White Lies ----- ---- GRIB, a WMO standard, is used by the Reanalysis Project for storing data. While GRIB has the great advantage in that it is self-documenting, it has some limitations. To overcome some of the limitations, we have been forced to lie to the grib encoder. I tried to make the white lies as minor as possible. Problem 1: GRIB considers initialized and un-initialized analyses to be different. (NCEP no longer initializes its analyses.) Unfortunately the GRIB standard defines the variance of initialized analyses but not the variance of un-initialized analyses. White Lie 1: The GRIB headers may say initialized analysis but they really mean un-initialized as NCEP no longer runs an initialization procedure. Problem 2: The daily maximum temperature (TMAX) is defined in GRIB as a product valid between 0 and 24 hours. The monthly mean TMAX is the mean of the daily TMAX and should be described as the mean of a product with a sampling frequency of 24 hours and that product is valid between 0 and 24 hours. However, that is not allowed in the GRIB header. White Lie 2: The GRIB head says TMAX is the mean of a 12 hour forecast (half way between 0 and 24 hours) with a sampling frequency of 24 hours. Note: this only affects TMAX and TMIN monthly means and variances. Problem 3: This is similar to problem 2 except with different sampling frequencies. Many quantities are averages from a 6-hour forecast interval (for example, precipitation, latent heat flux, and radiative fluxes). In the high temporal resolution files, they are described as an average from 0 to 6 hours. However, suppose you want the 00Z-06Z precipitation averaged over a month. Difficult to do in GRIB. White Lie 3: All quantities averaged from 0 to 6 hours are called 3-hour forecasts when monthly means and variances are computed. The average flux will be closer to the 3 hour forecast value than either the 0 or 6 hour values. Unfortunately there is an exception in the full archive. The 3-D flux file labels the 0-6 hour forecasts as 6 hour forecasts in the monthly means and variances. White Lie -> Truth ----- --- ----- 1) If the GRIB header says INITIALIZED analysis --> uninitialized analysis 2) If the GRIB header says TMAX or TMIN and the average of 12 hour forecasts --> average of daily TMAX or TMIN 3) If the GRIB header says 3 hour forecasts --> average of first 6 hours of forecast 4) If the GRIB header says 6 hour forecast and it's a 3-D flux file --> average of first 6 hours of forecast Note: Solution #4 is SOP on the second floor of an unnamed weather center in suburban Washington DC.