lagrangian-filtering

Temporal filtering of data in a Lagrangian frame of reference. Visit the documentation, or continue reading below.

Overview

This provides a library and a post-processing analysis tool for the removal of sub-inertial frequencies from data in a Lagrangian frame of reference. This may be useful, for example, for distinguishing stationary internal waves. At a high level, the algorithm looks like:

1. particle advection using OceanParcels
2. sampling of data (e.g. velocity, density) along particle tracks
3. temporal filtering
4. writing filtered data back to disk

Citing

The description and analysis of the Lagrangian filtering method has been published in JAMES, and can be cited as:

Shakespeare, C. J., Gibson, A. H., Hogg, A. M., Bachman, S. D., Keating, S. R., & Velzeboer, N. (2021). A new open source implementation of Lagrangian filtering: A method to identify internal waves in high-resolution simulations. Journal of Advances in Modeling Earth Systems, 13, e2021MS002616. https://doi.org/10.1029/2021MS002616

Installation

Using Conda

If you use Conda to manage Python packages, you may simply run conda install -c angus-g -c conda-forge lagrangian-filtering to install this package and its required dependencies. You may want to constrain this to its own conda environment instead, with conda create -n filtering -c angus-g -c conda-forge lagrangian-filtering. The environment can be activated with conda activate filtering and deactivated with conda deactivate.

Using pip or developing

If you don't use Conda, or are looking to develop this package, it is easier to get started using Python's pip package manager. Optionally, the package can be installed inside a virtualenv virtual environment, for cleaner separation from your native Python environment. There are very few dependencies, but a custom branch of OceanParcels is necessary to get acceptable performance at the moment, and the GCC compiler with OpenMP support is also required to compile the runtime kernels. During the development phase, I recommend installing this as a "development package", meaning that changes to your local checkout are instantly reflected in your Python environment.

1. Clone this repository git clone https://github.com/angus-g/lagrangian-filtering
2. Change to the directory cd lagrangian-filtering
3. (Optional) Create the virtualenv virtualenv env and activate it source env/bin/activate
4. Install the development version of the package pip install -e .
5. Install the dependencies required for parcels pip install -r requirements.txt

Upgrading

With the package installed in development mode, updating is as easy as git pull (or making local changes) in the lagrangian-filtering directory. If dependencies (particularly parcels) need to be updated, run pip install --upgrade --upgrade-strategy eager . to force installation of the new versions.

Usage

For the moment, it's easiest to set up the filtering workflow in a script or a jupyter notebook. An example looks like:

```python import filtering from datetime import timedelta

filenames = { "U": "/data/datawaveU.nc", "V": "/data/datawaveV.nc" } variables = {"U": "U", "V": "V"} dimensions = {"lon": "X", "lat", "Y", "time": "T"}

f = filtering.LagrangeFilter( "waves", filenames, variables, dimensions, samplevariables=["U"], mesh="flat", windowsize=timedelta(days=2).total_seconds() )

f() ```

This uses velocity data from the two specified files. Zonal velocity data from the U variable will be sampled and filtered, with a filtering window for 2 days on either side of each sample (i.e. a 4-day window for filtering).

3D input data

For 3D input data (i.e. with a Z dimension), there are a couple of options. Running the filtering as the example above will load only the first Z slice (likely the surface). For more precise control of the Z dimension, modify the dimensions dictionary, and specify the level to load in the indices dictionary. For example, to load only the 21st Z slice:

```python dimensions = {"lon": "X", "lat": "Y", "time": "T", "depth", "Z"} indices = {"depth": [20]}

f = filtering.LagrangeFilter( ... # other parameters as normal indices=indices, ... # any other keyword parameters ) ```

Beware that specifyng the depth dimension without restricting its indices will load data through full depth, however particles will only be seeded at the top. This would be a huge expense for no gain!