A Large‐Scale Analysis of Pockets of Open Cells and Their Radiative Impact

D Watson‐Parris,S A Sutherland,M W Christensen,P Stier,R Eastman

doi:10.1029/2020gl092213

D Watson‐Parris, S A Sutherland + Show 3 more

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https://doi.org/10.1029/2020gl092213

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Abstract

AbstractPockets of open cells sometimes form within closed‐cell stratocumulus cloud decks but little is known about their statistical properties or prevalence. A convolutional neural network was used to detect occurrences of pockets of open cells (POCs). Trained on a small hand‐logged data set and applied to 13 years of satellite imagery the neural network is able to classify 8,491 POCs. This extensive database allows the first robust analysis of the spatial and temporal prevalence of these phenomena, as well as a detailed analysis of their micro‐physical properties. We find a large (30%) increase in cloud effective radius inside POCs as compared to their surroundings and similarly large (20%) decrease in cloud fraction. This also allows their global radiative effect to be determined. Using simple radiative approximations we find that the instantaneous global annual mean top‐of‐atmosphere perturbation by all POCs is only 0.01 W/m2.

Highlights

Stratocumulus clouds play a vital role in the global energy balance (Randall et al, 1984) and can exist in two distinct regimes: open cells and closed cells (Agee et al, 1973), which can be considered two states of a coupled oscillator (Koren & Feingold, 2011)
Pockets of open cells sometimes form within closed-cell stratocumulus cloud decks but little is known about their statistical properties or prevalence
The model was run on all Moderate Resolution Imaging Spectrometer (MODIS) images which intersected the three main marine stratocumulus regions in the north-east Pacific, south-east Pacific and south-east Atlantic, as defined by Klein and Hartmann (1993), between 2005 and 2018

Summary

Introduction

Stratocumulus clouds play a vital role in the global energy balance (Randall et al, 1984) and can exist in two distinct regimes: open cells and closed cells (Agee et al, 1973), which can be considered two states of a coupled oscillator (Koren & Feingold, 2011). First coined in 2004 (Bretherton et al, 2004), pockets of open cells (POCs) are small regions of open cell clouds embedded in a uniform surrounding deck of closed cell clouds. Despite the importance of the marine stratocumulus decks on the global climate (Hansen et al, 2013; Randall et al, 1984; Stevens et al, 2005), studying POCs poses several difficulties due to their complex and ill-defined nature. Closed cells have a markedly higher albedo than open cells for a given cloud fraction (McCoy et al, 2017), and this is exacerbated by their greater cloud fraction (Rosenfeld et al, 2006). It has been proposed that anthropogenic aerosol could cause a large reduction in the number of POCs, and in turn lead to a large top-of-atmosphere radiative perturbation (Rosenfeld et al, 2006)

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