Fog climatology and stability index for Plovdiv 1991-2018

Despite the continuous improvement of weather prediction fog diagnosis and forecasting remains a challenge with large economic losses for public services and in particular aviation where the cost of flight delays and rescheduling is estimated to hundreds of million euros per year. Today the operational fog forecasting is mainly done with "in-house" developed tools, which is understandable due to the fog life cycle peculiarity. The aim of this work is to investigate the fog climatology for Plovdiv, Bulgaria for the period 1991 - 2018 and to use it for calculation a threshold value of stability index, which can be implemented as an operational forecast tool. The climatology shows well-defined seasonal behavior of the fog and that the majority of the fog registrations are with horizontal visibility below 200 m. A 10-year moving average of the fog registrations time series shows a decrease after 2012. Stability index values for various visibility ranges are calculated and compared. In the last decade, there are major improvements in horizontal and spatial resolution, microphysics, and initial conditions of the Numerical Weather Prediction models.  However, fog forecasting remains a challenge due to the small scale of the phenomena and local effects, which can remain unresolved by the models. One fog case in January 2013 is selected for numerical weather prediction simulations with the WRF model for the city of Plovdiv. The reliability of the index is evaluated both with observations and model data. It was found that while the index with its site-specific threshold value well describes the fog evolution, the WRF model has large deviations in temperature compared to the observations during daytime.