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 radiation ( Q ), global radiation ( G ), reflected global radiation ( R ), long‐wave atmospheric radiation ( A ) and thermal radiation ( E ) of a pine forest in Southern 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 management and in the case of the grass surface, the shifting of the site from a climatic garden to a horizontal roof. Both series of radiant fluxes are analyzed with respect to the influences of the weather (cloudiness, heat advection). 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, yielding the radiation efficiency Q / G , the albedo R / G =α and the normalized long‐wave net radiation ( A + E )/ G . Furthermore, the long‐term mean values and the long‐term trend of yearly mean values are discussed and, moreover, a comparison is made of individual monthly values. Q p is twice as large as Q g . The reason for this is the higher values of G and A above the pine forest and half values of α p compared to α g . E p is only a little greater than E g . The time series of the radiation fluxes show the following 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 of the site. This phenomenon is also dominated by the long‐wave radiation processes. Although the properties of both site surfaces alter, E p and E g remain 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 (pine forest). Compared to α g , α p shows only a small variability. The changes of α p can be easily explained as resulting from the influence on the albedo of alterations in cloudiness and the changes in the water storage in stand and soil produced by varying weather conditions. The influence of the forest management is only small and short‐termed. α g reacted with a leap on site merging, followed by a continuous drop. The drop of the grass surface albedo α g on top of the observatory is probably related to the gradual development of the newly sown lawn there and the resultant change in the specific hydrological balance of the soil. A decreasing 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 © 1999 Royal Meteorological Society