BUFR4 Descriptors

Introduction

As part of the BUFR format, a list of the included parameters is embedded within the file using the unexpandedDescriptors element of Section 3. This element takes a list of the parameters to be included and the order of those parameters. For example, the unexpanded descriptor list (in text, see format of descriptors below):

["unexpandedDescriptors"] = ["wigosID", "year", "month", "day", "hour", "minute", \
                              "latitude", "longitude", "pressure reduced to mean sea level"]

would specify that the station identifier (wigosID) followed by the date, time, location and then the pressure reduced to mean sea level would be included in the data section. For conciseness, aliases (or sequences in BUFR terminology) exist to group commonly reported parameters together, for example grouping the year, month and day together in the group date. Using sequences, the above example becomes:

["unexpandedDescriptors"] = ["wigosID", "date", "time", "location", "pressure reduced to mean sea level"]

where

["date"] = ["year", "month", "day"]
["time"] = ["hour", "minute"]
["location"] = ["latitude", "longitude"]

These sequences form building blocks that can be used to create much longer sequences using a small number of descriptors.

Format of BUFR descriptors

Whilst text strings have been used above to represent the parameters to report this has been done for ease of reading and explanation. Within the BUFR format these are specified by 6 digit codes of the form FXXYYY, with each of F, XX and YYY having specific meaning.

• F: Type of BUFR descriptor

  • 0: element descriptor (BUFR Table B);

  • 1: replication (or repetition) descriptor;

  • 2: operator (BUFR Table C);

  • 3: sequence descriptor (BUFR Table D)

• XX: Sub-table (class) within type of descriptor

• YYY: Index within in sub-table.

As an example, the table below shows the first few entries from BUFR Table B 01.

For the above example, using the FXXYYY notation, the list of abbreviated (or unexpanded) descriptors becomes:

["unexpandedDescriptors"] = [301150, 301011, 301012, 301023, 010051]

where:

[301150] = [001125, 001126, 001127, 001128] # (wigosID)
         = ["WIGOS identifer series", "WIGOS identifier issuer", "WIGOS identifier issue number", "WIGOS local identifier"]
[301011] = [004001,  004002, 004003] # (date)
         = ["year", "month",  "day"]
[301012] = [004004,   004005] # (time)
         = ["hour", "minute"]
[301023] = [    005002,      006002] # (location)
         = ["latitude", "longitude"]
[010051] = ["pressure reduced to mean sea level"]

Replication / repetition

Within the BUFR format, elements can be repeated using the replication descriptors (F=1 in FXXYYY). For example, we may want to repeat temperature and humidity measurements as part of an atmospheric profile or, alternatively, the daily minimum and maximum temperatures within a month. In some cases we may know the number of repetitions before encoding and all data of the same type may have the same number of repetitions. In this case the number of replications can be set before hand and included in the sequence. In other cases, there may be a variable number of repetitions and so the number is set at the time of encoding.

When using the replication descriptor (1XXYYY), the XX component indicates the number of following descriptors to repeat and the YYY component the number of replications. For example, to repeat the day of month, maximum and minimum temperatures 5 times we would use:

In expanded form, or without using the replication, this would be equivalent to:

If we do not known the number of replications, or there can be a variable number of replications, for a given sequence of descriptors we can set the YYY element (number of replications) to zero and follow the replication descriptor with a delayed replication factor.

This works in the same way as the regular replication except that the number of replications (n) is set at the the time of encoding and included in data. Often, within sequences, delayed descriptors are used to specify optional elements using the short delayed descriptor replication factor (031000) that takes a value of either 0 or 1.

Within the csv2bufr module the number of delayed replications needs to be set within the mapping file using the inputDelayedDescriptorReplicationFactor key. More information is provided on the mappings page.

Scope of descriptors

BUFR Table B descriptors within classes 0 - 8 contain metadata about the observations. For example, the location of an observations, the instrumentation used to make an observation or the time period over which an observation was made or averaged. These descriptors remain in force and apply to all subsequent elements until they are either reused or set to missing. For example the sequence [007032, 012001, 007032, 012001, 007032] could be used to record air temperature measurements at two different heights, e.g.:

# set height of sensor for following observations
[007032] = ["heightOfSensorAboveLocalGroundOrDeckOfMarinePlatform"] = 2.0
[012001] = ["airTemperature"] = 280.15 # air temperature at 2 m height
# redefine height of sensor to 10 m
[007032] = ["heightOfSensorAboveLocalGroundOrDeckOfMarinePlatform"] = 10.0
[012001] = ["airTemperature"] = 280.07 # air temperature at 10 m height
# cancel height of sensor, following observations will have an undefined height
[007032] = ["heightOfSensorAboveLocalGroundOrDeckOfMarinePlatform"] = None

Commonly used sequences

Listed below are some commonly used sequences:

• 307080: Sequence for representation of synoptic reports from a fixed land station suitable for SYNOP data.

• 315008: Sequence for the representation of data from moored buoys.

• 315009: Sequence for the representation of data from drifting buoys.

• As this documentation is developed further additional examples will be added.

Further information

The description of the BUFR operators (F = 2 in the FXXYYY notation) is beyond the scope of this documentation. For users wanting to define new sequences, including the use of the operators, it is recommended to refer to Volume I.2 of the WMO Manual on Codes. However, before defining a new sequence it is recommended to check if any of the existing sequence meet the user requirements. See the support page for information on how to get further information and support..