Monday, August 03, 2015

Super-typhoon Soudelor and a need for new hurricane & typhoon metrics

Super typhoon Soudelor is hitting sustained wind speeds of up to 178 miles per hour! That totally blows the Beaufort scale out of the water, and also leaves the Saffir-Simpson scale well behind, too.

There are two reasons why these scales are not too useful:
  1. They all have maximum values,
  2. The maximum values are tied with technologically based assumptions and purposes.
The Beaufort scale maxes out at "Hurricane force" winds that are anything of 72.9 mph or greater, and this maximum was set based on the technological limitations of shipping, for which the scale was developed. The idea was that anything greater than a category 12 was effectively as dangerous to ships as the winds at 72.9 mph, and so there was no reason for ship captains to worry about categories larger than 12.

The Saffir-Simpson scale maxes out at "Category 5 hurricane" winds that are anything of 157 mph or greater, and this maximum was set based on the technological limitations of building construction in the 1950s US. The idea was that anything greater than a Category 5 was effectively going to blow apart any building, and so there was no reason for having higher categories (despite an increasing number of buildings with the capacity to withstand 157mph and higher winds).

It's that "or greater" part that really is troubling to me. Why? Because it means that a hurricane with sustained winds of 157 mph is classified as a "Category 5" hurricane... right along with a super typhoon like Soudelor, which is reaching wind speeds of almost 180 mph.

Back in 2011, I noted that the scale for the Saffir-Simpson scale was somewhat linear, up to Category 5; but if we took that linear scale and extended it, we would be able to include a Category 6 (and even Category 7) type of storm:

Category 1: <95mph
Category 2: 96-110mph
Category 3: 111-130mph
Category 4: 131-150mph
Category 5: 151-175mph
Category 6: 176-205mph
Category 7: 206-235mph

Under this extended classification, Super typhoon Soudelor is a Category 6; one of only a few in recorded history, but potentially one of a growing number in a future with global warming.

Similarly, the Beaufort scale can be extended beyond the category 12. The Beaufort Scale progresses along an x-squared rate (Excel comes up with the equation: y = 0.4952x^2 + 5.2857x + 0.0382), giving us:

Beaufort Number
0: <7mph
1: 0.8-3.4mph
2: 3.5-7.4mph
3: 7.5-12.2mph
4: 12.3-17.8mph
5: 17.9-24.1mph
6: 24.2-31.0mph
7: 31.1-38.4mph
8: 38.5-46.4mph
9: 46.5-54.7mph
10: 54.8-63.6mph

11: 63.7-72.9mph
12: 73.0-83.7mph
13: 83.8-94.7mph
14: 94.8-106.3mph
15: 106.4-118.5mph
16: 118.6-131.3mph
17: 131.4-144.8mph
18: 144.9-158.8mph
19: 158.9-173.5mph
20: 173.6-188.8mph

Under this classification, Super typhoon Soudelor has a Beaufort number of 20! This is very different from just classifying it as a 12, solely because 12 arbitrarily is the largest value on the Beaufort scale.

Why worry?
Back in 2011, I wrote up a short extension of a 2005 paper in Nature, that indicated that the total number of hurricanes has been remaining the same, but that the strength of the hurricanes has been growing stronger. This means that there are a lower number of Category 1, 2, and 3 hurricanes now than in the past, but the number of Category 4 and 5 hurricanes has increased:

What is being measured here is storm intensity by proxy of hurricane Category. However, such a measure will not show the entire picture if Category 5 remains anything over 157mph, since this open-ended category definition would mask the rising intensity of hurricanes that is shown in the graph.

While this might seem an academic point, another way to think about this is to ask why the Richter scale doesn't have a maximum value? After all, if the Saffir-Simpson scale was built around the idea that structures wouldn't be able to sustain a force of a Category 5 hurricane, then why shouldn't the Richter scale max out at 7.0? And if the idea that the Richter scale should max out at an arbitrary number (like 7.0) sounds ludicrous, then why accept the idea that Category 5 in Saffir-Simpson (and Category 12 in Beaufort) are the maximum of the scale?

Especially in a future where the numbers of increasingly intensive hurricanes is only going to increase as the numbers of "lesser" hurricanes decrease?

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