STRUCTURAL-ANALYSIS OF AIRBORNE FLUX ESTIMATES OVER A REGION

Aircraft-based observations of turbulence fields of velocity, moisture , and temperature are used to study coherent turbulent structures that dominate turbulent transfer of moisture and heat above three differen t ecosystems. Flux traces are defragmented, to reconstruct the presume d full size (along the sampled transect) of these structures, and flux traces are simplified by elimination of those that contribute negligi bly to the flux estimate. Structures are analyzed in terms of size, sp atial distribution, and contribution to the flux, in the four ''quadra nt'' modes of eddy-covariance transfer (excess up/down and deficit up/ down). The effect of nonlinear detrending of moisture and temperature data on this ''structural analysis,'' over surfaces with heterogeneous surface wetness, is also examined. Results over grassland, wetland, a nd moist and dry agricultural land, show that nonlinear detrending may provide a more physically realistic description of structures. Signif icant differences are observed between structure size and associated r elative flux contribution, between moist and dry areas, with smaller s tructures playing a more important role over the moist areas. Structur e size generally increases with height, as spatial reorganization from smaller structures into larger ones takes place. This coincides with a gradual loss of surface ''signature'' (position and clustering of pl umes above localized source areas). The data are expected to provide a basis for an eventual statistical description of boundary-layer trans fer events, and help to interpret the link between boundary-layer tran sfer and hydrological surface conditions.