Day 84 – Q 1. What is the typical structure of a tropical cyclone? Discuss. How is the structure associated with the weather pattern of an area affected by a cyclone? Discuss
1. What is the typical structure of a tropical cyclone? Discuss. How is the structure associated with the weather pattern of an area affected by a cyclone? Discuss.
एक उष्णकटिबंधीय चक्रवात की विशिष्ट संरचना क्या है? चर्चा करें। चक्रवात से प्रभावित क्षेत्र के मौसम के पैटर्न के साथ संरचना कैसे जुड़ी है? चर्चा करें।
Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges. This is one of the most devastating natural calamities. They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and Willy-willies in the Western Australia.
- Tropical cyclones originate and intensify over warm tropical oceans. The energy that intensifies the storm, comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm.
- With continuous supply of moisture from the sea, the storm is further strengthened. On reaching the land the moisture supply is cut off and the storm dissipates.
- A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. The eye is a region of calm with subsiding air. Around the eye is the eye wall, where there is a strong spiralling ascent of air to greater height reaching the tropopause.
- The eye is surrounded by the “eye wall”, the roughly circular ring of deep convection, which is the area of highest surface winds in the tropical cyclone. Eye Wall region also sees the maximum sustained winds i.e. fastest winds in a cyclone occur along the eyewall region. The eye is composed of air that is slowly sinking and the eye wall has a net upward flow as a result of many moderate – occasionally strong – updrafts and downdrafts.
- Convection in tropical cyclones is organized into long, narrow rain bands which are oriented in the same direction as the horizontal wind. Because these bands seem to spiral into the center of a tropical cyclone, they are called “spiral bands”.
- Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above. A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands.
Tropical cyclones form in many parts of the world from initial convective disturbances sometimes referred to as cloud clusters. As the clusters evolve from a loosely organized state into mature, intense storms, they pass through several characteristic stages. Further, the structure is associated with the weather pattern in the following manner:
- Generally a larger scale (i.e., thousand kilometre) vortex already exists when the core develops and much of the research into tropical cyclone formation has examined the formation of the large-scale vortex in which the core forms. The distinction between core formation and large-scale vortex formation is important.
- Warm ocean waters (of at least 26.5°C ) throughout the upper 50 m of the tropical ocean must be present. The heat in these warm waters is necessary to fuel the tropical cyclone.
- The atmosphere must cool fast enough with height, such that it is potentially unstable to moist convection. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated and used for tropical cyclone development.
- The mid-troposphere (5 km altitude), must contain enough moisture to sustain the thunderstorms. Dry mid levels are not conducive to the continuing development of widespread thunderstorm activity.
- The disturbance must occur at a minimum distance of at least 500 km from the equator. For tropical cyclonic storms to occur, there is a requirement that the Coriolis force must be present. Remember that the Coriolis effect is zero near the equator and increases to the north and south of the equator. Without the Coriolis force, the low pressure of the disturbance cannot be maintained.
- There must be a pre-existing near-surface disturbance that shows convergence of moist air and is beginning to rotate. Tropical cyclones cannot be generated spontaneously. They require a weakly organized system that begins to spin and has low level inflow of moist air.
- There must be low values (less than about 10 m/s) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the rate of change of wind velocity with altitude.
Tropical Cyclones thus commonly develop in areas near, but not at the equator. As they move across the oceans their paths are steered by the presence of existing low and high pressure systems, as well as the Coriolis force and disrupt the normal life of the regions they pass through causing widespread damage.