Diurnal responses of tropical convective and stratiform rainfall to diurnally varying sea surface temperature

Two experiments were carried out using a two-dimensional cloud-resolving model to study the effects of diurnally varying sea surface temperature (SST) on diurnal variations of tropical convective and stratiform rainfall. Experiment SST29 is imposed by a constant SST of 29°C, whereas experiment SST29D is imposed by a diurnally varying SST with a time-mean of 29°C and a diurnal difference of 1°C. Both experiments are also zonally uniformly imposed by a zero vertical velocity and a constant zonal wind, and are integrated for 40 days to reach quasi-equilibrium states. The model domain mean surface rain rate is larger in SST29D than in SST29 in the late afternoon, when the ocean surface is warmer in SST29D. Convective-stratiform rainfall partitioning analysis reveals that the late-afternoon convective rainfall is larger in SST29D than in SST29, whereas the stratiform rainfalls are similar in both experiments. Further analysis of surface rainfall and cloud microphysical budgets over convective regions shows that, in the late afternoon, the larger amount of water vapor is pumped into the non-raining region through the larger surface evaporation associated with the warmer SST. This water vapor is then transported into convective regions to produce more vapor condensation and greater collection of cloud water by raindrops and larger convective rainfall in SST29D than in SST29.