CR 20:203-209 (2002)  -  doi:10.3354/cr020203

ABSTRACT: The shortwave radiation regime for 3 REKLIP sites at different altitudes in south-west Germany has been characterised. The study period extended from 1992 to 1995. Climatic variables used for the characterisation include surface albedo, cloud cover, relative sunshine duration, fog frequency, atmospheric clearness index and precipitation. Given clear sky conditions, the atmospheric clearness index amounted to 76% at the upper mountain site (1489 m above sea level [a.s.l.]) and 68.5% at the lowland site (212 m a.s.l.). Although the vertical profile of incoming solar radiation exhibited a positive gradient on clear sky days (independent of season), the net shortwave radiation showed a dissimilar trend in winter, when albedo values were significantly high at the mountain sites. In general, net shortwave radiation at the lowland site was 1.1 and 1.2 times that of the intermediate and upper mountain sites respectively. For each season, theoretically computed diffuse sky radiation increased with cloud cover, N, only to a maximum of about 5 octa, while the diffuse fraction, Kd, of solar radiation rose steadily with N. From April to October, the estimated monthly Kd increased with altitude, being vice-versa for the other months.

KEY WORDS: Solar radiation · Net shortwave radiation · Atmospheric clearness index · Diffuse fraction · Albedo · Relative sunshine duration · Cloud cover · REKLIP