A forecast improvement of a widespread fog event using ensemble simulations

In the second decade of January 2020, beginning on 12 January, a case of long-lasting continuous fog occurred in most regions of ​​Serbia. On 17 January, widespread fog was also observed at some stations in Croatia, Bulgaria, Romania, Hungary, and Slovakia. This fog event indicated the presence of a broad air mass whose characteristics favored the formation of regional fog. A high-pressure system with a warm core at upper levels, centred over western and central Europe, caused extremely stable weather with a strong temperature inversion that persisted throughout the fog period. Mean sea level pressure over regions of Serbia exceeded 1027 hPa during the event. The effects of ECMWF model initial and boundary conditions on simulations with the regional Eta model were examined. The ECMWF model forecast starting at 12 UTC, compared to the forecast starting at 00 UTC on 12 January, was slightly better in terms of fog duration and location, however, in both forecasts the fog dissipated after a few hours. Experiments with improved horizontal and vertical resolutions of the Eta regional model nested into the ECMWF model did not improve the forecast of this fog event. A series of ensemble simulations with the Eta model was conducted to investigate the effects of physical parameterizations on the fog forecast. The success of the fog forecast results was evaluated using surface and upper air fields. In these forecasts, fog occurrence was defined based on a liquid water content threshold (0.05 g m⁻³), which was converted to visibility using the Kunkel equation.