Numerical Simulations of Maritime Frontogenesis

Abstract

A hydrostatic primitive equation model initialized in a highly baroclinically unstable state was used to simulate maritime cyclogenesis and frontogenesis. In order to identify boundary layer physical processes important in maritime frontogenesis, several different simulations were performed. In an effort to isolate impacts due solely to the boundary layer, moist processes were not included. An adiabatic and inviscid simulation provided the control for these experiments. Two different boundary layer parameterizations were used: a K-theory parameterization featuring Richardson-number-dependent eddy diffusivity and a second-order closure scheme with prognostic equations for the turbulence quantities. Results indicated that strong warm and cold fronts formed in the adiabatic and inviscid case but that the vertical motion fields were weak. In the K-theory simulation, the results were somewhat more realistic with stronger vertical motion. In both the K-theory and second-order closure simulations, the boundary layer in the cold air was highly unstable and deep mixed layers formed in this region with a large generation of turbulence. The largest cross-front temperature gradients existed in the frontal zone above the mixed layer. These structures were in qualitative agreement with observations of maritime cold fronts over the northwest Pacific Ocean. The second-order closure simulations produced a shallower mixed layer in the cold air with a stronger, more narrow front and large vertical motion. These simulations were more consistent with observations. Results from the second-order closure simulations demonstrated that turbulent mixing of momentum was critical in reproducing the frontogenetic (and frontolytic) effects of the transverse secondary circulation.