| Under the background of global climate change,the characteristics of precipitation in Gansu Province have changed,so it is necessary to strengthen the research on the characteristics and causes of precipitation in different regions and different intensities,as well as to discuss the short-term climate prediction of regional precipitation.In this paper,the spatial-temporal characteristics of annual and summer precipitation in Gansu Province from 1961 to 2018 are analyzed,and the changes of summer precipitation events with different intensities(strong,moderate and weak),atmospheric circulation characteristics and water vapor conditions are compared in the eastern and western parts of Gansu Province.Focusing on the eastern part of Gansu province,a statistical-downscaling model for short-term prediction of precipitation is established by selecting forecasting factors using partial correlation method.The main conclusions are as follows:(1)The results show that the annual precipitation decreases from southeast to northwest,which is consistent with the trend of seasonal precipitation and precipitation is mainly concentrated in summer.Precipitation increases significantly from 1961 to 2018,with an interannual cycle of 2-3 years and a weekly oscillation of6-8 days within the year.EOF analysis shows that the first mode is consistent in the whole region(contributing 52.5%),and precipitation is more(less)in positive(negative)abnormal years.The second mode is the east-west reverse type(contributing 20.4%),and the dividing line is located at 104°E;The precipitation is more(less)in the west of the positive(negative)anomaly year,and less(more)in the east;After 2012,precipitation increases in western Gansu while decreases in eastern Gansu.Based on the second mode,Gansu province is divided into two regions with104°E as the boundary,and the characteristics of precipitation events with different intensities in the two regions are studied.The results show that moderate precipitation events play a leading role in the contribution of summer precipitation in the east and west,accounting for 47.43%and 46.60%,respectively.In the past 58 years,the frequency of strong and moderate precipitation events in western Gansu show an increasing trend,while the frequencies of weak events and precipitation events in the eastern part show decreasing trend.The moderate and weak events change the most significantly in the eastern and western regions,which are particularly significant after 2010.(2)The analysis of weather causes shows that the precipitation in the east and west of Gansu province is affected by the South Asian High(SAH),westerly jet,Ural ridge,western Pacific subtropical high(WPSH)and vortex.The study area is located on the right side of westerly jet and north of the SAH at 200h Pa.At 500h Pa,the WPSH is more northward,westward and stronger,the geopotential height anomaly of Ural ridge is more than 20 gpm,and negative geopotential height anomaly and small fluctuation in the westerly jet are near the western Gansu,conducive to cold waves and upward movement development.The pressure high to the north of the Tibet plateau enhances the northwest air flow in western Gansu,and the circulation on the right side of the vortex east of the plateau conveys warm and wet air to the north by relay.The favorable conditions of high and low altitude coordinate with each other to promote the occurrence of heavy precipitation.For precipitation events from strong to weak in the west,the ridge of the SAH moves southward,and the intensity of the WPSH,the small trough of the westerlies and the geopotential height anomaly gradually weaken.During the precipitation events in the east,the intensity of the SAH is obviously weaker than that in the west,and the small trough of westerly fluctuation is weakened,and the negative geopotential height anomaly is weakened and moved southward in Gansu,the intensity of WPSH weakens but its position is still northward or westward.In the east of the plateau,there is a weak low vortex and its position is southward.The decrease of the intensity of key factors in the circulation field leads to the change of precipitation intensity from strong to weak.(3)Further diagnosis of water vapor conditions and quantitative indexes of key weather systems show that the water vapor channels of precipitation in Gansu mainly include the northerly branch of westerly belt and the southerly branch of monsoon from the equatorial ocean.When the heavy precipitation events occur in the west,the strong fluctuation in the westerly jet lead to the strong water vapor transport in the north branch,and the westerly jet index is 4.42%weaker than the climatological value,which is favorable for the effect on the East Asian monsoon.At the same time,the SAH is 28.65%stronger than the climatic average and the monsoon indexes over East Asia and South Asia are 4.32%and 0.07%stronger,respectively.The monsoon flow is abnormally enhanced,and the water vapor transport from the southerly branch to western Gansu is significantly enhanced with the relay transport of the southerly warm and wet air from the right side of the vortex in east of the plateau.During the precipitation events in the west,the intensity of water vapor transport in the north and south decreases successively,and the intensity and range of water vapor convergence in the study area decrease accordingly.When various precipitation events occur in eastern Gansu,the SAH index is 14.58%-20.50%weaker than that in western Gansu.The indices of east Asian monsoon and South Asian monsoon are still more than0.75%and 0.69%stronger than the climatic state.However,due to the strengthening of the westerly jet or the weakening and disappearance of the vortex east of the plateau,the water vapor transport of the southerly branch and the northerly branch are obviously weakened.The water vapor flux of the south branch is stronger than that of the eastern one under the strong and medium events in the west,especially the water vapor flux of the south branch.(4)Focusing on the average precipitation in eastern Gansu,the factors includinge over 850 h Pa V wind field in eastern Australia(40°S?20°S,155°E?175°W)and7)e over Sea level pressure field in Central Asia(40°N?55°N,50°E?70°E)are selected by partial correlation method.The results by detrended correlation analysis show thate and7)e represent the variation of Australian high intensity and continental thermal low intensity in northern hemisphere summer,respectively.Both of these two factors influence precipitation in eastern Gansu by adjusting monsoon intensity.When the Australian high is stronger,the precipitation belt in China is southward,which leads to less precipitation in eastern Gansu.When the continental thermal low pressure is stronger,the difference of pressure between land and sea increases and the monsoon intensity is strengthened,which is conducive to precipitation in eastern Gansu.Finally,the binary linear regression model betweeneand7)e and precipitation in eastern Gansu province is established by linear regression mothod.The precipitation deviation is 10.71mm and the average deviation rate is 8.3%in the model establishment stage,and the precipitation deviation is11.14mm and the average deviation rate is 8.1%in the model verification stage.The precipitation variability in the eastern part of Gansu province is generally well simulated by this model,and the model maintains good stability and reliability by the independent test. |
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