Modeling dispersion from near-surface tracer releases at Cape Canaveral, Florida

Abstract

A trajectory-puff model is used to test the effects of horizontal variability of the transport winds and two different vertical dispersion schemes on the prediction of maximum 30-min-averaged ground-level concentration in the shoreline environment of Cape Canaveral, Florida. Data are used from three dispersion experiments conducted at the Cape Canaveral Air Station as part of the US Air Force's Model Validation Program. A total of 79 half-hour-averaged ground-level concentrations of tracer-gas from 11 near-surface tracer releases during unstable and near-neutral conditions are simulated. A result of this study is that a large difference is not seen in the results between the use of a horizontally uniform wind observed at the tracer release site and horizontally variable wind for puff trajectory calculations. The uniform wind gives a slightly more accurate average result, but only because the uniform wind (observed near the shoreline) has greater speed than the variable wind field. The puff-transport wind speed for the variable wind case would be increased if the effects of vertical wind shear are taken into account. It is shown that for unstable conditions, including near-neutral conditions, the assumption of rapid vertical mixing leads to an under prediction (negative bias) of the maximum ground-level concentrations by about a factor of 2.3 with a typical factor of about 11 scatter between predicted and observed values. If a less rapid vertical mixing is assumed, then the overall predictions equal the overall observations, i.e., near-zero bias with a typical factor of scatter of about 3.6. If the near-neutral data are removed from the evaluation statistics, then the assumption of rapid vertical mixing leads to an overprediction by a factor of about 0.03 with a scatter factor of about 4; the assumption of less rapid mixing leads to an overprediction by a factor of about 0.55 with a scatter factor of about 3. These results are consistent with the fact that on-shore flows are less turbulent than flows over land for the same stability class.