Monday 15 July 2013

Typhoon Soulik kills five people in south China.

Typhoon Soulik, which had already claimed four lives in Taiwan, has killed at least five people in south China after making landfall in Fujian Province on Saturday 13 July 2013, bringing with it winds of up to 220 kph. At least three people have died in Guangdong Province, where rainfall peaked at 292.5 mm and the Hanjiang River reached 43.68 m above flood level (the level at which the river is officially considered to be in flood). At least two more died in Jiangxi Province, where rainfall levels reached 354.3 mm. There have been no recorded fatalities in Fujian, where the typhoon first struck, though the high winds and 10 m waves are understood to have caused considerable damage to property.

The remains of a house destroyed by heavy rains in Liandun Village in Guangdong Province. Lian Zhicheng/Xinhua.

Southourn China is already suffering from widespread flooding, and Soulik has added to this. The low pressure above the storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as waterlogging soils on slopes leading to landslides. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.

Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.

These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it. They are categorised according to their wind speeds; using sustained wind strength (wind speeds maintained for a minute or more) rather than peak wind strength (the strongest gusts). A Super-Typhoon (or Super-Hurricane, or Super-Cyclone, depending on where these occur in the world), or Category 5 storm, has a sustained wind speed of 251 km/h or higher, a Category 3 Typhoon a sustained wind speed of 178-208 km/h.

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