Variability and trends of downward surface global solar radiation over the Iberian Peninsula based on ERA‐40 reanalysis

Abstract

A climate study of the incidence of downward surface global solar radiation (SSRD) in the Iberian Peninsula (IP) based primarily on ERA-40 reanalysis is presented. NCEP/NCAR reanalysis and ground-based records from several Portuguese and Spanish stations have been also considered. The results show that reanalysis can capture a similar inter-annual variability as compared to ground-based observations, especially on a monthly basis, even though annual ERA-40 (NCEP/NCAR) values tend to underestimate (overestimate) the observations with a mean relative difference of around 20 W m -2 (40 W m -2 ). On the other hand, ground-based measurements in Portuguese stations during the period 1964-1989 show a tendency to decrease until the mid-1970s followed by an increase up to the end of the study period, in line with the dimming/brightening phenomenon reported in the literature. Nevertheless, there are different temporal behaviours as a greater increase since the 1970s is observed in the south and less industrialized regions. Similarly, the ERA-40 reanalysis shows a noticeable decrease until the early 1970s followed by a slight increase up to the end of the 1990s, suggesting a dimming/brightening transition around the early 1970s, earlier in the south and centre and later in the north of the IP. Although there are slight differences in the magnitude of the trends as well as the turning year of the dimming/brightening periods, the decadal changes of ERA-40 fairly agree with the ground-based observations in Portugal and Spain, in contrast to most of the literature for other regions of the world, and is used in the climatology of the SSRD in the study area. NCEP/NCAR reanalysis does not capture the decadal variations of SSRD in the IP. The results show that part of the decadal variability of the global radiation in the IP is related to changes in cloud cover (represented in ERA-40).

An Iberian climatology of solar radiation obtained from WRF regional climate simulations for 1950–2010 period

“…with an increase of global radiation at the surface. Based on observations and ERA-40 reanalysis, Perdigão et al (2016) confirmed the existence of these two distinct periods in Portugal, known as dimming and brightening, but with an earlier turning year than the one described in the literature, in particular over the south of Portugal. This latter study revealed an increasing trend of surface global radiation located in the south of Portugal, without a pronounced decrease before the 1980s, in the period 1964-1986. Before reaching the Earth's surface, incoming solar radiation is scattered, absorbed and reflected in the atmosphere.…”

An updated review on recent trends in observational surface atmospheric variables and their extremes over Spain

“…Similar significant increases were observed in the mean seasonal series, with the highest rate of increase during summer (6.5 Wm −2 decade -1 ). These results confirm the "brightening" phenomenon in Spain over the last decades, a pattern that was recently confirmed by Perdigao et al (2016) using reanalysis datasets.…”

Long-Term Patterns and Trends of Shortwave Global Irradiance over the Euro-Mediterranean Region

“…In Europe, the timing of the transition from dimming to brightening has been reported around the mid-1980s (e.g., Ohmura [13]; Norris and Wild [12]). On the other hand, Perdigao et al [14], using ERA-40 reanalysis, found that the transition occurred around the early 1970s over the Iberian Peninsula. Additionally, Kambezidis et al [18], using monthly re-analysis datasets from the Modern Era Retrospective-Analysis for Research and Applications, did not find a reversal from dimming to brightening during 1980s in the Mediterranean basin.…”

“…The various studies analyzing long-term records of surface radiation measurements suggest a widespread decrease in surface solar radiation between the 1950s and 1980s ("dimming", e.g., [6][7][8][9][10][11][12][13][14][15][16]), with a partial recovery after 1980 at many locations ("brightening", e.g., [8][9][10][11][12][13][14][15][16][17][18][19][20]). There are some indications for an "early brightening" in the first part of the 20th century (e.g., [10,13,21]).…”