Roles of the Horizontal Advection on the Formation of Surface Fronts and on the Occlusion of a Cyclone Developing in the Baroclinic Westerly Jet

Abstract

The formation of fronts and the process of occlusion of an extratropical cyclone developing in the midlatitude baroclinic westerly jet are studied numerically with use of a dry primitive equation model.
The most unstable normal mode of the linear perturbation equations is adopted as the initial disturbance. This mode has the structure of a baroclinically unstable wave.
As the disturbance develops, there appear a strong, long cold front and a weak, short warm front. The intensification of horizontal potential temperature gradient and relative vorticity around these fronts are investigated. The generation term of the potential temperature gradient has positive value in both the regions. However, the generation term at the warm front is cancelled by the negative advection due to the strong northward flow to the east of the low. Thus the warm front does not become sharp and it also accompanies no positive relative vorticity line with as the result of strong upward flow at the warmer side of the front. On the other hand, there exists two-dimensional frontogenesis at the middle part of the cold front. At the southwest end, the cold front is elongated by the advection. At this stage, there exists another strong front to the northeast of the low centre. It is also shown that the advection is important for the formation of the front.
We can perform the experiment until the cyclone occludes. The vortex-like pattern is found in the vorticity field around the centre of the low. It is also explained with a combined effect of the generation term and the advection term of the horizontal temperature gradient and of the relative vorticity.
These structures found in the experiment correspond well to the cloud pattern formed in some of the midlatitude cyclones.