Evan's towering rainmakers studied closely

00:48, Dec 18 2012

It has tracked across nearly 3000 kilometres over the South Pacific, inflicted severe damage on two nations and it isn't finished yet but already scientists are closely examining Cyclone Evan.

The data coming from an array of satellites suggests that there was something unusual about Evan, the first named cyclone of the South Pacific 2012-2013 cyclone season.

The discussion also reveals just how intensely watched world weather is these days.

The US National Aeronautics and Space Administration is running mission page on Evan trying to work out what happened.

As it passed over Samoa, killing at least eight people and inflicting damage on a preliminary estimate of almost $200 million, the satellites were picking up "powerful storms and heavy rainmakers".

Evan was born, US time, on 12-12-12 but it had been spotted as a tropical depression a day earlier by NASA's Aqua satellite and the National Oceanic and Atmospheric Administration's GOES-15 near the French islands of Wallis and Futuna - halfway between Samoa and Fiji.


It was overcast on Wallis.

What is intriguing scientists is that another satellite, the Tropical Rainfall Measuring Mission (TRMM), passed over the tropical depression a day before it became a cyclone. It saw an eye-like structure in the system.

NASA says the debate now is whether a cyclone should have been declared then - a day before it actually was - because of TRMM's discovery.

Some in Samoa argue that the country did not have enough warning.

NASA got the first picture of Evan on December 13 - New Zealand time.

Its instruments were showing that at the top of big thunderstorms the temperature was minus 52 degrees C.

"Cloud top temperatures that cold indicate the highest, strongest storms with the heaviest rainfall within the tropical cyclone," NASA says.

Evan started to move to the east and strengthen: target Samoa.

An Atmospheric Infrared Sounder (AIRS) instrument on the NASA satellite picked Evan arriving over Samoa. It was measuring winds of 166.7 kph.

TRMM detected a clear eye in Evan on December 14.

Its 3-D Precipitation Radar found that the tallest thunderstorms shown around Evan's centre of circulation reached 16.5 km "indicating powerful storms and heavy rainmakers". Other thunderstorm cloud tops nearby were measured at 14.75 km (9.17 miles).

AIRS image showed a compact, circular area of strong thunderstorms around Evan's centre and that Evan's eye was about 11 kilometres wide.

"Imagery also showed tightly-curved deep convective (rising air that creates the storms that make up the cyclone) banding of thunderstorms were wrapping into the centre."

TRMM identified "hot towers" in the storm, hinting that it would continue to intensify.

A "hot tower" is a tall cumulonimbus cloud that reaches at least to the top of the troposphere, the lowest layer of the atmosphere which extends approximately 14.5 km high in the tropics.

They are called "hot" because they rise to such altitude due to the large amount of latent heat.

Water vapour releases this latent heat as it condenses into liquid.

NASA research shows that a tropical cyclone with a hot tower in its eyewall was twice as likely to intensify within six or more hours, than a cyclone that lacked a hot tower.

Yesterday as Evan rolled across Fiji, data was pouring into NASA.

AIRS data showed that temperatures at the top of thunderstorms were as cold as minus 63 degrees C over Fiji.

"Those thunderstorms were also generating heavy rainfall. JTWC noted that in addition to the heavy rainfall, Evan is generating very high surf that could also create major flooding," NASA says.

"Evan may be the strongest storm to affect Fiji since 1993, when Cyclone Kina killed 23 and left thousands without homes."

NASA says Evan is moving south and will continue weakening as vertical wind shear increases and sea surface temperatures drop.

"As the storm weakens and becomes extra-tropical, it is expected to speed up."

The Honolulu based Joint Typhoon Warning Centre expects Evan to move toward North Island, New Zealand and begin affecting the region by the end of the week.

Fairfax Media