By Paul Homewood
Surprisingly, there appears to have been little media coverage of Cyclone Winston, the category 5 storm which hit Fiji last Saturday, sadly leaving, on latest estimates, 42 dead.
It was a bit surprising then to find a small piece in the middle of the Sunday Telegraph, which claimed:
A powerful cyclone with wind gusts of 200 miles an hour has begun tearing through the Pacific nation of Fiji, prompting a nationwide curfew as authorities urged people to prepare for a “terrible event”.
The category five cyclone – believed to be the strongest ever recorded in the southern hemisphere – has already hit the outer islands of the South Pacific nation, bringing down trees and causing flooding.
The Met Office report only mentions 10-minute sustained wind speeds of 145 mph, which would leave it well short of being a record. Just since 1989, there have been five southern hemisphere cyclones which were more powerful: Monica, Zoe, Orson, Inigo and Pam. There have been many others of similar strength.
Bases on the JTWC satellite data, Zoe (2002) and Monica (2006) were the two most powerful cyclones since satellite measurements began, with one-minute sustained speeds of 180 mph. (This compares with Pam’s 165 mph last year)
And this is where the story starts to get a bit murky.
The not particularly reliable Mashable UK website states:
Tropical Cyclone Winston made history when it tore across Fiji’s main island of Viti Levu early on Sunday morning. It hit with staggering intensity, with estimated maximum sustained winds of up to 185 miles per hour, with gusts to 225 miles per hour.
This made it the strongest tropical cyclone on record in the entire southern hemisphere, based on wind speed estimates from the Joint Typhoon Warning Center.
According to Weather Underground, Tropical Cyclone Winston was also the second-strongest storm at landfall anywhere in the world, with only Super Typhoon Haiyan rated as more intense when it caused catastrophic damage to the Philippines in 2013. (Other storms in the northern hemisphere have been stronger prior to making landfall.)
And if we check the JTWC page , this is confirmed:
The intensity peaked at 160 Kt on midnight on the 22nd Feb. (160 Kt = 184 mph).
However, it is important to realise that, despite its name, Track History is nothing of the sort, as JTWC explain:
Track history for each storm is created from the operational warnings that are issued every six hours by NHC, CPHC , and JTWC . The positions and intensities are best estimates of those quantities when the warning is issued. THESE ARE NOT BEST TRACKS – having not been reanalyzed in any systematic manner
In other words, they simply reflect what was forecast.
However, on the same page, JTWC show the graph of the “theoretical” actual, calculated via the Dvorak technique, labelled Digital Dvorak Vmax:
This shows that the wind speed peaked more than a week before, at around 140 Kt (160 mph). Winston at that stage declined in strength, until it took an unexpected turn towards Fiji.
Crucially, however, on the 22nd Feb, when it supposedly reached 160 Kt, it only registered about 130 Kt.
This is not the first time we have discovered that “record breaking” storms have turned out to be nothing of the sort. It was only a few months ago, for instance, that we saw Hurricane Patricia being reported as the “strongest on record”. A few hours later, it made landfall on Mexico’s coast as a damp squib.
Nevertheless, Winston will no doubt remain, incorrectly, in the record books, and none of our supposedly intelligent media will even think to question it. It makes you wonder how many other cyclones have also been overestimated in recent years.
Comprehensive measurements of wind speeds for South Pacific cyclones have effectively only been available for the last three decades, at the most. Because of the geography, the vast majority of Pacific cyclones go nowhere near land, and certainly not at full strength. Consequently, prior to satellite monitoring, most cyclones never even got spotted, never mind measured.
I have long been suspicious of comparisons between the occasional land based measurements of the past (where often wind gauges are broken by the strongest winds) and satellite measurements nowadays, which can, in theory, measure every part of the storm, thus maximising the chance of finding the strongest wind speeds.
But there is another problem, and that is that satellite measurements are only theoretical, and therefore not directly comparable.
Worse still, this paper, A Pronounced Bias in Tropical Cyclone Minimum Sea Level Pressure Estimation Based on the Dvorak Technique, by James Kossin and Christopher Velden in 2003, reveals the following:
A recent study by Landsea et al. (2003) provides insight into the relationship between latitude and Dvorak technique wind estimates. By regressing Atlantic best track MSLP onto the concurrent best-track Vmax (in cases when aircraft data were available), they found that the empirical relationship between MSLP and Vmax employed by the Dvorak technique (which has no latitudinal dependence) generally provides wind that is too strong for a given pressure.
Put simply, satellite measurements have been exaggerating wind speeds.
If anybody would like to donate to help the victims of Winston, the Tearfund have their website open here.
Below is the list of southern hemisphere cyclones since 1997 which have had one-minute sustained wind speeds of 160 mph or more: