Subject: HALOE Production Processing

From: [email-masked] (David Winningham)
Date: Thu, 9 Apr 92 13:49:34 CDT

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>From "HALOE::RUSSELL"@PEM.space.swri.edu Thu Apr  9 12:43:56 1992
Date:    Thu, 9 Apr 1992 17:40:44 GMT
From: "HALOE::RUSSELL"@PEM.space.swri.edu
Subject: HALOE Production Processing
To: [email-masked], [email-masked]
X-Vmsmail-To: @UPI.DIS
Content-Length: 5394

           HALOE Production Processing and Data Status

As you probably have noted, we are now running the HALOE software
through Level 2 in production mode on the CDHF.  Also, we have
classified this as our Version 4 software since it is the version
we used at the Oxford intercomparison meeting. Therefore the
results have "some measure of validity" which is the criterion for
calling this Version 4.  I want to stress however, that even though
the data look reasonable, the HALOE team still has much work to do
to properly validate the data.

One major area of work is in correcting for aerosol effects on gas
retrievals.  We are making good headway on this but we have a long
way to go.  The Version 4 data being produced has no correction for
aerosol included.  The gas filter channels (HCl,HF,CH4,and NO) are
least affected but they are not unaffected.  We have to carefully
study the data in the aerosol region (e.g. 100mb to 10mb) and
assess what effect the aerosol has on our data.  This will take
time, detailed work, and comparisons with correlative measurements. 
THE HELP OF THE UARS SCIENCE TEAM IN LOOKING AT THE VALIDITY OF THE
DATA IN THIS AND OTHER WAYS IS WELCOMED.  You will see that some of
the time, the direct effect of aerosols on the signal/retrieval and
the indirect effect through perturbations to our pointer/tracker,
cause obviously invalid results.  THEREFORE, VIEW THE HALOE GAS
FILTER CHANNEL RESULTS BELOW ABOUT 10mb to 20mb AS VERY PRELIMINARY
AND IN SOME OBVIOUS CASES AS INVALID.

The effect of aerosols on radiomenter channel results (H2O,NO2, and
O3) is clear.  Since no correction is being made at present, the
gas retrievals show unrealistically large mixing ratios below about
10mb (and sometimes from about 7mb to 10mb) due to the fact that
the HALOE retrieval ignores the additional aerosol absorption and
considers the total absorption as due only to the gas.  We have
chosen to still do a retrieval for the lower stratosphere and
include it in the data base since (1) it shows the aerosol effect
and (2) the altitude where the effect starts varies with channel
and geographic location.  NOTE HOWEVER, THAT THE HALOE RADIOMETER
CHANNEL MIXING RATIO RESULTS AT AND BELOW 10MB (SOMETIMES 7mb TO
10mb) ARE CONTAMINATED BY AEROSOL ABSORPTION AND THEREFORE DO NOT
GIVE CORRECT GAS MIXING RATIOS.

A series of other improvements also need to be made to the
processing software including forward model updates to incorporate
the most recent spectral data base (current software uses 1989 data
bases in some cases) and modifications to the EGA data base being
used.  We will be evaluating the accuracy of the HALOE forward
model for the radiomenter channels by performing comparisons with
line-by-line calulations.   Other changes that need to be included
are temporal smoothing at the highest altitudes to reduce "top of
profile" errors, optimization of the optimal onion peeling
constaints and retrieval grid spacing to better capture atmospheric
variability, and inclusion of a HALOE temperature retrieval.  The
NMC temperature data stop at about 0.4mb which requires that
climatology be used at present for the higher altitudes.  This
could have a large effect especially on the nitric oxide results.

As a final note on the data status, very few comparisons have been
made with correlative data since since the data base has been very
sparse.  This situation is changing as campaigns are carried out
and more data accumulate in the CDHF.  This will be an important
focus of future HALOE validation efforts.  AGAIN THE HELP OF THE
UARS SCIENCE TEAM IN THIS EFFORT IS WELCOMED.    

A description of the HALOE level 1 and 2 files is now on the CDHF
in the directory HAL$DISK:[HALCMN.DOC].  The files in this
directory are:

  LEVEL12.TXT - A preliminary description of the file structure.  
                Users should read this before trying to run the   
                following program.

  READL2B.COM - Command procedure to compile and link the routines 
                in this directory.

  READL2B.FOR - The main program that calls the following routines. 
                RL2DAT.FOR, RL2SUM.FOR, RWL1HD.FOR
  
Several INCLUDE files and a test version of a level 2 file are
contained in these files.  The outline in the .TXT file and the
.COM file should be enough to enable users to use, and modify,
READL2B.FOR.  This file is now set to operate in an interactive
mode but it can be modified to operate in a batch mode and to write
a file in the users own format.

The quality data in the files are variances.  If the READL2B.FOR is
used to read the data however, standard deviation (SD) is returned
as agreed at the last UARS science team meeting.  Currently, the
negative sign attached to the SD does not indicate the point where
50% weighting of the apriori and measured signals occurs as agreed
at the meeting; this will be corrected in a future software
delivery.  The negative SD which is encountered at the lowest
altitude point indicates the starting altitude for the retrieval.

If there are questions about the mechanics of reading the data
please contact Ken Haggard, the HALOE Data Reduction Manager at
HALOE::Haggard.  If there are questions about data quality,
comments on data validation, or other information you feel would be
helpful in the validation effort, please contact me.

Jim Russell

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