WAVELET ANALYSIS ON ERRORS OF THE BULK AERODYNAMIC FLUX FORMULA OVER CANOPY FOR GCMS

A bulk aerodynamic formula for turbulent heat fluxes has been widely u sed in land surface process schemes for GCMs. It should have an import ant impact on GCM simulations since the surface turbulent exchange is a unique approach to linking the atmosphere with the underlying surfac e. In this paper, long-term recorded tower flux data from the Boreal E cosystem-Atmosphere Study 1994 field campaigns are used to assess time scale-dependent errors of the widely used traditional and recently mod ified bulk aerodynamic formulas for sensible heat fluxes over canopy b y the standard multiresolution wavelet analysis and recently developed stationary wavelet analysis. The assessed modified bulk formula inclu des parameterization of the free convection and turbulence in the very stable stratification. It is found that there is an obvious synoptic- timescale variation in the long-term recorded sensible heat fluxes. Un fortunately, neither the traditional bulk formula nor the modified for mula can simulate the contribution of this variation to the fluxes. Co untergradient phenomena with negative hear fluxes between the canopy a nd the atmosphere at the synoptic timescale are also discovered. The c oarse resolution or large depth used in the bulk formula under the sta ble condition is mainly responsible for the failure of the bulk aerody namic formula at the synoptic timescale. Studying and using stability- dependent zero-plane displacement thermal height may be an approach to improve the capability for a GCM to describe the sensible heat fluxes between the canopy and the atmosphere under the stable condition. Thi s analysis also confirms the advantage of the modified bulk formula by including free convection over the traditional formula. However, it i s found that both the traditional and modified formulas have significa nt errors in the stable and weak unstable conditions. Occurrence of th e countergradient phenomenon evidently aggravates the incapability of the bulk formula and causes strong artificial negative sensible heat f luxes.