Discussion: Modifying the Saffir Simpson Scale

Saffir Simpson Scale: How Should We Change It?

Posted by: leftlink, 11:38 PM GMT on August 31, 2012

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.

Categories:Climate Change Saffir-Simpson Scale Hurricane Isaac

Updated: 10:42 PM GMT on September 04, 2012

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About leftlink

Rich C., Based in Lowell, MA. A bit of a lurker here but have always been interested in winter storms and hurricanes since growing up near Cape Cod.

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