Long-term changes in net radiation and its components above a pine forest and a grass surface in Germany

Long-term measurements (1974-1993 and 1996, respectively) of the net radiat ion (Q), global radiation (G), reflected global radiation (R), long-wave at mospheric radiation (A) and thermal radiation (E) of a pine forest in South ern Germany (index p) and of a grass surface in Northern Germany (index g) are compared. The influence of changes in surface properties is discussed. There are, in the case of the pine stand, forest growth and forest manageme nt and in the case of the grass surface, the shifting of the site from a cl imatic garden to a horizontal roof. Both series of radiant fluxes are analy zed with respect to the influences of the weather (cloudiness, heat advecti on). To eliminate the different influence of the solar radiation of the two sites, it is necessary to normalize by means of the global radiation G, yi elding the radiation efficiency Q/G, the albedo R/G = alpha and the normali zed long-wave net radiation (A + E)/G. Furthermore, the long-term mean valu es and the long-term trend of yearly mean values are discussed and, moreove r, a comparison is made of individual monthly values. Q(p) is twice as larg e as Q(g). The reason for this is the higher values of G and A above the pi ne forest and half values of alpha(p) compared to alpha(g). E-p Is only a l ittle greater than E-g. The time series of the radiation fluxes show the fo llowing trends: Q(p) declines continuously despite a slight increase of G(p ). This is mainly due to the long-wave radiation fluxes. The net radiation of the grass surface Q(g) shows noticeably lower values after the merging o f the site. This phenomenon is also dominated by the long-wave radiation pr ocesses. Although the properties of both site surfaces alter, E-p and E-g r emain relatively stable. A(p) and A(g) show a remarkable decrease however. The reason for this is to be found in a modification of the heat advection, showing a more pronounced impact on the more continentally exposed site (p ine forest). Compared to alpha(g), alpha(p) shows only a small variability. The changes of alpha(p) can be easily explained as resulting from the infl uence on the albedo of alterations in cloudiness and the changes In the wat er storage in stand and soil produced by varying weather conditions. The in fluence of the forest management is only small and short-termed, alpha(g) r eacted with a leap on site merging, followed by a continuous drop. The drop of the grass surface albedo alpha(g) on top of the observatory is probably related to the gradual development of the newly sown lawn there and the re sultant change in the specific hydrological balance of the soil. A decreasi ng radiation efficiency Q/G at both sites is observed, which is principally due to an increase in - (A + E)/G. The investigation discovered changes in the long-wave radiation processes. Its influence in Q should lead to more attention to the long-wave radiation processes in routine monitoring of radiation. Copyright (C) 1999 Royal Met eorological Society.