The comprehensive radiation monitoring program comprising broadband, spectral and spectroscopic observations from the CIMO testbed site Lindenberg is presented. Long-term series of total solar and diffuse broadband radiation demonstrate the variations in the atmospheric transparency over the past 80 years such as the dimming in the 1950-1985 period followed by a continuous brightening. Total and direct shortwave radiation show significant trends at 3.5 and 9.3 Wm−2 per decade over the past 25 years, respectively. While the dimming and major parts of the brightening are most likely caused by variations in the aerosol optical depth, a possible decrease of the magnitude of the cloud radiative effect may contribute to the increase in solar radiation in the brightening period, especially in most recent years.

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