The Relationship Between Precipitation And Altitude Over East Of The Tibetan Plateau

As an important factor which determining the climatic distribution of precipitation,topography has a multi-scale and multi-dimensional complex influence on the cloud and rain process through a series of dynamic and thermal processes.Due to the complexity of topographic forcing,many widely recognized topographic and climatic effects remain in the stage of qualitative understanding,which leads to the defects of current numerical models in the simulation of precipitation in complex topographic areas.The interaction between topography of different scales and East Asia monsoon makes the relationship between topography and precipitation in China more complicated.Studying the spatial characteristics of the relationship between precipitation and topography is not only helpful to enhance the understanding of the dynamic and thermal processes that affect the precipitation over mountainous areas in China,but also provides detailed observation basis for the evaluation and understanding of the simulation of the precipitation process in mountainous areas.The quantitative relationship between precipitation and altitude over east of the Tibetan Plateau was systematically studied using hourly rain-gauge data,satellite data and ERA5 reanalysis data under different scales of terrain.And the performance of simulating the relationship between precipitation and altitude in numerical model is evaluated objectively and quantitatively.The main conclusions are as follows,Firstly,under the limited scale of(100km),the relationship between precipitation and altitude was analyzed quantitatively based on the Geographically Weighted Regression model.It was found that the relationship between precipitation frequency(intensity)and altitude has good regional consistency.And on the whole,the precipitation frequency(intensity)increases(decreases)with altitude over east of the Tibetan Plateau.The regional difference of the relationship between precipitation and altitude is very significant.By dividing the precipitation into short-afternoon precipitation events and long-nighttime precipitation events and then analyzing the relationship between precipitation of the two kinds of events and altitude,it was found that the precipitation amount of short-afternoon precipitation events mainly increases with altitude and the distribution of the relationship between precipitation amount of short-afternoon precipitation events and altitude has good regional consistency.While the regional consistency of the relationship between precipitation amount of long-nighttime precipitation events and altitude is poor,and there are more stations of which coefficients are negative than the stations of which coefficients are positive.After distinguishing the stations over large-scale topography and the stations over the eastern isolated topography according to the scale of topography,it can be seen that the precipitation amount of large-scale topography tends to increase in short-afternoon precipitation events and decrease in long-nighttime precipitation events compared with the eastern isolated topography.In order to compare the seasonal differences of the relationship between precipitation and altitude,the relationship between precipitation in cold season and altitude over the east of Tibetan Plateau was analyzed contrastively.It was found that compared with the warm season,there are mores stations of which the precipitation frequency decrease with altitude in cold season.And the correlation between precipitation and altitude in cold season is weaker than in warm season.Secondly,the variation characteristics of the precipitation along the large-scale terrains over the typical complex topography in China(North China and Southwest China)were analyzed.It was found that the characteristics of warm season precipitation over North China increase first and then decrease with altitude.And with the increasing of altitude,the location where the relationship between precipitation frequency and altitude has changed is higher than the location of precipitation amount.It can be seen that in the characteristics of diurnal variation,the precipitation amount over western alpine region of North China begins to spread downstream from afternoon and moves to eastern plain of North China at night and then strengthened.Then the variation of the precipitation with the altitude in short-afternoon precipitation events and long-nighttime precipitation events were concerned,it was found that the relationship between precipitation amount and altitude is opposite in the two kinds of precipitation events.Over Southwest China,the relationship between the warm season precipitation and altitude is not significant.But the precipitation over the eastern slope of the Yungui Plateau is slightly larger than that on the both sides of the eastern slope.There are significant differences between the precipitation amount(frequency)over the east region and the precipitation amount(frequency)over the west region.The cold season precipitation amount and frequency decrease with altitude.There is a little difference between the characteristic of the variation of the precipitation amount and frequency with the increasing altitude.The abrupt changes between the high value area and low value area of precipitation amount and frequency appear to move eastward during daytime.The smallest precipitation intensity in cold season appears over the eastern slope of the Yungui Plateau,and the largest precipitation intensity appears over the Yungui Plateau.By analyzing the features of large scale temperature and humidity field,it was found that the specific humidity is high and the environment gives priority to convergence in the lower troposphere over east of 104°E.And there is a local secondary circulation in the lower troposphere over the eastern region of Southwest China blocked by the topography,which is in favor of the formation of precipitation.Further analysis shows that the reason for the appearance of the high precipitation on cold season over the east of Southwest China may be related to the Kunming quasi-stationary front.The large precipitation area is located behind the Kunming quasi-stationary front.And the eastward movement of the large precipitation area during daytime is also related to the eastward movement of the Kunming quasi-stationary front.Finally,the ability of the 24 h predicting precipitation of the European Center for Medium-range Weather Forecast Integrated Forecast System model to reproduce the relationship between precipitation and altitude was evaluated based on the GWR model.It was found that the model can show the characteristic that precipitation intensity(frequency)mainly decreases(increases)with the increasing altitude.However,the negative(positive)regression relationship between precipitation intensity(frequency)and altitude is weaker(stronger)in model than that in observation of most stations,which is because of the difference of the underestimation(overestimation)of precipitation intensity(frequency)at different altitude.By comparing the relationship between precipitation in precipitation events and altitude in model and observation,it was found that the precipitation frequency of short-afternoon precipitation events in model is more overvalued over low-altitude regions,which making the model tend to weaken the positive regression relationship between precipitation frequency and altitude.In view of the long-nighttime precipitation events,the model has more stations of which precipitation frequency decrease significantly with altitude than observation.Moreover,the model significantly underestimates the value of the precipitation intensity decreasing with altitude of most stations.The evaluation results show that the GWR model provides an effective and objective method for evaluating the correlation between simulation deviation of precipitation and altitude in different terrain areas quantitatively.