Influence of surface heterogeneity on scalar dissimilarity in the roughness sublayer

While it is generally known that surface heterogeneityweakens the application ofMonin–Obukhov similarity (MOS), few studies have investigated how seasonal changes in the degree of surface heterogeneity at a particular site may influence the validity of the similarity application. Exploiting seasonal changes in forest function associated with senescence, we conduct a unique evaluation of the effects of surface heterogeneity on the validity of similarity theory at two sites through time. Using high frequency (10Hz) velocity and scalar time series collected within the roughness sublayer over mixed hardwood deciduous and coniferous forests during both periods of peak leaf area and senescence of deciduous foliage, we examined conformity with proposed universal flux-variance predictions and agreement amongst normalized standard deviations of different scalars (temperature, water vapour and carbon dioxide concentrations). Normalized scalar standard deviations were elevated above MOS flux-variance predictions, with more pronounced deviations observed during and following senescence, particularly in the case of CO2. Power-law scaling of normalized standard deviations as a function of stability was upheld and robust to seasonal changes in surface heterogeneity. However, dissimilarity of normalized standard deviations for the scalars increased during senescence, as heterogeneity in the source/sink field increased. Scalewise decomposition of scalar time series using wavelet analysis indicated that correlations between scalarswere conservative through much of the inertial cascade but decayed for eddies<10m. Senescence lowered correC. A. Williams (B) Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 80523-1499, USA e-mail: [email protected] C. A. Williams · J. D. Albertson Department of Civil andEnvironmental Engineering,DukeUniversity,Durham,NC, 27708-0287, USA T. M. Scanlon Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904-4123, USA 150 Boundary-Layer Meteorol (2007) 122:149–165 lations between scalars over awide range of eddy sizes. These results demonstrate how seasonal changes in surface physiology can cause a temporal production of heterogeneity in the source/sink field, thus weakening similarity applications in the roughness sublayer.