Re: [arts-users] Wind calculations (resent)

```Hi Jana,

Den ons 3 apr. 2019 kl 17:08 skrev Jana Mendrok <jana.mend...@gmail.com>:```
```
> Hi,
>
> thanks Richard for your feedback. I'm not sure, though, whether I really
> understand it (incl. wondering why you talk about jacobians here...).
>

I might have misunderstood the question slightly, but the easiest way to
understand this in the code is in relation to what a standard Jacobian
means, since it must relate to you the assumed internal signs and geometries

> we're trying to pin down where the wind is blowing in a 1D atm setup. 1D
> allows us to have vertical (w) and horizontal (v) winds that affect the
> signal. let's forget about the vertical, that one is clear. for the
> horizontal, there is two possible wind directions in a 1D case - a head
> wind (aka blowing in the observer's face) and a tail wind (aka blowing from
> the observer's back). which of these corresponds to a positive, which to a
> negative wind(_v) speed.
>

The documentation is too difficult for me to follow.  In the code, the
definition in the 1D case is the same as for the 3D fields.  Assuming
wind_u==0 is enforced somehow (I seldom use 1D geometry), a negative wind_v
value has a negative speed in the Doppler shift expression -- a blue shift
or a headwind -- and vice verse for a positive wind_v.

> starting from your "For the 'absolute' wind speed option, a positive
> retrieved sign is equivalent to a red-shift" and equating a red-shift with
> observer moving away from source (or source from observer...), a positive
> wind should mean a tail wind.
>

Yes.

> which actually seems indeed in agreement with info I dig from the ARTS
> documentation (not that easy to find and combine for a beginner...): wind_v
> doc says positive v-winds are winds blowing south to north (which alone
> isn't very helpful for 1D setups - there is no north in 1D...); sensor_los
> doc says for the 2D case (allowing positive and negative zenith angles - in
> order to distinguish two possible viewing directions) that positive angles
> are equivalent to viewing towards higher latitudes, ie towards north.
> assuming this is valid for 1D, too (ie in 1D the observer always looks
> towards north) means for the 1D case positive winds (blowing to north)
> equate tail winds (for an observer looking north), negative winds are head
> winds.
>

Indeed, the description seems coherent but difficult to follow.  Can you
point out where in the docs you found this?  It should be updated to
clearly read that a positive wind_v in 1D equates to a positive v in the
standard (1 - v/c) Doppler shift expression.

With hope,
//Richard

>
```
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