Study On The Impact Of Taihang Mountains On Torrential Rains In North China

Using daily precipitation observations and their reanalysis data from weather stations around China, this paper analyzes the spatial-temporal distribution and average circulation of the summer precipitation from1951to2007in North China, comparing the differences of rainstorms on the two sides of the Taihang Mountains in magnitude, frequency and water vapor budget. Taking into account the location, intensity and movement of heavy rains in the Taihang Mountains as well as forecasters’ experience with such weather, the rainstorm processes subject to the Taihang Mountains in North China have been classified with cases selected. A diagnoses and analysis of these cases has led to the identification of patterns of evolving circulations in rainstorm processes of different types, the identification of synoptically driven systems and the development of synoptic models for different types of torrential rains, providing a focus for type based rainstorm forecasting. In addition, the mesoscale model WRF and regional climate model RegCM3were employed to numerically simulate selected rainstorm cases of different types, design programs on terrain sensitivity experiments and analyze the changing physical field of a synoptic system over a changed terrain as well as the precipitation in terms of intensity, location and movement of precipitation. The findings are as follows:(1) Summer precipitation in North China experiences relatively wet, transitional and dry periods, with the rainfall in areas to the west of the Mountains being significantly less than that in areas to the east of the Mountains, and the frequency of torrential rains being significantly lower.(2) In accordance with the location, intensity and movement of rainstorms in the Taihang Mountains, the summer torrential rains in North China are divided into five types:rainstorm to the east of Taihang, rainstorm to the west of Taihang, rainstorm over Taihang, rainstorm on the two sides of Taihang, and rainstorm weakening when crossing Taihang. The case review shows that the ’rainstorm to the east of Taihang’ occurs most frequently.(3) Weather patterns differ under different rainstorm types:Class A under the ’rainstorm to the east of Tainhang’ usually occurs when the subtropical high is found to the south and the east, and the cold trough or low vortex moves eastward in the northwestern region. Moreover, low-level shear lines and jet streams get strengthened obviously after crossing the Taihang Mountains. The easterly airstream flowing westward and being blocked by Taihang makes a mesoscale convergence line or shear line on the ground surface. Class B under the ’rainstorm to the east of Taihang’ occurs when the eastern North China is controlled by a tuberous subtropical high. The subtropical high ridge line lies to the north. The southeast jet stream in the middle-low level of the subtropical high, which conveys moisture to North China, is lifted along the east slope of Taihang, developing into torrential rains. The’rainstorm to the west of Taihang’ occurs when the subtropical high is found to the west and the north, the westerly trough moving eastward being blocked. The massive energy built up by the instability in most of the west of Taihang is lifted by the surface mesoscale system and the terrain, creating extremely heavy rains. The ’rainstorm over Taihang’ occurs when the subtropical high is very intense, the Northwest to Southeast China is under the high, the North China is to the north of the high, and a trough in the westerly belt moves eastward. This type features mountainous local convective rainstorms. The’rainstorm on the two sides of Taihang’occurs when the west-east ribbon-shaped subtropical high is intense, the westerly trough over middle-high latitudes fluctuates, the south-west or south-east jet streams in the middle and low levels are strong, and the east slope of Taihang contributes to the rising east-south jet stream. The’rainstorm weakening when crossing Taihang’occurs when the subtropical high is intense and found to the west, continuously moving westward and northward, and the westerly trough moving eastward is weakened.(4) Numerical simulations indicate that the topography of Taihang, which is of great importance to rainstorm processes, alters physical fields of a synoptic system, making a difference to the location, intensity and movement of a rainstorm. A terrain affects different types of heavy rain processes differently. In the processes of Classes A and B under the’rainstorm to the east of Taihang’, the uplift of a mountain leads to the changing vertical circulation, the changing ascension and the changing intensity of low-level convergence and finally precipitation. A mountain blocks and slows down the movement of a precipitation system, thus relocating a storm centre. The topography of the Taihang Mountains accounts for the formation of the’rainstorm over Taihang’.