Space-time variability of subcanopy horizontal advection in moderately complex terrain

This study examines the space-time structure of the wind and temperature fields, as well as that of the resulting horizontal temperature gradients and advection of sensible heat in the subcanopy of a forest with a dense overstory in moderately complex terrain. The objectives of this study were to i) analyze the space-time variability of the observations, ii) evaluate the sensitivity of the results to network geometry and method of analysis, iii) shed some light on the physical mechanisms causing the variability. Data were collected from a sensor network of ten stations sampling horizontal wind speed and direction, and air temperature over several months. Observations were orthogonally decomposed into the multi-resolution basis set of time scales of dyadic width from 1 to 64 min duration and subsequently analyzed using bulk stochastic measures including two-point correlation and structure functions, and various indicators of space-time variability. Fundamental differences were found for the space-time variability between the wind and temperature fields. Motions dominating the flow and the transport of sensible heat occupy similar spatial, but different temporal scales. This mismatch aggravates the computation of meaningful estimates of horizontal advection and may explain the ambiguity and unresolved problems of the contribution of horizontal advective fluxes to the canopy energy and mass balances. Furthermore, both spatial temperature gradients and advective fluxes were demonstrated to be sensitive to network geometry and method of analysis. The assumption of linear spatial gradients was found to be incorrect and leads to increased space-time variability in both variables, at least at this site. The heterogeneity of the vegetated canopy as well as that of the local terrain had a significant impact on the micro-scale space-time structure of the flow and the temperature. A method is suggested to estimate the size of the control volume most promising to evaluate the advective components of the energy and mass balances. Although observed in a vegetated canopy, techniques and results may be transferred to other complex surfaces including urban canopies.