资源环境科技发展态势分析平台

Understanding the local valley wind regimes in the Hindu-Kush Karakoram Himalaya is vital for future predictions of the glacio-hydro-meteorological system. Here the Weather Research and Forecasting model is employed at a resolution of 1km to investigate the forces driving the local valley wind regime in a river basin in the Nepalese Himalaya, during July 2013 and January 2014. Comparing with observations shows that the model represents the diurnal cycle of the winds well, with strong daytime up-valley winds and weak nighttime winds in both months. A momentum budget analysis of the model output shows that in the summer run the physical drivers of the near-surface valley wind also have a clear diurnal cycle, and are dominated by the pressure gradient, advection, and turbulent vertical mixing, as well as a nonphysical numerical diffusion term. By contrast, the drivers in the winter run have a less consistent diurnal cycle. In both months, the pressure gradient, advection, numerical diffusion, and Coriolis terms dominate up to 5,000m above the ground. The drivers are extremely variable over the valley, and also influenced by the presence of glaciers. When glaciers are removed from the model in the summer run, the wind continues further up the valley, indicating how the local valley winds might respond to future glacier shrinkage. The spatial variability of the drivers over both months is consistent with the complex topography in the basin, which must therefore be well represented in weather and regional climate models to generate accurate outputs.

Plain Language Summary The rain and snow in Himalayan valleys, and the formation and melting of glaciers, are affected by the wind in the valleys. Exactly what drives this wind is not fully understood. Around the world, wind in valleys generally travels up the valley, and up the sides of mountains, during the day. This is due to the sun heating different areas by different amounts, creating areas of low and high air pressure. Here we use a computer simulation to determine whether the difference in pressure is the main cause of the acceleration of the wind in a valley in the Himalayas, or whether there are other forces which also affect the wind. We compare the simulation to measurements taken in the valley. We find that the pressure difference is the main process affecting the acceleration of the wind. However, the winds in the valley are also driven by the effects of turbulence and affected by the shape of the valley. The forces are consistent over the month, accelerating the wind in the morning and decelerating the wind in the afternoon every day.