Averaged Depth and Duration of Snow Cover in the Mountain Regions of Bulgaria

Abstract

In two consecutive papers, we tried to fill a knowledge gap in the snow research in the Bulgarian mountains and to estimate the current snow conditions in these regions. In the previous study, we encountered a significant increase in mean winter air temperatures at all elevations, weaker changes in winter precipitation, and almost no alteration in seasonal snow cover maxima, except for at the highest stations. In the current study, we focused on the mean monthly and seasonal snow cover depths and snow cover duration in terms of days with snow cover above different thresholds. Additionally, we investigated the seasonal mean minimal and maximal air temperatures and the percentages of different precipitation types. We used data from 23 mountain meteorological stations for the same period (1961/62−2020/21) and the Mann–Kendall test to assess for possible significant monotonic trends in the data, combined with Sen’s slope method to estimate the trend magnitude. The results are not homogeneous, but they confirm and complement the previous ones. No clear altitude dependence was found for the mean seasonal snow cover depth or snow cover duration. The greatest reductions in the former characteristic were observed in the highest regions (about 24% on average), and the greatest reductions in the latter one were observed in the interval from 900 to 1100 m a.s.l. Both maximal and minimal air temperatures increased everywhere, and the percentage of days with snowfall declined at all elevations. We also encountered distinct zones where changes in the investigated characteristics had a different direction or lower magnitude. These findings complement regional knowledge about the current snow cover conditions in the Bulgarian mountains.