A Combined Local and Nonlocal Closure Model for the Atmospheric Boundary Layer. Part I: Model Description and Testing

TLDR

The atmospheric boundary layer during convective conditions is highly uncertain in numerical weather and air‑quality models because turbulent transport spans a wide range of scales, and existing eddy‑diffusion or simple nonlocal schemes fail to capture both subgrid mixing and large‑scale plume transport. The authors developed a new ACM2 model that merges the original ACM nonlocal scheme with an eddy‑diffusion component to address this gap. ACM2 combines the original ACM nonlocal transport formulation with an eddy‑diffusion subgrid scheme, enabling simultaneous representation of supergrid and subgrid turbulent transport. One‑dimensional tests against large‑eddy simulations and 1999 C‑A‑S‑E field data show that ACM2 accurately reproduces PBL heights, flux and mean profiles, surface values, and performs equally well for meteorological variables and trace chemical concentrations, outperforming eddy‑diffusion models with only gradient‑adjusted nonlocal terms.

Abstract

Abstract The modeling of the atmospheric boundary layer during convective conditions has long been a major source of uncertainty in the numerical modeling of meteorological conditions and air quality. Much of the difficulty stems from the large range of turbulent scales that are effective in the convective boundary layer (CBL). Both small-scale turbulence that is subgrid in most mesoscale grid models and large-scale turbulence extending to the depth of the CBL are important for the vertical transport of atmospheric properties and chemical species. Eddy diffusion schemes assume that all of the turbulence is subgrid and therefore cannot realistically simulate convective conditions. Simple nonlocal closure PBL models, such as the Blackadar convective model that has been a mainstay PBL option in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) for many years and the original asymmetric convective model (ACM), also an option in MM5, represent large-scale transport driven by convective plumes but neglect small-scale, subgrid turbulent mixing. A new version of the ACM (ACM2) has been developed that includes the nonlocal scheme of the original ACM combined with an eddy diffusion scheme. Thus, the ACM2 is able to represent both the supergrid- and subgrid-scale components of turbulent transport in the convective boundary layer. Testing the ACM2 in one-dimensional form and comparing it with large-eddy simulations and field data from the 1999 Cooperative Atmosphere–Surface Exchange Study demonstrates that the new scheme accurately simulates PBL heights, profiles of fluxes and mean quantities, and surface-level values. The ACM2 performs equally well for both meteorological parameters (e.g., potential temperature, moisture variables, and winds) and trace chemical concentrations, which is an advantage over eddy diffusion models that include a nonlocal term in the form of a gradient adjustment.