INTERPRETATION OF CROWN RADIATION TEMPERATURES OF A DENSE DOUGLAS FIR FOREST WITH SIMILARITY THEORY FRED C. BOSVELD ∗ , A. A. M. HOLTSLAG and B. J. J. M. VAN DEN HURK Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE De Bilt, The Netherlands (Received in final form 24 May 1999) Abstract. 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 roughness length for heat, which is about equal to the roughness length for momentum. This value can be explained by the more efficient transport of heat relative to momentum in the roughness sublayer of the forest. Correcting for this effect we arrive at the classic value for homogeneous terrain of about 0.1 times the roughness length for momentum. For unstable cases the presence of enhanced mixing of heat in the roughness sublayer leads to a modified integral stability function for the dimensionless potential temperature difference between the surface and the top of the roughness sublayer. The observations give some evidence for this different stability behaviour. The analysis suggests that during daytime the radiative surface temperature and the aero- dynamic surface temperature are not significantly different when used to estimate fluxes. Daytime trunk space air temperature is satisfactory parameterized 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. Night time radiation temperatures at times strongly deviate from the expected behaviour based on similarity theory and the roughness length for heat, suggesting that the concept of a single surface temperature is too simple for such cases. Keywords: Forest, Surface radiation temperature, Roughness sublayer, Roughness length for heat, Excess resistance, Surface-layer similarity. 1. Introduction Surface temperature plays a crucial role in the description of energy transfer between the earth surface and the lowest layers of the atmosphere. By surface temperature, we imply the temperature at the interface between the air and the soil/canopy system. As such it can be a highly variable function of position. Con- ceptually at least three different surface temperatures may be distinguished: the radiation surface temperature, corresponding to the surface longwave emission; the aerodynamic surface temperature that determines the sensible heat transfer from the surface to the atmosphere, and the storage surface temperature that drives the change in the heat content of the soil and biomass. These temperatures are some kind of weighted average over the air soil/canopy interface, and they will differ in ∗ E-mail: bosveld@knmi.nl Boundary-Layer Meteorology 92: 429–451, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.