September 13th, 2018 by Michael Barnard
Currently, hurricane severity is measured by either the Saffir-Simpson scale or the Accumulated Cyclone Energy scale. Neither of them were really adequate in the 2017 Atlantic hurricane season, and it looks as if they will fail to capture the damage Florence will cause as well. Let’s explore why.
First, let’s look at all of the factors that a better model would have to have in order to help people understand their risk and to be useful in disaster preparedness. Then we’ll actually build a model with these factors and compare them to some historic hurricanes.
To be transparent, we’re building an inadequate model from scratch knowing it will be inadequate. It’s a useful thought exercise nonetheless and will assist in assessing other hurricane models that are in existence.
A potentially more useful measure would provide a risk rating based on wind velocity, size of hurricane, storm surge, wave height, and likely volumes of rain due to water vapor in the storm. These factors are interrelated to a greater and lesser degree, but each has unique risks. Size, surge, and rain are higher impact so would be weighted higher. The end system has to be relatively simple to explain, so let’s stick to a 1–5 scale, but average out and round ratings
The wind velocity would reasonably just use the Saffir-Simpson scale, as it was intelligently designed, is broadly used, and wind damage above Category 5 is relatively indistinguishable from Category 5 damage. There are also atmospheric limits on wind velocity that make additional points unnecessary.
Size of the hurricane is important. The broader the storm is, the more areas it will cause damage to. Hurricane Sandy was low on the Saffir-Simpson scale, but was the largest hurricane ever to form in the Atlantic at 1,800 km, and can be reasonable considered an upper boundary, with Typhoon Tip at 1,100 km being another data point. Tropical Cyclone Marco was only 18.5 km, so it would constitute the lowest boundary. The larger the hurricane, the larger the storm surge and amount of water it can hold. Given the impact of this, let’s weight double in the simplistic model.
Storm surge produces its own range of problems. It tends to be highest at the storm wall. Larger radius hurricanes tend to have higher storm surges, as do higher intensity hurricanes, and storm surges do their own level of damage. The highest ever recorded was 14.5 meters / 48 feet for Cyclone Mahina in Australia in 1899. Irma recorded 6 meter / 20 feet storm surges. Storm surge is one of the most damaging aspects of hurricanes, so it will be weighted double as well.
Wave height is an element. Bigger waves are much more destructive than smaller waves, and waves provide percussive damage that’s much different than the sustained storm surge damage. Wave height is additive to storm surge height as well. Hurricane Ivan generated 27 meter / 89 foot waves at sea. Hurricane Sandy, while low on the Saffir-Simpson scale, generated 9 meter / 30 foot waves in New York Harbor. Hurricane Florence has a recorded wave height of 25 meters / 83 feet at sea, but is diminishing in intensity thankfully.
The final component of our blended scale is the amount of rain likely to fall. Hurricane Harvey’s damage was predominantly from the rainfall. It didn’t have a high storm surge or hugely destructive waves or wind when it landed, it just sat there and dropped about 95 trillion liters of water on Houston and the surrounding area. That’s been compared to three weeks worth of the Mississippi River. Just as warmer water increases hurricane wind velocity, it also increases the amount of water vapor the hurricane sucks up only to drop down later. Due to high surface and deeper water temperatures in the Gulf of Mexico, much more water vapor was available to fuel rain. The size of a storm also affects how much vapor can be sucked up by a storm. The bigger the diameter, the larger the amount of water vapor. Double the diameter, quadruple the available water. And rainfall is one of the more serious problems of hurricanes as it scales up to high volumes, as with storm surge and waves, rainfall has fewer places to go quickly. What storm surge and waves don’t flood, rainfall has a good chance of. Average hurricanes can unleash 9 trillion liters of water a day. This factor will be weighted double as well after Harvey.
This gives a notional reference table which would look like this:
Running the numbers on some storms at their peaks using this approach as a first pass to test it gives the following results:
Assessing the results, we see that Sandy is much more convincingly modeled as a high severity storm. This aligns strongly with the actual damage and is two points above Sandy’s peak Saffir-Simpson rating. The model holds up so far.
Marco is in as a bottom-end data point and looks fine as far as it goes.
Katrina comes in at a rating of 4 as opposed to its peak Saffir-Simpson rating of 5. That’s still very high, but it was not as destructive as Sandy so it holds up. Inadequate preparation and response in New Orleans were as much the reason for Katrina’s destructiveness as Katrina’s scale and size. Katrina was a relatively small hurricane that scored a direct hit on New Orleans, so one way to think of this is that you really didn’t want to be underneath it regardless of its lower rating. This argues that this blended model is challenged simply because part of the value of ratings like this is to help motivate people to get out of the way.
Irma and Jose look very reasonable given what’s been observed. Irma is already one of the most destructive hurricanes to move through the Caribbean and will likely prove to be equal to Sandy or of greater cost when its all tallied up. Jose is a middling hurricane despite the wind speed, with a low surge especially reducing its likely destructiveness. Note that the water content of Jose wasn’t readily available, so it’s pegged at 3 for want of a better choice.
Finally, we get to the result which challenges the model’s usefulness. Harvey was rated a 3. A useful model would have had it rated at least as a four, at least as severe as Katrina, if not in the scale of Sandy or Irma. As the rainfall was the large problem with Harvey, I tested increasing the weight of water content of the hurricane to a factor of 3 and then 4 to see the results. Harvey didn’t budge from a rating of 3. Our test model is insensitive to peak damage categories, which might be a better approach. What would likely make the difference is an additional factor of velocity over the impacted areas. Harvey created a lot of damage not only due to the amount of rain, but also because it lingered over Houston.
Historically, the Saffir-Simpson scale used to accommodate more of this. Saffir was a structural engineer who did the base work in 1969 to figure out the impacts of wind on buildings and created the initial scale solely with wind velocity in 1969. He passed it on to the US National Hurricane Center, and Simpson, who was heading the NHC at the time, added storm surge and flooding impacts to the scale. That version of the scale was made public in 1973.
Then in 2009, the NHC decided to get rid of Simpson’s additions because of purported inaccuracies with the ratings for a couple of hurricanes. That became operational in 2010. Finally, in 2012, it changed again. Category 4 was expanded by a mile per hour in both directions. This allowed conversions from knots and rounding to not require fudging, which had been occurring previously.
Of course, there are other existing models which vary from the Saffir-Simpson model.
The most widely used is the accumulated cyclonic energy (ACE) measure. It takes the total wind velocity and related energy every six hours for the duration and assigns the hurricanes point scores up into the low hundreds. Looking at the examples, Sandy was rated 6.0875, Marco was rated 1.3175, Katrina was rated 20.0050, Harvey was 10.7225, Irma was 66.1075 and Jose was 23.6725. Once again, ACE doesn’t capture actual damage particularly well. Part of that is that it’s a cradle-to-grave measure, so a lot of the size can occur out to sea and not where humans are impacted. But a larger part is that ACE ignores everything except wind velocity when the size of the hurricane increases the damage over a much larger scale. This is an acknowledged shortcoming. It’s still useful for seeing increases of energy over time which can only be attributed to global warming at this point. Note that while there are shortcomings with the plot and data, the trends of both greater energy and an acceleration of that increase are clear.
The Hurricane Severity Index does deal with size, but it’s still dominated by wind speed with no factors made for storm surge, rain, or wave size. It’s quite likely that people who work and research in this space have concluded that size and intensity are all that are required to cover the other factors.
There’s also a new Cyclone Damage Potential Index which did more accurately predict that Harvey would be very severe, in the top 10% of storm impacts. It factors in how long destructive winds cover a specific area, which matches the lingering of Harvey over Houston, the factor which allowed Harvey to dump all of its trillions of liters of water in a single place. It still ignores water content, surge height, and wave height.
It’s possible wave height at least is unnecessary in a model based on this but the inclusion of breadth of a hurricane in both indicates that part of our assessment is correct. The exclusion of both surge height and moisture content are interesting. The impacts of climate change are creating not only more intense and bigger hurricanes, they are also increasing the correlated surge height and the separately correlated water load.
Saffir-Simpson is a useful model, but it’s obviously inadequate. It completely underrated Superstorm Sandy, the most destructive storm in US history. It underrated Hurricane Harvey, which was also incredibly destructive. And it set Irma as a 4 when it was more destructive than that.
When it comes to Hurricane Florence, the Saffir-Simpon scale did its primary job, which was to convince people something serious was coming and to evacuate. But at its current Category 2, it seems much less harmful and that’s completely inaccurate. It’s 600 km / 400 miles wide. Storm surge is estimated to be 3-4 meters/ 9-13 feet with equivalent waves.It has a massive amount of water vapor bound into it which will fall. It’s going to move very slowly down the coast, which means that the wind, waves, and rainfall will have more time to do accumulated damage.
Global warming is changing cyclones. We need a more effective measure to help understand and predict the damage. The Saffir-Simpson scale arguably went the wrong way in 2009. It’s not clear why.
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