Clouds significantly affect Earth's energy balance by absorbing, reflecting, and transmitting short- and long-wave radiation. They can either intensify or weaken the greenhouse effect. The overall impact of cloudiness on radiating transfer depends on macro- and micro-physical properties of clouds (e.g., optical thickness, altitude, water content, cloud drop effective radius) and still remains one of the greatest uncertainties in global climate projections.

Recent studies based on traditional, synoptic (surface) data have shown several statistically significant trends in cloud types (genera) frequency over Poland. These changes included an increase in high and convective clouds frequency, along with a decrease in the frequency of Stratus, Altostratus, and Nimbostratus. As the ability to observe mid and high-level clouds from the ground is limited due to clouds overlapping, in this research, we aim to explore whether these trends can be confirmed by satellite records.

This work evaluates cloud types frequency and amount, as well as cloud properties over Poland in the last two decades. We use Moderate Resolution Imaging Spectroradiometer (MODIS) cloud properties (cloud top pressure, CTP; cloud optical thickness, COT; cloud effective radius, CER; and cloud water path, CWP) and surface (SYNOP) observations of cloud genera from the country's 27 ground-based stations for the period 2003–2021.

It was found that while for some cloud types (Cirrus, Altostratus + Nimbostratus, and Cumulus) MODIS and SYNOP show similar trends over the last two decades, for other cloud types (Cumulonimbus, Altocumulus, Stratocumulus) the two sources of data are not consistent. Hence, it was concluded that they should be treated as independent rather than complementary.

It was demonstrated that the increase in high-level clouds over Poland, which has been observed by other authors who based their research on synoptic data, is not caused by a decrease in low- and mid-level cloud frequency but is confirmed by satellite records.

The results of the study indicate that, over the past two decades, CTP over Poland has consistently decreased by 7.3–9.7 hPa per decade. This decline primarily affects northern and north-western Poland, with local decreases reaching up to −40.0 hPa per decade. High clouds (CTP < 440 hPa) exhibited the most pronounced changes, with a statistically significant negative trend observed over 25–35% of Poland's area. The research findings are particularly important for understanding the radiative properties of clouds and their role in atmospheric energy balance.

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