Planetary Boundary Layer Depth Research Articles

CALOTRITON: a convective boundary layer height estimation algorithm from ultra-high-frequency (UHF) wind profiler data

Abstract. Long time series of observations of atmospheric dynamics and composition are collected at the French Pyrenean Platform for Observation of the Atmosphere (P2OA). Planetary boundary layer depth is a key variable of the climate system, but it remains difficult to estimate and analyse statistically. In order to obtain reliable estimates of the convective boundary layer height (Zi) and to allow long-term series analyses, a new restitution algorithm, named CALOTRITON, has been developed. It is based on the observations of an ultra-high-frequency (UHF) radar wind profiler (RWP) from P2OA with the help of other instruments for evaluation. Estimates of Zi are based on the principle that the top of the convective boundary layer is associated with both a marked inversion and a decrease in turbulence. Those two criteria are respectively manifested by larger RWP reflectivity and smaller vertical-velocity Doppler spectral width. With this in mind, we introduce a new UHF-deduced dimensionless parameter which weighs the air refractive index structure coefficient with the inverse of vertical velocity standard deviation to the power of x. We then search for the most appropriate local maxima of this parameter for Zi estimates with defined criteria and constraints such as temporal continuity. Given that Zi should correspond to fair-weather cloud base height, we use ceilometer data to optimize our choice of the power x and find that x=3 provides the best comparisons. The estimates of Zi by CALOTRITON are evaluated using different Zi estimates deduced from radiosounding according to different definitions. The comparison shows excellent results with a regression coefficient of up to 0.96 and a root-mean-square error of 71 m, which is close to the vertical resolution of the UHF RWP of 75 m, when conditions are optimal. In more complex situations, that is when the atmospheric vertical structure is itself particularly ambiguous, secondary retrievals allow us to identify potential thermal internal boundary layers or residual layers and help to qualify the Zi estimations. Frequent estimate errors are observed nevertheless; for example, when Zi is below the UHF RWP first reliable gate or when the boundary layer begins its transition to a stable nocturnal boundary layer.

Observations of Boundary Layer Convergence Lines and Associated Updrafts in the U.S. Southern Great Plains

Abstract Boundary layer convergence lines (CLs) are highly effective at deep-convection initiation (DCI), suggesting that their associated updraft properties differ from those of more widespread turbulent updrafts in the planetary boundary layer (PBL). This study exploits observations at the Atmospheric Radiation Measurement Southern Great Plains (ARM SGP) observatory in Oklahoma from 2011 to 2016 to quantify CL properties and their relation to turbulent PBL eddies preceding CL arrival. Two independent methods for estimating CL properties are developed at two locations in the SGP region, both relying on the assumption of a 2D circulation in the CL-normal plane but using different combinations of instruments. The first (the radar method) relies mainly on scanning radar data and is applied to 61 CLs passing near a high-resolution scanning radar based in Nardin, Oklahoma, while the second (the surface method) relies mainly on surface wind data and is applied to 68 CLs crossing the SGP facility in nearby Lamont, Oklahoma. Mean daytime (1000–1900 LST) CL width (∼2 km) and convergence magnitude (∼0.003 s−1) are similar for both methods, and mean daytime CL depth is ∼0.75 km. The two methods disagree at night (0000–1000 and 1900–2400 LST), where the surface method estimates wider and weaker CLs than the radar method. This difference may stem from the radar beam overshooting the shallow, highly stable nocturnal PBL. The largest CL updrafts are slightly wider (∼20%) and stronger (∼40%) than the largest PBL updrafts in the pre-CL period, generating 50%–100% larger updraft mass fluxes over most of the PBL depth. Significance Statement Deep convection is commonly initiated by boundary layer convergence lines (CLs), which are associated with intense surface-based wind convergence and strong updrafts that may lift air to saturation. Although CLs form regularly, they are far less common than ordinary, short-lived turbulent thermals in the daytime boundary layer. To better understand why CLs are so effective at deep-convection initiation, we observationally quantify their morphologies and strengths and compare these properties to those of surrounding turbulent updrafts. Perhaps surprisingly, the CLs are found to exhibit only slightly larger scales and strengths as the turbulent updrafts. Although these marginal increases help to explain the preference for storms to initiate along CLs, they likely are not the whole story.

Atmospheric Meridional Circulation Between South Asia and Tibetan Plateau Caused by the Change of Planetary Boundary Layer Depth

AbstractPlanetary boundary layer (PBL) influences the vertical exchanges of momentum, heat, water vapor, atmospheric pollutants, and the efficacy of climate forcing between land surface and atmosphere. Over the western Tibetan Plateau (TP), PBL can stretch into the middle‐upper troposphere. In this study, we investigate an effect of the PBL depth change on atmospheric circulations between South Asia and the TP through the formula derivation, the observation, and the model data set. The derivation first gives the PBL depth‐generalized pressure equation. It reveals an effect of PBL depth on the pressure change with time. The data analysis further shows that an increase of PBL depth in the western TP can cause the subsequent development of the PBL low (high) pressure anomalies in the northwestern TP (northern South Asia) and the upper‐tropospheric high (low) pressure anomalies over the TP (South Asia), with an anticlockwise meridional‐vertical circulation (MVC) anomaly from South Asia to the TP. In this process, the peak of PBL depth may be ahead of that of the low‐pressure occurrence, which may cause the MVC development to lag behind surface heating. The anticlockwise MVC anomaly strengthens the transport of water vapor from the lower troposphere over South Asia to the TP PBL, producing convective precipitation in the TP.

Simulating land-atmosphere coupling in the Central Valley, California: Investigating soil moisture impacts on boundary layer properties

Soil moisture links hydrologic and atmospheric processes and impacts important properties of the atmospheric boundary layer via turbulent land-atmosphere exchange. Research on land-atmosphere interactions and their impacts on the simulated boundary layer in semi-arid regions with substantial irrigation is relatively sparse. We use the Weather Research and Forecasting (WRF) model to evaluate the influence different land surface models (LSMs) and planetary boundary layer (PBL) schemes have on the performance of simulations through comparisons with multi-scale observations during a fifteen-day summertime period during 2016. The focus region for this study is the Central Valley (CV), California, which receives little to no rain in the summer and relies on widespread irrigation for agriculture. Results demonstrate that the LSM drives the differences between simulations, showing only minor variations with changing the PBL scheme. Simulations using the RUC (Rapid Update Cycle) and PX-NO (Pleim-Xu without soil moisture and soil temperature nudging) LSMs generated better comparisons with observed PBL depths. Contrasting RUC however, PX-NO better simulates surface fluxes and humidity, whereas Noah (Noah Unified) and Noah-MP (Noah Multiparameterization) simulate better temperatures despite relatively poor surface flux performance. For most quantities, indirect soil nudging in PX (Pleim-Xu) did not improve results compared to PX-NO, which may be related to soil moisture initialization, the nudging dataset, or a need for model improvements in arid regions. Despite these variations in performance statistics across simulations and quantities, we show that potential evapotranspiration (ETo) has robust performance statistics across simulations. This suggests that ETo depends more strongly on net radiation, which performs relatively well across simulations, than on wind, temperature, and humidity, and indicates a further disconnect between ETo and latent heat fluxes in WRF simulations. Finally, we suggest strategies to obtain the necessary observations to better understand the multi-scale dynamics in the CV and drive subsequent model development.

Relating geostationary satellite measurements of aerosol optical depth (AOD) over East Asia to fine particulate matter (PM<sub>2.5</sub>): insights from the KORUS-AQ aircraft campaign and GEOS-Chem model simulations

Abstract. Geostationary satellite measurements of aerosol optical depth (AOD) over East Asia from the Geostationary Ocean Color Imager (GOCI) and Advanced Himawari Imager (AHI) instruments can augment surface monitoring of fine particulate matter (PM2.5) air quality, but this requires better understanding of the AOD–PM2.5 relationship. Here we use the GEOS-Chem chemical transport model to analyze the critical variables determining the AOD–PM2.5 relationship over East Asia by simulation of observations from satellite, aircraft, and ground-based datasets. This includes the detailed vertical aerosol profiling over South Korea from the KORUS-AQ aircraft campaign (May–June 2016) with concurrent ground-based PM2.5 composition, PM10, and AERONET AOD measurements. The KORUS-AQ data show that 550 nm AOD is mainly contributed by sulfate–nitrate–ammonium (SNA) and organic aerosols in the planetary boundary layer (PBL), despite large dust concentrations in the free troposphere, reflecting the optically effective size and high hygroscopicity of the PBL aerosols. We updated SNA and organic aerosol size distributions in GEOS-Chem to represent aerosol optical properties over East Asia by using in situ measurements of particle size distributions from KORUS-AQ. We find that SNA and organic aerosols over East Asia have larger size (number median radius of 0.11 µm with geometric standard deviation of 1.4) and 20 % larger mass extinction efficiency as compared to aerosols over North America (default setting in GEOS-Chem). Although GEOS-Chem is successful in reproducing the KORUS-AQ vertical profiles of aerosol mass, its ability to link AOD to PM2.5 is limited by under-accounting of coarse PM and by a large overestimate of nighttime PM2.5 nitrate. The GOCI–AHI AOD data over East Asia in different seasons show agreement with AERONET AODs and a spatial distribution consistent with surface PM2.5 network data. The AOD observations over North China show a summer maximum and winter minimum, opposite in phase to surface PM2.5. This is due to low PBL depths compounded by high residential coal emissions in winter and high relative humidity (RH) in summer. Seasonality of AOD and PM2.5 over South Korea is much weaker, reflecting weaker variation in PBL depth and lack of residential coal emissions.

Evaluation and intercomparison of wildfire smoke forecasts from multiple modeling systems for the 2019 Williams Flats fire

Abstract. Wildfire smoke is one of the most significant concerns of human and environmental health, associated with its substantial impacts on air quality, weather, and climate. However, biomass burning emissions and smoke remain among the largest sources of uncertainties in air quality forecasts. In this study, we evaluate the smoke emissions and plume forecasts from 12 state-of-the-art air quality forecasting systems during the Williams Flats fire in Washington State, US, August 2019, which was intensively observed during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign. Model forecasts with lead times within 1 d are intercompared under the same framework based on observations from multiple platforms to reveal their performance regarding fire emissions, aerosol optical depth (AOD), surface PM2.5, plume injection, and surface PM2.5 to AOD ratio. The comparison of smoke organic carbon (OC) emissions suggests a large range of daily totals among the models, with a factor of 20 to 50. Limited representations of the diurnal patterns and day-to-day variations of emissions highlight the need to incorporate new methodologies to predict the temporal evolution and reduce uncertainty of smoke emission estimates. The evaluation of smoke AOD (sAOD) forecasts suggests overall underpredictions in both the magnitude and smoke plume area for nearly all models, although the high-resolution models have a better representation of the fine-scale structures of smoke plumes. The models driven by fire radiative power (FRP)-based fire emissions or assimilating satellite AOD data generally outperform the others. Additionally, limitations of the persistence assumption used when predicting smoke emissions are revealed by substantial underpredictions of sAOD on 8 August 2019, mainly over the transported smoke plumes, owing to the underestimated emissions on 7 August. In contrast, the surface smoke PM2.5 (sPM2.5) forecasts show both positive and negative overall biases for these models, with most members presenting more considerable diurnal variations of sPM2.5. Overpredictions of sPM2.5 are found for the models driven by FRP-based emissions during nighttime, suggesting the necessity to improve vertical emission allocation within and above the planetary boundary layer (PBL). Smoke injection heights are further evaluated using the NASA Langley Research Center's Differential Absorption High Spectral Resolution Lidar (DIAL-HSRL) data collected during the flight observations. As the fire became stronger over 3–8 August, the plume height became deeper, with a day-to-day range of about 2–9 km a.g.l. However, narrower ranges are found for all models, with a tendency of overpredicting the plume heights for the shallower injection transects and underpredicting for the days showing deeper injections. The misrepresented plume injection heights lead to inaccurate vertical plume allocations along the transects corresponding to transported smoke that is 1 d old. Discrepancies in model performance for surface PM2.5 and AOD are further suggested by the evaluation of their ratio, which cannot be compensated for by solely adjusting the smoke emissions but are more attributable to model representations of plume injections, besides other possible factors including the evolution of PBL depths and aerosol optical property assumptions. By consolidating multiple forecast systems, these results provide strategic insight on pathways to improve smoke forecasts.

On the mitigation potential and climatic impact of modified urban albedo on a subtropical desert city

Extreme urban heat alongside higher ambient temperatures in urban areas causes serious energy, comfort, health and environmental problems. The implementation of urban heat mitigation techniques can significantly reduce urban temperatures and counterbalance the impact of extreme urban heat. This study assesses the potential cooling ability of modified urban albedo strategies through the implementation of reflective and super reflective materials, as well as the global climatic impacts on a subtropical desert urban environment in Dubai, UAE. Three scenarios using low, average and high albedo modifications are designed and evaluated in parallel to a reference scenario. A physically-based mesoscale urban modeling system is used to assess the thermal and meteorological impacts of the albedo modifications during both the summer and winter seasons at a city scale. The reduction of ambient temperature during the peak of a summer day (14:00 LT) is shown to be 0.6 °C, 1.4 °C and 2.6 °C when urban albedo is increased by 0.20, 0.45 and 0.60 respectively. The winter cooling penalty ranges between 0.6 °C and 1.1 °C for the different albedo scenarios. The increase of the urban albedo also significantly reduces the planetary boundary layer (PBL) depth due to the loss of sensible heat and decreases the intensity of the convective mixing and advection flows from the desert to the city, improving the mitigation potential of the reflective materials; however this increases the risk of a higher pollutants concentration. A much higher mitigation potential is observed for the high-density parts of the city when compared to that of the low-density parts of the city. Irrespective of linear function in the drop of ambient temperature and changing fraction of global albedo, our results reported that the cooling potential of reflective materials is highly influenced by the climate, landscape, and urban characteristics of the cities.

Evaluating the Lagrangian Evolution of Subtropical Low Clouds in GCMs Using Observations: Mean Evolution, Time Scales, and Responses to Predictors

Abstract A Lagrangian framework is developed to show the daily-scale time evolution of low clouds over the eastern subtropical oceans. An identical framework is applied to two general circulation models (GCMs): the CAM5 and UKMET and a set of satellite observations. This approach follows thousands of parcels as they advect downwind in the subtropical trade winds, comparing cloud evolution in time and space. This study tracks cloud cover, in-cloud liquid water path (CLWP), droplet concentration N d , planetary boundary layer (PBL) depth, and rain rate as clouds transition from regions with predominately stratiform clouds to regions containing mostly trade cumulus. The two models generate fewer clouds with greater N d relative to observations. Models show stronger Lagrangian cloud cover decline and greater PBL deepening when compared with observations. In comparing frequency distributions of cloud variables over time, it is seen that models generate increasing frequencies of nearly clear conditions at the expense of overcast conditions, whereas observations show transitions from overcast to cloud amounts between 50% and 90%. Lagrangian decorrelation time scales (e-folding time τ) of cloud cover and CLWP are between 11 and 19 h for models and observations, although they are a bit shorter for models. A Lagrangian framework applied here resolves and compares the time evolution of cloud systems as they adjust to environmental perturbations in models and observations. Increasing subsidence in the overlying troposphere leads to declining cloud cover, CLWP, PBL depth, and rain rates in models and observations. Modeled cloud responses to other meteorological variables are less consistent with observations, suggesting a need for continuing mechanical improvements in GCMs.

Dust devils on Mars

Whirlwinds, familiar on Earth, can be informative probes of the Martian surface environment.

Impact of land surface physics on the simulation of boundary layer characteristics at a tropical coastal station

Abstract In this study the influence of land surface physics on the simulation of Planetary Boundary Layer (PBL) characteristics in the Advanced Research Weather Research and Forecast (WRF-ARW) is examined at the tropical costal station Kalpakkam. High resolution simulations were conducted using WRF-ARW with two land surface models (LSM) (Noah and Noah MP) for winter (2–5 February 2011) and South West (SW) monsoon (15–18 September 2010). Data from a Lower Atmospheric Wind Profiler (LAWP), GPS-Sonde, meteorological towers and sonic anemometer were used to analyze the PBL properties. Significant variation in the surface and PBL properties were noticed in the simulations with the two LSMs and various features of the PBL were produced better by Noah MP than Noah. The Noah because of higher exchange coefficients simulated more warm and humid boundary layers compared to the Noah MP. In the study region higher (lower) sensible heat flux is simulated compared to the latent and soil heat fluxes during the dry winter (rainy SW monsoon). The variation in the surface energy partitioning resulted in a variation in the simulated PBL depth by the two LSMs. The PBL simulated by Noah MP was shallower than the Noah. During winter higher Bowen ratio associated with the Noah MP resulted in stronger entrainment of upper dry air which suppressed the vertical growth of the PBL. Whereas in SW monsoon, the convective and mechanical turbulence simulated by Noah MP was less than that of Noah, which has lead to a shallow PBL in Noah MP. Overall, the Noah MP simulated the thermodynamical structure, winds and vertical extent of the PBL better than Noah. The improvement in simulations with Noah MP are due to the explicit treatment of plant canopy, snow and ground surface, tiling scheme for vegetation and bare soil, and due to moderate exchange coefficients for soil heat and moisture.

Winter VOCs and OVOCs measured with PTR-MS at an urban site of India: Role of emissions, meteorology and photochemical sources

Within the outline of air quality studies at metropolitan city, the mixing ratios of seven selected volatile organic compounds (VOCs) were measured during December 2015 (winter) at an urban site of Pune. The measurement of VOCs was conducted using a proton transfer reaction-quadrupole mass spectrometer (PTR-QMS). The study represents daily variability of ambient VOCs and their various associated emission sources. Diurnal profiles have differed from one VOC to another as the result of their different origins and the influence of different meteorological parameters (i.e. solar radiation, temperature) and planetary boundary layer height (PBL-H). The hourly mixing ratios of Oxygenated-VOCs (OVOCs) and aromatics were in the ranges of 0.6–29 ppbv and 0.13–14 ppbv, respectively with OVOCs accounted for up to 75% of total measured VOCs. The role of long-range transport from the clear Thar Desert and polluted Indo-Gangetic Plain (IGP) was observed during the episodes of 1–15 and 17–31 December 2015, respectively. VOCs showed the strong diurnal variations with peaks during morning and evening hours and lowest in the afternoon. In the evening period, high levels of aromatics coincided with the lowest OVOCs suggests the role of fresh vehicular emissions. Emission ratios of various VOCs as a function of temperature showed the role of different sources including the biogenic and photochemical production as well as the anthropogenic sources, respectively. The higher emission ratio of Δmethanol/Δacetonitrile at the study site suggests the long range transport of biomass burning plumes from the Indo-Gangetic Plain (IGP) during the 17–31, Dec. 2015. In addition to the pattern of emission, the diurnal and day-to-day variations of VOCs were influenced by the local meteorological conditions and depth of planetary boundary layer (PBL-H).

A High‐Resolution Simulation of Roll Convection Over the Yellow Sea During a Cold Air Outbreak

AbstractHorizontal convective rolls (HCRs) are significant for vertical transport of momentum, heat, and moisture, which is not fully understood partly due to lack of adequate observations. Previous high‐resolution modeling studies of roll convection mainly focused on idealized simulations. This study is one of the first simulations of HCRs with high‐resolution numerical weather prediction models in the real world, aiming to assess the modeling ability of realistic roll structure and investigate the characteristics of roll‐to‐cell transition. A real case of roll convection over the Yellow Sea during a cold air outbreak has been simulated, using the Weather Research and Forecasting (WRF) model at resolutions of 1 (WRF1) and 3 km (WRF3). Compared with the WRF3 simulation, the cloud water, vertical velocity, and the fine‐scale secondary circulations of HCRs are enhanced in WRF1 simulation, and the geometrical and physical parameters are consistent with the observations of satellite images (viewed as cloud streets) or previous studies. In this case, the 1‐km WRF simulation is able to reproduce the HCRs with a roll size of ~5 km. When the rolls transform to open cells downstream, the friction velocity and sensible heat flux gradually reduce following the prevailing wind, and the stability parameter –zi/L (the ratio of planetary boundary layer depth to Obukhov length) tends to increase with larger variances. During the roll‐to‐cell transition, the associated thermodynamic conditions are well simulated, with an average –zi/L ~ 22. In the future, reliable high‐resolution simulations can serve as an alternative to quantitatively investigate roll characteristics when observations are sparse or unavailable.

Estimate of boundary-layer depth in Nanjing city using aerosol lidar data during 2016–2017 winter

The planetary boundary-layer (PBL) structure was investigated using observations from single aerosol lidar, eddy covariance (EC) system, and automatic meteorological station (AWS) in the north of Nanjing city during an air pollution episode in 2016–2017 winter. Based on seven days’ observations under clear to polluted day, we present the temporal variations of the aerosol extinction profiles observed by lidar, and then inter-compare PBL depth retrieved from individual gradient methods. The results show that the gradient method (GM) generated the lowest PBL depth. In contrast to the cubic root gradient method (CRGM), which determined PBL depth ranging from 172 m to 1575 m during the observation period, the logarithm gradient method (LGM) and normalized gradient method (NGM) generated similar results and both tended to overestimate PBL depth on polluted days. The CRGM performed better than LGM and NGM in case of multiple backscatter layers and could detect low level layers, while the GM was biased at low heights probably due to the effect of lidar overlap function. Based on these measurements, the evolution of boundary layer structures and PBL depth over clean days and polluted days were compared. The results show that (1) on clean days, the strong surface turbulence exchange make the PBL depth fully developed and the PBL depth had obvious characteristics of diurnal variation; the maximum depth of PBL was 1560 m for CRGM; and (2) on polluted days, the high pressure system and lower wind was favorable to the accumulation of air pollutants, and thus generating less turbulence by reducing surface radiation. These conditions on polluted days led to smaller PBL depth than those on clean days, and the maximum depth of PBL was 660 m for CRGM. Besides, the diurnal variation of PBL depth on polluted days was weaker than those on clean days.

The Climatology of the Atmospheric Boundary Layer in Contemporary Global Climate Models

AbstractHere, we present the climatology of the planetary boundary layer depth in 18 contemporary general circulation models (GCMs) in simulations of the late-twentieth-century climate that were part of phase 5 of the Coupled Model Intercomparison Project (CMIP5). We used a bulk Richardson methodology to establish the boundary layer depth from the 6-hourly synoptic-snapshot data available in the CMIP5 archives. We present an ensemble analysis of the climatological mean, diurnal cycle, and seasonal cycle of the boundary layer depth in these models and compare it to the climatologies from the ECMWF ERA-Interim reanalysis. Overall, we find that the CMIP5 models do a reasonably good job of reproducing the distribution of mean boundary layer depth, although the geographical patterns vary considerably between models. However, the models are biased toward weaker diurnal and seasonal cycles in the boundary layer depth and generally produce much deeper boundary layers at night and during the winter than are found in the reanalysis. These biases are likely to reduce the ability of these models to accurately represent other properties of the diurnal and seasonal cycles, and the sensitivity of these cycles to climate change.

Evolution of the Planetary Boundary Layer on the northern coast of Brazil during the CHUVA campaign

This study aims to characterize the wind and thermodynamic structure of the Planetary Boundary Layer (PBL) on the northern coast of Brazil (NCB) via the CHUVA datasets. Three synoptic conditions were present in the NCB region between March 1 and 25, 2010: a dry period, the Upper Tropospheric Cyclonic Vortex (UTCV) and the Intertropical Convergence Zone (ITCZ). Nighttime precipitation accounted for 78% of the total precipitation observed in the month, mainly during the ITCZ. In general, the surface meteorological fields were few changed by intense weather events due to proximity to the ocean and the predominant contribution of the northeasterly trade winds. There was also a weak sea breeze signal that maintained the horizontal moisture flow in the studied area. On dry days, the PBL depth was higher, drier, and warmer, resulting in stronger winds below 500m. Moreover, trends throughout the period suggest that PBLs are near-neutral below 500m. However, the wind variability was intensified by up to 20% due to downdrafts and higher wind shears during the deep convection mechanisms derived by UTCV. Furthermore, ITCZ mixed rainfall cooled the PBL at approximately 2K, making it very stable according to the Richardson number classification adopted. The observed temporal and spatial scale represent challenges to the physical parameterizations used to improve numerical weather prediction models over tropical coastal areas.

Planetary boundary layer and circulation dynamics at Gale Crater, Mars

The Mars implementation of the Planet Weather Research and Forecasting (PlanetWRF) model, MarsWRF, is used here to simulate the atmospheric conditions at Gale Crater for different seasons during a period coincident with the Curiosity rover operations. The model is first evaluated with the existing single-point observations from the Rover Environmental Monitoring Station (REMS), and is then used to provide a larger scale interpretation of these unique measurements as well as to give complementary information where there are gaps in the measurements.The variability of the planetary boundary layer depth may be a driver of the changes in the local dust and trace gas content within the crater. Our results show that the average time when the PBL height is deeper than the crater rim increases and decreases with the same rate and pattern as Curiosity's observations of the line-of-sight of dust within the crater and that the season when maximal (minimal) mixing is produced is Ls 225°–315° (Ls 90°–110°). Thus the diurnal and seasonal variability of the PBL depth seems to be the driver of the changes in the local dust content within the crater. A comparison with the available methane measurements suggests that changes in the PBL depth may also be one of the factors that accounts for the observed variability, with the model results pointing towards a local source to the north of the MSL site.The interaction between regional and local flows at Gale Crater is also investigated assuming that the meridional wind, the dynamically important component of the horizontal wind at Gale, anomalies with respect to the daily mean can be approximated by a sinusoidal function as they typically oscillate between positive (south to north) and negative (north to south) values that correspond to upslope/downslope or downslope/upslope regimes along the crater rim and Mount Sharp slopes and the dichotomy boundary. The smallest magnitudes are found in the northern crater floor in a region that comprises Bradbury Landing, in particular at Ls 90° when they are less than 1 m s−1, indicating very little lateral mixing with outside air. The largest amplitudes occur in the south-western portions of the crater where they can exceed 20 m s−1. Should the slope flows along the crater rims interact with the dichotomy boundary flow, which is more likely at Ls 270° and very unlikely at Ls 90°, they are likely to interact constructively for a few hours from late evening to nighttime (∼17–23 LMST) and from pre-dawn to early morning (∼5–11 LMST) hours at the norther crater rim and destructively at night (∼22–23 LMST) and in the morning (∼10–11 LMST) at the southern crater rim.We conclude that a better understanding of the PBL and circulation dynamics has important implications for the variability of the concentration of dust, non-condensable and trace gases at the bottom of other craters on Mars as mixing with outside air can be achieved vertically, through changes in the PBL depth, and laterally, by the transport of air into and out of the crater.

The Vertical Dust Profile over Gale Crater, Mars.

We create a vertically coarse, but complete, vertical profile of dust mixing ratio from the surface to the upper atmosphere over Gale Crater, Mars, using the frequent joint atmospheric observations of the orbiting Mars Climate Sounder (MCS) and the Mars Science Laboratory (MSL) Curiosity rover. Using these data and an estimate of planetary boundary layer (PBL) depth from the MarsWRF general circulation model, we divide the vertical column into three regions. The first region is the Gale Crater PBL, the second is the MCS-sampled region, and the third is between these first two. We solve for a well-mixed dust mixing ratio within this third (middle) layer of atmosphere to complete the profile. We identify a unique seasonal cycle of dust within each atmospheric layer. Within the Gale PBL, dust mixing ratio maximizes near southern hemisphere summer solstice (Ls = 270°) and minimizes near winter solstice (Ls = 90-100°) with a smooth sinusoidal transition between them. However, the layer above Gale Crater and below the MCS-sampled region more closely follows the global opacity cycle and has a maximum in opacity near Ls = 240° and exhibits a local minimum (associated with the "solsticial pause" in dust storm activity) near Ls = 270°. With knowledge of the complete vertical dust profile, we can also assess the frequency of high-altitude dust layers over Gale. We determine that 36% of MCS profiles near Gale Crater contain an "absolute" high-altitude dust layer wherein the dust mixing ratio is the maximum in the entire vertical column.

The effects of horizontal grid spacing on simulated daytime boundary layer depths in an area of complex terrain in Utah

The influence of grid spacing on daytime planetary boundary layer (PBL) depths is investigated using 2 years of hourly output from a weather forecast system run operationally for an area of complex terrain in Utah. The model domain includes Dugway Proving Ground, the target area for the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program. Differences in PBL depths between a coarse- (10 km horizontal grid spacing) and a fine-grid (3.3 km) domain are expressed as a function of several parameters that describe the terrain variability, including the standard deviation and the Laplacian of the terrain elevation. PBL depths on fair weather days are larger in the coarse domain than in the fine-grid domain by more than 200 m over areas with unresolved ridges in the coarse-grid domain. Absolute differences are an order of magnitude larger in summer than in winter while relative differences are similar and on the order of 10%. The PBL depth differences can only be partly removed after correcting for the fine-grid terrain elevation in the coarse domain, indicating that unresolved atmospheric processes in the coarse-grid domain also account for the PBL depth differences. The results also demonstrate the importance of distinguishing between PBL depths and PBL heights when evaluating the performance of coarse-grid numerical models.

Investigation of Turbulence Parametrization Schemes with Reference to the Atmospheric Boundary Layer Over the Aegean Sea During Etesian Winds

The spatial structure of the marine atmospheric boundary layer (MABL) over the Aegean Sea is investigated using the Weather Research and Forecasting (WRF) mesoscale model. Two ‘first-order’ non-local and five ‘1.5-order’ local planetary boundary-layer (PBL) parametrization schemes are used. The predictions from the WRF model are evaluated against airborne observations obtained by the UK Facility for Airborne Atmospheric Measurements BAe-14 research aircraft during the Aegean-GAME field campaign. Statistical analysis shows good agreement between measurements and simulations especially at low altitude. Despite the differences between the predicted and measured wind speeds, they reach an agreement index of 0.76. The simulated wind-speed fields close to the surface differ substantially among the schemes (maximum values range from 13 to $$18\hbox { m s}^{-1}$$ at 150-m height), but the differences become marginal at higher levels. In contrast, all schemes show similar spatial variation patterns in potential temperature fields. A warmer (1–2 K) and drier (2–3 $$\hbox { g kg}^{-1})$$ layer than is observed, is predicted by almost all schemes under stable conditions (eastern Aegean Sea), whereas a cooler (up to 2 K) and moister (1–2 $$\hbox { g kg}^{-1})$$ layer is simulated under near-neutral to nearly unstable conditions (western Aegean Sea). Almost all schemes reproduce the vertical structure of the PBL and the shallow MABL (up to 300 m) well, including the low-level jet in the eastern Aegean Sea, with non-local schemes being closer to observations. The simulated PBL depths diverge (up to 500 m) due to the different criteria applied by the schemes for their calculation. Under stable conditions, the observed MABL depth corresponds to the height above the sea surface where the simulated eddy viscosity reaches a minimum; under neutral to slightly unstable conditions this is close to the top of the simulated entrainment layer. The observed sensible heat fluxes vary from −40 to $$25\hbox { W m}^{-2}$$ , while the simulated fluxes range from −40 to $$40\hbox { W m}^{-2}$$ ; however, all of the schemes’ predictions are close to the observations under unstable conditions. Finally, all schemes overestimate the friction velocity, although the simulated range (from 0.2 to $$0.5\hbox { m s}^{-1})$$ is narrower than that observed (from 0.1 to $$0.7\hbox { m s}^{-1})$$ .

On the Potential of 25 Years (1991–2015) of Rawinsonde Measurements for Elucidating Climatological and Spatiotemporal Patterns of Afternoon Boundary Layer Depths over the Contiguous US

The daytime planetary boundary layer (PBL) depth is an essential parameter in, for example, dispersion models and numerical weather prediction. We applied a recently developed technique to estimate afternoon PBL depths from 516,887 daily rawinsonde measurements obtained at 0000 UTC between 1 Jan 1991 and 31 Dec 2015 from 67 US sites. We discuss salient features in the PBL depth variability and examine the climatology of afternoon PBL depths as a function of geographical region and Köppen-Geiger climate regime. Results indicated a strong east to west PBL depth gradient, with higher PBL depths over the semiarid regions of the western US in spring and summer. Many sites located near coastlines exhibited no seasonal cycle due to marine boundary layer influences, and an inverse relationship was found between distance from the coast and observed afternoon PBL depth. Differentiating by climate type, BWk (arid, desert, and cold) has the largest median winter (1075 m) and summer (2500 m) PBL depths. Median PBL depths for Cfa (temperate, no dry season, and hot summers), Dfa (cold, no dry season, and warm summer), and Dfb (cold, no dry season, and hot summers) climate types are 750 m and 1000 m during winter and summer, respectively.