NCEP ENSEMBLE GRIB DATA INFO
Information on this page:
Getting NCEP Ensemble data
via anonymous ftp
NCEP Ensemble data are currently available via anonymous ftp on 2 different
servers:
Typically, the ftp server will have a few recent days worth of data on-line,
while the OSO server will likely only have the data from the current day.
The data are grouped together according to variable. For example, the file
enspost.T00Z.z500 will contain all 12 ensemble perturbation forecasts for
500 mb height for 00z for the day given by the directory name. The following
variables are available:
precipitation (precip)
precipitation probabilities (ensspf)
mean sea level pressure (prmsl)
rh 700 mb (rh700)
T 2 meters (t2m)
T Max, T Min (tmax, tmin)
T 850 mb (t850)
u-comp 10 meters, v-comp 10 meters (u10m, v10m)
u-comp 250 mb, v-comp 250 mb (u250, v250)
u-comp 500 mb, v-comp 500 mb (u500, v500)
u-comp 850 mb, v-comp 850 mb (u850, v850)
250 mb gp height (z250)
500 mb gp height (z500)
850 mb gp height (z850)
1000 mb gp height (z1000)
For each variable, 1 file is written for the 12 00z perturbations and 1
file is written for the 5 12z perturbations. In each file and for each
perturbation, data are included every 12 hours from valid time = 00h to
384h. GRIB index files are also included on these servers in the same directory
as the GRIB data files, and are identified by the "i" after the parameter
identifier in the file name (e.g., enspost.T00Z.z500i ).
In addition, a file containing probabilistic
quantitative precipitation forecasts (PQPF), derived from the ensemble,
is also available.
FTP Access to NCEP GRIB
decoding software
You can find NCEP GRIB decoding software at 2 different locations:
1. NCEP "nic" server: Do an anonymous ftp to nic.fb4.noaa.gov (140.90.50.22),
and cd to /pub/info. Download the file README.INFO. Read that and then
cd to the directory corresponding to the platform that you have (gribsgi,
gribsun, etc.).
2. A more recent version of the NCEP GRIB software package that contains
more bells and whistles can be downloaded from the web site at http://sgi62.wwb.noaa.gov:8080/reanl2/wd20tm/grib
by
downloading the latest version of the file "hp.gbpak.9702.tarfile" as a
binary file and expanding it using "tar xf". *** IMPORTANT NOTE *** This
version of the NCEP GRIB software has only been shown to work properly
on HP workstations and Cray computers. If you are working on a different
platform, you can try to use it, but it very well may not work. If it doesn't
work on your platform, then go with the first option of ftp'ing to the
NCEP nic server and downloading that software.
*** You can find help using NCEP's GRIB decoding
software by clicking
here ***
Understanding the Ensemble
GRIB PDS extension
Individual ensemble members are identified in GRIB by a special extension
to the GRIB header (the PDS) beyond byte 41. Typically, the PDS of a regular
GRIB file will only contain 28 bytes (also called octets). However, for
most ensemble applications, more than 28 bytes are needed to properly identify
the particular ensemble member, probability type, cluster, etc.
The ensemble flag is in the value of octet 26; If octet 26 = 2, then this
particular GRIB record contains a PDS extension with ensemble information.
The length of that extension depends on the type of information contained
in the record. For example, if there are no probability or clustering data
in this record and all you need to do is identify which ensemble member
is packed in this record, then all you need are octets 41-45. However,
if you need probability or clustering information, then you'll need to
access octets 46 through perhaps octet 86 (see the table below for more
detailed information).
You can use subroutine getgbex from the nw3lib for handling data with
the ensemble extension in the header.
Currently, there are no WMO standards for including this extra information
on ensemble data; below the PDS GRIB extension adopted at NCEP (which is
somewhat similar to that used at ECMWF) is described.
EXTENSION OF PDS SECTION FOR THE IDENTIFICATION OF ENSEMBLE PRODUCTS
|
Byte (or Octet)
|
Content
|
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
11 = Standard deviation with respect to ensemble mean
12 = Standard deviation with respect to ensemble mean, normalized
|
| 45 |
Spatial smoothing of product (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
4=Percentile lower value (below which the variable is expected to occur
with given probability)
5=Percentile upper value (above which the variable is expected to occur
with given probability)
6=Percentile interval (range of values between two percentiles) |
| 48-51 |
If octet 47=1 or 3: Lower limit for probability (unit as
in octet 9; floating point number)
If octet 47=4 or 6: Percentile lower limit (probability;
floating point number) |
| 52-55 |
If octet 47=2 or 3: Upper limit for probability (unit as
in octet 9; floating point number)
If octet 47=5 or 6: Percentile upper limit (probability;
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, shows, in binary (on/off) format, which members of the
ensemble belong to the current cluster. Membership table defined by product
generators. |
| 87-100 |
Reserved |
ADDITIONAL ENSEMBLE-RELATED CHANGES IN MAIN PDS DEFINITION
Please refer to the "GRIB Manual" (Office Note 388) for further details
|
|
Octet 9 (Indicator of parameter and units)
|
|
VALUE
|
PARAMETER |
UNITS |
ABBREV. |
191
|
Probability from ensemble (Octet 41=1) |
non-dim |
PROB |
| 192 |
Probability from ensemble, normalized with respect to climate expectancy
(Octet 41=1) |
non-dim |
PROBN |
193
|
Difference between two height levels |
gpm |
HGTD |
|
Octet 21 (Time Range Indicator)
|
|
VALUE
|
MEANING |
119
|
Standard deviation of N forecasts, all with the same reference time,
with respect to the time average of forecasts; the first forecast has a
forecast period of P1, the remaining forecasts follow at intervals of P2 |
|
Octet 26 (Identification of sub-center)
|
|
VALUE
|
MEANING |
2
|
NMC Ensemble Products (Perturbed forecasts and other products) (See
octet 41). i.e., if octet 26 = 2, then this GRIB record contains ensemble
data AND an extension to the PDS. |
|
Octet 29
|
|
BIT NUMBER
|
VALUE |
DEFINITION |
1
|
0 |
Full forecast field |
|
1 |
Forecast error field (forecast minus analysis) |
|
| 2 |
0 |
Original forecast field |
|
1 |
Bias corrected forecast field |
|
| 3 |
0 |
Original resolution retained |
|
1 |
Smoothed field |
|
| (Mean model error fields can be identified by setting the
first bit of octet 29 to 1 and specifying a time-averaging process in octets
19-24.) |