An integrated turbulence scheme for boundary layers with shallow cumulus applied to pollutant transport

A scheme is described which provides an integrated description of turbulent transport in free-convective boundary layers with shallow cumulus. The scheme uses a mass-flux formulation, as commonly found in cumulus schemes, and a first-order closure involving an eddy diffusivity. Both components are active in both the subcloud and cloud layers. The scheme is called "mass flux - diffusion." In the subcloud layer, the mass-flux component provides non-local transport. The scheme differs from others by a simpler cloud base closure, different lateral entrainment and detrainment rate profiles, and the use of a variable diffusivity in the cloud layer. The scheme has been implemented in a simple one-dimensional model. Results of simulations of a standard case which has been used for other model intercomparisons (ARM 21 June 1997) are shown and indicate that the scheme provides good results. The model also simulates the profile of a conserved scalar; this capability is applied to a case from the 1999 Southern Oxidants Study Nashville experiment, where it produces good simulations of vertical profiles of carbon monoxide in a cloud-topped boundary layer.