Vertical dependence of horizontal variation of cloud microphysics: observations from the ACE-ENA field campaign and implications for warm-rain simulation in climate models

Zhang, Zhibo; Song, Qianqian; Mechem, David B.; Larson, Vincent E.; Wang, Jian; Liu, Yangang; Witte, Mikael K.; Dong, Xiquan; Wu, Peng

In the current global climate models (GCMs), the nonlinearity effect of subgrid cloud variations on the parameterization of warm-rain process, e.g., the autoconversion rate, is often treated by multiplying the resolved-scale warm-rain process rates by a so-called enhancement factor (EF). In this study, we investigate the subgrid-scale horizontal variations and covariation of cloud water content (inline-formulaqc) and cloud droplet number concentration (inline-formulaNc) in marine boundary layer (MBL) clouds based on the in situ measurements from a recent field campaign and study the implications for the autoconversion rate EF in GCMs. Based on a few carefully selected cases from the field campaign, we found that in contrast to the enhancing effect of inline-formulaqc and inline-formulaNc variations that tends to make EF inline-formula> 1, the strong positive correlation between inline-formulaqc and inline-formulaNc results in a suppressing effect that tends to make EF inline-formula< 1. This effect is especially strong at cloud top, where the inline-formulaqc and inline-formulaNc correlation can be as high as 0.95. We also found that the physically complete EF that accounts for the covariation of inline-formulaqc and inline-formulaNc is significantly smaller than its counterpart that accounts only for the subgrid variation of inline-formulaqc, especially at cloud top. Although this study is based on limited cases, it suggests that the subgrid variations of inline-formulaNc and its correlation with inline-formulaqc both need to be considered for an accurate simulation of the autoconversion process in GCMs.

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Zhang, Zhibo / Song, Qianqian / Mechem, David B. / et al: Vertical dependence of horizontal variation of cloud microphysics: observations from the ACE-ENA field campaign and implications for warm-rain simulation in climate models. 2021. Copernicus Publications.

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