Marine boundary layer cloud property retrievals from high–resolution ASTER observations: Case studies and comparison with Terra–MODIS

Abstract. Retrievals of marine boundary layer (MBL) cloud microphysical and optical properties, based on high spatial resolution observations from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), are presented. The research–level retrieval algorithm, which is modified from the MODIS Data Collection 6 (C6) operational algorithms, is documented. An ASTER specific cloud masking scheme is implemented and the derived cloud covers agree well with those from an independent scheme reported in a previous ASTER cloud cover study by (Zhao and Di Girolamo, 2006). The viability of the retrieval algorithm is shown by a comparison of retrieved cloud optical thickness τAaM and effective droplet radius reff,AaM with the co-located, operational MODIS retrievals for 48 MBL cloud scenes with different degrees of cloud heterogeneity. The input for the retrieval is provided by aggregated, atmospherically corrected ASTER cloud top reflectances γ^0.86,AaM and γ^2.1,AaM. By means of the subpixel cloud cover Csub, which is derived from the ASTER cloud mask at its native resolution, the data set is divided into 52,254 overcast and 47,538 partially cloudy pixels. PDFs and scatter plots of γ^0.86,AaM, γ^2.1,AaM, and τAaM agree well with the corresponding MODIS products with Pearson's product–moment correlation coefficients of R > 0.980. A comparison for pixels with Csub = 1.0 yields a high agreement between reff,AaM and the MODIS retrievals with R = 0.972. However, for partially cloudy pixels there are significant differences between both products which can exceed 10 μm. Moreover, it is shown that the numerous delicate cloud structures in the example MBL scenes, resolved by the high–resolution ASTER retrievals, are smoothed by the MODIS observations. The overall good agreement between the research–level ASTER results and the operational MODIS C6 products proves the feasibility of MODIS–like retrievals from ASTER reflectance measurements and provides the basis for future studies concerning the scale–dependency of satellite observations and 3–dimensional radiative effects.

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Marine boundary layer cloud property retrievals from high-resolution ASTER observations: case studies and comparison with Terra MODIS

Atmospheric Measurement Techniques ◽

10.5194/amt-9-5869-2016 ◽

2016 ◽

Vol 9 (12) ◽

pp. 5869-5894 ◽

Cited By ~ 10

Author(s):

Frank Werner ◽

Galina Wind ◽

Zhibo Zhang ◽

Steven Platnick ◽

Larry Di Girolamo ◽

...

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Marine Boundary Layer ◽

Thermal Emission ◽

Three Dimensional ◽

Correlation Coefficients ◽

Retrieval Algorithm ◽

Research Level ◽

Boundary Layer Cloud ◽

Layer Cloud

Abstract. A research-level retrieval algorithm for cloud optical and microphysical properties is developed for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) aboard the Terra satellite. It is based on the operational MODIS algorithm. This paper documents the technical details of this algorithm and evaluates the retrievals for selected marine boundary layer cloud scenes through comparisons with the operational MODIS Data Collection 6 (C6) cloud product. The newly developed, ASTER-specific cloud masking algorithm is evaluated through comparison with an independent algorithm reported in [Zhao and Di Girolamo(2006)]. To validate and evaluate the cloud optical thickness (τ) and cloud effective radius (reff) from ASTER, the high-spatial-resolution ASTER observations are first aggregated to the same 1000 m resolution as MODIS. Subsequently, τaA and reff, aA retrieved from the aggregated ASTER radiances are compared with the collocated MODIS retrievals. For overcast pixels, the two data sets agree very well with Pearson's product-moment correlation coefficients of R > 0.970. However, for partially cloudy pixels there are significant differences between reff, aA and the MODIS results which can exceed 10 µm. Moreover, it is shown that the numerous delicate cloud structures in the example marine boundary layer scenes, resolved by the high-resolution ASTER retrievals, are smoothed by the MODIS observations. The overall good agreement between the research-level ASTER results and the operational MODIS C6 products proves the feasibility of MODIS-like retrievals from ASTER reflectance measurements and provides the basis for future studies concerning the scale dependency of satellite observations and three-dimensional radiative effects.

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Synoptic-scale controls of fog and low-cloud variability in the Namib Desert

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-3415-2020 ◽

2020 ◽

Vol 20 (6) ◽

pp. 3415-3438 ◽

Cited By ~ 1

Author(s):

Hendrik Andersen ◽

Jan Cermak ◽

Julia Fuchs ◽

Peter Knippertz ◽

Marco Gaetani ◽

...

Keyword(s):

Boundary Layer ◽

Cloud Cover ◽

Large Scale ◽

Marine Boundary Layer ◽

Synoptic Scale ◽

Namib Desert ◽

Data Set ◽

Air Masses ◽

Low Cloud ◽

Low Cloud Cover

Abstract. Fog is a defining characteristic of the climate of the Namib Desert, and its water and nutrient input are important for local ecosystems. In part due to sparse observation data, the local mechanisms that lead to fog occurrence in the Namib are not yet fully understood, and to date, potential synoptic-scale controls have not been investigated. In this study, a recently established 14-year data set of satellite observations of fog and low clouds in the central Namib is analyzed in conjunction with reanalysis data in order to identify synoptic-scale patterns associated with fog and low-cloud variability in the central Namib during two seasons with different spatial fog occurrence patterns. It is found that during both seasons, mean sea level pressure and geopotential height at 500 hPa differ markedly between fog/low-cloud and clear days, with patterns indicating the presence of synoptic-scale disturbances on fog and low-cloud days. These regularly occurring disturbances increase the probability of fog and low-cloud occurrence in the central Namib in two main ways: (1) an anomalously dry free troposphere in the coastal region of the Namib leads to stronger longwave cooling of the marine boundary layer, increasing low-cloud cover, especially over the ocean where the anomaly is strongest; (2) local wind systems are modulated, leading to an onshore anomaly of marine boundary-layer air masses. This is consistent with air mass back trajectories and a principal component analysis of spatial wind patterns that point to advected marine boundary-layer air masses on fog and low-cloud days, whereas subsiding continental air masses dominate on clear days. Large-scale free-tropospheric moisture transport into southern Africa seems to be a key factor modulating the onshore advection of marine boundary-layer air masses during April, May, and June, as the associated increase in greenhouse gas warming and thus surface heating are observed to contribute to a continental heat low anomaly. A statistical model is trained to discriminate between fog/low-cloud and clear days based on information on large-scale dynamics. The model accurately predicts fog and low-cloud days, illustrating the importance of large-scale pressure modulation and advective processes. It can be concluded that regional fog in the Namib is predominantly of an advective nature and that fog and low-cloud cover is effectively maintained by increased cloud-top radiative cooling. Seasonally different manifestations of synoptic-scale disturbances act to modify its day-to-day variability and the balance of mechanisms leading to its formation and maintenance. The results are the basis for a new conceptual model of the synoptic-scale mechanisms that control fog and low-cloud variability in the Namib Desert and will guide future studies of coastal fog regimes.

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Modeling microphysical effects of entrainment in clouds observed during EUCAARI-IMPACT field campaign

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-8489-2013 ◽

2013 ◽

Vol 13 (16) ◽

pp. 8489-8503 ◽

Cited By ~ 2

Author(s):

D. Jarecka ◽

H. Pawlowska ◽

W. W. Grabowski ◽

A. A. Wyszogrodzki

Keyword(s):

Boundary Layer ◽

Turbulent Mixing ◽

Marine Boundary Layer ◽

Cloud Droplet ◽

Droplet Radius ◽

Subgrid Scale ◽

Microphysics Scheme ◽

Local Conditions ◽

Field Campaign ◽

Les Model

Abstract. This paper discusses aircraft observations and large-eddy simulation (LES) modeling of 15 May 2008, North Sea boundary-layer clouds from the EUCAARI-IMPACT field campaign. These clouds are advected from the northeast by the prevailing lower-tropospheric winds and featured stratocumulus-over-cumulus cloud formations. An almost-solid stratocumulus deck in the upper part of the relatively deep, weakly decoupled marine boundary layer overlays a field of small cumuli. The two cloud formations have distinct microphysical characteristics that are in general agreement with numerous past observations of strongly diluted shallow cumuli on one hand and solid marine stratocumulus on the other. Based on the available observations, a LES model setup is developed and applied in simulations using a novel LES model. The model features a double-moment warm-rain bulk microphysics scheme combined with a sophisticated subgrid-scale scheme allowing local prediction of the homogeneity of the subgrid-scale turbulent mixing. The homogeneity depends on the characteristic time scales for the droplet evaporation and for the turbulent homogenization. In the model, these scales are derived locally based on the subgrid-scale turbulent kinetic energy, spatial scale of cloudy filaments, mean cloud droplet radius, and humidity of the cloud-free air entrained into a cloud, all predicted by the LES model. The model reproduces contrasting macrophysical and microphysical characteristics of the cumulus and stratocumulus cloud layers. Simulated subgrid-scale turbulent mixing within the cumulus layer and near the stratocumulus top is on average quite inhomogeneous, but varies significantly depending on the local conditions.

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Mind the gap – Part 1: Accurately locating warm marine boundary layer clouds and precipitation using spaceborne radars

Atmospheric Measurement Techniques ◽

10.5194/amt-13-2363-2020 ◽

2020 ◽

Vol 13 (5) ◽

pp. 2363-2379 ◽

Cited By ~ 3

Author(s):

Katia Lamer ◽

Pavlos Kollias ◽

Alessandro Battaglia ◽

Simon Preval

Keyword(s):

Boundary Layer ◽

Cloud Cover ◽

Marine Boundary Layer ◽

Short Pulse ◽

Pulse Mode ◽

Cloud Base ◽

Boundary Layer Clouds ◽

Highly Sensitive ◽

Eastern North Atlantic ◽

Long Pulse

Abstract. Ground-based radar observations show that, over the eastern North Atlantic, 50 % of warm marine boundary layer (WMBL) hydrometeors occur below 1.2 km and have reflectivities of < −17 dBZ, thus making their detection from space susceptible to the extent of surface clutter and radar sensitivity. Surface clutter limits the ability of the CloudSat cloud profiling radar (CPR) to observe the true cloud base in ∼52 % of the cloudy columns it detects and true virga base in ∼80 %, meaning the CloudSat CPR often provides an incomplete view of even the clouds it does detect. Using forward simulations, we determine that a 250 m resolution radar would most accurately capture the boundaries of WMBL clouds and precipitation; that being said, because of sensitivity limitations, such a radar would suffer from cloud cover biases similar to those of the CloudSat CPR. Observations and forward simulations indicate that the CloudSat CPR fails to detect 29 %–43 % of the cloudy columns detected by ground-based sensors. Out of all configurations tested, the 7 dB more sensitive EarthCARE CPR performs best (only missing 9.0 % of cloudy columns) indicating that improving radar sensitivity is more important than decreasing the vertical extent of surface clutter for measuring cloud cover. However, because 50 % of WMBL systems are thinner than 400 m, they tend to be artificially stretched by long sensitive radar pulses, hence the EarthCARE CPR overestimation of cloud top height and hydrometeor fraction. Thus, it is recommended that the next generation of space-borne radars targeting WMBL science should operate interlaced pulse modes including both a highly sensitive long-pulse mode and a less sensitive but clutter-limiting short-pulse mode.

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Exploring microphysical properties of marine boundary layer clouds through network analysis

10.5194/egusphere-egu21-13307 ◽

2021 ◽

Author(s):

Lucile Ricard ◽

Athanasios Nenes ◽

Jakob Runge ◽

Paraskevi Georgakaki

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Network Analysis ◽

Marine Boundary Layer ◽

Large Eddy Simulations ◽

Dynamical Structure ◽

Boundary Layer Clouds ◽

Microphysical Properties ◽

Spatial Coverage ◽

Large Eddy

<p>Aerosol-cloud interactions remain the largest uncertainty in assessments of anthropogenic climate forcing, while the complexity of these interactions require methods that enable abstractions and simplifications that allow their improved treatment in climate models. Marine boundary layer clouds are an important component of the climate system as their large albedo and spatial coverage strongly affect the planetary radiative balance. High resolution simulations of clouds provide an unprecedented understanding of the structure and behavior of these clouds in the marine atmosphere, but the amount of data is often too large and complex to be useful in climate simulations. Data reduction and inference methods provide a way that to reduce the complexity and dimensionality of datasets generated from high-resolution Large Eddy Simulations.</p><p>In this study we use network analysis, (the &#948;-Maps method) to study the complex interaction between liquid water, droplet number and vertical velocity in Large Eddy Simulations of Marine Boundary Layer clouds. &#948;-Maps identifies domains that are spatially contiguous and possibly overlapping and characterizes their connections and temporal interactions. The objective is to better understand microphysical properties of marine boundary layer clouds, and how they are impacted by the variability in aerosols. Here we will capture the dynamical structure of the cloud fields predicted by the MIMICA Large Eddy Simulation (LES) model. The networks inferred from the different simulation fields are compared between them (intra-comparisons) using perturbations in initial conditions and aerosol, using a set of four metrics. The networks are then evaluated for their differences, quantifying how much variability is inherent in the LES simulations versus the robust changes induced by the aerosol fields.&#160;</p>

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What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-13489-2019 ◽

2019 ◽

Vol 19 (21) ◽

pp. 13489-13506 ◽

Cited By ~ 3

Author(s):

Karmen Babić ◽

Norbert Kalthoff ◽

Bianca Adler ◽

Julian F. Quinting ◽

Fabienne Lohou ◽

...

Keyword(s):

Boundary Layer ◽

West Africa ◽

Climate Models ◽

Marine Boundary Layer ◽

Monsoon Season ◽

Onset Time ◽

Data Set ◽

Cold Air ◽

Low Level ◽

Stratus Clouds

Abstract. Nocturnal low-level stratus clouds (LLCs) are frequently observed in the atmospheric boundary layer (ABL) over southern West Africa (SWA) during the summer monsoon season. Considering the effect these clouds have on the surface energy and radiation budgets as well as on the diurnal cycle of the ABL, they are undoubtedly important for the regional climate. However, an adequate representation of LLCs in the state-of-the-art weather and climate models is still a challenge, which is largely due to the lack of high-quality observations in this region and gaps in understanding of underlying processes. In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA (Dynamics-aerosol-chemistry-cloud interactions in West Africa) ground-based field campaign was used for the first observational analyses of the parameters and physical processes relevant for the LLC formation over SWA. However, occasionally stratus-free nights occur during the monsoon season as well. Using observations and ERA5 reanalysis, we investigate differences in the boundary-layer conditions during 6 stratus-free and 20 stratus nights observed during the DACCIWA campaign. Our results suggest that the interplay between three major mechanisms is crucial for the formation of LLCs during the monsoon season: (i) the onset time and strength of the nocturnal low-level jet (NLLJ), (ii) horizontal cold-air advection, and (iii) background moisture level. Namely, weaker or later onset of NLLJ leads to a reduced contribution from horizontal cold-air advection. This in turn results in weaker cooling, and thus saturation is not reached. Such deviation in the dynamics of the NLLJ is related to the arrival of a cold air mass propagating northwards from the coast, called Gulf of Guinea maritime inflow. Additionally, stratus-free nights occur when the intrusions of dry air masses, originating from, for example, central or south Africa, reduce the background moisture over large parts of SWA. Backward-trajectory analysis suggests that another possible reason for clear nights is descending air, which originated from drier levels above the marine boundary layer.

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Characterization of the South Atlantic marine boundary layer aerosol using an aerodyne aerosol mass spectrometer

Atmospheric Chemistry and Physics ◽

10.5194/acp-8-4711-2008 ◽

2008 ◽

Vol 8 (16) ◽

pp. 4711-4728 ◽

Cited By ~ 98

Author(s):

S. R. Zorn ◽

F. Drewnick ◽

M. Schott ◽

T. Hoffmann ◽

S. Borrmann

Keyword(s):

Boundary Layer ◽

Sulfuric Acid ◽

High Resolution ◽

Marine Boundary Layer ◽

Methanesulfonic Acid ◽

Field Measurements ◽

Size Distributions ◽

Air Masses ◽

Unit Mass Resolution ◽

Mass Concentrations

Abstract. Measurements of the submicron fraction of the atmospheric aerosol in the marine boundary layer were performed from January to March 2007 (Southern Hemisphere summer) onboard the French research vessel Marion Dufresne in the Southern Atlantic and Indian Ocean (20° S–60° S, 70° W–60° E). We used an Aerodyne High-Resolution-Time-of-Flight AMS to characterize the chemical composition and to measure species-resolved size distributions of non-refractory aerosol components in the submicron range. Within the "standard" AMS compounds (ammonium, chloride, nitrate, sulfate, organics) "sulfate" is the dominant species in the marine boundary layer with concentrations ranging between 50 ng m−3 and 3 μg m−3. Furthermore, what is seen as "sulfate" by the AMS is likely comprised mostly of sulfuric acid. Another sulfur containing species that is produced in marine environments is methanesulfonic acid (MSA). There have been previously measurements of MSA using an Aerodyne AMS. However, due to the use of an instrument equipped with a quadrupole detector with unit mass resolution it was not possible to physically separate MSA from other contributions to the same m/z. In order to identify MSA within the HR-ToF-AMS raw data and to extract mass concentrations for MSA from the field measurements the standard high-resolution MSA fragmentation patterns for the measurement conditions during the ship campaign (e.g. vaporizer temperature) needed to be determined. To identify characteristic air masses and their source regions backwards trajectories were used and averaged concentrations for AMS standard compounds were calculated for each air mass type. Sulfate mass size distributions were measured for these periods showing a distinct difference between oceanic air masses and those from African outflow. While the peak in the mass distribution was roughly at 250 nm (vacuum aerodynamic diameter) in marine air masses, it was shifted to 470 nm in African outflow air. Correlations between the mass concentrations of sulfate, organics and MSA show a narrow correlation for MSA with sulfate/sulfuric acid coming from the ocean, but not with continental sulfate.

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Analysis of the MODIS Above-Cloud Aerosol Retrieval Algorithm Using MCARS

10.5194/gmd-2021-17 ◽

2021 ◽

Author(s):

Galina Wind ◽

Arlindo M. da Silva ◽

Kerry G. Meyer ◽

Steven Platnick ◽

Peter M. Norris

Keyword(s):

Boundary Layer ◽

Optical Thickness ◽

Marine Boundary Layer ◽

Single Case ◽

Ground Truth ◽

Retrieval Algorithm ◽

Boundary Layer Clouds ◽

Aerosol Retrieval ◽

Source Data ◽

Moderate Resolution Imaging Spectroradiometer

Abstract. The Multi-sensor Cloud and Aerosol Retrieval Simulator (MCARS) presently produces synthetic radiance data from Goddard Earth Observing System version 5 (GEOS-5) model output as if the Moderate Resolution Imaging Spectroradiometer (MODIS) was viewing a combination of atmospheric column inclusive of clouds, aerosols and a variety of gases and land/ocean surface at a specific location. In this paper we use MCARS to study the MODIS Above-Cloud AEROsol retrieval algorithm (MOD06ACAERO). MOD06ACAERO is presently a regional research algorithm able to retrieve aerosol optical thickness over clouds, in particular absorbing biomass burning aerosols overlying marine boundary layer clouds in the Southeastern Atlantic Ocean. The algorithm's ability to provide aerosol information in cloudy conditions makes it a valuable source of information for modeling and climate studies in an area where current clear sky-only operational MODIS aerosol retrievals effectively have a data gap between the months of June and October. We use MCARS for a verification and closure study of the MOD06ACAERO algorithm. Our simulations indicate that the MOD06ACAERO algorithm performs well for marine boundary layer clouds in the SE Atlantic provided some specific screening rules are observed. For the present study, a combination of five simulated MODIS data granules was used for a dataset of 13.5 million samples with known input conditions. When pixel retrieval uncertainty was less than 30 %, optical thickness of the underlying cloud layer was greater than 4 and scattering angle range within the cloud bow was excluded, MOD06ACAERO retrievals agreed with the underlying ground truth (GEOS-5 cloud and aerosol profiles used to generate the synthetic radiances) with a slope of 0.913, offset of 0.06, and RMSE = 0.107. When only near-nadir pixels were considered (view zenith angle within +/−20 degrees) the agreement with source data further improved (0.977, 0.051 and 0.096 respectively). Algorithm closure was examined using a single case out of the five used for verification. For closure, the MOD06ACAERO code was modified to use GEOS-5 temperature and moisture profiles as ancillary. Agreement of MOD06ACAERO retrievals with source data for the closure study had a slope of 0.996 with offset −0.007 and RMSE of 0.097 at pixel uncertainty level of less than 40 %, illustrating the benefits of high-quality ancillary atmospheric data for such retrievals.

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Sensitivity of PARASOL multi-angle photo-polarimetric aerosol retrievals to cloud contamination

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-7-10673-2014 ◽

2014 ◽

Vol 7 (10) ◽

pp. 10673-10714 ◽

Cited By ~ 2

Author(s):

F. A. Stap ◽

O. Hasekamp ◽

T. Röckmann

Keyword(s):

Cloud Cover ◽

Goodness Of Fit ◽

A Priori ◽

Synthetic Data ◽

Single Scattering ◽

Retrieval Algorithm ◽

Cloud Droplets ◽

Data Set ◽

Aerosol Parameters ◽

Cloud Screening

Abstract. An important problem in satellite remote sensing of aerosols is related to the need to perform an adequate cloud screening. If a cloud screening is applied that is not strict enough, the ground scene has the probability of residual cloud cover which causes large errors on the retrieved aerosol parameters. On the other hand, if the cloud screening procedure is too strict, too many clear sky cases, especially near-cloud scenes, will falsely be flagged cloudy. The detrimental effects of cloud contamination as well as the importance of aerosol cloud interactions that can be studied in these near-cloud scenes call for new approaches to cloud screening. Multi-angle, multi-wavelength photo-polarimetric measurements have a unique capability to distinguish between scattering by (liquid) cloud droplets and aerosol particles. In this paper the sensitivity of aerosol retrievals from multi-angle, photo-polarimetric measurements to cloud contamination is investigated and the ability to intrinsically filter the cloud contaminated scenes based on a goodness-of-fit criteria is evaluated. Hereto, an aerosol retrieval algorithm is applied to a partially clouded, synthetic data-set including partial cloud cover as well as non-cloud screened POLDER-3/PARASOL observations It is found that a goodness-of-fit filter, together with a filter on the coarse mode refractive index (mrcoarse > 1.335) and a cirrus screening adequately reject the cloud contaminated scenes. No bias nor larger SD are found in the retrieved parameters for this intrinsic cloud filter compared to the parameters retrieved in a priori cloud screened data-set (using MODIS/AQUA cloud masks) of PARASOL observations. Moreover, less high aerosol load scenes are misinterpreted as cloud contaminated. The retrieved aerosol optical thickness, single scattering albedo and Ångström exponent show good agreement with AERONET observations. Furthermore, the synthetic retrievals give confidence in the ability of the algorithm to correctly retrieve the micro-physical aerosol parameters.

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Twenty-Four-Hour Observations of the Marine Boundary Layer Using Shipborne NOAA High-Resolution Doppler Lidar

Journal of Applied Meteorology ◽

10.1175/jam2296.1 ◽

2005 ◽

Vol 44 (11) ◽

pp. 1723-1744 ◽

Cited By ~ 27

Author(s):

Volker Wulfmeyer ◽

Tijana Janjić

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Surface Temperature ◽

Sea Surface Temperature ◽

Wind Shear ◽

Marine Boundary Layer ◽

Sea Surface ◽

Doppler Lidar ◽

Sensible Heat ◽

Wind Profiles

Abstract Shipborne observations obtained with the NOAA high-resolution Doppler lidar (HRDL) during the 1999 Nauru (Nauru99) campaign were used to study the structure of the marine boundary layer (MBL) in the tropical Pacific Ocean. During a day with weak mesoscale activity, diurnal variability of the height of the convective MBL was observed using HRDL backscatter data. The observed diurnal variation in the MBL height had an amplitude of about 250 m. Relations between the MBL height and in situ measurements of sea surface temperature as well as latent and sensible heat fluxes were examined. Good correlation was found with the sea surface temperature. The correlation with the latent heat flux was lower, and practically no correlation between the MBL height and the sensible heat and buoyancy fluxes could be detected. Horizontal wind profiles were measured using a velocity–azimuth display scan of HRDL velocity data. Strong wind shear at the top of the MBL was observed in most cases. Comparison of these results with GPS radiosonde data shows discrepancies in the wind intensity and direction, which may be due to different observation times and locations as well as due to multipath effects at the ship’s platform. Vertical wind profiles corrected for ship’s motion were used to derive vertical velocity variance and skewness profiles. Motion compensation had a significant effect on their shape. Normalized by the convective velocity scale and by the top of the mixed layer zi, the variance varied between 0.45 and 0.65 at 0.4z/zi and decreased to 0.2 at 1.0z/zi. The skewness ranged between 0.3 and 0.8 in the MBL and showed in almost all cases a maximum between 1.0z/zi and 1.1z/zi. These profiles revealed the existence of another turbulent layer above the MBL, which was probably driven by wind shear and cloud condensation processes.

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