On the relationship between fog and low stratus and weather types over the Swiss Plateau

The relationship between persistent fog and low stratus (FLS) and weather types has been analysed in the extended winter season for the period 1957-2016. Ten automatic weather type classifications (WTCs) operationally computed at MeteoSwiss are considered. The capability to differentiate between days with and without FLS highly depends on the WTC. The best overall performance is found for WTCs based on sea level pressure and a large number of classes (GWT 26 MSL and CAP 27). Four out of 26 classes of GWT26MSL are able to represent 59% of all FLS days; eight classes already cover 85% of all FLS days. These FLS-prone classes are mainly characterized by a high pressure system near Switzerland and an easterly wind component. The frequency of FLS events is found to be roughly determined by the frequency of these weather types. Additionally, a clear seasonal cycle in the occurrence probability of FLS in those weather classes emerges. In December and January, the occurrence of the four most FLS-prone weather classes is a good predictor of interannual FLS varia-bility. A possible FLS forecast guidance is presented with tables showing the FLS fraction per weath-er class and month for all ten MeteoSwiss WTCs. There are indications that a part of the annual FLS frequency variability might not entirely be explained by the synoptic-scale flow alone and that other factors might have an influence. Further investigations are needed to shed more light on the effect of these factors and the robustness of the WTC approach with respect to this.