Algorithm Summary

Code for identifying atmospheric river (AR) outlines by applying size, shape, and transport direction criteria to fields of integrated water vapor transport (IVT) from gridded meteorological analyses.

Version 1 of the algorithm is described in Mattingly et al. (2018). The AR identification criteria are based on the Guan and Waliser (2015) and Mundhenk et al. (2016) algorithms, but with less strict requirements for minimum IVT threshold, object shape, and transport direction in order to capture ARs in the polar regions. This version (v2) includes the following updates:

• Reorganization of the code for speed and clarity
• Option to easily change AR identification parameters through the included configuration file
• Ability to calculate IVT on native model levels in addition to constant pressure levels
• Ability to identify ARs in the Southern Hemisphere as well as the Northern Hemisphere
• Default behavior is now to use IVT vector components for transport direction, rather than lower tropospheric mean wind vector
• Default cutoff latitude for AR meridional transport requirement is now the Arctic / Antarctic circle (66.56°N / -66.56°S), rather than 70°N / -70°S

To date, the algorithm has been successfully used to identify ARs in MERRA-2 and ERA5 reanalysis data, as well as ARTMIP Tier 2 paleoclimate simulations. Please feel free to contact me if you are interested in applying the algorithm to other gridded meteorological data sources.

Dependencies¶

The code requires a python 3 environment with the following packages:

• numpy
• scipy
• xarray
• pandas
• netcdf4-python
• scikit-image
• geopy
• hjson

Additionally, the directory containing the project code must be included in the user’s python path.

How to run¶

The AR identification procedure requires gridded fields of IVT magnitude, u/v-IVT components, and IVT values at the desired climatological percentile rank threshold (default: 85th percentile) as input. Optionally, lower-tropospheric mean wind may be used in place of u/v-IVT components for transport direction criteria.

The code is somewhat flexible with regard to the time properties of input and output files (the start time, end time, and interval between timesteps in hours); for data with 6-hourly or more frequent temporal resolution, structuring the data as monthly input and output files is recommended.

The typical work flow for producing AR data is:

• Save a copy of the AR_ID_config.hjson template configuration file and edit configuration options. These options include the input data source, data directories, input file characteristics, spatial domain and grid, IVT calculation parameters, and AR identification parameters. More details on each item can be found in the comments in the configuration file.
• Run calc_IVT.py <begin_time> <end_time> <timestep_hrs> <AR_ID_config_path> to calculate IVT u/v-components and vector magnitude from 3D input fields of u/v-wind components and specific humidity. This step can be skipped if IVT is provided as pre-calculated files, such as ARTMIP input data.
  • (Use calc_IVT_ERA5.py to calculate IVT magnitude from the u/v-IVT fields provided as pre-calculated fields in ERA5.)
• Run calc_IVT_at_percentiles.py <start_doy> <end_doy> <AR_ID_config_path> to calculate IVT values at the climatological percentile rank(s) specified in the configuration file (with climatology defined as the grid-cell-specific 31-day centered IVT distribution for the IVT_climo_start_year, IVT_climo_end_year, and IVT_climo_timestep_hrs specified in the configuration file).
  • (Use calc_IVT_at_pctiles_ARTMIP_tier2_paleo.py for ARTMIP Tier 2 paleo data.)
• Run ARs_ID.py <begin_time> <end_time> <timestep_hrs> <AR_ID_config_path> to identify final AR outlines.
  • (Use ARs_ID_ARTMIP_tier2_paleo.py for ARTMIP Tier 2 paleo data.)

Two important notes on running ARs_ID.py:

• The AR identification code must be run separately for the Northern and Southern Hemisphere. Minimum (maximum) latitude of NH (SH) AR output data is 10°N (-10°S). However, global input files (IVT, IVT at percentiles) can be used by the AR algorithm.
• The input files (IVT, IVT at percentiles) must span the entire globe zonally (e.g. MERRA-2 data, with resolution 0.5° lat by 0.625° lon, must have longitudes extending from -180°W to 179.375°E).

The scripts/ directory contains miscellaneous scripts that may be useful, including driver scripts for processing multiple years of data, downloading ERA5 data, chunking large ERA5 files for manageable processing, and working with ARTMIP data.