Comparison of turbulent statistics and spectra at two heights over a semi-arid grassland surface

Understanding the turbulent exchange of mass and energy from a surface remains essential to appropriate characterization and quantification of the components of surface energy balance. This is particularly true for arid and semi-arid regions that often contain surfaces that are highly heterogeneous, exhibit significant topographical features, abrupt changes in vegetation cover and contain large spatial and temporal gradients in the water and energy balance components. Collectively, all of the above can result in properties of turbulent exchange that significantly deviate from fundamental assumptions associated with eddy covariance measurements of energy and mass. As part of the Soil Moisture Experiment 2004 (SMEX04) eddy covariance measurements (10 Hz) at two heights (2 & 10 m AGL) in a sub-watershed basin in Walnut Gulch, AZ were conducted during the monsoon season over a heterogeneous grassland surface. Turbulence statistics, spectra, cospectra and fluxes of heat and water vapor were computed, evaluated and compared for the two heights. Unique spectral features related to near free convective conditions at the surface were observed and will be presented. The implications of these findings on the proper interpretation of eddy covariance data will be explored. The effects of surface heterogeneity in terms of source strength turbulent contributions to the measurements at two heights will also be discussed.