Analysis and Modeling of a Long-Lasting Fog Event over Beijing in February 2007

The evolution characteristics of a long-lasting fog event over Beijing during 20-22 February 2007 are investigated using the 5-min automatic visibility data and conventional meteorological observations. Data analysis results reveal that there is a close relationship between the development/evolution of this fog event and the weather conditions such as high humidity, light wind, and low temperature in the lower troposphere.Furthermore, numerical simulations of this event are carried out by using the Advanced Research WRF(ARW) V2.2-Noah LSM land-atmosphere coupled model.The model is two-way and two-nested with improvement in the description of the interface between the urban underlying surface and the atmospheric boundary layer. The horizontal grid spacings are 9 and 3 km for the two domains. The ground-based GPS precipitable water vapor content and automatic and conventional meteorological observations at the initial time are assimilated. The simulation results indicate that the control experiment with local data assimilation has successfully captured the spatial-temporal evolution of this fog event, especially the synoptic patterns and characteristics of the weather conditions (high humidity, light wind, and lower temperature). The results are well consistent with observations. Comparison of two experiments (with and without local data assimilation) shows that the model initial conditions are considerably modified with the WRF 3DVAR.The assimilation of local observations leads to significant improvement in the prediction of this event with better representation of static stability, enhanced southeasterly winds, concomitant warmer moisture in the south of Beijing, intensified temperature inversion between the lower and upper troposphere, and generally decreasing environmental temperature.