Studying the impact of precipitation on PM2.5 mass concentration in different regions can provide an important scientific support for the air quality assessment and forecast as well as pollution prevention in this region. Based on the hourly precipitation observation data and PM2.5 mass concentration monitoring data in five typical cities (Chengdu, Leshan, Yibin, Mianyang and Dazhou) of Sichuan Basin from 2016 to 2021, the scavenging effect of precipitation processes on PM2.5 was analyzed in different cities from some aspects including the occurrence time, duration, intensity of precipitation and the initial mass concentration of PM2.5. The results show that the proportion of positive scavenging processes of precipitation on PM2.5 increases with the increase of precipitation intensity or initial mass concentration of PM2.5 in Sichuan Basin, and the scavenging rate rises. Under the condition of air pollution, the scavenging effect of precipitation with intensity exceeding 1 mm·h-1 on PM2.5 improves obviously in Sichuan Basin, and the scavenging rate reaches 35.0%. The scavenging effect is positively correlated with the duration of precipitation processes, and the scavenging rate of precipitation processes with the duration more than 3 hours is 9.0%-18.0% higher than that of precipitation processes with the duration less than or equal to 3 hours. The probability of positive scavenging processes is higher in the early morning and afternoon in Sichuan Basin, and the precipitation processes in the early morning have better scavenging effect on PM2.5. In comparison, the proportion of positive scavenging processes is higher in Leshan and Yibin after the precipitation, and under different initial mass concentrations of PM2.5, the scavenging rate is significantly higher than that in other cities with the increase of precipitation duration.
This paper addresses the spatial and temporal variation of cloud cover at different heights over southern China and its relationship with precipitation, which is useful for understanding the role of clouds in precipitation and feedback mechanisms as well as provides a basis and foundation for the development of cloud water resources. Based on 12 years (1998-2009) data from the D dataset of International Satellite Cloud Climatology Project (ISCCP), the temporal and spatial distribution characteristics of total cloud cover, low cloud cover, medium cloud cover and high cloud cover were analyzed over Southern China. And based on the CMORPH-AWS merged gridded precipitation product which combined National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) morphing technique (CMORPH) with precipitation observations from automatic weather stations, the relationships between the cloud cover and precipitation intensity, precipitation frequency were analyzed over southern China. The results are as follows: (1) The spatial distribution of total cloud cover and medium cloud cover over southern China was similar. The high-value center was located over the junction of Sichuan Basin, Guizhou and Chongqing, while the low-value center was located in Yunnan. High clouds were distributed in the western part of southern China, and decreased from west to east. Low clouds were mainly distributed over the southeastern part of southern China. Overall, low cloud cover decreased from coastal area to inland area. (2) The seasonal variation of total cloud cover and high cloud cover was similar, with cloud cover more in summer and less in winter. Moreover, the high value center of total cloud cover shifted with the seasons. The high cloud cover changed seasonally. The moderate cloud cover and low cloud cover were more in winter and less in summer, with small seasonal changes. (3) With the increase of total cloud cover and high cloud cover, the precipitation intensity increased but the monthly precipitation frequency decreased. With the increase of medium cloud cover, the frequency of monthly precipitation increased, while effect on precipitation intensity varied little. (4) With the increase of cloud cover of stratocumulus and stratus clouds, precipitation intensity and monthly precipitation frequency decreased. With the increase of cloud cover of altocumulus, altostratus and deep convection clouds, precipitation intensity increased while monthly precipitation frequency decreased. In general, cloud cover and precipitation were closely related over southern China, especially total cloud cover and high cloud cover had a greater influence on precipitation intensity and precipitation frequency, while medium cloud cover only had a greater impact on precipitation frequency. The cloud cover of stratocumulus, stratus, altocumulus, altostratus, deep convection clouds had a greater influence on precipitation intensity and precipitation frequency, while cloud cover of cirrus and cirrostratus only had a greater impact on precipitation intensity.