Influence of nonlocal advection on surface fluxes over a vegetated surface: evidence from EBEX field data

We used two-level turbulence data (2.7 m, 8.7 m) and profile data to examine the influence of nonlocal advection on surface fluxes, turbulence characteristics and turbulence spectrum. The data were collected in an international Energy Balance EXperiment (EBEX) in July and August 2000 the San Joaquin Valley, California over a flood-irrigated flat cotton field with an area of 1.6km by 0.8 km. The typical meteorological conditions were cloud free skies with northerly winds and evaporation rates of up to 400 Wm-2. The experiment used an array of ten towers with along wind tower spacing of 200 m and a typically fetch of more than 400 m. In addition, temperature, specific humidity and wind profiles at 12 levels up to ~11 m from a nearby tower were measured to provide information about horizontal advection.

Strong soil moisture heterogeneity was present due to heterogeneous irrigation although the cotton canopy and the terrain were quite homogeneous. The results from spectra and cospectra analysis indicate the signatures of the non-local motions that originated in the upwind dry patches, and of the interaction between the local convection and non-local advection. This kind of interaction significantly affects the water vapor exchange in the surface layer with stronger influence at higher level than lower one (i.e., 8.7m and 2.7m). Both turbulence data and profile data suggest that this interaction between nonlocal advection and local convection is made through the entrainment of dry and warm air from the top of the internal boundary layer that develops due to the soil moisture heterogeneity.