Nighttime precipitation induced by a synoptic-scale convergence in the Central Tibetan Plateau

Abstract

In the 1998 monsoon season, precipitation from midnight through early morning was frequently observed in the central Tibetan Plateau (TP). It occurred on several successive days and tended to be accompanied by low-level easterly winds. Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis data also showed easterly component winds as a part of anti-cyclonic circulations in the northeastern plateau systematically occurring with a prevailing synoptic scale trough at around 100°E in the mid-latitudes. A numerical simulation by the Weather Research and Forecasting (WRF) model with a resolution of 15 km reproduced the development of a convergence flow over the central TP, and corresponding zonal cloud formation was confirmed by METEOSAT-IR images. The simulation results indicated that the precipitation system was developed through with dissipating nighttime low-level convective instability by low-level convergence flows, and sonde observations confirmed the existence of an instability layer. Sensitivity experiments revealed that the convergence pattern over the central TP was produced by the dynamical stagnation effect of the plateau to-pography to the northwesterly general flows from the mid-latitudes. Many former studies have explained nocturnal precipitation in monsoon Asia as a part of an orographically induced diurnal variation. However, this study characterized the nighttime precipitation over the central TP due to synoptic-scale convergence. Two reasons are given to explain the substantial precipitation enhancement in the night. One is the daytime thermal convections that depend strongly on the topography, which dissipate synoptic-scale convergences zone, and the other is the low-level moistening after the maturing of the monsoon season to increase nighttime convective instability. © 2009, Meteorological Society of Japan.