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For accumulated fields, such as precipitation, the GRIB packing can have a strange affect on the totals, even when the data are used on the native model reduced Gaussian grid. In some cases, the values concerned are sufficiently small that the issue can be ignored, but for others - for example when looking at the frequency of zero rain, or light drizzle on forecast day 10 - action is needed. The figures below highlight the issue is and its impacts:
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Figure 3(a): Illustration showing the correspondence between the lead times where negative precipitation accumulations occur (red points) and the points where the packing error, and hence the discretisation of the field, changes (indicated by the "step" changes in the blue line). The negative accumulations occur due to the subtraction of values from two different levels of descritisation. The example shown is for total precipitation from a 10-day ENS forecast control run. Refer also to the annotation. | Figure 3(b): Ambiguity in identifying zero rain areas on day 10 in one example ENS forecast control run (see legend) |
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