How well can regional fluxes be derived from smaller‐scale estimates?

Abstract

Regional surface fluxes are essential lower boundary conditions for large-scale numerical weather and climate models and are the elements of global budgets of important trace gases (Stewan. et al, 1989). Surface properties affecting the exchange of heat, moisture, momentum and trace gases vary with length scales from one meter to hundreds of kilometers. A classical difficulty is that fluxes have been measured directly only at points (towers) or along lines (from aircraft). The process of "scaling up" observations limited in space and/or time to represent larger areas has been done by assignings properties to surface classes and combining estimated or calculated fluxes using an areaweighted average. It is not clear that a simple area-weighted average is sufficient to produce the larye scale from the small scale, chiefly due to the effect of internal boundary layers, nor is it known how important the uncertainty is to large-scale model outcomes. Simultaneous aircraft and tower data obtained in the relatively simple terrain of the western Alaska tundra were used to determine the extent to which surface type variation can be related to fluxes of heat, moisture and other properties. Surface type was classified as lake or land with aircraft-borne infrared thermometer, and flight-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-specific uncertainties, regional-scale flux of heat and moisture using aircraft observations in our study area can be reasonably verified to be estimated correctly from linear combinations of smaller-scale estimates to within a factor of 1.5. Rights at lower levels or in a more comprehensive or systematic pattern might be able to resolve the contributions from individual surface types better, but an experiment to test any scaling-up hypothesis is difficult to devise.

Methodology in Climatology

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Subgrid‐scale variability in the surface energy balance of arctic tundra

“…For example, in a previous surface flux experiment in Alaskan arctic tundra, tower observations of surface energy and moisture exchange could not differentiate among the individual surface types (wet meadow, dry upland tundra, and lake) occurring within a 1-km flux source area. At the next larger scale, regional measurements from an aircraft, it was possible to decompose fluxes from contrasting surface types only very broadly [Moore et al, 1993]. On the other hand, in the same experiment, Fan et al [1992] found that the different tundra surface types varied by a factor of 2 or more in exchanges of trace gases with the atmosphere.…”

“…Although Moore et al [1993] concluded it may only be possible to decompose regional measurements into fluxes from component vegetation types very broadly, they also found that linear scaling may be valid for heat fluxes over tundra surfaces. This suggests that, if vegetation-type differences in surface energy partitioning and moisture exchange can be related to measurable properties of the surface, it may be feasible to predict regional fluxes from land-surface classifications and remote sensing data.…”

Ground based radon-222 observations and their application to atmospheric studies

“…In spite of considerable research effort (e.g. Raupach et al, 1992;Avissar and Chen, 1993;Moore et al, 1993;Philip, 1996;Mahrt, 1996), the problem of integrating observations spatially and temporally between scales of observation and application continues to confound scientists. It is a complex problem that necessitates an understanding of the processes that control the system at the scale of interest, and determination of the key regulating processes at the scale of observations (Baldocchi, 1993).…”