SEASONAL AND DIURNAL-VARIATIONS OF COHERENT STRUCTURES OVER A DECIDUOUS FOREST

Coherent structures in turbulent flow above a midlatitude deciduous fo rest are identified using a wavelet analysis technique. Coupling betwe en motions above the canopy (z/h = 1.5, where h is canopy height) and within the canopy (z/h = 0.6) are studied using composite velocity and temperature fields constructed from 85 hours of data. Data are classi fied into winter and summer cases, for both convective and stable cond itions. Vertical velocity fluctuations are in phase at both observatio n levels. Horizontal motions associated with the structures within the canopy lead those above the canopy, and linear analysis indicates tha t the horizontal motions deep in the canopy should lead the vertical m otions by 90-degrees. On average, coherent structures are responsible for only about 40% of overall turbulent heat and momentum fluxes, much less than previously reported. However, our large data set reveals th at this flux fraction comes from a wide distribution that includes muc h higher fractions in its upper extremes. The separation distance L(s) between adjacent coherent structures, 6-10h, is comparable to that ob tained in previous observations over short canopies and in the laborat ory. Changes in separation between the summer and winter (leafless) co nditions are consistent with L(s) being determined by a local horizont al wind shear scale.