scholarly journals Drizzle formation in stratocumulus clouds: effects of turbulent mixing

2015 ◽  
Vol 15 (17) ◽  
pp. 24131-24177 ◽  
Author(s):  
L. Magaritz-Ronen ◽  
M. Pinsky ◽  
A. Khain
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Liquid Water ◽  
Turbulent Mixing ◽  
Liquid Water Content ◽  
High Humidity ◽  
Air Volume ◽  
Stratocumulus Clouds ◽  
Microphysical Processes ◽  
Further Development

Abstract. The mechanism of drizzle formation in shallow stratocumulus clouds and the effect of turbulent mixing on this process are investigated. A Lagrangian-Eularian model of the cloud-topped boundary layer is used to simulate the cloud measured during flight RF07 of the DYCOMS-II field experiment. The model contains ~ 2000 air parcels that are advected in a turbulence-like velocity field. In the model all microphysical processes are described for each Lagrangian air volume, and turbulent mixing between the parcels is also taken into account. It was found that the first large drops form in air volumes that are closest to adiabatic and characterized by high humidity, extended residence near cloud top, and maximum values of liquid water content, allowing the formation of drops as a result of efficient collisions. The first large drops form near cloud top and initiate drizzle formation in the cloud. Drizzle is developed only when turbulent mixing of parcels is included in the model. Without mixing, the cloud structure is extremely inhomogeneous and the few large drops that do form in the cloud evaporate during their sedimentation. It was found that turbulent mixing can delay the process of drizzle initiation but is essential for the further development of drizzle in the cloud.

scholarly journals Drizzle formation in stratocumulus clouds: effects of turbulent mixing

2016 ◽  
Vol 16 (3) ◽  
pp. 1849-1862 ◽  
Author(s):  
L. Magaritz-Ronen ◽  
M. Pinsky ◽  
A. Khain
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Liquid Water ◽  
Turbulent Mixing ◽  
Liquid Water Content ◽  
High Humidity ◽  
Air Volume ◽  
Stratocumulus Clouds ◽  
Microphysical Processes ◽  
Further Development

Abstract. The mechanism of drizzle formation in shallow stratocumulus clouds and the effect of turbulent mixing on this process are investigated. A Lagrangian–Eularian model of the cloud-topped boundary layer is used to simulate the cloud measured during flight RF07 of the DYCOMS-II field experiment. The model contains ~ 2000 air parcels that are advected in a turbulence-like velocity field. In the model all microphysical processes are described for each Lagrangian air volume, and turbulent mixing between the parcels is also taken into account. It was found that the first large drops form in air volumes that are closest to adiabatic and characterized by high humidity, extended residence near cloud top, and maximum values of liquid water content, allowing the formation of drops as a result of efficient collisions. The first large drops form near cloud top and initiate drizzle formation in the cloud. Drizzle is developed only when turbulent mixing of parcels is included in the model. Without mixing, the cloud structure is extremely inhomogeneous and the few large drops that do form in the cloud evaporate during their sedimentation. It was found that turbulent mixing can delay the process of drizzle initiation but is essential for the further development of drizzle in the cloud.

scholarly journals Reflectivity and Liquid Water Content Vertical Decomposition Diagrams to Diagnose Vertical Evolution of Raindrop Size Distributions

2016 ◽  
Vol 33 (3) ◽  
pp. 579-595 ◽  
Author(s):  
Christopher R. Williams
Keyword(s):  
Water Content ◽  
Liquid Water ◽  
Liquid Water Content ◽  
Doppler Velocity ◽  
Size Distributions ◽  
Vertical Column ◽  
Vertical Decomposition ◽  
Air Motion ◽  
Raindrop Size ◽  
Microphysical Processes

AbstractThis study consists of two parts. The first part describes the way in which vertical air motions and raindrop size distributions (DSDs) were retrieved from 449-MHz and 2.835-GHz (UHF and S band) vertically pointing radars (VPRs) deployed side by side during the Midlatitude Continental Convective Clouds Experiment (MC3E) held in northern Oklahoma. The 449-MHz VPR can measure both vertical air motion and raindrop motion. The S-band VPR can measure only raindrop motion. These differences in VPR sensitivities facilitates the identification of two peaks in 449-MHz VPR reflectivity-weighted Doppler velocity spectra and the retrieval of vertical air motion and DSD parameters from near the surface to just below the melting layer.The second part of this study used the retrieved DSD parameters to decompose reflectivity and liquid water content (LWC) into two terms, one representing number concentration and the other representing DSD shape. Reflectivity and LWC vertical decomposition diagrams (Z-VDDs and LWC-VDDs, respectively) are introduced to highlight interactions between raindrop number and DSD shape in the vertical column. Analysis of Z-VDDs provides indirect measure of microphysical processes through radar reflectivity. Analysis of LWC-VDDs provides direct investigation of microphysical processes in the vertical column, including net raindrop evaporation or accretion and net raindrop breakup or coalescence. During a stratiform rain event (20 May 2011), LWC-VDDs exhibited signatures of net evaporation and net raindrop coalescence as the raindrops fell a distance of 2 km under a well-defined radar bright band. The LWC-VDD is a tool to characterize rain microphysics with quantities related to number-controlled and size-controlled processes.

scholarly journals The Spiderweb Structure of Stratocumulus Clouds

Atmosphere ◽  
2020 ◽  
Vol 11 (7) ◽  
pp. 730
Author(s):  
Georgios Matheou ◽  
Anthony B. Davis ◽  
João Teixeira
Keyword(s):  
Liquid Water ◽  
Evaporative Cooling ◽  
Liquid Water Content ◽  
Entrainment Rate ◽  
Liquid Water Path ◽  
Integral Boundary ◽  
Water Path ◽  
Large Eddy ◽  
Stratocumulus Clouds ◽  
Distinctive Structure

Stratocumulus clouds have a distinctive structure composed of a combination of lumpy cellular structures and thin elongated regions, resembling canyons or slits. The elongated slits are referred to as “spiderweb” structure to emphasize their interconnected nature. Using very high resolution large-eddy simulations (LES), it is shown that the spiderweb structure is generated by cloud-top evaporative cooling. Analysis of liquid water path (LWP) and cloud liquid water content shows that cloud-top evaporative cooling generates relatively shallow slits near the cloud top. Most of liquid water mass is concentrated near the cloud top, thus cloud-top slits of clear air have a large impact on the entire-column LWP. When evaporative cooling is suppressed in the LES, LWP exhibits cellular lumpy structure without the elongated low-LWP regions. Even though the spiderweb signature on the LWP distribution is negligible, the cloud-top evaporative cooling process significantly affects integral boundary layer quantities, such as the vertically integrated turbulent kinetic energy, mean liquid water path, and entrainment rate. In a pair of simulations driven only by cloud-top radiative cooling, evaporative cooling nearly doubles the entrainment rate.

Turbulent mixing at the top of stratocumulus clouds

1963 ◽  
Vol 17 (2) ◽  
pp. 212-224 ◽  
Author(s):  
J. S. Turner ◽  
I. K. Yang
Keyword(s):  
Liquid Water ◽  
Turbulent Mixing ◽  
Model Experiment ◽  
Small Decrease ◽  
Mixing Rate ◽  
Turbulent Region ◽  
Laboratory Results ◽  
Non Linear ◽  
Stratocumulus Clouds ◽  
New Feature

The process of mixing at the top of a turbulent cloud layer contains a new feature which has not been considered in previous studies of mixing; evaporation of liquid water can cause density changes which may affect the dynamics. A model experiment has been devised to study this problem, using liquids whose density behaviour is non-linear to simulate evaporation.The existence of a moist, stable, turbulent region above cloudtop can be explained using the laboratory results, which suggest that this region can be regarded dynamically as part of the cloud. Comparison of the rates of mixing in the model experiments with and without ‘evaporation’ suggests that evaporation could cause a small decrease in the mixing rate for a given density difference, but the change would be negligible in practice. This result also sheds some light on the mechanism of mixing, in both the linear and non-linear cases.

Surface-based remote sensing of the observed and the Adiabatic liquid water content of stratocumulus clouds

1990 ◽  
Vol 17 (1) ◽  
pp. 89-92 ◽  
Author(s):  
Bruce A. Albrecht ◽  
Christopher W. Fairall ◽  
Dennis W. Thomson ◽  
Allen B. White ◽  
Jack B. Snider ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Water Content ◽  
Liquid Water ◽  
Liquid Water Content ◽  
Stratocumulus Clouds

Drizzle in Stratiform Boundary Layer Clouds. Part I: Vertical and Horizontal Structure

2005 ◽  
Vol 62 (9) ◽  
pp. 3011-3033 ◽  
Author(s):  
R. Wood
Keyword(s):  
Boundary Layer ◽  
Water Content ◽  
Liquid Water ◽  
Liquid Water Content ◽  
Precipitation Rate ◽  
Cloud Base ◽  
Specific Reference ◽  
Horizontal Structure ◽  
Boundary Layer Clouds ◽  
Droplet Concentration

Abstract Detailed observations of stratiform boundary layer clouds on 12 days are examined with specific reference to drizzle formation processes. The clouds differ considerably in mean thickness, liquid water path (LWP), and droplet concentration. Cloud-base precipitation rates differ by a factor of 20 between cases. The lowest precipitation rate is found in the case with the highest droplet concentration even though this case had by far the highest LWP, suggesting that drizzle can be severely suppressed in polluted clouds. The vertical and horizontal structure of cloud and drizzle liquid water and bulk microphysical parameters are examined in detail. In general, the highest concentration of r > 20 μm drizzle drops is found toward the top of the cloud, and the mean volume radius of the drizzle drops increases monotonically from cloud top to base. The resulting precipitation rates are largest at the cloud base but decrease markedly only in the upper third of the cloud. Below cloud, precipitation rates decrease markedly with distance below base due to evaporation, and are broadly consistent in most cases with the results from a simple sedimentation–evaporation model. Evidence is presented that suggests evaporating drizzle is cooling regions of the subcloud layer, which could result in dynamical feedbacks. A composite power spectrum of the horizontal spatial series of precipitation rate is found to exhibit a power-law scaling from the smallest observable scales to close to the maximum observable scale (∼30 km). The exponent is considerably lower (1.1–1.2) than corresponding exponents for LWP variability obtained in other studies (∼1.5–2), demonstrating that there is relatively more variability of drizzle on small scales. Singular measures analysis shows that drizzle fields are much more intermittent than the cloud liquid water content fields, consistent with a drizzle production process that depends strongly upon liquid water content. The adiabaticity of the clouds, which can be modeled as a simple balance between drizzle loss and turbulent replenishment, is found to decrease if the time scale for drizzle loss is shorter than roughly 5–10 eddy turnover time scales. Finally, the data are compared with three simple scalings derived from recent observations of drizzle in subtropical stratocumulus clouds.

scholarly journals Combined Observational and Model Investigations of the Z–LWC Relationship in Stratocumulus Clouds

2008 ◽  
Vol 47 (2) ◽  
pp. 591-606 ◽  
Author(s):  
A. Khain ◽  
M. Pinsky ◽  
L. Magaritz ◽  
O. Krasnov ◽  
H. W. J. Russchenberg
Keyword(s):  
Velocity Field ◽  
Liquid Water Content ◽  
Radar Reflectivity ◽  
Cumulus Clouds ◽  
Entire Volume ◽  
Diffusion Growth ◽  
Wide Range ◽  
Retrieval Algorithms ◽  
Stratocumulus Clouds

Abstract In situ measurements indicate the complexity and nonunique character of radar reflectivity–liquid water content (Z–LWC) relationships in stratocumulus and cumulus clouds. Parameters of empirical (statistical) Z–LWC dependences vary within a wide range. Respectively, the accuracy of retrieval algorithms remains low. This situation is partially related to the fact that empirical algorithms and parameters are often derived without a corresponding understanding of physical mechanisms responsible for the formation of the Z–LWC diagrams. In this study, the authors investigate the processes of formation of the Z–LWC relationships using a new trajectory ensemble model of the cloud-topped boundary layer (BL). In the model, the entire volume of the BL is covered by Lagrangian parcels advected by a turbulent-like velocity field. The time-dependent velocity field is generated by a turbulent model and obeys the correlation turbulent laws. Each Lagrangian parcel represents the “cloud parcel model” with an accurate description of processes of diffusion growth–evaporation of aerosols and droplets and droplet collisions. The fact that parcels are adjacent to each other allows one to calculate sedimentation of droplets and precipitation (drizzle) formation. The characteristic parcel size is 50 m; the number of parcels is 1840. The model calculates droplet size distributions (DSDs), as well as their moments (e.g., aerosol and drop concentration, mass content, radar reflectivity) in each parcel. In the course of the model integration, Z–LWC relationships are calculated for each parcel, as well as the scattering diagram including all parcels. The model reproduces in situ observed types of the Z–LWC relationships. It is shown that different regimes represent different stages of cloud evolution: diffusion growth, beginning of drizzle formation, and stage of heavy drizzle, respectively. The large scattering of the Z–LWC relationships is found to be an inherent property of any drizzling cloud. Different zones on the Z–LWC diagram are related to cloud volumes located at different levels within a cloud and having different DSD. This finding allows for improvement of retrieval algorithms.

A PDF-Based Microphysics Parameterization for Simulation of Drizzling Boundary Layer Clouds

2009 ◽  
Vol 66 (8) ◽  
pp. 2317-2334 ◽  
Author(s):  
Anning Cheng ◽  
Kuan-Man Xu
Keyword(s):  
Boundary Layer ◽  
Liquid Water ◽  
Potential Temperature ◽  
Mixing Ratio ◽  
Total Water ◽  
Water Mixing ◽  
Mixing Ratios ◽  
Boundary Layer Clouds ◽  
Microphysical Processes ◽  
Analytical Expressions

Abstract Formulating the contribution of subgrid-scale (SGS) variability to microphysical processes in boundary layer and deep convective cloud parameterizations is a challenging task because of the complexity of microphysical processes and the lack of subgrid-scale information. In this study, a warm-rain microphysics parameterization that is based on a joint double-Gaussian distribution of vertical velocity, liquid water potential temperature, total water mixing ratio, and perturbation of rainwater mixing ratio is developed to simulate drizzling boundary layer clouds with a single column model (SCM). The probability distribution function (PDF) is assumed, but its parameters evolve according to equations that invoke higher-order turbulence closure. These parameters are determined from the first-, second-, and third-order moments and are then used to derive analytical expressions for autoconversion, collection, and evaporation rates. The analytical expressions show that correlation between rainwater and liquid water mixing ratios of the Gaussians enhances the collection rate whereas that between saturation deficit and rainwater mixing ratios of the Gaussians enhances the evaporation rate. Cases of drizzling shallow cumulus and stratocumulus are simulated with large-eddy simulation (LES) and SCM runs (SCM-CNTL and SCM-M): LES explicitly resolves SGS variability, SCM-CNTL parameterizes SGS variability with the PDF-based scheme, but SCM-M uses the grid-mean profiles to calculate the conversion rates of microphysical processes. SCM-CNTL can well reproduce the autoconversion, collection, and evaporation rates from LES. Comparisons between the two SCM experiments showed improvements in mean profiles of potential temperature, total water mixing ratio, liquid water, and cloud amount in the simulations considering SGS variability. A 3-week integration using the PDF-based microphysics scheme indicates that the scheme is stable for long-term simulations.

scholarly journals Observed correlations between aerosol and cloud properties in an Indian Ocean trade cumulus regime

2016 ◽  
Vol 16 (8) ◽  
pp. 5203-5227 ◽  
Author(s):  
Kristina Pistone ◽  
Puppala S. Praveen ◽  
Rick M. Thomas ◽  
Veerabhadran Ramanathan ◽  
Eric M. Wilcox ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Indian Ocean ◽  
Liquid Water ◽  
Radiative Forcing ◽  
Unmanned Aircraft ◽  
Cloud Base ◽  
High Humidity ◽  
Air Mass ◽  
High Pollution

Abstract. There are many contributing factors which determine the micro- and macrophysical properties of clouds, including atmospheric vertical structure, dominant meteorological conditions, and aerosol concentration, all of which may be coupled to one another. In the quest to determine aerosol effects on clouds, these potential relationships must be understood. Here we describe several observed correlations between aerosol conditions and cloud and atmospheric properties in the Indian Ocean winter monsoon season.In the CARDEX (Cloud, Aerosol, Radiative forcing, Dynamics EXperiment) field campaign conducted in February and March 2012 in the northern Indian Ocean, continuous measurements were made of atmospheric precipitable water vapor (PWV) and the liquid water path (LWP) of trade cumulus clouds, concurrent with measurements of water vapor flux, cloud and aerosol vertical profiles, meteorological data, and surface and total-column aerosol from instrumentation at a ground observatory and on small unmanned aircraft. We present observations which indicate a positive correlation between aerosol and cloud LWP only when considering cases with low atmospheric water vapor (PWV  < 40 kg m−2), a criterion which acts to filter the data to control for the natural meteorological variability in the region.We then use the aircraft and ground-based measurements to explore possible mechanisms behind this observed aerosol–LWP correlation. The increase in cloud liquid water is found to coincide with a lowering of the cloud base, which is itself attributable to increased boundary layer humidity in polluted conditions. High pollution is found to correlate with both higher temperatures and higher humidity measured throughout the boundary layer. A large-scale analysis, using satellite observations and meteorological reanalysis, corroborates these covariations: high-pollution cases are shown to originate as a highly polluted boundary layer air mass approaching the observatory from a northwesterly direction. The source air mass exhibits both higher temperatures and higher humidity in the polluted cases. While the warmer temperatures may be attributable to aerosol absorption of solar radiation over the subcontinent, the factors responsible for the coincident high humidity are less evident: the high-aerosol conditions are observed to disperse with air mass evolution, along with a weakening of the high-temperature anomaly, while the high-humidity condition is observed to strengthen in magnitude as the polluted air mass moves over the ocean toward the site of the CARDEX observations. Potential causal mechanisms of the observed correlations, including meteorological or aerosol-induced factors, are explored, though future research will be needed for a more complete and quantitative understanding of the aerosol–humidity relationship.

scholarly journals Vertically resolved concentration and liquid water content of atmospheric nanoparticles at the US DOE Southern Great Plains site

2017 ◽  
Author(s):  
Haihan Chen ◽  
Anna L. Hodshire ◽  
John Ortega ◽  
James Greenberg ◽  
Peter H. McMurry ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Water Content ◽  
Great Plains ◽  
Liquid Water ◽  
Ground Level ◽  
Liquid Water Content ◽  
Particle Formation ◽  
Water Soluble ◽  
New Particle Formation ◽  
Southern Great Plains

Abstract. Most prior field studies of new particle formation (NPF) have been performed at or near ground level, leaving many unanswered questions regarding the vertical extent of NPF. To address this, we measured concentrations of 11–16 nm diameter particles from ground level to 1000 m observed during the 2013 New Particle Formation Study at the Atmospheric Radiation Measurement Southern Great Plains site in Lamont, Oklahoma. The measurements were performed using a tethered balloon carrying two condensation particle counters that were configured for two different particle cut-off diameters. Those observations were compared to data from three scanning mobility particle sizers at the ground level. We observed that 11–16 nm diameter particles were generated at the top region of the boundary layer, and were then rapidly mixed throughout the boundary layer. We also estimate liquid water content of nanoparticles using ground-based measurements of particle hygroscopicity obtained with a Humidified Tandem Differential Mobility Analyzer and vertically resolved relative humidity (RH) and temperature measured with a Raman Lidar. Our analyses of these observations lead to the following conclusions regarding nanoparticles formed during NPF events at this site: (1) ground-based observations may not always accurately represent the timing, distribution, and meteorological conditions associated with the onset of NPF; (2) nanoparticles are highly hygroscopic, and typically contain up to 50 % water by volume, and during conditions of high RH combined with high particle hygroscopicity, particles can be up to 95 % water by volume; (3) increased liquid water content of nanoparticles at high RH greatly enhances the partitioning of water soluble species like organic acids into ambient nanoparticles.

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