The Southwest China Flood of July 2018 and Its Causes

Lijuan Wang,Wei Gu,Peiqiang Xu,Wen Chen,Lin Wang,Yuyun Liu

doi:10.3390/atmos10050247

Abstract

Excessive rainfall was observed over Southwest China in July 2018, leading to floods in several major tributaries of the Yangtze River and landslide and debris flow in the neighboring provinces. The rainfall during 7–11 July was unusually heavy and broke the record that can be traced back to 1961. The occurrence of the excessive rain can be attributed to the anomalous convection over the western North Pacific and the presence of a mid-latitude Rossby wave train. On one hand, the convection over the western North Pacific was anomalously strong in July 2018, and it could have excited the negative phase of the Pacific–Japan pattern and led to a northwestward shift of the western Pacific subtropical high. Hence, the water vapor transport toward inland China including Southwest China was enhanced, providing a favorable moisture environment for precipitation. On the other hand, a mid-latitude Rossby wave train was observed to propagate from Northern Europe towards East Asia, which was conducive to anomalous ascending motion over Southwest China via warm advection and differential vorticity advection, creating a favorable dynamical condition for precipitation. As a result, the combination of the two effects mentioned above led to the occurrence of the flood over Southwest China in July 2018.

Highlights

Summer is the primary rainy season for most parts of East Asia
Based on gridded precipitation data from China Meteorological Administration, atmospheric data from the NCEP/NCAR reanalysis dataset, and outgoing longwave radiation (OLR) data from NOAA, this study investigated the characteristics of the flood that occurred over Southwest China in July 2018 and its possible causes
It reveals that the rainfall in July 2018 was above average in the northern part of Northeast China, North China, and Southwest China and below average in other parts of eastern China (Figure 1a)

Summary

Introduction

Summer is the primary rainy season for most parts of East Asia. The anomalous summertime rainfall often causes meteorological hazards such as floods and droughts, exerting profound influences on society [1,2]. From the viewpoint of the summer mean, the characteristics and causes of this unusual rainy season have been analyzed thoroughly with an emphasis on the eastern and northern parts of China [6,7,8]. The precipitation data were from the daily temperature and precipitation dataset for China region on 0.5◦ × 0.5◦ grids, which was derived from the observation of 2472 meteorological stations in China (see Figure 1 in [28]). The interpolated monthly mean outgoing longwave radiation (OLR) data on 2.5◦ × 2.5◦ grids were used as a proxy for convection This OLR dataset that spans from 1979 to the present was provided by the National Oceanic and Atmospheric. 2018 is considered in this study, and all the anomalies are defined as the departures from the long-term means of 1981–2010

Observed Southwest China Flood in July 2018

July Mean

Flood During 7–11 July

Time series observedvorticity

Findings

Conclusions