Determination of Surface Fluxes from the Surface Radiative Temperature

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  • 1 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon
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Abstract

This study examines the bulk aerodynamic method for estimating surface fluxes of heat and moisture using the surface radiative temperature. The surface rediative temperature is often the only available surface temperature from field measurements. Models typically predict heat fluxes from the surface radiative temperature computed from the surface energy balance. In this study, the corresponding radiometric exchange coefficient and radiometric roughness height are computed from tower- and low-level aircraft data taken during four different field programs. The data analysis shows that the radiometric exchange coefficient does not increase with increasing instability. This is because the radiometric exchange coefficient must compensate for the large vertical temperature difference resulting from use of the surface radiative temperature.

The data analysis and scaling arguments indicate that the radiometric exchange coefficient for heat in the bulk aerodynamic formulation is closely related to θ*/Δθ for both stable and unstable conditions, where Δθ is the difference between the surface radiative temperature and the air temperature and θ* is the negative of the heat flux divided by the surface friction velocity. Application of Monin–Obukhov similarity theory with surface radiative temperature also reduces to a relatively circular internal relationship between the radiometric roughness height and θ*/Δθ. This roughness height is flow dependent and not systematically related to the roughness height for momentum.

As an additional complication, the observed radiometric exchange coefficient for heat depends on the relationship between the measured surface radiative temperature and the microscale distribution of surface radiative temperature in the footprint of the heat flux measurement. Analogous problems affect the prediction of the moisture flux based on the saturation vapor prssure at the surface radiative temperature.

Abstract

This study examines the bulk aerodynamic method for estimating surface fluxes of heat and moisture using the surface radiative temperature. The surface rediative temperature is often the only available surface temperature from field measurements. Models typically predict heat fluxes from the surface radiative temperature computed from the surface energy balance. In this study, the corresponding radiometric exchange coefficient and radiometric roughness height are computed from tower- and low-level aircraft data taken during four different field programs. The data analysis shows that the radiometric exchange coefficient does not increase with increasing instability. This is because the radiometric exchange coefficient must compensate for the large vertical temperature difference resulting from use of the surface radiative temperature.

The data analysis and scaling arguments indicate that the radiometric exchange coefficient for heat in the bulk aerodynamic formulation is closely related to θ*/Δθ for both stable and unstable conditions, where Δθ is the difference between the surface radiative temperature and the air temperature and θ* is the negative of the heat flux divided by the surface friction velocity. Application of Monin–Obukhov similarity theory with surface radiative temperature also reduces to a relatively circular internal relationship between the radiometric roughness height and θ*/Δθ. This roughness height is flow dependent and not systematically related to the roughness height for momentum.

As an additional complication, the observed radiometric exchange coefficient for heat depends on the relationship between the measured surface radiative temperature and the microscale distribution of surface radiative temperature in the footprint of the heat flux measurement. Analogous problems affect the prediction of the moisture flux based on the saturation vapor prssure at the surface radiative temperature.

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