1. Introduction
Over the last few decades, mean and heavy precipitation changes have been documented in Europe [
1,
2,
3] and in particular regions of Europe [
4,
5,
6,
7,
8]. The effect of heavy rainfall depends not only on its intensity but also on whether the days with rainfall are evenly distributed over time or whether they occur as consecutive days with rain, so-called wet spells [
9,
10]. Furthermore, the wet spells and their opposite, dry spells, together determine the European hydroclimate. Prolonged dry weather has negative impacts on society, including water security, wildfire risk, agriculture, and energy production. Extreme dry spells contribute to meteorological, agricultural, and hydrological droughts. Dry spells negatively affect water quality, and in combination with high temperatures, they can cause toxic algal blooms, which also decrease the oxygen content in the water [
11,
12,
13], threatening the lives of fish and other aquatic organisms. Prolonged wet weather, on the other hand, may favor flooding.
Wet and dry spell analysis usually includes their duration and frequency of occurrence [
10,
14,
15]. However, the time of their occurrence during the year is also essential. Changes in the timing of dry and wet spells have rarely been analyzed; their occurrence during the vegetation period is much more dangerous than during the rest of the year.
Contemporary warming significantly affects precipitation, not so much of its total, but to a much greater extent, in its distribution [
16]. There are many signs that precipitation events have become less frequent [
17], more intense [
18], and with higher extremes [
19,
20]. Studies examining dry and wet spell durations have presented that trends differ at different locations. Groisman and Knight [
21] have shown that the average duration of dry episodes during the warm season in the Central United States has significantly increased. Similar results were found in the Southwestern United States by Mc Cabe et al. [
22] and in Argentina by Llano and Penalba [
23]. In Europe, Schmidli and Frei [
7] have shown an increase in dry spell duration in Southern Switzerland, but only in spring. Zolina et al. [
10] have found that the duration of dry spells decreases in Scandinavia and Southern Europe during both winter and summer. However, in the Netherlands, dry periods are longer during both the warm and cold seasons. Breinl et al. [
11] analyzed the most extended dry spells in Western, Southern, and Northern Europe. They have shown that the declining trends dominate in Northern Europe, while in Southern and Central Europe they are very diverse.
According to the wet spells, Zolina et al. [
10] have shown the growth in their duration in Northern Europe and Central European Russia in winter and over the Netherlands during both winter and summer. In summer, wet spells have become shorter over Northern Russia and Scandinavia. Schmidli and Frei [
7] and Wibig [
14] showed an increase in wet-spell duration in the second half of the 20th century over the Swiss Alps and in Poland.
Paton [
24] has shown that the choice of spell definition significantly impacts the result. She suggested using precise definitions and being very careful when comparing results in the case of different definitions.
In Poland, dry and wet spells based on consecutive days with precipitation < 0.1 mm or >0 mm [
17] or the Standardized Precipitation Index (SPI) [
25,
26] were used, but there was a lack of papers on the spatial and temporal variability of the frequency of dry and wet days and spells. Dry spells occurred in Poland at a clear dominance of anticyclonic weather, wet in cyclonic circulation with advections from the south and west [
26]. The aim of this study is to fill this gap and present the spatial and temporal variability of the frequency of dry and wet days and spells against the background of changes in precipitation and atmospheric circulation. The data and methods used in this study are described in
Section 2.
Section 3 presents the following: spatial distribution and trends of precipitation (
Section 3.1), dry day frequencies and their changes (
Section 3.2), dry spell frequency changes (
Section 3.3), wet day frequencies and their changes (
Section 3.4), wet spell frequency changes (
Section 3.5), and relations of dry and wet day frequencies with selected atmospheric circulation indices (
Section 3.6). The discussion and summary are provided in
Section 4.
2. Data and Methods
The study is based on daily precipitation totals from 46 meteorological stations in Poland from 1966 to 2023. Their locations are presented in
Figure 1. The data are provided by the Institute of Meteorology and Water Management-National Research Institute (IMWM-NRI). Stations with complete data series and located below 1000 m a.s.l. were selected. From 1966 to 2023, site relocation was restricted within 1 km horizontally and 10 m vertically. Poland is a lowland country; the areas situated above 1000 m a.s.l. are less than 1%.
Seven circulation indices were used to investigate the influence of atmospheric circulation on precipitation and dry and wet day frequencies in Poland. The list is presented in
Table 1.
IMWM-NRI provides daily precipitation totals with an accuracy of 0.1 mm. In addition, they provide information on so-called unmeasurable precipitation when its sum is less than 0.1 mm. In this paper, dry days are defined as days with precipitation below 0.1 mm, and days with precipitation of at least 1 mm are defined as wet days. The threshold of 1 mm in the definition of wet days was applied for easier comparison with studies in other regions [
15,
30]. However, the days with precipitation between 0.1 and 0.9 mm were not analyzed. A dry spell at particular stations was defined as a sequence of at least seven consecutive dry days between the two nearest dates on which at least 0.1 mm of precipitation was recorded. A dry spell in Poland was defined as a sequence of at least seven consecutive days in which dry spells continued over at least 12 stations. A condition of 12 stations was chosen because it means that it is dry in an area covering more than 25% of the country. In the further part of this study, a dry spell is solely considered as a dry spell in Poland.
Adopting similar conditions in the definition of a wet spell caused only a few wet spells to be distinguished in the analyzed period. The requirements had to be significantly lowered. Ultimately, a wet spell at particular stations was defined as a sequence of at least five consecutive wet days between the two nearest dates on which precipitation below 1 mm was recorded. A wet spell in Poland was defined as a sequence of at least five consecutive days in which wet spells persisted at least nine stations. A condition of nine stations means it is wet in an area covering about 20% of the country. As in the case of dry spell, in the further part of this study, wet spell specifically refers to wet spell in Poland.
Trends in the monthly values of precipitation and dry and wet days at the stations were calculated using the Sen–Theil estimator of slope [
31,
32] and the Mann–Kendall test [
33]. Both are nonparametric methods that do not require a normal distribution of the analyzed time series. Non-parametric statistics are much less affected by outliers and give robust results for non-normally distributed series [
33].
Pearson’s correlation coefficient was used to analyze the relationships between the atmospheric circulation and the area average values of the monthly precipitation totals and monthly frequencies of dry and wet days. The area average values were calculated as averages of all the stations. The statistical significance of all the tests was assessed at the 0.05% level. All maps were prepared in Surfer 20 using the kriging method.
4. Discussion and Summary
The average annual precipitation total has not changed considerably in Poland. This is well visible when comparing the average annual total in the period of 1966–2023 from this study (623 mm) with the following results of other authors and different periods: 633 mm in 1881–1900 [
34], 605 in 1901–1980 [
35], 601 mm in 1891–1990, and 606 in 1951–1970 [
36], 590 mm in 1951–2000 [
37]. Similar results were observed in neighboring countries. In Germany, Brienen et al. [
38] noticed that tendencies in precipitation in Germany over the 20th century are variable and very sensitive to the considered period. It was shown that since 1950, annual precipitation has increased in Northern Europe and decreased in part of Southern Europe [
39]. However, despite the lack of significant changes in annual rainfall totals, many studies indicate that changes in precipitation in Germany depend on the season or even the month [
40,
41]. Many authors point out that the seasonality of rainfall changes; winter rainfall increases, and summer precipitation decreases [
34,
40,
42,
43].
Moving to a monthly time scale revealed that changes in individual seasons are not uniform. Winter begins in December, during which monthly precipitation totals increase in the north and slightly in the east, while it decreases in the south and slightly in the west. The area of increase in the number of dry days covers almost all of Poland except the coast. This suggests that over a significant area, warming is associated with fewer days with precipitation but higher daily totals. In January and February, an increase in precipitation and the number of wet days is clearly visible, with a decrease in the number of dry days. This trend is usually attributed to the entire winter.
Among the three spring months, the most substantial changes are observed in April. Precipitation is decreasing in almost the entire country, significantly in the north and west. In the same regions, the number of dry days is increasing, and the number of wet days is decreasing, but with lower intensity, suggesting a decrease in the abundance of precipitation, expressed by the average total per day with precipitation. In March, there is an apparent increase in dry days and a decrease in the number of wet days, most strongly in the center and on the coast. At the same time, precipitation totals are lower only on the coast and in the center, which is also likely due to changes in precipitation abundance. In May, no change in rainfall pattern is observed.
During the summer, precipitation decreases in the south and increases in the north. A similar picture characterizes changes in the frequency of dry and wet days. Such a north–south gradient of rainfall trends (wetter on the north/drier on the south) in the midlatitudes in Europe has been discussed by many authors [
39,
44]. In Poland, it is observed only in summer.
Changes in the autumn months are also quite varied. September is drier in the north of the country, with more rainfall in the south. The number of wet days decreases significantly in the north and remains virtually unchanged in the south. In contrast, dry days increase throughout the country, but most strongly in the north. Such an arrangement should also be associated with a change in rainfall abundance. In October, changes are relatively small. A slight increase in precipitation is observed in the south and west, as well as in the area of the Gulf of Gdansk at the mouth of the Vistula River. Consistent with this trend, changes occur in the frequencies of dry and wet days. Very strong changes are recorded in November everywhere, except in the southeastern parts of Poland. Precipitation decreases and the number of dry days increases accordingly. Furthermore, wet days decrease and these changes are noted to be significant over a wide area. The turn of autumn and winter, November and December, until recently were symbols of gloomy rainy weather. They are now increasingly drier, with the weather becoming more typical of October. Rainy weather is observed more and more often in January. The above considerations suggest that an analysis of the changes in abundance of precipitation could yield interesting results.
There is a great disproportion between the persistence and extent of dry and wet spells. In the case of dry spells, seven days were assumed as the minimum duration and occurrence in at least 12 stations, i.e., over 25% of the country area. Dry spells covered 5601 days, i.e., 96.5 on average per year and over 26% of all days. If the same condition were adopted in the definition of the wet spell, it would be possible to distinguish only a few of them, covering a total of 65 days, i.e., on average, just over one per year. Therefore, the requirements were reduced to 5 days and nine stations (20% of the country area). Despite this, only 1236 days were included in the wet spells, i.e., 21.3 on average per year and less than 6% of all days. This result confirms that dry spells are related to the presence of high-pressure systems over Poland, the persistence and spatial extent of which are usually longer than the duration of cyclonic weather [
26]. The only significant long-term trend in a series of different characteristics of dry and wet spells concerns the duration of the longest dry spell in the year. This result is consistent with the suggested increase in high-pressure system persistence in Central Europe [
45].
Preliminary analysis of the impact of atmospheric circulation on the occurrence of dry and wet spells indicates high seasonal variability. In the cold season, the Scandinavian type has the greatest influence; the presence of a high-pressure system over Scandinavia favors dry weather. In summer, precipitation and dry and wet spell occurrence are influenced by the NAO, AO, type EA, and the presence of a high-pressure system over Greenland. The influence of the EA/WR pattern can be noticed in spring and autumn. However, this issue requires more detailed analysis.