A-Train estimates of the sensitivity of the cloud-to-rainwater ratio to cloud size, relative humidity, and aerosols

Kevin M Smalley,Anita D Rapp

doi:10.5194/acp-21-2765-2021

Abstract

Abstract. Precipitation efficiency has been found to play an important role in constraining the sensitivity of the climate through its role in controlling cloud cover, yet its controls are not fully understood. Here we use CloudSat observations to identify individual contiguous shallow cumulus cloud objects and compute the ratio of cloud water path to rainwater (WRR) path as a proxy for warm-rain efficiency. Cloud objects are then conditionally sampled by cloud-top height, relative humidity, and aerosol optical depth (AOD) to analyze changes in WRR as a function of cloud size (extent). For a fixed cloud-top height, WRR increases with extent and environmental humidity following a double power-law distribution, as a function of extent. Similarly, WRR increases, holding average relative humidity at or below 850 mb constant. There is little relationship between WRR and AOD when conditioned by cloud-top height, suggesting that, once rain drop formation begins, aerosols may not be as important for WRR as cloud size and depth. Consistent with prior studies, results show an increase in WRR with sea-surface temperature. However, for a given depth and SST, WRR is also dependent on cloud size and becomes larger as cloud size increases. Given that larger objects become more frequent with increasing SST, these results imply that increasing precipitation efficiencies with SST are due not only to deeper clouds with greater cloud water contents but also to the propensity for larger clouds which may have more protected updrafts.

Highlights

Low cloud cover continues to be a dominant source of uncertainty in projecting future climate (e.g., Bony and Dufresne, 2005; Dufresne and Bony, 2008; Vial et al, 2013), with variations in shallow cumulus distributions explaining much of the differences in climate-model-derived estimates of climate sensitivity (e.g., Wyant et al, 2006; Medeiros and Stevens, 2011; Nam et al, 2012)
Building off work in Smalley and Rapp (2020) that analyzed the relationship between rain likelihood and cloud size, this study uses the higher-sensitivity radar of CloudSat in addition to Moderate Resolution Imaging Spectroradiometer (MODIS) observations to test the hypothesis that water path to rainwater (WRR) is higher in larger shallow cumulus clouds and is modulated by relative humidity (RH) and aerosol loading
This study uses the methodology described by Smalley and Rapp (2020) to classify a large global shallow cumulus cloud object dataset from CloudSat and determine the relationship between WRR, cloud extent, RH, and aerosol loading

Summary

Introduction

Low cloud cover continues to be a dominant source of uncertainty in projecting future climate (e.g., Bony and Dufresne, 2005; Dufresne and Bony, 2008; Vial et al, 2013), with variations in shallow cumulus distributions explaining much of the differences in climate-model-derived estimates of climate sensitivity (e.g., Wyant et al, 2006; Medeiros and Stevens, 2011; Nam et al, 2012). Both the large-scale and cloud microphysical definitions of precipitation efficiency are useful (Sui et al, 2005, 2007), variations in the ratio of cloud water to rainwater (WRR) in response to changes in evaporation can theoretically be used as a proxy for warm-rain efficiency based on the cloud microphysical definition From this coupled with LES results showing that shallow cumulus updrafts are more protected as clouds grow in size and/or RH increases, we hypothesize larger droplets will be evident closer to the cloud base and increase WRR in larger cloud objects, because the cloud core of larger cloud objects is more protected from entrainment. Building off work in Smalley and Rapp (2020) that analyzed the relationship between rain likelihood and cloud size, this study uses the higher-sensitivity radar of CloudSat in addition to MODIS observations to test the hypothesis that WRR is higher in larger shallow cumulus clouds and is modulated by RH and aerosol loading

Data and methods

Warm-rain relationship to extent

Limitations of analysis and observations

Summary and discussion