Diurnal cycle of precipitation and near-surface atmospheric conditions over the maritime continent: land–sea contrast and impacts of ambient winds in cloud-permitting simulations

A set of cloud-permitting-scale numerical simulations during January–February 2018 is used to examine the diurnal cycle (DC) of precipitation and near-surface variables (e.g., 2 m temperature, 10 m wind and convergence) over the Indo-Pacific Maritime Continent under the impacts of shore-orthogonal ambient winds (SOAWs). It is found that the DC of these variables and their variabilities of daily maxima, minima, and diurnal amplitudes vary over land, sea, and coastal regions. Among all variables, the DC of precipitation has the highest linear correlation with near-surface convergence (near-surface temperature) over coastal (noncoastal) regions. The correlations among the DCs of precipitation, wind, and heating are greater over the ocean than over land. Sine curves can model accurately the DCs of most variables over the ocean, but not over land. SOAWs act to influence the DC mainly by affecting the diurnal amplitude of the considered variables, with DC being stronger under more strengthened offshore SOAWs, though variable dependence and regional variability exist. Composite analysis over Sumatra reveals that under weak SOAWs, shallow clouds are dominant and cause a pre-moistening effect, supporting shallow-to-deep convection transition. A sea breeze circulation (SBC) with return flow aloft can develop rapidly. Cold pools are better able to trigger new updrafts and contribute to the upscale growth and inland migration of deep convection. In addition, warm gravity waves can propagate upward throughout the troposphere, thereby supporting a strong DC. In contrast, under strong SOAWs, both shallow and middle-high clouds prevail and persist throughout the day. The evolution of moistening and SBC is reduced, leading to weak variation in vertical motion and rainwater confined to the boundary layer. Large-scale winds, moisture, and convection are discussed to interpret how strong SOAWs affect the DC of Sumatra.