How Hurricane Sandy Can Become a ‘Frankenstorm’
By Adam Sobel
Guest Blogger
If Hurricane Sandy manages to make landfall on the East Coast of North America early next week, as the majority of the computer forecast models now have it, it probably won’t be right to call it a hurricane any more. Or even a tropical storm, even if the winds are “tropical storm strength.” To some degree, it probably will be a hybrid storm that shares characteristics of two parents: Sandy, a tropical cyclone (the broader term including hurricanes and tropical storms); and an extratropical “trough,” or upper-level low pressure system associated with a big wiggle in the jet stream.
Each type of storm – tropical and extratropical -- is named for the part of the world where it typically forms (“extratropical” just means “not tropical”) but the differences go deeper than that. A tropical cyclone gets its energy from the warm tropical sea surface. It will die if it goes over land, or cold water.
Forecast from the ECMWF model for this Sunday. The contours are surface pressure; colors are geopotential height on the 500 hPa pressure level, corresponding roughly to upper level pressure. The upper trough is the dip of the cooler colors southward and eastward over the eastern U.S., capturing Sandy (the bullseye in the pressure field off the Southeast U.S. coast).
Credit: Unisys Weather
An extratropical storm, on the other hand, doesn’t care as much what the temperature of the surface is beneath it, or even whether that surface is land or water. The extratropical storm gets its energy from the surface temperature contrast between the warm tropics and cold pole. The jet stream is tightly coupled to that temperature contrast. The contrast in temperatures between air masses is ultimately what drives the jet stream, and the stronger the temperature contrast, the stronger the jet stream will be.
The whole configuration of the jet stream and surface temperature contrasts is unstable; it can’t stay straight for long. It develops undulations, or wiggles, which contain high and low pressure centers. These are extratropical disturbances. The big upper-level “trough,” or southward dip in the jet stream, currently in place over the continental U.S., is a typical upper-level signature of such a disturbance.
The main thing to understand about extratropical storms is that they depend on contrasts in temperature: cold pole, warm equator, and sharp fronts in between. This makes them inherently asymmetric; something that’s only there because the temperature is different on two sides of it can’t have the same temperature all the way around. This asymmetry is evident in extratropical storm cloud patterns, which are typically comma-shaped rather than circular. Mature tropical cyclones, on the other hand, are as circularly symmetric as they can be.
As Sandy moves northward, it will move over cooler water. If this were all that were happening, Sandy would weaken, as tropical cyclones moving toward a pole typically do. At the same time, though, Sandy will come close enough to the upper trough now over the U.S. to interact with that trough in something like the way that an extratropical surface low normally would (see image above left).
When this happens, they will form a hybrid storm system with some tropical and some extratropical properties. Some energy will still come from the ocean surface, but some will now come from the pole-to-equator temperature contrast. This new energy source will enable Sandy to maintain its intensity, or maybe even increase it.
False-color infrared satellite image of the “perfect storm,” a tropical cyclone undergoing extratropical transition by interaction with an upper trough, on Oct. 30, 1991. Note the relatively small central circulation that still has some tropical cyclone-like appearance, and the much larger asymmetric comma-shaped cloud band to the north.
Credit: NOAA.
This process is called “extratropical transition.” It poses a lot of problems for forecasters. In the first place, the computer models aren’t that great at predicting exactly when it will happen. So predictions of intensity are uncertain, as the tropical cyclone may weaken before transition and then strengthen afterwards.
Structure predictions are similarly uncertain; tropical cyclones have very intense winds in a very narrow eyewall region, while extratropical ones have winds that usually aren’t quite as intense at their peak, but are quite strong over a much broader region. And tropical cyclones are much more symmetric than extratropical ones. A transitioning storm has some mixture of symmetry and asymmetry (see image above right).
Looking at the models now, it seems fairly certain that Sandy will undergo extratropical transition and wind up as a strong hybrid or extratropical storm. The models agree on that broad-brush conclusion. They do disagree on the storm’s track, and on just how intense it will be at landfall. This has to do with the details of the interaction of the tropical cyclone with the extratropical upper trough – not whether it will happen, but exactly how.
Adam Sobel is a professor at Columbia University, in Earth and Environmental Sciences and Applied Physics and Applied Mathematics. He is an atmospheric scientist who specializes in the dynamics of climate and weather, particularly in the tropics, on time scales of days to decades. He is author or co-author of more than 85 peer-reviewed articles and has received the Meisinger Award from the American Meteorological Society and the Excellence in Mentoring Award from Lamont-Doherty Earth Observatory of Columbia University. He lives in New York with his wife and two sons.
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