"Storm-relative" vs. "Surface-relative"

[Wthrman13]

I just looked at a visible satellite loop off of the NASA GHCC Satellite image web site:

http://weather.msfc.nasa.gov/GOES

I like to look at these loops with at least 25 images sped up to 5 frames per second or so, because it's a lot easier to see the locations of weak low-level circulations that way, at least to me. The eye is better able to pick up movement when the animation is more fluid.

Anyway, what I noticed immediately is that there is indeed a low-level circulation center associated with the remnants of Earl, RELATIVE to the movement of the wave axis. That is, if you subtracted the forward movement of the wave, the circulation would almost certainly be closed. You can see this "circulation" on the west side of the convection. Towards the end of the loop, as the sun is lowering, convection starts to erupt on the NE side of it.

Now, since the system is moving so fast to the west, the circulation is not closed with respect to the surface of the ocean. That is, if I were sitting out in a ship as this circulation center passed, the winds would continually have an easterly component, and would never shift to the west at any point. The most that would happen is that the easterly winds would weaken somewhat as the center approached, and pick up again as it passed.

This is where the ambiguity arises when the NHC classifies tropical cyclones without having good in situ observations. For weak systems such as this, it could have all the appearance of a tropical cyclone from satellite imagery, but due to the definition of a TC including a closed circulation center with respect to the surface, if it's moving too fast, it may not have one, and thus, not technically be a TC. However, I am convinced that the storm itself could "care less" how fast it is moving, and will still behave like a TC regardless. And, to those in the path of such storms, it makes little difference whether one is getting 45 mph winds from the east on the north side of an open wave, vs. 45 mph winds from the east on the north side of a named tropical storm. As I mentioned on another thread that's now hopelessly buried, it's the storm-relative environment that is most important to a storm's subsequent evolution. Really, using a definition of a closed circulation relative to the surface as a necessary condition for a TC is a matter only of convenience and consistency, and, in my opinion has little actual meteorological/physical relevance. Does anyone disagree? If so, please offer your comments.

P.S. I'm aware that fast-moving TC's such as this one (was) are prone to shearing effects because of the strong low-level easterly flow undercutting otherwise favorable mid-to-upper level flow, but given a scenario where the flow at all levels is fast and easterly, so that the *shear* is minimal, one could still h ave a situation where a system would not be classified as a TC simply and only because of it's forward movement causing it to not have a closed low-level circulation relative to the surface.

[BreinLa]

Found this on the web
http://mywebpages.comcast.net/wxman0071 ... lative.gif

[OtherHD]

That link bre posted is a great pictoral reference to what wthrman13 was talking about in his original post. I know the topic has been discussed a bit here before but not at length. This would be a great chance to get some additional pro input about whether NHC should just go by storm-relative circulation, or continue going with surface-relative. I'm too ill-informed to make a good opinion about the matter.

[Stormsfury]

The idea is relative to why that the NE quad on storms in the Northern Hemisphere is the more powerful side (and that IS factored in the maximum sustained winds) ...

http://www.aoml.noaa.gov/hrd/tcfaq/D6.html

[USAwx1]

I just looked at a visible satellite loop off of the NASA GHCC Satellite image web site:

http://weather.msfc.nasa.gov/GOES

I like to look at these loops with at least 25 images sped up to 5 frames per second or so, because it's a lot easier to see the locations of weak low-level circulations that way, at least to me. The eye is better able to pick up movement when the animation is more fluid.

Anyway, what I noticed immediately is that there is indeed a low-level circulation center associated with the remnants of Earl, RELATIVE to the movement of the wave axis. That is, if you subtracted the forward movement of the wave, the circulation would almost certainly be closed. You can see this "circulation" on the west side of the convection. Towards the end of the loop, as the sun is lowering, convection starts to erupt on the NE side of it.

Now, since the system is moving so fast to the west, the circulation is not closed with respect to the surface of the ocean. That is, if I were sitting out in a ship as this circulation center passed, the winds would continually have an easterly component, and would never shift to the west at any point. The most that would happen is that the easterly winds would weaken somewhat as the center approached, and pick up again as it passed.

This is where the ambiguity arises when the NHC classifies tropical cyclones without having good in situ observations. For weak systems such as this, it could have all the appearance of a tropical cyclone from satellite imagery, but due to the definition of a TC including a closed circulation center with respect to the surface, if it's moving too fast, it may not have one, and thus, not technically be a TC. However, I am convinced that the storm itself could "care less" how fast it is moving, and will still behave like a TC regardless. And, to those in the path of such storms, it makes little difference whether one is getting 45 mph winds from the east on the north side of an open wave, vs. 45 mph winds from the east on the north side of a named tropical storm. As I mentioned on another thread that's now hopelessly buried, it's the storm-relative environment that is most important to a storm's subsequent evolution. Really, using a definition of a closed circulation relative to the surface as a necessary condition for a TC is a matter only of convenience and consistency, and, in my opinion has little actual meteorological/physical relevance. Does anyone disagree? If so, please offer your comments.

P.S. I'm aware that fast-moving TC's such as this one (was) are prone to shearing effects because of the strong low-level easterly flow undercutting otherwise favorable mid-to-upper level flow, but given a scenario where the flow at all levels is fast and easterly, so that the *shear* is minimal, one could still h ave a situation where a system would not be classified as a TC simply and only because of it's forward movement causing it to not have a closed low-level circulation relative to the surface.

Absolutely awesome post, and VERY well written also!

[BreinLa]

Bump I find this so interesting I hated to see it get lost

[USAwx1]

Bump I find this so interesting I hated to see it get lost

yes i agree wthrman13's post was a HIGHLY informational, very useful and well articulated post.

[Wthrman13]

well, thank you