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...). > *scratches head* > 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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