Minimum turbulence assumptions and u* and L estimation for dispersion models during low-wind stable conditions

摘要

The U.S. Environmental Protection Agency (EPA) short-distance dispersion model, AERMOD, has been shown to overpredict by a factor of as much as 10 when compared with observed concentrations from continuous releases at the Oak Ridge, TN (OR), and Idaho Falls, ID (IF), field experiments during stable periods when wind speeds often dropped below 1 m/sec. Some of this overprediction tendency can be reduced by revising AERMOD's meteorological preprocessor's parameterizations of the friction velocity, u*, during low-wind stable conditions, thus increasing the calculated σ_V and σ_W and hence the lateral and vertical dispersion rates. Observations show that as the mean wind speed approaches zero at night, there is always significant σ_V and σ_W over time periods of 15 to 60min, while standard Monin-Obukhov Similarity Theory (MOST) predicts that σ_V and σ_W will approach zero. This paper focuses on the u* estimation methods and the minimum turbulence (σ_V and σ_W) assumptions in AERMOD (beta option 4) and two widely used U.S. operational dispersion models, AERMOD (v12345) and SCICHEM. The U.S. EPA has provided results of its tests with the OR and IF data, with its base AERMOD version and its December 2012 modified versions, which assume adjustments to the low-wind u* and increases in the minimum σ_V parameterization. SCICHEM has relatively small mean bias for both data sets. The revised AERMOD shows much less mean bias, agreeing more with SCICHEM.