Trends in the indices of precipitation phases under the background of current warming in Poland, 1966-2020

Precipitation phase depends on air temperature; therefore, the variabilities in snowfall and rainfall are counted among the indicators of current climate change. The shifts in the structure of precipitation phases have significant environmental implications and influence public perception of climate change. Based on data from 42 synoptic stations for the period 1966-2020, this study discusses trends in the indices of liquid (Lq), mixed (Mx) and solid (Sd) precipitation describing their totals (T), frequencies (Nd) and contribution to overall precipitation totals (T%) and to a number of days with overall precipitation (Nd%) from a seasonal perspective. Moreover, the influence of seasonal air temperature on long-term variability in the indices and changes in the probability of precipitation phases in 1 °C  wide ranges of daily air temperatures between  1966-1985 and 2001-2020 were also analysed. Precipitation phases were discriminated based on the observations of weather phenomena noted as current and past weather in SYNOP messages, considering days with precipitation ≥0.1mm.

In Poland, precipitation phases significantly reacted to current warming. Trend directions corresponded to warming; Thus, trends in liquid precipitation indices were positive, while trends in mixed and solid precipitation indices were negative, both resulting from redistribution of particular phases. The strongest and widespread were increasing trends in winter rains (+1.74 for LqNd, +5.35 mm for LqT, +3.51% for LqNd%, +4.08% for LqT% per decade) and decreasing trends in winter snowfalls (–1.92 days for SdNd, –3.12 mm for SdT, –3.65% for SdNd%, –3.26% for SdT% per decade). In winter, air temperature explained 60–80% variance in the frequency indices. In spring and autumn, increasing trends were found in LqNd% due to the transformation of solid to liquid phase, and in autumn due to melting of mixed and solid phases. In spring, air temperature explained at most 30–50% of the variance in the indices of snowfall frequency. Negative trends in mixed precipitation were significant in transitional seasons, being more prominent in MAM than in SON. A diminished percentage of winter mixed precipitation and summer rains on days with air temperature below 0 °C and 15 °C, respectively, was compensated by an increase in their frequency above these thresholds, thus producing no long-term trends. In most cases, trends in air temperature and indices of precipitation phases diminished the correlations between these variables, particularly in transitional seasons in the case of mixed and solid phases. Solid and mixed phases are becoming sporicidal phases in Polish transitional seasons.