RELATION OF VORTICITY TO WEATHER processes

Figure 1-9.-Contour-isotachch pattern for shear analysis. lines are streamlines or contours; dashed lines are isotachs. Figure 1-7 represents a symmetrical sinusoidal streamline pattern with isotachs parallel to contours. Therefore, there is no gradient of shear along the contours. In region I, the curvature becomes more anticyclonic downstream, reaching a maximum at the axis of the downstream ridge; that is, relative vorticity decreases from the trough to a minimum at the downstream ridge. The region from the trough to the downstream ridge axis is favorable for deepening. The reverse is true west of the trough, region II. This region is unfavorable for deepening. In figure 1-8 there is no curvature of streamlines; therefore, the shear alone determines the relative vorticity. The shear downstream in regions I and IV becomes less cyclonic; in regions II and III, it becomes more cyclonic. Regions I and IV are therefore favorable for deepening downstream. In region I of figure 1-9 both cyclonic shear and curvature decrease downstream and this region is highly favorable for deepening. In region III both cyclonic shear and curvature increase downstream and this region is unfavorable for deepening. In region II the cyclonic curvature decreases downstream, but the cyclonic shear increases. This situation is indeterminate without calculation unless one term predominates. If the curvature gradient is large and the shear gradient small, the region is likely to be favorable for deepening. Figure 1-10.-Contour-isotach pattern for shear analysis. In region IV, the cyclonic curvature increases downstream, but the cyclonic shear decreases, so that this region is also indeterminate unless one of the two terms predominates. In region I of figure 1-10 the cyclonic shear decreases downstream and the cyclonic curvature increases. The region is indeterminate; however, if the shear gradient is larger than the curvature gradient, deepening is favored. Region II has increasing cyclonic shear and curvature downstream and is quite unfavorable. In region III, the shear becomes more cyclonic downstream and the curvature becomes less cyclonic. This region is also indeterminate unless the curvature term predominates. In region IV, the shear and curvature become less cyclonic downstream and the region is favorable for deepening. 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 1-10