NASA: Hurricane Anatomy - Understanding Angular Momentum & Air Motion

[x_engineer]

TL;DR Summary

Two angular momentum impossibilities implied by the usual hurricane anatomy diagrams

Here is a NASA article on "Hurricane Anatomy"

https://earthobservatory.nasa.gov/features/Hurricanes/hurricanes_2.php
The first issue I have is the apparent reversal of the angular momentum when going from surface winds to the winds in the covering cloud cap.
In the northern hemisphere, surface winds are shown to be anti-clockwise when viewed from above, and increasing in intensity as one approaches the eyewall, and flowing towards the eyewall from outside. The wind then turns up, but remains anti-clockwise around the eye until it reaches the covering cap cloud layer, then starts flowing outward and decreasing in intensity. So far so good, everything is physical.

But then, every diagram I have seen shows the rotation flipping to clockwise before it gets to the edge of the cloud cap. Some even claim in the accompanying text that the cloud cap rotates clockwise. Does actual data show a clockwise rotation of the outer cloud cap? If so, what is the source of the clockwise torque to make this happen? Or is it just that people draw the right ward arrow denoting the decreasing angular velocity (not momentum!) too far to the right? If the point of the arrow still pointed a little bit in the anticlockwise direction things could be physical without a source of clockwise torque.

Assuming the above depiction is a mistake and the eyewall radius rotates anti-clockwise all the way to the "top" of the atmosphere, the second issue I have is the depiction of a downward flow in the center of the eye of the hurricane. This is inconsistent with calm conditions at the center of the eye. Since the source of this downward flow has anti-clockwise momentum, flows inward at the top, and there is nothing above to deliver a clockwise torque, the only possibility is increasing angular velocity as one approaches the center of the eye. However, an upward flow is consistent with calm conditions - this implies the surface flow is inward, but the angular momentum at the eyewall is bled away by friction with the surface.

So what is the true air motion? We are talking about the totally clear air in the eye of the storm and above the cloud cap,
and the direction of the cloud cap winds in the outer regions of the storm.

Does hurricane formation require pre-existing clockwise angular momentum about a latent eye in the upper atmosphere?

Niket Patwardhan

 

[jbriggs444]

You realize that angular momentum is not a conserved quantity in a rotating frame, right?

 

[x_engineer]

I forgot about that, thanks. I can see how a frame rotated 180° can cause the apparent rotation to flip.
(And for the outer cloud cap of a hurricane the timescale is also right). It might also explain the periodic rain bands.

Can we use the conservation of angular momentum inside the eyewall because those winds are generated over a much shorter timescale, so we can ignore the rotation of the frame?
(like ~2 rotations/hr instead of 1/day)

Then if the air above the cloud cap level at the eyewall is contra rotating we have an upper source of torque for killing rotation within the eye in a downdraft.

(I was thinking that the tradewinds might provide high altitude clockwise momentum with respect to an eye between them and the equator)

 

[anorlunda]

This diagram from Wikipedia suggests that al low altitudes, air flows into the center with CCW rotation. Then it rises to high altitudes and starts flowing outward with CW rotation. Does that make sense, reverse the flow direction and reverse the rotation too?

 

[TeethWhitener]

This diagram from Wikipedia suggests that al low altitudes, air flows into the center with CCW rotation. Then it rises to high altitudes and starts flowing outward with CW rotation. Does that make sense, reverse the flow direction and reverse the rotation too?

View attachment 249469

Yes, the inflow is CCW and the outflow is CW (in the Northern Hemisphere). Here’s a gif of Dorian where it’s relatively easy to see the CCW rotation of the main body of the storm and the CW rotation of the upper level cirrus clouds:
https://blog.weather.us/wp-content/uploads/2019/08/sat_20190831_0840_animation.gif

 

[Nugatory]

Does hurricane formation require pre-existing clockwise angular momentum about a latent eye in the upper atmosphere?

No. The Coriolis effect generates the necessary spin.

It also accounts for the why the outgoing and incoming winds rotate in opposite directions. In the northern hemisphere south-moving winds are deflected to the west and north-moving winds are deflected to the east. The incoming and outgoing winds are moving in opposite directions, so also are deflected in opposite directions.

 

[A.T.]

This diagram from Wikipedia suggests that al low altitudes, air flows into the center with CCW rotation. Then it rises to high altitudes and starts flowing outward with CW rotation. Does that make sense, reverse the flow direction and reverse the rotation too?

The image is a bit confusing, because it suggests that looking from the top you would see only the outflow cirrus shield rotating CW. But looking at the footage below, it seems that the rain bands extend beyond the cirrus shield and determine the visual impression of CCW rotation, while the cirrus shield doesn't have enough distinct features to determine the rotation direction.

Does anyone have footage where you actually see the opposite rotation of the cirrus shield?

 

[TeethWhitener]

Does anyone have footage where you actually see the opposite rotation of the cirrus shield?

Does the gif in post 5 not work? The thick cumulonimbus layer is the false color portion of that image, whereas the cirrus outflow can be seen as the wispy black and white portion on the outer edges of the false color portion. The video you posted actually shows the same thing. At the very beginning of the video, if you look at the eastern and southeastern edges of the rain bands, you can see a pretty clear clockwise rotation of the cirrus-level outflow. Keep in mind that, since it's an outflow, the rotation is not as pronounced as the CCW inflow.
Edit: actually, even better is right around 0:55 in the video you posted, on the eastern edge of the storm. You can see very clear banded cirrus/altocirrus clouds moving in a clockwise fashion.

 

[TeethWhitener]

Another reason it's more difficult to see the outflow than the inflow is that the outflowing air is much drier than the inflowing air (the warm humid surface air leaves most of its moisture behind in the storm as it cools and rises), so the outflow contains fewer clouds to show wind motion.

 

[A.T.]

Does the gif in post 5 not work?

Sorry. I missed the link. Indeed, it's more clear.

 

[Nugatory]

Does the gif in post 5 not work?

It works now that you've told me what to look for:

The thick cumulonimbus layer is the false color portion of that image, whereas the cirrus outflow can be seen as the wispy black and white portion on the outer edges of the false color portion. T

Silly, but I just wasn't seeing it until read this.

 

[x_engineer]

Not disputing that the diagrams show clockwise rotation in the upper layer clouds.

The question was how is it possible?
(Conservation of angular momentum implies if you start with CCW rotation you can't get to CW rotation without applying torque, and since the top of the atmosphere is vacuum, there cannot be any torque from there)

jbriggs444 gave me a useful reminder - angular momentum is NOT conserved in a rotating frame.
(And a frame attached to the Earth is a rotating frame! - 180° in 12 hours)

The question now is "what is the wind structure in the cloud free portion of the atmosphere above the cirrus cloudcap and within the eye?". Any meteorologist with access to weather simulation results should be able to answer that from the simulations, but you can't infer it from photos of an actual storm.