Shallow Cumulus Cloud Fields Are Optically Thicker When They Are More Clustered

TL;DR

This study shows that more clustered shallow trade cumulus clouds are optically thicker and reflect more sunlight, independently of cloud cover amount, which impacts climate modeling accuracy.

Contribution

It demonstrates that cloud organization influences optical thickness and radiative effects independently of cloud cover, using satellite data and high-resolution simulations.

Findings

Clustered clouds are geometrically thicker with larger liquid water paths.

More clustered clouds reflect up to 20 W/m² more shortwave radiation.

Cloud organization affects cloud optical properties independently of cloud cover.

Abstract

Shallow trade cumuli over subtropical oceans are a persistent source of uncertainty in climate projections. Mesoscale organization of trade cumulus clouds has been shown to influence their cloud radiative effect (CRE) through cloud cover. We investigate whether organization can explain CRE variability independently of cloud cover variability. By analyzing satellite observations and high-resolution simulations, we show that increased clustering leads to geometrically thicker clouds with larger domain-averaged liquid water paths, smaller cloud droplets, and consequently, larger cloud optical depths. The relationships between these variables are shaped by the mixture of deep cloud cores and shallower interstitial clouds or anvils that characterize cloud organization. Eliminating cloud cover effects, more clustered clouds reflect up to 20 W/m$^2$ more instantaneous shortwave radiation back to space.