Roles of Drop Size Distribution and Turbulence in Autoconversion Based on Lagrangian Cloud Model Simulations

D. Oh, Y. Noh

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1 Citation (Scopus)

Abstract

The roles of the drop size distribution (DSD) and turbulence in the autoconversion rate A are investigated by analyzing Lagrangian cloud model (LCM) data for shallow cumulus clouds. The correlations of DSD and turbulence with other cloud parameters are estimated, and they are applied to parameterize their effects on A. A new parameterization of A based on this analysis is proposed: (Formula presented.) with (Formula presented.), where (Formula presented.) is the cloud water mixing ratio, (Formula presented.) and (Formula presented.) are the number concentration and the volume mean radius of cloud droplets, (Formula presented.) is the dissipation rate, (Formula presented.) is the threshold value of (Formula presented.), H is the Heaviside step function, and X, (Formula presented.), and b are constants. Here, (Formula presented.) represents the effect of DSD via its correlation with (Formula presented.) and (Formula presented.), while (Formula presented.) represents the effect of gravitational collisional growth for a given DSD and turbulence. The correlation between turbulence and DSD makes b larger than expected from turbulence-induced collision enhancement only. The effects of DSD and turbulence and their correlations with (Formula presented.) and (Formula presented.) explain a wide range of exponent values of (Formula presented.) and (Formula presented.) in many existing parameterizations of A. The new parameterization is compared with the LCM data and applied to a bulk cloud model (BCM) while clarifying the difference between the cloud droplet mixing processes in the LCM and BCM. The importance of DSD and turbulence in raindrop formation in shallow cumulus clouds is shown by comparing the A results with and without these effects.

Original languageEnglish
Article numbere2022JD036495
JournalJournal of Geophysical Research: Atmospheres
Volume127
Issue number16
DOIs
Publication statusPublished - 2022 Aug 27

Bibliographical note

Publisher Copyright:
© 2022. American Geophysical Union. All Rights Reserved.

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Atmospheric Science
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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