Large-eddy simulations of the diurnal cycle of shallow convection and cloudiness over the tropical Indian Ocean

Abstract

A 3-D non-hydrostatic model (EULAG) with warm-rain bulk microphysics was used to study the diurnal cycle of shallow convection and cloudiness in the trade wind boundary layer over the Indian Ocean. Simulations were initialized with soundings obtained during the Indian Ocean Experiment (INDOEX). In the absence of diurnally varying large-scale forcing, the simulated diurnal cycles of vertical velocities, turbulent fluxes, condensation rate and cloudiness were characterized by distinct daytime reductions. Solar heating in the boundary layer stabilized the air, decreased the relative humidity and, therefore, suppressed cloud-layer turbulence and shallow cumulus convection. As a result, the condensation rate and cloud amount were reduced. Stronger thermal updraughts and turbulent fluxes caused by solar heating in the mixed layer triggered the recovery of cloudiness in the afternoon when the instability in the cloud layer increased. Sensitivity experiments showed that the principal cause of the daytime heating of the atmosphere and reductions in convection and cloudiness was not the diurnally varying surface fluxes nor the cloud radiative effects, but rather the gaseous absorption of solar radiation due mainly to water vapour in the spectral band of 2500-14 500 cm-1 and ozone in the ultraviolet and visible bands. Compared to the average short-wave heating of 0.1 K day-1 due to cloud droplets, gases enhanced solar heating by about 2.0 K day-1 in the cloud layer. However, depending on its direction and magnitude, large-scale vertical motion can highly modulate the diurnal cycles driven by solar heating, representing a big uncertainty in observing the diurnal cycle of shallow cumuli. Copyright © 2008 Royal Meteorological Society.