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D. Storr and G. Den Hartog

Abstract

Using 0.66 mb K−1 for γ, the psychrometric constant, can cause underestimates of evapotranspiration in excess of 5% at elevations above 1000 m. Graphs and equations are presented to prevent this error and to facilitate the calculation of evapotranspiration.

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M. Y. Leclerc, R. H. Shaw, G. Den Hartog, and H. H. Neumann

Abstract

This paper shows that the inclusion of thermal effects is necessary to correctly interpret the physical processes involved in the generation or suppression of Reynolds stress and turbulent kinetic energy inside a form canopy. In both of thew budgets, thermal effects are largest in the upper third of the canopy where the foliage is densest and the radiation load highest. The magnitude of the buoyant production term in both these budgets increases almost linearly with instability in the upper region of the canopy. The onset of stability exerts a strong influence on the behavior of the shear production in both the budgets of Reynolds stress and turbulent kinetic energy. In strong thermal stratification, the shear production term becomes a sink of Reynolds stress and turbulent kinetic energy in the lower half of the canopy.

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Piers Sellers, Forrest Hall, Hank Margolis, Bob Kelly, Dennis Baldocchi, Gerry den Hartog, Josef Cihlar, Michael G. Ryan, Barry Goodison, Patrick Crill, K. Jon Ranson, Dennis Lettenmaier, and Diane E. Wickland

The Boreal Ecosystem Atmosphere Study (BOREAS) is a largescale international field experiment that has the goal of improving our understanding of the exchanges of radiative energy, heat, water, CO2, and trace gases between the boreal forest and the lower atmosphere. An important objective of BOREAS is to collect the data needed to improve computer simulation models of the processes controlling these exchanges so that scientists can anticipate the effects of global change.

From August 1993 through September 1994, a continuous set of monitoring measurements—meteorology, hydrology, and satellite remote sensing—were gathered overthe 1000 × 1000 km BOREAS study region that covers most of Saskatchewan and Manitoba, Canada. This monitoring program was punctuated by six campaigns that saw the deployment of some 300 scientists and aircrew into the field, supported by 11 research aircraft. The participants were drawn primarily from U.S. and Canadian agencies and universities, although there were also important contributions from France, the United Kingdom, and Russia. The field campaigns lasted for a total of 123 days and saw the compilation of a comprehensive surfaceatmosphere flux dataset supported by ecological, trace gas, hydrological, and remote sensing science observations. The surface-atmosphere fluxes of sensible heat, latent heat, CO2, and momentum were measured using eddy correlation equipment mounted on a surface network of 10 towers complemented by four flux-measurement aircraft. All in all, over 350 airborne missions (remote sensing and eddy correlation) were flown during the 1994 field year.

Preliminary analyses of the data indicate that the area-averaged photosynthetic capacity of the boreal forest is much less than that of the temperate forests to the south. This is reflected in very low photosynthetic and carbon drawdown rates, which in turn are associated with low transpiration rates (less than 2 mm day−1 over the growing season for the coniferous species in the area). The strong sensible fluxes generated as a result of this often lead to the development of a deep dry planetary boundary layer overthe forest, particularly during the spring and early summer. The effects of frozen soils and the strong physiological control of evapotranspiration in the biome do not seem to be well represented in most operational general circulation models of the atmosphere.

Analyses of the data will continue through 1995 and 1996. Some limited revisits to the field are anticipated.

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