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AMSL vs. AGL in Manual Configuration

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AMSL vs. AGL in Manual Configuration

Postby gierschi@flightgear » Fri Mar 04, 2016 4:07 pm

Hey guys,

let's assume i am flying over a mountain with a height off 5000m AMSL and specify the following values in the Basic weather dialog:
boundary layer (AGL):
0ft 3kt
500ft 6kt

aloft layer (AMSL):
800ft 10kt
10000ft 20kt

It is rather clear that I have a wind value of 6kt in 5500m AMSL (500AGL) and in 5800m AMSL (10kt) with a linear interpolation in between. I've checked it and i think it works like this. But what happen when I climb on 5801m AMSL (801 AGL). Then a linear interpolation between 10 kt and 20 kt would result in a value of about 15.44 kt and thats a very big jump for a one meter higher. So how is flightgear considering the terrain in these wind calculation and what is the sense behind specifying the aloft layers in AMSL and not in AGL?? I hope you understand what i mean.

P.S.: In environment_ctrl.cxx a lot of interpolation is done but i am not understand that in detail. Maybe some of you do.
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Re: AMSL vs. AGL in Manual Configuration

Postby Thorsten » Fri Mar 04, 2016 6:16 pm

and what is the sense behind specifying the aloft layers in AMSL and not in AGL??

The sense is that the real atmosphere is very different in the boundary layer and outside of it, so there's a genuine class of phenomena happening inside a layer defined AGL and a different class of phenomena in layers defined approximately AMSL.

Which is to say, real aloft wind layers do not happen on constant altitude above ground but on constant pressure altitude surfaces for which MSL is a decent approximation.

(One very prominent example is that the diurnal temperature variation is almost exclusively confined to the boundary layer).

The BW model is a bit rough, AW is a bit more refined in that it dynamically shrinks the boundary layer thickness to zero around mountaintops, but the essential meteorology is quite correct.
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