Measured Constraints on Cloud Top Entrainment to Reduce Uncertainty of Nonprecipitating Stratocumulus Shortwave Radiative Forcing in the Southern Ocean

K J Sanchez,L M Russell,M Diao,G C Roberts

doi:10.1029/2020gl090513

Abstract

AbstractStratocumulus cloud top entrainment has a significant effect on cloud properties, but there are few observations quantifying its impact. Using explicit 0‐D parcel model simulations, initialized with below‐cloud in situ measurements, and validated with in situ measurements of cloud properties, the shortwave cloud radiative forcing (SWCF) was reduced by up to 100 W m−2 by cloud top entrainment in the Southern Ocean. The impact of entrainment‐corrected SWCF is between 2 and 20 times that of changes in the aerosol particle concentration or updraft at cloud base. The variability in entrainment‐corrected SWCF accounts for up to 50 W m−2 uncertainty in estimating cloud forcing. Measurements necessary for estimating the impact of entrainment on cloud properties can be constrained from existing airborne platforms and provide a first‐order approximation for cloud radiative properties of nonprecipitating stratocumulus clouds. These measurement‐derived estimates of entrainment can be used to validate and improve parameterizations of entrainment in Global Climate Models.

Highlights

Low clouds in the Southern Ocean (SO) are poorly simulated by general circulation models (GCMs) and tend to overestimate the amount of radiation absorbed by the SO (Bodas‐Salcedo et al, 2014; Haynes et al, 2011; Hyder et al, 2018; McCoy et al, 2014)
cloud droplet number concentration (CDNC) are always lower than the adiabatic simulations (Table 1; Figure 2) while Dv is consistent with adiabatic simulations, suggesting entrainment of dry air is reducing liquid water content (LWC) via the evaporation of cloud droplets
The adiabatic aerosol‐cloud‐parcel model (ACPM) overestimates the observed LWC, CDNC, and cloud droplet extinction; Dv remains remarkably similar to observed values, which supports evidence for inhomogeneous mixing (Brenguier et al, 2011; Burnet & Brenguier, 2007; Jia et al, 2019)

Summary

Introduction

Low clouds in the Southern Ocean (SO) are poorly simulated by general circulation models (GCMs) and tend to overestimate the amount of radiation absorbed by the SO (Bodas‐Salcedo et al, 2014; Haynes et al, 2011; Hyder et al, 2018; McCoy et al, 2014). This bias is driven by underestimates of cloud radiative forcing, which are likely due to errors in cloud microphysical properties such as droplet size and concentration, as well as cloud processes such as precipitation and entrainment (Mason et al, 2015; Vial et al, 2013).

Methods

Results

Conclusion