Saturday, December 02
12:32 AM

Understanding tropical cyclone ‘Shaheen’

2 Oct 2021 tropical storm

Oman braces itself today for Shaheen, the arriving tropical cyclone, though the exact strength and implication of this expected weather phenomenon with an unpredictable outcome is yet to be ascertained.

Be prepared, avoid venturing outdoors in the afternoon, and take precautions for your own safety. This includes not parking cars along any low-lying or unsafe areas, having enough food and water supply at home, and keeping candles and emergency lights charged, in case of a power outage.

The Civil Aviation Authority (CAA) has issued an alert which configures the arriving tropical storm Shaheen as a ‘category 1’ tropical cyclone. The CAA said that the storm will directly impact coastal areas, including the capital Muscat from Sunday.

According to the India Meteorological Department, the severe cyclonic storm ‘Shaheen’ was yesterday 240 km east-southeast of Chabahar Port (Iran) and seen to be moving west-northwestwards, skirting Makran coast. Thereafter, it was likely to re-curve west-southwestwards, move towards Oman coast across Gulf of Oman and expected to weaken gradually.

Shaheen means ‘falcon’

Shaheen is an Arabic word that means ‘falcon’ and was provided by Qatar as part of the naming system for tropical storms that form over the Arabian Sea and Bay of Bengal. The responsibility of naming tropical storms and cyclones that originate in these regions falls to the countries that border these bodies of water, a system adopted by the World Meteorological Organisation of the United Nations.

Understanding a tropical cyclone

Tropical cyclones, also known as typhoons or hurricanes, are among the most destructive weather phenomena. They are intense circular storms that originate over warm tropical oceans, and have maximum sustained wind speeds exceeding 120kmph and are accompanies by torrential rains. However, the greatest damage to life and property is not from the wind, but from secondary aspects, such as storm surges, flooding, landslides, etc.

In effect, tropical cyclones are low pressure systems that form over warm tropical waters. They typically form when the sea-surface temperature is above 26.5°C. Tropical cyclones can continue for many days, even weeks, and may follow quite erratic paths. However, a cyclone will dissipate once it moves over land or over cooler oceans.

Over the past two decades, storms, including tropical cyclones and hurricanes, were second only to earthquakes in terms of fatalities, killing over two hundred thousand people. During this time, storms also affected an estimated 726mn people worldwide, meaning they were injured, rendered homeless, displaced or evacuated during emergency measures.

Research, however, indicates that over the past 30 years, the proportion of the world’s population living on cyclone-exposed coastlines has increased 192 per cent, thus raising the risk of mortality and morbidity in the event of a tropical cyclone.

How do tropical cyclones form?

A cluster of thunderstorms can develop over warm tropical oceans. If that cluster persists in an area of low pressure, it can start rotating. If the conditions are just right, the cluster of thunderstorms can grow in size and sustain itself and then develop into a tropical cyclone.

Once developed, a tropical cyclone is like a giant, atmospheric heat engine. The moisture from the warm ocean acts as it’s fuel, generating huge amounts of energy as clouds form.

The rotating thunderstorms form spiral ‘rainbands’ around the centre (eye) of the cyclone where the strongest winds and heaviest rain are found (eye wall), transporting heat 15km or higher into the atmosphere. The drier cooler air at the top of the atmosphere becomes the exhaust gas of the heat engine.

The wind field of a tropical cyclone may be divided into three regions. First is a ring-shaped outer region, typically having an outer radius of about 160 km and an inner radius of about 30 to 50 km. In this region the winds increase uniformly in speed toward the centre. Wind speeds attain their maximum value at the second region, the eye wall, which is typically 15 to 30 km from the centre of the storm. The eye wall, in turn, surrounds the interior region, called the eye, where wind speeds decrease rapidly and the air is often calm.

Impacts of tropical cyclones

Tropical cyclones are dangerous because they can produce extreme winds, heavy rainfall with flooding and damaging storm surge that can cause inundation of low-lying coastal areas – in Muscat, typically Shatti Qurm, besides Ghubra and Azaiba along the seacoast.

Cyclones have gale force winds with wind gusts in excess of 90kmph around their centre. In the most severe cyclones, gusts can exceed 280kmph. These winds can cause extensive property damage and turn airborne debris into potentially lethal missiles. It is important to remember when the eye of a cyclone passes over a location, there will be a temporary lull in the wind, but that this will soon be replaced by destructive winds from another direction.

Heavy rainfall associated with the passage of a tropical cyclone can produce extensive flooding. This can cause further damage. The heavy rain can persist as the cyclone moves inland and weakens into a low pressure system, hence flooding due to an ex-tropical cyclone can occur a long way from where the cyclone made landfall.

Storm surge
Besides extreme winds, a tropical cyclone can cause the sea to rise well above the highest tide levels of the year when it comes ashore. These storm surges are caused mainly by strong, onshore winds and also reduced atmospheric pressure. Potentially, the storm surge is the most dangerous hazard associated with a tropical cyclone.

Tropical cyclone categories

Category 1

Negligible house damage. Damage to some crops, trees and caravans. A category one cyclone’s strongest winds are gales with typical gusts over open flat land of 90-125kmph.

Category 2

Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. A category two cyclone’s strongest winds are destructive winds with typical gusts over open flat land of 125-164kph.

Category 3

This is a severe cyclone causing structural damage. Power failures likely. A category three cyclone’s strongest winds are very destructive winds with typical gusts over open flat land of 165-224kph.

Category 4

This is also a severe cyclone causing significant roofing loss and structural damage. It carries dangerous airborne debris and causes widespread power failures. A category four cyclone’s strongest winds are very destructive winds with typical gusts over open flat land of 225-279kph.

Category 5

This is an extremely dangerous cyclone with widespread destruction. A category five cyclone’s strongest winds are very destructive winds with typical gusts over open flat land of more than 280kph.

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