This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Tuesday, October 30, 2012

Frankenstein Storm: Pressure in the eye is 943 millibars--so what?

Hurricane Sandy NOAA

11/03/2012: This now includes a correction about where the storm originated.

Well, to answer the question in the last post, I am indeed stuck in Washington D.C. My first hurricane experience! So far, it's the lull before the storm (but I only finished this post after the storm). A gentle drizzle when we woke up this morning (Monday), increasing rain throughout the morning, a lull between rainbands of the hurricane around noon, and increasing wind and rain again now, mid-afternoon. We journeyed out to see what is going on in the stores, and found that at Target, there were four items that racing (or, rather, had raced) off the shelves: water jugs (small bottles still seemed to be in ample supply), bread, batteries, and camping gear.
         I've been watching the pressure in the eye of the hurricane with interest because two days ago, a I reported that it was 972, and that one prediction was that it might go as low as 950 mb.  The last reading I saw was 943 mb, but the internet in the hotel where I'm staying has the speed of a phone line, so I haven't been able to update my stats. (I saw 941, but the official record appears to be 943 mb. It broke the record of 946 mb from the 1938 "Long Island Express" hurricane.)
        I'm going to try to explain what has gone on as my way of learning it, but first I refer you to a very authoritative source, that of meteorologist Cliff Mass. Excellent discussion! I am taking my discussion from a number of sources including Mass's blog, and the Seattle Times new service, which in turn, compiled its information from the Associated Press, The Washington Post, and Tribune Washington Bureau.
        Unlike most Atlantic hurricanes, Hurricane Sandy did not begin as a wave off the coast of west Africa, but rather started in the Caribbean. A low-pressure air trough was spun into a slow counterclockwise rotation by weak winds around October 19, and thunderstorms over the next few days sucked up energy from the warm Caribbean waters. On October 22, it became Tropical Depression 18, quickly developing into Hurricane Sandy which started moving north. The Gulf Stream is as much as 5 to 9 degrees warmer than normal for this time of year. It grew into a suze where tropical storm level winds (39-74 miles per hour) extended over a diameter of 940 miles. It is the largest Atlantic storm known to have made landfall, at least since the government began keeping records in 1988, and very likely well before that time.
        The puzzles to experts, and amateurs alike, were "why did it come so far inland so late in the year, and what's going to happen about it since most hurricanes like this head back out into the Atlantic where they peter out over the cold waters?"  They are usually chased out to sea by "onshore weather patterns," (a very unhelpful phrase in the Seattle Times article!). As I mentioned in my previous blog, there was a high-pressure system in the North Atlantic Oscillation west of Greenland, and a huge low pressure trough moving eastward across the U.S. The two combined to push/pull it westward. 
        Even though Cliff Mass's last post was on Sunday (Oct. 28), a day before the storm made landfall, his description/prediction of what would happen after landfall is intriguing (and you really need to see the color images on his blog to visualize this).  The storm transforms from a tropical storm with a warm core into an "extratropical storm" with a cold core.
        First point: tropical storms generally form in the tropics where there are not large gradients horizontally in temperature (tropical to subtropical at most). They derive their energy from the warmth and moisture of the oceans.
        Second point: midlatitude storms form in regions of large temperature gradients. If you travel through the mid-latitudes at this time of year, you go from the climate of New Orleans to that of, say, Juneau, Alaska! Midlatitude storms get their energy not from the warm oceans, but from the warmth on their south side and its contrast to the colder air on the north side. This temperature gradient fuels our mid-winter storms. These are cold-core storms.
        Cliff Mass predicted that Sandy would go an "amazing transition" from a warm-core to a cold-core storm (through what is called an "Extratropical Transition"). Many tropical storms weaken when they go through this transition, but Mass thinks not for Sandy. The two energy sources (ocean energy source and latitude-dependent energy source) work together for long  enough to result in an overall strengthening and expanding system.
        In his earlier October 26 blog, Mass compared/contrasted the predictions of two major models, considered to be the most important ones: The European Center model (ECM) and the US GFS model. Up until late last week, the US GFS model was predicting a low pressure of around 950 mbar that would swing up over Long Island on Tuesday morning, and then hang around inland, eventually moving northward on Wednesday. On the other hand, the ECM was forecasting a sub-940 mbar low pressure making landfall on central New Jersey, then circles around there before moving northward. To quote Mass "The European Center and GFS models tend to overdo the deepening in such situations, so I don't believe we will see anything below 950-960 mbar." Last I saw, the measured pressure in the low was 941 mbar. Good job ECM!
       Let's summarize: An eye pressure of 941 mbar, sustained winds at 90 mph, and a diameter of nearly 1000 miles. To the right, I post one version of the Saffir-Simpson Scale, though many others only have wind speed. Using wind-speed alone, this would be a category 1 scale. But, using pressure in the eye, it would have been a category 4. Am hoping some readers will pitch in and explain this one! Meanwhile, we mull over that 20 years ago, Halloween 1991, the "Perfect Storm" unfolded almost exactly like this one.

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