Analysis of an extremely dense regional fog event in Eastern China using a mesoscale model

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An unusually dense regional advection-radiation fog event over Anhui and the surrounding provinces in eastern China during Dec. 25–27, 2006, was investigated. At its mature stage, the fog covered most Anhui and parts of the surrounding provinces, reducing visibility to 100 m or less. It lasted more than 36 consecutive hours in some places. A mesoscale meteorological model (MM5), together with back-trajectory analysis, was used to investigate this fog event. The observations from a field station as well as hundreds of routine stations, along with two sets of visibility computing methods, were used to quantitatively and objectively validate the MM5 simulated liquid water content (LWC) and visibility. The verifications demonstrate that MM5 has a better fog predictability for the first day compared to the second day forecast, and better fog predictability compared to dense fog predictability with regard to the probability of detection (POD) and the threat score (TS). The new visibility algorithm that uses both LWC and number density of fog droplets significantly outperforms the conventional LWC-only based one in the fog prediction in terms of the POD score, especially for dense fog prediction. The objective verification in this work is the first time conducted for MM5 fog prediction, with which we can better understand the performance of simulated temporal and spatial fog coverage. The back-trajectory and sensitivity experiments confirm that subsidence and the steady warm and moist advections from southeast and southwest maintained the dense fog while the northwesterly dry wind resulted in dissipation of the fog.