4.2
Determining scalar fluxes over an old-growth forest using surface renewal
Donatella Spano, University of Basilicata, Potenza, Italy; and R. L. Snyder, P. Duce, K. T. Paw U, and M. Falk
The surface renewal (SR) method for estimating fluxes from canopies involves high frequency measurements of scalar parameters. The high frequency data are analyzed for ramp-like characteristics and the amplitude and inverse ramp frequency are used in basic energy or mass conservation equations to estimate fluxes. In recent years, good results were obtained for estimating sensible heat flux density (H) using temperature data. However, little information is available on using SR to estimate latent (LE) and CO2 flux density (Fc). The advantage in using the SR rather than the eddy covariance (EC) method is that a sonic anemometer is not required. To test the SR method for estimating H, LE, and Fc, data were collected above an old-growth forest at the Wind River crane site in Washington State. The coniferous forest canopy is approximately 70 m tall. Wind speed, temperature, humidity, and CO2 fluctuations were recorded using a Gill 3-D sonic anemometer and Licor 6262 infrared gas analyzer mounted near the canopy top. The same high frequency data were also used to determine H, LE, and Fc using the surface renewal method. The ramp amplitude and inverse ramp frequency were calculated during 30 minute sampling periods using a structure function. A good relationship between EC and SR estimates of H were observed when a weighting factor (a) for unequal heating within the canopy was employed. The surface renewal estimates of LE were comparable for both the SR and EC methods when using an a factor that was about the same as that used for the SR H calculations. During daytime CO2 uptake, the a factor required for Fc calculations was slightly smaller than that used for H and LE fluxes. However, at nighttime, when there was respiration, the a factor was near zero. Based on these experiments, it seems that the SR method offers a good alternative for estimating H, LE, and Fc when the a weighting factor is known.
Session 4, Theoretical and applied studies of interactions between the atmosphere and the land surface
Wednesday, 16 August 2000, 1:30 PM-5:15 PM
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