Energy budget above a high-elevation subalpine forest in complex topography

Components of the energy budget were measured above a subalpine coniferous forest over two complete annual cycles. Sensible and latent heat fluxes were measured by eddy covariance. Bowen ratios ranged from 0.7 to 2.5 in the summer (June-September) depending upon the availability of soil water, but were considerably higher (similar to3-6) during winter (December-March). Energy budget closure averaged better than 84% on a half-hourly basis in both seasons with slightly greater closure during the winter months. The energy budget showed a dependence on friction velocity (u*), approaching complete closure at u* values greater than 1 m s(-1). The dependence of budget closure on u* explained why energy balance was slightly better in the winter as opposed to summer, since numerous periods of high turbulence occur in winter. It also explained the lower degree of energy closure (similar to10% less) during easterly upslope flow since these periods were characterized by low wind speeds (U < 4 m s(-1)) and friction velocities (u* < 0.5 m s(-1)). Co-spectral analysis suggests a shift of flux density towards higher frequencies under conditions where closure was obtained. It is suggested that low frequency contributions to the flux and advection were responsible for the lack of day-time energy budget closure. These effects were reduced at high friction velocities observed at our site. Our ability to close the energy budget at night was also highly dependent on friction velocity, approaching near closure (similar to90%) at 0 values between 0.7 and 1.1 m s(-1). Below this range, the airflow within the canopy becomes decoupled with the flow above. Above this range, insufficient temperature resolution of the sonic anemometer obscured the small temperature fluctuations, rendering measurements intractable. (C) 2002 Elsevier Science B.V. All rights reserved.