Saffir Simpson Scale: How Should We Change It?
This blog will be used to share ideas on changing the Saffir Simpson scale so that it will more accurately warn the public about storms with a large area and duration (such as Ike or Issac). The first entry will be updated frequently and will contain the latest ideas for changing the scale.
Background:
Here are the existing categories of the scale, and observations people have made about the limitations of it:
1) The current scale, based completely on highest surface wind speed, says little about surge potential or duration. A 30-minute burst of convection could generate swings in wind speeds that just be temporary, or conversely a steady intensification can occur without any wind speed increase, as we saw with Isaac. A better system would take into account a growing wind field and also any large central pressure dips that occur just prior to public updates.
2) The current scale has an added flaw: a storm at 35kts and a storm at 63kts are given the same classification, requiring added effort by the public to tell whether a system is dangerous.
3) The "step-ups" from one hurricane category to another in the current scale are uneven. This leads to a lower number of "cat 2" storms than if the steps were equal.
4) [added 9/2] A storm that intensifying rapidly in terms of central pressure, but already has dispersed its energy to cover a large area, will not have its strongest winds near the center. In the case of Isaac, the strongest winds were 100-130 miles from the center for a couple of days. In such a storm there is lots of room for intensification in the center without affecting wind speed. IF winds near the core are just 45-50 due to poor organization of the eyewall, pressure can drop 10 or even 15mb without the storm getting its designation bumped up by Saffir Simpson.
5) [added 9/4] The current wind scale does not differentiate between a storm moving at 30mph and a storm moving at 5mph, with the latter likely to cause more damage.
The IKE (Integrated Kinetic Energy) scale addresses several of these flaws, however I don't think it was designed as a complete replacement for Saffir-Simpson. By focusing on surge, the IKE number if reported alone might underestimate the wind damage from small, compact storms. The IKE calculation requires observations from all four quadrants of a storm. This may not always be available on a timely fashion. Finally, the calculations needed will be a bit beyond what general members of the public can easily peform -- which is not exactly the best way to engage the public.
[Update on 9/4]
Here are some newly revised charts to describe an alternative to Saffir Simpson that attempts to correct current deficiencies, when it comes to warning the public:
Here is an initial list of proposed "intensity upgrades":
The idea for a "flight level wind" adjustment needs more review; the "size and wind field power factor" is a number that is larger for large-area storms, and it is defined as the radius of TS winds plus the radius of hurricane-force winds, multiplied by the top surface wind speed of the storm.
This is like the current scale except that a "strong tropical storm" category is added (suggested by Aussieweather), a category 6 is added, the categories are more uniformly spaced for wind, and most importantly, UNUSUAL SIZE OR STRENGTH can result in the storm's classification getting "bumped up" to a higher level even without a qualifying surface wind speed.
Reader Comments
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Other basins like the WPAC have TS(Tropical storm) and STS(Severe Tropical Storm) which accounts for the big gap between 35kts->63kts.
Cheers
Thanks, I wasn't aware of the STS designation for Australia. That is a good idea!
Here is a more uniform saffir simpson scale that includes a "strong tropical storm" designation:
By making each category (all the way to cat4) a 15kt increment you would get a more uniform distribution of categories as well. I understand that the categories were not even to allow the numbers to convert to more rounded mph numbers but I dont think this is necessary.
If a storm is measured at 66kts and it translates to 76mph, then why not report it as a 76mph storm? One thing people understand, as far as numbers go, is miles per hour because most people have driven at 50, 60, 70 mph before.
I have a few other ideas that I will post in the main masters blog.
I guess the idea behind using a numeric scale is that it would somehow provide a more "objective" number that is based on actual data as opposed to "emotional" factors. That way they can do a comparison from season to season, etc. and be able to examine trends knowing that the same criteria were used from year to year.
However it is also true that a storm headed for a coastline that is flat with houses at 3' elevation (with a concave and shallow continental shelf) has to be considered very differently from the same intensity storm headed for a coastline with no serious tides that has 50 foot cliffs along the shore with no buildings in danger.
Because of this I agree that the actual flood potential (based on specific areas of coast) will need to be announced more prominently with the approach of any significant storm, separately from any measure of overall storm strength. There are just too many factors to base flood potential solely on storm intensity.
Ike was a Cat 2 on landfall but had a surge of a Cat 4, so that would make it a Cat 2.S4
Irene on the other had was a Cat 1 on landfall and only brought minimal surge so that would make it a Cat 1.S1
Just a few more thoughts
The only problem is that a small number of people might be confused, and not know which of the two numbers is important for them. I would favor sticking to a single number... and if the number is on the higher side, that is good because it would cause people to then be alarmed enough to look at the actual effect of the storm on their area.
So someone on an island that has little surge due to small coastline and good drainage for flooding would see "cat 3" as the overall number first, but then have the opportunity to realize that the cat 3 was not purely for wind speed and would not require as much preparation. At the same time, the person on the coast of the mainland 100 miles away would give proper attention to the real danger.
Here is a chart for Andrew:
http://www.nhc.noaa.gov/1992andrew.html#TABLE1
Here is some data for Charley:
http://www.nhc.noaa.gov/archive/2004/pub/al032004 .public.019.shtml?
http://www.nhc.noaa.gov/archive/2004/pub/al032004 .public.018.shtml?
What other storms do you want me to add to the sample data? If you have links feel free to add those too.
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