Analysis Of The Influence Of Different Scale Topography On The Summer Monthly Precipitation Simulation In The Sichuan Basin

The influence of topography on precipitation has "scale" effect.The influence of macro and micro topography on precipitation are very different.Therefore,this thesis selected the Sichuan Basin as the research area,the macro trend of terrain was separated step by step as multi-level terrain by resampling,wavelet decomposition and two-dimensional empirical mode decomposition(BEMD)methods.Based on the station observation,the thesis discussed the impact of different levels of terrain on precipitation.Distributed models of summer monthly precipitation were established with the TRMM data as the macro item and the topographic factors of the multi-level terrain as the topographic items.Correlation coefficient,root mean square error and average absolute error with the verification station observation were used to estimate models and discuss the level of topographic factors that had the greatest impact on precipitation.The results shown that:(1)BEMD method was most suitable for constructing multi-level terrain and had the advantages of self-adaptability and keeping the data resolution unchanged.The method decomposed the terrain into high-frequency signal and low-frequency signal step by step,corresponding to undulating and broken surface and the macro trend of terrain.(2)Precipitation below 600-800 meters had a linear relationship with altitude,while the change of precipitation with altitude was not obvious at high altitude.The average of precipitation varied greatly with aspect,and the aspects corresponding to the maximum and minimum under multi-level terrain were relatively consistent.The aspects with the most and least precipitation were northwest and southeast in June respectively,while July and August were just in contrast.(3)The prevail precipitation direction(PPD)indicated that the water vapor came from westward direction in June,and it was uplifted by mountains in the eastern area of the basin to form a rainy area.With the onset of the monsoon,the water vapor from the southwest and southeast strengthened in June and August,increasing precipitation significantly and changing the spatial pattern.Mountains blocked water vapor from the northwest and formed a low-rain area in the Qinling Mountains.A rainy area and a high-value center appeared in Ya'an due to the "trumpet" topography.(4)The precipitation difference of windward slope and leeward slope was obvious under the meso-scale topography while that under the micro-topography was smaller.The precipitation on the leeward slope was even more than that on windward slope due to the unreasonable division with macro-topography.The range and spatial distribution characteristics of precipitation simulation results based on multi-level terrain were consistent with the station observation.The accuracy of models was ordered as followed:second-level > third-level > original and first-level > TRMM > fourth and fifth-level terrain.Therefore,the second-level topographic factors had the greatest impact on precipitation simulation,while that of the macro-topography had little impact.