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.

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.

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.

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.

3. Radical reorientation of the trough. This usually happens where the trough is initially NE-SW, resulting in a N-S and in some cases a NW-SE orientation after sufficient time (36 hours).

4. This situation may actually cut off a low in the lower area of the trough. This usually happens when the high-speed winds approaching the ridge are southwesterly and approach the ridge at a comparatively high latitude relative to the trough. This frequently reorients the trough line towards a more NE-SW direction. Usually, the reorientation of the trough occurs simultaneously with 1 and 2.

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