layer cloud and drizzle structures these cruises started with the east pacific investigation of climate epic 2001 field experiment followed by cruises in a similar area in 2003 and 2004 pan american climate studies pacs stratus cruises the sampling from these three cruises provides a sufficient dataset to study the variability occurring over this region this study compares observations from the 2004 cruise with those obtained during the previous two cruises observations from the ship provide information about boundary layer structure fractional cloudiness cloud depth and drizzle characteristics this study indicat

numerical simulation of the stratus to cumulus transition in the subtropical marine boundary layer part i boundary layer structure abstract a stratus to cumulus transition sct that resembles observations occurred in lagrangian numerical simulations of the subtropical marine boundary layer over the northeastern pacific ocean southwest of california the lagrangian approach involves translating the domain along the climatological boundary layer trajectory at a rate equal to the observed surface wind speed the sst is increased at a corresponding rate the simulations did not include drizzle the diurnal cycle divergence changes or mesoscale circulations and thus demonstrate that these processes are not essential for an sct a 2d numerical cloud model that can explicitly represent large convective eddies is used turbulence at scales smaller than the large eddies is parameterized using a third moment turbulence closure this type of model requires no cloud regime specific input and is computationally economical for multiday simulations the results suggest that there are four stages in the transition from the stratus topped boundary

stratocumulus cloud top height estimates and their climatic implications a depth dependent boundary layer lapse rate was empirically deduced from 156 radiosondes released duringsixmonth longresearchcruisestothe southeastpacificsamplingavarietyofstratocumulusconditions thelapse ratedependenceonboundarylayerheightisweak decreasingfromabestfitof7 6to7 2k km 21 as theboundarylayerdeepensfrom800mto2km ship basedcloud baseheightsupto800mcorrespondwellto liftingcondensationlevels indicatingwell mixedconditions withcloud bases 800moften 200 600m higher than the lifting condensation levels the lapse rates were combined with moderate resolution imaging spectrometer 11 mm derived cloud top temperatures and satellite microwave derived sea surface temperatures to estimate stratocumulus cloud top heights the october mean cloud top height structure of the southeast pacific was then spatially and diurnally characterized coastal shoaling is apparent but so is a significant along coast cloud top height gradient with a pronounced elevation of the cloud top heights above thearicabightat 208s diurnalcloud topheightvariations inferredfromirregular4 times dailysampling can locally reach 250 m in amplitude and they can help to visualize offshore propagation of free tropospheric vertical motions a shallow boundary layer associated with the chilean coastal jet expands to its north and west in the afternoon cloud top heights above the arica bight region are depressed in the afternoon which may mean that increased subsidence from sensible heating of the andes dominates an afternoon increase in convergence upward motion at the exit of the chilean coastal jet in the southeast atlantic during october thestratocumuluscloud topheightsaretypicallylowerthanthoseinthesoutheastpacific acoastaljetregion can also be identified through its low cloud top heights coastal shoaling of the south atlantic stratocumulus region is mostly uniform with latitude in keeping with the more linear namibian angolan coastline the southeast atlantic shallow cloudy boundary layer extends farther offshore than in the southeast pacific particularly at 158s

the influence of sea surface temperature on surface wind in the eastern equatorial pacific weekly to monthly variability abstract temporal correlations between near equatorial surface wind and sea surface temperatures sst at 11 w in the eastern pacific ocean are investigated using data from an array of moored sensors between 5 n and 5 s the signature of tropical instability waves with periods of 20 30 days is apparent in time series of sst and both the meridional and zonal wind components results indicate the existence of a band of pronounced horizontal divergence in the surface wind field associated with the large meridional sst gradient equatorial front normally located just north of the equator perturbations of the equatorial front by the instability waves induce fluctuations in the overlying winds evidence of the air sea coupling is stronger in time series of the meridional gradients of wind and sst than between time series of the variables themselves the meridional differencing serves as a high pass filter in the space domain which removes planetary scale wind fluctuations that are unrelated to the local sst

air sea heat exchange along the northern sea surface temperature front in the eastern tropical pacific abstract the atmospheric response to the oceanic forcing in the eastern pacific along the northern equatorial sea surface temperature sst front is investigated in terms of sensible and latent heat flux during the 6 month period from 28 july 1999 to 27 january 2000 and the 7 month period from 28 june 2000 to 27 january 2001 of particular interest is the atmospheric boundary layer abl response to oceanic tropical instability waves tiws that distort the sst front during may january in normal years in previous studies time series of surface heat fluxes clearly show the influence of tiws but the relationship to spatial patterns of sst and wind speed has been inferred only from sparse in situ data in this study satellite observations are used as a basis for compositing in situ data from moorings to compensate for the lack of a spatially dense mooring array the variability in the position of the sst front caused by westward propagating tiws enables fixed mooring locations to measure the abl response

cloudiness and marine boundary layer dynamics in the astex lagrangian experiments part i synoptic setting and vertical structure abstract a goal of the atlantic stratocumulus transition experiment astex southeast of the azores islands in the east central atlantic ocean during june 1992 was to examine the coupled evolution of cloud dynamical and thermodynamical vertical structure in a marine boundary layer mbl air mass as it advected from cold to warm water in the trade winds in two lagrangian observation periods during astex an unprecedentedly complete view of mbl and cloud evolution was achieved by nearly continuous aircraft coverage of such an air mass for 36 48 hours using three boundary layer aircraft supplemented by satellite ship and balloon observations during the first lagrangian period an accelerated stratocumulus to trade cumulus transition occurred in a clean marine air mass in the second lagrangian period a 200 hpa deep decoupled modified continental mbl persisted with almost no change in structure cumulus rising into intermittent stratocumulus were observed throughout the period the two contrasting a

direct observations of atmospheric boundary layer response to sst variations associated with tropical instability waves over the eastern equatorial pacific tropical instability waves tiws with a typical wavelength of 1000 km and period of 30 days cause the equatorial front to meander and result in sst variations on the order of 18 28c vertical soundings of temperature humidity and wind velocity were obtained on board a japanese research vessel which sailed through three fully developed sst waves from 1408 to 1108w along 28n during 21 28 september 1999 a strong temperature inversion is observed throughout the cruise along 28n capping the planetary boundary layer pbl that is 1 1 5 km deep temperature response to tiw induced sst changes penetrates the whole depth of the pbl in response to an sst increase air temperature rises in the lowest kilometer and shows a strong cooling at the mean inversion height as a result this temperature dipole is associated with little tiw signal in the observed sea level pressure slp the cruise mean vertical profiles show a speed maximum at 400 500 m for both zonal and meridional velocities sst based composite profiles of zonal wind velocity show weakened intensified vertical shear within the pbl that is consistent with enhanced reduced vertical mixing causing surface wind to accelerate decelerate over warm cold ssts taken together the temperature and wind soundings indicate the dominance of the vertical mixing over the slp driving mechanism based on the authors measurements a physical interpretation of the widely used pbl model proposed by lindzen and nigam is presented

lagrangian evolution of the northeast pacific marine boundary layer structure and cloud during cset abstractflight data from the cloud system evolution over the trades cset campaign over the pacific stratocumulus to cumulus transition are organized into 18 lagrangian cases suitable for study an

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

ice cover landscape setting and geological framework of lake vostok east antarctica abstract lake vostok located beneath more than 4 km of ice in the middle of east antarctica is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment melting and freezing at the base of the ice sheet which slowly flows across the lake controls the flux of water biota and sediment particles through the lake the influx of thermal energy however is limited to contributions from below thus the geological origin of lake vostok is a critical boundary condition for the subglacial ecosystem we present the first comprehensive maps of ice surface ice thickness and subglacial topography around lake vostok the ice flow across the lake and the landscape setting are closely linked to the geological origin of lake vostok our data show that lake vostok is located along a major geological boundary magnetic and gravity data are distinct east and west of the lake as is the roughness of the subglacial topography the physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake s circulation lake vostok is a tectonically controlled subglacial lake the tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small but significant amounts of thermal energy into the lake

wrf hindcasts of cold front passages over the arm eastern north atlantic site a sensitivity study abstractthe present study explores the ability of the weather research and forecasting wrf model to accurately reproduce the passage of extratropical cold fronts at the doe arm eastern north atlantic ena observation site on the azores an analysis of three case studies is performed in which the impact of the wrf domain size position of the model boundary relative to the ena site grid spacing and spectral nudging conditions are explored the results from these case studies indicate that model biases in the timing and duration of cold front passages change with the distance between the model domain boundary and the ena site for these three cases if the western model boundary is farther than 1500 km from the site the front becomes too meridional and fails to reach the site making 1000 or 1500 km the optimal distances in contrast integrations with small distances e g 500 km between the site and domain boundaries have inadequate spatial spinup i e the domain is too small for the model to pr

diagnosing monthly mean boundary layer properties from reanalysis data using a bulk boundary layer model abstract the mixed layer approach to modeling the planetary boundary layer pbl is particularly well suited to inversion topped pbls such as the stratocumulus topped boundary layer found off the west coast of america in the subtropical pacific ocean at northern and southern latitudes however a strong temperature inversion near 850 hpa the trade wind inversion is not confined to the stratocumulus regimes but has been observed over most parts of the subtropical tropical pacific ocean in this paper the authors test the ability of a simple bulk boundary layer model bblm to diagnose entrainment velocity cumulus mass flux and surface latent heat flux from monthly mean reanalysis data the pbl depth is estimated from geoscience laser altimeter system data the model is based on the conservation equations for mass total water mixing ratio and moist static energy the bblm diagnoses entrainment velocities between 1 and 8 mm s 1 in the stratocumulus and trade wind regions with increasing rates towar

simulation of seasonal variation of marine boundary layer clouds over the eastern pacific with a regional climate model abstractthe seasonal cycle of marine boundary layer mbl clouds over the eastern pacific ocean is studied with the international pacific research center iprc regional atmospheric model iram the results show that the model is capable of simulating not only the overall seasonal cycle but also the spatial distribution cloud regime transition and vertical structure of mbl clouds over the eastern pacific although the modeled mbl cloud layer is generally too high in altitude over the open ocean when compared with available satellite observations the model simulated well the westward deepening and decoupling of the mbl the rise in cloud base and cloud top of the low cloud decks off the peru and california coasts and the cloud regime transition from stratocumulus near the coast to trade cumulus farther to the west in both the southeast and northeast pacific in particular the model reproduced major features of the seasonal variations in stratocumulus decks off the peru and california coasts includin

spatial variability of atmospheric boundary layer structure over the eastern equatorial pacific abstract variations in the atmospheric boundary layer structure over the eastern equatorial pacific are analyzed using 916 soundings collected during the first global atmospheric research program global experiment unstable boundary layer structures are observed much more frequently in soundings north of the ocean front located near 2 5 n in the eastern equatorial pacific than in soundings south of the front an objective criterion is applied to identify the presence of the transition layer a weak stable layer near cloud base in the soundings the transition is observed in about 45 of the soundings in both the unstable and the inversion categories a comparison of soundings over the cold tongue with those over the itcz indicates that differences in static stability between these regions are limited to the layer from the surface to about 850 mb which is the mean height of the inversions capping the cloud layer over the cold tongue the cold tongue soundings on average are found to be drier from the sur

andean elevation control on tropical pacific climate and enso late cenozoic marine proxy data record a long term transition in the tropical pacific from el nino like conditions with reduced zonal sea surface temperature sst gradient deepened thermocline and reduced upwelling in the eastern equatorial pacific eep to conditions similar to modern this transition coincides with kilometer scale uplift of the central andes to understand whether the rise of the andes contributed to tropical pacific climate evolution we performed experiments with the national center for atmospheric research s community climate system model version 4 to quantify changes in tropical pacific climate and el nino southern oscillation as a function of andean elevations our results demonstrate that uplift increases the equatorial east west sst gradient and walker circulation the rise of the andes from 1 to 3 km increases the sst gradient by 0 8 c and walker circulation by 60 due to strengthened radiative cooling by enhanced low cloud formation in the eep this cooling effect is largest in the southeastern tropical pacific and accounts for about one half of the reconstructed sst cooling along the peru coast the uplift also strengthens upwelling north of the eep consistent with documented increases in biological productivity in this region and decreases the frequency of el nino southern oscillation and the number of strong el nino events simulated responses to andean uplift are generally consistent with the late cenozoic proxy records but too small in magnitude taken together our results indicate that andean uplift was likely one of the multiple factors that contributed to the long term evolution of both the mean climate state and the interannual variability in the tropical pacific

diagnosis of the surface momentum balance over the tropical pacific ocean abstract the purpose of this study is to evaluate the suitability of using linear drag as a proxy for surface friction in the observed climatological mean momentum balance over the tropical pacific ocean the linear drag parameterization of kinetic energy dissipation in the planetary boundary layer is widely used in simplified models of the tropical atmosphere and in numerous observational studies of the surface momentum balance climatological seasonal mean fields of sea level pressure and surface wind from the comprehensive ocean atmosphere data set are used to calculate the pressure gradient coriolis and acceleration terms in the momentum budget friction is derived as a residual it is found that when friction is parameterized as a linear dissipation of kinetic energy the damping time scale for the meridional wind is 2 3 times faster than the damping time for the zonal wind the preceding formulation fits the observations well especially in the trade wind regions it is suggested that the differ

silicon concentrations in uk surface waters abstract this paper describes the variations in silicon concentrations in uk waters for a wide range of catchment systems near pristine rural and agricultural and urban impacted systems the paper largely concerns silicon levels in streams rivers and lakes based on extensive data collected as part of several research and monitoring initiatives of national and international standing for a detailed study of an upland catchment in mid wales information on atmospheric inputs and groundwater chemistries is provided to supply background information to cross link to the surface water chemistry several hundred streams rivers and lakes are dealt with within the study dealing with the main types of freshwater riverine and lacustrine environments the streams rivers vary from small ephemeral runoff to the major rivers of the uk the geographical location of sites vary from local sites in mid wales to regional studies across scotland to the major eastern uk rivers entering the north sea and to acid sensitive upland sites across wales the english lake district scotland and northern ireland the surface waters range in silicon concentration from 0 to 19 mg si l 1 average for individual sites vary between 0 7 and 7 6 mg si l 1 and there are some clear variations which link to two primary processes 1 the relative inputs of groundwaters enriched in silicon and near surface waters more depleted in silicon and 2 plankton uptake of silicon during the summer months under baseflow conditions thermodynamic analysis reveals that the waters are approximately saturated with respect to either quartz or chalcedony except for two circumstances when undersaturation occurs firstly undersaturation occurs at ph less than 5 5 in the upland areas and this is because the waters are mainly sourced from the acidic organic soils which are depleted in inorganic minerals secondly undersaturation occurs in the lowland rivers when biological activity is at its highest and this leads to silicon removal from the water column quartz equilibrium can be approached at ph 5 5 mainly within the upland systems which are not aquifer recharge dominated however for the lowland systems that are groundwater recharge dominated it is chalcedony saturation which is approached and such saturation is often observed within groundwaters similar patterns of undersaturation in response to biological uptake are seen in lakes and the extent of silicon depletion increases with biological productivity chalcedony oversaturation can occur for some uk rivers under baseflow conditions and this probably links to a higher rate of weathering

mean structure and diurnal cycle of southeast atlantic boundary layer clouds insights from satellite observations and multiscale modeling framework simulations abstractthe mean structure and diurnal cycle of southeast se atlantic boundary layer clouds are described with satellite observations and multiscale modeling framework mmf simulations during austral spring september november hourly resolution cloud fraction cf and cloud top height ht are retrieved from meteosat 9 radiances using modified clouds and the earth s radiant energy system ceres moderate resolution imaging spectroradiometer modis algorithms whereas liquid water path lwp is from the university of wisconsin microwave satellite climatology the mmf simulations use a 2d cloud resolving model crm that contains an advanced third order turbulence closure to explicitly simulate cloud physical processes in every grid column of a general circulation model the model accurately reproduces the marine stratocumulus spatial extent and cloud cover the mean cloud cover spatial variability in the model is primarily explained by the boundary layer decoupling strength whereas a boundary layer

mixed layer mode simulation of eastern north pacific stratocumulus abstract the cloud topped mixed layer theory developed by lilly 1968 is employed in a slightly generalized form to simulate stratocumulus in the eastern north pacific the radiation parameterization involves cloud top longwave cooling and mixed layer shortwave heating long wave emissivity and shortwave absorption are functions of cloud thickness the model is integrated along climatological trajectories which traverse the area bounded by 145 w 115 w 20 n and 40 n the results of the integration include fields of cloud top and cloud base height mixed layer thermodynamic structure and convective and radiative fluxes where it is possible to compare model generated fields with the observations of neiburger et al 1961 general agreement is found the model predicted location of the southern limit due to cloud top instability of stratocumulus clouds is in agreement with observations

hydrochemical patterns of a small lake and a stream in an uplifting area proposed as a repository site for spent nuclear fuel forsmark sweden hydrochemical patterns of a small lake and a stream in an uplifting area proposed as a repository site for spent nuclear fuel forsmark sweden

simulating the transition from drizzling marine stratocumulus to boundary layer cumulus with a mesoscale model a case of coastal california summer season boundary layer cloud has been simulated with the u s navy coupled ocean atmosphere mesoscale prediction system and the results analyzed in the context of consistency with conclusions derived from large eddy simulation based les studies results show a pronounced diurnal cycle and fair agreement with satellite derived observations of liquid water path when drizzle processes are included a significant degree of mesoscale organization emerges in the form of cloud bands accompanied by a transition from a well mixed boundary layer topped by unbroken stratocumulus cloud into a more potentially unstable convective boundary layer regime the transition and the subsequent development of mesoscale variability is analogous to the drizzle induced cloud breakup produced in large eddy simulation studies the dynamics of the pure stratocumulus cloud are dictated by the model s subgrid parameterization while the more convective regime exhibits appreciable vertical velocities characteristic of an ensemble of cumulus updrafts the existence of convective updrafts is tied to a weak drizzle induced decoupling of the cloud and subcloud layer after which air of higher equivalent potential temperature ue can pool at the surface some similarities to the propagation of deep convection are also noted

cloud top entrainment instability mixing processes at cloud boundaries are thought to play a critical role in determining cloud lifetime spatial extent and cloud microphysical structure high fidelity direct numerical simulations by mellado j fluid mech 2010 this issue vol 660 pp 5 36 show for the first time the character and potency of a curious instability that may arise as a result of molecular mixing processes at cloud boundaries an instability which until now has been thought by many to control the distribution of climatologically important cloud regimes

conserved variable analysis of the convective boundary layer thermodynamic structure over the tropical oceans abstract an analysis of fgge dropwindsonde data using conserved thermodynamic variables shows mixing line structures for the convective boundary layer over the equatorial pacific deeper boundary layers show a double structure reversals of the gradients of mixing ratio and equivalent potential temperature above the boundary layer top are present in all the averages and suggest that the origin of the air sinking into the boundary layer needs further study

super refraction effects on gps radio occultation refractivity in marine boundary layers 1 with the combination of global coverage high vertical resolution and all weather capability gps radio occultation ro is an emerging satellite remote sensing technique that can probe the atmospheric boundary layer abl on a global basis however a systematic negative bias commonly referred to as n bias remains in derived refractivity profiles in the abl in this paper we present the n biases in cosmic ro soundings with respect to ecmwf analyses which show a seasonally varying pattern clustered over the oceans where super refraction sr often occurs a case study of coincident cosmic ro and radiosonde sounding confirms that the n bias is primarily caused by sr we also show that the high rate ro bending angle measurements can resolve the abl top at a vertical resolution better than 100 m a reconstruction method is applied to one case where sr occurs and significantly reduces the negative refractivity error in the abl

the subtropical stratocumulus topped planetary boundary layer a climatology and the lagrangian evolution abstractprior work has shown that deeper planetary boundary layers pbls are associated with cloud breakup and reduced droplet concentration in subtropical stratocumulus cloud decks motivating a need for a thorough understanding of pbl mechanics here 169 000 boundary layer trajectories are calculated in four eastern subtropical ocean basins following reanalysis winds at 925 mb 1 mb 1 hpa these trajectories combined with a twice daily cloud top height inferred pbl depth product allow for a comprehensive lagrangian analysis of the stratocumulus sc topped pbl as the cloud deck transitions from sc to trade cumulus cu month to month variations of this pbl product are strongly positively correlated with an independent pbl product derived from gps radio occultation a climatology shows the pbl deepening offshore in every region the yearly cycle of pbl depth varies in opposition to the yearly cycle of lower tropospheric stability lts but high frequency variation between lts and pbl depth is more

numerical simulations and a conceptual model of the stratocumulus to trade cumulus transition abstract a two dimensional eddy resolving model is used to study the transition from the stratocumulus topped boundary layer to the trade cumulus boundary layer the 10 day simulations use an idealized lagrangian trajectory representative of summertime climatological conditions in the subtropical northeastern pacific the sea surface temperature is increased steadily at 1 5 k day 1 reflecting the southwestward advection of the subtropical marine boundary layer by the trade winds while the free tropospheric temperature remains unchanged results from simulations with both a fixed diurnally averaged shortwave radiative forcing and a diurnally varying shortwave forcing are presented a two stage model for the boundary layer evolution consistent with these simulations is proposed in the first stage decoupling is induced by increased latent heat fluxes in the deepening boundary layer after decoupling cloud cover remains high but the cloudiness regime changes from a single stratocumulus layer to sporadic

on the role of sea surface temperature gradients in forcing low level winds and convergence in the tropics abstract we examine the importance of pressure gradients due to surface temperature gradients to low level p 700 mb flow and convergence in the tropics over time scales 1 month the latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall our approach is to consider a simple one layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically consistent with ecmwf analyzed data the top of the layer is taken at 700 mb the influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer horizontal convergence within the layer is taken up by cumulonimbus mass flux however the development of the cumulonimbus mass flux is associated with a short relaxation time o hr roughly the development time for such convection during this short time horizontal convergence acts to redistribute mass so as to reduce horizontal pressur

entrainment rayleigh friction and boundary layer winds over the tropical pacific abstract winds over the tropical pacific are interpreted using mixed layer theory the theory which posits that the surface winds can be derived in terms of a force balance among surface drag pressure gradients coriolis forces and the vertical mixing of momentum into the boundary layer entrainment is very successful in predicting the seasonal climatology of the surface winds the model is also used as a basis for interpreting previous results in particular the model illustrates why studies that model the momentum flux divergence as a rayleigh damping find optimal damping coefficients that are anisotropic a linear variant of the model which also incorporates entrainment but neglects the quadratic relation between the wind speed and the surface stress is also found to predict the surface winds skillfully in addition to improving the representation of the winds it leads to realistic representations of the divergence of the vector wind if the key parameters of the model the entrainment rate and t

time scales of clouds and cloud controlling variables in subtropical stratocumulus from a lagrangian perspective abstractthe lagrangian evolution of cloud cover and cloud controlling variables is well approximated using red noise processes with different autocorrelation time scales for each variable trajectories within the subtropical marine boundary layer are generated using winds from ecmwf re analysis data for low cloud decks in four eastern subtropical ocean basins cloud cover liquid water path and boundary layer depth are sampled at 12 h intervals using a train satellites and droplet concentration is sampled every 24 h lower tropospheric stability and vertical velocity are sampled concurrently using reanalysis data samples are converted to seasonal and diurnal anomalies data are spatially averaged over a range of length scales the e folding decay times for autocorrelation are calculated for each variable based on lag times of 12 24 36 and 48 h using lag 24 h and an averaging radius of 100 km 15 17 h for liquid water path and vertical velocity 19 h for cloud cover 24 25 h for bounda

the influence of sea surface temperature on surface wind in the eastern equatorial pacific seasonal and interannual variability abstract the climate of the eastern pacific exhibits a pronounced equatorial asymmetry boundary layer air originating in the southern hemisphere trades crosses the equator and flows into the intertropical convergence zone itcz whose southern limit is nearly always located at least 4 to the north of the equator the sea surface temperature sst distribution is characterized by a prominent cold tongue centered 1 s a strong frontal zone centered 2 n and a warm eastward current centered near 5 n the surface wind field exhibits a pronounced horizontal divergence as the air flows northward across the oceanic frontal zone these features vary in strength in response to the annual cycle and the el nino southern oscillation phenomenon the northward cross equatorial surface winds the cold tongue and the frontal zone all tend to be strongest during the cold season july through november during the cold season of the coldest years when the cold tongue is most prominent the cross equatorial flow t

the relationship between boundary layer stability and cloud cover in the post cold frontal region using nasa aqua modis and airs data the relationship between low level cloud cover cloud top below the 700 hpa level and boundary layer stability is explored in post cold frontal conditions a linear relationship is found between seasonal cloud cover and two separate measures of inversion strength the lower tropospheric stability lts and the estimated inversion strength eis for two specific regions in the north atlantic and pacific in quiescent and weakly subsiding conditions the relationship barely changes when considering dynamically active and subsiding post cold frontal conditions for the same regions to explore the generality of this result and increase sample size cold front centered composites of cloud cover and stability are constructed the northern and southern hemisphere seasonal cloud cover and stability distributions in the post cold frontal regions are then compared a fairly good correlation between cloud cover and eis is found in both hemispheres across all seasons suggesting that a linear relationship between cloud cover and inversion strength proposed for quiescent conditions exists also in more dynamically active subsiding post cold frontal conditions however for a given season and hemisphere the correlation between cloud cover and eis degrades in post cold frontal regions especially in the northern hemisphere at these scales other large scale factors tend to correlate better with cloud cover

lagrangian sources of frontogenesis in the equatorial atlantic front estimating the processes that control the north equatorial sea surface temperature sst front on the northern edge of the cold tongue in the tropical atlantic is a key issue for understanding the dynamics of the oceanic equatorial atlantic and the west african monsoon diagnosis of the frontogenetic forcings on a realistic high resolution simulation was used to identify the processes involved in the formation and evolution of the equatorial sst front the turbulent forcing associated with the mixed layer turbulent heat flux was found to be systematically frontolytic while the dynamic forcing associated with currents was found to be frontogenetic for the equatorial sst front nevertheless the low frequency component of the turbulent forcing was frontogenetic and initiated the sst front which was then amplified and maintained by the leading dynamic forcing this forcing was mainly driven by the meridional convergence of the northern south equatorial current nsec and the guinea current which points out the essential role played by the circulation in the equatorial sst front evolution the quasi biweekly variability of the equatorial sst front and its forcings were found to be more strongly coupled to the wind energy flux wef than to the surface wind stress in fact the wef controlled the convergence divergence of the nsec and guinea current and thus the meridional component of the leading dynamic forcing the wef explains the equatorial sst front development better than the wind does because it is a coupled ocean atmosphere process

marine atmospheric boundary layer height over the eastern pacific data analysis and model evaluation the atmospheric boundary layer abl height h is a crucial parameter for the treatment of the abl in weather and climate models about 1000 soundings from 11 cruises between 1995 and 2001 over the eastern pacific have been analyzed to document the large meridional zonal seasonal and interannual variations of h in particular its latitudinal distribution in august has three minima near the equator in the intertropical convergence zone itcz and over the subtropical stratus stratocumulus region near the west coast of california and mexico the seasonal peak of h in the itcz zone between 5 68 and 11 28n occurs in the spring february or april while it occurs in august between the equator and 5 68n comparison of these data with the 10 yr monthly output of the community climate system model ccsm2 reveals that overall the model underestimates h particularly north of 208n in august and september directly applying the radiosonde data to the ccsm2 formulation for computing h shows that at the original vertical resolution with the lowest five layers below 2 1 km the ccsm2 formulation would significantly underestimate h in particular the correlation coefficient between the computed and observed h values is only 0 06 for cloudy cases if the model resolution were doubled below 2 1 km however the performance of the model formulation would be significantly improved with a correlation coefficient of 0 78 for cloudy cases

les of a spatially developing atmospheric boundary layer application of a fringe method for the stratocumulus to shallow cumulus cloud transition abstractan arrangement of a large eddy simulation les is described that facilitates a spatially developing thermally stratified atmospheric boundary layer abl when the inflow and outflow boundary conditions are specified the les of stably stratified abl turns out to be challenging because spurious reflections of waves at the boundary accumulate inside the domain to tackle this problem a fringe method with an auxiliary les running concurrently is applied to enforce upstream downstream boundary conditions an artificial forcing term is applied within a fringe region located at the beginning of the main les domain in order to ensure statistically stationary inflow boundary conditions the auxiliary les which is horizontally homogeneous in a doubly periodic domain is used to determine the inflow condition of the main les domain the present scheme is used to provide an eulerian perspective of the stratocumulus to shallow cumulus cloud sc cu transition one of the key cloud regimes over the subtrop

surface based observations of mesoscale cumulus stratocumulus interaction during astex abstract data obtained from the island of santa maria in the azores during the atlantic stratocumulus transition experiment astex are used to describe cloud and boundary layer structure for a 24 h period on 15 june 1992 over the east central atlantic 37 n 25 10 w the evolution of the mesoscale cloud structure during a 24 h period in the vertical column above the surface site was characterized using a 94 ghz radar a laser ceilometer 3 h radiosonde ascents and surface micrometeorological instrumentation mesoscale circulations and drizzle were found to be key elements of the boundary layer clouds observed in this region during the late night and predawn hours of the study period a single layer of stratocumulus that averaged 200 m in thickness topped a well mixed marine boundary layer mesoscale cellular convection mcc which had ascending regions with horizontal dimensions of 7 km was observed during this period at sunrise decoupling was imposed on this mcc and extreme mesoscale variatio

processes controlling the mean tropical pacific precipitation pattern part i the andes and the eastern pacific itcz abstract the question of why the intertropical convergence zone itcz is generally north of the equator in the tropical pacific is addressed experiments with an atmospheric general circulation model coupled to idealized representations of the ocean show that the presence of the andes is enough to lower sea surface temperature sst off the west coast of south america through evaporation thus promoting a north south asymmetry with the itcz north of the equator which is amplified by interactions between the ocean and the atmosphere the evaporative cooling results mainly from the subsidence of low specific humidity air which is due in turn to the mechanical effect of the andes on the zonal mean flow the positive feedback from low level clouds on sst is an important factor for the efficiency of the mechanism described west of 120 w the presence of the rockies and himalayas produces a comparable forcing to that of the andes but this is not enough to reverse or neutralize the north south asymmetry se

using champ radio occultation data to determine the top altitude of the planetary boundary layer 1 a simple approach to derive the planetary boundary layer pbl top altitude from champ challenging minisatellite payload radio occultation ro data is presented our ro processing cuts off at an altitude typically 4 km below which the gps signals are affected by tracking errors this lowest processed altitude lpa is assumed to coincide with the pbl top we average lpas for the years 2001 to 2004 over 5 degree latitude longitude boxes and compare them to ecmwf analysis data the ecmwf pbl top was calculated from the relative humidity gradient with respect to altitude agreement between the data sets is good in terms of mean pbl height especially over sea the champ data shows the major features of pbl height with a realistic transition from stratocumulus regions to shallow and deep cumulus areas champ also shows a substantial amount of pbl height variability that may prove useful to study pbl dynamics

observed lagrangian transition of stratocumulus into cumulus during astex mean state and turbulence structure aircraft measurements made during the first lagrangian of the atlantic stratocumulus transition experiment astex between 12 and 14 june 1992 are presented during this lagrangian experiment an air mass was followed that was advected southward by the mean wind five aircraft flights were undertaken to observe the transition of a stratocumulus cloud deck to thin and broken stratocumulus clouds penetrated by cumulus from below from the horizontal aircraft legs the boundary layer mean structure microphysics turbulence structure and entrainment were analyzed the vertical profiles of the vertical velocity skewness are shown to illustrate the transition of a cloudy boundary layer predominantly driven by longwave radiative cooling at the cloud top to one driven mainly by convection due to an unstable surface stratification and cumulus clouds during the last flight before the stratocumulus deck was observed to be broken and replaced by cumuli the total water flux the virtual potential temperature flux and the vertical velocity variance in the stratocumulus cloud layer were found significantly larger compared with the previous flights to analyze the cloud top stability the mean jumps of conserved variables across the inversion were determined from porpoising runs through the cloud top these jumps were compared with cloud top entrainment instability criteria discussed in the literature it is suggested that enhanced entrainment of dry air is a key mechanism in the stratocumulus cumulus transition

water quality along a river continuum subject to point and diffuse sources summary the water quality along the river kennet in the thames basin of southern england was examined in terms of the influence of point and diffuse nutrient inputs the river is supplied mainly from a cretaceous chalk aquifer and hence the waters are of a calcium bicarbonate type the nitrate largely comes from agricultural sources with concentrations decreasing downstream due to plant uptake and probable denitrification in contrast soluble reactive phosphorus srp is largely associated with sewage inputs and concentrations increase downstream in line with effluents from major towns such as newbury and reading adjacent to the river in the lower half of the catchment is the kennet and avon canal and the two are in places hydrologically connected the canal inputs may influence calcium carbonate calcite precipitation and increase suspended sediment and particulate phosphorus concentrations in the river monitoring upstream and downstream of marlborough sewage treatment works stw showed that srp concentrations in the effluent were highly variable due to variable efficiency of p stripping and still sufficiently concentrated to dominate downstream river srp with potential impacts on stream ecology biological recovery in this river following p stripping at stws is complex and controlling those spikes in srp that are above a threshold of 100 g l 1 may be a critical requirement more stringent effluent targets than are currently recommended may be needed less than 800 g rp l 1 to achieve good ecological status in this river depending on srp concentrations upstream

a new parameterization for shallow cumulus convection and its application to marine subtropical cloud topped boundary layers part i description and 1d results abstract a new parameterization of shallow cumulus convection is presented the parameterization consists of a mass flux scheme based on a buoyancy sorting entrainment detrainment plume model the mass flux scheme is coupled to a 1 5 order turbulence closure model with an entrainment closure for convective boundary layers model performance is verified using single column model simulations at relatively high vertical resolution of pure trade cumulus convection and a cumulus to stratocumulus transition mixing rates cloud cover and vertical flux profiles as deduced from previously published large eddy simulation studies are well reproduced by the parameterization the model is used to demonstrate that height variations of lateral mixing rates can be successfully captured by a simple implementation of a buoyancy sorting mechanism in the updraft cloud model a companion paper describes the implementation of the scheme in a mesoscale model

boundary layer depth entrainment and decoupling in the cloud capped subtropical and tropical marine boundary layer abstract estimates of marine boundary layer mbl depth and degree of decoupling for two regions of the subtropical and tropical east pacific are presented using satellite observations from the moderate resolution imaging spectroradiometer modis and the tropical rainfall measuring mission trmm microwave imager tmi these data are combined in a novel way with ncep reanalysis data and a mixing line parameterization to estimate the mean entrainment rate we over the two regions mean entrainment rates vary geographically and have maxima just downwind of the californian coast we 4 5 mm s 1 and also in the core of the equatorial east pacific cold tongue where mean we exceeds 6 mm s 1 entrainment exceeds subsidence by 30 or less in the subtropical stratocumulus regions north of the equatorial cold tongue entrainment greatly exceeds subsidence producing a rapid deepening of the mbl as air flows over a marked sst gradient shallow mbls zi 500 700 m are found to be well mixed in general the deco

buoyancy reversal decoupling and the transition from stratocumulus to shallow cumulus topped marine boundary layers the transition in a marine boundary layer mbl from stratocumulus topped to shallow cumulus topped is investigated by using a large eddy simulation les model the experiments performed aim to examine the influence on the transition of 1 the probability of buoyancy reversal at the mbl top i e situations in which the mixture of two air parcels becomes denser than either of the original parcels due to phase change or other nonlinear processes involved in the mixing and 2 the degree of decoupling in the mbl i e the strength of a shallow stably stratified layer near cloud base our results suggest that a stratocumulus topped mbl is most likely to transit to a cumulus topped one when 1 there exists high probability of buoyancy reversal at the mbl top and 2 the mbl is decoupled due to large surface evaporation we argue that a parameterization that includes representation of those two effects combined has the potential to provide a simple way of predicting the mbl transition in climate models

observational evidence that enhanced subsidence reduces subtropical marine boundary layer cloudiness conventional wisdom suggests that subsidence favors the presence of marine stratus and stratocumulus because regions of enhanced boundary layer cloudiness are observed to climatologically co occur with regions of enhanced subsidence here it is argued that the climatological positive correlation between subsidence and cloudinessisnottheresultofadirectphysicalmechanismconnectingthetwo instead itarisesbecauseenhanced subsidence is typically associated with stronger temperature inversions capping the marine boundary layer and stronger temperature inversions favor greater cloudiness through statistical analysis of satellite cloud data and meteorological reanalyses for thesubsidenceregime over tropical 308s 308n oceans itisshown that enhanced subsidence promotes reduced cloudiness for the same value of inversion strength and that a stronger inversion favors greater cloudiness for the same value of subsidence using a simple conceptual model it is argued that

near surface density currents observed in the southeast pacific stratocumulus topped marine boundary layer abstractdensity currents i e cold pools or outflows beneath marine stratocumulus clouds are characterized using 30 days of ship based observations obtained during the 2008 variability of american monsoon systems vamos ocean cloud atmosphere land study regional experiment vocals rex in the southeast pacific an air density increase criterion applied to the improved meteorological imet sensor data identified 71 density current front core peak density and tail dissipating zones the similarity in speeds of the mean density current propagation speed 1 8 m s 1 and the mean cloud level advection relative to the surface layer wind 1 9 m s 1 allowed drizzle cells to deposit elongated density currents in their wakes scanning doppler lidar captured prefrontal updrafts with a mean intensity of 0 91 m s 1 and an average vertical extent of 800 m updrafts were often surmounted by low lying shelf clouds not connected to the overlying stratocumulus cloud the observed density currents were 5 10 time

observations of the variation in aerosol and cloud microphysics along the 20 s transect on 13 november 2008 during vocals rex abstractobservations are presented of the structure of the marine boundary layer mbl in the southeastern pacific made with the u k bae 146 aircraft on 13 november 2008 as it flew at a variety of altitudes along 20 s between the coast of chile and a buoy 950 km offshore during the variability of american monsoon systems vamos ocean cloud atmosphere land study vocals regional experiment rex the purpose of the study is to determine the variations along the 20 s transect in the clouds and boundary layer on this particular day as compared to the typical structure determined from the composite studies the aircraft flew in three regions on this day relatively continuous thick stratocumulus clouds open cells and closed cells results show three particular features first the results of the cloud microphysics are consistent with the typical behavior showing a decrease in aerosol particles by a factor of 3 4 and a decrease in cloud droplet number concentration westward from the coast from about 200

relation between mean boundary layer structure and cloudiness at the r v valdivia during astex abstract the relationship between boundary layer thermodynamic structure and cloud fields and their diurnal variation are explored using seven days of data from dw r v valdivia during the atlantic stratocumulus transition experiment cloudiness is at a maximum before dawn when the boundary layer bl has the thermodynamic structure of a partially mixed conditionally unstable stratocumulus layer which is close to mean saturation below the inversion cloudiness falls during the daytime and in the late afternoon the bl has two distinct layers a warmer drier cloud layer characteristic of trade cumulus above a more well mixed subcloud layer the observed mean profiles are consistent with an earlier suggestion that there is a cloud cover transition once the bl mixing line slope exceeds half that of the moist adiabat in contrast the bl structure in the ecmwf model for the same week has a much drier warmer more stable cloud layer than the observations

a new parameterization for shallow cumulus convection and its application to marine subtropical cloud topped boundary layers part ii regional simulations of marine boundary layer clouds abstract the impact of physical parameterizations on simulations of cloud topped marine boundary layers is investigated using the fifth generation pennsylvania state university ncar mesoscale model mm5 three month mm5 simulations of the northeast and southeast pacific during june august 1987 are boundary forced with time varying ecmwf analyses runs with four planetary boundary layer pbl parameterizations already implemented in mm5 are compared with runs using new parameterizations of boundary layer turbulence and shallow cumulus convection shcu described in a companion paper numerous modifications to the mm5 that allow it to be used as a regional climate model are described the simulated 3 month mean shortwave cloud radiative forcing swcf and vertical structure of cloud topped boundary layers in the northeast pacific are sensitive to the pbl shallow convection schemes all four current mm5 pbl schemes the blackadar medium range forecast mrf burk thompson and gayno seaman schemes produc

marine ice sheet dynamics part 1 the case of rapid sliding marine ice sheets are continental ice masses resting on bedrock below sea level their dynamics are similar to those of land based ice sheets except that they must couple with the surrounding floating ice shelves at the grounding line where the ice reaches a critical flotation thickness in order to predict the evolution of the grounding line as a free boundary two boundary conditions are required for the diffusion equation describing the evolution of the grounded ice thickness by analogy with stefan problems one of these conditions imposes a prescribed ice thickness at the grounding line and arises from the fact that the ice becomes afloat the other condition must be determined by coupling the ice sheet to the surrounding ice shelves here we employ matched asymptotic expansions to study the transition from ice sheet to ice shelf flow for the case of rapidly sliding ice sheets our principal results are that the ice flux at the grounding line in a two dimensional ice sheet is an increasing function of the depth of the sea floor there and that ice thicknesses at the grounding line must be small compared with ice thicknesses inland these results indicate that marine ice sheets have a discrete set of steady surface profiles if they have any at all and that the stability of these steady profiles depends on the slope of the sea floor at the grounding line

atmospheric response to mesoscale sea surface temperature anomalies assessment of mechanisms and coupling strength in a high resolution coupled model over the south atlantic many aspects of the coupling between the ocean and atmosphere at the mesoscale on the order of 20 100 km remain unknown while recent observations from the southern ocean revealed that circular fronts associated with oceanic mesoscale eddies leave a distinct imprint on the overlying wind cloud coverage and rain the mechanisms responsible for explaining these atmospheric changes are not well established here the atmospheric response above mesoscale ocean eddies is investigated utilizing a newly developed coupled atmosphere ocean regional model consortium for small scale modeling regional ocean modelling system cosmo roms configured at a horizontal resolution of 10 km for the south atlantic and run for a 3 month period during austral winter of 2004 the model simulated changes in surface wind cloud fraction and rain above the oceanic eddies are very consistent with the relationships inferred from satellite observations for the same region and time from diagnosing the model s momentum balance it is shown that the atmospheric imprint of the oceanic eddies are driven by the modification of vertical mixing in the atmospheric boundary layer rather than secondary flows driven by horizontal pressure gradients this is largely due to the very limited ability of the atmosphere to adjust its temperature over the time scale it takes for an air parcel to pass over these mesoscale oceanic features this results in locally enhanced vertical gradients between the ocean surface and the overlying air and thus a rapid change in turbulent mixing in the atmospheric boundary layer and an associated change in the vertical momentum flux

summertime post cold frontal marine stratocumulus transition processes over the eastern north atlantic abstractmarine boundary layer mbl cloud morphology associated with two summertime cold fronts over the eastern north atlantic ena is investigated using high resolution simulations from the weat

statistical analyses of satellite cloud object data from ceres part iv boundary layer cloud objects during 1998 el nino abstract three boundary layer cloud object types overcast stratocumulus and cumulus that occurred over the pacific ocean during january august 1998 are identified from the clouds and the earth s radiant energy system ceres single scanner footprint data characteristics of each cloud object type matched with atmospheric states are examined for large regions in the tropics and subtropics and for different size categories stratocumulus cloud objects dominate the entire boundary layer cloud population in all regions and size categories overcast cloud objects which have the largest average size are more prevalent in the subtropics and near the coastal regions while cumulus cloud objects are prevalent over the open oceans and the equatorial regions particularly within the small size categories cloud objects with equivalent diameters less than 75 km are excluded in the analysis the differences between the tropical and subtropical statistical distributions of cloud properties are small for liquid wate

epic 95 w observations of the eastern pacific atmospheric boundary layer from the cold tongue to the itcz abstract the atmospheric boundary layer abl along 95 w in the eastern equatorial pacific during boreal autumn is described using data from the east pacific investigation of climate epic 2001 with an emphasis on the evolution of the thermodynamic abl properties from the cold tongue to the cold advection region north of the sea surface temperature sst front surface sensible and latent heat fluxes and wind stresses between 1 s and 12 n are calculated from data from eight ncar c 130 research aircraft flights and from tropical atmosphere ocean tao buoys reduced surface wind speed and a 10 m s 1 jet at a height of 500 m are found over the equatorial cold tongue demonstrating the dependence of the surface wind speed on surface stability the abl exhibits a maximum in cloud cover on the north downwind side of the warm sst front at 1 3 n turbulent mixing driven by both surface buoyancy flux and radiative cooling at the cloud tops plays a significant role in maintaining the depth and structure of t

the influence of cross equatorial pressure gradients on the location of near equatorial convection tomas and webster note that in regions where there is a substantial cross equator surface pressure gradient there is locally anticyclonic absolute vorticity on the low pressure side of the equator and thus the flow in this region meets the parcel criterion for inertial instability they hypothesize that the atmospheric response to this absolute vorticity distribution is a convergence divergence doublet in the boundary layer the convergence centre results in strong convection and thus is very important in determining the strength and location of convection i e the intertropical convergence zones whereas the divergence centre results in suppressed convection in this work a zonally symmetric model of the boundary layer is developed to investigate this hypothesis further an analysis of a linearized version of the model indicates that although the observed flow in these regions meets the parcel criterion for instability it does not satisfy the linear stability criteria because of the stabilizing influence of dissipation and the finite vertical scale much greater shear than that observed is required for linear instability it is noted however that when the parcel criterion is met one of the nonlinear terms which was neglected in the linear analysis may have an important influence on the flow several experiments are performed integrating the full nonlinear model relaxing back to a pressure distribution having a cross equatorial gradient to test whether the observed vorticity wind and convergence distributions can be well simulated it is found that the simulations reach a steady state in which there is a region of locally anticyclonic absolute vorticity on the low pressure side of the equator this absolute vorticity distribution results in an anomalous acceleration of the meridional wind and a convergence divergence doublet similar to that observed by tomas and webster this response is explained as resulting from accelerations that act to bring the zonal flow out of geostrophic balance as it passes through the region of locally anticyclonic absolute vorticity the response of the meridional wind to the strength of the pressure gradient is quasi linear for small and somewhat larger values of forcing but between these values there is a range where the response is highly nonlinear i e large increases in the strength of the meridional wind result from small changes in the pressure gradient it is suggested that this nonlinear response may play a role in the observed sudden onset of the monsoon circulations the nonlinear model is also forced using pressure distributions taken from observations the resulting steady state simulated vorticity wind and convergence distributions closely match the observations for the cases of pressure distributions taken from the east pacific and east atlantic during july when forced using the pressure distribution from the indian ocean region during july the model produces an unrealistically strong meridional wind response some improvement is obtained by including the latitudinally dependent zonal pressure gradient force which is moderately strong in this region finally the model is forced using a pressure distribution taken from observations in the central pacific ocean during july in this case the cross equator pressure gradient is near zero

terrestrial ecosystems and the global biogeochemical silica cycle 1 most research on the global si cycle has focused nearly exclusively on weathering or the oceanic si cycle and has not explored the complexity of the terrestrial biogeochemical cycle the global biogeochemical si cycle is of great interest because of its impact on global co2 concentrations through the combined processes of weathering of silicate minerals and transfer of co2 from the atmosphere to the lithosphere a sizable pool of si is contained as accumulations of amorphous silica or biogenic silica bsi in living tissues of growing plants known as phytoliths and after decomposition of organic material as remains in the soil the annual fixation of phytolith silica ranges from 60 200 tmol yr 1 and rivals that fixed in the oceanic biogeochemical cycle 240 tmol yr 1 internal recycling of the phytolith pool is intense with riverine fluxes of dissolved silicate to the oceans buffered by the terrestrial biogeochemical si cycle challenging the ability of weathering models to predict rates of weathering and consequently changes in global climate consideration must be given to the influence of the terrestrial bsi pool on variations in the global biogeochemical si cycle over geologic time and the influence man has had on modifying both the terrestrial and aquatic biogeochemical cycles

coupling of the radiative convective and surface fluxes over the equatorial pacific abstract an idealized energy balance model for a closed circulation is first presented to illustrate the coupling between the net tropospheric radiative cooling the surface fluxes and the mean subsidence away from the precipitation zones then a one dimensional diagnostic model and a radiation model with boundary layer clouds are combined to explore this coupling for a specific region using mean sounding data over the tropical pacific the radiatively driven subsidence rate at the top of the convective boundary layer is approximately 35 mb day 1 0 04 pa s 1 and is largely independent of boundary layer cloud fraction the sensitivity of the corresponding convective heat flux profiles to the mass divergence profile and cloud fraction within the boundary layer is explored reasonable assumptions give realistic surface sensible and latent heat fluxes for this region of approximately 10 and 130 w m 1 the paper illustrates the important background climatic control of the radiation field on the tropical surfa

observations of coupling between surface wind stress and sea surface temperature in the eastern tropical pacific satellite measurements of surface wind stress from the quikscat scatterometer and sea surface temperature sst from the tropical rainfall measuring mission microwave imager are analyzed for the three month period 21 july 20 october 1999 to investigate ocean atmosphere coupling in the eastern tropical pacific oceanic tropical instability waves tiws with periods of 20 40 days and wavelengths of 1000 2000 km perturb the sst fronts that bracket both sides of the equatorial cold tongue which is centered near 1 8s to the east of 1308w these perturbations are characterized by cusp shaped features that propagate systematically westward on both sides of the equator the space time structures of these sst perturbations are reproduced with remarkable detail in the surface wind stress field the wind stress divergence is shown to be linearly related to the downwind component of the sst gradient with a response on the south side of the cold tongue that is about twice that on the north side the wind stress curl is linearly related to the crosswind component of the sst gradient with a response that is approximately half that of the wind stress divergence response to the downwind sst gradient the perturbed sst and wind stress fields propagate synchronously westward with the tiws this close coupling between sst and wind stress supports the wallace et al hypothesis that surface winds vary in response to sst modification of atmospheric boundary layer stability