Cloud Levels: High Middle and Low
                  ----- ------  ---- ------ --- ---


Historically the MRF model saved cloud data for high, middle and low
clouds.  One could then assume the clouds had random overlap to get
the total cloud fraction (CF).

   total CF = 1 - (1 - high CF) * (1 - middle CF) * (1 - low CF)
            "random cloud overlap"

The Reanalysis model recognizes two basic types of clouds, convective and
RH.  The convective clouds are based on the convective parameterizations
and the RH clouds occur when the RH exceeds a critical value.  While
the cloud cover from RH clouds can be modeled as "random overlap", the
convective clouds are better modeled as "maximum overlap".  (At least
that is how the radiation code works.)  Unfortunately this change occurred
before NCEP moved to grib output.  At that time, changing the binary output 
files was a hanging offense.  Imagine changing the binary files and suddenly 
all the code that read those files suddenly failed.  In addition, any new 
code would be incompatible with the old binary files.  Since the total
cloud cover is the most commonly used diagnostic, the high/middle/low cloud 
fractions were redefined so that a random overlap would produce the 
approximate total cloud cover as seen by the radiation code.  The
redefinition is


low_CF =  low_CF_RH            			if no convective clouds
       =  0.80 * (low_CF_RH + low_CF_cn - low_CF_RH*low_CF_cn)

mid_CF =  mid_CF_RH            			if no convective clouds
       =  0.35 * (mid_CF_RH + mid_CF_cn - mid_CF_RH*mid_CF_cn)

high_CF =  high_CF_RH            		if no convective clouds
       =  0.35 * (high_CF_RH + high_CF_cn - high_CF_RH*high_CF_cn)

CF_RH = cloud fraction from large-scale clouds
CF_cn = cloud from from convective clouds

As you can see from the redefinition, the CF shows a large discontinuity
depending on whether convective clouds are present.  In addition, a deep
convective cloud would show up as a combination of deep and shallow
clouds.  This makes the the high/middle/low CF nearly useless EXCEPT for 
calculating the total cloud fraction.  (If you knew whether the layer
had convective clouds, you could recover the cloud fractions.  However,
obtaining this information is very involved.)

Summary:  do not use the high/middle/low cloud fractions except for 
calculating the total cloud fraction.  (Since the total cloud fraction 
is a standard output, it is best not to use the high/middle/low cloud
fractions.)  The clouds as seen by the radiation codes are not available 
which is an oversight on our part.