Study of fog in Bulgaria by using the GNSS tropospheric products and large scale dynamic analysis

The fog formation, development, and dissipation are studied by employing the synergy between surface observations and vertically Integrated Water Vapor (IWV) from Global Navigation Satellite Systems (GNSS). Selected are three fog cases in February and November 2012 and the fog development in 4 locations in north Bulgaria is analysed. It is found that the IWV tends to decrease during fog formation, and densification. Increase of IWV leads to fog dispersion and can be a result of evaporation or advection of new humid air mass. The mixing ratio also decreases during the fog formation and increases during dissipation but has a distinct diurnal variability, which limits its short range forecasting potential. IWV is found to have a very high sensitivity to both air mass transformation and/or advection at altitude. In one case it is found that the arrival time of a new air mass at altitude is of key importance for further fog development or suppression. The change of the air mass leads to change of the diurnal cycle of surface parameters like temperature thus controlling the fog life cycle. Further complication of fog diagnosis is introduced by a dynamic component, reflecting the orography difference in west and east part of Bulgaria. The behaviour of the IWV and mixing ratio can be a valuable additional tool in decision making processes for very short range fog diagnosis and prognosis. For monitoring fog life cycle hourly or sub-hourly data-sets will be an advantage.