Sensitivity of Landsat-Scale Energy Balance to Aerodynamic Variability in Mountains and Complex Terrain

Authors


  • Paper No. JAWRA-12-0113-P of the Journal of the American Water Resources Association (JAWRA). Received May 4, 2012; accepted October 22, 2012. © 2013 American Water Resources Association.
  • Discussions are open until six months from print publication.

Abstract

The speed and direction of air flow through complex terrain are difficult to define. Both impact sensible and latent heat flux exchanges at the surface. Evapotranspiration (ET) models such as Mapping EvapoTranspiration at high Resolution with Internalized Calibration (METRIC™) estimate ET as a residual of the surface energy process and are thus sensitive to aerodynamics, including terrain-induced impacts on roughness governing convective heat transfer (H). There is a need to explore the sensitivities of H estimation and thereby ET estimation to wind speed and terrain roughness in mountainous areas and to determine the merit of operating complex mesoscale wind field models in conjunction with the energy balance process. A sensitivity analysis is explored in METRIC where we increased wind speed in proportion to a relative elevation parameter and we increased aerodynamic roughness to assimilate impacts of relative terrain roughness, estimated in proportion to standard deviation of elevation within a 3 km locality. These aerodynamic modifications increased convective heat transfer in complex terrain and reduced estimated ET. In other sensitivity runs, we reduced estimated wind speed on estimated leeward slopes. Estimated ET with and without these sensitivity adjustments is shown for mountainous areas of Montana and Nevada. Changes in ET ranged from little change (<5%) for lower slopes to about 30% reductions on windward slopes and 25% increases on leeward slopes for some mid to high elevations in the Montana application.

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