Spatial And Temporal Characteristics Of Summer Graded Precipitation Over The East Of Northwest China

Based on the daily precipitation data collected from 59 stations in the eastern part of northwest China during the recent 50 years (1965-2014), the rainfall intensity can be classified into three levels:light rain, moderate rain and heavy rain. From the perspective of climatology, we analyze the spatial and temporal characteristics of the three-level precipitation amounts, precipitation days, and precipitation intensity during summers of the 50 years in the areas, and we discuss their relations and the spatial pattern differences of typical drought-and-flood years through partial correlation coefficient calculation and composite analysis. Results are as follows:(1) The spatial distribution of amount, days and intensity for different grades of precipitation in the east of northwest China were uneven and decreased from south to north in varying degrees. The similarity coefficients of total precipitation amount to graded precipitation amount, days and intensity were more than 0.88.(2) The light rain occurred most frequently and its precipitation days accounted for 81% of total summer precipitation days, the moderate rain took the second place with 14%, and the heavy rain ranked the third (5%). However, the precipitation amounts of light, moderate and heavy rain were roughly equal and each of them contributed about one third of total precipitation amount in summer. The partial correlation coefficients of summer precipitation amount to moderate and heavy precipitation days were higher than 0.75 and reached the significant level. It showed that changes of moderate and heavy precipitation days could roughly reflect change of total precipitation amount in summer.(3) Summer precipitation amount was mainly affected by moderate and heavy precipitation amount. Total precipitation days changes were consistent with light precipitation days changes and decreased over the whole eastern part of northwest China. And the increase of total precipitation intensity was derived from the increase of heavy precipitation intensity. Different graded precipitation amounts and days had an abrupt change around 1996.(4) In flood (drought) years, graded precipitation amounts and days were more (less) than normal. The increase (decrease) of total precipitation amount was mainly caused by the increase (decrease) of moderate, heavy precipitation days and amounts in summer over the east of northwest China.(5) The spatial distribution of water vapor content for different grades in the east of northwest China were uneven and decreased from south to north to different degrees. There was significant growth from non-rain to light rian, light rain to moderate rain while non-obvious growth between heavy rain and moderate rain. It indicated that water vapor content increased with the rising level of precipitation. However, precipitation extreme events like heavy rain were depended by a combination of reasons over the east of northwest China.