Tropical Cyclones

Quick Notes

Structure: cyclones develop in the presence of these conditions:

• warm water of tropical oceans (>26.5C) → collects energy through convection (?)
• unstable atmosphere, cooling fast enough to cause thunderstorms
• moist middle atmosphere (humidity)
• low vertical wind shears, and little change in wind direction with change in height

Tropical Cyclone Genesis potential index (GPI) → accurate, but only at low resolutions

Often these are good for spatial correlation but bad for temporal one (difficulty in predicting inter-annual events) Solution is to use evolutionary algorithms to obtain Pareto Front of possible solutions (all possible optimal trade-offs between spatial and temporal optimality) → all solutions still have non acceptable temporal resolution.

Physics

A tropical cyclone is a storm system that rotates rapidly, featuring a low-pressure center, intense winds, and an organized series of thunderstorms that cause intense rain and sudden gusts. The term tropical refers to the geographical origin of these systems, which form almost exclusively over tropical seas, while cyclone refers to their winds moving in a circle, around a central eye, with surface winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern one. These cyclones have a diameter most often found between 100 and 2,000 km. The powerful swirling winds of a tropical cyclone, as the ones shown in Figure 2.3, arise due to the Earth’s rotation imparting angular momentum as air moves towards the axis of rotation. These storms are generally most severe when over or near water and quickly lose intensity when moving over land. Damage can result from strong winds, rain, high waves, and storm surges, all of which are phenomena of rising water caused by high-speed winds pushing water towards the coast.

These tropical storms are low-pressure regions in the troposphere. The pressure is the lowest near the surface, while at the center of these storms sea level pressures are among the lowest ever observed. These systems are called ”warm core” because the environment near their center is warmer than the ambient temperature at all heights. At the periphery of the storm, the air may be nearly calm; however, because of the Earth’s rotation, the air possesses non-zero absolute angular momentum. As the air flows radially inwards, it starts rotating cyclonically so as to conserve angular momentum effectively. At a certain distance from the centre of the storm, air starts moving upwards towards the top of the troposphere. The air, once lifted, moves away from the storm’s centre and forms a layer of high clouds called ”cirrus clouds”. These processes ultimately create a wind field that is almost symmetrical around the storm’s centre. Wind speeds are low at the centre, increase moving outwards towards the radius of maximum winds and then decay more gradually with radius.