Influences of drizzle on stratocumulus cloudiness and organization. Issue 13 (3rd July 2017)

• Record Type: Journal Article
• Authors: Zhou, Xiaoli; Heus, Thijs; Kollias, Pavlos
• Is Part Of: Journal of geophysical research. Volume 122:Issue 13(2017)
• Page Start: 6989
• Page End: 7003
• Publication Date: 2017-07-03
• Subjects: marine stratocumulus -- boundary layer -- mesoscale organization -- drizzle -- large eddy simulation; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/2017JD026641
• Languages: English
• ISSNs: 2169-897X

Abstract

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. The cloud scales are greatly increased when the boundary layer is too deep to maintain well mixed. Key Points: Raindrop evaporation below 300 m amplifies subcloud variances and forms cumulus‐like clouds below stratocumulus, while it does not alter the mesoscale organization of cloud layer; no significant moist cold pool dynamic forcing is observed An N c threshold is observed below which removal of cloud water via precipitation efficiently reduces cloud depth, allowing evaporation to become efficient and quickly remove the remaining thin clouds, leading to a fast transition from closed cells to open cells Mesoscale organization on an insufficient scale biases the mean properties toward less cloud water as a result of additional entrainment via forced circulations