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Some Very Deep Category 1-2 Atlantic Hurricanes Since 2008


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#1
bluewave

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There have been some very deep category 1-2 hurricanes in the Atlantic since 2008. Several storm
interactions with drier air have occurred over this time frame . While it may be the case that several
several mid-century hurricanes had overestimated wind speeds, recent years do raise some
interesting questions. Does increased stability in the Tropical Atlantic allow more dry air to wrap into
TC circulations leading to a changed wind to pressure relationship in some storms? This may be showing
up in reduced precipitation rates for the Tropical Atlantic since 2008.

These are the hurricanes that had a very deep pressure for the category that they were at a some point
during their lifecycle:

Year...Storm...Category...Pressure in MB

2008

Ike..........2...944

2010

Alex...2...948
Earl...2....949
Igor...2....939

2011

Irene...2...942

2012

Isaac...1....968
Sandy...1...940


Charts showing the increased stability in the Tropical Atlantic the last few years:

2011

[attachment=78890:11.png]

2012

[attachment=78891:12.png]




Reduced precipitation rates for the Tropical Atlantic since 2008:

[attachment=78841:0812.png]

[attachment=78842:0307.png]

[attachment=78843:9502.png]


-AMO

[attachment=78844:6394.png]


Last +AMO

[attachment=78845:4862.png]

#2
Ed Lizard

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I'd think you'd have to screen out storms that have expanded wind fields from land interaction (Ike, Alex) and/or are no longer purely tropical cyclones (Irene, Sandy) from that list.



I am not familiar with, nor am going to Wki, all the storms on that list I don't remember well. How many were purely tropical and didn't experience major land interactions that were anonymously deep for the max sustained winds. If there aren't many, looking at the indices to explain something that isn't becoming more common may be over-analyzing.

#3
aslkahuna

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In WPAC we certainly had that occur with Bolaven and Songba both of which passed over Okinawa. Both had sub 950 pressures but with Bolaven you had to try hard to find even Cat 1 winds while the second storm MIGHT have produce Cat 2 sustained. Dry air was certainly and issue with Bolaven and quite likely with the other storm.

Steve

#4
Analog96

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I'd think you'd have to screen out storms that have expanded wind fields from land interaction (Ike, Alex) and/or are no longer purely tropical cyclones (Irene, Sandy) from that list.



I am not familiar with, nor am going to Wki, all the storms on that list I don't remember well. How many were purely tropical and didn't experience major land interactions that were anonymously deep for the max sustained winds. If there aren't many, looking at the indices to explain something that isn't becoming more common may be over-analyzing.


Earl was definitely not fully-tropical because it was brushing Eastern New England.

#5
Amped

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Maximum Sustained winds are usually proportional to the eyewall pressure gradient. A 975mb storm can have a stronger eyewall pressure gradient than a 955mb storm.

I am not surprised about Irene or Sandy given the NJ coast water temps. Put those storms over 30c waters and the winds will mix down.


A lot of this also has to do with the Vortex structure, which is a fascinating but complex topic. Plenty of good research papers on it.



#6
bluewave

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I'd think you'd have to screen out storms that have expanded wind fields from land interaction (Ike, Alex) and/or are no longer purely tropical cyclones (Irene, Sandy) from that list.



I am not familiar with, nor am going to Wki, all the storms on that list I don't remember well. How many were purely tropical and didn't experience major land interactions that were anonymously deep for the max sustained winds. If there aren't many, looking at the indices to explain something that isn't becoming more common may be over-analyzing.


The cases of land interaction have been interesting the last few years. At least since 1995, Ike and Isaac
had a large wind fields after interaction with Cuba, but were not majors post interaction in the Gulf. Hurricane
Ike stands out for having a sub 950 pressure for a time post Cuba while at a Category 2. Hurricane Isaac
I believe was the deepest Gulf category 1 in the Gulf before a weakening process set in.

1995-2005 featured several majors post land interaction in the Gulf, but you may be correct in saying that it
is really difficult to measure without a sophisticated modeling study to see if it's significant or not.

I included Sandy since she was a 954 category 1 while still south of Cuba. And Irene went from 956 Cat 3
down to cat 2 942 while still south of 30 degrees north.

#7
Sickman

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I would also think that better observations in recent years has led to a better understanding of pressure vs. surface wind measurements. I have a feeling that some maximum winds from 25-30+ years ago may be inflated based solely on pressure measurements.

#8
tornadotony

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I would also think that better observations in recent years has led to a better understanding of pressure vs. surface wind measurements. I have a feeling that some maximum winds from 25-30+ years ago may be inflated based solely on pressure measurements.

This x 10000

#9
HurricaneJosh

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I would also think that better observations in recent years has led to a better understanding of pressure vs. surface wind measurements. I have a feeling that some maximum winds from 25-30+ years ago may be inflated based solely on pressure measurements.

It's what I been sayin', whiteboy.

#10
wxmx

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For large cyclones, I agree...for small ones and/or where ambient pressures were high, probably there was some underestimation.

#11
HurricaneJosh

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For large cyclones, I agree...for small ones and/or where ambient pressures were high, probably there was some underestimation.

But the thing is, the reanalysis formulas already take into account size when deriving wind speed. Basically, larger storms get penalized and smaller storms get a bonus. It's not like the reanalysis people don't take size into account-- they do. And yet... I still wonder if many of the reanalyzed storms have been too generously assessed.

#12
HurricaneJosh

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When I ran the reanalysis formulas on some recent storms, like Irene 2011 (in NC) and Alex 2010 (in MX), I got some spectacular errors-- really huge. The formulas grossly overestimated the winds in both storms. For example, based on central pressure, RMW, latitude, intensity trend, and forward speed, Alex should have been an azz-spankin' Cat 4.

Ike 2008 (in TX) actually wasn't so bad-- the formulas got that one pretty close. I haven't run the formulas on Isaac 2012 yet.

#13
thewxmann

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When I ran the reanalysis formulas on some recent storms, like Irene 2011 (in NC) and Alex 2010 (in MX), I got some spectacular errors-- really huge. The formulas grossly overestimated the winds in both storms. For example, based on central pressure, RMW, latitude, intensity trend, and forward speed, Alex should have been an azz-spankin' Cat 4.

Ike 2008 (in TX) actually wasn't so bad-- the formulas got that one pretty close. I haven't run the formulas on Isaac 2012 yet.

What the heck? Wasn't Alex a large cyclone with central pressure in the 940s? I wouldn't think that would support more than Cat 3 in the formula...

Not sure how winds would depend on latitude. I would also think that history (i.e. land interaction in the previous 24 hr) ought to figure into the formula as well.

#14
HurricaneJosh

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What the heck? Wasn't Alex a large cyclone with central pressure in the 940s? I wouldn't think that would support more than Cat 3 in the formula...

Alex's RMW (7 n mi) was quite small, and reanalysis uses RMW as the size metric for calculating pressure-based wind values.

Not sure how winds would depend on latitude. I would also think that history (i.e. land interaction in the previous 24 hr) ought to figure into the formula as well.

Read the reanalysis papers. Latitude is an important factor in the wind formulas (Brown et al. (2006)). Lower latitude = higher winds. Not my rules.

But I'm surprised you'd be surprised about this. We all know that a 940-mb 'cane deep in the tropics is generally going to have higher winds than a 940-mb 'cane at 35N.

#15
HurricaneJosh

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Just to show you what I mean...

Alex 1) had a small RMW (7 n mi), 2) was strengthening at landfall, and 3) was fairly far S (~24N). Except for forward speed-- which was average-- all of the metrics were positive. (To add to this, Alex had no land interaction in the days prior to its Tamaulipas landfall-- it was coming in clean off the Gulf-- so that was yet another reason one would have expected higher winds.)

The Brown et al. (2006) formula yields 117 kt for a strengthening, 946-mb cyclone S of 25N, and the small RMW-- which is more than 50% smaller than climo-- adds another 5 or 10 kt. So that yields ~125 kt, whereas best track was 95 kt-- a whopping 30-kt difference.

#16
Sunny and Warm

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Just to show you what I mean...

Alex 1) had a small RMW (7 n mi), 2) was strengthening at landfall, and 3) was fairly far S (~24N). Except for forward speed-- which was average-- all of the metrics were positive. (To add to this, Alex had no land interaction in the days prior to its Tamaulipas landfall-- it was coming in clean off the Gulf-- so that was yet another reason one would have expected higher winds.)

The Brown et al. (2006) formula yields 117 kt for a strengthening, 946-mb cyclone S of 25N, and the small RMW-- which is more than 50% smaller than climo-- adds another 5 or 10 kt. So that yields ~125 kt, whereas best track was 95 kt-- a whopping 30-kt difference.

Hmm indeed Josh. If the Brown formula cannot hold for a percentage of recent storms, then how can we trust the reanalysis results at all. Even one major miss by the formula as you are suggesting with Alex is cause to stop reanalysis and look again at the Brown formula. JMO.

#17
HurricaneJosh

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Hmm indeed Josh. If the Brown formula cannot hold for a percentage of recent storms, then how can we trust the reanalysis results at all. Even one major miss by the formula as you are suggesting with Alex is cause to stop reanalysis and look again at the Brown formula. JMO.

Well... Just to be clear, I think the reanalysis work is great, and I think their methodologies are the best we can expect for pre-satellite cyclones with scant surface obs. The formulas work pretty well, and I think they get us in the right ballpark most of the time. And, also, the researchers are good at making adjustments based on other factors.

But, yeah, there are going to be some misses. But the reanalysis folks are up front about this-- they don't pretend that the new best-track data are 100% accurate. And despite the inevitable errors, the reanalyzed data are still much better than what we had, and much better than what's available for any other basin. So, overall, it's awesome and relatively reliable.



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