SPATIO-TEMPORAL ANALYSIS OF COLD SPELLS IN UKRAINE OVER A LONG-TERM PERIOD

Abstract

Background. Despite the significant negative impact of cold spells on human health and the potential for catastrophic consequences following their occurrence, these extreme atmospheric phenomena remain insufficiently studied in Ukraine, which determines the high relevance of their investigation. The aim of this study is to analyze the spatiotemporal distribution of cold spells across Ukraine over a long period.

Methods. In this study, the identification of cold spells is based on the Heat and Cold Wave Index, which is calculated for each location based at least three consecutive days during which both minimum and maximum daily air temperatures simultaneously exceed the 90th percentile (for heat waves) or fall below the 10th percentile (for cold spells), defined from a 30-year climatological baseline period (1981–2010). Seasonal intensity (I2) and annual intensity (I3) were used to characterize cold spells. To reach the objectives of this study, the ClimUAd dataset for 1961–2020 was used, containing daily minimum, maximum, and mean air temperature values from 178 meteorological stations across Ukraine, interpolated onto a 0.1° × 0.1° grid. A total of 70 points, evenly distributed (2–3 points per region) across the territory of Ukraine, were selected for analysis. Data processing was carried out using scripts developed by the authors in the Python programming language. The spatial distribution of cold spells characteristics was mapped using QGIS and SagaGIS software.

Results. Based on a spatiotemporal analysis of cold spells over the territory of Ukraine it was shown that in 1991–2020 the number of cold spells  decreased by 40% compared to 1961–1990. During the cold season, the duration of cold spells was longer than in the warm season. In the modern period (1991–2020), a decrease in cold spells duration is observed, more pronounced in the cold season. The longest cold spells were recorded in 1963 (average duration of 5.7 days), 1985 (5.4 days), and 1987 (4.5 days). For the period 1991–2020, the mean seasonal intensity (I2) of cold waves in the warm season ranged from 1.8 °C to 10.2 °C, with a decrease of approximately 2–8 °C compared to the previous climatic period. In the cold season, the mean seasonal intensity decreased within the range of 9 to 28 °C during 1991–2020. It was also shown that the longest (38 days) and most intense (seasonal intensity I2 – 149.5 °C, annual intensity I3 – 9.8 °C) cold spells over the period 1961–2020 occurred in February–March 1985. This event affected the entire territory of Ukraine, with temperatures in some days dropping to –34 °C in the Sumy and Kharkiv regions.

Сonclusions. Thus, a decrease in the frequency, duration and intensity of cold spells in Ukraine has been observed (particularly since the 1980s), which confirms the impact of global warming on the region’s thermal regime and is consistent with the trends in the changing characteristics of cold spells worldwide.