Shortwave cloud effect and its trends in Southern Poland (2002-2021)
Authors: Zajączkowski Dariusz , Łupikasza Ewa 
Published: June 2022

&lt;p&gt;The shortwave cloud effect (SCE) is understood as the difference between potential and measured global radiation. The time-space variability of SCE is mainly modified by cloudiness and its type. Proper diagnosis of SCE is a crucial factor from the point of view of climate change modelling, as cloudiness is particularly important in the Earth's energy balance. However, the impact of cloudiness on the changes in SCE is still not well understood.&lt;/p&gt;&lt;p&gt;This research quantifies the SCE depending on cloudiness and cloud types in GZM (Metropolis G&amp;#243;rno&amp;#347;l&amp;#261;sko-Zag&amp;#322;&amp;#281;biowska) which represent the most urbanized part of Poland. Moreover, the study also assesses long-term variability and trends in SCE and cloudiness. The research is based on unique hourly data of global radiation (W/m&lt;sup&gt;2&lt;/sup&gt;) which, due to its specific location in the center of GZM, represent urban area conditions. The hourly potential global radiation was calculated using the Bird and Hulstrom model. Then the SCE (W/m&lt;sup&gt;2&lt;/sup&gt;) was calculated as the difference between the potential and the measured global radiation. Data on cloudiness, cloud level (CL-Low, CM-Mid, CH-High), and its type were obtained from the synoptic station in Katowice, located 9.6 km from the FNS station. Trends for SCE, cloudiness, and cloud types were calculated by using Mann-Kendall Test. The data covers the years 2002-2021.&lt;/p&gt;&lt;p&gt;In 2002-2021, the annual SCE reaches &amp;#8211;145.3 W/m&lt;sup&gt;2&lt;/sup&gt;. During the year, SCE varied from &amp;#8211;203.2 (May) to &amp;#8211;76.7 (December) W/m&lt;sup&gt;2&lt;/sup&gt;. The strongest SCE was &amp;#8211;335.2 W/m&lt;sup&gt;2&lt;/sup&gt; in May 2010, which may have been due to the contribution of the Eyjafjallaj&amp;#246;kull volcano eruption (Iceland), whose volcanic ash reduced the transparency of the atmosphere, strengthening the SCE. The weakest SCE occurred in December 2013, reaching &amp;#8211;51.3 W/m&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&lt;p&gt;The SCE during the occurrence of a given cloud level showed that the SCE is the strongest during CL occurrence. The annual average of SCE reached &amp;#8211;95.6 W/m&lt;sup&gt;2&lt;/sup&gt;. The weakest annual SCE was found for CM and CH, reaching &amp;#8211;23.0 and &amp;#8211;32.5 W/m&lt;sup&gt;2&lt;/sup&gt;, respectively. In the annual course of SCE during the occurrence of CL, the SCE was strongest from June to August, reaching &amp;#8211;150 W/m&lt;sup&gt;2&lt;/sup&gt;. The SCE for CM was most intense (&amp;#8211;36.1 W/m&lt;sup&gt;2&lt;/sup&gt; on average) from July to October, which coincided with increased CM level cloud frequency in this part of the year.&lt;/p&gt;&lt;p&gt;The trend analysis revealed a reduction in monthly SCE from +6.6 (January) to +27.6 (July) W/m&lt;sup&gt;2&lt;/sup&gt; per decade (SCE takes negative values, thus positive trends indicate weakening of the effect). The weakening of SCE in these particular months may have been linked to the significant decrease (p&lt;0.05) in the CL frequency (&amp;#8211;39.6 hours per decade). Strengthening of SCE in September (&amp;#8211;9.4 W/m&lt;sup&gt;2&lt;/sup&gt; per decade) may have been related to the increase (p&lt;0.1) of CH (26.0 hours per decade).&lt;/p&gt;