Variability of turbulence within a daytime stable internal boundary layer over a heterogeneously irrigated cotton field in EBEX

During the Energy Balance Experiment (EBEX) conducted in the San Joaquin Valley, California in August of 2000, the patch-by-patch, flood irrigation in a flat cotton field created the underlying surface with heterogeneous soil moisture, leading to the dry (warm)-to-wet (cool) transition within the cotton field under northerly winds. Moreover, the existence of an extremely dry, large bare soil field upstream beyond the cotton field created an even larger step transition from the bare soil field to the cotton field. In the morning, the ASL was unstable while in the afternoon a stably internal boundary layer (SIBL) was observed in our site due to an “oasis effect” and horizontal advection effect. We investigated the variability of turbulence using two-level turbulence data at 2.7 m and 8.7 m in EBEX.

The results from spectra and cospectra analysis indicate the signatures of submeso motions on the ASL turbulence. These submeso motions cause the flows to be non-stationary particularly when the background wind is weak and are associated with changes in wind directions. These submeso motions post significant impacts on turbulence characteristics and turbulent exchange of momentum, heat, and water vapor. Variations in turbulent exchanges are largely dependent upon intensity and the ways submeso motions interact with local turbulence. Our results indicate that the influences of submeso motions have different influences on momentum flux and heat/water vapor flux. Additionally, intermittency is presented in the daytime SIBL and is analyzed in details to illustrate its contributions to flux exchanges.