VALDRIFT—A Valley Atmospheric Dispersion Model

VALDRIFT (valley drift) is a valley atmospheric transport, diffusion, and deposition model. The model is phenomenological—that is, the dominant meteorological processes governing the behavior of the valley atmosphere are formulated explicitly in the model, although in a highly parameterized fashion. The key meteorological processes treated are 1) nonsteady and nonhomogeneous along-valley winds and turbulent diffusivities, 2) convective boundary layer growth, 3) inversion descent, and 4) nocturnal temperature inversion breakup. The model is applicable under relatively cloud-free, undisturbed synoptic conditions in which the winds in the valley are predominantly along the valley’s axis. The model is configured to operate through one diurnal cycle for a single narrow valley. The inputs required are the valley topographic characteristics, pollutant release rate as a function of time and space, wind speed and direction as functions of time measured at one height, lateral and vertical turbulent eddy diffusivities as functions of stability, and the valley temperature inversion characteristics at sunrise. The outputs are three-dimensional concentration fields and ground-level deposition fields as functions of time. The scientific foundations of VALDRIFT are given in this paper along with a brief discussion of the model inputs and outputs. Air concentrations estimated by VALDRIFT compare favorably with results from a tracer experiment conducted in a deep mountain valley.