Understanding differences of ECOR and EBBR measurements and their impact on large-scale forcing
Tang, Shuaiqi — Pacific Northwest National Laboratory
Xie, Shaocheng — Lawrence Livermore National Laboratory
Area of research:
Surface latent (LH) and sensible (SH) heat fluxes are the key elements in characterizing heat and water exchanges between the atmosphere and the underlying surface. They are also used as the key constraints in objective variational analysis to derive the large-scale forcing for cloud modeling studies. However, large uncertainties exist in the measured turbulent fluxes due to instrument limitations, synoptic conditions, and surface type representations. This has limited their use in studying surface and boundary-layer processes and impacted the derived large-scale forcing fields.
Understanding the uncertainty in measured turbulent fluxes and its impact on large-scale forcing will improve both interpretation of the observational evidence explored by these quantities in land-atmosphere interaction and understanding of potential uncertainties in derived large-scale forcing fields and therefore model simulations.
Large differences are found in surface turbulent fluxes measured by the eddy correlation flux measurement system (ECOR) and the energy balance Bowen ration station (EBBR). They are mainly attributed to the different underlying surface types. When both ECOR and EBBR are downwind of the same surface type, the measured fluxes agree quite well; when they are downwind of different surface types, the measured fluxes differ significantly. Among all stations at ARM's Southern Great Plains observatory, ECOR measures mostly 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 the 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. 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. This impact prevails in summertime on non-precipitating days, when the surface turbulent fluxes play an important role in the atmosphere.