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Weather and Thunderstorm behavior

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Weather and Thunderstorm behavior

Postby wlbragg » Tue Jan 20, 2015 10:32 pm

First I want to say, I think it is things like FG's advanced weather systems that distinguishes us from other sims and that we should do what we can to build on and polish what we already have. The base for so many things in FG have been painstakingly laid out and it is up to current and future "power users" to put the polish on.

First, in simple terms, is turbulence pretty much just random x/y/z plane positioning by factor of slider or is something else?

Second, my main focus of this post.
When I use Thunderstorm setting in the weather drop-down, I get rain and some of what appear to be Cumulus clouds to about 6000 ft (basic weather). I don't see any Cumulonimbus clouds unless I use advanced weather, no problem, I'm just trying to determine if my weather is behaving as designed.
I shouldn't expect Cumulonimbus, cloud base and rain shaft in basic mode?

In advanced mode the Cumulonimbus clouds only appear to go to about 9300-9500 ft instead of the typical or achievable 50,000 ft.
Is this too difficult or expensive to model?

In advanced mode I also see a reasonably good representation of a cloud base and rain shaft. But I am getting a detachment of the rain shaft from the cloud base, a gap, (picture available if needed).
Is this EASILY correctable?

Is the visible rain shaft the actual rain falling from the cloud or a representation?

The description says
"A hot and damp summer day with thunderstorms developing in the late afternoon. Be prepared for reduction of visibility in showers and strong gusts near thunderstorms"
Understanding I can potentially model wind-shear with the advanced weather dialog.
Are there any of the typical updrafts, inflows, outflows, micro-bursts or wind-shear modeled into the wind patterns associated with these types of systems using this weather drop-down selection?

If not, regarding the last question, is it reasonably practicable to do so?

Thanks to all for your dedication and hard work.
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Re: Weather and Thunderstorm behavior

Postby daveculp » Wed Jan 21, 2015 1:42 am

The only full scale thunderstorm model that I know of is the AI thunderstorm, which is about 30K ft tall, but could easily be enlarged. It looks OK from about 50 or more miles away (or used to. It's been years since I looked at it and Flightgear ' s haze and LOD rules have changed over time) but up close it looks pretty cheesy. It had lightning and turbulence, but I don't know if that still works. I was hoping someone would figure out a way to make something that big look OK up close without resorting to thousands of vertices, but maybe that's what it'll take to improve the looks.
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Re: Weather and Thunderstorm behavior

Postby wlbragg » Wed Jan 21, 2015 2:12 am

I never thought to try that one, I'll give it a try.
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Re: Weather and Thunderstorm behavior

Postby radi » Wed Jan 21, 2015 7:54 am

wlbragg wrote in Tue Jan 20, 2015 10:32 pm:First, in simple terms, is turbulence pretty much just random x/y/z plane positioning by factor of slider or is something else?

I'm not sure I understand your question completely. Assume zero winds, no turbulence, and an airplane in unaccelerated flight. Then, with respect to the wing, the oncoming air has basically constant velocity (== TAS) and angle, which generates constant lift and drag forces that balance weight and thrust. The sum of forces is zero, there is no acceleration, the airplane continues to travel at a constant velocity.

If there's winds but no turbulence, the air's velocity (with respect to Earth) is non-zero but constant and the resulting forces are also constant, resulting in a smooth ride. However, turbulence means the air's velocity is unsteady. Even though the airplane might be travelling at (an approximately) constant speed with respect to Earth, the air's velocity with respect to the wing will then also be also unsteady, causing unsteady lift and drag forces. Since weight and thrust stay approximately constant, the sum of forces is no longer zero and the resulting accelerations move the airplane around.

I don't know what our FDMs are actually doing to model turbulence, but I can imagine they superimpose random fluctuations (maybe with a certain spectrum) on the TAS vector.
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Re: Weather and Thunderstorm behavior

Postby Thorsten » Wed Jan 21, 2015 11:44 am

First, in simple terms, is turbulence pretty much just random x/y/z plane positioning by factor of slider or is something else?


Turbulence is handled by the FDM, so it's quite different between JSBSim and YaSIm. I understand that at least the JSBSim turbulence model is quite complex and state-of-the-art (or so Jon told me).

I shouldn't expect Cumulonimbus, cloud base and rain shaft in basic mode?


No, I don't think basic weather has any Cb definitions - they're fairly tricky to assemble.

In advanced mode the Cumulonimbus clouds only appear to go to about 9300-9500 ft instead of the typical or achievable 50,000 ft.


There's three types of Cb-towers defined - small, forming ones, medium active ones and large decaying ones.

That's two small ones:

Image

And a large one in the distance:

Image

The medium variant goes up to 28.000 ft or so, I think the large ones even higher (though I think not up to 50.000). In general, they're all on the expensive side (using 1000+ quads each), which means before the cloud LOD system dropping quads they were designed with the bare minimum of quads possible. Which means if you do cloud LOD (which we all have set by default) they appear ragged and a bit mutilated.

Generally, modeling Cb is a thankless business, as half of the people end up complaining that the clouds don't look realistic, and the other half that the scenario is too slow to fly it. For the same reason, I don't think the big Cb tower appears often in the weather system as it stands.

In advanced mode I also see a reasonably good representation of a cloud base and rain shaft. But I am getting a detachment of the rain shaft from the cloud base, a gap, (picture available if needed).
Is this EASILY correctable?


Don't think so... Probably one needs to rework the whole strategy and dynamically generate a quad of the right size for the rain. Trying to squeeze a fixed size model into the variable gap between cloud and terrain isn't a very good strategy. If the rain is detached from the base, it's not that bad - if it separates from the ground, that looks really odd... So I think it priorizes touching the ground.

Is the visible rain shaft the actual rain falling from the cloud or a representation?


What do you mean with 'actual falling rain'? We don't have any actual falling rain modeled anywhere in FG, it's always conjurer's tricks.

Are there any of the typical updrafts, inflows, outflows, micro-bursts or wind-shear modeled into the wind patterns associated with these types of systems using this weather drop-down selection?


Some - not many though. Updrafts and turbulence mainly, I think if they're part of a front, then winds correlate a bit.

If not, regarding the last question, is it reasonably practicable to do so?


Yeah. I guess the bottomline is that Cb clouds and T-storm weather scenarios could use some love - they're really quite old and could be reworked using a few more modern tools of the weather system and tuning of course. It's probably a weekend worth of effort to bring this up to standards.
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Re: Weather and Thunderstorm behavior

Postby Johan G » Wed Jan 21, 2015 1:52 pm

wlbragg wrote in Tue Jan 20, 2015 10:32 pm:Is the visible rain shaft the actual rain falling from the cloud or a representation?

Thorsten wrote in Wed Jan 21, 2015 11:44 am:What do you mean with 'actual falling rain'? We don't have any actual falling rain modeled anywhere in FG, it's always conjurer's tricks.

I do not think I have to guess that "actual falling rain" would be pretty expensive... :wink:
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Re: Weather and Thunderstorm behavior

Postby wlbragg » Wed Jan 21, 2015 9:10 pm

Thank guys, excellent replies.

actual falling rain

When flying through rain in general, from the cockpit I see individual rain drops (thousands of them). Not knowing or understanding how those are created, they appear to be individual raindrops. If they were and if you saw that from a distance, you might see a "rain shaft". Obviously, your answers indicate that in the case of the thunderstorm rain shaft that is some kind of a semi-transparent model.

LATE EDIT:
cloud LOD (which we all have set by default)

Is this in the c source or am xlm definition?

Using the Weather dialog I get this next image.
Are the hard lines on the rain shaft preventable?
Image

I tried both bigstorm.xml and big_thunderstorm.xml in fgdata\Models\Weather.

bigstorm.xml gave me
Image

big_thunderstorm.xml gave me
Image

I haven't had any time to look at the code and see if I could figure out what is wrong. But either I have a system problem or the code's mucked up.

Just for something interesting to see.
Back a couple years ago they had the Norman, OK tornado that caused so much devastation. Here in Kansas some of us take our weather seriously. I have a few tools I run on those potentially violent days to try and keep my family safe. One of which is a program called AWeather. On that day I was watching the sky and capturing radar data. I put together a gif of the radar indicated tornado, both reflectivity and wind.
Reflect, no tornado path
Reflect, with tornado path
Wind, no tornado path
Wind, with tornado path

There is a mode the radar can be set to that can display a debris ball it the debris ball is large enough. I think I saved at least an image somewhere. I'll see if I can find it and post it.
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Re: Weather and Thunderstorm behavior

Postby wlbragg » Thu Jan 22, 2015 6:46 am

Just a side note, the AI scenario generated thunderstorms movement worked well. They drift through the area nicely. I can envision some individual cloud morphing and cell decline and regeneration.

While I still would like to get the user created AI Thunderstorms to look as good as the Advanced Weather generated ones, using the advanced weather to generate thunderstorms has worked much better, graphically anyway. But there is an issue I'm not sure of.

Are the rain shafts designed to be under the cloud?
Image

This one is more of what I expect a Cumulonimbus to look like. I have yet to see the anvil style one.
Image
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Re: Weather and Thunderstorm behavior

Postby Thorsten » Thu Jan 22, 2015 7:51 am

When flying through rain in general, from the cockpit I see individual rain drops (thousands of them). Not knowing or understanding how those are created, they appear to be individual raindrops. If they were and if you saw that from a distance, you might see a "rain shaft".


Yeah, that's a particle system putting a series of billboards into a small curtain around the camera. There's no seeing that from the distance, that's simulating rain 'here' and even if a rain front were to start 10 meters from the camera you'd see nothing as long as the camera is still dry. Conjurer's tricks - all of it.

Are the rain shafts designed to be under the cloud?


Broadly speaking yes. There's small print to it though.

Quoting myself:

Probably one needs to rework the whole strategy and dynamically generate a quad of the right size for the rain. (...) I guess the bottomline is that Cb clouds and T-storm weather scenarios could use some love - they're really quite old and could be reworked using a few more modern tools of the weather system and tuning of course.


Is this in the c source or am xlm definition?


Property tree in /sim/rendering/cloud-*

Are the hard lines on the rain shaft preventable?


Not without a major effort. You're seeing a textured sheet intersecting with a mesh - that mathematically results in a line. You can blur the line by jiggling with the texture - if you know at every time for every sheet how far the lower edge is above the terrain. Or you can render volumetric rain distributions - which will suck up a large chunk of performance - more than volumetric fog in fact.
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Re: Weather and Thunderstorm behavior

Postby wlbragg » Fri Jan 23, 2015 9:06 pm

I've dug through the Advanced Weather Code, found most of the pertinent pieces and have a few more questions.

In the image I posted above, (bigstorm.xm), is the Cumulonimbus cloud model displaying correctly or is it broke?
If it is correct then I suggest we get rid of that model and replace it with one of the three Cb cloud assemblies already available in advanced weather.
Can that be done?

Any idea why the Cumulonimbus model in the second image I posted, (big_thunderstorm.xml), is behaving so badly?

The typical AI Scenario has the following params
Code: Select all
<type>thunderstorm</type>

<model>Models/Weather/bigstorm.xml</model>
or
<model>Models/Weather/big_thunderstorm.xml</model>

<latitude>37.716</latitude>
<longitude>-96.976</longitude>
<speed>30.0</speed>
<altitude>5000.0</altitude>
<heading>49.0</heading>
<diameter-ft>21000.0</diameter-ft>
<height-msl>38000.0</height-msl>
<strength-norm>1.0</strength-norm>


bigstorm.xml and big_thunderstorm.xml are calling select animations on their respective .ac models.
bigstorm.xml aoppears to be static and big_thunderstorm.xml has nasal driving its motion.

Is there a way we can use the Advanced Weather System's way of creating or "assembling" a Cb storm but with the AI Scenario front end?
Kind of a mix and match, instead of calling the select animation on a model, use nasal to assemble an Advanced Weather Cb and animate it with nasal using the same xml params the scenario has available now?

Is the Advanced Weather are Cb storms moving through the area or are they static?
If moving, what was the criteria used for direction, size, strength, etc. Is that done with a clamped randomization of the necessary params?
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Re: Weather and Thunderstorm behavior

Postby daveculp » Sat Jan 24, 2015 4:11 am

The big storm model was made in the pre-OSG days, when transparency worked differently. It used to look better, but never looked great.
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Re: Weather and Thunderstorm behavior

Postby Thorsten » Sat Jan 24, 2015 8:26 am

Is there a way we can use the Advanced Weather System's way of creating or "assembling" a Cb storm but with the AI Scenario front end?


No - clouds aren't models these days, they're 'just' geometry to the renderer. Stuart made them that way because it's a factor 10 faster than introducing them as models. But that means they don't do model-specific things (collision tests, animations, property control...)

Is the Advanced Weather are Cb storms moving through the area or are they static?


They move with the current windspeed like any other cloud. Cb clouds are no different from almost all other cloud types (except Cirrus). In fact, the main problem with the rain layers is that they're /not/ clouds but models, so they behave differently (and don't co-move with the clouds).

If moving, what was the criteria used for direction, size, strength, etc. Is that done with a clamped randomization of the necessary params?


Sorry, I don't understand the question.

Clouds move with the windspeed at the current airplane position (note that they don't ever move with the actual windspeed at their position - you can work out yourself that this'd shear a Cb apart in a few minutes, because in reality it isn't subject to external winds, it makes its own wind system, dynamically stabilizing it, which is way too complicated for us to model).
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