Hurricane eye subsidence discussion (continued):

[Wthrman13]

Air Force Met, Mike

Well, I took a close look at the Willoughby paper linked earlier:

Willoughby, H. E, 1998: Tropical cyclone eye thermodynamics. Mon. Wea. Rev., 126, 3053–3067.

and he argues that the air in the eye of a hurricane subsides slowly, either from dynamic or thermodynamic effects (the dynamic effects due partially to centrifuging were discussed previously in mine and Air Force Met's posts in the previous thread). He also does confirm that the circulation in the eye is thermally indirect (subsiding warm air). Interestingly, he does argue against the traditional view that the subsiding air in the eye comes directly from the tropopause level all the way down to the inversion layer above the boundary layer, and instead argues that the air subsides slowly and is an effect of outflowing air at low-levels below the inversion. Here is a portion from the introduction to the paper:

"The conventional view of the eye’s thermodynamics is that air detrains from the top of the eyewall and sinks inside the eye to the lower troposphere where it is entrained back into the eyewall. Inward mixing from the eyewall is hypothesized to force the subsidence and maintain the moisture and momentum budgets of the subsiding air (Miller 1958 ; Malkus 1958 ; Holland 1997 ). In this interpretation, the recirculation is rapid enough to replenish the eye’s volume many times over the hurricane’s lifetime. The original argument for rapid replenishment of the air in the eye was that the calm air inside the eye did not appear to share in the translation of the vortex as a whole (Malkus 1958 ), and that the eye moved by continuously reforming. More recent observations from aircraft equipped with inertia navigation equipment show clearly that the low-level wind is a superposition of circulation about the axis of rotation and the translation of the axis (Willoughby and Chelmow 1982 ) so that it is kinematically possible for a mass of air to move with the eye.

Here observed eye soundings are examined; they suggest that the dry air above the inversion has a long lifetime inside the eye, experiences only a few kilometers total subsidence, and mixes only weakly with the moist air from the eyewall. The air below the inversion seems, by contrast, to derive largely from the eyewall, or from frictional inflow layer below it, through a complicated process of mixing and evaporatively driven subsidence. A conceptual model based upon this interpretation can synthesize observed eye soundings and calculate realistic hydrostatic pressure falls from the eyewall inward to the axis of vortex rotation."

In other words, he argues, from observations, that the eye does indeed have descending air, albeit rather slowly and brings into question the idea that the air descends all the way from the tropopause to the boundary layer.

Further on in the paper he asks,

"How does the two-layer thermodynamic structure of the eye come about? Measurements of chemical tracers suggest long residence times for air in the upper reaches of the eye (Newell et al. 1996 )"

This is consistent with what mike was saying earlier (about the cirrus just sitting there), but it's not the whole story:

"The convective updrafts in the eyewall are buoyant with respect to the air around the eye, but not with respect to the eye itself. Collectively, they act as a “heat pump” that does work on the eye by pulling air out around the bottom of the eye to force thermally indirect descent. Since the updrafts do not rise in the warmest air inside the eye, the warm anomaly that causes the lowest hydrostatic pressure does not necessarily limit eyewall convection. When the convection is intense, net flux from the eye lowers the inversion and promotes descent above the inversion, warming and drying the eye. When the convection weakens, net frictional inflow and mixing into the eye raise the inversion, cooling the eye and filling it with cloud. This interpretation is consistent with the idea that convection causes the pressure fall between the eyewall and the axis of vortex rotation through compensating subsidence and adiabatic warming concentrated in the eye."

Thus, it is the convection in the eyewall that forces the thermally indirect descent inside the eye. However, it is slow and weak. It seems to me that even with this view of things, you would still get subsidence aloft above the eye at the tropopause, just fairly weak subsidence.

And finally, from the conclusion of the paper:


"The soundings and interpretation presented here suggest strongly that the thermodynamics of tropical cyclone eyes do not obey conventional models in which air detrains from the eyewall near the tropopause, sinks through most of the depth of the troposphere inside the eye—acquiring moisture and momentum as needed to maintain its bulk properties—and is entrained back into the eyewall at the bottom. Conventional models assume implicitly that the time required to cycle air though the eye is short compared with the lifetime of the eye."

So, what we have here is very slow subsidence in the eye, as a more or less passive response to the strong convection in the eyewall. This paper does not address whether that subsidence extends all the way to the top of the eye near the tropopause or not. My guess is, from reasons given earlier, that it does, and it explains why intensifying hurricanes typically clear out their eyes of cirrus.

[mike18xx]

Thread title: "Re: Hurricane eye subsidence discussion (continued)"

Continued from: http://www.storm2k.org/phpbb2/viewtopic.php?t=68540&start=60

Interestingly, he does argue against the traditional view that the subsiding air in the eye comes directly from the tropopause level all the way down ...snip... In other words, he argues, from observations, that the eye does indeed have descending air, albeit rather slowly and brings into question the idea that the air descends all the way from the tropopause to the boundary layer...snip...This is consistent with what mike was saying earlier (about the cirrus just sitting there),

You 'da Man!

.....The convective updrafts in the eyewall are buoyant with respect to the air around the eye, but not with respect to the eye itself. Collectively, they act as a “heat pump” that does work on the eye by pulling air out around the bottom of the eye to force thermally indirect descent.

I.e., "eddying".

[weatherwindow]

is it fair to say that the lower the inversion layer the more intense, ie lower central presure and clearer eye?.................rich

[Air Force Met]

And I've never argued this...about the cirrus. I have been speaking of the reason behind the clearing of the eye in the entire column. I have been discussing the result...which is clearing. The process that involved the actual clearing is descending air. The process(es) that cause that air to descend is the debatable point...but there is no debate that the air is descending. Descending air MUST dry.

"by pulling air out around the bottom of the eye to force thermally indirect descent."...
"He also does confirm that the circulation in the eye is thermally indirect (subsiding warm air)."

So the only thing this paper says that is outside of what we have been saying is it reaching all the way from the trop...and that is debatable as he admits.

So lets get down to brass tacks. What is the reason (not the process that causes the reason) that clouds clear out at 20,000 feet, or 10,000 fett...30,000 feet? Forget the cirrus at 60,000...why do the clouds disappear at those levels in your opinion? Is it because they are pulled out? Or is it because they evaporate because descending air warms (and that descending air is caused by whatever process you want it to be)...or both?

The reason I ask is this quote: "I don't know what "descending motion" you're talking about it; hurricane eyes form due to centrifuging; there may be some slight downward motion..."

Is it your contention also that descending air in the eye does not cause clouds to disappear because the dewpoint depression increases? Again...I am not speaking of one level of the eye...but the entire column.

Is it also your contention that the fact air is much drier in the eye than the eyewall...and much warmer would have no impact on cloud formation? If not...why? If so...why is it warmer?

It could be we have a chicken or egg problem here...or a causality problem.

[Air Force Met]

is it fair to say that the lower the inversion layer the more intense, ie lower central presure and clearer eye?.................rich

That is a good general rule. And it is helpful to realize the type of inversion in an eye is a subsidense inversion.

[Wthrman13]

Thread title: "Re: Hurricane eye subsidence discussion (continued)"

Continued from: http://www.storm2k.org/phpbb2/viewtopic.php?t=68540&start=60

Interestingly, he does argue against the traditional view that the subsiding air in the eye comes directly from the tropopause level all the way down ...snip... In other words, he argues, from observations, that the eye does indeed have descending air, albeit rather slowly and brings into question the idea that the air descends all the way from the tropopause to the boundary layer...snip...This is consistent with what mike was saying earlier (about the cirrus just sitting there),

You 'da Man!

.....The convective updrafts in the eyewall are buoyant with respect to the air around the eye, but not with respect to the eye itself. Collectively, they act as a “heat pump” that does work on the eye by pulling air out around the bottom of the eye to force thermally indirect descent.

I.e., "eddying".

Note, I did NOT say (and neither did the Willoughby paper for that matter) that the regime of downward motion does not extend all the way from the tropopause to the boundary layer in the eye, only that the descent is slow overall. In fact, I've been browsing through a couple other papers:

Liu, Y, Zhang D.-L, and Yau M. K, 1999: A multiscale numerical study of Hurricane Andrew (1992). Part II: Kinematics and inner-core structures. Mon. Wea. Rev., 127, 2597–2616.

Da-Lin Zhang, Yubao Liu and M. K. Yau. 2000: A Multiscale Numerical Study of Hurricane Andrew (1992). Part III: Dynamically Induced Vertical Motion. Monthly Weather Review: Vol. 128, No. 11, pp. 3772–3788.

and they seem to indicate, through high resolution simulations, that there is indeed downward motion, particularly just inside the inner edge of the eyewall, at the top of the eye that helps maintain the downdraft in the center. Furthermore, they show that the adiabatic warming due to this subsidence maintains the warm core in the eye and thus the very low central pressures (i.e. it's not the mechanical centrifugal eddying that produces the low pressure, but the maintenance of the warm core due to slow subsidence). Without this compensating subsidence, the warm core cannot be maintained and the central pressure rises.

This has turned out to be a much more complicated issue than I even previously thought. You'd do well to have a look at those papers, as I did.

[mike18xx]

That Isabel loop is the shizzits as far as I am concerned: stuff is just blasting out of there like it's rocket-assisted off an out-of-control potter's wheel; the radial grooving and clear-air exhaust-induced waves in the canopy are simply incredible.

http://cimss.ssec.wisc.edu/tropic/archive/2003/storms/isabel/movies/isabel-eye.html

[Air Force Met]

Yes...it is a very complicated issue. I would like to know though...what are we discussing? It originally started out as basically why are there no clouds in the eye. As all the papers confirmed...its descending air. The CAUSE of that descending air is disputable...but descending air always dries and warms...it's basic thermodynamics...and this has to lead to an increased dewpoint depression which causes clouds to vanish. The first post of Mike's I was responding to was when he asked about what descending air? And said the eye cleared due to centrefuging. I assumed (maybe wrong) that he was speaking of all the clouds in the entire column in a mature hurricane.

Again...this could be a chicken and the egg argument...with one side on the chicken's and another on the egg's...and basically both sides are right but not doing a very good job at what they are saying.

[Air Force Met]

That Isabel loop is the shizzits as far as I am concerned: stuff is just blasting out of there like it's rocket-assisted off an out-of-control potter's wheel; the radial grooving and clear-air exhaust-induced waves in the canopy are simply incredible.

http://cimss.ssec.wisc.edu/tropic/archive/2003/storms/isabel/movies/isabel-eye.html

So...why are there no clouds at 20,000 feet in the center of the storm...which is about 40,000 feet below the level you can actually see? What is occuring at that level that creates clear skies...lower dewpoints...higher temps and higher dewpoint depressions. What is occuring at THAT level to cause those things?
edit...and don't say eddying...I want the thermodynamic properties and processes. If you say eddying...then explain eddying using thermodynamic properties to describe WHY the eddying causes those things (like eddying in the vertical does...yada yada yada...)...

[mike18xx]

I may have mistakenly assumed you were asserting that sinking air causes eyes. (I've had arguments before which proceeded from that base premise, and I'm "keyed" to jump upon similar-sounding lines of argumentation.)

So..eh, sorry about that.

[jasons2k]

Thanks guys for thread.

[Wthrman13]

Yes...it is a very complicated issue. I would like to know though...what are we discussing? It originally started out as basically why are there no clouds in the eye. As all the papers confirmed...its descending air. The CAUSE of that descending air is disputable...but descending air always dries and warms...it's basic thermodynamics...and this has to lead to an increased dewpoint depression which causes clouds to vanish. The first post of Mike's I was responding to was when he asked about what descending air? And said the eye cleared due to centrefuging. I assumed (maybe wrong) that he was speaking of all the clouds in the entire column in a mature hurricane.

Again...this could be a chicken and the egg argument...with one side on the chicken's and another on the egg's...and basically both sides are right but not doing a very good job at what they are saying.

Yes, I apologize for perhaps taking the discussion off on too much of a tangent. As such, I don't think mike, you, or I are actually disagreeing too much. If there is compensating descent from the tropopause level in the eye of Isabel in that shot, you wouldn't necessarily see it, because it is compressing and drying, so no cirrus would be there as a tracer. Thus, mike's assertion that the satellite shows no inward directed motion cannot be taken as evidence that there is no such motion. Our argument from mass continuity still stands as far as I'm concerned. Indeed the Liu papers I referenced above discuss the presence, revealed through high resolution simulations of Andrew, of a return flow aloft above the outer edge of the eyewall. This involves radially inward-directed flow towards the center of the eye, just like we've been saying, although in their schematic they don't show it going all the way to the center (nor would I expect it to), though it does contribute to the overall downdraft in the center of the eye. Again, this flow is descending as it is flowing inward, thus any inward flowing cirrus aloft would quickly be evaporated and wouldn't show up in the satellite loops.

[Air Force Met]

I may have mistakenly assumed you were asserting that sinking air causes eyes. (I've had arguments before which proceeded from that base premise, and I'm "keyed" to jump upon similar-sounding lines of argumentation.)

So..eh, sorry about that.

Go back and read what I said...I never said that. Notice I said "The idea that it is "centrifuging" alone is a myth."...ALONE is the key word. ALone means it is the only reason...and there are no others...like descending air.

Sinking air does not cause the eye alone either (also go back and you'll see I never said that either). I know the initial process is centrefuging...what I am talking about is the entire eye structure...the reason the clouds disappear...the reason teh dewpoints are lower...the reason the dewpoint depressions and temp are higher...IN MATURE STORMS (those with nice clear eyes). I also admitted that centrefuging does aid in the downward vertical motion...but part of the downward vertical motion is also caused by the colliding air that (while mostly sent outward) meets over the eye as the updraft meets the trop.

Every paper I quoted was basically saying this. Again...I was never talking about weak tropical storms or even ugly looking hurricanes (like Emily was this morning)...because they don't have the mechanisms going yet to complete the cycle.

So...no problem. I just wanted to make sure you understood that the clouds in mature storms weren't being pulled out to create a nice clear spot...and that this was occuring because of that process and no other process. I had a prof at Texas A&M who said that once...and I had to sick Dr. Steve Lyons on him (when he was a prof there) to straighten him out

edit...for those aggies who might be lurking....it was Dr. Dourich (OK...It's been 20 years and I can't remember how to spell his name...but he was the guy (ok...one of them) that you couldn't understand really well ).

[Wthrman13]

I may have mistakenly assumed you were asserting that sinking air causes eyes. (I've had arguments before which proceeded from that base premise, and I'm "keyed" to jump upon similar-sounding lines of argumentation.)

So..eh, sorry about that.

Actually, this is the chicken and egg problem that Air Force Met is referring to. I don't think anyone working in the field currently is confident enough to assert what exactly initiates the development of the eye: is it the centrifuging of air beginning in low levels, or is the development of subsidence in the core of the developing TC? From what I can gather, the two processes are dynamically linked and it's not entirely clear whether one actually precedes the other. This is typical of many problems in meteorology, actually, and causes no end of headache and debate in the literature.

Apologies if I got anyone's blood up, but this discussion was just too interesting to pass up

[Air Force Met]

This involves radially inward-directed flow towards the center of the eye, just like we've been saying, although in their schematic they don't show it going all the way to the center (nor would I expect it to), though it does contribute to the overall downdraft in the center of the eye. Again, this flow is descending as it is flowing inward, thus any inward flowing cirrus aloft would quickly be evaporated and wouldn't show up in the satellite loops.

And I have a GREAT streamline analysis in a text book that illustrates this perfectly...but I can't find one on the net...and I can't scan the one I got and post it (scanners down)....but it is a streamline of the 100mb level...and it shows the streamlines leaving the eyewall on the inside...some going inward...and converging...and the rest going out...first cyclonically then turning anticyclonically. The part where they diverge is where the eye clears and the clouds disappear....which backs up the point of you can't see the if the air is converging over the sat pic or not...because the cirrus is not there...so you can't use it as a marker.

If you...or anyone else...have a link to that image...I could really use it. I used to have it digitally in a hurricane PPT presentation I gave to new forecasters...but we had a data loss when Allison flooded our wx building in 2001...and now I don't have it.

[mike18xx]

http://cimss.ssec.wisc.edu/tropic/archive/2003/storms/isabel/movies/isabel-eye.html

So...why are there no clouds at 20,000 feet in the center of the storm...which is about 40,000 feet below the level you can actually see? What is occuring at that level that creates clear skies...lower dewpoints...higher temps and higher dewpoint depressions. What is occuring at THAT level to cause those things?

Chicken: Rotational centrifuging creates a partial vacuum.
Egg: Drag at the eyewall/eye boundry induces eddying within the eye.

Otherwise, the eye appears capped: Ascending air wrung of moisture but still buoyant due to incomplete expansion shoots right out of the top of the eyewall, and shoots outward in all directions (very discernable, IMO, with Isabel). This particular air, I would submit, is not descending into the eye, and that what downdrafting there is inside the eye if a function of eddying underneath this cap.

[Air Force Met]

Ok...if it shoots outward from any given point in all directions...some of it is going to shoot inward towards the center and meet other air shooting inward. What happens to that air?

Also...why are there no clouds. What caused them to disappear? Please explain the thermodynamics...please don't jsut say eddying causes it...what ABOUT the eddying causes the temps to rise (creating a vacuum lower temps...per Charles' Law) and what about it causes the dewpoint depression to increase?

[Air Force Met]

OK...bedtime. I think this discussion would have been over a long time ago if we were all sitting in front of a dry erase board explaining what we were trying to type.

Sometimes eggheads (of which I am one)...don't explain ourselves too well.

Gotta get up early. Night Night.

[Stratosphere747]

I hate to interject into what is probably the most intriguing conversation that this site has ever seen.

But wanted to give some props to Dr. Steve Lyons. TWC is constantly being discredited by the majority of this site, and he at times is the butt of everyones negativity.

His only downfall is that he is not a "on-camera personality" combined with the fact that he wants to detail his reasoning in a more scientific manner, which at times seems to trip him up because he is trying to explain things to the common man.

I think he is one of the best out there. Then again I'm an Aggie as well...

Could'nt help but notice that AFM...

[mike18xx]

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