Diurnal Variations of Thermal Roughness Height over a Grassland |
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| Authors: | Jielun Sun |
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| Institution: | (1) National Center for Atmospheric Research, Boulder, CO, 80307-3000, U.S.A. |
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| Abstract: | The thermal roughness height associated with the surface radiation temperature has been previously found to vary between different surface types. This study finds that the thermal roughness height varies diurnally even over a homogeneous senescent grassland. The corresponding roughness length for momentum is relatively constant.Both the aerodynamic temperature and the surface radiation temperature are found to be closely related to the air temperature in the middle of the grass canopy. However, the aerodynamic temperature is strongly influenced by the horizontally integrated heat transfer, while the surface radiation temperature represents the integrated thermal emission through the grass depth within the field of view of the radiometer. The aerodynamic temperature is less sensitive to variations and measurement errors in sensible heat flux, wind speed, and air temperature than the thermal roughness height. We find that formulating the aerodynamic temperature in terms of the surface radiation temperature is better posed for use in the bulk formula than using the surface radiation temperature directly and adjusting the thermal roughness length. |
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| Keywords: | Roughness length Aerodynamic temperature Surface layer Monin– Obukhov similarity theory |
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