The Influence of Planetary Boundary Layer Physics on Frontal Structure in the Hoskins-Bretherton Horizontal Shear Model

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Abstract
A series of numerical experiments with the Hoskins-Bretherton horizontal shear model of frontogenesis in an, amplifying, two-dimensional baroclinic wave is performed. The analytic solutions from the Boussinesq, semi-geostrophic model provide initial conditions for numerical integrations with a two-dimensional, dry version of the fully compressible, hydrostatic primitive equation (PE) model of Anthes and Warner with 40 km horizontal resolution. The PE model is integrated 1) without planetary boundary layer (PBL) physics; 2) with a one-layer bulk-drag scheme; and 3) with a high-vertical-resolution PBL model. The lower boundary is thermally insulated in order to isolate the effect of the internal mixing of heat in the PBL. The simulation with the high-resolution PBL physics resolves several realistic features including 1) a narrow updraft at the top of the PBL above the sea-level pressure trough at the warm edge of the frontal zone; 2) a stable layer capping the PBL to the rear of the frontal zone; and 3) slightly unstable or neutral lapse rates in the PBL behind the front and stable lapse rates in the PBL ahead of the front. A diagnostic analysis of the frontogenesis indicates that the fine structure resulting from adding PBL physics can be attributed to the frictionally driven, ageostrophic inflow in the PBL toward the surface pressure trough in which the frontal zone is located. A finding of particular interest is that the stability patterns in the PBL on either side of the front evolve independently of sensible heating at the surface.

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