Fast Response of East Asian Precipitation in June 2020 to Local and Remote Aerosol Emission Reductions during COVID-19

Abstract

In this study, the fast response of East Asian summer precipitation to COVID-induced aerosol emission reductions is examined using the Community Earth System Model version 2.2 (CESM2.2). The emission reductions decreased aerosol optical depth and cloud cover over northern China in June 2020. The troposphere became warmer, strengthening the land-sea thermal contrast and anomalous southerly winds. The subtropical westerly jet accelerated and shifted southwards, favoring the low-level convergence, upward air motions, and subsequent condensational heating over the Yangtze River Basin (YRB). The feedback of condensational heating in return strengthened the convergence and ascent. The western North Pacific Subtropical High was intensified, which further enhanced the moisture advection and convergence over the YRB. Both the enhanced moisture convergence and ascent increased precipitation over the YRB during June 2020. Furthermore, local and remote emission reductions show different impacts on convection and moisture transport over the YRB. The emission reductions over China caused stronger convective precipitation (1.15 mm/day versus 0.63 mm/day), but weaker larger-scale precipitation (1.17 mm/day versus 2.24 mm/day) than the emission reductions outside China did. In addition to the emission reductions, the sea surface temperature (SST) anomalies in 2020 also play an important role in increasing precipitation over the YRB, contributing about 42.8%. And the relative contribution from SST anomalies even increases under the COVID-induced emission scenario.