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