Retired software engineer. "What is that?", you may ask. It's someone who has time to blog about the weather...
By: Bogon , 9:14 PM GMT on November 06, 2012
In the aftermath of the Frankenstorm, several WU bloggers have attempted a post-storm analysis. You might be interested in Barefootontherock's perspective, Bryan Norcross' commentary, or Christopher Burt's comparisons.
Doc Masters took a turn, as did guest blogger Lee Grenci. Why should I be any different?
The obvious answer is, "Please, no, I've had enough!" If that's how you feel, I advise you to click elsewhere. Here it comes. I promise to keep it short.
The interesting thing about Sandy was her transition from hurricane to nor'easter, or from barotropic to baroclinic storm. The difference is a matter of temperature. In a tropical setting all the air is warm. The classic tropical storm is a symmetrical pinwheel. In October off the east coast of North America, cold air is a factor. It's a baroclinic environment. Storms are lopsided. They adopt the yin-yang shape of the Taijitu.
The dividing line between tropical and temperate zones is the subtropical jet. As she moved north of the Bahamas, Sandy had to cross beneath the subtropical jet stream. That was the beginning of the transition.
At the time hurricane advisories reported that Sandy was subject to high wind shear on the order of fifty knots. In the animation above you can see a band of dry blue cutting across the neck of tropical orange on Sandy's southeast side. By the time Sandy reaches a position off the Carolina coast, the tropical connection has been completely severed. Sandy has become an island of tropical warmth in a temperate sea.
By the time Sandy passes Cape Hatteras, she has lost her symmetry. Cold air wrapping from west to east across her south side begins to displace the tropical air northward. As she approaches landfall in New Jersey, all of the tropical moisture is confined to an elliptical region on her north side. Here is an animation from my previous blog, which shows the process more clearly.
At landfall all of the milky white tropical convection gets swept into the broad rain band on the west side. What remains is purely baroclinic. A picture of the storm a few hours after landfall shows the yin-yang swirl of a nor'easter.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.