EXECUTIVE SURVEY OF THE TUCSON AGGREGATION WORKSHOP

'Aggregation' refers to spatial averaging of some heterogeneous surfac e variable to obtain an effective value representative of an area. The effect of surface heterogeneity an interactions between land and atmo sphere is relevant to near-surface hydrology, ecology, and climate, an d is the common theme of the papers in this issue. Even though the ful l effect of heterogeneity must be neglected owing to limited spatial r esolution of large-scale models, it is important to understand when an d how the presence of heterogeneity requires recognition in any aggreg ate representation. In March 1994, a workshop, which has come to be kn own as the 'Tucson Aggregation Workshop', was convened to assess the s tate of the art in aggregation research, and the papers in this issue are the product of that workshop. The principal findings of the worksh op can be summarized as follows: 1. substantial progress has been made in producing aggregated representations of flat terrain. Simple aggre gation rules applied to surface properties have given rise, in some st udies, to simulated surface energy fluxes that are within 10% of fluxe s produced from models with full representation of heterogeneity.2. Ag gregation rules are relatively straightforward in the ease of patch-sc ale heterogeneity (variability on the order of hundreds to thousands o f meters) of vegetative characteristics which control surface exchange s, although aggregation of soil hydraulic properties and possibly of s oil moisture remains problematic. In addition, some of the effects of meso-scale heterogeneity (variability on the order of 10-100 km) in su rface cover will need to be addressed through more complicated types o f parameterization. 3. There is convincing evidence that the regional energy balance (over, say, 10(5) km(2)) is insensitive to gentle topog raphy, provided that surface vegetation and water availability are uni form, but in mountainous terrain the influence of topography on near-s urface meteorology must be considered. 4. It appears that the value of simple combinations of remotely sensed radiances representing areal-a verage measurements are influenced only slightly by unresolved variabi lity, although the averaging of some derived variables based on these radiances offers a greater challenge, especially with sparse canopies.