NUMERICAL SIMULATIONS OF MARITIME FRONTOGENESIS

A hydrostatic primitive equation model initialized ina highly baroclin ically unstable state was used to simulate maritime cyclogenesis and f rontogenesis. In order to identify boundary layer physical processes i mportant 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 s imulation provided the control for these experiments. Two different bo undary layer parameterizations were used: a K-theory parameterization featuring Richardson-number-dependent eddy diffusivity and a second-or der closure scheme with prognostic equations for the turbulence quanti ties. 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 reali stic with stronger vertical motion. In both the K-theory and second-or der closure simulations, the boundary layer in the cold air was highly unstable and deep mixed layers formed in this region with a large gen eration of turbulence. The largest cross-front temperature gradients e xisted in the frontal zone above the mixed layer. These structures wer e in qualitative agreement with observations of maritime cold fronts o ver 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 c onsistent with observations. Results from the second-order closure sim ulations demonstrated that turbulent mixing of momentum was critical i n reproducing the frontogenetic (and frontolytic) effects of the trans verse secondary circulation.