Note that the associated height rises or falls occur
downstream and to the left of the flow, as illustrated in
figure 1-3.
Divergence Identification (Downstream
Straightline Flow)
The technique for determining the areas of
divergence consists in noting those areas where winds
of high speed are approaching weaker downstream
gradients that are straight. When inertia carries a
high-speed parcel of air into a region of weak gradient,
it possesses a Coriolis force too large to be balanced by
the weaker gradient force, It is thus deflected to the
right. This results in a deficit of mass to the left. The
parcels that are deflected to the right must penetrate
higher pressure/heights and are thus slowed down until
they are in balance with the weaker gradient. Then they
can be steered along the existing isobaric or contour
channels.
Divergence Identification (Weak Downstream
Cyclonically Curved Flow)
If the weak downstream gradients are cyclonically
curved, the divergence resulting from the influx of
high-speed wind is even more marked due to the
additional effect of centrifugal forces.
Divergence Identification (Downstream
Anticyclonically Curved Flow)
The effect of centrifugal forces on anticyclonically
curving high-speed parcels is of extreme importance in
producing overshooting of high-speed air from sharply
curved ridges into adjacent troughs, causing pressure
rises in the west side of the troughs.
Divergence Identification (Strong Winds)
If high-speed parcels approach diverging
cyclonically curved contours, large contour falls will
occur downstream to the left of the high-speed winds.
Eventually a strong pressure gradient is produced
downstream, to the right of the high-speed winds,
chiefly as a result of pressure falls to the left of the
direction of high-speed winds in the cyclonically curved
contours with weak pressure gradient. Usually the
deflection of air toward higher pressure is so slight that
it is hardly observable in individual wind observations.
However, when the pressure field is very weak to the
tight of the incoming high-speed stream, noticeable
angles between the wind and contours may be observed,
especially at lower levels, due to transport of momentum
downward as a result of subsidence, where the gradients
are even weaker. This occurs sometimes to such an
extent that the wind flow is considerably more curved
anticyclonically than the contours. In rare cases this
results in anticyclonic circulation centers out of phase
with the high-pressure center. This is a transitory
condition necessitating a migration of the pressure
center toward the circulation center. In cases where the
high-pressure center and anticyclonic wind flow center
are out of phase, the pressure center will migrate toward
the circulation center (which is usually a center of mass
convergence).
It is more normal, however, for the wind component
toward high pressure to be very slight, and unless the
winds and contours are drawn with great precision, the
deviation goes unnoticed.
Overshooting
High-speed winds approaching sharply curved
ridges result in large height rises downstream from the
ridge due to overshooting of the high-speed air. It is
known from the gradient wind equation that for a given
pressure gradient there is a limiting curvature to the
trajectory of a parcel of air moving at a given speed.
Frequently on upper air charts, sharply curved
stationary ridges are observed with winds of high speed
approaching the ridge.
The existence of a sharply
curved extensive ridge usually means a well-developed
trough downstream, and frequently a cold or cutoff low
exists in this trough. The high-speed winds approaching
the ridge, due to centrifugal forces, are unable to make
the sharp turn necessary to follow the contours. These
winds overshoot the ridge anticyclonically, but with less
curvature than the contours, resulting in their plunging
across contours toward lower pressure/heights
downstream from the ridge. This may result in anyone
of a number of consequences for the downstream
trough, depending on the initial configuration of the
ridge and trough, but all of these consequences are based
on the convergence of mass into the trough as a result
of overshooting of winds from the ridge.
Four effects of overshooting areas follows:
1. Filling of the downstream trough. This happens
if the contour gradient is strong on the east side of the
trough; that is, a blocking ridge to the east of the trough.
2. Acceleration of the cutoff low from of its
stationary position. This usually occurs in all cases.
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