scholarly journals Remote Sensing of Sea Salt Aerosol below Trade Wind Clouds

2019 ◽  
Vol 76 (5) ◽  
pp. 1189-1202 ◽  
Author(s):  
Marcus Klingebiel ◽  
Virendra P. Ghate ◽  
Ann Kristin Naumann ◽  
Florian Ditas ◽  
Mira L. Pöhlker ◽  
...  

Abstract Sea salt aerosol in the boundary layer below shallow cumulus clouds is remotely observed with a Ka-band cloud radar at the Barbados Cloud Observatory and is detected in 76% of the measurements over 1 year. Carried by convection, sea salt particles with a diameter larger than 500 nm show an upward motion of 0.2 m s−1 below shallow cumulus clouds for a 2-day case study. Caused by an increasing relative humidity with increasing altitude, the sea salt particles become larger as they move closer to the cloud base. By using combined measurements of a Ka-band cloud radar and a Raman lidar, the retrieved equivolumetric diameter of the hygroscopically grown sea salt particles is found to be between 6 and 11 μm with a total number concentration of 20 cm−3 near cloud base. Assuming a fixed shape parameter, a size distribution of sea salt particles under high-relative-humidity conditions below cloud base is estimated and agrees with measurements taken by a dry-deposition sampler and online aerosol observations. The methods outlined in this paper can be used in future studies to get a better understanding of the vertical and temporal sea salt distribution in the boundary layer and sea salt aerosol–cloud interaction processes.

2016 ◽  
Vol 16 (13) ◽  
pp. 8643-8666 ◽  
Author(s):  
Eunsil Jung ◽  
Bruce A. Albrecht ◽  
Graham Feingold ◽  
Haflidi H. Jonsson ◽  
Patrick Chuang ◽  
...  

Abstract. Shallow marine cumulus clouds are by far the most frequently observed cloud type over the Earth's oceans; but they are poorly understood and have not been investigated as extensively as stratocumulus clouds. This study describes and discusses the properties and variations of aerosol, cloud, and precipitation associated with shallow marine cumulus clouds observed in the North Atlantic trades during a field campaign (Barbados Aerosol Cloud Experiment- BACEX, March–April 2010), which took place off Barbados where African dust periodically affects the region. The principal observing platform was the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter (TO) research aircraft, which was equipped with standard meteorological instruments, a zenith pointing cloud radar and probes that measured aerosol, cloud, and precipitation characteristics.The temporal variation and vertical distribution of aerosols observed from the 15 flights, which included the most intense African dust event during all of 2010 in Barbados, showed a wide range of aerosol conditions. During dusty periods, aerosol concentrations increased substantially in the size range between 0.5 and 10 µm (diameter), particles that are large enough to be effective giant cloud condensation nuclei (CCN). The 10-day back trajectories showed three distinct air masses with distinct vertical structures associated with air masses originating in the Atlantic (typical maritime air mass with relatively low aerosol concentrations in the marine boundary layer), Africa (Saharan air layer), and mid-latitudes (continental pollution plumes). Despite the large differences in the total mass loading and the origin of the aerosols, the overall shapes of the aerosol particle size distributions were consistent, with the exception of the transition period.The TO was able to sample many clouds at various phases of growth. Maximum cloud depth observed was less than ∼ 3 km, while most clouds were less than 1 km deep. Clouds tend to precipitate when the cloud is thicker than 500–600 m. Distributions of cloud field characteristics (depth, radar reflectivity, Doppler velocity, precipitation) were well identified in the reflectivity–velocity diagram from the cloud radar observations. Two types of precipitation features were observed for shallow marine cumulus clouds that may impact boundary layer differently: first, a classic cloud-base precipitation where precipitation shafts were observed to emanate from the cloud base; second, cloud-top precipitation where precipitation shafts emanated mainly near the cloud tops, sometimes accompanied by precipitation near the cloud base. The second type of precipitation was more frequently observed during the experiment. Only 42–44 % of the clouds sampled were non-precipitating throughout the entire cloud layer and the rest of the clouds showed precipitation somewhere in the cloud, predominantly closer to the cloud top.


2015 ◽  
Vol 72 (8) ◽  
pp. 3051-3072 ◽  
Author(s):  
Campbell D. Watson ◽  
Ronald B. Smith ◽  
Alison D. Nugent

Abstract A sharp reduction in precipitation was observed on the island of Dominica in the Caribbean during a 2011 field campaign when the trade winds weakened and convection transitioned from mechanically to thermally driven. The authors propose four hypotheses for this reduction, which relate to (i) the triggering mechanism, (ii) dry-air entrainment, (iii) giant sea-salt aerosol, and (iv) small-island-derived aerosol. The plausibility of the first three hypotheses is the focus of this study. Aircraft observations show the dynamics of the orographic cumulus clouds at flight level are surprisingly similar, irrespective of how they are triggered. However, the orographic cumulus clouds are consistently shallower when the trade winds are weak, which the authors attribute to a drier and shallower cloud layer compared to days with stronger trade winds. The strong negative influence of dry-air entrainment in a drier environment on cumulus depth and liquid water content is qualitatively demonstrated using an entraining plume model and the WRF Model. Although the models appear more sensitive than observations to entrainment and cloud size, the sensitivity tests have some resemblance to observations. The authors also find evidence of sea-salt aerosol entering the base of marine cumulus on strong wind days using an aircraft-mounted lidar and other instruments. Although each hypothesis is plausible, the complex interplay of these processes makes determining the controlling mechanisms difficult. Ultimately, the authors’ analysis rejects the hypothesis (i) triggering, while supporting (ii) entrainment and (iii) sea-salt aerosol.


2016 ◽  
Author(s):  
Eunsil Jung ◽  
Bruce A. Albrecht ◽  
Graham Feingold ◽  
Haflidi H. Jonsson ◽  
Patrick Chuang ◽  
...  

Abstract. Shallow marine cumulus clouds are by far the most frequently observed cloud type over the Earth's oceans; but they are poorly understood and have not been investigated as extensively as stratocumulus clouds. This study describes and discusses the properties and variations of aerosol, cloud, and precipitation associated with shallow marine cumulus clouds observed in the North-Atlantic trades during a field campaign (Barbados Aerosol Cloud Experiment- BACEX, March–April, 2010), which took place off of Barbados where African dust periodically affects the region. The principal observing platform was the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter (TO) research aircraft, which was equipped with standard meteorological instruments, a zenith pointing cloud radar and probes that measured aerosol, cloud, and precipitation characteristics. The temporal variation and vertical distribution of aerosols observed from the 15 flights, which included the most intense African dust event during all of 2010 at Barbados, showed a wide range of aerosol conditions. During dusty periods, aerosol concentrations increased substantially in the size range between 0.5 μm and 10 μm (diameter), particles that large enough to be effective giant cloud condensation nuclei (CCN). The 10-day back trajectories showed three distinct air masses with distinct vertical structures associated with air masses originating in the Atlantic (typical maritime air mass with relatively low aerosol concentrations in the marine boundary layer), Africa (Saharan Air Layer), and mid-latitudes (continental pollution plumes). Despite the large differences in the total mass loading and the origin of the aerosols, the overall shapes of the aerosol particle size distributions were consistent, with the exception of the transition period. The TO was able to sample many clouds at various phases of growth. Maximum cloud depth observed was less than ~ 3 km, while most clouds were less than 1 km deep. Clouds tend to precipitate when the cloud is thicker than 500–600 m. Distributions of cloud field characteristics (depth, radar reflectivity, Doppler velocity, precipitation) were well identified in the reflectivity-velocity diagram from the cloud radar observations. Two types of precipitation features were observed for shallow marine cumulus clouds that may impact boundary layer differently: first, a classic cloud-base precipitation where precipitation shafts were observed to emanate from the cloud base; second, cloud-top precipitation where precipitation shafts emanated mainly near the cloud tops, sometimes accompanied by precipitation near the cloud base. The second type of precipitation was more frequently observed during the experiment. Only 42–44 % of the clouds sampled were non-precipitating throughout the entire cloud layer and the rest of clouds showed precipitation somewhere in the cloud, predominantly closer to the cloud top.


2020 ◽  
Author(s):  
Claudia Acquistapace ◽  
Tobias Boeck

<p>Trade wind cumulus clouds play a vital role in the Earth's radiation budget and produce up to 20% of the total precipitation in the tropics. However, we still don't know how they will respond to global warming. Precipitation from trade wind cumuli can alter cloud macroscopic properties and the boundary layer structure and dynamics.</p><p>Precipitation development in models is very uncertain, being dependent on simulation setup and microphysics. In particular, the autoconversion scheme dramatically affects precipitation flux, cloud structure, and organization. Currently, no evaluations of the different autoconversion schemes with observations reduced the uncertainties in rain processes. Precipitation can impact convection organization and circulation intensity with massive effects on climate sensitivity and its evaporation determines the intensity of cold pools and influences the cloud field organization. It is hence key to quantify evaporation rates and their spatiotemporal variability. Parametrizations of evaporation below cloud base are available but strongly depend on the drop size distribution of raindrops. Also, in the observations, evaporation rates are hard to observe directly.</p><p>Here, we would like to present the potential given by the observations collected on the Maria S. Merian ship during the EUREC4A campaign to estimate evaporation rates and provide advanced multi-sensor observations of rain onset and development. The synergy of multiple in-situ and remote-sensing from the ship as well as aircraft observations available will allow to constrain the autoconversion scheme in LES models and reduce the uncertainties connected to rain processes. Moreover, quantification of evaporation rates will clarify the role of precipitation in moisturizing the boundary layer in trade wind regions.</p><p> </p>


2016 ◽  
Vol 144 (2) ◽  
pp. 681-701 ◽  
Author(s):  
Virendra P. Ghate ◽  
Mark A. Miller ◽  
Ping Zhu

Abstract Marine nonprecipitating cumulus topped boundary layers (CTBLs) observed in a tropical and in a trade wind region are contrasted based on their cloud macrophysical, dynamical, and radiative structures. Data from the Atmospheric Radiation Measurement (ARM) observational site previously operating at Manus Island, Papua New Guinea, and data collected during the deployment of ARM Mobile Facility at the island of Graciosa, in the Azores, were used in this study. The tropical marine CTBLs were deeper, had higher surface fluxes and boundary layer radiative cooling, but lower wind speeds compared to their trade wind counterparts. The radiative velocity scale was 50%–70% of the surface convective velocity scale at both locations, highlighting the prominent role played by radiation in maintaining turbulence in marine CTBLs. Despite greater thicknesses, the chord lengths of tropical cumuli were on average lower than those of trade wind cumuli, and as a result of lower cloud cover, the hourly averaged (cloudy and clear) liquid water paths of tropical cumuli were lower than the trade wind cumuli. At both locations ~70% of the cloudy profiles were updrafts, while the average amount of updrafts near cloud base stronger than 1 m s−1 was ~22% in tropical cumuli and ~12% in the trade wind cumuli. The mean in-cloud radar reflectivity within updrafts and mean updraft velocity was higher in tropical cumuli than the trade wind cumuli. Despite stronger vertical velocities and a higher number of strong updrafts, due to lower cloud fraction, the updraft mass flux was lower in the tropical cumuli compared to the trade wind cumuli. The observations suggest that the tropical and trade wind marine cumulus clouds differ significantly in their macrophysical and dynamical structures.


2005 ◽  
Vol 5 (5) ◽  
pp. 8811-8849
Author(s):  
J. Vilà-Guerau de Arellano ◽  
S.-W. Kim ◽  
M. C. Barth ◽  
E. G. Patton

Abstract. The distribution and evolution of reactive species in a boundary layer characterized by the presence of shallow cumulus over land is studied by means of two large-eddy simulation models: the NCAR and WUR codes. The study focuses on two physical processes that can influence the chemistry: the enhancement of the vertical transport by the buoyant convection associated with cloud formation and the perturbation of the photolysis rates below, in and above the clouds. It is shown that the dilution of the reactant mixing ratio caused by the deepening of the atmospheric boundary layer is an important process and that it can decrease reactant mixing ratios by 10 to 50 percent compared to very similar conditions but with no cloud formation. Additionally, clouds transport chemical species to higher elevations in the boundary layer compared to the case with no clouds which influences the reactant mixing ratios of the nocturnal residual layers following the collapse of the daytime boundary layer. Estimates of the rate of reactant transport based on the calculation of the integrated flux divergence range from to −0.2 ppb hr−1 to −1 ppb hr−1, indicating a net loss of sub-cloud layer air transported into the cloud layer. A comparison of this flux to a parameterized mass flux shows good agreement in mid-cloud, but at cloud base the parameterization underestimates the mass flux. Scattering of radiation by cloud drops perturbs photolysis rates. It is found that these perturbed photolysis rates substantially (10–40%) affect mixing ratios locally (spatially and temporally), but have little effect on mixing ratios averaged over space and time. We find that the ultraviolet radiance perturbation becomes more important for chemical transformations that react with a similar order time scale as the turbulent transport in clouds. Finally, the detailed intercomparison of the LES results shows very good agreement between the two codes when considering the evolution of the reactant mean, flux and (co-)variance vertical profiles.


2003 ◽  
Vol 3 (3) ◽  
pp. 2963-3050 ◽  
Author(s):  
R. Sander ◽  
W. C. Keene ◽  
A. A. P. Pszenny ◽  
R. Arimoto ◽  
G. P. Ayers ◽  
...  

Abstract. The cycling of inorganic bromine in the marine boundary layer (mbl) has received increased attention in recent years. Bromide, a constituent of sea water, is injected into the atmosphere in association with sea-salt aerosol by breaking waves on the ocean surface. Measurements reveal that supermicrometer sea-salt aerosol is depleted in bromine by about 50% relative to conservative tracers, whereas marine submicrometer aerosol is often enriched in bromine. Model calculations, laboratory studies, and field observations strongly suggest that these depletions reflect the chemical transformation of particulate bromide to reactive inorganic gases that influence the processing of ozone and other important constituents of marine air. However, currently available techniques cannot reliably quantify many \\chem{Br}-containing compounds at ambient concentrations and, consequently, our understanding of inorganic Br cycling over the oceans and its global significance are uncertain. To provide a more coherent framework for future research, we have reviewed measurements in marine aerosol, the gas phase, and in rain. We also summarize sources and sinks, as well as model and laboratory studies of chemical transformations. The focus is on inorganic bromine over the open oceans, excluding the polar regions. The generation of sea-salt aerosol at the ocean surface is the major tropospheric source producing about 6.2 Tg/a of bromide. The transport of  Br from continents (as mineral aerosol, and as products from biomass-burning and fossil-fuel combustion) can be of local importance. Transport of degradation products of long-lived Br-containing compounds from the stratosphere and other sources contribute lesser amounts. Available evidence suggests that, following aerosol acidification, sea-salt bromide reacts to form Br2 and BrCl that volatilize to the gas phase and photolyze in daylight to produce atomic Br and Cl. Subsequent transformations can destroy tropospheric ozone, oxidize dimethylsulfide (DMS) and hydrocarbons in the gas phase and S(IV) in aerosol solutions, and thereby potentially influence climate. The diurnal cycle of gas-phase \\Br and the corresponding particulate Br deficits are correlated. Higher values of Br in the gas phase during daytime are consistent with expectations based on photochemistry. Mechanisms that explain the widely reported accumulation of particulate Br in submicrometer aerosols are not yet understood. We expect that the importance of inorganic Br cycling will vary in the future as a function of both increasing acidification of the atmosphere (through anthropogenic emissions) and climate changes. The latter affects bromine cycling via meteorological factors including global wind fields (and the associated production of sea-salt aerosol), temperature, and relative humidity.


2018 ◽  
Vol 75 (7) ◽  
pp. 2235-2255 ◽  
Author(s):  
Neil P. Lareau ◽  
Yunyan Zhang ◽  
Stephen A. Klein

Abstract The boundary layer controls on shallow cumulus (ShCu) convection are examined using a suite of remote and in situ sensors at ARM Southern Great Plains (SGP). A key instrument in the study is a Doppler lidar that measures vertical velocity in the CBL and along cloud base. Using a sample of 138 ShCu days, the composite structure of the ShCu CBL is examined, revealing increased vertical velocity (VV) variance during periods of medium cloud cover and higher VV skewness on ShCu days than on clear-sky days. The subcloud circulations of 1791 individual cumuli are also examined. From these data, we show that cloud-base updrafts, normalized by convective velocity, vary as a function of updraft width normalized by CBL depth. It is also found that 63% of clouds have positive cloud-base mass flux and are linked to coherent updrafts extending over the depth of the CBL. In contrast, negative mass flux clouds lack coherent subcloud updrafts. Both sets of clouds possess narrow downdrafts extending from the cloud edges into the subcloud layer. These downdrafts are also present adjacent to cloud-free updrafts, suggesting they are mechanical in origin. The cloud-base updraft data are subsequently combined with observations of convective inhibition to form dimensionless “cloud inhibition” (CI) parameters. Updraft fraction and liquid water path are shown to vary inversely with CI, a finding consistent with CIN-based closures used in convective parameterizations. However, we also demonstrate a limited link between CBL vertical velocity variance and cloud-base updrafts, suggesting that additional factors, including updraft width, are necessary predictors for cloud-base updrafts.


2020 ◽  
Author(s):  
Marcus Klingebiel ◽  
Heike Konow ◽  
Bjorn Stevens

<p>Mass flux is a key parameter to represent shallow convection in global circulation models. To estimate the shallow convective mass flux as accurately as possible, observations of this parameter are necessary. Prior studies from Ghate et al. (2011) and Lamer et al. (2015) used Doppler radar measurements over a few months to identify a typical shallow convective mass flux profile based on cloud fraction and vertical velocity. In this study, we extend their observations by using long term remote sensing measurements at the Barbados Cloud Observatory (13° 09’ N, 59° 25’ W) over a time period of 30 months and check a hypothesis by Grant (2001), who proposed that the cloud base mass flux is just proportional to the sub-cloud convective velocity scale. Therefore, we analyze Doppler radar and Doppler lidar measurements to identify the variation of the vertical velocity in the cloud and sub-cloud layer, respectively. Furthermore, we show that the in-cloud mass flux is mainly influenced by the cloud fraction and provide a linear equation, which can be used to roughly calculate the mass flux in the trade wind region based on the cloud fraction.</p><p> </p><p>References:<br>Ghate,  V.  P.,  M.  A.  Miller,  and  L.  DiPretore,  2011:   Vertical  velocity structure of marine boundary layer trade wind cumulus clouds. Journal  of  Geophysical  Research: Atmospheres, 116  (D16), doi:10.1029/2010JD015344.</p><p>Grant,  A.  L.  M.,  2001:   Cloud-base  fluxes  in  the  cumulus-capped boundary layer. Quarterly Journal of the Royal Meteorological Society, 127 (572), 407–421, doi:10.1002/qj.49712757209.</p><p>Lamer, K., P. Kollias, and L. Nuijens, 2015:  Observations of the variability  of  shallow  trade  wind  cumulus  cloudiness  and  mass  flux. Journal of Geophysical Research: Atmospheres, 120  (12), 6161–6178, doi:10.1002/2014JD022950.</p>


2005 ◽  
Vol 62 (6) ◽  
pp. 1976-1988 ◽  
Author(s):  
Larry K. Berg ◽  
Roland B. Stull

Abstract A new parameterization for boundary layer cumulus clouds, called the cumulus potential (CuP) scheme, is introduced. This scheme uses joint probability density functions (JPDFs) of virtual potential temperature (θυ) and water-vapor mixing ratio (r), as well as the mean vertical profiles of θυ, to predict the amount and size distribution of boundary layer cloud cover. This model considers the diversity of air parcels over a heterogeneous surface, and recognizes that some parcels rise above their lifting condensation level to become cumulus, while other parcels might rise as noncloud updrafts. This model has several unique features: 1) cloud cover is determined from the boundary layer JPDF of θυ versus r, 2) clear and cloudy thermals are allowed to coexist at the same altitude, and 3) a range of cloud-base heights, cloud-top heights, and cloud thicknesses are predicted within any one cloud field, as observed. Using data from Boundary Layer Experiment 1996 and a model intercomparsion study using large eddy simulation (LES) based on Barbados Oceanographic and Meteorological Experiment (BOMEX), it is shown that the CuP model does a good job predicting cloud-base height and cloud-top height. The model also shows promise in predicting cloud cover, and is found to give better cloud-cover estimates than three other cumulus parameterizations: one based on relative humidity, a statistical scheme based on the saturation deficit, and a slab model.


Sign in / Sign up

Export Citation Format

Share Document