Parameterization of Convective Momentum Transport in Highly Baroclinic Conditions

Abstract

The organization of convection by vertical shear strongly affects the transport of horizontal momentum, yet the concept of organization has received little attention in convective parameterization. Here the focus is on open-cellular convection in cold-air outbreaks under strongly baroclinic conditions that occur to the rear of midlatitude cyclones. The principles derived herein are envisaged to apply to baroclinic advection at large in similar shear flows. Open-cellular convection, forced by surface fluxes of sensible and latent heat and organized by unidirectional shear, is simulated using a three-dimensional cloud-resolving numerical model. The effects of the in-cloud pressure field, organization, and shear on momentum transport are quantified for the simulated clouds. A simple analytic model of blocked overturning provides dynamical insight into downgradient momentum transport by the simulated three-dimensional cumulonimbus ensemble. The overturning circulation having vorticity of the same sign as the ambient shear maintains the far-field flow, while the blocking effect decelerates in-cloud momentum. The combined effect is to maintain the mean flow somewhat below its undisturbed value. The numerical results and analytic predictions are used to evaluate and interpret two parameterization schemes for convective momentum transport used in operational global weather prediction and climate models at the U.K. Meteorological Office and the European Centre for Medium-Range Weather Forecasts. Finally, the authors comment on how the downgradient momentum transport by three-dimensional cumulus convection contrasts with momentum-mixing concepts as well as countergradient momentum transport by two-dimensional squall lines.