Impacts of Coronavirus-driven Reduction in Aerosols on Precipitation in the Western United States

The western United States, particularly the coastal areas from Washington to California, received much less precipitation than normal during February-March 2020. At the same time, the COVID-19 pandemic has led to improved air quality in pandemic-ravaged countries because of a reduction in aerosols due to the shutdown of industries and drastically reduced traffic. Aerosols transported from East Asia, as well as local aerosols, have been reported to enhance precipitation in the western U.S. by aerosol-cloud-precipitation interactions. Our objective is to examine whether there is a causal link between aerosol reduction due to COVID-19 and the reduction in precipitation over that region.

To explore the role of remote and local aerosols on reduced precipitation across the western U.S., we will use the Weather Research and Forecasting (WRF) model coupled with Chemistry (WRF-Chem) version 3.8.1. WRF-Chem simulates the emission, transport, mixing, and chemical transformation of trace gases and aerosols simultaneously with meteorology. We will rely on NASA's satellite data, especially in terms of water vapor, precipitation and aerosol products. We will use NASA's MERRA-2 as initial and boundary conditions for the atmospheric states (e.g., winds, temperature, geopotential height, humidity). The near real-time Whole Atmosphere Community Climate Model (WACCM) outputs will provide initial and boundary conditions for trace gases and aerosols at a 6-hourly scale.

We will consider four experiments to isolate the relative contribution of the aerosols. In these runs, the circulation (e.g., zonal and meridional winds, geopotential height) is the same but they have different settings for transported and local aerosols. Comparing the four experiments will allow us to assess the relative roles of aerosols in forcing the reduced precipitation during February-March 2020.