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Categorising cyclones

Dr Sushil K Sharma
Saturday, March 03, 2018

OBSESSION with the category of a tropical cyclone should not be the emphasis when worrying about the impacts from an imminent cyclone in your area. The category of the tropical cyclone, whether it is three or five often does not matter as far as impact forecasting is concerned.

The impacts that you will have in your area from any imminent cyclone forecast for your nation will depend on many complex factors that a normal layperson would have extreme difficulty in understanding generally.

Often people extrapolate a Category 5 as the most damaging, without realising that it matters only if the eye of the cyclone is expected to pass over your location. Nations and island groups are often very large entities, often spread over many thousands of square kilometres of land and sea area.

The track of the cyclone will determine the impact and its variation, not only based upon whether it is a Category 1, 2, 3 ,4 or 5 but dependent upon other features of the system like the diametre of the cyclone, the size of the eye, the speed of movement of the cyclone centre on its track, the area of gales, storm and hurricane force winds, whether system over sea or land, location of major cities, town and population areas relative to the track, and the amount of storm surge and flooding.

The strongest winds often are towards the front periphery of the direction that the system is moving towards, often with very strong pressure and wind gradients indeed. This is where your most dangerous hurricane force winds of Category 3-5 are confined in.

The backward trailing edge has a very wide area of gales with very slack wind and pressure gradients relative to the front edge of the system, which is relatively of a much smaller area of sharp gradients from gale, storm to hurricane force winds.

Thus a cyclone is not usually a circular saw-tooth type of symmetric system, as many people think. However stronger Category 5 systems such as Severe Tropical Cyclone Gita and STC Winston, established these characteristics in the absence of interference from outside atmospheric dynamics.

The speed of passage of a cyclone through a nation, whether landfalling or passing nearby in the sea, is also extremely important for impact assessment. Often this point is missed by planners, development partners and aid organisations.

The impact by STC Winston and STC Gita was limited because of its fast passage at 10-15 knots. However, systems generally move slowly only at 3-7 knots in our regions, especially while still at lower latitudes, and when on a more westerly course.

This means a slow moving Category 2-3 cyclone will do a higher amount of impact damage, then a very fast moving Category 5 passing the same reference point. Also of note is that a Category 2 system, for example, can be of a very large diametre (e.g. 800km) and slow moving while a very mobile Category 5 could be a very small diametre (200 km) only.

A weak cyclone but slow moving may lead to enormous floods in a nation, because of the prolonged rain period.

The impact thus to you personally and your family is totally dependent on many other factors and there should be no notion of becoming self-secure with a false-sense of security, just because the cyclone in your area is not of a high category.

Always check your position in relation to the track and also determine which side of the track you are and figure out the impact risks.

Thus, note that tropical cyclone characteristics are very complex for a lay person to fully understand all the guiding principles and limitations.

One point I will repeat here is that it would be fool hardy to take less precautions if the cyclone is only a Category 2, 3 or even 1.

The most important thing that you can do for your family is to figure out where you are physically, in relation to the lifecycle and path of the cyclone. The fact that you have a Category 5 cyclone over your nation does not necessarily mean that the entire nation will experience Category 5 winds.

Further even if you have a Category 5 traversing overhead your location, please note that all locations will not experience the same amount of wind force and related damage because of the earth's friction layer.

The terrain plays a great part in enhancing or even decreasing the wind force because of the varying topography which will "shelter" some locations, like on the lee side of a mountain or valley or major structure and further enhance winds in more exposed places, especially winds towards the foot of a mountain facing the onslaught of the winds.

Our WMO category 1-5 classification is dependent on the 10-minute sustained wind average at the time of the observation.

A 2100 UTC observation for example, according to the WMO rules, is the average of the winds from 2050 UTC to 2100 UTC, reporting as the average in this period and the individual highest gust during this period, for the 2100 UTC synoptic report data.

Thus during a cyclone with 100 knot sustained winds during a 10-minute averaging period, the gust would be about 150 knots (50 per cent added to 100 knots) and the lower value would be 50 knots (50 per cent subtracted from 100 knots).

In our part of the world including Australia and New Zealand, Category 1-5 refers to gale force winds (Category 1), storm force winds (Category 2) and hurricane force winds (Category 3, 4 and 5).

However, the Americans use the Saffir-Simpson Hurricane Wind scale, where their Category 1-5 is used only for hurricane force winds. Rather than 10-minute wind averages, they use a much finer and small time-scale one-minute wind average scale.

It is intuitively clear that all the cyclones classified using this scale for our region's cyclones, will elevate all our intensities towards the higher side by at least 25 per cent, reporting our cyclones as more intense than the 10-minute averaging scheme, as recommended by the WMO, used is used for our region.

Thus there should not be any direct comparisons of intensities of cyclones of the Atlantic Basin, for example, compared with our southwest Pacific basin, on a category basis — as their Category 1-5 is not the same as our Category 1-5.

Surface wind measurement, in our part of the world, is as stipulated by the WMO rules. Wind is measured at 10 metres above the ground with no obstructions 200 metres either side of the instruments. The reading is a 10-minute average value.

A proper WMO class 1 designated meteorological weather observing station would have an anemometre attached to anemograph where every movement of the wind vane, would automatically record on a chart the varying speed and direction.

Because of natural variability associated with the Atlantic Multi-Decadal Oscillation, we have cycles where we have 20-30 years of active cyclonic periods and then a similar period of less activity.

This oscillation means that the Atlantic is expected to cool in the future, obscuring links among cyclone activity and global warming.

Recent computer modelling studies predict fewer tropical cyclones, if the ocean heats up further as a result of global warming. Frequency drops by six to 34 per cent this century, whereas intensity rises by 2-11 per cent.

Today, water is the biggest concern then wind when it comes to property destruction and loss of life.

"Run from the water and hide from the wind" is a pretty good simple advice for all of you reading this article.

* Dr Sushil K Sharma is a WMO accredited class 1 professional meteorologist and is an associate professor of meteorology at the Fiji National University. Views expressed are his and not of this newspaper or his employer.

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