HOW WELL CAN REGIONAL FLUXES BE DERIVED FROM SMALLER-SCALE ESTIMATES

Regional surface fluxes are essential lower boundary conditions for la rge-scale numerical weather and climate models and are the elements of global budgets of important trace gases (Stewart et al., 1989). Surfa ce properties affecting the exchange of heat, moisture, momentum and t race gases vary with length scales from 1 m to hundreds of kilometers. A classical difficulty is that fluxes have been measured directly onl y at points (towers) or along lines (from aircraft). The process of '' scaling up'' observations limited in space and/or time to represent la rger areas has been done by assigning properties to surface types and combining estimated or calculated fluxes using an area-weighted averag e. Because of nonlinear influences, such as the effect of internal bou ndary layers, it is not clear that a simple area-weighted average is s ufficient to produce the large scale from the small scale, nor is it k nown how important the uncertainty is to large-scale model outcomes. S imultaneous aircraft and tower data obtained in the relatively simple terrain of the western Alaskan tundra were used to determine the exten t to which surface type variation can be related to regional-scale flu xes of heat, moisture, and other properties. Surface type was classifi ed as lake or land with an aircraft-borne infrared thermometer, and fl ight-level heat and moisture fluxes were related to surface type. The magnitude and variety of sampling errors inherent in eddy correlation flux estimation place limits on how well any flux can be known even in simple geometries. Because of the presence of intrinsic and site-spec ific uncertainties, regional-scale flux of heat and moisture using air craft observations in our study area can be reasonably verified to be estimated correctly from linear combinations of smaller-scale estimate s only to within a factor of 1.5. Flights at lower levels or in a more comprehensive or systematic pattern might be able to resolve the cont ributions from individual surface types better, but an experiment to t est any scaling-up hypothesis is difficult to devise.