Anomalous holiday precipitation over southern China

Abstract. The Chinese Spring Festival (CSF) is the most important festival in China. Officially, this holiday lasts approximately one week. Based on the long-term station observations from 1979 to 2012, this manuscript reports that during the holidays, the precipitation over southern China (108° E–123° E and 21° N–33° N, 155 stations) has been significantly reduced. The precipitation frequency anomalies from the fourth day to the sixth day after Lunar New Year's Day (i.e., days [+4, +6]) were found to decrease by −7.4 %. At the same time, the daily precipitation amounts experienced a reduction of −0.62 mm d−1 during days +2 to +5. The holiday precipitation anomalies are strongly linked to the relative humidity (ΔRH) and cloud cover. The station observations of the ΔRH showed an evident decrease from day +2 to +7, and a minimum appeared on days [+4, +6], with a mean of −3.9 %. The ΔRH vertical profile displays a significant drying below approximately 800 hPa. Between 800 hPa and 1000 hPa, the mean ΔRH is −3.9 %. The observed station daytime low cloud cover (LCC) evidently decreased by −6.1 % during days [+4, +6]. Meanwhile, the ERA-Interim daily LCC also shows a comparable reduction of −5.0 %. The anomalous relative humidity is mainly caused by the lower water vapor in the lower-middle troposphere. Evident negative specific humidity anomalies persist from day −3 to day +7 in the station observations. The average specific humidity anomaly for days [+4, +6] is −0.73 g kg−1. When the precipitation days exclude the mean, the anomaly remains significant, being −0.46 g kg−1. A significant deficit of water vapor is observed in the lower troposphere below 700 hPa. Between 800 hPa and 1000 hPa, the mean specific humidity dropped by −0.70 g kg−1. This drier lower-middle troposphere is due to anomalous northerly winds. Authors have proposed that the anomalous atmospheric circulation is likely related to the holiday aerosol anomaly. Station and satellite observations show that the East Asian aerosol concentrations during the CSF decrease evidently, the largest reduction occurring on days [−3, −1]. At the same time, a concurrent cooling is observed in the lower troposphere. In addition, an anomalous low pressure tilting westward occurs in the troposphere over East Asia. The anomalous cold advection seems to help trigger/strengthen a cyclonic circulation anomaly, which is responsible for the northerly winds and the less precipitation around the holidays. This possible mechanism needs further clarification by elaborate observation analysis and modeling.