Observational study of the relationship between entrainment rate and relative dispersion in deep convective clouds

TitleObservational study of the relationship between entrainment rate and relative dispersion in deep convective clouds
Publication TypeJournal Article
Year of Publication2018
AuthorsGuo X.H, Lu C.S, Zhao T.L, Liu Y.G, Zhang GJ, Luo S.
JournalAtmospheric Research
Volume199
Pagination186-192
Date Published2018/01
Type of ArticleArticle
ISBN Number0169-8095
Accession NumberWOS:000415908300014
Keywordsaircraft measurements; autoconversion; Cumulus clouds; droplet size distributions; effective radius; parameterization; spectral dispersion; stratocumulus clouds; threshold behavior; vertical velocity
Abstract

This study investigates the influence of entrainment rate (lambda) on relative dispersion (epsilon) of cloud droplet size distributions (CDSD) in the 99 growing precipitating deep convective clouds during TOGA-COARE. The results show that entrainment suppresses epsilon, which is opposite to the traditional understanding that entrainment-mixing broadens CDSD. To examine how the relationship between epsilon and lambda is affected by droplets with different sizes, CDSDs are divided into three portions with droplet radius < 3.75 mu m (N-1), radius in the range of 3.75-12.75 mu m (N-2) and 12.75-23.25 mu m (N-3), respectively. The results indicate that although the droplet concentration at different sizes generally decrease simultaneously as lambda increases, the variation of standard deviation (sigma) depends mainly on N-3, while the mean radius (r(m)) decreases with decreasing N-3, but increases with decreasing N-1. So the influence of entrainment on CDSD causes a more dramatical decrease in a than that in r(m), and further leads to the decrease of a as entrainment enhances. In addition, a conceptual model of CDSD evolution during entrainment mixing processes is developed to illustrate the possible scenarios entailing different relationships between a and lambda. The number concentration of small droplets and the degree of evaporation of small droplets are found to be key factors that shift the sign (i.e., positive or negative) of the epsilon-lambda relationship.

DOI10.1016/j.atmosres.2017.09.013
Short TitleAtmos. Res.
Student Publication: 
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