ON388 - APPENDIX C

NCEP USE OF THE EXTENDED GRIB PDS SECTION
FOR DESCRIBING ENSEMBLE OUTPUT

Elements of individual ensemble members are identified in GRIB by a special extension to the PDS section beyond octet 40. Typically, the PDS of a regular GRIB file will only contain 28 bytes (also called octets).  However, for most ensemble applications, more than 28 octets are needed to properly identify the particular ensemble member, probability type, cluster, etc.

This appendix details NCEP's use of the extended GRIB PDS section (octets 41 to 86) to describe ensemble model data.  Currently there are no WMO standards for including this extra information on ensemble data in GRIB1.
 
 

EXTENSION OF PDS SECTION FOR ENSEMBLE PRODUCTS 

Octet 

Description 

41 

Identifies application 

1 = Ensemble 
42 

Type 

1 = Unperturbed control forecast 
2 = Individual negatively perturbed forecast 
3 = Individual positively perturbed forecast 
4 = Cluster 
5 = Whole ensemble 
43 

Identification number 

If Byte 42 = 1, byte 43 will = 1 or 2: 
1 = High resolution control forecast 
2 = Low resolution control forecast

 If Byte 42 = 2 or 3, byte 43 will = 1,2,3,4 or 5: 
This number indicates the identification number 
for this member of a pair. That is, there is a 
first negative perturbation and a first positive 
perturbation (n1 and p1). 
44  Product Identifier

1 = Full field (individual forecast) / Unweighted mean (cluster/ens) 
2 = Weighted mean (of raw forecasts)
3 = Full field (individual forecast of  bias correction) : the values are modified by considered bias
4 = Weighted mean ( of bias corrected forecasts)
5 = Weights: statistically decided weights for each ensemble member
  6 = Climate percentile: percentile values from climate distribution
  7 = daily climate mean: from 40-year NCEP/NCAR reanalysis
  8 = daily climate standard deviation from daily mean: from 40-year NCEP/NCAR reanalysis
11 = Standard deviation with respect to ensemble mean 
12 = Standard deviation with respect to ensemble mean, normalized
21 = maximum value of all members
22 = minimum value of all members
23 = The ensemble forecast value for X% probability
(X is given in byte 45 - integer value 0 - 100)
24 = The ensemble mode forecast (mode = 3*medium - 2*mean)

45 

Spatial Smoothing of Product or Probability (if byte 44 = 23)

(number of highest total wavenumber included) 

255 = Original resolution retained 

Octet 46 and above are used only if needed 

Probability Section 

46 

Probability product definition: 

If octet 9=191 or 192, defines variable in terms of octet 9 table (see below for more details)
47 

Probability type 

1 = Probability of event below lower limit 
2 = Probability of event above upper limit 
3 = Probability of event between lower and upper limits 
48-51  Probability lower limit (unit as in octet 9; floating point number)
52-55  Probability upper limit (unit as in octet 9; floating point number)
56-60  Reserved 

Octet 61 and above are used only if octet 42=4 or 5 

Clustering Section 

61 

 Ensemble size (number of members, including controls) 
62  Cluster size (number of members) 
63  Number of clusters 
64  Clustering Method

1 = AC 
2 = RMS 
65-67  Northern latitude of clustering domain (in thousandths of degree) 
68-70  Southern latitude of clustering domain (in thousandths of degree) 
71-73  Eastern longitude of clustering domain (in thousandths of degree) 
74-76  Western longitude of clustering domain (in thousandths of degree) 
77-86  Cluster Membership
(If octet 42=4)

Indicates in binary (on/off) format, which members of the 
ensemble belong to the current cluster. Membership 
table defined by product generators.