Impacts of snow cover and frozen soil in the Tibetan Plateau on summer precipitation in China

摘要

This paper presents an analysis of the mechanisms and impacts of snow cover and frozen soil in the Tibetan Plateau on the summer precipitation in China, using RegCM3 version 3.1 model simulations. Comparisons of simulations vs. observations show that RegCM3 well captures these impacts. Results indicate that in a more-snow year with deep frozen soil there will be more precipitation in the Yangtze River Basin and central Northwest China, western Inner Mongolia, and Xinjiang, but less precipitation in Northeast China, North China, South China, and most of Southwest China. In a less-snow year with deep frozen soil, however, there will be more precipitation in Northeast China, North China, and southern South China, but less precipitation in the Yangtze River Basin and in northern South China. Such differences may be attributed to different combination patterns of melting snow and thawing frozen soil on the Plateau, which may change soil moisture as well as cause differences in energy absorption in the phase change processes of snow cover and frozen soil. These factors may produce more surface sensible heat in more-snow years when the frozen soil is deep than when the frozen soil is shallow. The higher surface sensible heat may lead to a stronger updraft over the Plateau, eventually contributing to a stronger South Asia High and West Pacific Subtropical High. Due to different values of the wind fields at 850 hPa, a convergence zone will form over the Yangtze River Basin, which may produce more summer precipitation in the basin area but less precipitation in North China and South China. However, because soil moisture depends on ice content, in less-snow years with deep frozen soil, the soil moisture will be higher. The combination of higher frozen soil moisture with latent heat absorption in the phase change process may generate less surface sensible heat and consequently a weaker updraft motion over the Plateau. As a result, both the South Asia High and the West Pacific Subtropical High will be weaker, hence causing more summer precipitation in northern China but less in southern China.