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DEVORAC  May 2012

DEVORAC May 2012

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Subject:

Re: Agreement on consistent Channel numbering necessary

From:

Gareth Thomas <[log in to unmask]>

Reply-To:

Gareth Thomas <[log in to unmask]>

Date:

Wed, 30 May 2012 17:09:36 +0100

Content-Type:

text/plain

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Hi Matthias, all,

Here are my thoughts on this, which differ slightly from Matthias' - I 
suspect because I have been dealing with the surface albedo and 
emissivity side of things.

Since the primary aim of the preprocessing is to run ORAC on the "same" 
(well, similar!) set of heritage channels from a range on instruments, 
it makes sense to me that we define a set on index numbers for these 
channels across all 3 instruments:
1 = 0.67 microns
2 = 0.87
3 = 1.6
4 = 3.7
5 = 11
6 = 12
This makes dealing with ancillary data (like the surface albedo) a lot 
easier, because its independent of a particular instrument. A selection 
of these index numbers should be defined on the command line (or 
whatever), providing a common way of selecting different mixes of 
channels across the different instruments: this is how I'd deal with the 
3A,3B channel question with AVHRR.

The surface data files have their own "band" number systems, which are 
not (in general) related to any of the instrument channel indexes 
(although, since we're using MODIS surface albedo data, its numbering 
follows the MODIS convention). Thus, the routines for reading this data 
need to be hard-wired to return the correct channels, and this is much 
easier if you have a wavelength index number.

Of course, the instrument specific channel numbers/identifiers need to 
be carried around to, as do the entry numbers in the RTTOV coefficient 
files. I can't really see any way around having these instrument 
specific index numbers hard-wired into the instrument specific set up 
routines.

In order to minimise the amount of hard-wired index number selections 
(and thus number of changes that would need to be made to the 
preprocessing if additional channels were added to the list), I think 
another couple of indexes or masks need to be specified as well. In 
particular, I suggest defining two masks which define which channels are 
"shortwave" and which "longwave" (or solar and thermal, if you prefer). 
Ie. for the channels listed above, these would be:
shortwave = (/ 1, 1, 1, 1, 0, 0 /)
longwave  = (/ 0, 0, 0, 1, 1, 1 /)
Don't forget that 3.7 microns is both shortwave and longwave!

This may be self-evident, I would also suggest that we require that the 
channels be stored in wavelength order. This allows the use of array 
slicing to extract the shortwave or longwave channels.

Regarding Matthias' point 4, the system that has been used in ORAC in 
the past is:
  -90 < lat <  90
-180 < lon < 180
Absolute azimuth is measured clockwise from north
Relative azimuth has zero lying where the satellite and sun lie in the 
same direction when viewed from the surface, and ranges from 0 to 180 
degrees.

Cheers

Gareth

On 24/05/12 12:43, Jerg Matthias wrote:
> Hello everybody,
>
> While programming it occured to me that we should discuss the channel numbering to find a consistent solution for the different parts of the software (Preprocessing, RTTOV, ORAC). Here are some thoughts about it to start the discussion:
>
> 1.) It seems the most natural way is to use the original instrument numbering. While this is straightforward for MODIS it is more difficult for AVHRR as there are channels 3A and 3B. 3B seems to be attached to the files as channel 6 and they ar not always assigned meaningful number as they are switched on/off. All channels are read in presently but there is a flag which chooses for the preprocessing if either all channels are processed, all-3A or all-3B. Any thoughts on that? What about AATSR?
> 2.) To read the RTTOV coefficeint files the instrument channel numbers have to be mapped to the entry number of the respective channel in the coefficient file. This is presently hardwired in the setup routines of the respective instruments. I think this is OK so far. When calling RTTOV this again changes as channels there are just counted starting with 1.
> 3.) How shall we connect the preprocessing and ORAC in this respect?
> 4.) Another thing which has nothing to do with channels but which should be clarified and unified in the preprocessing is the definition of the angles and lat,lon.
>
>
> Cheers,
>
> Matthias
>
> Dr. Matthias Jerg
> ESA Climate Change Initiative
> Satellite Application Facility on Climate Monitoring (CM-SAF)
>
> Deutscher Wetterdienst (DWD)
> Frankfurter Straße 135
> D-63067 Offenbach
>
> Tel.:  +49-69-8062-2502
> Fax:   +49-69-8062-2506
> E-mail: [log in to unmask]

-- 
Address: Dr G E Thomas
          Atmospheric, Oceanic & Planetary Physics
          Clarendon Laboratory
          Parks Road
          Oxford OX1 3PU
          England

Email: [log in to unmask]
Telephone: +44/0 1865 272 894
Fax:       +44/0 1865 272 923

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