Differences in Eddy-Correlation and Energy-Balance Surface Turbulent Heat Flux Measurements and Their Impacts on the Large-Scale Forcing Fields at the ARM SGP Site

Abstract

Differences in the surface turbulent heat fluxes measured by the eddy correlation flux measurement system (ECOR) and the energy balance Bowen ratio system (EBBR) are examined using 12 years of continuous measurements collected at multiple stations in the Atmospheric Radiation Measurement (ARM) program Southern Great Plains (SGP) surface observational network. The flux measurements are found strongly impacted by their upwind surface types, which vary with wind direction. For collocated ECOR and EBBR at the central facility, the fluxes measured by ECOR and EBBR agree much better when their upwind fetches are over the same surface type. Among all stations at SGP, ECOR measures more over winter wheat fields while EBBR measures mostly over grassland. The different seasonality of growth cycles between winter wheat and grass causes systematic differences in measured fluxes between ECOR and EBBR. These differences impact the derived large-scale forcing as illustrated in a constrained variational analysis, in which the state variables have to be adjusted according to different fluxes to keep the column-integrated energy and moisture budgets in balance. This impact prevails in summertime on nonprecipitating days. A single-column model test shows that model-simulated boundary layer development is impacted by using the large-scale forcing data of different surface turbulent fluxes. It is recommended to include both ECOR and EBBR measurements to better represent the domain-mean turbulent fluxes and atmospheric budgets of energy and water vapor at the SGP.