Coherent eddies and temperature structure functions for three contrasting surfaces .2. Renewal model for sensible heat flux

Sensible heat, latent heat, and other scalar fluxes cannot be measured within short dense canopies, e.g., straw mulches, with standard approaches such as eddy correlation, Bowen ratio-energy balance, aerodynamic, and variance methods. However, recently developed surface renewal models, that are based on the fact that most of the turbulent transfer within and above canopies is associated with large-scale coherent eddies, which are evident as ramp patterns in scalar time series, offer a feasible solution. We present a new air renewal model that calculates sensible heat flux at different heights within and above a canopy from the average cubic temperature structure function, sampled at a moderate rate, and measured average friction velocity. The model is calibrated and tested with data measured above and within a Douglas-fir forest and above a straw mulch and bare soil. We show that the model describes half-hour variations of sensible heat flux very well, both within the canopy and roughness sublayers and in the inertial sublayer, for stable and unstable atmospheric conditions. The combined empirical coefficient that appears in the model has an apparently universal value of about 0.4 for all surfaces and heights, which makes application of the model particularly simple. The model is used to predict daytime and nighttime sensible heat flux profiles within the straw mulch and within a small bare opening in the mulch.