Global solar irradiance arriving on the ground plays a key role in the radiative energy balance of the Earth-Atmosphere system. The knowledge of its magnitude, availability and distribution at the earth’s surface is most crucial to the design, development and estimation of the performance of solar energy conversion systems. Apart from astronomical and geographical factors, the global solar irradiance measured on an horizontal surface is largely dependent on the nature and extent of cloud cover; albedo of the underlying surface; atmospheric scattering by air molecules, smoke and dust as well as atmospheric absorption by Ozone, water vapor and carbon-dioxide. Although these varying factors coupled with the temporal and spatial fluctuation of the global transmissivity make it somewhat problematic to predict the actual value of global irradiance for a given locality with absolute accuracy, the study of medium to long term meteorological parameters does serve as a viable tool in an attempt at reproducing the characteristic variability of shortwave solar irradiance.

The regional climate project (REKLIP) is a joint project between meteorological institutions in Germany, France and Switzerland and has the objective of investigating the long term regional, temporal and altitudinal distribution of the energy balance components, among others, within the Upper Rhine valley as well as the surrounding mountainous areas. Results emanating from the solar radiation budget at three of the REKLIP measurement sites maintained by the Meteorological Institute, University of Freiburg, Freiburg, Germany and located at Bremgarten (47°N, 7°, 212 m a. s. l.), Geiersnest (47°N, E, 870 m a. s. l.) and Feldberg (N, , 1489 m a. s. l.) in southwest Germany are here been reported. The global solar radiation was measured during the period spanning from 1991 to October 1996. In addition, hourly records of cloud amount in octal for the period January 1991 to February 1993 for Bremgarten and January 1991 to October 1994 for Feldberg were utilized in the course of this work.

The mean diurnal, monthly, annual cumulative and inter-annual variation of global solar irradiance at all three locations are presented. As expected, the annual march of the measured solar radiation do exhibit strong seasonal dependence, rising from a minimum in January, reaching a maximum in mid-summer and finally falling to a minimum in December The annual means of daily total of global radiation at the three sites are generally consistent and agree to within 6.5% for Bremgarten, 11.2 % for Geiersnest and 7.6 % for Feldberg. The mean seasonal values of daily total of global solar radiation whose summer values are estimated at 19.5 MJm^-2d^-1 for Bremgarten, 18.5 MJm^-2d^-1 for Geiersnest and 16.7 MJm^-2d^-1 for Feldberg are observed to reflect altitude-dependence vis a vis the prevailing weather conditions at each of the sites. In the same vein, the vertical profile of global radiation under mean and selected weather and seasonal conditions within the area of investigation are presented. Also reported are the characteristic variations of the clearness index K_T at the three locations. The annual maximum of K_T is recorded in the summer month of July at Bremgarten and Geiersnest. As shown in Fig. 1 below, the clearness index K_T for July at the referenced locations are 0.53, 0.50 and 0.44 for Bremgarten (Br), Geiersnest (Ge) and Feldberg (Fe) respectively. Whereas the relatively low values of the clearness index obtained for Feldberg during the summer months are consequent on convective cumulus clouds and the attendant precipitation, the lower values recorded at Bremgarten and Geiersnest during the winter months are the resultant effects of radiation fogs at these locations indicative of an anticyclonic temperature inversion. In view of the comparatively higher values of the clearness index recorded during summer months at Bremgarten and Geiersnest, solar collectors and concentrators are expected to have high performance during this period at these locations.

Fig. 1: Annual variation of the clearness index K_T at Bremgarten, Giersnest and Feldberg in southwestern Germany (1992-1995)

Keywords: Global solar radiation, radiative energy balance, global transmissivity, clearness index, solar concentrators and atmospheric scattering.