Diurnal Variations in Rainfall and Precipitation Asymmetry of Tropical Cyclones in the Northwest Pacific Region

This study investigates diurnal variations of tropical cyclone precipitation in the northwest Pacific (NWP) region, including the South China Sea (SCS) and adjacent landmasses. Diurnal cycles of TC rainfall show significant land- sea contrasts. The primary peak of areal mean TC rain rate occurs in the early morning [0600 local time (LT)] and the afternoon (1500 LT) over the ocean and land, respectively. Both the total and heavy TC precipitation extend farther inland in the afternoon, while nocturnal heavy TC rain is more confined to the coast. A significant semidiurnal cycle of TC precipitation is observed over the ocean (i.e., a secondary peak near 1800 LT). The diurnal cycle of TC rainfall also depends on precipitation frequency, intensity, and radial distance from the TC center. Over the ocean, although TC precipitation intensity shows a pronounced diurnal cycle, its precipitation frequency exhibits virtually no diurnal variation. Over land, TC precipitation frequency markedly peaks in the afternoon (1500 LT), whereas its precipitation intensity interestingly maximizes in the early morning (0300-0600 LT). Diurnal variations of TC asymmetric rainfall structure are consistent with diurnal changes of vertical wind shear. Over the SCS, maximum precipitation located in the downshear-left quadrant and is the most extensive in the morning. However, this heavy rain area shrinks and shifts downshear-ward in the afternoon, consistent with changes of the magnitude (reduced) and direction (clockwise) of the shear. In contrast, TCs over the open ocean of the NWP have little diurnal variability of precipitation asymmetry, due mainly to a diurnally invariant shear environment.