The thermal roughness height corresponding to the surface radiation temperature over a relatively homogeneous senescent grassland varies diurnally while the roughness length for momentum is relatively constant. The aerodynamic temperature, which is defined as the extrapolated air temperature down to the roughness length for momentum following Monin-Obukhov similarity theory, is linearly correlated with the surface radiation temperature except under very weak wind conditio
ns. 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.
The aerodynamic temperature is close to the air temperature measured within the middle of the grass canopy, which has a stronger diurnal variation compared to the air temperatures at the grass top and bottom. The surface radiation temperature is dominated by the thermal emission from the middle of the grass layer, which penetrates through the cooler grass top.