Abstract: Located in arid and semi-arid area, the eastern part of northwest China has scarce precipitation and is easily influenced by drought and flood. 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 precipitation intensity can be classified into three levels:light rain(0.1-9.9 mm per day); moderate rain(10.0-24.9 mm per day); heavy rain (≥25.0 mm per day). From the perspective of climatology, we analyze the spatial patterns 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 show as follows:(1) 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.(2) 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.(3) 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.(4) In flood(drought) years, graded precipitation amounts and days were more(less) than normal. With the rising level of precipitation, the percentages of precipitation amount anomaly and precipitation days anomaly increased(decreased)rapidly while the similarity coefficients of total precipitation amount anomaly to precipitation amount anomaly and precipitation days anomaly increased. 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. However, the above analysis was mainly about the spatial distribution of graded summer precipitation amount, precipitation days and precipitation intensity. As for the temporal variation of graded summer precipitation, further study and improvement are also awaited.

Key words: east of northwest China, boreal summer, graded precipitation, spatial distribution, drought and flood