Evolution and forecast of a radiation fog event in Shanghai

Based on the conventional observational data at the surface and upper levels,satellite cloud images and numerical modelling products,the evolution characteristics and causes of a radiation heavy fog event from November 29 to December 1,2015 in Shanghai and the coastal region are investigated.The heavy fog process was predicted using the WRF (Weather Research and Forecasting) model and ECMWF (European Center for Medium-range Weather Forecasting) models.The results show that the heavy fog event is mainly caused by the radiation cooling effect and is characterized by strong intensity and long duration of near-sea fog.The heavy fog event can be divided into three periods:the formation of radiation fog on land and coastal region at the nighttime on November 29,the disappearance of fog on land at day on November 30,and the advection and enhancement of fog from near sea to coastal region in the evening on November 30 till the end of this fog event in the morning on December 1.This heavy fog event forms earlier than normal radiation fog events,mainly because of the continuous descending of low cloud ceiling and radiation cooling.Poor radiation heating and increasing humidity maintain the fog over the near sea surface for a long time period.When the fog over the near sea area moves towards the coastal land at night,the fog advection and radiation cooling effects result in the heavy fog again.Both the WRF model and the high-resolution EC model were not very successful to predict this heavy fog event,probably due to the poor simulation of humidity at lower levels.Thus,synthesis analysis using cloud images and AWS (Automatic Weather Station) data at the coastal region is still an important mean for the short-time forecast of coastal fog,before the numerical weather prediction models are accurate enough to meet the operational requirements.