Tracks of negative upper-tropospheric PV anomalies (PVAs$^{-}$) over the Northern Hemisphere in ERA5 reanalysis (1979-2021)

The identification and tracking of PVAs$^{-}$ is based on ERA5 reanalysis data on model levels from the European Centre for Medium-Range Weather Forecasts (ECMWF) with a horizontal resolution of 0.5° and a temporal resolution of 3 hours (Hersbach et al., 2020). The following data covering the period 1979 to 2021 are stored in this archive:

(1) Fields of upper-tropospheric PV anomalies: A vertical, weighted average is performed based on model level ERA5 data between 500 and 150hPa. Anomalies are calculated based on a 30-day centered running-mean climatology (1979-2021). This variable (vertically-averaged PV anomaly; ‘VAPVA’; in PV Units) is stored in the netCDF files in the archive.

(2) Identified negative, upper-tropospheric PV anomaly objects: Due to the interest in their link to atmospheric blocking, negative anomalies as 2D objects (PVAs$^{-}$) are identified using a percentile threshold (described in detail in Hauser et al., 2024). These objects are stored along the fields of upper-tropospheric PV anomalies in the netCDF files in the variable ‘labels’. Note that the label 0 indicates no presence of a PVA$^{-}$ and the labels >0 represent the track ID (see next bullet point).

(3) Tracks of PVAs$^{-}$: The tracking of PVAs$^{-}$ is based on contour overlap and considers a handling of splitting and merging events (see Hauser et al., 2023 for details and a schematic figure on the tracking algorithm). Each txt-files in this archive represents the track information for one year. The structure of the file is based on time, i.e. for each point in time it provides information on which track ID is active, and whether it is a new track, the end of a track or a continuation of a track. There is also information on splitting and merging between the previous and current time step. Detailed information on how to read these files is given in the README.txt file in the same archive.

(4) Helpful python functions to handle the PVA$^{-}$ tracks: In a jupyter notebook (demonstration.ipynb, demonstration.pdf), we provide some basic functions and an example on how to work with the tracking data, e.g. how to retrieve the lifetime of a PVA$^{-}$ track ID and how to calculate basic characteristics such as center of mass (latitude-longitude coordinates), amplitude, size, etc. along the track.

References:
Hauser, S., Teubler, F., Riemer, M., Knippertz, P., and Grams, C. M. (2024): Life cycle dynamics of Greenland blocking from a potential vorticity perspective, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-2945.

Hersbach, H., Bell, B., Berrisford, P., et al (2020): The ERA5 global reanalysis. Quarterly Journal of the Royal Meteorological Society; 146: 1999–2049. https://doi.org/10.1002/qj.3803.