Air parcel trajectories for the expedition ARTofMELT, Arctic Ocean, 2023 — raw data

Sonja Murto, Michael Tjernström

10-day backward and forward air parcel trajectories were calculated along the ship track of the Swedish icebreaker Oden during the ARTofMELT (Atmospheric Rivers and the Onset of Arctic Melt) expedition in the Fram Strait pack ice during the spring transition, 8 May to 13 June of 2023. Trajectories were calculated both from the ship's position and within a circle with origo at the ship and a radius of 100 km.

Trajectories from the ship position were initialised every 1-hour. Trajectories from within a circle were initialised every 6-hour at locations 30 km equidistant from each other. All trajectories were initialised at 14 different vertical starting levels. The trajectories were calculated with the Lagrangian Analysis Tool LAGRANTO on ERA5 data — fifth generation of ECMWF atmospheric reanalysis data.

This dataset contains the raw native data for all of the four different sets of trajectory runs. Data tracked and calculated along the tracks by the trajectory model, e.g., longitude, latitude, pressure, temperature and relative humidity, are included. Four associated user-friendly datasets, with additional surface variables interpolated along the tracks but reduced to 7-day trajectories, are available separately for each set of trajectory run.

The data enable analysis of air mass origin and air mass transformation of air parcels starting or ending up at and around Oden during the ARTofMELT expedition in the Arctic spring of 2023.

Related objects

Datasets

• Backward trajectories from the ship
• Forward trajectories from the ship
• Backward trajectories from within a circle around the ship
• Forward trajectories from within a circle around the ship

Code

• Scripts for creating and plotting netCDF files

Citation

Sonja Murto, Michael Tjernström (2024) Air parcel trajectories for the expedition ARTofMELT, Arctic Ocean, 2023 — raw data. Dataset version 1. Bolin Centre Database. https://doi.org/10.17043/oden-artofmelt-2023-trajectories-raw-1

References

Murto S, Tjernström M (2024a) Backward trajectories along the ship track using the LAGRANTO model, for the expedition ARTofMELT, Arctic Ocean, 2023. Dataset version 1. Bolin Centre Database. https://doi.org/10.17043/oden-artofmelt-2023-trajectories-backward-ship-1

Murto S, Tjernström M (2024b) Forward trajectories along the ship track using the LAGRANTO model, for the expedition ARTofMELT, Arctic Ocean, 2023. Dataset version 1. Bolin Centre Database. https://doi.org/10.17043/oden-artofmelt-2023-trajectories-forward-ship-1

Murto S, Tjernström M (2024c) Backward trajectories within a circle around the ship using the LAGRANTO model, for the expedition ARTofMELT, Arctic Ocean, 2023. Dataset version 1. Bolin Centre Database. https://doi.org/10.17043/oden-artofmelt-2023-trajectories-backward-circle-1

Murto S, Tjernström M (2024d) Forward trajectories within a circle around the ship using the LAGRANTO model, for the expedition ARTofMELT, Arctic Ocean, 2023. Dataset version 1. Bolin Centre Database. https://doi.org/10.17043/oden-artofmelt-2023-trajectories-forward-circle-1

Murto S (2024) Scripts for creating and plotting netCDF files from air parcel trajectories for the expedition ARTofMELT, Arctic Ocean, 2023. Software version 1.0.0. Bolin Centre Code Repository. https://doi.org/10.57669/oden-artofmelt-2023-trajectories-1.0.0

Sprenger M, Wernli H (2015) The LAGRANTO Lagrangian analysis tool – version 2.0. Geoscientific Model Development 8:2569–2586. https://doi.org/10.5194/gmd-8-2569-2015

Data description

This dataset is provided in one .zip file that contains four .zip files, one for each set of trajectory run. The total file size for the entire compressed dataset is about 1.11 GB. File sizes for the different trajectory runs are given in the list below:

• Trajectories_raw_text_files_backward_AoM23_from_Oden.zip: 888 text files for backward trajectories initialised from Oden (total compressed size about 82 MB, size of one text file is 274 kB)
• Trajectories_raw_text_files_forward_AoM23_from_Oden.zip: 888 text files for forward trajectories initialised from Oden (total compressed size about 84 MB, size of one text file is 274 KB)
• Trajectories_raw_text_files_backward_AoM23_from_circle.zip: 148 text files for backward trajectories initialised from within a circle around Oden (total compressed size about 469 MB, size of one text file is about 9 MB, varying depending on the number of trajectories within the circle)
• Trajectories_raw_text_files_forward_AoM23_from_circle.zip: 148 text files for forward trajectories initialised from within a circle around Oden (total compressed size about 563 MB, size of one text file is about 9 MB, varying depending on the number of trajectories within the circle)

An explanation of the four sets of trajectory runs, the naming rule and a list of all variable names, with short explanations and units, can be found in an associated Readme_Traj_raw.txt file.

Data file structure

The structure of the text files is the same for all trajectory runs.

File names

Each text file is named according to the initialisation time and the type of trajectory, in the following way:

• traj_ARTofMELT_ERA5_YYYYMMDD_HH_XW_type, where
  • YYYYMMDD_HH is the date and hour for trajectory initialisation
  • XW can be either BW (backward) or FW (forward)
  • type can be either fromOden or circle.

All times are in UTC.

File content

• Line 1 contains information of the trajectory run setup, further explained in the Readme_Traj_raw.txt file.
• Line 2 is a line break
• Line 3 is a header row
• Line 4 is a row with hyphens
• Line 5 is a line break
• Line 6 and onwards contains the data along a trajectory at hourly intervals.
• Values on one row refer to the same time step along the trajectories.
• Space is used as value separator.
• A line break separates one trajectory from the next trajectory, with a new trajectory always starting at time 0.00 (at the initialisation time and location).

The trajectories in one text file are presented according to the chosen vertical level, starting with the trajectory initialised from the surface. For these trajectory runs, the 14 starting vertical levels are following:

• every 10 hPa between 10 and 50 hPa AGL
• every 50 hPa between 100 and 300 hPa AGL
• every 100 hPa between 600 and 300 hPa.

For trajectories initialised along the ship track, only 14 trajectories are provided in the text files. In the case of several trajectories starting from the same vertical level, which is true for trajectories started within a circle around the ship (31⁠ – ⁠37 trajectories pro vertical starting level), all trajectories initialised from the same vertical starting level are presented after each other prior to moving to the next starting vertical level.

Variables

Variables in the text file are divided into primary variables traced directly by the model and variables derived within the model using built-in functions.

• Primary variables traced by the model
  • default output variables: time, longitude, latitude and pressure
  • traced output variables: air temperature and specific humidity
• Variables calculated by the model using the primary variables
  • air potential temperature and relative humidity
  • distance from start

Detailed list:

• Primary variables traced by the model
  • time Time along the trajectories relative to the reference date and initialisation time (in hours). Negative hours indicate backwards in time.
  • lon Longitude along the trajectories (degrees_east)
  • lat Latitude along the trajectories (degrees_north)
  • p Pressure along the trajectories (hPa)
  • T Air temperature along the trajectories (K). This variable is traced directly from the input data files.
  • Q Specific humidity along the trajectories (g/kg). This variable is traced directly from the input data files.
• Calculated variables traced by the model
  • TH Air potential temperature along the trajectories (K).
  • DIST0 Spherical distance of the trajectory from the starting position (km)
  • RH Relative humidity along the trajectories (%)

Dimensions

The dimension for the trajectories are time along the tracks (from 0 to −240 or 240), where a time of 0.00 h refers to the trajectory initialisation time. Initialisation time is between 8 May and 13 June 2023. Trajectories are initialised every 1-hour at the location of Oden, and every 6-hour for trajectories within the circle around Oden.

Comments

Trajectory model

Trajectories are calculated by the LAGRANTO model (Sprenger & Wernli, 2015) on ERA5 data on model levels with its original temporal and spatial resolution (1 h and 0.25 degrees). Trajectory computations are performed on ECMWF’s high-performance computing facility (HPCF). In order for LAGRANTO to run, the meteorological input data need to be converted into a format that is readable for LAGRANTO. Apart from the compulsory 3D wind fields (zonal, meridional and vertical winds) and surface pressure, the input data files for LAGRANTO include air temperature and specific humidity. These variables are directly traced within the LAGRANTO model, whereas other variables traced along the tracks are derived by the model with built-in functions. To avoid air parcels to get stuck at the same position in case of hitting the ground, a "jumping flag" is given as an optional parameter in the trajectory calculations, meaning that the air parcel in such a case will be raised a little from the ground in order to continue its travel.

Usefulness

This dataset contains the raw text data files for all the four different trajectory runs.

Associated processed data and code

The four associated user-friendly datasets, in NetCDF format, derived from the raw text files, are available separately. Note that these NetCDF data files include additional ERA5 data on surface levels that are interpolated along the tracks and that the length of the trajectories are narrowed to 7-days. The associated datasets are:

1. Backward trajectories started every hour along the ship track, by Murto & Tjernström (2024a)
2. Forward trajectories started every hour along the ship track, by Murto & Tjernström (2024b)
3. Backward trajectories started every 6-hours within a circle around the ship, by Murto & Tjernström (2024c)
4. Forward trajectories started every 6-hours within a circle around the ship, by Murto & Tjernström (2024d)

The computer codes in Python, written by Murto (2024e), used to derive the processed datasets in NetCDF format from the raw data files are also available. In the same code repository, we also provide python codes for reading in the netCDF files and plotting the trajectories, as well as example figures to illustrate the trajectories.

Data creator's role

Trajectories were computed post expedition from the ship's GPS location along the track of the ARTofMELT 2023 expedition. The set-up for the trajectory runs was performed by one of the members of the Stockholm University Meteorology team onboard Oden:

• Sonja Murto (dataset creator)

Additionally:

• Michael Tjernström (chief scientist; project PI)

Acknowledgements

Anna Lewinschal (scientific programmer at Stockholm University) provided with example raw-code for runs on the ECMWF's supercomputing facility. The original setup of the LAGRANTO model to the supercomputer facility was done by Moritz Deinhard (researcher at Karlsruher Institut für Technologie, Germany).

The expedition

Data were produced for the ARTofMELT 2023 expedition on board the Swedish icebreaker Oden, which was organized by the Swedish Polar Research Secretariat from 8 May through 14 June, 2023.