Question about Eyewall Replacement Cycles

[Berwick Bay]

How often (in general) will a major hurricane undergo an eyewall replacement cycle? My impression is about every 36 hours or so. Am I close, or way off? Anything else you can tell me about the variablitly of the cycle in regards to strength of storm (more or less frequent), location of storm (deep tropics as opposed to sub-tropical lats), size of storm (perhaps smaller storms having more frequent cycles??) or indivuduality of the storm (each storm is different and has its own "biological clock"
relative to frequency of cycle. Thanks.

[Derek Ortt]

depends upon many variables

the PV distribution, water vapor distribution, rainband orientation, possibly vortex Rossby Wave propagation and interaction.

There are many factors which affect eyewall replacement cycles.

[Berwick Bay]

So Derek, back to the original question about frequency of ERC's. Can we make a generalization about the frequency of eyewall replacement cycles in major hurricanes? If so, what would be the closest estimate of that frequency? Or, is this simply impossible right now due to the variables you listed. In other words, are we completely in the dark about when to expect an ERC in a major hurricane?

[WindRunner]

If you're looking for a general figure, 36-48 hours is about right. But, in reality, it does vary from storm to storm, and sometimes even day to day within a storm.

In terms of variability, well, I've noticed storms that are getting some negative environmental effects are somewhat more likely to go through one or will do so more frequently, especially if those negative effects are surface-based (i.e. SSTs, land interaction). Storms that are very well formed - annular hurricanes - will rarely undergo these cycles, due to their perfect (or close to it) nature/symmetry. Also, a smaller/shrinking eye is another possible sign that a ERC may be about to start, though that is also a sign of a strengthening storm and therefore the associated ERC may still be aways off yet. Typically, though, this is true because of the nature of an ERC - that the inner (smaller) eyewall has to collapse inward on itself before the outer (larger) eyewall can take over.

Otherwise, though, we really just have to realize what it means once when we see one starting. Intensity forecasting does not take into account ERCs right now, as you will see the NHC mention the possibility that a storm will weaken due to an ERC, even though the intensity forecasts is steady through the first 24-48 hours. Not enough is known about the phenomenon to really say when they are going to happen, hence we have to roll with the blows . . .

[Berwick Bay]

Thank you Windrunner for that well-thought out reply. So the 36-48 hour frequency seems about right (generally speaking). I was intrigued by something else that you said. I believe you mentioned that "annular storms" (and I'm not really familiar with that term for storms) or very perfectly formed systems have eyewall replacement cycles that are "rare". Thats interesting because I thought that with all of these systems, the energy constraints upon the eye core neccesitated a systematic eyewall replacement. But if I read your post right, with major storms in ideal conditions we might expect very few replacement cycles???

[Derek Ortt]

They can occur almost one right after the other. It really depends upon the storm structure and dynamics, as well as the large scale environment.

[wobblehead]

Perhaps we can focus and discuss this aspect of a storm during an active storm this season.

[wxmann_91]

Good question. I suspect atmospheric instability (takes into account SST's and environmental lapse rates) along with the amount of low level convergence/upper diffluence could influence the tendency for an outer eyewall to develop in the first place

[Tampa Bay Hurricane]

I think Eyewall replacement cycles will happen if you
have ssts and favorable conditions like in Katrina
the Eyewall replacement cycle made it go
from having a small eye to a
gigantic eye within 24 hours.


Also same in wilma within 24 hours
of pinhole eye the eyewall got
really large.

[Tampa Bay Hurricane]

oh and one more thing the eyewall
replacement cycles make the windfields
expand over a much larger area...
like in katrina and wilma

[WindRunner]

Thank you Windrunner for that well-thought out reply. So the 36-48 hour frequency seems about right (generally speaking). I was intrigued by something else that you said. I believe you mentioned that "annular storms" (and I'm not really familiar with that term for storms) or very perfectly formed systems have eyewall replacement cycles that are "rare". Thats interesting because I thought that with all of these systems, the energy constraints upon the eye core neccesitated a systematic eyewall replacement. But if I read your post right, with major storms in ideal conditions we might expect very few replacement cycles???

For the period of time they are annular, yes. The term annular references a storm that has near-perfect form, defined as being without any visible feederbands and relatively symmetric. A storm, however, does not maintain its annular form forever and the end of an annular period will often contain an ERC as the storm weakens from its annular strength. The big thing to remember with annular storms is that little to no change occurs in the storms structure and strength during that period.

And yes, generally speaking, 36-48 hours is a good timeframe. However, like Derek mentioned, they can occur one after another (I believe Katrina was doing this near landfall, for a notable example), or they can be seperated by a good bit, though rarely is it more than 48 hours. I believe a fair estimate would be to say it is far more likely to see ERCs occur more frequently than every 36 hours than for such a period to be more than 48.

The best thing to do in order to get a feel for the timing of ERCs is to watch them in a storm that spends a good period of time at Cat 3+ . . . if nothing else, the WPAC should provide a few good examples . . . though I'm sure most here would like the Atlantic to do so as well.

[Derek Ortt]

Frances was a good example of a storm that went through many EWRCs

If it was concluded that Katrina was undergoing an eyewall replacement, that was about 48 hours after its previous one. I think Rita was about to start one, but we'll never know since it crashed into Texas and it had 25KT of shear at the time of landfall, would have been about 36 after its previous

[WindRunner]

Frances was a good example of a storm that went through many EWRCs

If it was concluded that Katrina was undergoing an eyewall replacement, that was about 48 hours after its previous one.

Frances is a better one, good call. But I though recon found 3 wind velocity peaks in Katrina in one of their last passes before landfall - i.e. indicating a dying inner, a (briefly) mature wall, and a forming outer? I'll have to go back and take a look at the vortexes and see . . .

[Berwick Bay]

Thanks again to Windrunner and Derek for your input on Eyewall Replacement Cycles. If we do have 3 or more major storms this year I'm going to be paying particular attention to the ERC's. Would like to check our ability to anticipate them as they occur for individual storms in real time.

[Derek Ortt]

I think there was one in the SW quadrant.

The eyewall had collapsed, likely due to the increasing shear and dry air. I have a question regarding the outer peak (did I make a mistake by classifying that as non convective), so I cannot comment on that at this time

[WindRunner]

I think there was one in the SW quadrant.

The eyewall had collapsed, likely due to the increasing shear and dry air. I have a question regarding the outer peak (did I make a mistake by classifying that as non convective), so I cannot comment on that at this time

Yeah, you're right, that really wasn't much of a 3rd wall, as it is a) so far removed and b) so low with its velocities - i.e. FL winds jumped to the low 80s from 69 a good 20 minutes out from the secondary wall. I can see why you would question such a statement, as it really doesn't qualify . . .


And glad we could be of help, Berwick.