‘Beauty’ of her eye
17 February, 2018, 12:00 am
BEAUTIFUL is what I would say, regarding the eye of Severe Tropical Cyclone Gita, when viewed from 13,600km in the sky using the Japanese Himawari-8 Geostationary satellite.
However there is an expression that “beauty is in the eye of the beholder” used to express the fact that not all people agree on what is attractive or not.
As a tropical cyclone expert and scientist I have extra reasons than most to admire the beauty of Gita’s eye, which in relation to the persistence of the large visible cloud-free eye three days and the large 100km diametre; makes the Category 5 severe tropical cyclone, very unique in our region.
Symmetric, wide, open to the sky with cloudless centre and warm downward wind drafts drying the air and keeping fine conditions and even the sun shining through the eye. A 100km wide diametre, moving at 10km/hr, it would take nearly 10 hours before the back edge of the eye-wall arrived.
This aspect is quite deceptive with the general public often not realising that they are in the cyclone’s eye. Some may have felt the system had passed as the calm conditions and fine sunny weather inside the eye of the super cyclone, nearly five-10 hours, may given them a false sense of security.
Gita’s eye is so big that it would take nearly 10 hours for it to pass. Not only big is big but it has persisted as such for about five days.
The area of coverage of the “eye” of 100 km wide diameter is pi times the radius squared (A = p r²) = 3.14 x (50×50) = 7854 square kilometers, which is about four times the size of Samoa or nearly half the size of the area of Fiji.
To my knowledge, this prolonged appearance of a cloud-free eye in a severe tropical cyclone for our region is unusual. Beautiful, clear and crisp edges of the eye of Gita, continue to surprise me as I question the whereabouts and the absence of the upper level wind shear and steering fields.
Having said that, we must note that in super cyclones like Gita will make its own fields and create its own upper level wind dynamics, meaning the system itself starts to control the elements and the environment, rather than the elements controlling and modulating the system. This is the reason why it has not succumbed to the elements so soon.
It is also of note that Gita has been drawing through its rain feeder bands intense warm air and moisture content, water vapour, via the northeastern lined trough towards northern Cooks and another one linked to the convergence zone north of Fiji. Much needed sustenance is thus being which will keep the system going and also causing much squally conditions over Fiji.
The relatively warm seas and absence of any landmass to cut off Gita’s energy sup means the elements continue to provide more than enough moisture, latent energy and sustenance.
The diametre of the cyclone, not to be confused with the diametre of the eye, is now much larger in size, at 800 km wide. Noumea is now getting covered in clouds with gusts and passing rains coming from the south.
Severe TC Gita, with estimated 10-minute sustained winds of 200 km/hr (108 knots, 124 miles per hour) gusting to 300 km/hr (162 knots, 186 miles per hour), remains relentless showing no sign of recurving towards our expected climatological path, south westerly to southerly and then southeasterly quadrant.
Some southward movement of about 50km appears noted on the satellite pictures of February 16, 2018.
However mature severe cyclones do often move in a slightly “zigzag” manner, called the “trochoidal motion”. The factors that appear to influence trochoidal motion of mature and severe cyclones include:
1) The large-scale flow pattern in which the system is embedded;
2) Asymmetric cyclone features;
4) Vertical tilt of systems; and
5) Cyclone intensity.
However for operational objective or subjective cyclone forecasting, these small scale changes are often smoothed out.
At some stage, Gita does not pose an immediate to threat to New Zealand but the fact that it has moved so far to the west means that New Zealand can no longer be spared anymore from it passing to the northeast of Auckland or even Norfolk Island.
It appears the anticlockwise arch it will take will now be a much wider one than expected taking the system as a storm bearing extra-tropical low pressure system much south of Auckland, through the North Island in its southeasterly passage next week.
In its relentless strength, resilience and persistent straight line track from the Southern Cooks to its present point, one can literally put a ruler and draw a straight line through the above points, that is how straight was Gita’s 1500 km track to the present point; moving westwards at 270 degrees for straight two to three days, initially at 20-25 km/hr (11-13 knots) and then slowing to half that speed, at 10-12 km/hr (5-7 knots). It appears to be now travelling at 25km/hr again.
Gita continues to keep to the very warm waters of the Pacific tracking parallel in a latitudinal basis, and still tracking westwards at the same latitude of around 22 degree south.
The very well-defined eye is a beautiful feat for meteorologists and weather forecasters, who often have to look within the embedded clouds to try to delineate the centre of a cyclone, to accurately work out its position, and using this initial position, make forecasts of the future movement.
With a visible eye of Gita being evident for the past five days, it has been a boon to scientists. Forecasters have been 100 per cent accurate in the positioning the centre of cyclone Gita because of its central location being clearly delineated from the existence and visibility of a clear eye.
It is my submission to readers that tropical cyclones are not necessarily malicious, and often play a major role in the redistributing of heat, moisture, energy and momentum to ensure t our closed system earth remains in equilibrium and also inhabitable for human beings.
If it was not for systems like tropical cyclones, the earth would freeze over at the poles and heat up beyond human habitation in the equatorial regions. These cyclonic systems are the major synoptic tools via which heat, moisture, energy and momentum are transported southwards to higher latitudes.
With the rotating Earth using the Coriolis Effect, our planet is in equilibrium. It is sufficient to mention here that the majority of cyclones do not affect human beings and neither do they make landfall as their genesis regions are only in areas of Earth’s vast empty oceans.
* Dr Sushil K Sharma BA MA MEng (RMIT) PhD (Melbourne) is a WMO accredited Class 1 professional meteorologist, and is an associate professor of meteorology at the Fiji National University. The views expressed are his and not of FNU or this newspaper.