Abstract
The diurnal features of rainfall over the Himalayas have been widely investigated, but their triggers remain unclear. In this work, we divided the Himalayas and surroundings into four regions, including the plains, foothills, slopes, and plateau, and investigated the above issues. The results show that the rainfall total is controlled by large-scale monsoon flows while its meridional distribution is regulated by terrain circulations. The afternoon rainfall peak in the plains and foothills is linked with the intersection of two monsoon flows. The southward-shifting rainfall peak, which occurs from midnight to early morning in the slopes and foothills, is affected by the nighttime downslope flow and the strong Bay of Bengal monsoon flow in the morning. The evening rainfall peak in the plateau and high-altitude slopes is thought to be a result of the atmospheric layer being at its moistest at that time.
Significance Statement
During the South Asian summer monsoon season, the Himalayas are affected by two large-scale monsoon flows as well as unique topographic circulations. We want to understand how these complex circulations act on diurnal variations of orographic precipitation. The diurnal cycle of rainfall over the Himalayas and surroundings shows three prominent south-to-north peaks, which are caused by significantly different thermodynamic conditions. The southward-shifting diurnal rainfall peak over the Himalayan slopes and foothills, which contributes the most to the arc-shaped orographic rain belt, is triggered by the nighttime downslope flow and strengthened by the strong Bay of Bengal monsoon flow in the morning. The result highlights the entangled impact of monsoon and terrain circulations on orographic precipitation.
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