Diurnal Variations of Precipitation Grouped into Cloud Categories around the Japanese Archipelago in the Warm Season

Abstract

Diurnal variations of precipitation from 1988 to 1993 are investigated using the radar echo composite chart covering the Japanese archipelago and adjacent seas at intervals of one or three hours. We focus on studying expansive areas on land and sea connected by eighteen digitized radars, and determining variations of precipitation due to the following cloud categories based on a horizontal scale and maximum precipitation intensity: large-scale precipitation clouds originating in quasi-horizontal circulation and cumulus-scale precipitation clouds originating in convection.
The cumulus-scale precipitation cloud, defined as small precipitation clouds with strong intensity, pre-dominates in afternoon peaks over four major islands of Japan excluding some points, the south Korean Peninsula, and Okinawa at about 26N. This is attributed to boundary layer heating by solar radiation during the daytime. Other precipitation clouds mainly composed of the large-scale precipitation cloud exhibit a diurnal cycle peaking in the morning, regardless of land-sea circulation.
Westerly wind fields over Japan have two effects on diurnal variations. Westerly winds over northeastern Japan maximize the cumulus-scale precipitation cloud over the east; and cause an evening peak over the eastern inshore by driving clouds east. Large-scale precipitation cloud driven by westerly wind often pass through the Nansei Islands from the northwest in the afternoon between May and July, and are responsible for afternoon peak precipitation in these islands.