Nonlocal-Closure Schemes for Use in Air Quality and Environmental Models

Abstract

The description of the atmospheric boundary layer (ABL) processes, understanding of complex boundary layer interactions, and their proper parameterization are important for air quality as well as many other environmental models. In that sense single-column vertical mixing models are comprehensive enough to describe processes in ABL. Therefore, they can be employed to illustrate the basic concepts on boundary layer processes and represent serviceable tools in boundary layer investigation. When coupled to 3D models, single-column models can provide detailed and accurate simulations of the ABL structure as well as mixing processes. Description of the ABL during convective conditions has long been a major source of uncertainty in the air quality models and chemical transport models. There exist two approaches, local and nonlocal, for solving the turbulence closure problem. While the local closure assumes that turbulence is analogous to molecular diffusion in the nonlocal-closure, the unknown quantity at one point is parameterized by values of known quantities at many points in space. The simplest, most popular local closure method in Eulerian air quality and chemical transport models is the K-Scheme used both in the boundary layer and the free troposphere. Since it uses local gradients in one point of model grid, K-Scheme can be used only when the scale of turbulent motion is much smaller than the scale of mean flow (Stull, 1988), such as in the case of stable and neutral conditions in the atmosphere in which this scheme is consistent. However, it can not: (a) describe the effects of large scale eddies that are dominant in the convective boundary layer (CBL) and (b) simulate counter-gradient flows where a turbulent flux flows up to the gradient. Thus, K-Scheme is not recommended in the CBL (Stull, 1988). Recently, in order to avoid the K-scheme drawbacks, Alapaty (Alapaty, 2003; Alapaty & Alapaty, 2001) suggested a “nonlocal” turbulent kinetic energy (TKE) scheme based on the K-Scheme that was intensively tested using the EMEP chemical transport model (Mihailovic & Jonson, 2005; Mihailovic & Alapaty, 2007). In order to quantify the transport of a passive tracer field in three-dimensional simulations of turbulent convection, the nonlocal and non-diffusive behavior can be described by a transilient matrix whose elements contain the fractional tracer concentrations moving from one subvolume to another as a function of time. The approach was originally developed for and applied to geophysical flows known as turbulent transilient theory (T3) (Stull, 1988; Stull & Driedonks, 10