Evaluation of United Arab Emirates WRF two-way nested model on a set of thick coastal fog situations

United Arab Emirates (U.A.E.) is suffering almost all the year by the consequences of visibility reduction caused either by persistent dust and haze episodes or by the occurrence of thick fogs especially over marine and costal regions. Visibility drop due to fog usually happens during late night and early morning and could last several hours before dissipation. The U.A.E coastal fogs are generally advective caused by the north westerly surface flow draining huge amounts of moisture from the Arabian Gulf sea strait to the 600 km U.A.E western and southern coasts. The large contrast in temperature between land and sea especially during winter season accelerates the condensation process over land near the surface, and contributes to the formation of low stratiform clouds close to the surface since the very beginning of night time. The Weather Research and Forecasting model is among the tools used by the forecasters for fog prediction. It offers a large number of prognostic and diagnostic fog predictability parameters. However, UAE WRF is not always successful in predicting fog formation. Recently, during February and March 2008, among more than 10 thick fog situations, only 6 were correctly predicted. This study aims the scientific evaluation of multiple WRF fog ingredients like physical parameterizations schemes (microphysics, radiation, planetary boundary layer, surface, horizontal diffusion), horizontal and vertical resolution, assimilation techniques (3D-Var, Nudging), over a series of 2008 winter and spring fog situations. It has been found that the inner most nest with very fine resolution (1.4km), an adapted microphysics scheme (Lin et al.), an adequate planetary boundary scheme (Yonsei University