Characteristics of Precipitation Systems and Their Environment Observed during the Onset of the Western North Pacific Summer Monsoon in 2008

Abstract

This paper has investigated the structure, propagation, orientation, and rainfall of precipitation systems and environmental wind and thermodynamic structures observed by the Doppler radar and the radiosonde system on board the R/V Mirai around a fixed site (12°N, 135°E) during the onset of the western North Pacific summer monsoon in 2008. The monsoon onset occurred in mid-June and was accompanied by a remarkable change in large-scale circulation over the observational area.Statistical analyses indicate that precipitation systems evolved deeply and widely upon the monsoon onset. The development of mesoscale convective systems contributed to the formation of much more rainfall. Precipitation systems tended to move along with low-level winds during the pre-onset and the early-onset periods and with mid-level winds during the latter-onset period. The increase in the height and the area of precipitation systems possessed much stronger correlation with the evolution of the low-level shear and the mid-level humidity than with other environmental variables.The internal structure of two precipitation systems that occurred in the environment with the distinct low-level shear and mid-level humidity has also been analyzed. The shear-parallel precipitation system during the early-onset period occurred in the environment with the dry middle troposphere and the weak low-level shear and was characterized by the descending rear inflow and weaker updrafts. On the other hand, the shear-normal precipitation system during the latter-onset period occurred in the environment with the moist middle troposphere and the strong low-level shear and was featured with the elevated rear inflow and stronger updrafts.According to the observational results, possible interaction processes between precipitation systems and their environment during the monsoon onset are discussed.