17:15 〜 18:30
[ACG32-P02] The simulation for the nocturnal precipitation over the Himalayas
キーワード：アジアモンスーン、山岳降水、降水日周期
Summer precipitation and clouds developed over the Himalayas and its surrounding region, which is valuable water resource in Asia, have a role in a maintenance of the Asian monsoon circulation. The precipitation dominates between the midnight and early morning over the Himalayan foothills, while it has double peaks during the afternoon-evening and night over the Himalayan slopes. Simulations with a coarse resolution are difficult to simulate this diurnal cycle in precipitation over the Himalayas. In this study, the 2km resolution experiment was conducted using the Weather Research and Forecasting (WRF) model ver. 3.9.1.1 during the mature monsoon season for 8 years from 2003 to 2010 to evaluate the diurnal cycle in precipitation, particularly nocturnal precipitation dominated over the Himalayan slopes and foothills. Then, we diagnose a local-scale physical process for the nocturnal precipitation occurrence.
Comparing with the satellite estimated precipitation from GPM IMERG (V06B), a spatial distribution of the climatological nocturnal precipitation is simulated well in the WRF model although the absolute value of precipitation is underestimated. Diurnal variations in the simulated precipitation averaged over the slopes and foothills are consistent with that in the GPM IMERG and the in-situ observation. When the precipitation dominated over the Himalayan slopes, a monsoon depression over the northwest of the Bay of Bengal contributes to the moisture transport to the slopes. During the daytime, the moisture transported by the synoptic-scale flow causes orographic precipitation at the Himalayan mountain ridges in the model, of which altitude is 2000-2500 m. The simulated precipitation expands widely in the Himalayan slopes from evening until night, and it also occurs a lower-altitude mountain in front of the Himalayas. Meanwhile, the moisture is directly provided from the synoptic-scale monsoon westerlies when the nocturnal precipitation dominated over the Himalayan foothills. In this case, the nocturnal precipitation is formed over the foothills by the convergence between the synoptic-scale monsoon flow and the local-scale downslope wind from the Himalayas. The evaporation cooling near the surface and the spatial heterogeneity of the radiative cooling at the ground would enhance the downslope wind locally. In addition, the cloud convections were likely to be maintained by the latent heating from the cloud and the radiative cooling at the cloud top. The sensitivity experiments suggested that both representation of topography and numbers of vertical layer affected the spatial distribution of the nocturnal precipitation in the model.
Comparing with the satellite estimated precipitation from GPM IMERG (V06B), a spatial distribution of the climatological nocturnal precipitation is simulated well in the WRF model although the absolute value of precipitation is underestimated. Diurnal variations in the simulated precipitation averaged over the slopes and foothills are consistent with that in the GPM IMERG and the in-situ observation. When the precipitation dominated over the Himalayan slopes, a monsoon depression over the northwest of the Bay of Bengal contributes to the moisture transport to the slopes. During the daytime, the moisture transported by the synoptic-scale flow causes orographic precipitation at the Himalayan mountain ridges in the model, of which altitude is 2000-2500 m. The simulated precipitation expands widely in the Himalayan slopes from evening until night, and it also occurs a lower-altitude mountain in front of the Himalayas. Meanwhile, the moisture is directly provided from the synoptic-scale monsoon westerlies when the nocturnal precipitation dominated over the Himalayan foothills. In this case, the nocturnal precipitation is formed over the foothills by the convergence between the synoptic-scale monsoon flow and the local-scale downslope wind from the Himalayas. The evaporation cooling near the surface and the spatial heterogeneity of the radiative cooling at the ground would enhance the downslope wind locally. In addition, the cloud convections were likely to be maintained by the latent heating from the cloud and the radiative cooling at the cloud top. The sensitivity experiments suggested that both representation of topography and numbers of vertical layer affected the spatial distribution of the nocturnal precipitation in the model.