A non-local closure model for vertical mixing in the convective boundary layer

A simple non-local closure model for vertical mixing in Convective Boundary Layers (CBL) has been developed specifically for application in regional or mesoscale atmospheric chemistry models. The model, named the Asymmetrical Convective Model (ACM), is based on the concept that vertical transport within the CBL is inherently asymmetrical. Upward transport by buoyant plumes originating in the surface layer is simulated by mixing from the lowest model layer directly to all other layers in the CBL. Downward transport, however, proceeds only to the next layer in order to emulate gradual compensatory subsidence. The ACM is similar to the model developed by Blackadar (1978, 4th Symp. on Atmospheric Turbulence, Diffusion and Air Quality, pp. 443–447, Reno, Am. Meteorol. Soc.) but differs in its treatment of downward transport. The realism of the ACM is tested through comparisons to large-eddy simulations of several idealized test cases. These tests show that while the ACM shares the Blackadar model's ability to simulate rapid transport upward from the surface layer to all levels in the CBL, it is clearly superior in its treatment of material emitted from elevated sources either within or above the CBL. The ACM is also tested in the context of the Regional Acid Deposition Model (RADM) both to determine sensitivity to different CBL mixing schemes and to compare to vertically resolved aircraft measurements. These tests demonstrate quicker upward transport of ground-level emissions by the ACM as compared to the eddy diffusion scheme currently used in RADM. The ACM also affects ozone photochemistry in the boundary layer resulting in lower ozone concentrations in areas of high NO_x emissions.