Interannual variability of total and low cloudiness over large cities of Ukraine: an intercomparison of observations and reanalysis

Abstract

Cloud cover is one of the important climate indicators. The characteristics of the cloud cover regime can be used to determine the peculiarities of atmospheric circulation, the planetary albedo of the climate system, the effective radiation of the underlying surface and the possible distribution of precipitation, so quantifying changes in cloud cover in modern conditions is relevant. Changes in circulation indices should affect changes in both the quantitative indicators of cloud cover and the repeatability of the main cloud types. An analysis of climatic studies of cloud cover and its actual changes over the territory of Ukraine, which were carried out by Ukrainian scientists, showed that they are based only on ground-based observations. The features of the spatial distribution of individual characteristics were obtained using long-term observations of the same length at all weather stations of the Ukrainian hydrometeorological network. Ukrainian studies of cloud cover fluctuations due to climate change are based on ground observations. Moreover, the length of the rows depends on the goal of the study and the time of its completion. This paper is devoted to the analysis of total and low cloudiness for each month between different decades in relation to the climate norm (1981-2010). The input data for this study was total cloud cover and low cloud cover from ERA5 [https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels-monthly-means?tab=overview] for each month of the year for the period 1981-2020 years. The horizontal grid resolution was 0.25 degrees. The data of total and low cloud cover measured in points (from 0 to 10) were also used, which were obtained from the archive of Ukrainian hydrometeorological observation network, which is situated in the Borys Sreznevsky Central Geophysical Observatory. The average monthly data from 15 meteorological stations for the period from 1981 to 2020 were collected: Kyiv, Kharkiv, Odesa, Dnipro, Zaporizhzhia, Kryvyi Rih, Lviv, as well as Kropyvnytskyi, Lutsk, Mykolaiv, Sumy, Uzhhorod, Kherson, Chernivtsi, and Chernihiv. An analysis of cloud cover changes over the past four decades (1981-1990, 1991-2000, 2001-2010, 2011-2020) compared to the climate norm of 1981-2010 showed that they were oscillating over the territory of Ukraine. In some decades both a decrease and an increase of total and low cloudiness were observed in different parts of the country. However, for certain months, there are tendencies toward a decrease in cloud cover over time. These tendencies are most typical for spring and summer. Partially similar changes can be noted for the autumn-winter period. The decrease of low cloud cover prevails over the total cloud cover.