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20031203: Processing Cloud IR data



>From: Kwang-Chung Eddie Harm <address@hidden>
>Organization: Harvard
>Keywords: 200312032123.hB3LNop2005172 McIDAS AXFORM

Eddie,

>       I am using AXFORM command in order to process cloud IR data.  I
>first used axform option of 'temperature'.  Could you please tell me what
>the threshold value which will differentiate the mountain top snow and
>clouds ?

There is no set temperature that will distinguish mountain top snow
temperature and cloud temperatures.  You could use sounding data from
stations close to the mountain in question to estimate what the
temperature of the snow might be and go from there.  The other thing
you could do is to compare VIS and IR images for the same time and
location and determine where snow is from the VIS image and compare
areas known to be mountain top and see what the IR Temperatures are
there and see how they correlate with cloudy regions.

>How about 'BRIT' option of AXFORM command ?  what is the difference
>between 'Temperature' option and 'Brit' option ?

Brightness is the measure of each pixel as 1-byte values.  GOES imagery,
for instance, has 10 bits of resolution for each pixel, so values
at each location can range from 0 to 1023.  Brightnesses are the 10-bit
values scaled to 8 bits or 1 byte, so the values range from 0-255.
For calibrated images (those for which there is not a calibration
block in the image AREA file), there is a fixed relationship between
brightness and measures like Temperature (for IR images).  In this
case, one could interpret the brightnesses in the exactly same manner
as the Temperature.  For uncalibrated images, the same would not be true.

>Can you please tell me
>where I can find the description of these output files ( for 'BRIT' and
>"TEMPERATURE' option )  ?

CSU/CIRA has some excellent modules on interpreting the values from
GOES imagery.  I would do a Google (tm) search for GOES brightness
and go from there.  The information you get back will provide a lot
of background on satellite imagery interpretation that would likely
be most helpful.

Cheers,

Tom Yoksas


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