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[1] Typhoon Rusa (2002) is simulated using a threedimensional mesoscale model (MM5), and the characteristics of gravity waves generated by convection associated with the typhoon are investigated. The gravity waves in the stratosphere propagate in two directions, northwestward and southeastward according to the convective bands propagating in the same directions, although the typhoon itself moves north-northeastward. Spectral analyses show that the inertio-gravity waves (IGWs) in the stratosphere generated by Rusa have a dominant horizontal wavelength of 300-600 km, a vertical wavelength of 3 -11 km, and a period of 6 -11 hrs. A large fraction of the IGWs is filtered out in the upper troposphere and stratosphere mainly due to the critical-level filtering process. The decreased magnitude of the momentum flux with height in a non-filtered region is likely due to the damping process, with a minor contribution by the wave breaking process that can occur exclusively near the criticallevel phase speeds. Citation:

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Seoul, the most populous city in South Korea, has been practicing social distancing to slow down the spread of coronavirus disease 2019 (COVID-19). Fine particulate matter (PM2.5) and other air pollutants measured in Seoul over the two 30 day periods before and after the start of social distancing are analyzed to assess the change in air quality during the period of social distancing. The 30 day mean PM2.5 concentration decreased by 10.4% in 2020, which is contrasted with an average increase of 23.7% over the corresponding periods in the previous 5 years. The PM2.5 concentration decrease was city-wide and more prominent during daytime than at nighttime. The concentrations of carbon monoxide (CO) and nitrogen dioxide (NO2) decreased by 16.9% and 16.4%, respectively. These results show that social distancing, a weaker forcing toward reduced human activity than a strict lockdown, can help lower pollutant emissions. At the same time, synoptic conditions and the decrease in aerosol optical depth over the regions to the west of Seoul support that the change in Seoul’s air quality during the COVID-19 social distancing can be interpreted as having been affected by reductions in the long-range transport of air pollutants as well as local emission reductions.

[1] Sensitivity of typhoon-induced gravity waves to cumulus parameterizations is examined using a mesoscale model (MM5). For this, Typhoon Rusa (2002) is simulated with four cumulus parameterizations (Kain-Fritsch, Grell, Anthes-Kuo, and Betts-Miller schemes) and the characteristics of typhoon-induced gravity waves are compared. The experiments show differences in rainband structure and vertical motion, resulting in different forcing spectra for zonal wavelength and period. As a result, induced stratospheric gravity waves are different in amplitude and spectral shape. However, the difference is not as large as that in the forcing spectrum, since a large portion of the waves generated by major forcing components is filtered out by the background wind, which is nearly the same in all the experiments. Instead, variation in zonal wavelength and period of forcing modifies the characteristics of stratospheric gravity waves by changing damping time scale in the nonfiltered region. Citation: