Hypothetical question about closed circulation

[Stormavoider]

A tropical system is is moving amongst an air mass that is moving east at 25 knots. There is an obvious surface circulation moving with it. At the northern most edge of this feature, surface winds are east at 45 knots. At the southern most edge the winds are calm (0).
Is this a closed LLC???

[vbhoutex]

Not if the winds are calm at the southern edge. A closed circulation requires wind movement in the correct direction for a low in all quadrants to be declared closed. It doesn't really have anything windwise to do with the air mass it is moving in/with. A TC moves through several different "air masses" during its' life time, but the definiton of closed circulation has to do with the TC itself. If someone knows better and can cite a source feel free to correct me, but that is my understanding of a closed circulation when one is talking about tropical cyclones.

[Stormavoider]

Not if the winds are calm at the southern edge. A closed circulation requires wind movement in the correct direction for a low in all quadrants to be declared closed. It doesn't really have anything windwise to do with the air mass it is moving in/with. A TC moves through several different "air masses" during its' life time, but the definiton of closed circulation has to do with the TC itself. If someone knows better and can cite a source feel free to correct me, but that is my understanding of a closed circulation when one is talking about tropical cyclones.

Thanks for the reply. That's what I have gathered from the experts' comments on this board. It doesn't seem quit right to me though.

[wxmann_91]

A tropical system is is moving amongst an air mass that is moving east at 25 knots. There is an obvious surface circulation moving with it. At the northern most edge of this feature, surface winds are east at 45 knots. At the southern most edge the winds are calm (0).
Is this a closed LLC???

That's a very good point there. That is not a closed LLC. And that is why fast-movering disturbances usually don't develop a closed center and become TD's.

[Wthrman13]

This question appears to be a source of endless confusion for several people on this board, and it is indeed easy to get confused by it. I have made posts in the past to attempt to try to clear it up. You seem to understand the gist of it. Basically, it boils down to the difference between storm-relative and ground-relative winds. A fast-moving storm can easily have a closed circulation with respect to it's own moving reference frame, but yet have no actual closed circulation with respect to the ground, if the storm-relative winds on the side opposing it's forward motion are in fact less than the actual forward motion. In essence, the ground relative winds are the only ones that matter to us as far as sensible weather at ground-level is concerned, and the NHC will always report and use ground relative winds when discussing a particular storm, and this includes whether or not they classify a storm based on a closed (again, ground-relative) circulation.

Storm-relative winds, on the other hand, are useful when describing how a storm "sees" its environment and in regards to its overall level of organization. Furthermore, when watching satellite images, we humans tend to automatically adjust unconciously to the storm's overall motion, or whatever other feature we are tracking, and thus our brains tend to interpret winds in the storm-relative framework. Thus, when I'm watching satellite images, and want to get an idea of the actual ground relative winds (at least as best as I can), I always try to focus on one small part of a storm and track the cloud motion relative to a fixed point on the ground (or sea), or a lat/lon grid if that's available on the image. It's a lot easier to do this if you aren't focused on the storm as a whole.

Hope that helps,

Dan

[Stormavoider]

Thank you! Great explanations. This makes it very clear how the NHC does or does not declare closed circulation. I feel though if "storm relative" closed circulation exists a system has reached a milestone in development irregardless of ground (open water) relativity. This hypothetical system now has the ability to wrap storm "components" around it's self. It now only needs to increase its axial speed beyond the storm relative speed to be declarable.

[Air Force Met]

This question appears to be a source of endless confusion for several people on this board, and it is indeed easy to get confused by it. I have made posts in the past to attempt to try to clear it up. You seem to understand the gist of it. Basically, it boils down to the difference between storm-relative and ground-relative winds. A fast-moving storm can easily have a closed circulation with respect to it's own moving reference frame, but yet have no actual closed circulation with respect to the ground, if the storm-relative winds on the side opposing it's forward motion are in fact less than the actual forward motion. In essence, the ground relative winds are the only ones that matter to us as far as sensible weather at ground-level is concerned, and the NHC will always report and use ground relative winds when discussing a particular storm, and this includes whether or not they classify a storm based on a closed (again, ground-relative) circulation.

Storm-relative winds, on the other hand, are useful when describing how a storm "sees" it's environment and in regards to it's overall level of organization. Furthermore, when watching satellite images, we humans tend to automatically adjust unconciously to the storm's overall motion, or whatever other feature we are tracking, and thus our brains tend to interpret winds in the storm-relative framework. Thus, when I'm watching satellite images, and want to get an idea of the actual ground relative winds (at least as best as I can), I always try to focus on one small part of a storm and track the cloud motion relative to a fixed point on the ground (or sea), or a lat/lon grid if that's available on the image. It's a lot easier to do this if you aren't focused on the storm as a whole.

Hope that helps,

Dan

I second this explaination. I basically said the same thing yesterday. We, as humans, tend to see storm-relative rather than ground relative.

Here is a tip. If you really want to see if a system has storm or ground relative circulation...pick out the individual cloud elements on a vis or IR-2 image (at night). Many times people say it has a closed LLC...but if you look at the individual elements...you can actually see the clouds south of the vorticity center still moving west...just real slow.

This is what I do. Take a fine pointed vis-a-vis and trace the coud elements on your screen. It wipes off as long as you don't leave it (make a test dot first...in a bottom corner!). Zoom in and follow an individual cloud. You'll be surprised at the number of systems you thought were closed that weren't.

NOAA had a VHS tape on this very phenom back in the early 90's. It was GREAT training. NOt sure if it is still around in a COMET form or not.