Maintenance mechanisms for the early-morning maximum summer rainfall over southeast China

During the summer months, diurnal rainfall variation over southeast China is frequently characterized by a major peak in the afternoon and a minor peak in the early morning. While the afternoon rainfall maximum is generally recognized to be mainly modulated by the diurnally varying wind introduced by land-sea and mountain-valley differential heating, causes of the early-morning rainfall are not well documented. In this study, variation in the semi-diurnal harmonic of rainfall is found to be more important than variation in the diurnal harmonic of rainfall for determining the timing of the early-morning rainfall peak. Diagnoses of the atmospheric thermodynamic conditions indicate that late-night vertical differential thermal advection and semi-diurnal variation in land-sea differential radiative heating/cooling are the major reasons for reduction in stability in the early morning and, in turn, facilitate the formation of an early-morning maximum in rainfall. Computation of the water vapor budget suggests further that the early-morning maximum over southeast China is mainly maintained by the semi-diurnal harmonic of water vapor flux transported from the South China Sea. Copyright © 2011 Royal Meteorological Society.