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Note  

 
Note that the problem described here affects only fields accumulated from the start of the forecast. .In particular, the ERA5 short forecast accumulations should do not suffer from the same problem because they are packed and archived as accumulations since the previous post processing (archiving) step i.e. they are already deaccumulated before packing. 
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When data are packed in GRIB using the simple packing method, the values are packed by storing differences from the reference value (the minimum of all the values) to a specified fixed number of bits referred to as the "bits per value" (ecCodes key: bitsPerValue). Thus the packing introduces a discretisation of the values with an associated packing error (ecCodes key: packingError) such that:
Tip  

 

The packing error depends on the range of vafieldslues field values and the number of bits per value used to pack the values.
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Accumulated model values  Accumulated packed GRIB values  Deaccumulated packed values  packing error  




 All accumulations start with a zero value at step=0
 The values at all points are packed exactly
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Accumulated model values  Accumulated packed GRIB values  Deaccumulated packed values  packing error  



 0.03125 
 The model values increase at all points except the north west
 Not all values can be represented exactly using 8 bits per value
 In particular, the value of 0.031250 at the southern point is exactly the packing error and its packed value becomes 0.062500
 In this case, the packed values are all multiples of 2*packing error=0.0625 so values of 4.55, 5.9 and 2.2 are not stored precisely
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Accumulated model values  Accumulated packed GRIB values  Deaccumulated packed values  packing error  



 0.06250 
 Only the model value at the north east point increases
The increase in the range of values increases the packing error by a factor of 2
Now all packed values are multiples of 2*packing error=0.125
 The value at the central point of 4.55 is now packed as 4.5 compared to a packed value of 4.5625 at the previous step  a negative accumulation of 0.0625
 The value at the south west point of 2.2 is now packed as 2.25 compared to a packed value of 2.1857 at the previous step  a positive accumulation of +0.0625
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Accumulated model values  Accumulated packed GRIB values  Deaccumulated packed values  packing error  



 0.12500 
 Again only the model value at the north east point increases further from 20.0 to 40.0
 The increase in the range of values increases the packing error.
 Now all packed values are multiples of 2*packing error=0.25
 The model value at the eastern point is unchanged at 5.9 but now its packed value is 6.0 giving an apparent positive accumulation of +0.125
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