Opady marznące i gołoledź w Hornsundzie (SW Spitsbergen)

Artykuł przedstawia charakterystykę opadów marznących i gołoledzi w strefie polarnej na podstawie danych z Polskiej Stacji Polarnej w Hornsundzie. Analizie poddano opady w 34 sezonach zimowych (od 1982/83 do 2015/16 roku). W okresie tym wystąpiło łącznie 197 dni z opadami powodującymi gołoledź. Zjawiska te występowały od września do czerwca, z największym nasileniem od października do maja. Zmiana liczby dni z gołoledzią w następujących po sobie sezonach zimowych była bardzo zróżnicowana (od 0 do 17 dni na sezon). W badanym okresie nie zaobserwowano istotnej tendencji zmian liczby dni z opadami marznącymi. Najczęściej opady takie występowały przy cyklonalnym typie cyrkulacji: adwekcji mas powietrza z kierunków od S do W oraz przy zatoce niżowej i centrum niżu w rejonie Spitsbergenu. Typową sytuacją sprzyjającą pojawieniu się gołoledzi było zbliżanie się niżu znad Morza Grenlandzkiego lub Morza Norweskiego z frontem ciepłym, przed którym występowała strefa opadów marznących.

The article discusses the climatological and synoptic characteristics of freezing rain and freezing drizzle causing glaze at Hornsund. Freezing precipitation and glaze are relatively rare phenomena, but they can expose human activity in the polar regions to serious risks. The study is based on data from the Polish Polar Station Hornsund. The synoptic analysis of selected cases of supercooled precipitation presented in the article was performed on the basis of surface synoptic charts, absolute topography charts at 700 and 850 hPa levels, and aerological diagrams. The type of circulation which co-occurred with each day with glaze was determined using a calendar prepared by T. Niedźwiedź. Over the course of the 34 winter seasons under investigation (between 1982/83 and 2015/16), there was a total of 197 days with supercooled precipitation. Air temperatures ranged between -8.5° and 1.2°C, and predominantly between -3° and 0°C, when freezing precipitation under investigation were observed. In a typical situation, supercooled precipitation followed a long thawless period. There were also frequent cases when several days of thaw and liquid precipitation were followed by freezing rain turning into snowfall with simultaneous air temperature drops. One feature characterising weather with glaze is the occurrence of sudden and multi-directional changes of temperature, which are almost completely determined by advection. Freezing precipitation occurs both when air temperatures drop below zero and when they rise above freezing point. Glaze can appear in Hornsund from early September until early June, with the highest likelihood observed between October and May. The change in the number of days with glaze observed in the successive winters was variable (ranging from 0 to 17 days per season). The occurrence of freezing precipitation in the area of the Polish Polar Station is nearly exclusively correlated with circulatory conditions. Most such phenomena are associated with cyclonic activity and advection of warm air from above the Norwegian Sea and the Greenland Sea. Occasionally, liquid precipitation may turn into freezing rain as a consequence of a local factor, for example the presence of cold surface air caused by sea ice in the Hornsund fjord or the influx of freezing air carried by katabatic winds from the Hans Glacier, with simultaneous southerly and westerly advection of warm air masses with rainfall. No significant trend was observed as regards multi-annual changes in the number of days with freezing rain. Such rain was observed most frequently during the winter season of 2002/03 (17 days in total), and was absent altogether during three seasons (1983/84, 1984/85 and 2015/16). There is also no noticeable correlation between the number of days with glaze and the gradual increase in average air temperatures over successive winters and years. As is demonstrated by studies of changes in the circulation over Spitsbergen conducted by T. Niedźwiedź (2013), the values of westerly and southerly circulation indexes went up in the years 1951-2012, which is evidenced by an increased frequency in the corresponding westerly and southerly advection. There is also a rising trend in the activity of cyclones in winter. The above elements seem to contribute to an increased frequency of days with glaze. However, there is no observable growth in the numbers of these phenomena.