Budget
                                 ------


Balancing an atmospheric budget can prove to be almost as difficult as 
balancing the federal budget.  A lot of pain and hard work is involved in 
both.  The first step is to understand how NCEP's assimilation system works 
(see ASSIM.SYS).  Next is understand that the GRIB files will normally not 
have a enough precision for a residual calculation (see PRECSION) or even a 
direct calculation of the increment.  The first step in calculating an precise 
(who said accurate?) budget is obtain the sigma files of the analyses and the 
first guess (same as 6-hour forecasts).  These files have the full precision 
data can be used to (1) calculate the increments accurately, and (2) avoid 
vertical interpolation errors.  Unfortunately the sigma files are stored as 
spectral data on sigma layers.  Who said balancing the budget would be 
painless?  Be aware that if you use other fields in the budget they may have 
the GRIB  precision problem. 



General Budget Equation:

   dA
   --   = source - sinks + increment
   dt

   source = source term averaged over the 0 to 6-hour forecast interval
   sink   = sink   term averaged over the 0 to 6-hour forecast interval
   increment = artificial term
               what is needed to get the 6 hour forecast to look like
               the analysis

For some of the source/sinks, the model already saves the 6 hour average (with 
the GRIB precision problem).  For dynamic terms (for example water budget), 
the 6-hour average moisture flux has to be calculated from the winds and the 
humidity.  The best estimate is the mean flux from the sigma analysis and 6 
hour sigma forecast.

See FLUX3D.