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首页> 外文期刊>Journal of hydrometeorology >Cloud-Resolving-Model Simulations of Nocturnal Precipitation over the Himalayan Slopes and Foothills
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Cloud-Resolving-Model Simulations of Nocturnal Precipitation over the Himalayan Slopes and Foothills

机译：Cloud-Resolving-Model Simulations of Nocturnal Precipitation over the Himalayan Slopes and Foothills

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摘要

A numerical experiment with a 2-km resolution was conducted using the Weather Research and Forecasting (WRF) Model to investigate physical processes driving nocturnal precipitation over the Himalayas during the mature monsoon seasons between 2003 and 2010. The WRF Model simulations of increases in precipitation twice a day, one in the afternoon and another around midnight, over the Himalayan slopes, and of the single nocturnal peak over the Himalayan foothills were reasonably accurate. To understand the synoptic-scale moisture transport and its local-scale convergence generating the nocturnal precipitation, composite analyses were conducted using the reanalysis dataset and model outputs. In the synoptic scale, moisture transport associated with the westward propagation of low pressure systems was found when nocturnal precipitation dominated over the Himalayan slopes. In contrast, moisture was directly provided from the synoptic-scale monsoon westerlies for nocturnal precipitation over the foothills. The model outputs suggested that precipitation occurred on the mountain ridges in the Himalayas during the afternoon and expanded horizontally toward lower-elevation areas through the night. During the nighttime, the downslope wind was caused by radiative cooling at the surface and was intensified by evaporative cooling by hydrometeors in the near-surface layer. As a result, convergence between the downslope wind and the synoptic-scale flow promoted nocturnal precipitation over the Himalayas and to the south, as well as the moisture convergence by orography and/or synoptic-scale circulation patterns. The nocturnal precipitation over the Himalayas was not simulated well when we used the coarse topographic resolution and the smaller number of vertical layers. SIGNIFICANCE STATEMENT: Nocturnal precipitation is observed over the Himalayan slopes and foothills during summer. We investigated physical factors to cause the nocturnal precipitation there using a reanalysis dataset and a numerical simulation with 2-km horizontal resolution. Water vapor was intensively transported to the Himalayas between midnight and morning in the synoptic scale, and then generated precipitation under the influence of mountain topography. A downslope wind, which occurred in association with nocturnal cooling of land surface, enhanced a moisture convergence and caused precipitation over the Himalayan slopes and foothills. Evaporative cooling from the precipitation also contributed the cooling in the near-surface layer, which assisted an intensification of the nocturnal downslope wind.

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