Comparing Gustav to Katrina (Jeff Masters)

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http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1056&tstamp=200809

We got very lucky with Gustav--it could have been another Katrina. Both Gustav and Katrina had similar diameters (not radii) of tropical storm force winds at landfall--440 miles. However, Katrina affected the coast with a region of hurricane force winds 170 miles across--45% larger than the 115 miles of coast affected by Gustav (Figure 4). Both storms passed over some very high heat content waters in the Gulf of Mexico--Katrina, over a Loop Current eddy, and Gustav, over the Loop Current itself. Why didn't Gustav explode into a Cat 5 monster storm when it crossed the Loop Current yesterday? Well, when a hurricane has a well-formed circular eyewall that is aligned vertically from the surface to the upper atmosphere, it acts as a very efficient heat engine that can take heat out of the ocean and convert it to the kinetic energy of its winds. When Katrina hit its Loop Current eddy, the hurricane was under low wind shear and had an ideal structure like this for taking advantage of the heat energy offered to it. Gustav, on the other hand, had just crossed Cuba when it hit the Loop Current. Gustav was under about 15 knots of wind shear, which it had been able to hold off, thanks to its tight, well-formed eyewall. However, passage over Cuba disrupted the eyewall structure just enough to allow the upper-level winds shearing it to penetrate into the heart of the hurricane. These winds ripped up the eyewall and tilted it, so that the surface eye was no longer underneath the upper-atmosphere eye. A tilted eyewall structure is not able to act as an efficient heat engine until it can get itself lined up more vertically, so Gustav was unable to take advantage of the warm Loop Current waters it was traversing. It's like when your car engine is not firing on all cylinders and you hit the gas pedal--nothing happens. Once Gustav finally did align its eyewall vertically and armored itself against the effects of the wind shear, it had passed beyond the Loop Current and was over cooler waters of much lower heat content. Thus, Gustav was not able to intensify much before landfall. The computer models that predicted a Category 4 hurricane at landfall could easily have been correct, had the shear been a few knots less when Gustav crossed Cuba.



Figure 4. Comparison of the sizes of Hurricane Gustav and Hurricane Katrina shortly before landfall. The outermost black heavy line denotes the 34 knot radius of tropical storm force winds, while the black heavy line marking the beginning of orange colors (64 knots) denotes the region of hurricane force winds. Both Gustav and Katrina had similar diameters (not radii) of tropical storm force winds at landfall--440 miles. However, Katrina affected the coast with a region of hurricane force winds 170 miles across--45% larger than the 115 miles of coast affected by Gustav. Image credit: NOAA/AOML/HRD.