Notes on the Vertical Structure of Cyclones and Anticyclones

The vertical structure of a cyclone or an anticyclone mainly depends on the temperature distribution within it. On the basis of the horizontal distribution of temperature within the system, cyclones or anticyclones are classified in the following categories which have been diagrammatically:-

(1) Cold-core cyclone.

(2) Warm-core cyclone.

(3) Cold core anticyclone.

(4) Warm-core anticyclone.

The pressure and temperature distribution at the surface within these systems helps the weather scientist tell as to what may be the conditions aloft. It also proves to be an invaluable aid to meteorologists in weather forecasting.

(1) Cold-core cyclone:

In this type of cyclones, the temperature is lowest at the centre and increases towards the outer margin. Because of the contraction of air column at the centre and its expansion on the outer side of the low the air pressure registers rapid changes upward. Thus, with increasing elevation the barometric slope becomes steeper.

If such lows are produced near the earth’s surface, they will increase in intensity aloft. If these lows form at some distance from the surface, then they grow weaker near the surface, or they disintegrate. Ordinarily the cold-core cyclones form in the upper air. The semi-permanent Icelandic or Aleutian lows are typical examples of cold-core cyclones.

(2) Warm-core cyclones:

In these low pressure systems the temperature is highest at the centre, and decreases towards the outer margin. In this pressure system, the air expands more over the warm central region than over the cool edges.

Thus, the isobars’ spacing becomes wider and wider aloft until the low disappears. The vertical pressure gradient at the centre is, therefore, low. In a warm core cyclone the rate of decrease in air-pressure varies with altitude.

At a certain height from the earth’s surface, the horizontal pressure gradient vanishes so that the isobars become horizontal and the low itself becomes non-existent. At higher levels the low turns into a high.

Such lows generally originate at or near the surface. The lows lying over the desert regions of eastern California or Arizona are the best examples of warm-core cyclones. This variety of cyclone has a relatively larger rate of movement than the cold-core cyclones.

(3) Cold-core anticyclone:

The cold-core highs, as they are called, have low temperatures at their centers. They are generally shallow and are capped by lows.

At sea-level a cold- core high weakens with elevation and may change into a high-level low. Thus, a low forms over a high. The isobars’ spacing above the central core is less than that towards the outer edge.

If the highs with a cold-core form in the upper air, they intensify downward. The cold-core anticyclones develop over the Arctic and Antarctic regions in the winter.

On occasions the cold- core highs move into the warmer regions in low latitudes where they turn into deep warm-core anticyclones by a process which is known in meteorology as dynamic anticyclogenesis.

(4) Warm-core anticyclone:

The highest temperatures in this type of circulation observed at the centre. The temperature gradually decreases from the centre outward. Vertical spacing of isobars at the centre is relatively greater.

At higher elevations the isobars bulge out upward so that the slope of the isobaric surfaces becomes steeper. That is why a sea-level high with a warm core intensifies with increasing elevation. On the other hand, an anticyclone aloft in a warm-air column diminishes in intensity or disappears towards the ground.

The anticyclones of the tropical regions generally have a warm-core. In the south- eastern states of the United States of America and South California, such highs form during the summer. According to Byers, the stratosphere above the warm-core highs is exceptionally cold and high so that there is high pressure.