20000225: Solar zenith angles in McIDAS

>From: Xinli Li <address@hidden>
>Organization: University of Hawaii
>Keywords: 200002252127.OAA12741 McIDAS-X solar zenith

Xinli,

>I have installed the McIDAS-X sucessfully.  Thank you for your help!

You are welcome.

>Now I am using it to display the real satellite images. I have a
>question about McIdas-X.
>
>Is lt possible to calculate the azimuth angles from each pixel of the
>real image to satellite by using McIdas-X, and how?

This is not something that is readily available in McIDAS, but other sites
have done it.  I sent off an email to the one site that is doing this on
a regular basis asking them to share the code they developed with you and
the rest of the McIDAS community.

>This is very important to us, I didn't find any things about this
>function on the McIdas-X documents.

I will get back to you with the code/technique as soon as I hear from the
site I contacted.

>Best regard,
>Xinli Li

Tom

>From address@hidden  Thu Mar  2 14:58:34 2000
>Date: Thu, 2 Mar 2000 16:58:26 -0500 (EST)
>From: "Jennie L. Moody" <address@hidden>
>To: address@hidden
>Subject: Re: 20000225: solar zenith angles for satellite imagery

Tom,

We did this a couple different ways, orginally we had a grid of zenith
angles calculated once offline (they don't change unless somebody moves
a satellite), but under version 7.6 of McIDAS that stopped working,
however, there were some new math functions (that return the lat and
log on a grid point) that made it pretty easy to calculate the zenith
angles dynamically.  So now a new grid of zenith angles is made each
time the product gets made. We use a set of equations from Kidder and
VonderHaar's book Satellite Meteorology.  Its section 2.5.2 where they
define the zenith angle based on a number of vectors to a point a earth
(in this case, tracking atenna's, in our case, grid cell locations).
Anyway, Tony worked on coding this and so I have sent him a copy of
your message.  He will be in touch with you about making the code
available, either as a contrib, or whatever other way it might be
helpful.  Its pretty simple code to get the zenith angle, now, how
people use the zenith angle to make "corrections" to radiances is
another whole issue.  In our case, we were able to rely on the results
of some research by other investigators (Soden and Bretherton) who had
used a radiative transfer model.