On the cumulus diurnal cycle over the tropical warm pool

An idealized cloud-resolving model experiment is executed to study the prominent cumulus diurnal cycle in suppressed regimes over the tropical warm pool. These regimes are characterized by daytime cumulus invigoration and cloud-layer moistening connected with enhanced diurnal cycles in shortwave radiative heating (SW) and sea surface temperature (SST). The relative roles of diurnally varying SW and SST in this cumulus diurnal cycle are assessed, wherein radiation is modeled and SST is prescribed. Large-scale subsidence is parameterized using the spectral weak temperature gradient (WTG) scheme, such that large-scale vertical motion (w(wtg)), and hence subsidence drying, is modulated by diurnal changes in diabatic heating. A control simulation exhibits daytime cumulus invigoration that closely matches observations, including midday cloud-layer moistening. This cumulus invigoration is composed of two distinct modes: (1) a midday nonprecipitating (forced) mode of predominately shallow clouds, driven by the peak in SST and surface fluxes as the mixed layer deepens and dries; and (2) a precipitating late-afternoon (active) mode characterized by deeper clouds in connection with a more moist cloud layer. This cloud-layer moistening is driven by the daytime relaxation of w(wtg) subsidence, which is prompted by the midday peak in SW. The transition from the surface flux-driven forced mode to the active precipitating mode is accompanied by a transition from relatively small-scale boundary layer circulation cells to larger cells that are highly modulated by cold pools, consistent with observations. When the diurnal cycle is removed, clouds are persistently shallower with virtually no rainfall, emphasizing the inherent nonlinearity of the cumulus diurnal cycle.