Arkadiusz M. Tomczyk

Warm spells in Northern Europe in relation to atmospheric circulation

This study describes warm spells in Northern Europe and determines the synoptic situations that cause their occurrence. In this article, a relatively warm day was defined as a day when the maximum temperature exceeded the 95th annual percentile, and a warm spell (WS) was considered to be a sequence of at least five relatively warm days. In the analysed multiannual period and within the investigated area, 24 (Kallax) to 53 (Oslo) WSs were observed. The occurrence of WSs was mainly connected with positive anomalies of sea level pressure and a 500-hPa isobaric surface, displaying the presence of high-pressure systems. This occurrence was also accompanied by positive T850 anomalies.

Frost periods and frost-free periods in Poland and neighbouringcountries

Abstract The main objective of this article is to analyse the multiannual variability of frost occurrence and the length of the frost-free period in Poland and neighbouring countries between 1971 and 2010. A frost day was de- fined as a day onwhich the minimum temperaturewas below 0°C and the maximum temperature was above 0°C. On average, between March and November, there were 30 frost days in Poland and neighbouring countries, and their number ranged between 12 and 61. In the analysed multiannual period, there was a statistically significant tendency for the last spring frost to be earlier. Conversely, the first autumn frost occurred increasingly late, although the changes were not statistically significant. All but three stations (Augsburg, Kaliningrad, Olsztyn) recorded an increase in the length of frost-free periods, which was statistically significant in 51% of stations. The pressure condition conducive to the occurrence of frosts was anticyclonic circulation, as confirmed by the SLP and z500 hPa maps which were drawn up, and the Grosswetterlagen (GWL) classification types which were distinguished.

If not NAO then what?—regional circulation patterns governing summer air temperatures in Poland

The correlation between the daily/monthly North Atlantic Oscillation (NAO) index and the daily/monthly anomalies of the summer (JJA) surface air temperatures in Poland is close to 0.00, being positive in most of the country (but hardly exceeding 0.15) and negative in the southeast. Therefore, the regional scale circulation patterns other than NAO were determined using principal component analysis (PCA) to establish the relationship between the air circulation and the daily anomalies of the mean, maximum and minimum air temperatures in Poland. Regression and correlation analysis were applied to define the strengths and spatial distributions of these relationships. The best results were obtained for the mean and maximum daily temperature anomalies, for which the Pearson correlation coefficient (r) between the modelled and observed values exceeded 0.75 in the vast areas of central and southeastern Poland, while, over the rest of the country, it amounted to 0.7. The weaker influence of the circulation on air temperatures is observed in the northwestern part of the country, along the Baltic Sea shore (r < 0.65). The weakest results were obtained for the anomalies of the daily minimum temperatures (r equals 0.5–0.6 over most of the country and drops below 0.4 in the northwest). Furthermore, the influences of each PCA-based regional circulation pattern on the variabilities of the surface temperatures were analysed. The circulation pattern recognized as the second principal component, distinguished in the positive phase by the high-pressure conditions with the anticyclonic centre located right over central and southeastern Poland, revealed the strongest positive relationship with the air temperature.

Thermal conditions relative to atmospheric circulation in the Christmas period in Poland

Abstract The main objective of this article was to determine thermal conditions in the Christmas period and the impact of atmospheric circulation on their formation. The article is based on data on the mean daily air temperature for 16 stations in Poland for the 1966-2014 multiannual period. In this period, no statistically significant changes of air temperature were observed, neither in December, nor the Christmas period. The occurrence of averagely warmer holidays was associated with the inflow of maritime polar air masses from the western sector, while colder holidays occurred with the inflow of continental polar air masses from the eastern sector.

Impact of macro-scale circulation types on the occurrence of frosty days in Poland

The research aimed at determining variability of occurrence of frosty days in Poland and defining the impact of macro-scale circulation types on these days occurrence from December to March. The study used daily data concerning the minimum, maximum and mean daily air temperature for 15 stations located in Poland from 40 winter seasons between 1970/71 and 2009/10. In the predominantly large area of Poland; in the analysed multiannual period, a decrease in the number of frosty days was noticed; still, these changes were not statistically significant. The conducted research study showed that the North Atlantic Oscillation and the Scandinavian type had the most significant impact on the number of frosty days in Poland.

Synoptyczne uwarunkowania intensywnych opadów śniegu w wybranych regionach Europy Synoptic conditions underpinning intensive snowfalls in selected regions of Europe

The aim of the work described here has been to determine synoptic situations associated with intensive snowfalls in selected regions of Europe. Specifi cally, an analysis was carried out for three stations representing regions with different climatic conditions in winter. Synoptic conditions were characterized on the basis of atmospheric pressure at sea level, the height of the 500 hPa isobaric area and air temperature at the 850 hPa isobaric level. Typical features of synoptic situations leading to intensive snowfalls in Belgrade are negative SLP anomalies and a lowering height of the 500 hPa isobaric area over SouthernEurope, as indicating the presence of a low-pressure system. Heavy snowfalls in Suwalki, Poland, are in turn associated with the occurence of a low-pressure system centred on the south-eastern Baltic Sea. Finally, the occasional snowstorms characterising Ubachsberg in The Netherlands are associated with extensive low-pressure troughs that bring in humid and cold air masses from the north and north-west.