Influence of surface heterogeneity on scalar dissimilarity in the roughness sublayer

Abstract

Whileitisgenerallyknownthatsurfaceheterogeneityweakenstheapplica- tionofMonin-Obukhovsimilarity(MOS),fewstudieshaveinvestigatedhowseasonal 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 rough- ness sublayer over mixed hardwood deciduous and coniferous forests during both periods of peak leaf area and senescence of deciduous foliage, we examined con- formity 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 waveletanalysisindicatedthatcorrelationsbetweenscalarswereconservativethrough much of the inertial cascade but decayed for eddies <10m. Senescence lowered corre-