Methods to Estimate Surface Fluxes of Momentum and Heat from Routine Weather Observations for Dispersion Applications under Stable Stratification

We examine the efficacy of two methods commonly used to estimate the vertical turbulent fluxes of momentum and sensible heat from routinely observed mean quantities in the surface layer under stable stratification. The single-level method uses mean wind speed and temperature measurements at a single height, whereas the two-level method uses mean wind speed measurements at a single height and mean temperature measurements at two heights. These methods are used in popular meteorological processors such as the U.S. Environmental Protection Agency approved AERMET and CALMET to generate inputs for dispersion simulations. We use data from a flux station of the U.K. Met Office at Cardington for comparison. It is found that the single-level method does not describe the flux variation in the weakly stable regime at all, because of its assumption that the temperature scale, i.e. the ratio of the kinematic sensible heat flux to the friction velocity, is constant, which is plausible only under strongly stable conditions. On the other hand, the two-level method provides a physically realistic variation of the fluxes with stability, but the required temperature measurements at two levels are usually not available on a routine basis. If measurements of the standard deviation of temperature are also available, in addition to the mean temperature at a single level, then they can be usefully employed in a third (single-level) method, with the consequence that the computed fluxes are very similar to those obtained from the two-level method. An improvement to the original single-level method is considered, and flux calculations under low wind conditions are also discussed.