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Title: Influences of drizzle on stratocumulus cloudiness and organization [Influences of drizzle on cloudiness and stratocumulus organization]

Abstract

Large-eddy simulations are used to study the influence of drizzle on stratocumulus organization, based on measurements made as part of the Second Dynamics and Chemistry of the Marine Stratocumulus field study-II. Cloud droplet number concentration ( N c) is prescribed and considered as the proxy for different aerosol loadings. Our study shows that the amount of cloudiness does not decrease linearly with precipitation rate. An N c threshold is observed below which the removal of cloud water via precipitation efficiently reduces cloud depth, allowing evaporation to become efficient and quickly remove the remaining thin clouds, facilitating a fast transition from closed cells to open cells. Using Fourier analysis, stratocumulus length scales are found to increase with drizzle rates. Raindrop evaporation below 300 m lowers the cloud bases and amplifies moisture variances in the subcloud layer, while it does not alter the horizontal scales in the cloud layer, suggesting that moist cold pool dynamic forcings are not essential for mesoscale organization of stratocumulus. Furthermore, the cloud scales are greatly increased when the boundary layer is too deep to maintain well mixed.

Authors:
ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [3]
  1. McGill Univ., Montreal, QC (Canada)
  2. Cleveland State Univ., Cleveland, OH (United States)
  3. State Univ. of New York at Stony Brook, Stony Brook, NY (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
OSTI Identifier:
1405931
Report Number(s):
BNL-114420-2017-JA
Journal ID: ISSN 2169-897X; R&D Project: 2019‐BNL-EE630EECA-Budg; KP1701000; TRN: US1702900
Grant/Contract Number:
SC0012704; SC0013489
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Geophysical Research: Atmospheres
Additional Journal Information:
Journal Volume: 122; Journal Issue: 13; Journal ID: ISSN 2169-897X
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; marine stratocumulus; boundary layer; mesoscale organization; drizzle; large eddy simulation

Citation Formats

Zhou, Xiaoli, Heus, Thijs, and Kollias, Pavlos. Influences of drizzle on stratocumulus cloudiness and organization [Influences of drizzle on cloudiness and stratocumulus organization]. United States: N. p., 2017. Web. doi:10.1002/2017JD026641.
Zhou, Xiaoli, Heus, Thijs, & Kollias, Pavlos. Influences of drizzle on stratocumulus cloudiness and organization [Influences of drizzle on cloudiness and stratocumulus organization]. United States. doi:10.1002/2017JD026641.
Zhou, Xiaoli, Heus, Thijs, and Kollias, Pavlos. 2017. "Influences of drizzle on stratocumulus cloudiness and organization [Influences of drizzle on cloudiness and stratocumulus organization]". United States. doi:10.1002/2017JD026641.
@article{osti_1405931,
title = {Influences of drizzle on stratocumulus cloudiness and organization [Influences of drizzle on cloudiness and stratocumulus organization]},
author = {Zhou, Xiaoli and Heus, Thijs and Kollias, Pavlos},
abstractNote = {Large-eddy simulations are used to study the influence of drizzle on stratocumulus organization, based on measurements made as part of the Second Dynamics and Chemistry of the Marine Stratocumulus field study-II. Cloud droplet number concentration (Nc) is prescribed and considered as the proxy for different aerosol loadings. Our study shows that the amount of cloudiness does not decrease linearly with precipitation rate. An Nc threshold is observed below which the removal of cloud water via precipitation efficiently reduces cloud depth, allowing evaporation to become efficient and quickly remove the remaining thin clouds, facilitating a fast transition from closed cells to open cells. Using Fourier analysis, stratocumulus length scales are found to increase with drizzle rates. Raindrop evaporation below 300 m lowers the cloud bases and amplifies moisture variances in the subcloud layer, while it does not alter the horizontal scales in the cloud layer, suggesting that moist cold pool dynamic forcings are not essential for mesoscale organization of stratocumulus. Furthermore, the cloud scales are greatly increased when the boundary layer is too deep to maintain well mixed.},
doi = {10.1002/2017JD026641},
journal = {Journal of Geophysical Research: Atmospheres},
number = 13,
volume = 122,
place = {United States},
year = 2017,
month = 6
}

Journal Article:
Free Publicly Available Full Text
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  • This paper considers the production of drizzle in stratocumulus clouds in relation to the boundary-layer turbulent kinetic energy and in-cloud residence times. It is shown that drizzle production in stratocumulus of the order of 400 m in depth is intimately related to the verticle velocity structure of the cloud eddies. In a series of two-dimensional numerical experiments with fixed cloud condensation nucleus concentrations, the effect on drizzle production of enhanced or diminished verticle velocities is simulated. Rather than do this by simulating clouds exhibiting more or less energy, the authors modify drop terminal velocities in a manner that conserves themore » fall velocity relative to the air motions and allows droplet growth to occur in a similar dynamical environment. The results suggest that more vigorous clouds produce more drizzle because they enable longer in-cloud dwell times and therefore prolonged collision-coalescence. In weaker clouds, droplets tend to fall out of the cloud before they have achieved significant size, resulting in smaller amounts of drizzle. In another series of experiments, the authors investigate the effects of the feedback of drizzle on the boundary-layer dynamics. Results show that when significant amounts of drizzle reach the surface, the subcloud layer is stabilized, circulations are weaker, and the boundary layer is not well mixed. When only small amounts of drizzle are produced, cooling tends to be confined to the region just below cloud base, resulting in destabilization, more vigorous circulations, and a better mixed boundary layer. The results strongly suggest that a characteristic time associated with collision-coalescence be incorporated into drizzle parameterizations. 40 refs., 12 figs., 1 tab.« less
  • 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 andmore » 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. Lastly, 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.« less
  • Here, a case study of persistent stratocumulus over the Azores is simulated using two independent large-eddy simulation (LES) models with bin microphysics, and forward-simulated cloud radar Doppler moments and spectra are compared with observations. Neither model is able to reproduce the monotonic increase of downward mean Doppler velocity with increasing reflectivity that is observed under a variety of conditions, but for differing reasons. To a varying degree, both models also exhibit a tendency to produce too many of the largest droplets, leading to excessive skewness in Doppler velocity distributions, especially below cloud base. Excessive skewness appears to be associated withmore » an insufficiently sharp reduction in droplet number concentration at diameters larger than ~200 μm, where a pronounced shoulder is found for in situ observations and a sharp reduction in reflectivity size distribution is associated with relatively narrow observed Doppler spectra. Effectively using LES with bin microphysics to study drizzle formation and evolution in cloud Doppler radar data evidently requires reducing numerical diffusivity in the treatment of the stochastic collection equation; if that is accomplished sufficiently to reproduce typical spectra, progress toward understanding drizzle processes is likely.« less