Module encoder 

GRIB2 data encoder.

The complexity of GRIB2

GRIB2 is a data format specifically designed to store grid-point values related to meteorological data. Therefore, it is easy to imagine that, in addition to the values at each grid point, information such as the latitude and longitude of each point, the date and time, and meteorological elements must also be included as metadata. Furthermore, even for grid-point values represented as arrays of real numbers, GRIB2 generally does not store floating-point values as-is. Instead, the values are rounded based on their precision and then compressed and stored as an array of integers–that is, discrete numerical values.

Consequently, unlike highly versatile formats such as NetCDF and HDF, which allow for the flexible serialization of large amounts of numerical data, or Gzip and Xz, which can freely compress arbitrary data, GRIB2 is a format with specific constraints. On the other hand, GRIB2 is a format whose specifications prevent situations where “a value has been obtained but its meaning is unclear.” It can also be described as a format capable of “representing” meteorological data by incorporating not only raw numerical values but also the complexities of the real world, such as missing values and precision information.

Goal of this module

The goal of this module is to enable users to create GRIB2 data–which has many parameters and some configurable options within its constraints–using an API that is as user-friendly as possible (although some parts are currently not user-friendly).

Internally, a single GRIB2 dataset (message) is composed of a collection of blocks called “sections.” However, this module’s high-level API avoids using the term “section” and instead focuses on “what information needs to be configured” and “how you want to represent the meteorological data.”

High-level and low-level APIs

This module provides 3 structs: SingleGrib2Message, MultiGridGrib2Message, and MultiProductGrib2Message. By providing all the necessary information to these structs (which may be a bit of a challenge), you can create GRIB2 messages. These structs constitute a high-level API designed to accommodate the most common use cases. If your use case does not fit these scenarios, you will need to implement the low-level API WriteGrib2Message and its sub-traits: WriteGrib2MessageIterL1, WriteGrib2MessageIterL2, and WriteGrib2MessageIterL3.

Generating data sets comprising multiple elements

It is rare to have only one type of grid point value data for a given time that you want to generate and provide. As shown below, in many cases, you will likely want to generate and provide multiple types of grid point value data together.

• Different meteorological elements (temperature, pressure, humidity, wind speed in 2 directions)
• Different forecast times (1 hour later, 2 hours later, 3 hours later)
• Different altitude levels (1000 hPa, 925 hPa, 850 hPa)

We will refer to this type of data here as “data sets comprising multiple elements.” The term “element” here is used in a general sense and is not limited to meteorological elements.

When representing data sets comprising multiple elements in GRIB2, there are 2 main options, or 3 when broken down in detail:

• A. Include them in a single message
• B. Create separate messages (but store them in a single file)
• C. Create separate messages (and store them in separate files)

If you wish to use option A, use MultiGridGrib2Message or MultiProductGrib2Message (or the low-level API) to include the set of multiple elements in a single message. If you want to implement options B or C, simply use SingleGrib2Message for each element. You can implement option B by writing to the same output destination consecutively, and option C by writing to separate output destinations.

Structs

GpvEncodeOutput

Data obtained through GPV encoding. Instances are typically used to write GRIB2 data via the methods defined in WriteGrib2DataSections.

GpvEncoder

GPV data encoder.

LatLonGrid

An auxiliary struct for creating the definition of a latitude/longitude grid (def::grib2::template::param_set::LatLonGrid) in a more intuitive way.

MultiGridGrib2Message

A writer for GRIB2 messages containing multiple submessages with different grids.

MultiProductGrib2Message

A writer for GRIB2 messages containing multiple submessages that share a grid.

SingleGrib2Message

A writer for GRIB2 messages that contain only one submessage.

Enums

ComplexPackingStrategy

Strategies applied when performing complex packing on numerical sequences. Complex packing is a method that divides a sequence of numbers into groups and efficiently compresses each group to improve the overall compression ratio of the data.

EncodingMethod

GpvEncodeParams

SimplePackingStrategy

Strategies applied when performing simple packing on numerical sequences. Simple packing is a method for discretizing continuous numerical values as integers, and various approaches can be taken during this process.

SpatialDifferencingOption

Traits

WriteGrib2DataSections

A serializer that writes the byte sequence of sections concerning GPV data to the output buffer.

WriteGrib2GridDef

A functionality to write the byte sequence of Section 3 (Grid Definition Section) of a GRIB2 message.

WriteGrib2Ident

A functionality to write the byte sequence of Section 1 (Identification Section) of a GRIB2 message.

WriteGrib2LocalUse

A functionality to write the byte sequence of Section 2 (Local Use Section) of a GRIB2 message.

WriteGrib2Message

A functionality to write an entire GRIB2 message. This trait works in conjunction with WriteGrib2MessageIterL1, WriteGrib2MessageIterL2, and WriteGrib2MessageIterL3 to write the message.

WriteGrib2MessageIterL1

A functionality to write elements of the L1 iterator in a GRIB2 message. This trait works in conjunction with WriteGrib2Message, WriteGrib2MessageIterL2, and WriteGrib2MessageIterL3 to write the message.

WriteGrib2MessageIterL2

A functionality to write elements of the L2 iterator in a GRIB2 message. This trait works in conjunction with WriteGrib2Message, WriteGrib2MessageIterL1, and WriteGrib2MessageIterL3 to write the message.

WriteGrib2MessageIterL3

A functionality to write elements of the L3 iterator in a GRIB2 message. This trait works in conjunction with WriteGrib2Message, WriteGrib2MessageIterL1, and WriteGrib2MessageIterL2 to write the message.

WriteGrib2PointValues

A functionality to write the byte sequence of Section 5 (Data Representation Section), Section 6 (Bit-map section), and Section 7 (Data Section) of a GRIB2 message.

WriteGrib2ProductDef

A functionality to write the byte sequence of Section 4 (Product Definition Section) of a GRIB2 message.