Understanding Differences of ECOR and EBBR Measurements and Their Impact on Large-scale Forcing

Tang, S., Lawrence Livermore National Laboratory

Surface Properties

Warm Boundary Layer Processes

Tang S, S Xie, M Zhang, Q Tang, Y Zhang, S Klein, D Cook, and R Sullivan. 2019. "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." Journal of Geophysical Research: Atmospheres, 124(6), 10.1029/2018JD029689.


(top) Satellite image of the collocated ECOR and EBBR stations at the SGP Central Facility and the wind direction when they view the same surface type (grass, red arrow) and different surface types (ECOR views cropland while EBBR views grassland, blue arrow). (bottom) 2004-2015 daytime mean LH and SH measured by ECOR and EBBR filtered by different wind directions.



(top) Satellite image of the collocated ECOR and EBBR stations at the SGP Central Facility and the wind direction when they view the same surface type (grass, red arrow) and different surface types (ECOR views cropland while EBBR views grassland, blue arrow). (bottom) 2004-2015 daytime mean LH and SH measured by ECOR and EBBR filtered by different wind directions.

Science

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.

Impact

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.

Summary

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.