Extended Abstract (100K)9.6
Characteristic Turbulence Spectra Above and Below a Tamarisk Canopy
John H. Prueger, USDA/ARS, Ames, IA; and W. Eichinger, L. E. Hipps, J. R. Cleverly, J. L. Hatfield, S. Bawazir, D. I. Cooper, and W. P. Kustas
Many riparian eco-tones in the southwest U.S. are dominated by dense stands of Tamarisk. Accurately predicting water loss through Tamarisk evapotranspiration is essential to effective river management attempting to meet growing demands of an increasingly scarce resource. Tamarisk stands typically represent spatially complex challenges for modeling and measurement campaigns and in semi-arid regions turbulence exchange processes for mass and scalars are routinely influenced by the advection of sensible heat. Over a 5 m tall Tamarisk canopy in an semi-arid environment, the depth of the roughness sub-layer is thin and can vary in response to advection or the entrainment of intermittent eddies from the PBL. A study was developed to evaluate turbulence structures above and below a Tamarisk canopy under varying local atmospheric conditions and how it affects sensible and latent heat flux exchange from a Tamarisk stand. A 12 m tower was erected in the middle of a dense stand of Tamarisk located along the Rio Grande in the Bosque del Apache Wildlife Refuge in central New Mexico. Two eddy covariance (EC) systems and all pertinent ancillary measurements to complete the surface energy balance were mounted on a single tower, 3 m above the canopy and 1 m above the ground surface below the canopy. In addition, a Raman lidar was deployed near the tower to measure multi-dimensional water vapor fields. The lidar produces visual images of water vapor transport using two-dimensional scans of ranged lidar returns. Times series data from the eddy covariance instrumentation were acquired at a rate of 20 Hz. Power spectra and co-spectra of various turbulence components were computed for periods representing the full range of diurnal conditions above and below the canopy. Lidar and spectra results were integrated to characterize turbulence properties for this unique environment. Preliminary results indicate that the roughness sub-layer is temporally and spatially dynamic. A common occurrence was that at times the EC was well within the roughness sub-layer, while at other times clearly out of the layer, thus complicating the interpretation of the EC results within the prescribed assumptions of eddy covariance. Distinct turbulence characteristics were found at both EC levels. Rapid decoupling between the space below (surface) and above the canopy (atmosphere) was observed to evolve rapidly after sunset. Effects on computed fluxes of sensible and latent heat flux will be presented.
Session 9, Turbulence and Dispersion in Canopies Part 2
Thursday, 23 May 2002, 1:30 PM-4:00 PM
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