Schematic representation of the development of a wave cyclone along a frontal zone. It depicts all the four stages of the development of a travelling wave cyclone (extratropical cyclone).
However, the diagram represents graphically the life cycle of an extratropical cyclone in the northern hemisphere. The four stages in the life cycle of an extratropical cyclone are: (1) the initial state, (2) the incipient stage, (3) the mature stage, and (4) the occlusion stage.
(1) The initial Stage:
In the initial stage the polar and the tropical air currents on the opposite sides of the polar front blow parallel to the isobars and the front.
In the cold air mass to the north of the polar front the flow of air is from east to west. In the warm air mass to the south of the front the flow of air is from west to east. Therefore the wave disturbance is produced; the front is quasi-stationary and is in perfect equilibrium.
The wedge of cold air mass lies under the warm air. There is complete absence of wind shift. The weather is fine. However, along the slanting surface of discontinuity where the opposing air currents meet, there is a sudden change in the direction of wind. This is called wind-shear.
(2) The incipient stage:
In the second stage a wave has formed on the front. Cold air is turned in a southerly direction and warm air in a northerly direction.
There is an encroachment of each air mass into the domain of the other. This results in the readjustment in the pressure field as a result of which the isobars become almost circular in shape.
A cyclonic circulation is initiated around a low centre at the apex of the wave. The whole cyclonic vortex is carried along with the winds prevailing in the warm-air region at approximately the speed of the geostrophic component of the wind.
It may be pointed out that the new depression developing at the crest of the wave is called the nascent cyclone. The process of the birth of a new cyclone is commonly called cyclogenesis.
(3) The mature stage:
In the third stage the intensity of cyclone increases. The curvature and amplitude of the wave also undergo a marked increase. The air in the warm sector starts flowing from the southwest towards the colder air flowing from the southeast.
Now, the cyclone is fully developed. There are well marked warm and cold sectors. The idealized circulation of a mature cyclone is shown in Figure 35.4c. The warm air in this stage moves faster than the cold air.
The direction of movement is perpendicular to the warm front. In fact, the warm air is moving into a region previously occupied by the cold air. In the rear of the cyclone cold polar air is under- running the air of the warm sector, thus a cold front is generated there. Each of these fronts is convex in the direction of its movement.
Throughout the cyclone, there is an ascending air along the entire surface of discontinuity. If the rising air mass is moist, there will be cloudiness and precipitation along the warm as well as cold fronts as shown by the shaded areas.
The precipitation released at the warm front is more widespread and steady, whereas the cold front precipitation is confined to a narrow zone.
Since the position of the cold front advances faster than the warm front, the warm sector becomes progressively narrower. This is the beginning of occlusion. This particular phenomenon marks the maturity of the cyclone. This is obviously the period of maximum intensity.
(4) The occlusion stage:
In the final stage the advancing cold front ultimately overtakes the warm front which results in the formation of an occluded front. Occlusion starts first near the apex of the wave where warm front is closest to the cold front.
Gradually the process of occlusion comes down to the more open part of the two fronts. Thus, the warm sector is slowly pinched off and finally the two cold air masses mix across the front. This eliminates the occluded front. Now, the cyclone dies out. The life span of a single frontal cyclone is normally about five to seven days.