Hurricane eye subsidence discussion (continued):

[mike18xx]

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.

I like Lyons. If only they'd clone him and John Hope....

[mike18xx]

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?

I'd surmise that as pressures lower and inbound wind-speeds increase, rotational-deflection force increases by the square to the point that beyond a certain velocity, inbound parcels are unable to approach more closely to the center, and centrifuging begins -- air parcels become subject to orbital mechanics as much as Boyle's gas laws. (Of course, a circular eyewall is a very efficient convective mechanism compared to disorganized convection, and therefore enables lower pressures and subsequent further compaction of the system to an equillibrium point where the eyewall's shrinking size is balanced against the amount of atmosphere it can process and maintaining the necessary low pressure to sustain the system at that level.)

[SCUBAdude]

Hi all, been a while since I've posted so please bear with me. I'm a little confused about one part of the sinking air process in the eye. If the air is descending and warming, causing evaporation of the clouds in the eye, then wouldn't that raise the air pressure in the eye? Or is it being evacuated so fast in the eye wall so fast that it can't catch up? I've always understood that sinking air caused higher pressures at the surface and if that's what's happening in the case of a hurricane trying to replace the air that's being evacuated then one would think that air would be just pouring in from the upper atmosphere to try and meet the need. And as a result increase the pressure in the eye to a level more than the eye wall.
I'm just curious about this and I've never had a chance to ask anyone in the know about it before to clear it up.
TIA
SD

[Wthrman13]

Hi all, been a while since I've posted so please bear with me. I'm a little confused about one part of the sinking air process in the eye. If the air is descending and warming, causing evaporation of the clouds in the eye, then wouldn't that raise the air pressure in the eye? Or is it being evacuated so fast in the eye wall so fast that it can't catch up? I've always understood that sinking air caused higher pressures at the surface and if that's what's happening in the case of a hurricane trying to replace the air that's being evacuated then one would think that air would be just pouring in from the upper atmosphere to try and meet the need. And as a result increase the pressure in the eye to a level more than the eye wall.
I'm just curious about this and I've never had a chance to ask anyone in the know about it before to clear it up.
TIA
SD

One more post for me, and then I'm going to bed as well, or maybe try to get some more work done before then: we'll see

Anyway, to answer your question, although it is rather counterintuitive, the sinking motion in the eye actually helps to maintain the low pressure there. The reason is, and bear with me here, the sinking air warms as it sinks, because it is being compressed. This sinking air is warmer than its surroundings at the same level, so it is less dense (The air at the same level in the eyewall is actually more dense because of the weight of all the liquid and ice water in the clouds there). This means that the column of air in the eye of the hurricane is overall less dense than a column of air, say, in the eyewall, and also compared to a column of air on the outskirts of the hurricane. Since the pressure at the surface, barring large vertical accelerations, is essentially the weight of the column of air above a unit area of the surface, it follows that the pressure at the surface inside the eye is lower than it's surroundings. Without the sinking air in the eye to maintain the warm core, the eye would quickly flood with cooler, denser air from outside, raising the surface pressure in the center.

To look at it another way, note that sinking air doesn't itself cause higher pressure at the surface, but rather net convergence into the column of air above a point at the surface causes higher pressure. That is, if air is being put in at the top faster than it is being taken out at the bottom, the pressure will rise at the surface. The pressure will also rise even in the presence of rising motion, provided that air is being taken in at the surface at a faster rate than it is diverging aloft.

Hope that helps,

Dan

[mike18xx]

Without the sinking air in the eye to maintain the warm core, the eye would quickly flood with cooler, denser air from outside, raising the surface pressure in the center.

Unless the eye is itself a partial-vaccum created by centrifuging borne of rotational-deflection forces increasing by the square of velocity and preventing spiralling inflow winds from reaching the lowest central pressure zone (the eye) at all until the system weakens and "fills in".

[Wthrman13]

Without the sinking air in the eye to maintain the warm core, the eye would quickly flood with cooler, denser air from outside, raising the surface pressure in the center.

Unless the eye is itself a partial-vaccum created by centrifuging borne of rotational-deflection forces increasing by the square of velocity and preventing spiralling inflow winds from reaching the lowest central pressure zone (the eye) at all until the system weakens and "fills in".

Ok, I said I was going to bed, but oh well

But you still need to maintain that central low pressure somehow, so you have an inward-directed pressure gradient force to balance the centrifugal force of the air flowing around the eye (cyclostrophic balance, or gradient balance if you include the Coriolis force). Otherwise the eye would just unravel. That central low pressure is maintained hydrostatically by the warm core in the center of the cyclone, which in turn is maintained by the subsidence warming in the eye, which in turn is a response to the intense thermally direct convection in the eyewall, and so on...

Ok, now I'm going to bed

[SCUBAdude]

Thanks Wthrman13, I do understand exactly what you're saying and I can't believe it didn't click until you explained it. The air converging vertically above a given point was what I was missing. Air is not pilling up on itself above the eye even though there is subsidence. And when I think of subsidence I think of sinking air forced down by more air above it. That's what threw me. It's basically the same thing I've learned in dive physics about psi at a given depth and how that changes when diving at high altitude for the same depth because of the difference in atmospheric pressure above a given body of water.
Thanks again,
SD.

[mike18xx]

But you still need to maintain that central low pressure somehow, so you have an inward-directed pressure gradient force to balance the centrifugal force of the air flowing around the eye (cyclostrophic balance, or gradient balance if you include the Coriolis force). Otherwise the eye would just unravel. That central low pressure is maintained hydrostatically by the warm core in the center of the cyclone, which in turn is maintained by the subsidence warming in the eye, which in turn is a response to the intense thermally direct convection in the eyewall, and so on...

Low pressure is maintained by that which drew the whole darn thing together in the first place: sustained convective release of latent heat of condensation and expansion aloft creating a suction-draw at the surface; an eye merely appears when centrifugal force exceeds inflow velocity.