Interpretation of crown radiation temperatures of a dense Douglas fir forest with similarity theory

Infrared crown radiation temperatures as observed over a dense Douglas fir forest are analyzed in the context of similarity theory and the concept of transport resistances. As such we obtain a rather high value of the roughne ss length for heat, which is about equal to the roughness length for moment um. This value can be explained by the more efficient transport of heat rel ative to momentum in the roughness sublayer of the forest. Correcting for t his effect we arrive at the classic value for homogeneous terrain of about 0.1 times the roughness length for momentum. For unstable cases the presenc e of enhanced mixing of heat in the roughness sublayer leads to a modified integral stability function for the dimensionless potential temperature dif ference between the surface and the top of the roughness sublayer. The obse rvations give some evidence for this different stability behaviour. The ana lysis suggests that during daytime the radiative surface temperature and th e aerodynamic surface temperature are not significantly different when used to estimate fluxes. Daytime trunk space air temperature is satisfactory pa rameterized with the concept of gusts and with surface renewal analysis. As such it is related to the sensible heat flux and the storage heat flux. Ni ght time radiation temperatures at times strongly deviate from the expected behaviour based on similarity theory and the roughness length for heat, su ggesting that the concept of a single surface temperature is too simple for such cases.