Executive summary of the Tucson Aggregation Workshop

‘Aggregation’ refers to spatial averaging of some heterogeneous surface variable to obtain an effective value representative of an area. The effect of surface heterogeneity on interactions between land and atmosphere is relevant to near-surface hydrology, ecology, and climate, and is the common theme of the papers in this issue. Even though the full effect of heterogeneity must be neglected owing to limited spatial resolution of large-scale models, it is important to understand when and how the presence of heterogeneity requires recognition in any aggregate representation. In March 1994, a workshop, which has come to be known as the ‘Tucson Aggregation Workshop’, was convened to assess the state of the art in aggregation research, and the papers in this issue are the product of that workshop. The principal findings of the workshop can be summarized as follows:

1. 1. substantial progress has been made in producing aggregated representations of flat terrain. Simple aggregation rules applied to surface properties have given rise, in some studies, to simulated surface energy fluxes that are within 10% of fluxes produced from models with full representation of heterogeneity.

2. 2. Aggregation rules are relatively straightforward in the case of patch-scale heterogeneity (variability on the order of hundreds to thousands of meters) of vegetative characteristics which control surface exchanges, although aggregation of soil hydraulic properties and possibly of soil moisture remains problematic. In addition, some of the effects of meso-scale heterogeneity (variability on the order or 10–100 km) in surface cover will need to be addressed through more complicated types of parameterization.

3. 3. There is convincing evidence that the regional energy balance (over, say, 10⁵ km²) is insensitive to gentle topography, provided that surface vegetation and water availability are uniform, but in mountainous terrain the influence of topography on near-surface meteorology must be considered.

4. 4. It appears that the value of simple combinations of remotely sensed radiances representing areal-average measurements are influenced only slightly by unresolved variability, although the averaging of some derived variables based on these radiances offers a greater challenge, especially with sparse canopies.