On dispersion above a forest—Measurements and methods

ABSTRACT

Data collected over a mixed conifer/deciduous forest at the U.S. Department of Energy’s Savannah River Site in South Carolina using sonic anemometry reveal that on-site and real-time measurements of the velocity component standard deviations, σ_v and σ_w, are preferred for dispersion modeling. Such data are now easily accessible, from the outputs of cost-effective and rugged sonic anemometers. The data streams from these devices allow improvements to conventional methodologies for dispersion modeling. In particular, extrapolation of basic input data from a nearby location to the site of the actual release can be facilitated. In this regard reliance on the velocity statistics σ_v and σ_w appears to be preferred to the conventional σ_θ and σ_ϕ. In the forest situations addressed here, the uncertainties introduced by extrapolating initializing properties (u, θ, σ_θ, and σ_ϕ, or alternatively, σ_v and σ_w) from some location of actual measurement to some nearby location where an actual release occurs are similar to those associated with the spread of the plume itself and must be considered in any prediction of the likelihood of downwind concentration (exposure) exceeding some critical value, i.e., a regulatory standard. Consideration of plume expansion factors related to meander will not necessarily cause predicted downwind maxima within a particular plume to be decreased; however, the probability of exposure to this maximum value at any particular location will be reduced. Three-component sonic anemometers are affordable and reliable, and are now becoming a standard for meteorological monitoring programs subject to regulatory oversight. The time has come for regulatory agencies and the applied dispersion community to replace the traditional discrete sets of dispersion coefficients based on Pasquill stability by the direct input of measured turbulence data.

Implications: The continued endorsement of legacy Pasquill-Gifford stability schemes is presently under discussion among professional groups and regulatory agencies. The present paper is an attempt to introduce some rationality, for the case of a forested environment.

Acknowledgment

This paper was finalized shortly after the untimely passing of one of its authors, Allen Weber. Allen had a distinguished career over many decades; his cheerful countenance and professional contributions will be missed by his many colleagues.

Additional information

Notes on contributors

B.B. Hicks

Bruce Hicks started in atmospheric physics as a participant in Australian micrometeorological studies of the 1960s, and most recently served as the Director of the Air Resources Laboratory of the US National Oceanic and Atmospheric Administration.Chuck Hunter is the manager of the Atmospheric Technologies Group of Savannah River National Laboratory, working on improving computer models for forecasting dispersion.The late Allen Weber was a leading figure within the atmospheric dispersion community, working at (and subsequently as a consultant with) Savannah River National Laboratory.