Top Of Planetary Boundary Layer Research Articles

A preliminary study of turbulent coherent structures and ozone air quality in Seoul using the WRF-CMAQ model at a 50 m grid spacing

The effects of turbulent coherent structures on daytime ozone air quality in Seoul, South Korea are investigated through a case study using the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model at a horizontal grid spacing of 50 m. In Seoul, sea breeze and convective structures develop in the daytime. Due to the different wind directions above and below the planetary boundary layer (PBL) top, eddies are formed at the PBL top when updrafts related to convective structures reach above the PBL top. Air at lower level, which has higher concentrations of O3 precursors and lower O3 concentration than air at upper level, is transported upward by convective structures and eddies at the PBL top. Some of the transported air reaches above the PBL top, resulting in the vigorous chemical production of O3 above the PBL top in the afternoon. An integrated process rate analysis is performed to examine the impacts of turbulent coherent structures on O3 concentration in detail. The chemical production of O3 generally appears in updraft areas except near the surface because O3 precursors at lower level are transported by updrafts. Below the height of ~1 km, the horizontal advection of air from other areas to updraft areas increases O3 concentration in updraft areas because O3 concentration in updraft areas is generally lower than that in other areas. Slightly above the PBL top, air with low O3 concentration diverges from updraft areas and air with high O3 concentration at upper level is transported downward by downdrafts around updraft areas.

Wind turning in the atmospheric boundary layer over land

A climatology of the boundary‐layer wind‐turning angle over land is presented based on radiosonde observations from 800 stations in the Integrated Global Radiosonde Archive (IGRA). The dependence of the wind turning on a suite of parameters is analyzed. Results from previous studies indicating the importance of the planetary boundary layer (PBL) stratification for the angle of wind turning are confirmed here. A clear increase in the wind‐turning angle with wind speed, particularly for stratified conditions, is also evident. According to Rossby number similarity theory, the cross‐isobaric angle for a neutral and barotropic boundary layer decreases with increasing surface Rossby number, Ro. The IGRA observations indicate that this dependence on Ro might partly be linked to the dependence of the stratification on the wind speed, a dependence that seems to prevail even for the high wind speeds, a criterion that traditionally is used to approximate a neutral PBL. The vertical distribution of the turning of the wind is analyzed using the high‐resolution Stratospheric Processes And their Role in Climate (SPARC) data. For unstable cases, there is a maximum in the directional wind shear around the PBL top, whereas for the most stable class of cases there is a maximum near the surface. The wind‐turning angles from observations are also compared with values obtained from ERA‐Interim reanalysis fields, also presented over ocean. ERA‐Interim underestimates the magnitude of the wind‐turning angles as well as the range. Furthermore, the midlatitude cross‐isobaric mass transport is estimated using the IGRA data. This transport is generally underestimated by ERA‐Interim, likely related to the too small wind‐turning angles.

Optimization of vertical grid setting for air quality modelling in China considering the effect of aerosol-boundary layer interaction

The feedback between ambient aerosols and planetary boundary layer (PBL) meteorology has been proven to play a critical role in the enhancement of haze pollution. Vertical distribution of aerosols as well as temperature stratification are vital to understand aerosol – boundary layer interaction (ABI) and its impact on air quality deterioration. In current regional air quality models, the default vertical grid setting is relatively coarse and decreases progressively with altitude. However, the ABI is sensitive to aerosol layer at specific altitudes, i.e. around the top of PBL. This work aims to explore optimized vertical grid setting for better characterizing ABI and its role in air quality degradation. A single column model (SCM) is used for sensitivity tests considering the balance between model performance and computational cost. This optimized grid setting is then applied in three-dimensional air quality modelling in eastern China. Compared with default configuration, the optimized one is demonstrated to perform much better in characterizing temperature stratification and extreme fine particle (PM2.5) concentration as well as its diurnal variation during haze episodes. Specifically, the averaged decrease in PBL height and increment in surface PM2.5 concentration are about 5% and 20% while applying optimized setting, thereby reducing the mean bias from 30 to 5 μg/m3 in PM2.5 concentration. The improvements could be attributed mainly to more accurate profiles of aerosol and its heating effect, and thus stabilized lower atmosphere. The optimization of vertical grid setting could be applied to areas with high concentration of absorbing aerosols such as eastern China and India, which could help better predict extreme near-surface pollution episodes.

Analysis of the Planetary Boundary Layer Height during DISCOVER-AQ Baltimore–Washington, D.C., with Lidar and High-Resolution WRF Modeling

AbstractThe daytime planetary boundary layer (PBL) was examined for the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) Baltimore (Maryland)–Washington, D.C., campaign of July 2011 using PBL height (PBLH) retrievals from aerosol backscatter measurements from ground-based micropulse lidar (MPL), the NASA Langley Research Center airborne High Spectral Resolution Lidar-1 (HSRL-1), and the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. High-resolution Weather Research and Forecasting (WRF) Model simulations with horizontal grid spacing of 1 km and different combinations of PBL schemes, urban parameterization, and sea surface temperature inputs were evaluated against PBLHs derived from lidars, ozonesondes, and radiosondes. MPL and WRF PBLHs depicted a growing PBL in the morning that reached a peak height by midafternoon. WRF PBLHs calculated from gridded output profiles generally showed more rapid growth and higher peak heights than did the MPLs, and all WRF–lidar differences were dependent on model configuration, PBLH calculation method, and synoptic conditions. At inland locations, WRF simulated an earlier descent of the PBL top in the afternoon relative to the MPL retrievals and radiosonde PBLHs. At Edgewood, Maryland, the influence of the Chesapeake Bay breeze on the PBLH was captured by both the ozonesonde and WRF data but generally not by the MPL PBLH retrievals because of generally weaker gradients in the aerosol backscatter profile and limited normalized relative backscatter data near the top height of the marine layer.

A climatological view of the vertical stratification of RH, O<sub>3</sub> and CO within the PBL and at the interface with free troposphere as seen by IAGOS aircraft and ozonesondes at northern mid-latitudes over 1994–2016

Abstract. This paper investigates in an innovative way the climatological vertical stratification of relative humidity (RH), ozone (O3) and carbon monoxide (CO) mixing ratios within the planetary boundary layer (PBL) and at the interface with the free troposphere (FT). The climatology includes all vertical profiles available at northern mid-latitudes over the period 1994–2016 in both the IAGOS (In-service Aircraft for a Global Observing System) and WOUDC (World Ozone and Ultraviolet Radiation Data Centre) databases, which represents more than 90 000 vertical profiles. For all individual profiles, apart from the specific case of surface-based temperature inversions (SBIs), the PBL height is estimated following the elevated temperature inversion (EI) method. Several features of both SBIs and EIs are analysed, including their diurnal and seasonal variations. Based on these PBL height estimates (denoted h), the novel approach introduced in this paper consists of building a so-called PBL-referenced vertical distribution of O3, CO and RH by averaging all individual profiles beforehand expressed as a function of z∕h rather than z (with z the altitude). Using this vertical coordinate system allows us to highlight the features existing at the PBL–FT interface that would have been smoothed otherwise. Results demonstrate that the frequently assumed well-mixed PBL remains an exception for both chemical species. Within the PBL, CO profiles are characterized by a mean vertical stratification (here defined as the standard deviation of the CO profile between the surface and the PBL top, normalized by the mean) of 11 %, with moderate seasonal and diurnal variations. A higher vertical stratification is observed for O3 mixing ratios (18 %), with stronger seasonal and diurnal variability (from ∼ 10 % in spring–summer midday–afternoon to ∼ 25 % in winter–fall night). This vertical stratification is distributed heterogeneously in the PBL with stronger vertical gradients observed at both the surface (due to dry deposition and titration by NO for O3 and due to surface emissions for CO) and the PBL–FT interface. These gradients vary with the season from the lowest values in summer to the highest ones in winter. In contrast to CO, the O3 vertical stratification was found to vary with the surface potential temperature following an interesting bell shape with the weakest stratification for both the lowest (typically negative) and highest temperatures, which could be due to much lower O3 dry deposition in the presence of snow. Therefore, results demonstrate that EIs act as a geophysical interface separating air masses of distinct chemical composition and/or chemical regime. This is further supported by the analysis of the correlation of O3 and CO mixing ratios between the different altitude levels in the PBL and FT (the so-called vertical autocorrelation). Results indeed highlight lower correlations apart from the PBL–FT interface and higher correlations within each of the two atmospheric compartments (PBL and FT). The mean climatological O3 and CO PBL-referenced profiles analysed in this study are freely available on the IAGOS portal for all seasons and times of day (https://doi.org/10.25326/4).

Impacts of Shallow Convection Processes on a Simulated Boreal Summer Climatology in a Global Atmospheric Model

This study investigates the impacts of shallow convection schemes on a simulated seasonal climatology in the Global and Regional Integrated Model system (GRIMs). The eddy-diffusivity scheme of Tiedtke (TDK) is evaluated, focusing on the dependency upon deep convection schemes. Drying and warming near the top of the planetary boundary layer (PBL) and opposing effects above are observed. The height of PBL is reduced due to the increase of thermal stability near the PBL top. The weakened PBL turbulence is partly compensated with the increased downward solar radiation due to the reduction of low clouds. These effects are pronounced over the oceans, which leads to the modulation of tropical precipitation. It is found that the original TDK scheme shows similar behavior regardless of the choice of deep convection schemes. A revised TDK scheme that explicitly couples the PBL and shallow convection processes is proposed and evaluated. The proposed scheme generally improves the simulated climatology over the results with the original TDK scheme, along with further improvement in the case of the revised deep convection scheme. Our results indicate that the role of the shallow convection scheme needs to be carefully examined to improve the performance of atmospheric models, with a focus on modulated PBL and deep convection processes.

Global climatology of planetary boundary layer top obtained from multi-satellite GPS RO observations

Accurate estimation of the planetary boundary layer (PBL) top is essential for air quality prediction, weather forecast, and assessment of regional and global climate models. In this article, the long-term climatology of seasonal, global distribution of PBL is presented by using global positioning system radio occultation (GPSRO) based payloads such as Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), Communication/Navigation Outage Forecast System (C/NOFS), TerraSAR-X, and The Gravity Recovery and Climate Experiment (GRACE) from the year 2006–2015. We used Wavelet Covariance Transform (WCT) technique for precise PBL top identification. The derived PBL top from GPSRO data is rigorously evaluated with GPS radiosonde data over Gadanki. Significant seasonal variation is noticed in both radiosonde and GPSRO observations. Further, we compared the PBL obtained GPS RO with global radiosonde network and observed very good correlation. The number of occultations reaching down to 500 m and retrieval rate of PBL top from WCT method is very high in mid-latitudes compared to tropical latitudes. The global distribution of PBL top shows significant seasonal variation with higher during summer followed by spring, fall, and minimum in winter. In the vicinity of Inter Tropical Convergence Zone (ITCZ), the PBL top is high over eastern Pacific compared to other regions. The ERA-Interim reanalysis data underestimate the PBL top compared to GPS RO observations due to different measurement techniques. The seasonal variation of global averaged PBL top over land and ocean shows contrasting features at different latitude bands.

UAS as a Support for Atmospheric Aerosols Research: Case Study

Small drones (multi-copters) have the potential to deliver valuable data for atmospheric research. They are especially useful for collecting vertical profiles of optical and microphysical properties of atmospheric aerosols. Miniaturization of sensors, such as aethalometers and particle counters, allows for collecting profiles of black carbon concentration, absorption coefficient, and particle size distribution. Vertical variability of single-scattering properties has a significant impact on radiative transfer and Earth’s climate, but the base of global measurements is very limited. This results in high uncertainties of climate/radiation models. Vertical range of modern multi-copters is up to 2000 m, which is usually enough to study aerosols up to the top of planetary boundary layer on middle latitudes. In this study, we present the benefits coming from usage of small drones in atmospheric research. The experiment, described as a case study, was conducted at two stations (Swider and Warsaw) in Poland, from October 2014 to March 2015. For over 6 months, photoacoustic extinctiometers collected data at both stations. This enabled us to compare the stations and to establish ground reference of black carbon concentrations for vertical profiles collected by ceilometer and drone. At Swider station, we used Vaisala CL-31 ceilometer. It delivered vertical profiles of range corrected signal, which were analysed together with profiles acquired by micro-aethalometer AE-51 and Vaisala RS92-SGP radiosonde carried by a hexacopter drone. Near to the surface, black carbon gradient of approx 400 (upmug/m^3)/100 m was detected, which was below the ceilometer minimal altitude of detection. This confirmed the usefulness of drones and potential of their support for remote sensing techniques.

Correcting negatively biased refractivity below ducts in GNSS radio occultation: an optimal estimation approach towards improving planetary boundary layer (PBL) characterization

Abstract. Global Navigation Satellite System (GNSS) radio occultation (RO) measurements are promising in sensing the vertical structure of the Earth's planetary boundary layer (PBL). However, large refractivity changes near the top of PBL can cause ducting and lead to a negative bias in the retrieved refractivity within the PBL (below ∼ 2 km). To remove the bias, a reconstruction method with assumption of linear structure inside the ducting layer models has been proposed by Xie et al. (2006). While the negative bias can be reduced drastically as demonstrated in the simulation, the lack of high-quality surface refractivity constraint makes its application to real RO data difficult. In this paper, we use the widely available precipitable water (PW) satellite observation as the external constraint for the bias correction. A new framework is proposed to incorporate optimization into the RO reconstruction retrievals in the presence of ducting conditions. The new method uses optimal estimation to select the best refractivity solution whose PW and PBL height best match the externally retrieved PW and the known a priori states, respectively. The near-coincident PW retrievals from AMSR-E microwave radiometer instruments are used as an external observational constraint. This new reconstruction method is tested on both the simulated GNSS-RO profiles and the actual GNSS-RO data. Our results show that the proposed method can greatly reduce the negative refractivity bias when compared to the traditional Abel inversion.

DUTOS TROPOSFÉRICOS E A ESTRUTURA DA CAMADA LIMITE PLANETÁRIA NA REGIÃO DE PETROLINA

The planetary boundary layer (PBL) structure is investigated focusing on days with low-tropospheric ducts occurrence in the Petrolina-PE region, in the period from 1 to 29 April 2014. Ducts identification and the estimative of the PBL top/height and its dynamic and thermodynamic characterization is based on 12 UTC upper-air soundings launched at the altitude station of Petrolina. Surface ducts are associated with high instability and wind speed intensification in the surface layer. Ducts at the PBL top (elevated ducts) are associated with a decrease in the moisture content of the air and a wind speed maximum, above the PBL, in the low-troposphere.

Roles of convective heating and boundary-layer moisture asymmetry in slowing down the convectively coupled Kelvin waves

Mechanisms for an in-phase relationship between convection and low-level zonal wind and the slow propagation of the convectively coupled Kelvin wave (CCKW) are investigated by analyzing satellite-based brightness temperature and reanalysis data and by constructing a simple theoretical model. Observational data analysis reveals an eastward shift of the low-level convergence and moisture relative to the CCKW convective center. The composite vertical structures show that the low-level convergence lies in the planetary boundary layer (PBL) (below 800 hPa), and is induced by the pressure trough above the top of PBL through an Ekman-pumping process. A traditional view of a slower eastward propagation speed compared to the dry Kelvin waves is attributed to the reduction of atmospheric static stability in mid-troposphere due to the convective heating effect. The authors’ quantitative assessment of the heating effect shows that this effect alone cannot explain the observed CCKW phase speed. We hypothesize that additional slowing process arises from the effect of zonally asymmetric PBL moisture. A simple theoretical model is constructed to understand the relative role of the heating induced effective static stability effect and the PBL moisture effect. The result demonstrates the important role of the both effects. Thus, PBL-free atmosphere interaction is important in explaining the observed structure and propagation of CCKW.

Radiative driving of shallow return flows from the ITCZ

AbstractThe tropical overturning circulation over the central‐east Pacific not only extends through the troposphere but also includes a shallow component outflowing from the ITCZ near the planetary boundary layer (PBL) top, the Shallow Meridional Circulation (SMC). Idealized simulations with the Weather Research and Forecasting (WRF) model, which qualitatively reproduce this two‐component circulation, are used to investigate the mechanism driving the SMC by altering model parameterization schemes (PBL, cumulus and radiation), the solar heating rate, and the SST gradient. A radiative‐driving model for subsidence is tested and found to quantitatively predict the altitude and strength of the SMC in all experiments. This includes experiments with a changed SST gradient, in which the changes in temperature profile and clouds near PBL top in the subsiding region altered radiative cooling so as to make the SMC vary in proportion to the SST gradient. These results indicate that the SMC is strongly controlled by radiative cooling rather than independently by SST patterns or convective processes, at least in this model setup. Experiments with altered solar heating of the atmosphere affected the strength of the SMC strongly, while changes in cumulus parameterization had a weaker effect. Parameterization changes influenced the SMC mainly by altering shallow cloud amount and humidity in subsidence regions, but vertical gradients of temperature also affect the radiative driving of the SMC in at least one case. The results highlight that atmospheric radiative heating is of prime importance for determining the character of deep and shallow overturning.

Improved Low-Cloud Simulation from the Community Atmosphere Model with an Advanced Third-Order Turbulence Closure

Abstract In this study, a simplified intermediately prognostic higher-order turbulence closure (IPHOC) is implemented in the Community Atmosphere Model, version 5 (CAM5), to provide a consistent treatment of subgrid-scale cloud processes, except for deep convection. The planetary boundary layer (PBL) height is prognosticated to better resolve the discontinuity of temperature and moisture above the PBL top. Single-column model tests show that fluxes of liquid water potential temperature and total water, cloud fraction, and liquid water content are improved with this approach. The simplified IPHOC package replaces the boundary layer dry and moist turbulence parameterizations, the shallow convection parameterization, and the liquid-phase part of the cloud macrophysics parameterization in CAM5. CAM5-IPHOC improves the simulation of the low-level clouds off the west coasts of continents and the storm track region in the Southern Hemisphere (SH). The transition from stratocumulus to cumulus clouds is more gradual. There are also improvements on the cloud radiative forcing, especially shortwave, in the subsidence regime. The improvements in the relationships among low cloud amount, surface relative humidity, lower tropospheric stability, and PBL depth are seen in some stratocumulus regions. CAM5-IPHOC, however, produces weaker precipitation at the South Pacific convergence zone than CAM5 because of less energy flux into the SH atmosphere. The more downward surface shortwave radiative cooling and the less top-of-the-atmosphere longwave cloud radiative heating in the SH relative to the Northern Hemisphere explains the anomalous cooling and the lesser energy flux into the SH, which is related to the underestimate of extratropical middle/high clouds in the SH.

A Unified Convection Scheme (UNICON). Part I: Formulation

Abstract The author develops a unified convection scheme (UNICON) that parameterizes relative (i.e., with respect to the grid-mean vertical flow) subgrid vertical transport by nonlocal asymmetric turbulent eddies. UNICON is a process-based model of subgrid convective plumes and mesoscale organized flow without relying on any quasi-equilibrium assumptions such as convective available potential energy (CAPE) or convective inhibition (CIN) closures. In combination with a relative subgrid vertical transport scheme by local symmetric turbulent eddies and a grid-scale advection scheme, UNICON simulates vertical transport of water species and conservative scalars without double counting at any horizontal resolution. UNICON simulates all dry–moist, forced–free, and shallow–deep convection within a single framework in a seamless, consistent, and unified way. It diagnoses the vertical profiles of the macrophysics (fractional area, plume radius, and number density) as well as the microphysics (production and evaporation rates of convective precipitation) and the dynamics (mass flux and vertical velocity) of multiple convective updraft and downdraft plumes. UNICON also prognoses subgrid cold pool and mesoscale organized flow within the planetary boundary layer (PBL) that is forced by evaporation of convective precipitation and accompanying convective downdrafts but damped by surface flux and entrainment at the PBL top. The combined subgrid parameterization of diagnostic convective updraft and downdraft plumes, prognostic subgrid mesoscale organized flow, and the feedback among them remedies the weakness of conventional quasi-steady diagnostic plume models—the lack of plume memory across the time step—allowing UNICON to successfully simulate various transitional phenomena associated with convection (e.g., the diurnal cycle of precipitation and the Madden–Julian oscillation).

Atmospheric boundary layer top height in South Africa: measurements with lidar and radiosonde compared to three atmospheric models

Abstract. Atmospheric lidar measurements were carried out at Elandsfontein measurement station, on the eastern Highveld approximately 150 km east of Johannesburg in South Africa throughout 2010. The height of the planetary boundary layer (PBL) top was continuously measured using a Raman lidar, PollyXT (POrtabLe Lidar sYstem eXTended). High atmospheric variability together with a large surface temperature range and significant seasonal changes in precipitation were observed, which had an impact on the vertical mixing of particulate matter, and hence, on the PBL evolution. The results were compared to radiosondes, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) space-borne lidar measurements and three atmospheric models that followed different approaches to determine the PBL top height. These models included two weather forecast models operated by ECMWF (European Centre for Medium-range Weather Forecasts) and SAWS (South African Weather Service), and one mesoscale prognostic meteorological and air pollution regulatory model TAPM (The Air Pollution Model). The ground-based lidar used in this study was operational for 4935 h during 2010 (49% of the time). The PBL top height was detected 86% of the total measurement time (42% of the total time). Large seasonal and diurnal variations were observed between the different methods utilised. High variation was found when lidar measurements were compared to radiosonde measurements. This could be partially due to the distance between the lidar measurements and the radiosondes, which were 120 km apart. Comparison of lidar measurements to the models indicated that the ECMWF model agreed the best with mean relative difference of 15.4%, while the second best correlation was with the SAWS model with corresponding difference of 20.1%. TAPM was found to have a tendency to underestimate the PBL top height. The wind speeds in the SAWS and TAPM models were strongly underestimated which probably led to underestimation of the vertical wind and turbulence and thus underestimation of the PBL top height. Comparison between ground-based and satellite lidar shows good agreement with a correlation coefficient of 0.88. On average, the daily maximum PBL top height in October (spring) and June (winter) was 2260 m and 1480 m, respectively. To our knowledge, this study is the first long-term study of PBL top heights and PBL growth rates in South Africa.

The seasonal cycle of planetary boundary layer depth determined using COSMIC radio occultation data

AbstractThe seasonal cycle of planetary boundary layer (PBL) depth is examined globally using observations from the Constellation Observing System for the Meteorology, Ionosphere, and Climate (COSMIC) satellite mission. COSMIC uses GPS radio occultation to derive the vertical profile of refractivity at high vertical resolution (~100 m). Here, we apply an algorithm to determine PBL top height and thus PBL depth from the maximum vertical gradient of refractivity. PBL top detection is sensitive to hydrolapses at nonpolar latitudes but to both hydrolapses and temperature jumps in polar regions. The PBL depths and their seasonal cycles compare favorably with selected radiosonde‐derived estimates at tropical, midlatitude, and Antarctic sites, adding confidence that COSMIC can effectively provide estimates of seasonal cycles globally. PBL depth over extratropical land regions peaks during summer consistent with weak static stability and strong surface sensible heating. The subtropics and tropics exhibit a markedly different cycle that largely follows the seasonal march of the Intertropical Convergence Zone with the deepest PBLs associated with dry phases, again suggestive that surface sensible heating deepens the PBL and that wet periods exhibit shallower PBLs. Marine PBL depth has a somewhat similar seasonal march to that over continents but is weaker in amplitude and is shifted poleward. The maximum seasonal amplitude over oceans occurs over the Arctic. Over subtropical/tropical oceans there is seasonal asymmetry about the equator, with winter maxima in the Northern Hemisphere but fall maxima in the south. The seasonal march of PBL depth is largely modulated by the seasonal cycle of static stability in the extratropics and by the monsoon circulations at tropical and subtropical latitudes.

A thermal plume model for the Martian convective boundary layer

The Martian planetary boundary layer (PBL) is a crucial component of the Martian climate system. Global climate models (GCMs) and mesoscale models (MMs) lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the “thermal plume” model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single‐column models. The aim of this physically based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in large‐eddy simulations (LESs). We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near‐surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking into account stability and turbulent gustiness to calculate surface‐atmosphere fluxes. Those new parameterizations for the surface and mixed layers are validated against near‐surface lander measurements. Using a thermal plume model moreover enables a first‐order estimation of key turbulent quantities (e.g., PBL height and convective plume velocity) in Martian GCMs and MMs without having to run costly LESs.

The Vertical Structure of Tropical Urban Heat Island with LES

Abstrac t The Large-eddy simulation (LES) model is used to investigate the influence of the Tropical Urban Heat Island (UHI) in the vert ical structure of the Planetary Boundary Layer (PBL) under adiabatic and non-saturated conditions. An idealized UHI is represented by two-dimensional patches of heat fluxes at the surface defining variable Bowen rat io along the horizontal do main. The results indicated that the heterogeneities are able to induce the format ion of intense updraft over the center of warm patch. Th is buoyant thermal transports the mo isture fro m the lower to the upper part of the PBL and penetrate the entrainment zone. Consequently, the urban-breeze circulat ion can contribute to the clouds development at the top of the PBL over the UHI.

Estimates of North American summertime planetary boundary layer depths derived from space‐borne lidar

The planetary boundary layer (PBL) mediates exchanges of energy, moisture, momentum, carbon, and pollutants between the surface and the atmosphere. This paper is a first step in producing a space‐based estimate of PBL depth that can be used to compare with and evaluate model‐based PBL depth retrievals, inform boundary layer studies, and improve understanding of the above processes. In clear sky conditions, space‐borne lidar backscatter is frequently affected by atmospheric properties near the PBL top. Spatial patterns of 5‐year mean mid‐day summertime PBL depths over North America were estimated from the CALIPSO lidar backscatter and are generally consistent with model reanalyses and AMDAR (Aircraft Meteorological DAta Reporting) estimates. The rate of retrieval is greatest over the subtropical oceans (near 100%) where overlying subsidence limits optically thick clouds from growing and attenuating the lidar signal. The general retrieval rate over land is around 50% with decreased rates over the Southwestern United States and regions with high rates of convection. The lidar‐based estimates of PBL depth tend to be shallower than aircraft estimates in coastal areas. Compared to reanalysis products, lidar PBL depths are greater over the oceans and areas of the boreal forest and shallower over the arid and semiarid regions of North America.

Observations and Large-Eddy Simulations of Entrainment in the Sheared Sahelian Boundary Layer

At the top of the planetary boundary layer, the entrainment of air, which incor- porates dry and warm air from the free troposphere into the boundary layer, is a key process for exchanges with the free troposphere since it controls the growth of the boundary layer. Here, we focus on the semi-arid boundary layer where the entrainment process is analyzed using aircraft observations collected during the African Monsoon Multidisciplinary Analysis experiment and large-eddy simulations. The role of the entrainment is specifically enhanced in this region where very large gradients at the planetary boundary-layer top can be found due to the presence of the moist, cold monsoon flow on which the dry, warm Harmattan flow is superimposed. A first large-eddy simulation is designed based on aircraft observations of 5 June 2006 during the transition period between dry conditions and the active monsoon phase. The simulation reproduces the boundary-layer development and dynamics observed on this day. From this specific case, sensitivity tests are carried out to cover a range of conditions observed during seven other flights made in the same transition period in order to describe the entrainment processes in detail. The combination of large-eddy simulations and observations