Nocturnal wind structure over land and dispersion of pollutants: an analytical study

An analytical cum empirical formulation (AE formulation) is proposed to provide estimates of wind structure and plume width (and σy) over land for dispersion overnight. The formulation is an extension of our earlier studies (McNider, 1993, Atmospheric Environment 27A, 2199–2214; Singh, 1993, Boundary-Layer Met. 63, 397–423) with the basic difference that the latter formulation required the use of the mesoscale numerical model (MSNM) (McNider, 1988, Atmospheric Environment 11, 2445–2462), whereas the present study is a self-contained formulation. The study shows that the model results are significantly dependent on the initial wind profile at t = ti (around the time of sunset) when the inertial oscillations are expected to be triggered in the residual layer due to the surface heat flux reversal. The analysis shows that the results in respect of σy obtained from the AE formulation (based on the logarithmic/hyperbolic profile as an initial input) are in good agreement with those obtained from the MSNM. During the initial stages of the nocturnal dispersion ( first hour after sunset), when the traditional Briggs empirical formulations for σy are valid, it is shown that the AE formulation based on the logarithmic profile gives results very close to those obtained from the Briggs formulation. Since the AE formulation is a simple scheme and requires data routinely available at the meteorological stations, it can be conveniently used by the operational scientists and the regulatory agencies to provide estimates of the nocturnal wind structure and width of plumes dispersed over land.