Aerosol impacts on fog microphysics over the western side of Taiwan Strait in April from 2015 to 2017

Abstract

Fog has drawn much attention from scholars and the public; it can reduce visibility to less than 1 km and affect air, sea, and land transportation and economy. In late winter and spring, fog events frequently affect intensive human activity in the coastal areas of Eastern China. In this study, numerical simulations from the Weather Research and Forecasting model were combined with observations to investigate the effects of aerosol on the microphysical properties of fog over the Taiwan Strait in April from 2015 to 2017. The mean values of observed liquid water content, number concentration of fog droplets, and mean diameter fog droplets were 0.11 g/m3, 2.2 × 108 #/m3, and 13.9 μm. The observed relationships between fog microphysical properties were close to the simulation results in the urban aerosol experiment, suggesting that the fog events at Kinmen were significantly affected by urban aerosols. The simulation results indicated that the wind blew from the south, and the air temperature was about 1°C higher than the sea surface temperature on foggy days. A stable condition accompanied by a shallow planetary boundary layer was favorable to fog formation. In the urban aerosol experiment, higher number density of aerosols resulted in more but smaller droplets and increased the LWC. An increase in aerosols reflected shortwave radiation, resulting in a colder and wetter atmosphere during fog events in the daytime. This led to a thicker fog layer and an increased number of fog droplets in the urban aerosol experiment. Foggy days and microphysical properties at Kinmen significantly differed between the studied years. As suggested by the correlations between fog-day anomalies and the corresponding Niño 3.4 index and Oceanic Niño Index, this interannual difference might be attributable to the 2015 to 2016 El Niño–Southern Oscillation (ENSO). Further observations are required to ascertain the relationship between fog events, fog microphysics, and ENSO events.