Hail Growth Trajectory - Myth?

[Mike Marz]

This tweet - https://twitter.com/VORTEXJeff/status/1030151540361834501 - was posted in the ST Discord this evening and brought up the discussion of hail stone growth / trajectory through a storm. Many of us have always thought, or been told, that stones can cycle up and down through an updraft - giving the layered rings appearance. Having seen hail stones of all shapes, sizes, and appearances, I am still not 100% sure on the trajectory or processes behind hail growth. I've seen the layered appearance, I've seen no layers at all. I've seen clumped together stones that look like 10 or 11 quarter sized pieces clumped together to make a jagged baseball sized stone. I've seen hail that bounces and hail that is soft and just explodes upon impact to pavement. So, aside from seeing hail while chasing, I have no scientific educational expertise whatsoever in this matter. I just wanted to throw this out to the forum to see what others think. Is it a myth that hail cycles up and down in the storm? Is it a single trajectory?

 

[Mark Blue]

I’ve seen the same things as you have Mike regarding the variety of different hail stones. I was taught that hail stones get caught in the updraft and keep growing in size until the updraft can no longer sustain the weight of the hail stone and it falls to earth. Sounds like some scientists have a new theory about the process. Thanks for sharing!

 

[Warren Faidley]

I would suspect there are a number of processes involved, not just one. I recall the fighter pilot who bailed out in a severe storm and how he was carried through the storm. What amazes me is how rapidly some developing storms can produce damaging hail.

 

[Marc R. O'Leary]

I would suspect there are a number of processes involved, not just one. I recall the fighter pilot who bailed out in a severe storm and how he was carried through the storm. What amazes me is how rapidly some developing storms can produce damaging hail.

Warren - Do you have a link to said fighter pilot story?

 

[Mark Blue]

I think it’s here Marc: http://www.weatherimagery.com/blog/ejects-into-thunderstorm/

Please correct me if I’m wrong Warren. Thanks.

 

[Dan Robinson]

Honestly, this is the first time I've ever heard this being challenged as a myth, and I consider myself to be well-read in severe storms scientific literature. The "recirculating" idea never seemed to be that well defined, and has the hallmarks of of a myth like so many we have in the realm of weather and science (oversimplified, no source). It's probably more of a colloquialism like "tornado on the ground" or "power flashes are all exploding transformers" that has never been so publicly challenged until now. If it has been previously, I sure don't remember seeing it.

 

[Warren Faidley]

So I'm confused. If a hailstone travels upward through the storm, it has to fall at some point. When it falls it could accumulate additional mass from encountering freezing precipitation, or in some cases the mass is reduced from encountering a warmer environment. So in theory, the hailstone could gain or lose mass depending on the environment but it's only a two way trip?

 

[Mark Blue]

After reading Lt Col Rankin’s experience in the up/downdraft cycle what stuck with me was his description of going through a multitude of these cycles. The link in the article about the hang glider, Ewa Wisnierska, paralleled what Rankin experienced to a large extent, with length of time tumbling up and down dependent on the updraft strength.

This certainly doesn’t answer Warren’s questions, it’s more or less me thinking out loud. We need to have Felix Baumgartner deploy in an updraft with the appropriate gear to solve the mystery!

 

[Dan Robinson]

Here are a couple of relevant papers. To summarize, current thought is that the updraft ingests an "embryo" in the form of a small frozen droplet that originates in another part of the storm, then the hailstone grows/adds layers throughout its upward and then downward trajectory. So in a sense, there is at least one "recirculation" cycle happening in that description. The idea of the hailstone being suspended for an interval of time by the updraft in one region of the cloud is also supported in the literature, which would indicate at least some minor cycling up and down on the small scale at the top of its trajectory (I picture it like those suspended beach-ball-above-a-ducted fan displays you see at science centers). So, there's no large-scale cycling as in a hailstone traveling thousands of feet up and down repeatedly, but it's also not a simple, single parabolic trajectory.

Based on that, I'd label the myth as "partly true".

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JD022004

https://journals.ametsoc.org/doi/full/10.1175/MWR-D-16-0027.1

 

[Jeff Duda]

See also https://twitter.com/wxJeffDuda/status/1030637576976097280

And really, it makes sense that a hailstone does not repeatedly exit the updraft, fall a great distance, then re-enter the updraft. Perhaps for any given stone this happens once or maybe twice, but such occurrences are likely to be coincidental or serendipitous (if you see it that way). The reason is that it would be hard to explain such a constant large scale hydrometeor circulation within a storm. What process is going to cause a hailstone to repeatedly re-enter the updraft? Once a stone is ejected from the updraft it necessarily will possess momentum carrying it away from the updraft. The only way it could get back into the updraft is through storm-relative flow, but storm-relative flow is not isotropic (with the exception of Beltrami flow, which is never realized IRL). Only storm-relative flow that points towards the updraft can result in an ejected hail stone re-entering the updraft. Stones ejected in a region where storm-relative flow does not point towards the updraft have no chance of re-entering. In a typical severe weather environment, there might be levels within the lowest 3-4 km where storm-relative winds point back towards the updraft (there's probably no point in considering stones moving in and out above the freezing level), but those storm relative winds are typically not very strong...maybe 20-30 kts on average (larger in cases of extremely strong and curved wind profiles), and again, this may only occur in a pretty limited set of levels and azimuthal directions around the updraft. Not only does this configuration result in a small probability of any single stone being recirculated, but such storm-relative winds are not likely strong enough to push that many stones back into the updraft, especially once they become large (due to both higher horizontal inertia and terminal fall speeds). And even if a stone does make it back towards the updraft, it needs to get into the core of the updraft where upward velocities are strong enough to stop its descent and then accelerate it back upward, meaning there is a pretty narrow region where this can occur.

Is it possible for stones to recirculate a few times? I'm sure it is, but again, such occurrences would be by chance. Perhaps some of the largest stones that fall are those for which this actually happens, but this is not a dominant growth mechanism.