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Differences between NARR A and B Files

Table of Contents

Naming Convention
GrADS and the file differences

Naming Convention

For each analysis time .grb files are named:

narr-a_YYYYMMDD_HH00_000.grb and
narr-b_YYYYMMDD_HH00_000.grb

YYYYMMDDHH is the date code which corresponds to the analysis time or the starting time of the forecast for model predicted quantities such as flux or precipitation. The possible HH are 0,03, 06, 09, 12, 15, 18, and 21. All time are in UTC.

One GrADS control file is used for each day

narr-a_YYYYMMDD_0000_000.ctl
narr-b_YYYYMMDD_0000_000.ctl

The eight 3-hour time increments covered by the specific day (YYYYMMDD), are defined as forecast hours 1 though 8. However, these files actually contain no forecast data.

Top of Page GrADS and the file differences

The data for each analysis time is split into two files so that the data will be compatible with the software program, GrADS. The "narr-b" file is much smaller and contains fields that are similar to those in the larger file except they have some differences that GrADS does not recognize. For example, the "narr-b" file contains 3 hour forecast of the sensible heat flux whereas the larger file contains the average from the 0-3 hour forecast.

GrADS has only one time variable.

Example:

file A: analysis at 2004-01-01-00Z
file B: 3 hour forecast valid at 2004-01-01-00Z

GrADS allows you to combine data sets together so you can make time series. If we combined all the "A" files together, one could set the time to 2004-01-01-18Z and make a plot. On the other hand if the A and B files were combined together, GrADS would get confused if you set the time to 2004-01-01-18Z. GrADS would ask, do you mean the analyses, the forecast valid at that time or even the forecast started from that time?

To avoid this GrADS limitation, all the data were put into the "A" files except the forecasts that would cause the timing confusion. The average person would only need the "A" files and only people who want to know some hydrological analyses increments would need the "B" files.