Solar dimming/brightening effect over the Mediterranean1979 − 2012
Written by H.D. Kambezidisa, D.G. Kaskaoutisa, et al.
This work investigates the evolution and trends in the surface net short-wave radiation (NSWR, surface solar radiation – reflected) over the Mediterranean Basin during the period 1979 − 2012 using monthly re-analysis datasets from the Modern Era Retrospective-Analysis for Research and Applications (MERRA) and aims to shed light on the specific role of clouds on the NSWR trends.
The solar dimming/brightening phenomenon is temporally and spatially analyzed over the Mediterranean Basin. The spatially-averaged NSWR over the whole Mediterranean Basin was found to increase in MERRA by +0.36 Wm−2 per decade, with higher rates over the western Mediterranean (+0.82 Wm−2 per decade), and especially during spring (March-April-May; +1.3 Wm−2 per decade). However, statistically significant trends in NSWR either for all-sky or clean-sky conditions are observed only in May.
The increasing trends in NSWR are mostly associated with decreasing ones in cloud optical depth (COD), especially for the low (<700 hPa) clouds. The decreasing COD trends (less opaque clouds and/or decrease in absolute cloudiness) are more pronounced during spring, thus controlling the increasing tendency in NSWR.
The NSWR trends for cloudless (clear) skies are influenced by changes in the water-vapor content or even variations in surface albedo to a lesser degree, whereas aerosols are temporally constant in MERRA. The slight negative trend (not statistically significant) in NSWR under clear skies for nearly all months and seasons implies a slight increasing trend in water vapor under a warming and more humid climatic scenario over the Mediterranean.
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