Hi Jana, Hi all,
The definition of the winds do not change between 1D, 2D and 3D. That
would at least lead to messy code.
This is in fact a question of the azimuth angle of the sensor. There is
also a north direction in 1D and the sensor is assumed to be looking
exactly North for 1D. This is explained in Sec 3.2 of AUG:
1D ... The sensor is assumed to by directed towards the North pole,
corresponding to an azimuth angle of 0 ◦.
Maybe few are looking in AUG so I just also added this information to
the built-in doc of sensor_los.
Bye,
Patrick
On 2019-04-03 17:08, Jana Mendrok wrote:
Hi,
thanks Richard for your feedback. I'm not sure, though, whether I really
understand it (incl. wondering why you talk about jacobians here...).
*scratches head*
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.
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.
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.
to confirm this, Rita, you could have a look where the line peak moves
(prefereably in a high-spectral resolution simulation) for positive and
negative winds, respectively. for positive it should move to lower
freqs, for negative to higher.
best wishes,
Jana
On Wed, Apr 3, 2019 at 12:03 PM Richard Larsson <ric.lars...@gmail.com
<mailto:ric.lars...@gmail.com>> wrote:
Hi Rita,
For the directional components, the sign of a retrieved component
with the ARTS Jacobian is positive along the axes themselves. The
direction the sensor is looking only influences the scale of the
Jacobian, not the sign of the direction of the retrieval
calculation. If you look parallel or anti-parallel to the field,
the values inside the Jacobian switch signs accordingly to work on
the external field. As an example, you should be able to make
measurements at several different directions and, if you assume the
field is uniform, retrieve just one field without any transformation
on the components themselves inside the calculations.
For the 'absolute' wind speed option, a positive retrieved sign is
equivalent to a red-shift, or a Doppler shift below unity if you
wish. This is much messier to work with during ARTS iterations, so
make sure your signal is easy to interpret if you use it. This is
of course closer to the physics of what is going on with the signal
itself, since all we really can see of the wind is how much it is
blue- or red-shifting the signal.
With hope,
//Richard
Ps. Oliver, please sent another email when it is fixed.
Den ons 3 apr. 2019 kl 13:36 skrev Oliver Lemke
<oliver.le...@uni-hamburg.de <mailto:oliver.le...@uni-hamburg.de>>:
Hi all,
Sorry if you received the mail below already. We're currently
investigating a mail delivery problem to Gmail adresses. If
you've already received the mail from Rita yesterday please
disregard this mail.
Cheers,
Oliver
> From: Rita Edit Kajtar <rita.edit.kaj...@ltu.se
<mailto:rita.edit.kaj...@ltu.se>>
> Subject: Wind calculations
> Date: 2 April 2019 at 10:36:54 CEST
> To: "arts_users.mi@lists.uni-hamburg.de
<mailto:arts_users.mi@lists.uni-hamburg.de>"
<arts_users.mi@lists.uni-hamburg.de
<mailto:arts_users.mi@lists.uni-hamburg.de>>
>
>
> Hello!
>
>
> I have a short question regarding wind calculations.
>
> What is the general convention assumed for determining the
sign of the wind speeds having the measuring instrument as the
reference point? Are the winds blowing towards the instrument
considered positive and those blowing away from the instrument
negative or the other way around?
>
> The ARTS documentation provides information for how the sign
should be considered regarding the four cardinal directions, but
not relative to a measuring device.
>
>
> Best regards,
> Rita Kajtar
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--
=====================================================================
Jana Mendrok, Ph.D. (Geoscience)
Email: jana.mend...@gmail.com <mailto:jana.mend...@gmail.com>
Phone : +46 (0)708 860 729
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