2005-09-05

Hurricanes and SST, technical

The RC "Katrina" post is up - go look and comment, if you haven't already. I want to talk about one issue, see comment #34 Higher temperatures mean higher energy levels, but in thermodynamics, it is the difference in temperatures that determines how much power a system outputs... explain the temperature difference that drives hurricane systems and how GW is expected to increase that temperature difference. The point being, is it *SSTs* going up that you expect to drive more hurricanes, or is it the *difference* between SSTs and "the top", whatever that might be? My answer was Much of the power for the hurricanes comes from latent heat: the condensation of atmospheric moisture as air parcels are raised. Warmer air leads to more moisture (roughly exponentially), so no T diff would be needed and I still like it, but it doesn't seem to have found much favour. Isaac Held said it is temperature differences that are fundamental, and not, for the most part, the temperature itself... It is the temperature difference between the ocean surface and that of the upper level outflow that is crucial. From this perspective one can see that the issue of hurricane intensity is linked with that of trends in upper tropospheric temperatures.... To which I reply I'm sticking with my view for the moment, bolstered by the fact that everyone seems to correlate hurricanes against SST and I've never seen anyone correlating them against upper temps....

From this, I hope its clear that I'm unsure about all this... if anyone feels inclined to supply useful info, please comment.

So: the idea that everyone is talking about SSTs and no-one is talking about upper air temps is meta-evidence that they don't much matter, but that is not in itself terribly convincing. What I want to do is look at some nice words by Kerry "intensity" Emanuel himself. See http://wind.mit.edu/~emanuel/anthro2.htm (which will, if you look carefully, link you to a PDF of his Nature paper if you want it). And in fact the one we want turns out to be this about theoretical max intensity, and the section physical basis of limit calculations.

First off, the efficiency E is (T_s - T_0)/T_s. T_s is the "heat source" - the ocean; and T_0 is the exit temperature. So at first sight this is promising for the "upper air"-ists. But wait... equation (4) is for the max wind speed, but T_0 has gone and T_s is still there. OK, its because T_0 is wrapped up in E. And... "Finally, one has to estimate E. To do this, one has to first estimate T_0, the temperature at which air flows out of the top of the storm; this is also equivalent to estimating the altitude of the storm top. This accomplished by finding out how high up in the eyewall the air remains warmer than the distant environment, and is straightforward to assess given the vertical profile of temperature in the storm environment." Which I take to mean that he is doing some parcel-ascent calculations based on T_s and the atmos profile.

Lets also try a PDF of a Nature paper of his from 1987 here. Again, we have the E factor (e.g. eq (1) or (4)). Again, the calc of E has come from T_s, profiles, and parcel ascent. And a bit later we get, paraphrased, "The pattern of intensity largely matches the SST pattern, but non-linearly, because E is largely a function of the SST". Then he goes on to argue that the portion of changes in intensity (under climate change) due to climatic changes in E will be small.

So... all in all I still think I'm right and Emanuel agrees with me. But I'm willing to be told otherwise, if you know better.

ps: I haven't forgotten my two questions in the Q.

1 Comments:

Anonymous Anonymous said...

Kerry Emanuel's FAQ

http://wind.mit.edu/~emanuel/anthro2.htm

includes:
"Q: Is the intensity of hurricanes increasing with time?

A: There is some evidence that it is. Records of hurricane activity worldwide show an upswing of both the maximum wind speed in and the duration of hurricanes. The energy released by the average hurricane (again considering all hurricanes worldwide) seems to have increased by around 70% in the past 30 years or so, corresponding to about a 15% increase in the maximum wind speed and a 60% increase in storm lifetime. "

I am puzzled by the numbers. I thought PDI varied with wind speeds cubed while energy varied with wind speeds squared. Why isn't the 70% more like 1.15^2 * 1.6 - 1

Is most of the extra 60% duration spent at low strength rather than maintaining the same shape of a graph of wind speed against time?
Can you explain those numbers?

crandles

6:37 pm  

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