Observation and simulation study on the macro-microphysical characteristics of a coastal fog offshore Zhejiang Province of China

A coastal fog patch occurred offshore Zhejiang Province of China on 1 April 2021, and was investigated by using observations from automatic weather stations, a millimeter-wave radar and a fog droplet spectrometer and the Weather Research and Forecasting model simulations. The macro-microphysical characteristics of the fog in five stages, i.e., light fog, formation, burst, mature and dissipation, have been studied. The millimeter-wave radar displays that the spatial scale of the fog episode is about 6 km, with vertical thickness and duration <700 m and 3 h, respectively. Cloud base lowering is essential for the fog burst reinforcement. Condensational growth is the main microphysical process for the fog formation and burst, however, the collision-coalescence process is important as well, with stronger intensity in the burst stage. Both observations and simulations indicate that the fog formed because of moisture convergence related to sea breeze developing along the Zhejiang coast. The convergence is not only favorable for the fog formation, but also for weak ascent and low cloud formation, which in turn intensifies the inversion layer by latent heat releasing, then further enhances descent in the boundary layer. A sufficiently moist subcloud layer capped by a strong inversion, together with downward motion are important factors for the cloud base lowering. The top-cooling entrainment is the main reason for the fog dissipation. Finally the macro-microphysical processes of the fog episode are summarized.