automatic detection of atmospheric boundary layer height using ceilometer backscatter data assisted by a boundary layer model this work tackles the problem of the automated detection of the atmospheric boundary layer bl height h from aerosol lidar ceilometer observations a new method the bayesian selective method bsm is presented it implements a bayesian statistical inference procedure which combines in a statistically optimal way different sources of information firstly atmospheric stratification boundaries are located from discontinuities in the ceilometer backscattered signal the bsm then identifies the discontinuity edge that has the highest probability to effectively mark the bl height information from the contemporaneous physical boundary layer model simulations and a climatological dataset of bl height evolution are combined in the assimilation framework to assist this choice the bsm algorithm has been tested for 4 months of continuous ceilometer measurements collected during the base alfa project and is shown to realistically diagnose the bl depth evolution in many different weather conditions a standard one dimensional processing of the ceilometer signal without the a priori support of the dynamical and climatological bl models often fails to correctly detect h with the greatest inaccuracies occurring at night time when residual layers can generate very strong signals which are then classified by an automated application of the gradient or of the wavelet analysis as the most probable bl height the bsm approach instead carries information on the low climatological probability to find elevated bl depths at night and penalizes the selection of these points moreover this method is able to correctly convey information along the temporal dimension thus filling data gaps using earlier and subsequent ceilometer information for the retrieval copyright 2011 royal meteorological society

the influence of idealized heterogeneity on wet and dry planetary boundary layers coupled to the land surface abstract this manuscript describes numerical experiments investigating the influence of 2 30 km striplike heterogeneity on wet and dry convective boundary layers coupled to the land surface the striplike heterogeneity is shown to dramatically alter the structure of the convective boundary layer by inducing significant organized circulations that modify turbulent statistics the impact strength and extent of the organized motions depend critically on the scale of the heterogeneity relative to the boundary layer height zi the coupling with the land surface modifies the surface fluxes and hence the circulations resulting in some differences compared to previous studies using fixed surface forcing because of the coupling surface fluxes in the middle of the patches are small compared to the patch edges at large heterogeneity scales zi 18 horizontal surface flux gradients within each patch are strong enough to counter the surface flux gradients between wet and dry patches allowing the formation of

a comparison of shear and buoyancy driven planetary boundary layer flows abstract planetary boundary layer pbl flows are known to exhibit fundamental differences depending on the relative combination of wind shear and buoyancy forces these differences are not unexpected in that shear instabilities occur locally while buoyancy force sets up vigorous thermals which result in nonlocal transport of heat and momentum at the same time these two forces can act together to modify the flow field in this study four large eddy simulations less spanning the shear and buoyancy flow regimes were generated two correspond to the extreme cases of shear and buoyancy driven pbls while the other two represent intermediate pbls where both forces are important the extreme cases are used to highlight and quantify the basic differences between shear and convective pbls in 1 flow structures 2 overall statistics and 3 turbulent kinetic energy tke budget distributions results from the two intermediate les cases are used to develop and verify a velocity scaling and a tke budget mode

the turbulent structure and diurnal growth of the saharan atmospheric boundary layer the turbulent structure and growth of the remote saharan atmospheric boundary layer abl is described with in situ radiosonde and aircraft measurements and a large eddy simulation model a month of radiosonde data from june 2011 provides a mean profile of the midday saharan abl which is characterized by a well mixed convective boundary layer capped by a small temperature inversion 1k and a deep near neutral residual layer the boundary layer depth varies by up to 100 over horizontal distances of a few kilometers due to turbulent processes alone the distinctive vertical structure also leads to unique boundary layer processes such as detrainment of the warmest plumes across the weak temperature inversion which slows down the warming and growth of the convective boundary layer as the boundary layer grows overshooting plumes can also entrain freetropospheric air into the residual layer forming a second entrainment zone that acts to maintain the inversion above the convective boundary layer thus slowing down boundary layer growth further a single column model is unable to accurately reproduce the evolution of the saharan boundary layer highlighting the difficulty of representing such processes in large scale models these boundary layer processes are special to the sahara and possiblyhot dry desertenvironmentsingeneral andhaveimplicationsforthelarge scalestructureofthesaharan heat low the growth of the boundary layer influences the vertical redistribution of moisture and dust and the spatial coverage and duration of clouds with large scale dynamical and radiative implications

coherent structures in the boundary and cloud layers role of updrafts subsiding shells and environmental subsidence coherent structures such as updrafts downdrafts shells and environmental subsidence in the boundary and cloud layers of shallow convection are investigated using a new classification method using large eddy simulation data the new method first filters out background turbulence and small scale gravity waves from the coherent part of the flow composed of turbulent coherent structures and large scale transporting gravity waves then the algorithm divides this coherent flow into updrafts downdrafts shells subsidence ascendance and four other flow structures using an octant analysis the novel method can systematically track structures from the cloud free boundary layer to the cloud layer thus allowing systematic analysis of the fate of updrafts and downdrafts the frequency and contribution of the coherent structures to the vertical mass flux and transport of heat and moisture can then be investigated for the first time updrafts subsidence and downdrafts subsiding shells to a lesser extent are shown to be the most frequent and dominant contributors to the vertical transport of heat and moisture in the boundary layer contrary to previous perspective environmental subsidence transport is shown to be weak in the cloud layer instead downdrafts shells are the main downward transport contributors especially in the trade inversion layer the newly developed method in this study can be used to better evaluate the entrainment and detrainment of individual or an ensemble of coherent structures from the unsaturated boundary layer to the cloud layer

large eddy simulation of conditionally neutral boundary layers a mesh resolution sensitivity study abstractlarge eddy simulation les is used to model turbulent winds in a nominally neutral atmospheric boundary layer at varying mesh resolutions the boundary layer is driven by wind shear with z

effects of resolution on the simulation of stratocumulus entrainment three dimensional large eddy simulations less of a smoke cloud beneath a strong inversion are used to investigate how grid resolution affects simulated entrainment rate these simulations are generalizations of a 1995 gewex cloud system studies intercomparison of models of smoke clouds radiative cooling at the top of the smoke cloud induces convection in the smoke cloud in a manner closely analogous to that in marine stratocumulus capped boundary layers the simulations are compared to a laboratory analogue for a range of inversion strengths and radiative forcings the inverse richardson number scaling of non dimensional entrainment rate found in the laboratory experiments is largely captured by the numerical model but the modelled entrainment rates for strong inversions are two large by a factor of two better agreement is obtained when the numerical model resolves the undulations of the inversion driven by the underlying convection by this criterion most les studies of subtropical maritime stratocumulus made so far are under resolved for determining the bulk entrainment rate a nested grid model which has fine resolution only in the entrainment zone is as effective as a model with uniformly fine resolution if horizontal resolution is much coarser than the vertical resolution modelled entrainment rates are less than those obtained with simulations which have comparable vertical and horizontal resolutions

convective boundary layer structure in the presence of wind following swell the marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area where they were initiated by momentum transferred to the ocean wave field during storms the atmospheric boundary layer during wind following swell and various stability states has been investigated using large eddy simulation les data the dominant energy containing motions in the near neutral atmospheric boundary layer over flat terrain are known to be dominated by near ground shear induced regions of high and low speed flow wind fields and momentum fluxes from les for swell dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary layer winds and fluxes a mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of les data coherent detached eddies from the directly wave induced motions near the surface are found to maintain an upward momentum transfer this mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions in this way it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell this has implication for the turbulence length scales during wind following swell copyright 2012 royal meteorological society

characterization of wind shear effects on entrainment in a convective boundary layer direct numerical simulations are used to characterize wind shear effects on entrainment in a barotropic convective boundary layer cbl that grows into a linearly stratified atmosphere we consider weakly to strongly unstable conditions z enc l ob gtrsim 4 where z enc is the encroachment cbl depth and l ob is the obukhov length dimensional analysis allows us to characterize such a sheared cbl by a normalized cbl depth a froude number and a reynolds number the first two non dimensional quantities embed the dependence of the system on time on the surface buoyancy flux and on the buoyancy stratification and wind velocity in the free atmosphere we show that the dependence of entrainment zone properties on these two non dimensional quantities can be expressed in terms of just one independent variable the ratio between a shear scale unicode stix x0394 z i s equiv sqrt 1 3 unicode stix x0394 u n 0 and a convective scale unicode stix x0394 z i c equiv 0 25z enc where unicode stix x0394 u is the velocity increment across the entrainment zone and n 0 is the buoyancy frequency of the free atmosphere here unicode stix x0394 z i s and unicode stix x0394 z i c represent the entrainment zone thickness in the limits of weak convective instability strong wind and strong convective instability weak wind respectively we derive scaling laws for the cbl depth the entrainment zone thickness the mean entrainment velocity and the entrainment flux ratio as functions of unicode stix x0394 z i s unicode stix x0394 z i c these scaling laws can also be expressed as functions of only a richardson number n 0 z enc unicode stix x0394 u 2 but not in terms of only the stability parameter z enc l ob

a case study of convective boundary layer development during ihop_2002 numerical simulations compared to observations abstract results are presented from a combined numerical and observational study of the convective boundary layer cbl diurnal evolution on a day of the international h2o project ihop 2002 experiment that was marked by the passage of a dryline across part of the oklahoma and texas panhandles the initial numerical setup was based on observational data obtained from ihop 2002 measurement platforms and supplementary datasets from surrounding locations the initial goals of the study were as follows i numerical investigation of the structure and evolution of the relatively shallow and homogeneous cbl east of the dryline by means of large eddy simulation les ii evaluation of les predictions of the sheared cbl growth against lidar observations of the cbl depth evolution and iii comparison of the simulated turbulence structures with those observed by lidar and vertically pointing radar during the cbl evolution in the process of meeting these goals complications associated with comparisons betwee

high tio 2 metadiabase dikes of the hudson highlands new york and new jersey possible late proterozoic rift rocks in the new york recess

effect of planetary rotation on oceanic surface boundary layer turbulence abstracta large eddy simulation les model is configured to investigate the effect of the horizontal northward component of earth s rotation on upper ocean turbulence the focus is on the variab

where is the interface of the stratocumulus topped pbl abstract various locally defined not horizontal mean interfaces between the stratocumulus topped pbl and the free atmosphere are investigated using a fine resolution large eddy simulation with a vertical grid spacing of about 4 m the local cloud top height is found to be always below the height where the maximum gradient of the local sounding occurs and the maximum gradient height is always below the interface where pbl air can reach via turbulent motions the distances between these local interfaces are of significant amount a few tens of meters on average air between the cloud top and maximum gradient interfaces is fully turbulent unsaturated but rather moist air between the maximum gradient and turbulent mixing interfaces consists of turbulent motions that are intermittent in space and time the simulated flow shows no clearly defined interface that separates cloudy turbulent air mass from clear nonturbulent air above even locally

an alternative scaling for unsteady penetrative free convection 1 the daytime evolution of the atmospheric boundary layer under high pressure anticyclonic synoptic systems over a homogeneous terrain can be successfully described by a penetrative free convection model evolving from an initially stably stratified environment in this article dimensional analysis has been employed to derive a new set of scaling parameters which are functions of external time dependent fluid properties and boundary conditions this novel theoretical framework has been adopted to compare laboratory scale experiments performed using a thermally controlled water tank and a large eddy simulation les numerical model both these have been developed for the characterization of the instabilities associated with the development of the convective boundary layer cbl the characteristic length and time scales of the thermal plumes their spatial distribution and interaction with the overlying stable layer are analyzed the proposed scaling parameters appear to be representative of the bulk and turbulent properties of the cbl as confirmed by both numerical and laboratory results

long term observations of the convective boundary layer using insect radar returns at the sgp arm climate research facility abstract a long term study of the turbulent structure of the convective boundary layer cbl at the u s department of energy atmospheric radiation measurement program arm southern great plains sgp climate research facility is presented doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the cbl the observations cover four summer periods 2004 08 and are classified into cloudy and clear boundary layer conditions profiles of vertical velocity variance skewness and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions a conditional sampling method is applied to the original doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport overall the derived turbulent statistics are consistent with previous aircraft and lidar observations the obs

interaction between turbulent flow and sea breeze front over urban like coast in large eddy simulation turbulent flow and its interaction with a sea breeze front sbf over an urban like coast with a regular block array were investigated using a building resolving computational fluid dynamics model it was found that during daytime with an offshore ambient flow streaky turbulent structures tended to grow within the convective boundary layer cbl over a warm urban surface ahead of the sbf the structures were organized as streamwise streaks at an interval of a few hundred meters which initiated at the rooftop level with strong wind shear and strengthens in the cbl with moderate buoyancy the streaks then interacted with the onshore propagating sbf as it made landfall the sbf which was initially characterized as a shallow and quasi linear feature over the sea developed three dimensional structures with intensified updrafts at an elevated frontal head after landfall frontal updrafts were locally enhanced at intersections where the streaks merged with the sbf which greatly increased turbulent fluxes at the front the frontal line was irregular because of merging tilting and transformation effects of vorticity associated with streaky structures inland penetration of the sbf was slowed by the frictional effect of urban like surfaces and turbulent flow on land the overall sbf intensity weakened after the interaction with turbulent flow these findings aid understanding of local weather over coastal cities during typical sea breeze conditions

daytime development of the boundary layer over a plain and in a valley under fair weather conditions a comparison by means of idealized numerical simulations abstractthe daytime thermal structures of the valley boundary layer vbl and of the convective boundary layer cbl above a plain as revealed by idealized large eddy simulations are compared simulations in the two environments consider similar thermal forcing thus allowing an analysis of the atmospheric heating processes in the vbl and cbl in light of the volume effect theory traditionally invoked to explain the larger diurnal temperature ranges observed in valleys it is found that after an equal input of thermal energy the atmospheric volumes affected by thermal perturbations in the cbl and in the vbl are comparable although the boundary layer top is higher in the vbl than in the cbl the average vbl depth is approximately equal to the cbl depth since the ground elevation is nonuniform in the valley accordingly the volume averaged potential temperature increments in the cbl and vbl are comparable nevertheless surface air temperature variations are larger in the vbl while differences in th

daytime heat transfer processes over mountainous terrain abstractthe daytime heat transfer mechanisms over mountainous terrain are investigated by means of large eddy simulations over idealized valleys two and three dimensional topographies corresponding to infinite and finite valleys are used in order to evaluate the influence of the along valley wind and the valley surroundings on the heat transfer processes the atmosphere is coupled to an interactive land surface allowing for dynamic feedback on the surface fluxes the valley heat budget is analyzed both from a local and bulk perspective and the flow is reynolds decomposed into its mean and turbulent component the analysis clarifies recent issues of contention regarding the heating of the valley atmosphere the flow decomposition allows one to clearly distinguish between the different heating processes those associated with the mean flow such as advection induced cooling by the upslope flows and the warming induced by the compensating subsidence and those associated with the turbulent motions the

wind shear effects on radiatively and evaporatively driven stratocumulus tops abstractdirect numerical simulations resolving meter and submeter scales in the cloud top region of stratocumulus are used to investigate the interactions between a mean vertical wind shear and in

dynamics of sheared convective boundary layer entrainment part i methodological background and large eddy simulations abstract the reported study examines the dynamics of entrainment and its effects on the evolution of the dry atmospheric convective boundary layer cbl when wind shear is present the sheared cbl can be studied by means of direct measurements in the atmosphere laboratory studies and numerical techniques the advantages and disadvantages of each technique are discussed in the present paper which also describes the methodological background for studying the dynamics of entrainment in sheared cbls for the reported study large eddy simulation les was chosen as the primary method of convective entrainment investigation twenty four les runs were conducted for cbls growing under varying conditions of surface buoyancy flux free atmospheric stratification and wind shear the simulations were divided into three categories cbl with no mean wind ns cbl with a height constant geostrophic wind of 20 m s 1 gc and cbl with geostrophic wind shear gs in the simulated cases the sheared cbls grew fastes

a multiple mass flux parameterization for the surface generated convection part i dry plumes abstract the present study introduces a parameterization for the mixing in the surface generated free convection regime including dry and moist convection cases the parameterization explicitly represents the plumes that drive the convective turbulence ascending parcels that have similar properties in the surface layer are grouped into updrafts with a determined fractional area the updrafts ascend according to an entraining plume model and may eventually overturn the plume model can either use a time or a length scale for the lateral entrainment closure the plumes environment is assumed to be horizontally uniform a one dimensional framework is used to evaluate the parameterization in the case of a dry convective boundary layer cbl the predicted variances are underestimated presumably due to the uniform environment assumption the contrast between the strong upward motions and the compensating subsidence is simulated and the skewness of w increases realistically with height the plumes dyn

regional bowen ratio controls on afternoon moist convection a large eddy simulation study this study examines the effect of the regional bowen ratio the ratio of the domain averaged surface sensible heat flux shf to latent heat flux lhf on afternoon moist convection with a temporally evolving but spatially uniform surface available energy over a mesoscale domain under a weak capping inversion we run large eddy simulation of the afternoon convective boundary layer cbl we first prescribe a small of 0 56 a wet surface and then the reversed large of 1 80 a dry surface by switching the shf and lhf fields the perturbation fields of the fluxes are prescribed with the fourier spectra of 3 is horizontal wave number strong mesoscale heterogeneity and 0 homogeneity the large cases have strong vertical buoyancy fluxes and produce more vigorous updrafts in the heterogeneous large surface case with the removal of convective inhibition over a mesoscale subdomain of large shf deep convection develops in the heterogeneous small surface case convective clouds develop but do not progress into precipitating convection in the homogeneous surface cases randomly distributed shallow clouds develop with significantly more and thicker clouds in the large case co spectral analyses confirm the more vigorous turbulent thermals in the large cases and reveal that the moisture advection by the surface heterogeneity induced mesoscale flows makes the correlation between mesoscale temperature and moisture perturbations change from negative to positive which facilitates the mesoscale pool of high relative humidity air just above the cbl top a necessary condition for deep convection

role of surface friction on shallow nonprecipitating convection abstractthe role of surface friction on shallow nonprecipitating convection is investigated using a series of large eddy simulations with varying surface friction velocity and with a cloud identification algorithm as surface friction intensifies convective rolls dominate over convective cells and secondary overturning circulation becomes stronger in the subcloud layer thus transporting more moisture upward and more heat downward between the subcloud and cloud layers identifying individual clouds using the identification algorithm based on a three dimensional topological analysis reveals that intensified surface friction increases the number of clouds and the degree of tilting in the downstream direction highly intensified surface friction increases wind shear across the cloud base and induces cloud tilting which leads to a vertically parabolic profile of liquid water mixing ratio instead of the classical two layer structure conditionally unstable and trade inversion layers furthermore cloud ti

entrainment into mixed layers part ii a new closure abstract a new entrainment closure for convectively mixed boundary layers particularly those topped by stratocumulus clouds is developed and calibrated against results of large eddy simulations carried out by c h moeng the closure differs principally in two ways from those developed by j w deardorff s nicholls and others the prediction of interface buoyancy flux from its vertical integral is modified by a height weighting term this accounts for the greater entrainment produced by buoyancy flux near the cloud top as compared to that generated by surface heating the relationship between buoyancy flux at the top of a cloud layer and entrainment is reconsidered entrainment increases with the wetness of the cloud top which is evidently a function of cloud top liquid water content and the cloud top jumps in potential temperature and total water content this result is related to findings by s nicholls and j d turton though using a somewhat different approach and with different interpretatio

a combined eddy diffusivity mass flux approach for the convective boundary layer a better conceptual understanding and more realistic parameterizations of convective boundary layers in climate and weather prediction models have been major challenges in meteorological research in particular parameterizations of the dry convective boundary layer in spite of the absence of water phase changes and its consequent simplicity as compared to moist convection typically suffer from problems in attempting to represent realistically the boundary layer growth and what is often referred to as countergradient fluxes the eddy diffusivity ed approach has been relatively successful in representing some characteristics of neutral boundary layers and surface layers in general the mass flux mf approach on the other hand has been used for the parameterization of shallow and deep moist convection in this paper a new approach that relies on a combination of the ed and mf parameterizations edmf is proposed for the dry convective boundary layer it is shown that the edmf approach follows naturally from a decomposition of the turbulent fluxes into 1 a part that includes strong organized updrafts and 2 a remaining turbulent field at the basis of the edmf approach is the concept that nonlocal subgrid transport due to the strong updrafts is taken into account by the mf approach while the remaining transport is taken into account by an ed closure large eddy simulation les results of the dry convective boundary layer are used to support the theoretical framework of this new approach and to determine the parameters of the edmf model the performance of the new formulation is evaluated against les results and it is shown that the edmf closure is able to reproduce the main properties of dry convective boundary layers in a realistic manner furthermore it will be shown that this approach has strong advantages over the more traditional countergradient approach especially in the entrainment layer as a result this edmf approach opens the way to parameterize the clear and cumulus topped boundary layer in a simple and unified way

mixing of cordierite granitoid and pyroxene gabbro and fractionation in the santa olalla tonalite andalucia abstract the asymmetricaly zoned santa olalla pluton consists of several facies gabbro a tonalitic group that includes both hornblende diorite and cordierite bearing quartz monzonite granitic dykes and enclaves enclaves are an ubiquitous component and in places they occur in swarms the tonalites and enclaves display extremely abundant disequilibrium microstructures pyroxene replaced by actinolite actinolite by hornblende hornblende by biotite biotite by hornblende and both calcic plagioclase and biotite phenocrysts have been corroded in a disequilibrium melt phase most of these microstructures can develop during the evolution of a single magma but the replacement of biotite suggests that crystal melt disequilibrium through magma mixing is responsble hornblende and actinolite occur in clots enclosing pyroxene the growth of actinolite was probably metastable at magmatic temperatures and replacement of pyroxene was controlled by diffusion of al subsequently hornblende replaced actinolite with a melt phase present plagioclase microstructures and compositions are consistent with batches of phenocrysts immersed in a disequilibrium melt that resulted from a bulk compositional change in the magma with which they re equilibrated bulk rock geochemistry indicates that the series gabbro through tonalite to granitic dykes cannot be simply related by a single fractionation trend gabbros vary by pyroxene fractionation the tonalites are a mixture of a cordierite bearing s type magma and a somewhat fractionated pyroxene gabbro thus it is h hybrid type plutonism not i type the granitic dykes developed after mixing by segregation of a residual melt from the hybrid tonalites there is evidence of some degree of fractionation of the tonalite facies after mixing the presence of aplites granitic dykes and divergence of incompatible trace elements from simple mixing ratios no variation can be attributed to restite unmixing

large eddy simulation of the onset of the sea breeze this paper describes results from a large eddy simulation les model used in an idealized setting to simulate the onset of the sea breeze as the les is capable of simulating boundary layer scale threedimensional turbulence along with the mesoscale sea breeze circulation a parameterization of the planetary boundary layer was unnecessary the basic experimental design considers a rotating uniformly stratified resting atmosphere that is suddenly heated at the surface over the land half of the domain to focus on the simplest nontrivial problem the diurnal cycle effects of moisture interactions with large scale winds and coastline curvature were all neglected in this study the assumption of a straight coastline allows the use of a rectangular computational domain that extends to 50 km on either side of the coast but only 5 km along the coast with 100 m grid intervals so that the small scale turbulent convective eddies together with the mesoscale sea breeze may be accurately computed through dimensional analysis of the simulation results the length and velocity scales characterizing the simulated sea breeze as functions of the externally specified parameters are identified

structure of the entrainment zone capping the convective atmospheric boundary layer the authors use large eddy simulation les to investigate entrainment and structure of the inversion layer of a clear convectively driven planetary boundary layer pbl over a range of bulk richardson numbers ri the les code uses a nested grid technique to achieve fine resolution in all three directions in the inversion layer extensive flow visualization is used to examine the structure of the inversion layer and to illustrate the temporal and spatial interaction of a thermal plume and the overlying inversion it is found that coherent structures in the convective pbl that is thermal plumes are primary instigators of entrainment in the ri range 13 6 ri 43 8 at ri 5 13 6 strong horizontal and downward velocities are generated near the inversion layer because of the plume interface interaction this leads to folding of the interface and hence entrainment of warm inversion air at the plume s edge at ri 5 34 5 the inversion s strong stability prevents folding of the interface but strong horizontal and downward motions near the plume s edge pull down pockets of warm air below the nominal inversion height these pockets of warm air are then scoured off by turbulent motions and entrained into the pbl the structure of the inversion interface from les is in good visual agreement with lidar measurements in the pbl obtained during the lidars in flat terrain field experiment a quadrant analysis of the buoyancy flux shows that net entrainment flux or average minimum buoyancy flux wu min is identified with quadrant iv w2u1 0 motions that is warm air moving downward plumes generate both large negative quadrant ii w1u2 0 and positive quadrant iii w2u2 0 buoyancy fluxes that tend to cancel the maximum vertical gradient in potential temperature at every x y grid point is used to define a local pbl height zi x y a statistical analysis of zi shows that skewness of zi depends on the inversion strength spectra of zi exhibit a sensitivity to grid resolution the normalized entrainment rate we w where we and w are entrainment and convective velocities varies as ari21 with a 0 2 in the range 13 6 ri 43 8 and is in good agreement with convection tank measurements for a clear convective pbl the authors found that the finite thickness of the inversion layer needs to be considered in an entrainment rate parameterization derived from a jump condition

large eddy simulations of real world episodes in complex terrain based on era reanalysis and validated by ground based remote sensing data abstracta computationally efficient and inexpensive approach for using the capabilities of large eddy simulations les to model small scale local weather phenomena is presented the setup uses the

coherent structures in large eddy simulations of a nonprecipitating stratocumulus topped boundary layer abstractthe properties of coherent convective structures are analyzed in a nonprecipitating marine nocturnal stratocumulus topped boundary layer stbl with a series of high resolution large eddy simulations less a new classification method based on octant analysis using vertical velocity and two passive scalars is introduced to systematically define convective structures in both the cloudy and the cloud free regions it is therefore possible to detect and track updrafts downdrafts and their turbulent shells both ascending and subsiding together with the entraining air from the inversion layer or the free troposphere the geometrical and thermodynamical characteristics e g areal fraction temperature liquid and total water mixing ratio buoyancy of those structures are then accurately described and particular attention is given to their respective contributions to the turbulent transport of mass heat and moisture it is shown that updrafts downdrafts and entrainment are equally important

entrainment process of carbon dioxide in the atmospheric boundary layer received 2 march 2004 revised 7 june 2004 accepted 23 june 2004 published 23 september 2004 1 aircraft and surface measurements of turbulent thermodynamic variables and carbon dioxide co2 were taken above a grassland in a convective atmospheric boundary layer the observations were analyzed to assess the importance of the entrainment process for the distribution and evolution of carbon dioxide in the boundary layer from the observations we were able to estimate the vertical profiles of the fluxes the correlation coefficients and the skewness of the virtual potential temperature the specific humidity and the carbon dioxide these profiles indicate that important entrainment events occurred during the observed period the data were also used to estimate the budgets for heat moisture and carbon dioxide by studying this observational data we find that the entrainment of air parcels containing lower concentrations of water vapor and carbon dioxide significantly dries and dilutes the concentration of these two constituents in the boundary layer this process is particularly important in the morning hours which are characterized by a rapidly growing boundary layer the observations show that the co2 concentration in the boundary layer is reduced much more effectively by the ventilation with entrained air than by co2 uptake by the vegetation we quantify this effect by calculating the ratio of the entrainment flux of co2 to the surface flux of co2 bc wc e wc o a value of bc equal to 2 9 is estimated at around 1300 utc from the vertical profile of the carbon dioxide flux we corroborate this observational evidence by reproducing the observed situation using a mixed layer model the mixed layer model also yields the variation in time of bc during the morning the ventilation process is more important than the co2 uptake by the vegetation bc 1 whereas in the afternoon the assimilation by grass at the surface becomes the dominant process bc 1 this research points out the relevance of the entrainment process on the budget of carbon dioxide in the lower troposphere and the relevance of boundary layer dynamics in controlling the diurnal variation of carbon dioxide index terms 0315 atmospheric composition and structure biosphere atmosphere interactions 0322 atmospheric composition and structure constituent sources and sinks 0368 atmospheric composition and structure troposphere constituent transport and chemistry keywords entrainment carbon dioxide mixed layer model citation de arellano j v g b gioli f miglietta h j j jonker h k baltink r w a hutjes and a a m holtslag 2004 entrainment process of carbon dioxide in the atmospheric boundary layer j geophys res 109 d18110