Letters I've Written Never Meaning To Send

I was cleaning up my email account for this site when I noticed I had an old email from January 2007 in my draft folder. Thinking it was something I had already sent I almost delted it without reading it. I'm glad I didn't as it was something I was working on that I never wrote about in another context. Who I was thinking of sending it to I don't remember, but as it dealt with hurricanes it was probably Chris Landsea or Roger Pielke.

Here is what I wrote:

I was reading over this: Emanuel, K. A., 2005: Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 686-688. and more specifically this: Online supplement to this paper.

In the supplement Emanuel says the following:

For the first decade or so of airborne reconnaissance, surface winds were estimated mostly by visual inspection of the sea surface. Beginning in the early 1950s, radar altimeters aboard the aircraft made possible an accurate determination of the aircraft’s absolute altitude. When combined with direct pressure measurements, this gives a good estimate of geopotential height at flight level. Surface pressure can then be estimated using empirical relationships between surface and flight level pressure. This technique, developed during the 1950s, was used without significant modification through the end of aircraft reconnaissance in the western North Pacific and until the advent of accurate dropwindsondes in the North Atlantic. Minimum surface pressure estimates were converted to maximum sustained surface wind using semi-empirical wind-pressure relations which, however, have evolved with time. For the North Atlantic, Landsea 3 has documented a change in that took place in 1970, leading to lower wind speed estimates.

Now, as I read this, Emanuel is taking umbrage with the pre-1970 wind speed estimates, and not with the pressure measurements. But if that is so his adjusted wind speed results are a little strange, to say the least.

If you look at the pre-1973 storms with the lowest pressure, say those lower than
911 hPa, Emanuel gives adjusted wind speeds as follows:

~165 0
~160 0
~155 0
~150 3
~145 2
~140 1
~135 0
~130 2
~125 2

Now for the post 1973 storms (which are OK with Emanuel) you get the following:

~165 1
~160 2
~155 2
~150 1
~145 1
~140 0
~135 0
~130 0
~125 0

So are we supposed to believe that prior to 1973 half of the ten Atlantic hurricanes with the lowest pressures (under 911 hPa) had top wind speeds less than 145 knots, while after 1973 none of the seven lowest pressure storms had a top wind speed under 145 knots? We are also asked to believe that none of the pre-1973 storms had top winds over 150 knots, while 71% of post-1973 storms were greater than 150 knots.

In fact, if Emanuel's scheme were adopted than a full 40% of the pre-1973 storms under 911 hPa should really be categorized as Category 4 storms.

This strikes me as absurd. It amounts to arguing that the physics of storms have changed.

That is where I left off. Possibly I meant to check something else and never got back to it. Who knows?

Still, when I re-read this stuff, I'm pretty sure I was on to something.