RELATION OF VORTICITY TO WEATHER PROCESSES

Vorticity not only affects the formation of cyclones and anticyclones, but it also has a direct bearing on cloudiness, precipitation, pressure, and height changes.

Vorticity is used primarily in forecasting cloudiness and precipitation over an extensive area. One rule states that when relative vorticity decreases downstream in the upper troposphere, convergence is taking place in the lower levels. When convergence takes place, cloudiness and possibly precipitation will prevail if sufficient moisture is present.

One rule using vorticity in relation to cyclone development stems from the observation that when cyclone development occurs, the location, almost without exception, is in advance of art upper trough. Thus, when an upper level trough with positive vorticity advection in advance of it overtakes a frontal system in the lower troposphere, there is a distinct possibility of cyclone development at the surface. This is usually accompanied by deepening of the surface system. Also, the development of cyclones at sea level takes place when and where an area of positive vorticity advection situated in the upper troposphere overlies a slow moving or quasi-stationary front at the surface.

The relationship between convergence and divergence can best be illustrated by the term shear. If we consider a flow where the cyclonic shear is decreasing downstream (stronger wind to the right than to the left of the current), more air is being removed from the area than is being fed into it, hence a net depletion of mass aloft, or divergence. Divergence aloft is associated with surface pressure falls, and since this is the situation, the relative vorticity is decreasing downstream. We may state that surface pressure falls where relative vorticity decreases downstream in the upper troposphere, or where advection of more cyclonic vorticity takes place aloft. The converse of this is in the case of convergence aloft.

In this chapter we expanded on the subjects of convergence, divergence, and vorticity, which were first presented in the AG2 manual, volume 1. Our discussion first dealt with convergence and divergence as simple motions. The dynamics of convergent and divergent flow was covered, along with a discussion of wind directional shear and wind speed shear. Convergence and divergence as complex motions were then presented. Rules of thumb on convergence and divergence relative to surface and upper air features were covered. The last portion of the chapter dealt with vorticity. Definitions of relative vorticity and absolute vorticity were covered. Vorticity effects on weather processes was the last topic of discussion.

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