Sandy Inconsistency

[Frank2]

What I don't understand is how a Cat 1 hurricane could have a pressure and surge of a Cat 3 or 4, even though it was only a Cat 3 when in the Caribbean Sea? As we know it made landfall at Santiago de Cuba as a strong Cat 3, but because it approached the coast at a direct angle it likely spent most of it's surge at landfall, and once it emerged into the Atlantic and crossed the outer Bahama Islands never did reintensify or attain the deep convection that it had as it approached Cuba, so very unusual that while it never strengtened to anything above a Cat 1 wind-wise, it's central pressure continued to drop...

I've never seen that happen, and would be interested to know what others think, though my guess is that the MLC and LLC became separated while over Cuba and never did become vertically aligned for the remainder of it's life cycle...

Frank

[wxman57]

Frank,

It's all about pressure gradient (change in pressure with distance) vs. absolute pressure. When Sandy move north of Cuba its wind field expanded significantly, which relaxed the pressure gradient. Therefore, even with a lower central pressure, the distance over which the pressure changed from outside the storm to the center was increased significantly, which resulted in lower winds than a similar storm with a compact wind field and tighter pressure gradient. A tornado, for example, may have a central pressure only 50mb lower than its surroundings but it can produce winds of 200 mph. It's the fact that the pressure may drop from 1010mb to 960mb over a span of a few hundred yards that causes the strong wind.

Oh, and there's no such thing as a Cat 3 or 4 storm surge. Storm surge is not a function of Saffir-Simpson classification. Though wind speed is a factor, it's more the size of the wind field and its orientation toward the coast which determines storm surge, not the peak wind.

[Frank2]

Thanks - that all makes sense - I knew the pressure gradient wind was part of the effect, but the increasing size of the windfield versus the speed of the wind has often been too complex for me to understand completely (other than broad systems have a lower wind speed around the center than compact ones), but your tornado explanation makes it clear...

Somehow when I worked at the NHC as a tech the systems of the 80's were either classic CV systems that were easy to follow, or sheared El Nino systems that were weak and died out before causing too much trouble - don't think we ever had a Sandy, though we did have Gloria which for awhile appeared to be another 1938 hurricane scenario, but caused much controversy by ending up to be not nearly as bad as had been forecast...

http://en.wikipedia.org/wiki/File:Gloria_1985_track.png

P.S. And thanks for that correction to my careless storm surge comment - Vic Wiggert would hit me over the head with a rolled up storm surge map for saying what I said (lol)...

Thanks,

Frank

[Ptarmigan]

Hurricane Charley had a central pressure of 941 millibars with winds of 150 mph. It was due to its small size. On the other hand, Hurricane Sandy had central pressure of 940 millibars with 90 mph winds. The pressure gradient is less with a larger storm like Sandy. It also depends on ambient pressure. For example, Hurricane Gustav had a central pressure of 941 millibars with 155 mph winds and was a large storm, but the ambient pressure was lower than where Charley was in.

Code: Select all

AL0304 CHARLEY   081312 2004 24.4  82.9  95  969  10  10 1012 100  90 90 75 75  50 50 40 40  25 25 15 15 *   159.
AL0304 CHARLEY   081318 2004 26.1  82.4 125  947  10   5 1012 100  40 75 75 50  30 40 40 25  20 20 10 10 *    55.
AL0304 CHARLEY   081400 2004 28.1  81.6  75  970  10 -99 1012 100  40 75 40 40  30 40 30 25  20 20 20 10 *   -82.

Code: Select all

AL0708 GUSTAV    083012 2008 20.7  81.6 110  955  20 -99 1007 250 140120 75120  90 90 45 60  50 30 25 30 *   149.
AL0708 GUSTAV    083018 2008 21.6  82.5 125  943  15  25 1008 250 150120 90120 100 90 45 60  60 40 25 30 *    91.
AL0708 GUSTAV    083100 2008 22.7  83.5 120  950  15  20 1008 275 160120100135 100 90 60 75  60 40 30 35 *   -10.
AL0708 GUSTAV    083106 2008 23.6  84.4 105  960  15  25 1008 275 175135120135 120 75 60 75  45 30 30 30 *   118.

ftp://rammftp.cira.colostate.edu/demari ... k_atlc.txt

[Frank2]

Your mention of ambient pressure brings back memories because it varies with the time of day, and similar to the smokestack we'd see from the old NHC - at sunset it appeared taller than at midday, because the density of the air had increased when the air cooled...

As you said, size versus wind speed and central pressure and ambient pressure - interesting that's for certain...

Very complex processes at work, and I'll guess that Sandy's lifecycle will be studied for a long time by many...

Frank

[Frank2]

P.S. In all fairness to myself (lol), I worked at the NHC between 25 and 30 years ago, so much that was learned back then has since drifted away, and as a map plotter we were not required or expected to know the science as well as a meteorologist, but it was fun (except for the bad hours - ugh)...

Fortunately they were good to work with and surprised that I'd leave, but during my decade with the HRD and NHC my personal life changed a great deal and the time came to move on - sometimes I wished I'd stayed, but the bad hours were taking their toll physically (I'm not a night person) and had I stayed would have probably been a diabetic by now (my poor blood sugar would dive at about 4 a.m. - the body wasn't mean to be awake at those hours, and after 5 years of that off and on work on the mid shift it was time for me to go)...

Anyway - it was fun working for many of the pioneers in tropical meteorology, (Neil Frank, Bob Burpee, John Hope, Bob Case, etc.), and glad to have been at the right place at the right time - always a good thing...

Now, as for expanding wind fields and storm surge, after thinking about the posts yesterday I still have to think that the entire issue with Sandy was exceptional, true it followed natural laws as far as was explained to me (above), but as Brian Norcross and others said, it was a very rare combination of events, for reasons we do not understand, I'm sure...

Frank

[Blown Away]

IMO, Sandy was a remarkable system w/ amazing energy, but when mother nature produces a system that brings 150-200mph winds to the surface, that is truly amazing to me!

[brunota2003]

IMO, Sandy was a remarkable system w/ amazing energy, but when mother nature produces a system that brings 150-200mph winds to the surface, that is truly amazing to me!

Here are some neat, generalized energy facts provided by HRD:

It turns out that the vast majority of the heat released in the condensation process is used to cause rising motions in the thunderstorms and only a small portion drives the storm's horizontal winds.

Method 1) - Total energy released through cloud/rain formation:

An average hurricane produces 1.5 cm/day (0.6 inches/day) of rain inside a circle of radius 665 km (360 n.mi) (Gray 1981). (More rain falls in the inner portion of hurricane around the eyewall, less in the outer rainbands.) Converting this to a volume of rain gives 2.1 x 1016 cm3/day. A cubic cm of rain weighs 1 gm. Using the latent heat of condensation, this amount of rain produced gives
5.2 x 1019 Joules/day or
6.0 x 1014 Watts.

This is equivalent to 200 times the world-wide electrical generating capacity - an incredible amount of energy produced!

Method 2) - Total kinetic energy (wind energy) generated:

For a mature hurricane, the amount of kinetic energy generated is equal to that being dissipated due to friction. The dissipation rate per unit area is air density times the drag coefficient times the windspeed cubed (See Emanuel 1999 for details). One could either integrate a typical wind profile over a range of radii from the hurricane's center to the outer radius encompassing the storm, or assume an average windspeed for the inner core of the hurricane. Doing the latter and using 40 m/s (90 mph) winds on a scale of radius 60 km (40 n.mi.), one gets a wind dissipation rate (wind generation rate) of
1.3 x 1017 Joules/day or
1.5 x 1012Watts.

This is equivalent to about half the world-wide electrical generating capacity - also an amazing amount of energy being produced!

Either method is an enormous amount energy being generated by hurricanes. However, one can see that the amount of energy released in a hurricane (by creating clouds/rain) that actually goes to maintaining the hurricane's spiraling winds is a huge ratio of 400 to 1.

Source:
http://www.aoml.noaa.gov/hrd/tcfaq/D7.html

Now remember, that is over a single DAY! So, on average, a mature 90 mph Category 1 hurricane generates over 200 times the amount of energy generated by humans in the entire WORLD!

[vbhoutex]

P.S. In all fairness to myself (lol), I worked at the NHC between 25 and 30 years ago, so much that was learned back then has since drifted away, and as a map plotter we were not required or expected to know the science as well as a meteorologist, but it was fun (except for the bad hours - ugh)...

Fortunately they were good to work with and surprised that I'd leave, but during my decade with the HRD and NHC my personal life changed a great deal and the time came to move on - sometimes I wished I'd stayed, but the bad hours were taking their toll physically (I'm not a night person) and had I stayed would have probably been a diabetic by now (my poor blood sugar would dive at about 4 a.m. - the body wasn't mean to be awake at those hours, and after 5 years of that off and on work on the mid shift it was time for me to go)...

Anyway - it was fun working for many of the pioneers in tropical meteorology, (Neil Frank, Bob Burpee, John Hope, Bob Case, etc.), and glad to have been at the right place at the right time - always a good thing...

Now, as for expanding wind fields and storm surge, after thinking about the posts yesterday I still have to think that the entire issue with Sandy was exceptional, true it followed natural laws as far as was explained to me (above), but as Brian Norcross and others said, it was a very rare combination of events, for reasons we do not understand, I'm sure...

Frank

I am not sure we will ever understand those processes, no matter how much studying is done. One thing we do know and are told in no uncertain times over and over-Mother Nature is ALWAYS in control.

[Frank2]

Yes, this was unusual, that's for certain!

I just happened to see the below, written by Brian Norcross (link for the entire blog just posted to Twitter by Jim Cantore - and makes me feel less of a dunce for wondering about the unusual set of circumstances):

"Sandy was behaving as forecast through most of the day last Monday. It was an off-the-charts unusual storm - something like a hurricane embedded in a nor'easter - but the forecast nailed it in almost every way. Then something changed Monday afternoon.

Instead of slowing down as forecast, it took off like a rocket ship heading for the South Jersey coast. On that pace and track, the center would have crossed the coast near Cape May about 6 PM. But the center hung something of a right and it took 2 hours longer to get to the coastline farther north, 5 miles from Atlantic City. At least that was the National Hurricane Center analysis at the time it was happening.

Additionally, at 7 PM, with the center less than 20 miles offshore, the NHC declared Sandy "post-tropical". In other words, in their analysis, the technical, meteorological structure of the system wasn't enough of a hurricane anymore to continue with that classification. Well... maybe, maybe not.

There were complicated things going on near landfall and it's going to take detailed analysis to figure out what the center of the hurricane part of Sandy did, and how it related to the nor'easter part of the storm. This type of post-analysis happens with every storm, but in this case it could have big ramifications.

If the conclusion is that Sandy was still a hurricane - meaning the structure of the system was mostly tropical, not mostly nor'easter - what does that do to insurance deductibles? In many cases big deductibles kick in when a "hurricane" makes landfall, as opposed to a tropical storm or some other freak-job of a storm structure. Hurricane warnings can play into it as well.

There have already been very official sounding proclamations by governors and other optimists that the big deductibles will NOT apply since it wasn't a hurricane. There is the potential for a big mess here if the science proves otherwise.

Hurricane Hunters were flying through the center of the hurricane part of Sandy right up until landfall, so there will be lots of data to analyze. It usually takes some months before the final conclusions are formulated."