Pluvial conditions in Wroclaw , Poland

The paper presents rainfall conditions for the city of Wroclaw (Poland) in the multi-annual period 1960–2017. A decreasing tendency of annual precipitation totals on the level of –12.5 mm/decade and decreasing trend of the monthly total in November of –2.9 mm/decade has been shown. In remaining months, no statistically significant change trends were observed. The maximum daily rainfall amounts were recorded in summer months: 56.4 mm in June, 74.4 mm in July and 67.5 mm in August. The analysis of variability in the number of precipitation with daily amounts exceeding 10 mm showed statistically significant decreasing trends – a decrease in the number of days in the amount of 0.69 per decade (linear regression). In the case of rainfall exceeding 20 mm and 30 mm no statistically significant trends were noted – the number of days with such heavy precipitation is practically constant and amounts to an average of 3.84 and 1.44 precipitation per year.


Introduction
From the point of view of urban hydrology, the most important climate features are atmospheric precipitation [1,2].For the proper recognition of pluvial conditions of urban areas, a climatological analysis of multi-annual series of rainfall data is necessary [3][4][5].
According to the recommendations of The World Meteorological Organization, a minimum period of 30 years should be adopted for climatological studies [6].
The latest research on climate change in Europe on the basis of over 50 years of summer measurement series indicates that maximum daily rainfall, depending on the altitude above sea level, are highly variable and range from 37.6 to 520 mm.The number of short-term but intense rainfall increases continuously, and since 1960, on 89% of measurement stations more than one case of daily precipitation greater than 50 mm has been registered [7,8].
In Poland up to 2010, an increase in the number of days with precipitation is observed, with an explicit fluctuation of monthly and annual precipitation totals.Trends of changes are usually statistically irrelevant, regardless of whether the trends are downward or upward [9,10].In the paper [11] a decreasing tendency of annual totals change was shown for Wroclaw.
The aim of this study is to characterize the pluvial conditions of the city of Wroclaw.The work includes the analysis of daily rainfall totals recorded at the synoptic station in Wroclaw over a 58-year period.Both daily and monthly totals of precipitation were analyzed, as well as a number of days with precipitation.

Materials and methods
The meteorological station of the Institute of Meteorology and Water Management (IMWM) in Wroclaw is a part of national measurement and observation network at national hydrological and meteorological service.The station coordinates are: 51-06 N, 16-54 E; terrain altitude: about 120 m above sea level.In this study, daily precipitation records from this IMWM meteorological station from 1960-2017 long-term period were used as a research material.
In order to determine trends of changes in rainfall time series, both linear regression and non-parametric Mann-Kendalls test were used [12,13].Changes (increases or decreases) at a significance level above 95% are considered as statistically relevant.Changes at the significance level from 90 to 95% are assumed to be close to statistical significance, while changes in the significance level from 75 to 90% are assumed to be a tendency to change.Changes at the level of significance below 75% are considered statistically insignificant and consequently without a specific direction of change [9].

Results
The average annual sum of the analyzed precipitation series amounts to H = 568 mm.For comparison, on the multi-annual period 1971-2000, which appears in many climatological studies, i.e. the Climate Atlas of Poland [17], a corresponding precipitation total is equal to 570 mm, while for the earlier 1961-1990 the sum amounts to 590 mm and for the current period of 1981-2010 the average precipitation total is 537 mm.
The course of annual rainfall totals and their trend is presented in Fig. 1.Linear regression and Mann-Kendalls test reveal no statistically significant change trends, but only a declining trend (-12.4 mm/decade and -12.6 mm/decade respectively).Due to very similar values of directional coefficients, trend lines determined by both methods are practically overlapping.A summary of average, maximum and minimum monthly rainfall totals is presented in Table 1.The table also shows the percentage of monthly rainfall in the annual total (h / H) and the years in which the maximum and minimum values occurred.In 1960-2017, the monthly precipitation totals were qualified according to the accepted classification: 32 times as extremely dry (4.6%), 93 very dry (13.4%),129 dry (18.5%),258 normal (37.1%),66 wet (9.5%), 52 very wet (7.5%) and 66 extremely wet (9.5%).Most often the most dry month was October (6 times), very dry September (12 times), dry December (16 times), normal June (28 times), wet March and December (8 times each), very wet February (8 times) while the extremely wet September (10 times).The structure of variability of monthly precipitation totals with reference to the average perennial values according to the adopted classification is shown in Figure 4. Analysis of variability of precipitation sums in individual months did not show statistically significant change trends except for November, for which a downward trend The smallest average daily totals are recorded in the winter months (1.00 mm in December, 0.92 mm in January and 0.90 mm in February), and the highest in the summer months (2.40 mm in June, 2.86 mm in July and 2.28 mm in August).The course of average daily sums in the following days of the year is shown in Figure 5.The maximum daily rainfall amounts in a given month are many times higher than the average values.In the winter months they are 32.0 mm, 20.0 mm and 19.5 mm respectively, and in summer 56.4 mm, 74.4 mm and 67.5 mm respectively.The course of maximum daily totals recorded in Wroclaw in the years 1960-2017 in the following days of the year is shown in Figure 6.Analysis of the variability of the number of precipitation with daily sums exceeding 10 mm (Fig. 7) showed statistically significant decreasing trends -a decrease in the number of days by 0.69 per decade (linear regression).In the case of rainfall exceeding 20 mm and 30 mm, no statistically significant trends were recorded -the number of days with such heavy rainfall is practically constant and amounts to 3.84 and 1.44 precipitation per year (on average).

Conclusions
The work assessed rainfall conditions in Wroclaw based on measurement data recorded in 1960-2017.The analyzes presented allowed to formulate the following conclusions:  in line with the rain zone's rainfall regime, the highest values of monthly and daily sums are found in Wroclaw in the warm season of the year (V-X);  annual precipitation show a statistically insignificant decreasing tendency of approx.
12.6 mm changes per decade;  there is a strong temporal and spatial differentiation of the degree of humidity of individual months in the multi-year analysis;  negative deviations from the norm of individual months occur more frequently than positive anomalies, which corresponds with the decreasing tendency of changes;  the most frequent rainfall in Wroclaw are weak, very weak and moderate precipitation -usually occurring in winter;  days with precipitation greater than 10 mm occur most often in the summer period (in particular in July), statistically significant trends of decreasing numbers of days with precipitation above 10 mm have been demonstrated;  the number of days with precipitation exceeding 20 and 30 mm over the years does not change.

Fig. 1 .
Fig. 1.Annual precipitation in Wroclaw in 1960-2017.In the years 1974-1981, the longest period of precipitation higher than average occurred, with the maximum (over the whole analyzed period) annual precipitation of 776.2 mm (208.6 mm above the average) in 1977.The longest period with lower than average rainfall amounts occurred in years 1988-1994, with the minimum annual rainfall of 380.8 mm (186.8 mm below the mean) observed in 1982.Deviations from the average amount of precipitation are shown in Fig. 2.
5 mm in January and 25.4 mm in February), and the highest in the summer (71.9 mm in June, 88.7 mm in July and 70.6 mm in August).Very important (in relation to averages) are the maximum values, which were respectively 100.0 mm, 95.7 mm and 47.8 mm for winter months and 185.4 mm, 250.8 mm and 229.3 mm for summer months.The minimum, average and maximum monthly rainfall amounts, as well as their median and percentiles of 25% and 75% are shown as a box plot in Figure 3.

Fig. 4 .
Fig. 4. The range of rainfall classes calculated by means of the Kaczorowska classification.

Fig. 5 .
Fig. 5.The course of average daily sums in subsequent days of the year in Wroclaw from 1960 to 2017 (black linemoving average from 15 days).

Fig. 6 .
Fig. 6.The course of maximum daily totals in subsequent days of the year in Wroclaw from 1960 to 2017 (black linemoving average from 15 days).The lowest values (among daily maxima from a multi-year period) of 3.4 mm were recorded on 03.01.2006 and 26.01.2008.The absolute maximum of 74.4 mm was recorded on 20.07.2001.It should be noted that this sum accounted for 84% of the monthly average for the month of July.In Wroclaw in the years 1960-2017 a total of 8295 days with precipitation were recorded, of which the highest were classified as very weak (0.1-1.0 mm), weak (1.1-5.0 mm) and moderate (5.1-10.0mm) -which occurred respectively 2892, 3497 and 1102 times.Daily rainfall exceeding 10 mm was already less: 581 moderately strong (10.1-20.0mm), 139 strong (20.1-30.0mm), 71 dangerous (30.1-50.0mm), 11 constituting flood hazard (50.1-70.0mm) and 2 floods (70.1-100.0mm).In the analyzed period no disastrous daily precipitations (over 100 mm) were recorded.
Days with rainfall exceeding 10 mm occur most often during the summer, especially in July.The winter months are characterized by the highest frequency of very weak and weak precipitation.The comparison of the average number of days with precipitation of individual categories registered in Wroclaw in the years 1960-2017 is presented in tab. 3.

Table 1 .
Monthly rainfall totals against the background of the annual total (I-XII) in Wroclaw in 1960-2017.
For monthly rainfall amounts, the lowest average values are recorded in the winter (31.1 mm in December, 28.

Table 2 .
.9 mm/per decade (linear regression) was calculated.In addition, declining trends close to statistical significance were demonstrated in June and August and growing in January (Mann-Kendalls test).The average and maximum daily rainfall values recorded in Wroclaw in particular months of the year in 1960-2017 are summarized in tab. 2. Daily sums of rainfall in Wroclaw in the years 1960-2017.

Table 3 .
Average number of days with fall of individual categories in Wroclaw in 1960-2017.