Study On The Characteristics And Process Of Precipitation In The Valley Of The Central Himalayas

As the natural barrier of the Tibetan Plateau,the Himalayas block most of the water vapor from entering the plateau,while there are many north-south valleys between the peaks,which play an important channel role in transporting water vapor from South Asia to higher altitudes and inland of the plateau.Due to the lack of support from ground observation data,not only the precipitation characteristics of the valleys are unclear and the water vapor transport process cannot be understood,but also it is difficult to effectively validate satellite precipitation products,making it difficult to apply the products to the investigation of hydro-meteorological processes in the region.In this study,a precipitation observation network in the mountainous region of the central Himalaya with extremely complex topography was established.Through the collected precipitation data,the comprehensive use of high-resolution WRF simulations,satellite remote sensing precipitation products and reanalysis datasets,the precipitation characteristics of valleys and the uncertainties of ground precipitation observations were carried out.The main results are as follows:1.An observation network including 15 rain gauges was established in the high-altitude of the Yadong Valley on the south slope of the central Himalaya to obtain high temporal resolution measurements since 2016.Based on this data,two unique precipitation features are found.(1)The Yadong Valley has very significant amount of pre-monsoon precipitation,with March-May precipitation accounting for 25-40% of total annual precipitation,which is significantly higher than the proportion of pre-monsoon precipitation in typical monsoon regions of South Asia and the Tibetan Plateau(～20%).(2)The diurnal variation of precipitation during the monsoon period depends on the altitude.As the altitude rises,the diurnal variation of precipitation gradually transits from the nighttime peak at low altitude to two precipitation peaks occurring in the afternoon and at night at high altitude,and the afternoon peak may exceed the nighttime peak.This differs from the predominantly nocturnal rainfall in South Asia and the Tibetan Plateau.2.Combining observation data and high-resolution numerical simulation(1.5 km horizontal grid spacing),it is found that(1)the simulation cannot accurately reflect the spatial distribution of precipitation but can express the spatial variability of precipitation in complex terrain.The latter provides a new method for determining the uncertainty of the assessment of precipitation products.(2)Compared with the drastic spatial variability of precipitation,the spatial variability of diurnal and seasonal variability of precipitation is relatively small.In addition,the thesis further reveals the relationship between the spatial variability of precipitation and the topography and temporal scale.3.The region with dual diurnal peaks of monsoon precipitation and the relevant processes are revealed.The dual diurnal peaks of precipitation during the monsoon period are prevalent in central Himalayan valleys.During the monsoon,the water vapor in the valley is near saturated.When nighttime downslope winds occur,the water vapor converges over the valley to produce precipitation,resulting in a nighttime precipitation peak.The afternoon precipitation peak at higher elevations corresponds to the strong valley winds at lower elevations,where the stronger daytime valley winds at lower elevations transport the water vapor northward and promote the generation of afternoon precipitation at higher elevations.4.The areas where a large amount of pre-monsoon precipitation exists and the reasons for its occurrence are clear.Considerable pre-monsoon precipitation is widespread over the region from the high altitude of east-central Himalaya to southeastern Tibet.During the pre-monsoon season,low-level southwesterly airflow exists from the Bay of Bengal to the southern foothills of the east-central Himalaya.Under the driving of strong pre-monsoon valley winds,warm and humid water vapor is transported to the higher elevations of the valley and condenses to produce large amounts of precipitation.