Min central pressre not the best indication of intensity...

[John Peters]

I was wondering what your opinions were on whether statistical data in terms of cyclone intensity should be based on minimum pressure. Last year there were two examples that in my opinion somewhat discredit this method.

Dean achieved an 145 kt winds (165 mph) in the eastern Caribbean at about 0600Z on the 18th of August. At this time the minimum central pressure was estimated at 930 mb. Dean's central pressure continued to fall throughout the day, reaching 923 mb at 1100z, and by 1800z it had dropped to 918 mb; however, estimated surface winds decreased from 145 kt to 120 kt over this time, declining as the storm deepened.

Felix bottomed out at 929 mb, with sustained surface winds of 175-180 mph; many storms have achieved lower central pressures without the ability to sustain comparatively intense surface winds.

Should minimum central pressure be used to gauge storm intensity?

 

[Jim Leonard]

Tropical cyclones during their imature rapid deepening stage in the deep tropics tend to have higher winds while the central pressure is relalitively high.
This is because the storm is usually has a much smaller inner core at this time and the pressure gradient is most steep near the inner core. As the storm matures the inner core winds tend to spread out and decrease even though the central pressure may be lower than earlier. The worst time for a coastline is have a hurricane cross the coast during this rapid deepening phase, Charley and Andrew are recent examples. During hurricane Wilma when the pressure went down to 882mb with a eye diameter of only 2 miles probably had surface winds over 200mph but from what I heard many dropwindsondes were lost during that mission.

 

[John Peters]

Tropical cyclones during their imature rapid deepening stage in the deep tropics tend to have higher winds while the central pressure is relalitively high.
This is because the storm is usually has a much smaller inner core at this time and the pressure gradient is most steep near the inner core. As the storm matures the inner core winds tend to spread out and decrease even though the central pressure may be lower than earlier. The worst time for a coastline is have a hurricane cross the coast during this rapid deepening phase, Charley and Andrew are recent examples. During hurricane Wilma when the pressure went down to 882mb with a eye diameter of only 2 miles probably had surface winds over 200mph but from what I heard many dropwindsondes were lost during that mission.

thanks for the response jim. Do you believe there is a relationship between eye diameter, pressure, and intensity; in other words, given two storms with equal central pressures, one with a large eye and one with a smaller eye, would the smaller storm produce more intense surface winds (due to a tighter pressure gradient)?

Also, would a higher surrounding ambient pressure relative to the storm's central pressure result in higher surface winds? I know felix achieved category five status at a pretty low latitude, and i suspect this may have to do with discrepancies between dopwindsonde data and dvorak estimates of central pressure.

 

[Patrick Marsh]

Mathematically (and meteorologically) intensity is defined as the Laplacian of the pressure field. In other words, it is the gradient of the pressure gradient. The larger the Laplacian, the more intense a storm system.

 

[Jim Leonard]

thanks for the response jim. Do you believe there is a relationship between eye diameter, pressure, and intensity; in other words, given two storms with equal central pressures, one with a large eye and one with a smaller eye, would the smaller storm produce more intense surface winds (due to a tighter pressure gradient)?

Also, would a higher surrounding ambient pressure relative to the storm's central pressure result in higher surface winds? I know felix achieved category five status at a pretty low latitude, and i suspect this may have to do with discrepancies between dopwindsonde data and dvorak estimates of central pressure.

John
That is correct, the smaller the eye diameter which would have the lower pressures over a smaller area would have higher winds, tighter pressure gradient.
Yes, the higher surrounding ambient pressures would result in higher surface winds.
Typhoons that occur in the western pacific from late October through December tend to have stronger surface winds because of the ambient surrounding pressures are higher than they are during the summer months. Also during late season they are steered westward for many days by strong high pressure systems to the north.

 

[John Peters]

Mathematically (and meteorologically) intensity is defined as the Laplacian of the pressure field. In other words, it is the gradient of the pressure gradient. The larger the Laplacian, the more intense a storm system.

interesting. I knew all that vector calculus would come in handy some day

 

[Justin E. Reed]

Hey all I'm sayin is I demand research into Charley's landfall. He was the closest the U.S. has seen to a category 5, I tend to think he was, right at landfall. The max sustained winds were 145mph, with a 944 pressure. I still think he was 160 sustained, but whom am I to say.

 

[John Peters]

Hey all I'm sayin is I demand research into Charley's landfall. He was the closest the U.S. has seen to a category 5, I tend to think he was, right at landfall. The max sustained winds were 145mph, with a 944 pressure. I still think he was 160 sustained, but whom am I to say.

the US has seen 3 category fives:
Camile, Andrew, and Labor Day 1935

 

[Justin E. Reed]

the US has seen 3 category fives:
Camile, Andrew, and Labor Day 1935

I'm incredibly well aware of that, I just demand an investigation into Charley is all.

 

[John Peters]

Hey all I'm sayin is I demand research into Charley's landfall. He was the closest the U.S. has seen to a category 5, I tend to think he was, right at landfall. The max sustained winds were 145mph, with a 944 pressure. I still think he was 160 sustained, but whom am I to say.

my apologies Justin, i figured you'd be well aware of that. I was just thrown off by the above statement.

 

[Chad Cowan]

I'm incredibly well aware of that, I just demand an investigation into Charley is all.

Two questions for you...

1) How can you be 'incredibly well aware' of something that you contradict in your previous statement?

2) To whom are you demanding this investigation?

 

[Jim Leonard]

Hurricane Charley was a solid cat-4. Even though there were gusts over 140kts the sustained winds at landfall were rated at 130kts or 150mph. Because of the rapid intensifying phase right up to landfall some of the gusts were quite a bit higher in a very few cells in the eyewall. This is common in rapidly intensifying tropical cyclones. I first noticed this characteristic during the passage of hurricane Cleo in Miami which was another compact hurricane. Cleo rapidly intensified during the last few hours before making a direct hit on Miami in 1964. All during the day the storm was pretty steady at 85 to 90 mph, in about 3 hours up to landfall the storm went up to sustained winds of 110mph. Some areas in North Miami sustained damage from gusts estimated as high as 145mph. The central pressure was 967mb at landfall which is relatively high for a 110mph storm.

 

[Justin E. Reed]

I retract my previous statements, I am a lil overzealous about Charley I have to admit. One thing we can all agree on, he was definetly the most intense hurricane to make a daytime, mainland US landfall since, well, before I was born. If only Andrew would have waited 2 more hours, we could have seen how furious he really was. To a certain degree, many are thankful he hit during the dark, so you couldn't see what was going on, so a mixed blessing I guess.

 

[MClarkson]

Personally I like using pressure as the primary way to rate a storm. Obviously every storm is different. A tight storm will have higher winds while a loose storm lower, but the loose storm will hit more people with those winds and may cause higher or just as high surge due to a longer fetch and the storm has probably been around a while.

The one main comment I have is that pressure can be measured directly, and accuratly. These days as long as there is a plane around, which there usually is at almost all times as storms threaten land, and sometimes in the deep ocean, we get extremely reliable pressure reports. When a storm landfalls, a barometer is more survivable and historically we have far more good pressure reports than wind speeds. Barometers also have been around longer. Anemometers must be exposed and often fail before high end hurricane winds arrive.

Surface winds are rarely sampled, and even then only a small portion of the storm. The best way to sample surface winds is with an eyewall dropsonde. Usually surface wind is an estimate based on upper level winds, and we all know the vertical wind profile is not exact. Surface landfall recordings are spotty, especially as you go back in time. And that still leaves the bulk of the storm unsampled. Like with tornados, the best wind estimates often come after the fact, from looking at the damage.

So especially for historical context, i'm all for central pressure.

 

[Jack Beven]

Central pressure is certainly easier to measure in a tropical cyclone than the winds. But it is the winds that do the damage and the winds that push the storm surge onto the coast.

We're making strides in being able to measure hurricane winds, but we still have a long way to go. Instruments like the stepped frequency microwave radiometer will help us with surface wind estimates, once we figure out what we are seeing in the data (a problem I had with Hurricane Felix).

One thing that I think gets lost in the debate of how strong Katrina was at landfall is that Katrina at 920 mb and weakening was a much different storm than Katrina at 920 mb and strengthening. There were some remarkable changes in both the horizontal and vertical structure of the storm - changes that were evident even with the wind measuring systems we have today. I think will be seeing more of such changes as the wind measuring technology improves in the future.

Jack Beven

Personally I like using pressure as the primary way to rate a storm. Obviously every storm is different. A tight storm will have higher winds while a loose storm lower, but the loose storm will hit more people with those winds and may cause higher or just as high surge due to a longer fetch and the storm has probably been around a while.

The one main comment I have is that pressure can be measured directly, and accuratly. These days as long as there is a plane around, which there usually is at almost all times as storms threaten land, and sometimes in the deep ocean, we get extremely reliable pressure reports. When a storm landfalls, a barometer is more survivable and historically we have far more good pressure reports than wind speeds. Barometers also have been around longer. Anemometers must be exposed and often fail before high end hurricane winds arrive.

Surface winds are rarely sampled, and even then only a small portion of the storm. The best way to sample surface winds is with an eyewall dropsonde. Usually surface wind is an estimate based on upper level winds, and we all know the vertical wind profile is not exact. Surface landfall recordings are spotty, especially as you go back in time. And that still leaves the bulk of the storm unsampled. Like with tornados, the best wind estimates often come after the fact, from looking at the damage.

So especially for historical context, i'm all for central pressure.