Performance of Steady-State Dispersion Models Under Low Wind-Speed Conditions

We examine the performance of two steady-state models, a numerical solution of the advection-diffusion equation and the Gaussian plume-model-based AERMOD (the American Meteorological Society/Environmental Protection Agency Regulatory Model), to predict dispersion for surface releases under low wind-speed conditions. A comparison of model estimates with observations from two tracer studies, the Prairie Grass experiment and the Idaho Falls experiment indicates that about 50% of the concentration estimates are within a factor of two of the observations, but the scatter is large: the 95% confidence interval of the ratio of the observed to estimated concentrations is about 4. The model based on the numerical solution of the diffusion equation in combination with the model of Eckman (1994, Atmos Environ 28:265-272) for horizontal spread performs better than AERMOD in explaining the observations. Accounting for meandering of the wind reduces some of the overestimation of concentrations at low wind speeds. The results deteriorate when routine one-level observations are used to construct model inputs. An empirical modification to the similarity estimate of the surface friction velocity reduces the underestimation at low wind speeds.