Effect of the Coastal Large-Scale Environment on the Tropical Diurnal Cycle

Environmental conditions supporting offshore propagation of diurnal precipitation near tropical coastlines are examined. In particular, the effect of the near-coastal background wind, moisture, and surface wind speed and fluxes on offshore precipitation propagation is assessed for the Philippines, northern Australia, and Panama Bight region near Colombia. Reanalysis fields, satellite precipitation, surface wind speed (from the CYGNSS satellite), and flux observations, and the Cloud Model 1 (CM1) are used in this work. In general, a moist offshore environment and enhanced wind-driven surface fluxes support offshore propagation of strong diurnal convective disturbances. Near the west coast of Luzon, a weak offshore wind in the lower free troposphere also supports offshore propagation, as often occurs in the transition phases of the boreal summer intraseasonal oscillation from suppressed to enhanced daily mean convection. Vertically-integrated moist static energy budget analysis is used to support these results. Sensitivity tests with the CM1 verify the importance of weak offshore flow and a moist offshore environment for supporting offshore propagation of diurnal precipitation.