Wednesday, September 03, 2008

Hanna Karenina

Every happy hurricane is the same. Every unhappy hurricane is unhappy in its own way
.............. with apology to Leo Tolstoy


Despite its brutish nature, a tropical cyclone is a delicate creature, especially in its formative days. The seed can be a somewhat simple thing - a nice tropical wave, or perhaps a remnent low from a cold front - a bit of low pressure and a bit of spin. As with humans, conception is the fun and easy part, but managing growth and development is tougher.

A lot of things need to be just right. For nutrition, our baby needs nice warm ocean water, a whole lot of it, the warmer and deeper the better. A good latitude is also critical - too close to the equator and you are in the no spin zone. It needs at least ten degrees from the equator to get some good rotation on, and of course if it is too far from the equator, there just isn't any warm enough water.

Those basics are just the environment, though. Baby H also needs to fire up its own convection - some nice powerful thunderstorms. This is natural to it, but there are problems that can intervene. If the upper air is descending in the area, that suppresses thunderstorms.

Among the worst threats, though, are those arid twins, dry desert air and desert dust. Those guys suck the moisture right out of a budding cyclone's lungs. Even if it manages to get past those adolescent hurdles there is another huge hurdle awaiting - wind shear.

A mature tropical cyclone is a complex and sophisticated heat engine, and like any heat engine it needs a hot reservoir and a cold. Here the warm ocean serves as the heat reservoir and the the upper atmosphere as the cold. Once it gets going, the hurricane sucks heat and moisture out of the ocean like crazy by means of whipping it into a churning froth with its powerful winds. The winds in turn are powered by the ascent of that warm, moist air, and the heat released by condensing moisture as that air ascends. Hurricanes are the tallest structures in the troposphere, and that height means they have a very cold reservoir at the top, colder than -80 degrees. That cold reservoir can only function if the pipeline all the way up works, and that's where wind shear does its evil work.

If upper level winds are blowing crosswise to the lower atmosphere, the tops of thunderstorms get blown off, and the heat engine loses its cold reservoir, and the machine stops. This, in fact, is the story of our plucky heroine, the once and future Hurricane Hanna.

We have followed Hanna from modest but promising beginnings as a young but vigorous tropical wave. The sharp eyes at the NHC spotted her as a comer from the get go, and gave her her first number (91L, or maybe 92L - it's complicated) way back when. She proceeded through many vicissitudes, falling off the charts at times, and eventually, just as she was growing up into a tropical storm, being overshadowed by her older brother Gustav.

It was her own fault, of course. She got into a little dance with an unscrupulous upper level low (ULL) who kept blowing her away with his shear. Later, mighty Gus did the same.

Now she has mostly rid herself of both, only to have upstart younger brother Ike jump from TS to major hurricane right behind her. She may yet become a respectable category one or two on her own, but these things always end tragically - usually for all concerned.

In a few days, she will fatally throw herself upon Florida, Georgia, or the Carolinas and end it all. We won't mourn her though. She already is a killer, and likely to do more damage before she's through.