Preprints
https://doi.org/10.5194/acp-2021-656
https://doi.org/10.5194/acp-2021-656

  06 Aug 2021

06 Aug 2021

Review status: this preprint is currently under review for the journal ACP.

Subgrid-scale Horizontal and Vertical Variations of Cloud Water in Stratocumulus Clouds: A case study based on LES and comparisons with in-situ observations

Justin A. Covert1, David B. Mechem1, and Zhibo Zhang2,3 Justin A. Covert et al.
  • 1Department of Geography and Atmospheric Science, University of Kansas, Lawrence, KS, United States
  • 2Joint Center for Earth Systems Technology, UMBC, Baltimore, MD, United States
  • 3Department of Physics, UMBC, Baltimore, MD, United States

Abstract. Stratocumulus clouds in the marine boundary layer cover a large fraction of ocean surface and play an important role in the radiative energy balance of the Earth system. Simulating these clouds in Earth system models (ESMs) has proven to be extremely challenging, in part because cloud microphysical processes such as the autoconversion of cloud water into precipitation occur at the scales much smaller than typical ESM grid sizes. An accurate autoconversion parameterization needs to account for not only the local microphysical process (e.g., the dependence on cloud water content qc and cloud droplet number concentration Nc), but also the sub-grid scale variability of the cloud properties the determine the process rate. Accounting for subgrid-scale variability is often achieved by the introduction of a so-called enhancement factor E. Previous studies of E for autoconversion focused more on its dependence on cloud regime and ESM grid size, but largely overlooked the vertical dependence of E within the cloud. In this study, we use a large-eddy simulation (LES) model, initialized and constrained with in situ and surface-based measurements from a recent airborne field campaign, to characterize the vertical dependence of the horizontal variation of qc in stratocumulus clouds and the implications for E. Similar to our recent observational study (Zhang et al., 2021), we found that the inverse relative variance of qc, an index of horizontal homogeneity, generally increases from cloud base upward through the lower 2/3 of the cloud, and then decreases in the uppermost 1/3 of the cloud. As a result, E decreases from cloud base upward, and then increases towards the cloud top. We apply a decomposition analysis to the LES cloud water field to understand the relative roles of the mean and variances of qc in determining the vertical dependence of E. Our analysis reveals that the vertical dependence of the horizontal qc variability and enhancement factor E is a combined result of condensation growth throughout the lower portion of the cloud and entrainment mixing at cloud top. The findings from this study indicate that a vertically dependent E should be used in ESM autoconversion parameterizations.

Justin A. Covert et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-656', Anonymous Referee #1, 24 Aug 2021
  • RC2: 'Comment on acp-2021-656', Anonymous Referee #2, 31 Aug 2021
  • RC3: 'Comment on acp-2021-656', Anonymous Referee #3, 03 Sep 2021

Justin A. Covert et al.

Justin A. Covert et al.

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Short summary
Stratocumulus play an important role in Earth's radiative balance. The simulation of these cloud systems in climate models is difficult due to the scale at which cloud microphysical processes occur compared to model grid sizes. In this study, we use large-eddy simulation to analyze subgrid-scale variability of cloud water and its implications on a cloud water to drizzle model enhancement factor E. We find current values of E may be too large and that E should be vertically dependent in models.
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