Convergence and Divergence Above 500 Millibars

Study the 300-mb (or 200-mb) chart to determine areas of convergence and divergence. Note these areas of convergence and divergence.

Convergence and divergence are covered in chapter 1 of this TRAMAN, and also in the AG2 TRAMAN, volume 1. As a review of the effects of convergence and divergence, and the changes in intensity of troughs and ridges, we have the following rules:

. Divergence and upper height falls are associated with high-speed winds approaching cyclonically curved weak contour gradients. Divergence results in height falls to the left of the high-speed current.

. Convergence and upper height rises are associated with low-speed winds approaching straight or cyclonically curved strong contour gradients and with high-speed winds approaching anticyclonically curved weak contour gradients.

FORECASTING THE MOVEMENT OF UPPER LEVEL FEATURES

The movement of upper level features is discussed in the following text.

Areas of high pressure possess certain characteristics and traits. The following text discusses these indicators for areas of high pressure.

SEMIPERMANENT HIGHS. The semipermanent, subtropical highs are ordinarily not subject to much day-to-day movement. When a subtropical high begins to move, it will move with the speed and in the direction of the associated long wave ridge. The movement of the long wave ridge has already been discussed. Also, seasonal movement, though slower and over a longer period of time, should be considered. These highs tend to move poleward and intensify in the summer, and move equatorward and decrease in intensity in the winter.

BLOCKS. Blocks will ordinarily persist in the same geographic location. Movement of blocks will be in the direction of the strongest winds; for example, eastward when the westerlies are strongest, and westward when the easterlies are strongest. The speed of movement of these systems can usually be determined more accurately by extrapolation, Extrapolation should be used in moving the highs under any circumstance, and the results of this extrapolation should be considered along with any other indications. Some indications of intensity changes that are exhibited by lower tropospheric charts (700-500 hPa) are as follows:

. Intensification will occur with warm air advection on the west side; weakening and decay will occur with cold air advection on the west side; and there is little or no change in the intensity if the isotherms are symmetric with the contours. This low tropospheric advection is not the cause of the intensity change but is only a indicator. The cause is at higher levels; for example, intensification is caused by high-level cold advection and/or mass convergence.

. Under low zonal index situations, a blocking high will normally exist at a northern latitude and will have a pronounced effect on the systems in that area; in general it will slow the movement

. Under high zonal index situations, there is a strong west to east component to the winds, and systems will move rapidly.

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