Abstract
The climatological surface solar radiation (SSR; also called global radiation), which is largely dependent on cloud conditions, is an important indicator of the solar energy production potential. In the Baltic area, previous studies have indicated lower cloud amounts over seas than over land, in particular during the summer. However, the existing literature on the SSR climate or how it translates into solar energy potential has not paid much attention to how the SSR behaves quantitatively in relation to the coastline. In this paper, we have studied the climatological land–sea contrast of the SSR over the Baltic area. For this, we used two satellite climate data records, CLARA-A2 and SARAH-2, together with a coastline data base and ground-based pyranometer measurements of the SSR. We analyzed the behaviour of the climatological mean SSR over the period 2003–2013 as a function of the distance to the coastline. The results show that off-shore locations on average receive higher SSR than inland areas and that the land–sea contrast in the SSR is strongest during the summer. Furthermore, the land–sea contrast in the summer time SSR exhibits similar behavior in various parts of the Baltic. For CLARA-A2, which shows better agreement with the ground-based measurements than SARAH-2, the annual SSR is 8% higher 20 km off the coastline than 20 km inland. For summer, i.e., June–August, this difference is 10%. The observed land–sea contrast in the SSR is further shown to correspond closely to the behavior of clouds. Here, convective clouds play an important role as they tend to form over inland areas rather than over the seas during the summer part of the year.
Highlights
Clouds are more prevalent over seas than over land [1,2]
From the satellite cloud climatology of Karlsson [6], it appears that the strongest land–sea contrast in clouds and solar radiation prevails during summer
While it can be noted that the months May–July exhibit the highest solar radiation, and corresponds to ’summer’ in terms of the solar radiation climate of this region (e.g., [16]), we here choose to use the standard definitions of seasons in our analysis
Summary
Over mid-to-high latitudes in the Northern Hemisphere, surface-based synoptic weather reports show higher cloud amount over land than over seas [3]. In Sweden, the land–sea contrast in cloud behavior was studied already by Ångström in 1928 [4] He analyzed measurements of bright sunshine duration across the Swedish west coast and found that Vinga, a lighthouse in the outermost archipelago, had 11% more sunshine than Kålltorp, which is located slightly east of the central parts of Gothenburg. The distance between these two locations is ca 27 km. An early sunshine climatology over Finland, presented by Lunelund in 1941 [5], indicates a land–sea contrast in the duration of bright sunshine, with relatively high values along the coastline and in the archipelago
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