The EF-Scale Ratings Brouhaha

In the wake of upgrades to the EF-scale ratings based on mobile Doppler radar-measured windspeeds, there's been a lot of back and forth on the topic of (a) whether or not to use such measurements at all in the rating process, and (b) if mobile Doppler radar measurements are to be used, just how should this be done?

Item (a) is relevant since the NWS Director some time ago issued a directive to the field offices in which use of mobile Doppler radar windspeed measurements should not be used.  The following is a recent directive from the NWS Director, Dr. Louis Uccellini:

Directive NWSI 10-1604, Post-Storm Data Acquisition, requires us to use the EF scale, which is an impact-based rating assigned to a tornado after extensive investigation of the damage it caused. EF ratings are determined by observed damage rather than measured wind because we have no consistent way to measure wind speed for every tornado that occurs. Adhering to NWSI 10-1604 ensures we continue to use consistent methodology throughout the country for assigning EF ratings.

Tornado research is an exciting and rapidly evolving area of science, and we are able to capture more information about the character of tornadoes than ever before. We are updating Directive NWSI 10-1604 to allow the option of including this new information, when available, in the narrative of tornado summaries. The new policy will allow NWS to document available data that are scientifically valid and reliable without changing the objective and consistent EF assessment. Until the update is finalized into policy, WFOs should continue to follow current policy.

This is essentially a management directive from the agency head to the field offices, and it naturally must be obeyed by the NWS employees, without regard to the scientific merits for such a choice.  Personally, I find it absurd to force the offices to ignore the only "direct" measurements of wind speed in a tornado they're ever likely to have.  I'll have more to say about "direct" shortly.  The EF-Scale was never intended to be a damage scale, but rather is a windspeed scale.  The problem always has been that we have so few actual windspeed observations we must use damage to infer the windspeed - otherwise, only a tiny number of events could be rated.  That was Fujita's contribution to the science of tornadoes, and it has been useful (albeit controversial) ever since it was introduced in 1971.

The "consistency" argument is traditional within the NWS as a reaction to technological innovation.  I could provide numerous examples of the wrong-headedness of this policy, but I'll try to keep this as concise as possible.  For instance, should we degrade the data obtained by WSR-88D radars to that from WSR-57s simply to maintain consistency with the older data sets?  Should we disregard the dual polarity information of the new upgrades to the WSR-88D radars just to maintain consistency with the old versions of the radar?  When something new and exciting comes on line, its capabilities should be embraced by the agency, not rejected as inconsistent with older technology!

Turning to item (b), the primary concern is that the mobile Dopplers "measure" the winds within a sample volume well above the standard 10 m anemometer height, and the EF-Scale is supposed to be based on the virtually non-existent anemometer-based windspeeds at that height for a 3-second gust.  By and large, this is a meaningless definition since anemometer measurements in tornadoes are very, very, very rare - a tornado is an anemometer-hostile environment!  [I need to point out that no absolutely "direct" measurement of wind speed is ever possible.  Anemometer output is electronic signals associated with its rotation rate, which must be calibrated to the wind speed.  Any instrument, even in situ systems, are not direct measurements and entail a lot of issues (sensitivity, accuracy, response time, etc.)  Doppler radar windspeed estimates are not absolutely direct, either of course, and are a form of remote sensing, which is why they can be used in tornadoes.]

So the issue becomes:  what relationship exists between the Doppler radar wind measurements and the actual wind at the 10 m level averaged for three seconds [which is virtually never observed but must be inferred from a highly nonlinear relationship with damage]?  As of this moment, research is underway to try to determine this as unambiguously as possible - it will never be completely unambiguous, of course.  There are reasons to believe that windspeeds  might actually increase at decreasing heights as we go downward from where the Doppler measurements are taken.  The details of that windspeed profile remain to be established and there likely is variability from one tornado to another, or even from time to time during the life cycle of a single tornado.  It's unlikely some single profile would actually be observed at all times for every tornado!  Of course, theoretically, the wind must be zero at a height of 0 m but the winds just above that level must increase quite rapidly with height if they are to become capable of damage at 10 m.

Given that the research is not yet complete (and when is research ever truly complete?), it could be argued that it's premature to use the Doppler measurements and the suggestion to keep them but not use them is at least not entirely ridiculous.  However, all the anxiety about the consistency of the EF-Scale ratings strikes me as rather silly.  The existing record is laced with numerous inconsistencies for a host of reasons.  Denying the value of the most direct measurements of windspeed in tornadoes in order to maintain consistency with an inconsistent data set strikes me as silly.  You can argue we shouldn't introduce yet another source of inconsistency but I say we should take advantage of new techology as soon as possible and not get trapped into the "consistency argument" I discussed earlier.

Many years ago, earthquake intensity was "measured" with a damage scale that had many of the same problems as the original F-Scale.  It eventually was replaced with various objective measures of earthquake intensity and has passed into the dustbin of history.  No one ever suggested degrading the Richter Scale to match the old intensity scale for the sake of "consistency" with the older system.  The extent to which Doppler radar measurements will be able eventually to supercede damage estimate is unknown, but it's likely they never will become capable of being used for every tornado to map out the detailed space-time distribution of windspeeds.  Nor will the relationship of their measurements to the mostly hypothetical 3-second gust from an anemometer at 10 m ever be known perfectly.  But to ignore them or defer their use in EF-Scale assessment just for consistency's sake makes absolutely no sense to me.