Variability of Sunshine Duration in Western Ukraine

Abstract

The duration of sunshine is a crucial radiation characteristic of climate, serving as an indicator for both regional atmospheric circulation and surface atmospheric pollution variability. Ground observations using a heliograph, spanning a long duration, enable the detection of changes in the impact of climate-forming factors on modern warming. Since the 1980s, more precise, high-resolution CMSAF satellite data has allowed for the determination of spatial variability in sunshine duration over recent decades. From 1371 to 1934 hours of sunshine per year were observed in Western Ukraine during 1961–1990, with the southwestern regions of the Transcarpathian lowland being the sunniest. Some years exceeded 2000 hours, indicative of a Mediterranean climate. Typical latitudinal changes were not found, but instead, a noticeable development of continentality, expressed in an increase in annual sunshine duration from west to east was found. Over the year in 1961–1990, significant differences in sunshine duration exist. Generally, the highest values occur in July, exceeding 250 hours in the southeast, southwest, and even in the northwest. The northwest and other western weather stations show high sunlight variability in June and July, linked to North Atlantic cyclones and increased cloudiness. The lowest seasonally determined total duration of sunshine during 1961–1990 occurred in December (<50 hours), with low values and a high number of days without sunshine (up to 20 days) typical for low-mountain valleys of the Carpathians and the northwest (27–38 hours). The most complex distribution is in autumn and spring, with a well-expressed latitudinal distribution from north to south: an increase in autumn and a decrease in spring due to transformation of seasonal circulation processes. In the last thirty years, changes in sunshine duration in Western Ukraine correspond to pan-European trends associated with global brightening. These changes are generally insignificant throughout the year, except for January and the western part of the region. In July, a slight statistically significant increase in the duration of sunshine was detected only in the west and in the foothills during 1961–2020. We associate these features with the higher frequency of anticyclonic types of weather and therefore with fewer cloudy days. These changes are noticeable in the Prut-Dniester highland and the eastern Podilska highland. The spatial distribution of sunlight duration indicates variability near the Carpathian Mountain system and above the highlands. The manifestation of the barrier function of mountains is manifested in an increase in values in winter due to altitudinal differentiation and a decrease in summer. Disctinctive changes east from the influence of the Carpathians are observed in the summer period. Similar peculiarities on a smaller scale were found over the Khotyn and Podilska highlands. The seasonal distribution of sunshine duration in Chernivtsi from 1991–2020 shows significant variability in spring and autumn. In the first decade of this period, low values corresponded to the low solar activity of the 22nd solar cycle. The last two decades, with significant fluctuations, especially in May, August, and September, and extremely high values in separate years, did not coincide with the least active recent solar cycles. Instead, they indicate the variability of circulation systems, especially in the west of the region. Identifying the impact of this factor, along with the growing atmospheric pollution in recent years, constitutes the perspective of our research. Keywords: sunshine duration, climate variability, regional atmospheric circulation, atmospheric pollution, satellite data.