Space-time variability of the wind, temperature, and horizontal advection of sensible heat in the subcanopy of a dense coniferous forest

Over the past decades many studies in the micrometeorological and applied flux communities have focused on the proper experimental implementation and analytical evaluation of the mass conservation principle to quantify land surface-atmosphere exchange. As a result, terms other than the turbulent vertical flux are now routinely included in estimates of the exchanges of momentum, sensible heat, carbon dioxide, and water vapor. Despite significant advances particularly for nocturnal observations, e.g., through accounting for horizontal and vertical advection, little attention has been given to the spatiotemporal variability of the exchange. The objectives of this study were to i) characterize all motions contributing to the transport and mixing in the subcanopy including turbulent, submeso and mesoscale processes covering time scales from milliseconds to hours and spatial scales from few centimeters to kilometers, ii) evaluate to what extent the wind and temperature fields, as well as the resulting advective transport of sensible heat vary with space and time given the roughness and the heterogeneity of the vegetation; and c) evaluate the effectiveness of currently deployed experimental methods of the micrometeorological and biogeochemical communities to capture this variability. Observations were made using a network of ten identical stations each consisting of a 2-D sonic anemometer and an air temperature sensor sampled continuously at 0.1 Hz starting July 2008 covering a domain size of 250 x 200 m centered around a main eddy covariance tower at an AmeriFlux research site in Oregon USA. Data were quality-filtered and subject to a Multi-Resolution Decomposition (MRD) using dyadic modes (1, 2, 4,…, 64min) to evaluate the variability separately for each time scale. Results showed that the systematic (64min mode) variability of the wind field is much greater then that of air temperature, indicating that the microsetting of each individual station has a large effect on the observed wind speeds and directions. Spatial correlation of the air temperature field was dominated by the diurnal course of temperature driven by the changes in the radiative components. The variability of horizontal advection of sensible heat was found to be large for all modes and independent of separation distance between stations suggesting that the advection is dominated by micro-scale processes and its estimates cannot be improved by increasing the sampling domain. However, three different regimes of the nocturnal subcanopy temperature field were identified that may prove useful to improve nocturnal estimates of subcanopy turbulent mixing.