ASI AMSR2 (Univ. Bremen)¶

University of Bremen ASI (ARTIST Sea Ice) Sea Ice Concentration from AMSR-E/AMSR2, version 5.4

Available

At a glance¶

Note on period: The v5.4 record covers AMSR-E from 2002-06 to 2011-09 and AMSR2 from 2012-07 to present (gap between the two sensors from 2011-10 to 2012-07). All AMSR-E and AMSR2 data were reprocessed in 2018 with identical settings as v5.4; earlier near-real-time data are v5.3 (from 2017-05-02) and v5.2. The yearly bundles on PANGAEA run to 2019 for AMSR2; later years are on the University of Bremen server.

Overview¶

Daily Arctic and Antarctic sea ice concentration produced by the University of Bremen with the ASI (ARTIST Sea Ice) algorithm, which uses the polarization difference of the AMSR-E and AMSR2 89 GHz channels. Its 6.25 km grid is four times finer than the 25 km grids of the NOAA/NSIDC CDR and the OSI SAF climate records, so it resolves ice edges, polynyas and leads much better. Updated every morning, it is widely used for icebreaker routing, field campaign planning and operational ice charting, and is available without login as HDF4, GeoTIFF, netCDF and PNG maps. It was not designed as a climate data record, so long-term trends are best checked against the 25 km climate records.

What changes with the successor¶

ASI AMSR2 v5.4 → ASI AMSR3 (preliminary) Planned¶

Transition: 2026-08

With the AMSR2 host GCOM-W in its latter phase and JAXA releasing products from the successor AMSR3 (GOSAT-GW) since 2026-06-30, the University of Bremen has started posting preliminary AMSR3-based ASI concentrations in an amsr3/ folder. As of 2026-10-13, AMSR2 v5.4 is still being updated, and no official version name, switch-over date or reprocessing plan for the AMSR3 product has been announced.

Sources: University of Bremen data server, amsr3/ folder · JAXA G-Portal — GOSAT-GW/AMSR3 Products Now Available (2026-06-30) · JAXA — GCOM-W satellite page (Latter Phase In Operation) · OSI SAF — Implications of AMSR2 being replaced by AMSR3 on OSI SAF products

ASI v5.2 and v5.3 (near real time) → ASI v5.4¶

Transition: 2018

In 2018 all AMSR-E and AMSR2 data for both poles were reprocessed with the same parameters, settings and software as v5.4. Near-real-time data produced before that are v5.3 (from 2017-05-02) and v5.2.

Sources: ASI Version 5 Sea Ice Concentration User Guide (V0.9.4) · University of Bremen data archive page (value encoding)

Available variables¶

The user guide says only that each daily file holds one two-dimensional sea ice concentration field (%) and does not give the variable name inside the netCDF files, so no name is listed here (check with xarray). According to the data archive page, concentrations are scaled 0–100 with land and missing values set to 120 (older files: 0–200, land/missing 255). Latitudes and longitudes for the HDF4 files are in the separate grid_coordinates/ folder.

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

Daily grids (n6250, s6250; HDF4, GeoTIFF, netCDF)¶

Per-orbit swaths (swath, netCDF)¶

The full variable list is in the official documentation.

How to get the data¶

• University of Bremen data server (HTTPS) — Direct HTTPS download (e.g. n6250/netcdf/2026/asi-AMSR2-n6250-20261012-v5.4.nc; HDF4, GeoTIFF and PNG under n6250/2026/oct/Arctic/) No login needed
  • No login needed. Under n6250 (Arctic) and s6250 (Antarctic), HDF4, GeoTIFF and PNG files sit in year/month/region folders, and yearly netCDF folders are under netcdf/, with past years also bundled as yearly zip files. The same structure is reachable at https://seaice.uni-bremen.de/data/.
• University of Bremen data server (anonymous FTP) — Anonymous FTP (user name anonymous, no password) No login needed
  • Same folder structure as HTTPS. For bulk downloads the archive page suggests recursive tools such as wget -r.
• PANGAEA — AMSR2 ASI v5.4 yearly netCDF (Arctic and Antarctic) — Web download (DOI-registered archive, 2012-07 to 2019-12) No login needed
  • The Antarctic dataset is https://doi.org/10.1594/PANGAEA.898400; AMSR-E (2002-06 to 2011-09) is PANGAEA.919777 (Arctic) and 919778 (Antarctic).
• University of Bremen data browser (map viewer) — Web maps (browse daily concentration maps)

Example code¶

Downloads one Arctic 6.25 km netCDF file from the University of Bremen server and opens it. Because the user guide does not name the variable, the example picks the largest data variable as the concentration field; run print(ds) first to check the name.

import urllib.request
import xarray as xr

day = "20261012"
url = f"https://data.seaice.uni-bremen.de/amsr2/asi_daygrid_swath/n6250/netcdf/{day[:4]}/asi-AMSR2-n6250-{day}-v5.4.nc"
urllib.request.urlretrieve(url, "asi.nc")

ds = xr.open_dataset("asi.nc")
print(ds)                                                   # check variable names and projection

sic = max(ds.data_vars.values(), key=lambda v: v.size)      # 2-D sea ice concentration (%)
sic = sic.where(sic <= 100)                                 # drop land/missing (120)
print("Cells with concentration >= 15%:", int((sic >= 15).sum()))

Practical tips¶

• Compared with the NOAA/NSIDC CDR (../nsidc-cdr-sic/) and OSI SAF (../osisaf-sic/) in the same tab, the difference is resolution and purpose. Those two are 25 km climate records with consistency across sensors, and they provide per-cell uncertainty (OSI SAF) or a standard deviation between algorithms (NSIDC). ASI uses 89 GHz to reach 6.25 km, but it is not an inter-calibrated long-term record and has no per-cell uncertainty variable. Use the climate records for long-term extent and area trends, and ASI for detailed structures such as ice edges, leads and polynyas.
• The user guide gives an error of about 25% at 0% concentration and about 5.7% at 100%, below 10% above 65% concentration. Low concentrations carry large relative errors, so take care when locating the 15% edge precisely.
• To remove spurious ice from atmospheric water vapour and clouds, concentration is set to 0 where the gradient ratios exceed thresholds (GR37V/18V > 0.045, GR23V/18V > 0.04) or the Bootstrap concentration is 5% or less, and outside the maximum ice extent for that month over the past 40 years. A value of 0% is therefore partly the result of these filters and masks.
• Within about 20 km of the coast, land emission can produce non-zero concentration over open water. Filter this band in East Asian seasonal ice zones with complex coastlines such as the Sea of Okhotsk, and in narrow bays. Daily regional 3.125 km maps exist for the Sea of Okhotsk (SeaOfOkhotsk folders under n3125), but that grid was not reprocessed in v5.4.
• There is a gap of about nine months between AMSR-E (2002–2011) and AMSR2 (2012–), and AMSR2 brightness temperatures are converted to AMSR-E equivalents. State how the gap and the sensor boundary are handled when building a combined series.
• The server's SeaIceConcentration/ASI_CDR/ folder holds a separate 1991–2024 6.25 km EASE2 sea ice concentration record (v1, posted 2026-08) in which ASI is adapted to SSM/I and SSMIS. It uses different sensors and a different grid from the AMSR2 v5.4 product described here, so do not mix the two. No DOI or official citation has been given yet.
• During the move to AMSR3, the same date may give different values in the amsr2/ and amsr3/ folders. When checked on 2026-10-13 the readme files in amsr2/ and amsr3/ had swapped contents, so identify the sensor from the file name (asi-AMSR2-…, amsr3-…) rather than the folder name.

Notices and change history¶

References for your paper¶

Dataset citation¶

AMSR2 ASI v5.4 Arctic (PANGAEA) — DOI 10.1594/PANGAEA.898399

@misc{melsheimer2019asi_arctic,
  author    = {Melsheimer, Christian and Spreen, Gunnar},
  title     = {{AMSR2} {ASI} sea ice concentration data, {Arctic}, version 5.4 ({NetCDF}) (July 2012 - December 2019)},
  year      = {2019},
  publisher = {PANGAEA},
  doi       = {10.1594/PANGAEA.898399}
}

AMSR2 ASI v5.4 Antarctic (PANGAEA) — DOI 10.1594/PANGAEA.898400

@misc{melsheimer2019asi_antarctic,
  author    = {Melsheimer, Christian and Spreen, Gunnar},
  title     = {{AMSR2} {ASI} sea ice concentration data, {Antarctic}, version 5.4 ({NetCDF}) (July 2012 - December 2019)},
  year      = {2019},
  publisher = {PANGAEA},
  doi       = {10.1594/PANGAEA.898400}
}

AMSR-E ASI v5.4 Arctic (PANGAEA, for 2002–2011) — DOI 10.1594/PANGAEA.919777

Reference papers¶

ASI algorithm paper (requested by the University of Bremen) — DOI 10.1029/2005JC003384

@article{spreen2008asi,
  author  = {Spreen, Gunnar and Kaleschke, Lars and Heygster, Georg},
  title   = {Sea ice remote sensing using {AMSR-E} 89-{GHz} channels},
  journal = {Journal of Geophysical Research: Oceans},
  volume  = {113},
  number  = {C2},
  pages   = {C02S03},
  year    = {2008},
  doi     = {10.1029/2005JC003384}
}

Acknowledgements¶

To help maintain the service, the University of Bremen asks publications to cite Spreen et al. (2008) and acknowledge the source (University of Bremen, seaice.uni-bremen.de). Data from 2020 onward taken from the server have no DOI, so state the version (v5.4) and the download date.

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• University of Bremen — AMSR-E/AMSR2 sea ice concentration page
• ASI Version 5 user guide (PDF)
• University of Bremen — data archive (FTP, HTTPS, file formats)
• University of Bremen — news archive
• Grid coordinate files (latitude and longitude for HDF4)
• This site — OSI SAF sea ice concentration (25 km climate record, AMSR3 transition)
• This site — NOAA/NSIDC CDR sea ice concentration (25 km climate record)
• This site — SMOS thin sea ice thickness (University of Bremen and AWI)