Statistical Analysis of Oceanic Rainfall Characteristics in the Baiu Season Utilizing TRMM PR Data

Abstract

Temporal variations of rainfall characteristics over the ocean to the south of the Japan Archipelago during the Baiu season are quantitatively analyzed, and their relationships to the variations of environmental conditions are discussed. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data are utilized for the precipitation, upper-air observation data at six oceanic stations of the Japan Meteorological Agency (JMA), and the reanalysis data produced by JMA are utilized for the environmental data, to study the nine Baiu seasons from 1998 to 2006. Calendar-day averaged time series of nine-year TRMM PR data show a significant increase of tall rain in the latter period of the Baiu season, which accompanies the destabilization of the environmental thermodynamic conditions. Utilizing the values of Convective Available Potential Energy (CAPE), Baiu periods are classified into the earlier-period type (EPT) days and the latter-period type (LPT) days, such that EPT/ LPT days are those with negative/positive CAPE anomalies from the entire Baiu-period average. Between the two periods, the convective rain ratio and the stratiform rain characteristics change significantly. In the EPT days, weak stratiform rain associated with the Baiu front is dominant and characterized by an intensity of ~2.5 mm h -1 and a rain-top height (RTH) of ~5 km. In the LPT days, rain is dominated by cloud clusters along the Baiu front with larger contribution from the convective rain, associated with stronger and deeper stratiform rain characterized by ~5.0 mm h -1 intensity and 7.0-7.5 km RTH. Four rain types are classified with the rainfall characteristics obtained from TRMM PR data at 1 degree x 1 degree grids, with the thresholds of convective rain ratio at 35% and rainfall intensity at 2.5 mm h -1 . Type 1 represents the weak stratiform rain along the Baiu front, type 2 rain has well organized cloud clusters, and type 3-rain corresponds to relatively shallow convective rain often found under the influence of the subtropical high. The type-4-rain region can be interpreted as a temporally-varying mixture of rain types 1 and 2. Statistics of rainfall types and environmental conditions suggest that dominant rainfall characteristics may be diagnosed utilizing environmental variables such as the equivalent potential temperature at 1000 hPa and the mean sea-level pressure.