Microphysics of fog bursting in polluted urban air

Abstract

The goal of this work is to study sudden fog visibility (Vis) changes called fog bursting occurred during polluted urban air conditions. Fog bursting refers to a sudden drop of fog Vis below 500 m over 30 min. A comprehensive field observational campaign called Costal City Fog Microphysical Research (CCFMR) was carried out over the city of Tianjin, China during winter of 2016–2017. The six fog bursting cases occurred during the project suggest that droplet number concentration (Nd) and liquid water content (LWC) were higher than these of other conditions. The mass concentration of PM2.5 (NPM2.5) was also higher before the fog bursting events. The fog droplet spectra (FDS) followed a Junge distribution. During fog bursting events, Vis, surface air temperature (Ta) and dew point depression (Ta-Td) exhibited decreasing trends. On the other hand, mean and turbulence kinetic energy (MKE and TKE), as well as friction velocity (u∗) had increasing trends in time. After the fog bursting, environment stability changed from the neutral stage to weakly unstable stage, and fog became deeper and optically thicker. The FDS broadened drastically between 20 and 35 μm diameter, and smaller Nd increased up to 1 order of magnitude. It is concluded that fog bursting is positively affected by combination of decreasing Ta by radiative cooling, increasing RH, turbulence mixing, and high value of NPM2.5. Results from this work suggest that fog bursting conditions need to be studied in detail by numerical weather forecast (NWP) models that currently lack of these conditions.