Exceptionally cold and warm spring months in Kraków against the background of atmospheric circulation (1874-2022)

In the changing climate, exceptionally warm (EWMs) and dry spring months are increasingly observed. At the same time, exceptionally cold months (ECMs) are less frequent, although their impact on a warming climate becomes significant. Due to the role that such climatic anomalies play in the environment and their effects on human activity, it is very important to explain the causes of their occurrence. For this reason, in this study, the authors have attempted to determine the circulation conditions favourable to the occurrence of extremely cold (ECM) and warm (EWM) spring months in Kraków in the years 1874-2022. The study used the average temperature of individual spring months (March-May), as well as types of atmospheric circulation and air masses from the daily Calendar of Atmospheric Circulation Types for southern Poland. A distinct increase in spring air temperature (0.181 $^{\circ}C$/10 years) and its individual months (0.162-0.191 $^{\circ}C$/10 years) was confirmed. It was accompanied by a significant increase in the occurrence of EWM and a decrease in ECM. It was also found that the direction of air advection and the related temperature characteristics of air masses have the greatest impact on the occurrence of exceptionally cold or warm months. A slight positive effect of zonal circulation on the temperature increase at the beginning of the spring season and the advection of air from the south in April and east in May was found. In the case of the coldest months (ECMs), low temperatures most often developed in the presence of advection from the NW-N-NE directions

Strong anticyclones and deep cyclones over Svalbard in the years 1971–2015

This paper presents annual and long-term variability in the occurrence of cyclone and anticyclone centres over Svalbard during the period 1971–2015. The study is based on average daily air pressure values at sea level for 12 grid points and on the typology of synoptic situations. Particular emphasis is placed on the occurrence of deep and very deep cyclones (with a pressure of ≤ 990 hPa and ≤ 970 hPa, respectively) and strong and very strong anticyclones (with a pressure of ≥ 1030 hPa and ≥ 1035 hPa, respectively). On average there were about 17.5 days with a cyclone and 4.9 days with an anticyclone centre over Svalbard annually. The results of the analysis show no clear trend in the number of cyclones moving over the study area, but there was an observable increase in the frequency of anticyclones. There were also no major changes in the annual maximum of occurrence of the pressure systems under study

Frosty, Freezing and Severe Freezing Days and Their Synoptic Implications in Małopolska, Southern Poland, 1951–2000

This study discusses the occurrence of days with unique thermal characteristics for the period 1951–2000. The authors investigated longterm variability, probability of occurrence and synoptic conditions favourable to frosty (tmin≤0˚C ∧ tmax>0˚C), freezing (tmax<0˚C) and severe freezing (tmax<-10˚C) days at six stations in the southern part of Poland. The occurrence of frosty days is characterized by the highest diversity, both in spatial and temporal terms and these days depend on the landform to the highest degree. The number of freezing days ranged from 28–30 in the foothills of the Carpathians to 147 at 2000 m a.s.l. in the Tatra Mountains, with severe freezing days from ca. 2 to ca. 18 days respectively, though no distinct trends were noted in their long-term progression. The anticyclonic situations and air mass advection from the southern sector (frosty days), from the east, south-east and north (freezing and severe freezing days) were the most favourable for the days examined

Thunderstorms and thunderstorm precipitations in southern Poland

This paper addresses spatial and temporal variability in the occurrence of thunderstorms and related precipitation in southern Poland between 1951 and 2010. The analysis was based on thunderstorm observations and daily precipitation totals (broken down into the few ranges) from 15 meteorological stations. It was found that precipitation accompanied an overwhelming majority of thunderstorms. The most frequent range of thunderstorm precipitation totals was 0.1–10.0 mm which accounted for 60% of all values while precipitation higher than 20.0 mm accounted only for ca. 8%. During the study period, long-term change in the number of days with thunderstorm precipitation within a certain range displayed no clearcut trends. Exceptions included: 1) an increase in the number of days with thunderstorm precipitation in the lowest range of totals (0.1–10.0 mm) at Katowice, Tarnów, Rzeszów and Lesko and decrease at Mt. Kasprowy Wierch, 2) an increase in the range 10.1–20.0 mm at Zakopane and 20.1–30.0 mm at Opole, 3) a decrease of the top range (more than 30.0 mm) at Mt. Śnieżka. It was found that the heaviest thunderstorm precipitation events, i.e. totalling more than 30 mm, and those events that covered all or most of the study area, occurred at the time of air advection from the southern or eastern sectors and a passage of atmospheric fronts

Zróżnicowanie przestrzenne i zmienność wieloletnia występowania burz w Polsce (1949-1998)

The main aim of this study was to present the thunderstorms’ spatial and temporal occurrence in Poland in the period 1949—1998. The paper also attempts to assess an impact of atmospheric circulation on the variability of number of days with thunderstorm. At the same time the study fills a gap in Polish climatological research into thunderstorm occurrence and variability based on the long-term series. The research was based on records on thunderstorms from the period 1949—1998, gathered at 56 meteorological stations which belong to the Institute of Meteorology and Water Management. The main indicator analysed in the study was the day with a thunderstorm. In the second part of the study, devoted to the influence of synoptic situations during which the thunderstorms occurred, the types of synoptic situations elaborated by B. Osuchowska-Klein (1978, 1991, and 1998) for the period 1949—1998 were used. Other sources used in the project were the types of synoptic situations by T. Niedźwiedź (1999) for the period 1949—1998, indices of atmospheric circulation by Z. Ustrnul (1999) for the period 1949-1995, index of North-Atlantic Oscillation (NAO) from the period 1949—1997 (North Atlantic Oscillation, 1998) and synoptic maps from the Institute of Meteorology and Water Management published as Biuletyn synoptyczny PIHM (then IMGW) (Biuletyn..., 1959—1979), Mapy synoptyczne (Mapy..., 1952—1981) and Codzienny biuletyn meteorologiczny (Codzienny biuletyn..., 1980—1981) from the years 1952-1981. The analysis of 50-year series of number of days with thunderstorm, carried for stations from all over Poland, revealed some aspects of spatial variability of that element. It also allowed to find areas with different probability of the thunderstorm occurrence and to define the differentiation of noted numbers of days with thunderstorm in shorter observation periods. Spatial analysis of the occurrence of number of days with thunderstorm in Poland showed that even though in Poland there are 24 such days on average, the number varies from 15 in North-West to 33 in South-East of the country. The largest values of the index are noted at the stations in the Carpathian Mts., e.g. 54 days in 1963 at Kasprowy Wierch Mt., while the smallest — at the Baltic Sea seaside, e.g. 4 days in 1976 in Ustka and 5 days in Kołobrzeg and Gdańsk (in 1976 and 1977, respectively). It turned out, however, that in spite of high number of days with thunderstorm in southern Poland, its inter-annual variability is much smaller than for northern areas. In shorter observation series and in particular years, the spatial pattern of number of days with thunderstorm may vary significantly from the mean one from the multi-annual period. Therefore the impression of increase or decrease of the frequency of thunderstorm occurrence at a certain station depends to a large extent on its geographical location and on the weather type prevailing in a certain year. It was also found out that only at the seaside and in the mountains, where the local conditions determine significantly the number of the days with thunderstorm, their values are always the lowest and the highest in the country, respectively. Spatial distribution of number of days with thunderstorm in seasons and months is related to annual cycle of thunderstorm occurrence. In winter, at all stations, days with thunderstorm occur sporadically, while in summer their distribution is similar to the annual one. Transitional periods of spring and autumn are characterised mainly by higher number of days with thunderstorms in spring, but also typical spatial distribution of the element in autumn: the highest number of days with thunderstorm is noted at the seaside, not in southern Poland. Special attention was paid to the multi-annual variability of days with thunderstorm occurrence. It was marked not only in annual and seasonal changes of the element in Poland and at the particular stations; also different types of its temporal and spatial patterns were found. No tendency was found in the multi-annual course of mean yearly number of days with thunderstorm in Poland, and the values usually do not differ much from the value typical for the climatic-geographical region in which Poland is situated. Periods with significantly higher number of days with thunderstorm are also difficult to find. It can only be stated that at the beginning of the analysed 50-year period the yearly numbers of days with thunderstorm were the smallest and the mean 50-year value was exceeded most often in 1960’s and 1970’s. The minimum value occurred in 1976 (17.5 days), and the maximum one in 1963 (31.3 days). The only season in which the number of days with thunderstorm increased was winter. This is in accordance with the tendencies observed in the whole Europe and most probably is due to the increase of the western circulation in winter. Multi-annual variability of the number of days with thunderstorm at particular stations differed significantly from the mean one for Poland, especially in southern part of the country (correlation coefficient < 0.20), while it was similar to it in northern Poland and in Wielkopolska Lowland (correlation coefficient > 0.70). Large differences in the number of days with thunderstorm at particular stations were found, even if the stations were located quite close to each other. Again, it is difficult to find periods when number of days with thunderstorm would be significantly larger than the 50-year value in the regions or at certain stations. Apart from the stations in Kielce and WIodawa no increasing or decreasing tendencies were found. Therefore the hypothesis that the last years of the 20th century were marked with exceptional increase of the number of days with thunderstorm cannot be confirmed. The annual variability of number of days with thunderstorm was used to distinguish the period of the highest thunderstorm activity (April-September), when 96% of all ca ses are noted. In Poland, on average, the number of days with thunderstorm reaches from 1.2 day in April to 5.6 days in July. However, more detailed analysis allowed to find out six types of its annual pattern. The types differ mainly in the occurrence of the monthly maximum in number of days with thunderstorm, which was noted from May to July in particular years, but also the element’s pattern in the months of the period of maximum thunderstorm activity was different. The analysis of the frequency of number of days with thunderstorm in particular days of the year proved large influence of the station’s location on the annual course of the thunderstorms occurrence. The smallest differences among stations are found in southern and south-eastern Poland, while the largest ones in north-western areas. In the North the period of the highest frequency of days with thunderstorm is by 15—30 days longer, and the decrease in autumn slower than in the South. The assessment of impact of atmospheric circulation on the occurrence of days with thunderstorm concerned variability of this index, both in space and time. In the study the types of atmospheric circulation conducive to the formation of thunderstorms were estimated. The differentiation of inter-air mass thunderstorm and thunderstorm connected with atmospheric front occurrence was also presented. Synoptic situations have less influence on the thunderstorm occurrence than it is usually suspected. The obtained results suggest that future research into the synoptic conditions of days with thunderstorm should be more concerned with air masses and values of other meteorological indicators. The most favourable are cyclonic situations with the advection of air masses from the South (D) or South-East (F) (probability of the thunderstorm occurrence is then 18% and 19.5%, respectively). The most unfavourable are situations with the high pressure center (G) over Poland (6%). There is little correlation between number of days with thunderstorm and the frequency of particular circulation types. Only for the seaside and southern regions there was an influence of eastern advection on the number of days with thunderstorm, while the influence of the western advection (dominating in mid-latitudes) was marked only in south-eastern regions. The analysis of relations between the variability of number of days with thunderstorm and the changes of the circulation indices proved the regularities already presented. The eastern advection has the largest influence on the occurrence of the days with thunderstorm, especially in southern Poland. In the Sudety Mts. southern advection is also favourable, while the increase of cyclonic situations causes larger number of days with thunderstorm only in north-eastern Poland. The results of the study let us divide Poland into 5 thunderstorm regions, which are characterised by similar variability of number of days with thunderstorm. The division was made using K-means method. The hunderstorm regions were distinguished with three criteria characterizing the occurrence of days with thunderstorm in Poland: multi-annual variability, yearly course of the days with thunderstorm and the frequency of synoptic situations accompanied by thunderstorms. Depending on the criterion, 5 or 6 regions were obtained. However, their limits are very similar to each other. The most significant differences among the divisions are the delimitation of Carpathian—Zamość Region and the division of the seaside into two smaller regions due to the analysis of the variability of number of days with thunderstorm in the multi-annual period. As the regions distinguished according to different criteria were similar, finally the territory of Poland was divided into 5 thunderstorm regions. The most important criterion used to classify a station to a certain region was yearly course of the days with thunderstorms, then the multi-annual variability of the index. The mentioned variability of number of days with thunderstorm was used to distinguish, within the South-Eastern Region, a Carpathian Region, which is characterized with significantly different number of days with thunderstorm and its smallest variability in the multi-annual period. Summarising, it should be stated that in Poland the number of days with thunderstorm is characterised by higher spatial differentiation than multi-annual variability. Apart from some regions, the types of atmospheric circulation do not control the variability of the examined element to a larger extent. Most probably only significant climatic changes could influence its pattern. The obtained results indicate also a great role of the local environment in the thunderstorm occurrence’s variability in Poland

Strong High Pressure Systems Influencing the Weather in Poland During the Period 1971-2000

This paper shows preliminary results of research into the occurrence of strong anticyclonic systems that influenced the weather in Poland during the period 1971–2000. The study was based on NCEP/NCAR reanalysis data, including daily values of the 1000 and 500 hPa geopotential heights, maps of mentioned geopotential heights and maps of sea-level field pressure. With the use of these data a number of exceptionally strong high-pressure systems were identified, together with their areas of origin and subsequent development patterns. They were then broken down into five groups with similar dynamics. The numbers of systems in each group were not found to follow any significant change trends in the long term. The greatest differences between groups were identified in terms of their annual occurrence rates and centre pressure values.A619110

Silne wyże nad Europą (1951-2010)

Badania zostały przeprowadzone w ramach projektu badawczego nr N N306 047939 finansowanego przez Ministerstwo Nauki i Szkolnictwa Wyższego — Narodowe Centrum NaukiThe aim of this study was to determine the temporal and spatial variability of strong and very strong highs in an area of the Euro-Atlantic sector limited by the coordinates 20°N—85°N and 50°W—75°E. The research was based on a rich set of data including, at its core, the average daily air pressure values at sea level at 1377 grid points of a 2.5° × 2.5° grid derived from NCEP/NCAR reanalyses. That data set was supplemented by the values of the 500 hPa isobaric level’s height at the grid points mentioned above. Additionally, a wide range of synoptic maps for different pressure surfaces was used. The study covers the period 1951—2010. The significant spatial and temporal variability of the air pressure over Europe and the Atlantic required the finding of a suitable method to identify strong or very strong anticyclonic systems. The results obtained during this study suggest that defining criteria are highly dependent on both the geographical location and the season of the year. Finally, it was assumed that a pressure system would qualify as a strong high if its pressure was equal or higher than 1030 hPa and as a very strong high if the pressure was at least 1035 hPa. The annual course of the number of days meeting a given criterion became the basis for the division of the Euro-Atlantic region into four main anticyclonic regions of the Euro-Atlantic sector and a number of subregions. The most characteristic differences between regions involved pressure values in strong and very strong highs, their annual patterns and long-term variability of the number of days with strong and very strong highs. The highest pressure values were found in regions featuring the strongest continental highs (more than 1060 hPa) and the lowest values in areas of the most active cyclonicity, i.e. the Iceland region (IIIA) and the Southern region (IV). The Azores High region (IIAW) stands out with the highest average pressure (1018 hPa) at the centre. Areas of oceans and the Southern Region (IV), which covers nearly the entire area below 50°N, stand out from the rest of the sector. They are dominated by cyclonic activity and strong highs only occur for 30—40 days per year. Strong and very strong anticyclones were mainly recorded during the cool half of the year (Oct.—Mar.). The annual maximum of the occurrence of strong and very strong highs is in winter with the exception of the Arctic where it is shifted to springtime. During the final decades of the study period, there has been a change in the timing of the annual maximum of the number of days with these systems, as well as a change in the timing of the onset and end of the season of the greatest frequency of strong and very strong highs. Where long-term variability of strong highs were clear-cut they occurred primarily in the cool half of the year (Oct.—Mar.). They mainly included a decrease in the annual number of strong and very strong highs in the Greenland region (I) by more than four days per 10 years and an increase in that number in the Azores High (IIAW), Asian (IIAE) and Southern (IV) region. The increase began in the 1970s and was the greatest in the Southern region (up to 6 days per 10 years). Air pressure spatial pattern types on days with strong and very strong highs were identified for each region in order to account for the role of atmospheric circulation as a climate control and for relationships existing between individual elements of circulation. Each of the types involved two main distinctive features, namely the spatial distribution and the strength of the most important Euro-Atlantic highs, i.e. Greenland, Azores and Asian, as well as the Iceland low. A comparison of the types of pressure pattern on days with strong and very strong highs reveals that some of them are so similar to each other that they might be regarded as common to the entire Euro-Atlantic sector. This is particularly true of the types occurring during the cool half of the year. Also some of the pressure pattern types are similar to general circulation types identified in this sector by other authors. This would mean that they should be counted among significant features of this area’s atmospheric circulation. Duration is an important characteristic describing the occurrence of pressure systems. Among the 8406 strong or very strong highs that occurred in the Euro-Atlantic sector during the study period, the dominant duration was 1—3 days and slightly more than 50% of all of them were shorter than 7 days. It was also found that the duration of these systems increased with their strength to the point that for systems with a pressure 1045 hPa it averaged at 6 days. High pressure systems in the cool half of the year (Oct.—Mar.) lasted longer than the high pressure systems in the warm half of the year. The most persistent highs develop over Greenland and in the Asian High region (IIAE). In contrast to the rapid movement of cyclones, high pressure systems travel at a much slower pace but cover a far greater area. For this reason, when investigating the occurrence of strong and very strong high pressure systems, the author also identified areas of the most frequent occurrence of their centres, thus indirectly identifying the paths of these systems. The areas so-far identified formed two distinct zones: • A belt stretching from the Azores High area on the Atlantic Ocean across Europe to the East European Lowland (Voyeykov’s axis) and • Greenland. During the cool half of the year, the number of strong and very strong highs of a local nature clearly increases. These anticyclones occur over the Iberian interior, the Alps, a part of the Balkan Peninsula, the Anatolian Upland, the Caucasus and the Scandinavian Peninsula. An analysis of the geographical extent of these two zones revealed that their location was related strongly to the annual cycle of change in atmospheric circulation and, consequently, of the highs in question. For the first time, the results of the research project presented in this paper allowed the researchers to identify changes in the occurrence of strong and very strong highs, one of the most important elements of atmospheric circulation, over such a large spatial scale and time. The results clearly showed that in the Euro-Atlantic sector weather conditions were influenced more by the annual pattern than the long-term variability of these two pressure systems and by the great deal of regional variation discovered in their frequency of occurrence.Ministerstwo Nauki i Szkolnictwa Wyższego — Narodowe Centrum Nauk

Atmospheric fronts over Poland (2006-2015)

The paper presents the spatial and temporal variations in the occurrence of fronts and days with no fronts over Poland in 2006-2015. The research was based on a database of the atmospheric fronts that moved over five selected stations located in the outermost regions of Poland and in the centre of the country. The database was created as a result of an analysis of synoptic charts from the website of the German weather service (Deutscher Wetterdienst; DWD). As is shown by the results, atmospheric fronts moved over Poland on approximately 65% of days of the year. However, their frequency in the individual regions was almost half of this, ranging from 33-39%. The annual variations in the number of fronts depended largely on the location of a given area and the atmospheric circulation prevailing there. In most years the maximum frequency of occurrence of atmospheric fronts was observed in November-January, and the lowest frequency was seen in February and September. The research confirms that there is a clear predominance of cold fronts, with warm and occluded fronts forming at around half the frequency of cold fronts. One characteristic feature is a decrease in the number of occluded fronts and days with different types of fronts moving from the north of Poland southwards. In the period under study, more than 80% of the sequences of days with atmospheric fronts included up to 6 days, even though there were also cases when fronts passed over Poland on 20 consecutive days

Persistence of high pressure systems over Europe in the years 1901-2018

Jedną z głównych przyczyn zmian warunków klimatycznych poszczególnych regionów są zmiany cyrkulacji atmosferycznej, zwłaszcza intensywności i częstości występowania układów barycznych. Z tego powodu celem niniejszej pracy jest określenie prawidłowości występowania układów wysokiego ciśnienia nad Europą w latach 1901-2018. Badania przeprowadzono na podstawie średnich dobowych wartości ciśnienia atmosferycznego na poziomie morza z 14 stacji. Za dzień z wysokim ciśnieniem uznano taki, w którym ciśnienie wynosiło przynajmniej 1020 hPa. Szczególną uwagę zwrócono na występowanie dni z ciśnieniem równym lub wyższym niż 1030 hPa (silny wyż) i 1035 hPa (bardzo silny wyż). Niezależnie od przyjętego kryterium nie stwierdzono wyraźnych tendencji zmian liczby dni z rozpatrywanymi wartościami ciśnienia, a ich częstość i przebieg roczny wskazywały na wyraźną dominację układów wysokiego ciśnienia w chłodnej połowie roku. Dni z analizowanymi progowymi wartościami ciśnienia najczęściej grupowały się w ciągach krótkotrwałych (1- lub 2-5-dniowych) i poza kilkoma stacjami położonymi w środkowej części badanego obszaru nie stwierdzono zmian ich występowania. Na większości stacji nie zaobserwowano także zauważalnej zmiany trwałości układów wysokiego ciśnienia. Niewielki jej wzrost odnotowano jedynie w Heraklionie, Wiedniu i Dublinie.One of the main causes of climate change in the individual regions are changes in atmospheric circulation, especially the strength and frequency of baric systems occurrence. For this reason, the objective of the research was to determine the regularities of the high pressure systems occurrence over Europe in the years 1901-2018. The analysis was based on the average daily values of atmospheric pressure measured at sea level from 14 meteorological stations. A day with high pressure system was considered to be one in which the pressure value was equal to or higher than 1020 hPa. Particular attention was paid to the occurrence of days with a pressure equal to or higher than 1030 hPa (strong high) and 1035 hPa (very high high). Regardless of the criterion adopted, there were no clear trends of changes in the number of days considered, and their frequency and annual course indicated a clear dominance of high pressure systems occurring in the cool half of the year. Days with the analyzed pressure values most often occurred in short-term sequences of days (1- or 2-5-days) and except for a few stations located in the center of the area under consideration, no significant trend of their changes was found. At most stations there also was no noticeable change in the persistence of high pressure systems. A slight increase was recorded only in Heraklion, Vienna and Dublin

Variability of frost-free season in Poland in the period 1951-2006

In the study tendencies in changes of frost-free season at 20 meteorological stations in Poland in the period 1951-2006 were examined. Both changes in the beginning and the end of frost-free season and its length were analysed. At 13 of those examined stations the frost-free season were beginning sooner and sooner (16 days/50 years an average), whereas the tendencies in changes of the end of analysed season lack such a clear distinction. Nevertheless, the length of frost-free season was noticed to be longer from about 15 days in Legnica to about 32 in Łódź. Th e most signifi cant of the described changes occurred in the west part of the country and the lowest in the east part