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
