Foehn classification and climatology in Sofia for 1975–2014

Abstract

Foehn is a warm, dry, and downslope wind blowing in the lee side of a mountain range. It is a well known example of a local atmospheric circulation. The foehn wind is also an extreme weather event, and its forecasting is an important task for the short-range weather forecaster. The foehn in Bulgaria is observed on the northern slopes of the mountains, as a result of warm air advection from the south and southwest. Its occurrence is highest north of the Vitosha and Balkan mountains. In this study, a synoptic classification of the meteorological conditions leading to foehn in the central meteorological station in Sofia for the period 1975–2014 is made. Foehn climatology is prepared, and in addition, an evaluation of the foehn as an extreme weather event by wind gust is presented. For the period 1975–2014, there were 298 days with foehn in Sofia, which resulted from 220 synoptic cases. A manual foehn classification was developed with four major types. Type I is associated with the Mediterranean cyclone with the highest frequency – 52% of the foehn days. Foehn climatology gives average annual number of 7.5 foehn days but with a large variance between decades. The lowest annual number of days (4.5) is registered for the 2005–2014 period, and it was associated with the lowest recorded wind gust (22 m/s).