Long-term variability of Altocumulus lenticularis clouds in Katowice and atmospheric circulation conditions

The aim of this paper is to analyse the annual and long-term variability in the occurrence of these clouds in the Silesian Upland (on the example of Katowice) as well as to indicate its causes related to atmospheric circulation. The source material comes from the IMGW-PIB. The paper uses data on mid-level cloud cover in Katowice in the years 1971-2020. Altocumulus lenticularis (Ac len) clouds, coded CM = 4, were analysed for eight time points a day (every 3 hours) during the study period. The study also used data on air masses, weather fronts, and types of atmospheric circulation based on the Calendar of atmospheric circulation types for southern Poland compiled by Niedźwiedź (2024). Most occurrences of Ac len were observed in autumn, and least in summer. The highest average number of days and number of cases of Ac len were recorded in October and November, while the lowest in April. In terms of the diurnal distribution, these clouds occurred most frequently between 6:00 and 15:00 UTC. Based on a 50-year dataset, a statistically significant decreasing trends for both number of cases and number of days were found for this type of cloud cover (-2.7 days per 10 years). The maximum number of occurrences was found in 1975, and the minimum in 2016. The highest conditional probability of occurrence of a lenticular cloud in Upper Silesia was found for advection of air masses from the south and southwest in cyclonic pressure systems (>25%) and for advection of a warm polar maritime air mass over southern Poland (20%).