mllam-data-prep

This package aims to be a declarative way to prepare training-data for data-driven (i.e. machine learning) weather forecasting models. A training dataset is constructed by declaring in a yaml configuration file (for example example.danra.yaml) the data sources, the variables to extract, the transformations to apply to the data, and the target variable(s) of the model architecture to map the data to.

The configuration is principally a means to represent how the dimensions of a given variable in a source dataset should be mapped to the dimensions and input variables of the model architecture to be trained.

The configuration is given in yaml-format and the file specification is defined using python3 dataclasses (serialised to yaml using dataclasses-wizard) and defined in mllam_data_prep/config.py.

Installation

To simply use mllam-data-prep you can install the most recent tagged version from pypi with pip:

python -m pip install mllam-data-prep

If you want support for creating datasets in parallel using dask.distributed you can install the package with the dask-distributed extra:

python -m pip install mllam-data-prep[dask-distributed]

Developing mllam-data-prep

To work on developing mllam-data-prep it easiest to install and manage the dependencies with pdm. To get started clone your fork of the main repo locally:

git clone https://github.com/<your-github-username>/mllam-data-prep
cd mllam-data-prep

Use pdm to create and use a virtualenv:

pdm venv create
pdm use --venv in-project
pdm install

All the linting is handelled by pre-commit which can be setup to automatically be run on each git commit by installing the git commit hook:

pdm run pre-commit install

Then branch, commit, push and make a pull-request :)

Usage

The package is designed to be used as a command-line tool. The main command is mllam-data-prep which takes a configuration file as input and outputs a training dataset in the form of a .zarr dataset named from the config file (e.g. example.danra.yaml produces example.danra.zarr). The format for the config is described below. The package can also be used as a python module to create datasets in a more programmatic way by calling mllam_data_prep.create_dataset() directly (see below).

Command-line usage

python -m mllam_data_prep example.danra.yaml

Example output:

Creating large datasets (with dask.distributed)

If you will be creating datasets larger than a few 100MB you may want to use dask.distributed.LocalCluster to parallelise the creation of the dataset. This can be done by setting the --dask-distributed-local-core-fraction flag to a value between 0.0 and 1.0. This will create a local dask.distributed cluster with the number of workers set to the number of cores on the machine multiplied by the fraction given. For example, to use 50% of the cores on the machine you would run:

python -m mllam_data_prep example.danra.yaml --dask-distributed-local-core-fraction 0.5

Unfortunately, the number of cores to use can only be worked out by trial and error, but a good starting point is to use 50% of the cores on the machine and then if you notice warnings suggesting that workers are running out of memory you should reduce the fraction of cores used (so that each worker has more memory available). You can also adjust the fraction of the total system memory allocated with --dask-distributed-local-memory-fraction (default is 0.9).

When you run the above command the console will print a URL to the dask dashboard, which you can open in a browser to monitor the progress of the dataset creation (and see the memory usage of the workers).

Usage as a python module

The package can also be used as a python module to create datasets directly, for example to create training datasets during training. The main function to use is mllam_data_prep.create_dataset(config) which takes a mllam_data_prep.Config as input and returns a xarray.Dataset object. For example:

import mllam_data_prep as mdp

config_path = "example.danra.yaml"
config = mdp.Config.from_yaml_file(config_path)
ds = mdp.create_dataset(config=config)

Configuration file

A full example configuration file is given in example.danra.yaml, and reproduced here for completeness:

schema_version: v0.2.0
dataset_version: v0.1.0

output:
  variables:
    static: [grid_index, static_feature]
    state: [time, grid_index, state_feature]
    forcing: [time, grid_index, forcing_feature]
  coord_ranges:
    time:
      start: 1990-09-03T00:00
      end: 1990-09-09T00:00
      step: PT3H
  chunking:
    time: 1
  splitting:
    dim: time
    splits:
      train:
        start: 1990-09-03T00:00
        end: 1990-09-06T00:00
        compute_statistics:
          ops: [mean, std, diff_mean, diff_std]
          dims: [grid_index, time]
      validation:
        start: 1990-09-06T00:00
        end: 1990-09-07T00:00
      test:
        start: 1990-09-07T00:00
        end: 1990-09-09T00:00

inputs:
  danra_height_levels:
    path: https://mllam-test-data.s3.eu-north-1.amazonaws.com/height_levels.zarr
    dims: [time, x, y, altitude]
    variables:
      u:
        altitude:
          values: [100,]
          units: m
      v:
        altitude:
          values: [100, ]
          units: m
    dim_mapping:
      time:
        method: rename
        dim: time
      state_feature:
        method: stack_variables_by_var_name
        dims: [altitude]
        name_format: f"{var_name}{altitude}m"
      grid_index:
        method: stack
        dims: [x, y]
    target_output_variable: state

  danra_surface:
    path: https://mllam-test-data.s3.eu-north-1.amazonaws.com/single_levels.zarr
    dims: [time, x, y]
    variables:
      # use surface incoming shortwave radiation as forcing
      - swavr0m
    dim_mapping:
      time:
        method: rename
        dim: time
      grid_index:
        method: stack
        dims: [x, y]
      forcing_feature:
        method: stack_variables_by_var_name
        name_format: f"{var_name}"
    target_output_variable: forcing

  danra_lsm:
    path: https://mllam-test-data.s3.eu-north-1.amazonaws.com/lsm.zarr
    dims: [x, y]
    variables:
      - lsm
    dim_mapping:
      grid_index:
        method: stack
        dims: [x, y]
      static_feature:
        method: stack_variables_by_var_name
        name_format: f"{var_name}"
    target_output_variable: static

Apart from identifiers to keep track of the configuration file format version and the dataset version (for you to keep track of changes that you make to the dataset), the configuration file is divided into two main sections:

• output: defines the variables and dimensions of the output dataset produced by mllam-data-prep. These are the variables and dimensions that the input datasets will be mapped to. These output variables and dimensions should match the input variables and dimensions expected by the model architecture you are training.
• inputs: a list of source datasets to extract data from. These are the datasets that will be mapped to the architecture defined in the architecture section.

The output section

output:
  variables:
    static: [grid_index, static_feature]
    state: [time, grid_index, state_feature]
    forcing: [time, grid_index, forcing_feature]
  coord_ranges:
    time:
      start: 1990-09-03T00:00
      end: 1990-09-09T00:00
      step: PT3H
  chunking:
    time: 1
  splitting:
    dim: time
    splits:
      train:
        start: 1990-09-03T00:00
        end: 1990-09-06T00:00
        compute_statistics:
          ops: [mean, std, diff_mean, diff_std]
          dims: [grid_index, time]
      validation:
        start: 1990-09-06T00:00
        end: 1990-09-07T00:00
      test:
        start: 1990-09-07T00:00
        end: 1990-09-09T00:00

The output section defines three things:

1. variables: what input variables the model architecture you are targeting expects, and what the dimensions are for each of these variables.
2. coord_ranges: the range of values for each of the dimensions that the model architecture expects as input. These are optional, but allows you to ensure that the training dataset is created with the correct range of values for each dimension.
3. chunking: the chunk sizes to use when writing the training dataset to zarr. This is optional, but can be used to optimise the performance of the zarr dataset. By default the chunk sizes are set to the size of the dimension, but this can be overridden by setting the chunk size in the configuration file. A common choice is to set the dimension along which you are batching to align with the of each training item (e.g. if you are training a model with time-step roll-out of 10 timesteps, you might choose a chunksize of 10 along the time dimension).
4. Splitting and calculation of statistics of the output variables, using the splitting section. The output.splitting.splits attribute defines the individual splits to create (for example train, val and test) and output.splitting.dim defines the dimension to split along. The compute_statistics can be optionally set for a given split to calculate the statistical properties requested (for example mean, std) any method available on xarray.Dataset.{op} can be used. In addition methods prefixed by diff_ (so the operational would be listed as diff_{op}) to compute a statistic based on difference of consecutive time-steps, e.g. diff_mean to compute the mean of the difference between consecutive timesteps (these are used for normalisating increments). The dims attribute defines the dimensions to calculate the statistics over (for example grid_index and time).

The inputs section

inputs:
  danra_height_levels:
    path: https://mllam-test-data.s3.eu-north-1.amazonaws.com/height_levels.zarr
    dims: [time, x, y, altitude]
    variables:
      u:
        altitude:
          values: [100,]
          units: m
      v:
        altitude:
          values: [100, ]
          units: m
    dim_mapping:
      time:
        method: rename
        dim: time
      state_feature:
        method: stack_variables_by_var_name
        dims: [altitude]
        name_format: f"{var_name}{altitude}m"
      grid_index:
        method: stack
        dims: [x, y]
    target_architecture_variable: state

  danra_surface:
    path: https://mllam-test-data.s3.eu-north-1.amazonaws.com/single_levels.zarr
    dims: [time, x, y]
    variables:
      # shouldn't really be using sea-surface pressure as "forcing", but don't
      # have radiation varibles in danra yet
      - pres_seasurface
    dim_mapping:
      time:
        method: rename
        dim: time
      grid_index:
        method: stack
        dims: [x, y]
      forcing_feature:
        method: stack_variables_by_var_name
        name_format: f"{var_name}"
    target_architecture_variable: forcing

  ...

The inputs section defines the source datasets to extract data from. Each source dataset is defined by a key (e.g. danra_height_levels) which names the source dataset, and the attributes of the source dataset:

• path: the path to the source dataset. This can be a local path or a URL to e.g. a zarr dataset or netCDF file, anything that can be read by xarray.open_dataset(...).
• dims: the dimensions that the source dataset is expected to have. This is used to check that the source dataset has the expected dimensions and also makes it clearer in the config file what the dimensions of the source dataset are.
• variables: selects which variables to extract from the source dataset. This may either be a list of variable names, or a dictionary where each key is the variable name and the value defines a dictionary of coordinates to do selection on. When doing selection you may also optionally define the units of the variable to check that the units of the variable match the units of the variable in the model architecture.
• target_architecture_variable: the variable in the model architecture that the source dataset should be mapped to.
• dim_mapping: defines how the dimensions of the source dataset should be mapped to the dimensions of the model architecture. This is done by defining a method to apply to each dimension. The methods are:
  • rename: simply rename the dimension to the new name
  • stack: stack the listed dimension to create the dimension in the output
  • stack_variables_by_var_name: stack the dimension into the new dimension, and also stack the variable name into the new variable name. This is useful when you have multiple variables with the same dimensions that you want to stack into a single variable.