Jonathan Pleim
ARL

A New Combined Local and Non-Local Boundary Layer Model: ACM2

Convective boundary layers (CBL) are characterized by both small-scale (sub-grid) turbulence and large-scale turbulence extending to the depth of the CBL. Thus, meteorology models such as MM5 and WRF include several PBL schemes that include some sort of non-local or counter gradient component. Simple non-local closure models, such as the Blackadar convective model and the original Asymmetric Convective Model (ACM), have been used in MM5 for many years. The new version of the ACM (ACM2) includes the non-local scheme of the original ACM combined with an eddy diffusion scheme. Thus, the ACM2 is able to represent both the super-grid scale and sub-grid scale components of turbulent transport in the CBL. While the performance of the ACM2 for potential temperature is similar to eddy diffusion schemes with a gradient adjustment term (e.g. YSU, MRF, Holtslag and Boville), the ACM2 is uniquely applicably to any atmospheric quantity including humidity, micrometers, momentum, and trace chemical species.

Testing the ACM2 in one-dimensional form and comparing to large-eddy simulations and field data from CASES-99 demonstrates that the new scheme accurately simulates PBL heights, profiles of fluxes and mean quantities, and surface-level values. MM5 and WRF simulations show good overall performance compared with ground level observations. Comparisons to PBL heights derived from radar wind profilers for ICARTT-2004 and TexAQS-II demonstrate generally accurate simulation of diurnal PBL height evolution. Application of the ACM2 in the Community Multiscale Air Quality (CMAQ) model ensures consistent treatment of PBL processes for meteorological and chemical species by the WRF-CMAQ system.