modeling thermals for simulation

[flyzguy]

I've been researching thermal modeling techniques for development of various soaring simulations. Much of the recent work done so far by Allen at NASA and others from the 80's involving taking humidity measurements across the ocean have provided a good groundwork. This work, along with convective layer meteorology, does a pretty good job of quantifying thermal size, strength, lift distribution shape, spacing, and probability over the course of a year. The one thing I feel is lacking from most of what I have found is the ability to model thermals of differing shapes, moving with ground velocity, and the inflow/outflow at different heights. I have found a lot of very good descriptive work to answer the question: how fast do thermals move? how long do they last? where does air enter and leave a thermal? But not a lot of quantitative research. For simulation purposes, I think it's especially important to get the relationship between thermal ground speed and prevailing wind speed. Does anyone know of any measurements taken of this? It would be great, for example, if all the pilots at a soaring contest used data loggers. If you don't know about any data, maybe you could just share qualitatively what you believe from experience as to thermal movement. Do they stay anchored? Do they move at a fraction of the prevailing wind speed?

[vintage1]

Pretty sure that the general large ones move at the prevailing wind speed.

But small localised ones over e.g. parking lots stay fixed..


WE always know that if a steady wind dies back, ones coming. And when the wind speeds up even more, sinks coming. I mentioned this to a pilot friend and he confirmed it. Model leapt up a couple of feet.


Oddly enough, the birds know this too. There is a distinct change in birdsong...

[MCarlton]

Would a thermal in a wind follow the "ream of paper" analogy, in that the upper portion of the thermal would move further in a sort of exponential manner, whilst the formation point may remain static?

It would also make sense that if a thermal has a boundary, that cool air would be sucked into the thermal through the pressure differential between warm and cool air. Higher in altitude, as the thermal loses heat, this effect would be lessened I assume.

That would, to me, indicate that there would also be an effect whereby cool air at altitude would be sucked down into the core of the thermal. So inside the thermal, would that cold air from altitude would be colder than the air outside the thermal at lower altitude, thus there could be a pressure differential at the very base of the thermal, which could cause a cold air mass to be ejected from the thermal (hence the heavy "sink" on the thermal boundary. Could it also be possible that any cold air drawn in from high altitude, could also retain its lower density and thus contribute to the apparent "sink"?

In my minds eye, thermals "look" more akin to a non revolving tornado like a funnel, with the base more or less anchored, with the upper portion blowing away, much like the way smoke appears to drift from a chimney.

[BMatthews]

There's so many forms of lift that most sailplane fliers have encountered that to model them all would take much of the rest of your life.

But the standard classic sort of bubble that forms from a hot spot on the ground and then breaks away from the ground and forms a rising tall, slanted torus of warm air drifts at the wind speed. The size and final specific shape seems to depend on how the thermal was made and how much energy it has stored at the time of the break.

[Cory]

I'm far from an expert on this, but here's my thoughts. Since all cumulus clouds are formed by thermals, and cumulus clouds can travel great distances from where they are first formed, I'd suspect that they often do not stay attached to the hot spot that formed them. However, I could see a possibility that a cloud may recieve energy from many successive thermals and not just from the initial one. Any thoughts on this?

[BMatthews]

I'm far from an expert on this, but here's my thoughts. Since all cumulus clouds are formed by thermals, and cumulus clouds can travel great distances from where they are first formed, I'd suspect that they often do not stay attached to the hot spot that formed them. However, I could see a possibility that a cloud may recieve energy from many successive thermals and not just from the initial one. Any thoughts on this?


An emphatic YES on all counts. Also there's a very wide variation on thermal strength as you can well imagine. And I've been under the clouds formed by them and when you stare at the sky long enough you'll see that they grow, move sideways in response to other close by and stronger thermals, join with other clouds, grow, shrink, sometimes split and finally fade away...... typically about 2 minutes before the time you're looking for..... :D

[flyzguy]

a lot of the literature I have read have suggested this thermal joining idea:
http://www.rcsoaring.com/docs/thermals_2006.pdf

The attractive force between two thermals that join would seem to suggest low pressure, which would also mean in-flow. If two thermals were both ejecting air outward from their cores, it would be hard to image them being attracted to one another.

[BMatthews]

Remember that there's a whole circulation of the air, not JUST the rising air. If you've ever seen the temperature generated systems that occur and interact in a bowl of miso soup you'll have a better appreciation for what is going on in a big region of air that is full of thermals.

And don't forget that while there may be high altitude thermals occuring and "flying" by there can be other things happening at lower levels underneath the higher level activity. I've flown in lower level conditions that up to around 500'ish feet had absolutely nothing at all in common with the clouds shrinking and swelling and joining that were occuring up at 1500 and up feet.

ANY model for thermal activity you come up with will be at best a simplistic version of the real activity happening in the air on any given day.

Yes there is an inrush at the bottom of a thermal as the air is drawn into the sharply rising warm core. This air rises and if there's enough energy in the "column" it warms the inrushing air and the thermal grows and expands both in horizontal and vertical size. Eventually they loose contact with the ground and it seems like that is when they move into their middle life period and rise as a single thermal and may join with other thermals. But eventually they spread so much they lose their energy density or the temperature falls without the ground warmed inrush and they fizzle out.

The inner core is rising but the air that spills out the top and cools in the higher levels falls back down the outside of the warm core. This is why there's typically downdrafts in the region of a thermal that can be as strong as the rising core.