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The Product Requirements Editor provides an estimate of both the number of fields and the total volume of the products requested.

For GRIB products:

• the number of fields is estimated by expanding the request and evaluating the total number of products;
• The volume of data that will be produced is estimated by applying the post-processing requested (e.g., using the GRID and AREA keywords specified) to sample data  corresponding to the data requested

For BUFR products:

• the volume estimate of Weather Parameter products is based on an average size of message;
• Tropical Cyclone product estimates are based on an average number of Tropical Cyclone products produced in a one year period.

In most cases, this approach is expected to produce a reasonably accurate estimate of both the number of fields and the total volume.  The exception will be in the case where parameters that can take constant values are requested,  in which case volume will be overestimated.

Constant fields in GRIB

Some parameters take constant values at all grid points.  Such parameters are packed efficiently in GRIB by storing only the single reference value rather than the same value for each grid point.  As a consequence, the GRIB messages for there parameters are smaller than will be estimated by the volume estimator.

Typical parameters that can take constant values include:

• accumulated parameters such a precipitation or radiation parameters at step=0 where all values are zero;
• cloud parameters on model or pressure levels at high altitudes where all values are zero.

In particular, for the cloud parameters on model levels, where between 30% and 50% of the levels may have constant values, there can be a significant overestimate of the volume.

A full list of parameters that may produce constant global fields

Analysis fields

Forecast fields