Modeling the diurnal cycle of shallow convection and cloudiness in trade wind boundary layer over the Indian Ocean

A three-dimensional non-hydrostatic Eulerian/semi-Lagrangian (EULAG) anelastic cloud-resolving model with warm-rain bulk microphysics was used to study the diurnal cycle of shallow convection and cloudiness in trade wind boundary layer over the Indian Ocean. Simulations with100 m horizontal and 40 m vertical resolution and with a 40 km2 by 3 km domain were initialized with soundings obtained during the Indian Ocean experiment (INDOEX) and sea surface temperatures estimated from the microwave imager on the Tropical Rainfall Measuring Mission (TRMM) satellite. Without considering large-scale forcing, the simulated diurnal cycles of turbulent fluxes and cloudiness were characterized by significant daytime reductions and midday minima. The daytime average turbulent fluxes were over 30% less than the nocturnal average, and the daytime cloud coverage (liquid water path) was reduced by 13% (24%). Periodic pulsations were superimposed on the diurnal cycles. A series of sensitivity experiments were conducted with constant and diurnally varying surface fluxes, with and without large-scale vertical motion, and including and excluding cloud radiative effects to show the quantitative impacts of these factors on circulation and turbulence, and of the dynamics on the diurnal cycle of convection and cloudiness. The experiments showed that the diurnally varying surface fluxes and the cloud radiative effects were not the principal cause of the daytime reduction in cloudiness, and that solar heating in the conditionally unstable layer and the inversion layer was primarily responsible for the daytime reductions by suppressing circulation and turbulence. Stronger thermal updrafts and turbulent fluxes in the mixed layer triggered the recovery of cloudiness in the late afternoon when the stability in the conditionally unstable layer decreased Depending on its direction and magnitude, large-scale vertical motion can either enhance or eliminate the daytime reduction of cloudiness. A diurnally varying large-scale vertical velocity can drive the diurnal variation of cloudiness but it is not necessarily the same pattern as that driven by solar forcing.