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Title: Evaluating source area contributions from aircraft flux measurements over heterogeneous land cover by large eddy simulation

item BERTOLDI, G - Collaborator
item Kustas, William - Bill
item ALBERTSON, J - Duke University

Submitted to: Boundary Layer Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2012
Publication Date: 5/3/2013
Citation: Bertoldi, G., Kustas, W.P., Albertson, J.D. 2013. Evaluating source area contributions from aircraft flux measurements over heterogeneous land cover by large eddy simulation. Boundary Layer Meteorology. 147:261-279.

Interpretive Summary: Variation in land cover conditions has a significant effect on water and energy exchange, which in turn can have significant impact on local to regional weather and climate affecting agricultural crops. Both the measurement and modeling of moisture and heat exchange and their variation over a landscape as related to land use/land cover patterns is shown to have significant uncertainties due to the atmospheric transport processes having different affects on water vapor and heat exchange in relation to land use patterns. A methodology is proposed that considers these effects, and is shown to improve the interpretation of both the measurement and modeling of water and energy exchange for complex landscapes. This will ultimately improve weather forecast model predictions and the ability to assess crop conditions.

Technical Abstract: The estimation of spatial patterns in surface fluxes from aircraft observations poses several challenges in presence of heterogeneous land cover. In particular, the effects of turbulence on scalar transport and the different behavior of passive (e.g. moisture) versus active (e.g. temperature) scalars may lead to large uncertainties in the source area/flux footprint estimation for sensible (H) and latent (LE) heat flux fields, respectively. This study uses Large Eddy Simulation (LES) of the land-atmosphere interactions to investigate the ABL processes that are likely to create differences in airborne-estimated H and LE footprints. We focus on 32 m-elevation aircraft flux observations collected over a study site in central Oklahoma during the SGP97 experiment. Aircraft-model comparisons provide evidence of a difference in source area for turbulent H and LE fluxes. The LES produces reasonable representations of the observed fluxes, and hence provides credible evidence of the observed differences in the H and LE footprints. Those differences can be quantified analyzing the change in the sign of the spatial correlation of the H and LE fields simulated by the LES model. Dry patterns in relatively moist surroundings are able to generate strong, but localized sensible heating. However, whereas H at the aircraft elevation is still in phase with the surface, LE presents a more complicated connection to the surface as the dry updrafts are converging the surrounding moist air. Both the observational and LES model evidence support the concept that under strongly advective conditions, H and LE fluxes measured at the top of the surface layer (~50m) can come from very different upwind source areas—effectively contradicting surface layer self-similarity theory for scalars. The results indicate that under certain environmental conditions footprint models will need to predict differing source area/footprint contributions between active (H) and passive (LE) scalar fluxes by considering land surface heterogeneity and ABL dynamics.