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

In Poland the phase in which precipitation occurs impacts the availability of water in its reservoirs, the potential for hydrological droughts, snowfall flooding and winter recreation. The influence of warming on the occurrence of precipitation phases on the Polish scale has not been studied so far. 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 a number of days with overall precipitation (Nd%) from a seasonal perspective. Moreover, the influence of seasonal and daily air temperatures on long-term variability in the precipitation phase indices was assessed. The precipitation phase was discriminated based on the observations of weather phenomena noted as current and past weather considering days with precipitation ≥ 0.1 mm. Precipitation phases significantly reacted to current warming. The strongest and widespread were increasing trends in rains (+1.74 for LqNd, +5.35 mm for LqT, +3.51% for LqNd%, +4.08% for LqT% per decade) and decreasing trends in snowfalls (−1.92 for SdNd, −3.12 mm for SdT, −3.65% for SdNd%, −3.26% for SdT% per decade) in winter when the warming was weakest but average temperature was close to freezing point. In winter, air temperature explained 60%–80% of the variance in the frequency of snowfall and rainfall. Daily air temperature > –2 °C contributed to trends in winter rainfall while daily air temperature > 1 °C most contributed to trends in winter snowfall. No trends in winter mixed precipitation were due to opposite changes in its frequency above and below 1 °C. In spring, the increase in a liquid phase was due more to the melting of a solid and mixed phase, while in autumn, it was due more to the transformation of a mixed phase into a liquid phase. 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 spring than in autumn and were mostly driven by changes in daily air temperatures between 0 and 5 °C. A diminished percentage of summer rains on days with air temperatures below 15 °C was compensated by their increase above that threshold thus producing no long-term trends. Warming significantly weakened the relationships between precipitation phases and average