Wavelet Analysis of Coherent Structures at the Atmosphere-Forest Interface

Abstract

Wavelet analysis was applied to turbulence data for temperature and vertical velocity within and above a deciduous forest. This method appears to provide an objective technique for examining thermal and flow fields associated with coherent structures occurring near the forest. The two-dimensional unfolding in time and scale by the wavelet transform illustrates discrete warm and cool centers associated with organized updrafts and downdrafts, which have similar patterns but different magnitudes at different heights. Wavelet variances computed for temperature and velocity at different heights appear to have local maximum values corresponding to certain time scales, which are self-consistent and useful for objective determination of the principal time scale of the structures. Within the canopy, the principal time scales of the structures determined by this technique are 56?60 s and 40?44 s for the temperature and vertical velocity fields, respectively. These time scales are close to those determined by the multilevel detection scheme used in a previous analysis. The temperature structures above the canopy have a shorter duration, but the rate of the decrease in the time scale with increasing height appears to be proportional to the increase in mean wind speed. The horizontal size of the structure determined by the product of local wind speed and the detected principal time scales is in the range of 83?112 m. The time scale of the structures identified in vertical velocity appears to be consistently smaller than that in the thermal field. The canopy structures show a smooth connection in the scale change with circulations of lower frequency (about 5?7 min) and merge into updrafts and downdrafts of these larger-scale circulations.