Boundary Layer Features of One Winter Fog in the Yangtze River Delta, China

The goal of this work is to analyze the boundary layer features and large-scale synoptic conditions affecting fog conditions over two adjacent meteorological sites (Taicang and Xiangcheng near the Yangtze River Delta). These sites had significant differences in moisture advection conditions and inversion layer intensity during this fog episode which occurred during November 24–25, 2018. A cold high pressure center moving eastward was linked to the fog. The surface convergence line moved, resulting in a significant difference in fog formation and maintenance between the two stations. Another is that a surface inversion capped the fog layer. The surface inversion layer at Taicang formed between 17:00 and 08:00 LST (local time) under the influence of moist cold air advection. During inversion formation, its top height reached 200 m, and the maximum inversion gradient was 2.0 °C 100 m−1. Xiangcheng was under the influence of a weak convergence area before the formation of the dense fog; therefore, inversion failed to occur. Results suggest that during large-scale fog events, the formation and development of fog in different regions can be dominated by the physical processes occurring. It is concluded that large-scale and mesoscale conditions interact extensively with each other, characterizing the fog life cycle. This result is supported by the fog layer thickness (50 m versus 200 m) that occurred at the two nearby locations of Taicang and Xiangcheng stations.