The eye like (on visible) feature of certain intense Nor'Easters.

[Ed Lizard]

OK, basic explanation, what causes an eye like feature (which looks nowhere near as good on IR, as its mostly low clouds to the South) on certain intense Nor'Easters?

[am19psu]

OK, basic explanation, what causes an eye like feature (which looks nowhere near as good on IR, as its mostly low clouds to the South) on certain intense Nor'Easters?

The same thing that causes it in hurricanes - cyclostrophic balance... i.e. the balance between the centrifugal force and the pressure gradient force

[LocoAko]

I believe that is the warm seclusion stage of a Shapiro-Keyser style cyclone. Don't know too much about this, but here is some relevant reading:

http://en.wikipedia.org/wiki/Extratropical_cyclone#Warm_seclusion

[isohume]

Hurricane eyes are formed through mostly dynamical processes...ie: downward moving air (subsidence warming) from the anticyclone aloft.

[am19psu]

Hurricane eyes are formed through mostly dynamical processes...ie: downward moving air (subsidence warming) from the anticyclone aloft.

The secondary circulation only forms once cyclostrophic(ish) balance has been achieved

[isohume]

The secondary circulation only forms once cyclostrophic(ish) balance has been achieved

Cyclostrophic flow is assumed in small-scale events such as tornadoes, water spouts, dust devils, etc. Cyclostrophic balance assumptions throw out friction and Coriolis.

That's not the case with a large tropical system, esp as it moves moves north. The strengthening anticyclone aloft is a response to the deepening convection...the anticyclone then creates a positive feedback loop by forcng llvl air into the convection and also creating a subsidence zone at it's center...or relatively large scale downward motion.

[am19psu]

Cyclostrophic flow is assumed in small-scale events such as tornadoes, water spouts, dust devils, etc. Cyclostrophic balance assumptions throw out friction and Coriolis.

That's not the case with a large tropical system, esp as it moves moves north. The strengthening anticyclone aloft is a response to the deepening convection...the anticyclone then creates a positive feedback loop by forcng llvl air into the convection and also creating a subsidence zone at it's center...or relatively large scale downward motion.

Sure, as it gains latitude. At 10N, though, Coriolis is negligible. Obviously, convection is playing a role in warming the eye, deepening the system and generating positive vorticity below max warming and negative vorticity above (WISHE stuff), but without cyclostrophic(ish) balance, the secondary circulation would never set up.

[isohume]

Sure, as it gains latitude. At 10N, though, Coriolis is negligible. Obviously, convection is playing a role in warming the eye, deepening the system and generating positive vorticity below max warming and negative vorticity above (WISHE stuff), but without cyclostrophic(ish) balance, the secondary circulation would never set up.

I think you're confusing the term cyclostropic balance with other balanced forces. Cyclostropic balance refers to small scale, short-lived events...where the rotation doesn't have to be cyclonic...ie: no Coriolis and no friction on those scales are assumed.

http://itg1.meteor.wisc.edu/wxwise/AckermanKnox/chap6/cyclostrophic.html

[am19psu]

I think you're confusing the term cyclostropic balance with other balanced forces. Cyclostropic balance refers to small scale, short-lived events...where the rotation doesn't have to be cyclonic...ie: no Coriolis and no friction on those scales are assumed.

http://itg1.meteor.w...lostrophic.html

That's why I'm saying -ish. Yeah, it's technically gradient wind balance, but in the incipient stages of tropical cyclogenesis, the Coriolis term is an order of magnitiude less than the PGF and CF and so when you do the math, it's negligible. It's the same in the core of a warm seclusion as above - PGF and CF are about an order of magnitude larger - so yes, it is technically gradient wind balance, but you don't get clouds in the "eye" because of the large centrifugal force near the center.

[isohume]

That's why I'm saying -ish. Yeah, it's technically gradient wind balance, but in the incipient stages of tropical cyclogenesis, the Coriolis term is an order of magnitiude less than the PGF and CF and so when you do the math, it's negligible. It's the same in the core of a warm seclusion as above - PGF and CF are about an order of magnitude larger - so yes, it is technically gradient wind balance, but you don't get clouds in the "eye" because of the large centrifugal force near the center.

I always thought the secondary circulation in a hurricane was created by the conversion of heat energy to mechanical energy...much like a heat engine.

The convection rises (heat energy)...hits the trop, spirals out (mech energy)...with some measure of heat energy lost to space as well. Then the cooler/denser air begins to sink in the center, where the outward spiraling is minimal, creatng the secondary circ aided by increasing subsidence from the developing anticyclone aloft...leading to a warm core cloud free region. And as long as the incoming heat energy is greater than the heat energy lost...the heat engine (hurricane) will continue.

[am19psu]

I always thought the secondary circulation in a hurricane was created by the conversion of heat energy to mechanical energy...much like a heat engine.

The convection rises (heat energy)...hits the trop, spirals out (mech energy)...with some measure of heat energy lost to space as well. Then the cooler/denser air begins to sink in the center, where the outward spiraling is minimal, creatng the secondary circ aided by increasing subsidence from the developing anticyclone aloft...leading to a warm core cloud free region. And as long as the incoming heat energy is greater than the heat energy lost...the heat engine (hurricane) will continue.

You are absolutely correct, that is the positive feedback mechanism (WISHE). But why does the subsidence focus in the center and why does a coherent secondary circulation exist? That's where the gradient wind balance comes in.

[isohume]

You are absolutely correct, that is the positive feedback mechanism (WISHE). But why does the subsidence focus in the center and why does a coherent secondary circulation exist? That's where the gradient wind balance comes in.

I can see your point. With gradient balance and the pgrad inward...the net circulation will be outward from the center and the mass will be forced outward as well due to centrifugal forces. So the eye formation is a well coordinated combination of dynamic and thermal processes, which makes sense.

[Ed Lizard]

I believe that is the warm seclusion stage of a Shapiro-Keyser style cyclone. Don't know too much about this, but here is some relevant reading:

http://en.wikipedia....#Warm_seclusion

I have heard rumors that a famous HPC met has done much of the Wiki writing about weather. Something resembling a warm core structure would seem involved.

Warm seclusion

A warm seclusion is the mature phase of the extratropical cyclone lifecycle. This was conceptualized after the ERICA field experiment of the late 1980s, which produced observations of intense marine cyclones that indicated an anomalously warm low-level thermal structure, secluded (or surrounded) by a bent-back warm front and a coincident chevron-shaped band of intense surface winds.^[28] The Norwegian Cyclone Model, as developed by the Bergen School of Meteorology, largely observed cyclones at the tail end of their lifecycle and used the term occlusion to identify the decaying stages.^[29]

Warm seclusions may have cloud-free, eye-like features at their center (reminiscent of tropical cyclones), significant pressure falls, hurricane force winds, and moderate to strong convection. The most intense warm seclusions often attain pressures less than 950 millibars (28.05 inHg) with a definitive lower to mid-level warm core structure.^[28] A warm seclusion, the result of a baroclinic lifecycle, occurs at latitudes well poleward of the tropics.

As latent heat flux releases are important for their development and intensification, most warm seclusion events occur over the oceans; they may impact coastal nations with hurricane force winds and torrential rain.^[27][30] Climatologically, the Northern Hemisphere sees warm seclusions during the cold season months, while the Southern Hemisphere may see a strong cyclone event such as this during all times of the year.

In all tropical basins, except the Northern Indian Ocean, the extratropical transition of a tropical cyclone may result in reintensification into a warm seclusion. For example, Hurricane Maria of 2005 reintensified into a strong baroclinic system and achieved warm seclusion status at maturity (or lowest pressure).^[31]