The Role of Radiation in Influencing Tropical Cloud Distributions in a Radiative–Convective Equilibrium Cloud-Resolving Model

Observations by Johnson et al. depict regions of active tropical convection as possessing increased relative humidity through a deep layer and reduced low-level static stability when compared to nonconvecting regions. Shallow cumulus clouds, congestus clouds, and deep convection all coexist within these convecting regions. This investigation explores the effect that radiation might have on the tropical cloud distributions by using large-domain (20 000 km) radiative–convective equilibrium cloud-resolving model (RCE-CRM) experiments that reproduce similar moisture, stability, and cloud structures to those observed. Radiation is found to significantly increase the amount of shallow and intermediate-level clouds (tops between 1.5 and 5 km) by increasing low-level stability and thus promoting additional low-level cloud detrainment. The mechanism by which radiation stabilizes the low levels is found to differ between convectively suppressed and active regions. In convectively suppressed regions, strong relative humidity gradients within the trade inversion layer produce a low-level cooling maximum that further stabilizes the stable layer, much as proposed by Mapes and Zuidema. In convectively active regions, sufficiently moist columns with no relative humidity gradients are also found to produce a low-level cooling maximum that stabilizes the lower levels. This cooling maximum is due to the complicated effects of the water vapor continuum and is sensitive to the absolute moisture path. Because of the dependence on absolute moisture, the radiative enhancement of shallow and intermediate-level clouds in convectively active regions is potentially sensitive to SSTs.