*3.2. Potentially and Particularly Favorable Hydrometeorological Conditions for the Occurrence of Aeolian Processes*

Temporal and spatial analysis of hydrometeorological conditions for initiation of aeolian processes revealed a greater annual average of events on the coast of the Pomeranian Bay (Swinouj´ ´ scie, 92 days) and the Gda ´nsk Bay (Hel, 96 days) than in the open coastal zone (Kołobrzeg, 64 days; Ustka, 87 days) (Table 1). Along the entire coast, the greatest number of days potentially beneficial for initiation of aeolian processes was found in the decade of 1971–1980 (annual average of 93 days) and the least days in the decade of 2001–2010 (annual average of 79 days). This happened on the most days in 1975 (average of 125 days) and the least days in 2004 (average 65 days). For the occurrence of hydrometeorological conditions potentially favorable to initiation of aeolian processes, all thermal, precipitation, wind, and sea criteria must be fulfilled simultaneously.


**Table 1.** Average number of days per year with conditions potentially favorable to initiation of aeolian processes in the Polish Baltic coastal zone.

This is why the occurrence of all these conditions at once in the entire Baltic coastal zone is significantly rarer (annual average of 85 days) than their occurrence individually (sea level, average 360 days; air temperature, average 319 days; wind speed, average 296 days; and precipitation, average 128 days). During the analyzed period, there was a statistically significant (*p* < 0.05) increase in the number of days per year with an average air temperature of >0 ◦C (r ~ +0.3), and a decrease in the number of days per year with a maximum wind speed of ≥4 m·s−<sup>1</sup> (r ~ −0.5). Only in Ustka was there an increase in the number of days per year with a wind speed of ≥4 m·s−<sup>1</sup> (r ~ +0.9). The remaining hydrometeorological conditions, i.e., precipitation of 0 mm/2 days and sea level <570 cm, exhibited a statistically insignificant (*p* > 0.05) downward trend in the number of days per year, with a low correlation (r < −0.3).

Analysis of the number of days per year with favorable conditions for initiation of aeolian processes revealed a statistically insignificant downward trend for Swinouj´ ´ scie and Hel (r ~ <sup>−</sup>0.2), and a statistically significant downward trend for Kołobrzeg (r ~ −0.7). Only for the central coast in the region of Ustka was a statistically significant upward trend recorded (r ~ +0.4) (Figure 6).

**Figure 6.** Annual trend of days with potentially favorable conditions for initiation of aeolian processes in the Polish Baltic coastal zone.

Temporal and spatial analysis of hydrometeorological conditions particularly conducive to intensification of aeolian processes revealed a greater average annual number of events in Swinouj´ ´ scie and Hel (4 days) than in Ustka (3 days) and Kołobrzeg (1 day) (Table 2). Conditions particularly conducive to intensification of aeolian processes occurred on the most days in the decade of 1971–1980 (annual average of 5 days), and on the least days in the decade of 1991–2000 (annual average of only 1 day). This happened on the most days in 1976 (average of 9 days) and the least in 1990, 1991, and 1997 (no days). Intensification of aeolian processes in the coastal zone may only occur when all hydrometeorological criteria are fulfilled simultaneously. Such events in the entire Baltic coastal zone are thus very rare (annual average of 3 days). The individual hydrometeorological conditions of these events occur more frequently (air temperature—average of 290 days, precipitation—average of 281 days, sea level—average of 140 days, and wind speed—average of just 10 days). During the analyzed period, there was a statistically significant (*p* < 0.05) increase in the number of days per year with a minimum daily air temperature of >0 ◦C (r ~ +0.3), and a decrease in the number of days per year with a maximum wind speed of ≥10 m·s−<sup>1</sup> (r ~ −0.6). Only in Ustka was there an increase in the number of days per year with a wind speed of ≥10 m s−<sup>1</sup> (r ~ +0.5). The remaining hydrometeorological conditions, i.e., precipitation of <5 mm/2 days and sea level <502 cm, exhibited a statistically insignificant (*p* > 0.05) downward trend in the number of days per year, with a low correlation (r < −0.3).


**Table 2.** Average number of days per year with conditions particularly conducive to intensification of aeolian processes in the Polish Baltic coastal zone.

Analysis of the number of days per year with conditions conducive to intensification of aeolian processes revealed a statistically significant downward trend for Kołobrzeg and Hel (r ~ −0.6), and a statistically significant upward trend for Ustka (r ~ +0.5) (Figure 7). For Swinouj´ ´ scie, this trend was statistically insignificant.

**Figure 7.** Annual trend of days with conditions particularly conducive to intensification of aeolian processes in the Polish Baltic coastal zone.

The occurrence of hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes in the Polish Baltic coastal zone clearly depended on the seasons (Figure 8). During the cold half-year, especially from November to March, the number of days with conditions for the occurrence of aeolian processes was lower than from April to October. The cold half-year is the storm period, during which, despite higher wind speeds, storm surges are not conducive to aeolian processes, as they limit the availability of sandy beach sediments. Furthermore, during this period, there is a higher frequency of days with precipitation, as well as low temperatures causing ground frost. In the warm half-year, there are significantly fewer storm surges and days with precipitation, and no days with freezing temperatures. Conditions particularly favorable to the initiation and intensification of aeolian processes occurred especially in the spring, from April to June. At the beginning of spring, hydrometeorological conditions for the occurrence of aeolian processes were exceptionally favorable, as vegetation was not fully developed at the beginning of the season, and therefore did not limit the availability of sandy sediments for aeolian processes. Additionally, the inflow of dry masses of continental air from the Northeast were frequently recorded during this time.

**Figure 8.** Seasonal dynamics of the number of days with conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes in the Polish Baltic coastal zone—a case study on Swinouj´ ´ scie 1961–2010.

For the Polish Baltic coastal zone, analysis of the general trend in occurrence of hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes (based on the maximum number of days per year with conditions for aeolian processes recorded by all four measurement stations) revealed statistically significant patterns (Figure 9). For hydrometeorological conditions potentially favorable to initiation of aeolian processes, an upward trend was observed. However, for hydrometeorological conditions particularly conducive to initiation of aeolian processes, a downward trend was observed. The hydrometeorological conditions from 1961 to 2010 were thus characterized by a certain increase in the number of events (days) with conditions for the occurrence of aeolian processes, albeit of relatively low intensity.

**Figure 9.** Trend in the number of days with hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes in the Polish Baltic coastal zone.

#### **4. Discussion**

Research on the mechanism of aeolian processes in the coastal zone (especially quantitative values and the laws governing the movement of particles by wind) conducted in natural conditions [11,42,43], as field experiments [5,10,15,34,44,45], and especially as laboratory experiments [46,47] has often yielded divergent results. The number of variables determining the mechanism of aeolian processes in the coastal zone is much higher than in desert areas, especially in laboratory conditions (wind tunnels).

Field observations have shown that aeolian processes have positive and negative effects in geomorphological transformations of the coastal zone. The first group contains reconstruction of the beach after storms and during interstorm periods [9]. At this time, sand can be transported by winds from the beach to the dunes [1,9,10,34,45,48]. Ripple marks form on the surface of the beach, as well as aeolian shadows behind various obstacles, transverse sand patches, or even small barchans [10]. The second group contains lowering of the beach surface by seaward and alongcoast winds [4,9,49]. The visible effect of lowering of the beach is aeolian pavement and microshadows behind small obstacles [10].

During a storm build-up, the transverse profile of the beach (width and height) is of great importance; the beach can be flooded completely. During weakening of the storm, the aeolian processes act the most quickly on the upper beach fragment at the base of the dunes or cliff. During large storms, at sea level of 570 cm, sea waves cause erosion of sand dunes [1,41]. Such a situation occurs in many places on the Polish coast, even on beaches with a width of up to 70 m, which have low altitudes.

The threshold hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes determined in the study are a kind of generalization. The threshold values were determined based on a review of literature concerning field research on the dune coastal zone [5,9,10,38], and original research on the cliff coastal zone [2,3]. We can assume that the hydrometeorological criteria used in this study are very general, but appropriate for the South Baltic coastal zone.

Analysis of the variability of hydrometeorological conditions over time on the Polish coast revealed patterns similar to those found in other studies on the South Baltic coast [50–52]. Current trends and forecasts of climatic conditions in the South Baltic coastal zone indicate that for every 10-year interval, the average annual air temperature will increase by 1.9 ◦C, and the annual sum of precipitation will decrease by 4% (meaning around 30 mm) [53]. The trend of increasing temperatures, especially during winter (1.7–2.3 ◦C), will limit the number of days on which freezing temperatures freeze the ground, and as a consequence, improvement of conditions unfavorable to the functioning of aeolian processes. Furthermore, the drop in precipitation will most likely reduce the number of days with precipitation, and thus increase the occurrence of conditions favorable to aeolian processes. Additionally, the observed increase in sea level by 3–4 cm every 10 years [41] may reduce the width of beaches and the availability of sandy sediments for aeolian processes. The increase in the level of the South Baltic Sea is insignificantly higher than the forecasted rise in global sea level, which is estimated at 1–2 mm·a−<sup>1</sup> [54].

The high number of days with conditions potentially favorable to aeolian processes from 1971 to 1980 was accompanied by high annual intensity of potential aeolian transport. In this decade, the value of aeolian transport in Swinouj´ ´ scie was estimated at around 1075 t·m−1—from ~80 t·m−<sup>1</sup> in 1972 to ~150 t·m−<sup>1</sup> in 1977 [4]. During the same period in Kołobrzeg, there were significantly fewer hydrometeorological events favorable to initiation of aeolian processes. The average annual value of aeolian transport was around 395 t·m<sup>−</sup>1—from ~30 t·m−<sup>1</sup> in 1980 to ~60 t·m−<sup>1</sup> in 1975 [4]. According to Reference [4], 80% of the total aeolian transport takes place during 8% of the year (for about 1 month). The period of potential increase in aeolian transport (1 month) therefore constituted 30% to 50% of the average annual number of days with conditions favorable to aeolian processes (92 days in Swinouj´ ´ scie and 64 days in Kołobrzeg).

#### **5. Conclusions**

The relationship between hydrometeorological conditions and aeolian processes in the coastal zone is not linear. Extremely high wind speed does not always generate extreme aeolian erosion, transport, and accumulation on beaches and the slopes of dunes and cliffs, especially when strong winds are accompanied by storm surges and significant precipitation. The geomorphological effects of wind in the coastal zone are determined by many other factors that disturb the relationship between hydrometeorological conditions and aeolian processes. Among the most important of these factors are: Morpholytic conditions on beaches and the slopes of dunes and cliffs; surface exposure to wind; land cover from vegetation in different seasons; the dynamics and frequency of previous extreme hydrometeorological events; and human activity (e.g., hydraulic engineering).

Temporal and spatial analysis of hydrometeorological conditions determining initiation and intensification of aeolian processes in the South Baltic coastal zone in Poland yielded the following conclusions:


Regional climate models (RCM) predict climate change in the upcoming few decades involving the increase of temperature and precipitation [55]. The efficiency of daily rainfall is also to increase [56], and their frequency will be lower [57]. More frequent and longer periods of heat waves are predicted [16] as well as the possibility of long periods of drought [58], which will be conducive to the functioning of aeolian processes. The number of days potentially beneficial and particularly favorable to aeolian processes is likely to increase. An increase in their intensity is also expected.

Considering the current trend of rising sea levels, climatic changes, and the increasing frequency of extreme hydrometeorological events in the South Baltic coastal zone, it is not possible to reliably forecast the frequency of aeolian processes. However, the results of this study may be useful for determining how the coast functions, especially its beaches, coastal dunes, and moraine cliffs, whose low resistance to aeolian erosion is characteristic for the Baltic coast from Germany to Estonia.

**Author Contributions:** Conceptualization, M.H. and J.T.; Data curation, M.H. and J.T.; Formal analysis, M.H. and J.T.; Funding acquisition, M.H. and J.T.; Investigation, M.H., J.T. and M.R.; Methodology, M.H.; Project administration, M.H. and J.T.; Resources, M.H. and J.T.; Validation, J.T.; Visualization, M.H., J.T. and M.R.; Writing—original draft, M.H. and J.T.; Writing—review and editing, M.H and M.R.

**Funding:** The APC was funded by the Polish Ministry of Science (Project Supporting Maintenance of Research Potential of the Department of Physical Edu., Health and Tourism at Kazimierz Wielki University no. BS/2016/N1).

**Acknowledgments:** Hydrometeorological data were obtained from the Institute of Meteorology and Water Management, National Research Institute in Warsaw. We would like to thank the four reviewers for very helpful comments that improved our paper substantially.

**Conflicts of Interest:** The authors declare no conflict of interest.
