Metop ASCAT ocean winds¶

Metop Advanced Scatterometer (ASCAT) ocean surface winds — EUMETSAT OSI SAF L2 wind products

Available

At a glance¶

Note on period: Near-real-time Metop-A ASCAT winds started as a demonstration product on 2007-03-28, became operational on 2008-12-03 and ended on 2021-11-15. Metop-B followed with a first development version on 2012-11-14 and operational status on 2013-05-27, and Metop-C with a demonstration on 2019-02-12 and operational status in October 2019. The reprocessed Metop-A climate data record OSI-150 starts on 2007-01-01; its end date is given as 2014-12-31 on the OSI SAF page but as March 2014 (2014-04-01) on the KNMI page and at PO.DAAC.

Overview¶

ASCAT is a C-band (5.255 GHz) scatterometer on Europe's Metop weather satellites. Beams pointing in three azimuth directions illuminate two swaths, each about 550 km wide, on either side of the ground track, and the 10 m wind speed and direction are retrieved from sea-surface roughness. EUMETSAT OSI SAF produces 25 km (OSI-102) and 12.5 km coastal (OSI-104) L2 winds at KNMI about 2 hours after sensing, and NASA PO.DAAC and Copernicus Marine redistribute or grid them. The record, which began with Metop-A in 2007, is widely assimilated in reanalyses and weather forecasting, used to evaluate ocean-model wind forcing and to study wind stress and curl, and is a key input to multi-satellite wind analyses such as CCMP.

What changes with the successor¶

Metop-A ASCAT (ASCAT-A) → Metop-B/C ASCAT (ASCAT-B, ASCAT-C)¶

Transition: 2021-11

Metop-A ASCAT winds, available since 2007, ended on 2021-11-15. The record continues with Metop-B, flying alongside since 2012, and Metop-C, operational since 2019. The three ASCATs share the same design but differ slightly in calibration, which is why RSS produced a separate inter-calibrated A/B/C climate record (V2.1/V2.1.1).

Sources: OSI SAF — Metop-C ASCAT 25 km Winds (OSI-102-c) product history · EUMETSAT — Metop-A end of mission (2021-11-15) · EUMETSAT — decision to operate Metop-A in a drifting orbit · RSS — ASCAT-A, -B and -C ocean surface wind CDR announcement (2021-08-16) · Ricciardulli & Manaster (2021), Remote Sensing

OSI SAF ASCAT winds (CMOD5.n, 10 m neutral winds) → OSI SAF ASCAT winds (CMOD7, 10 m stress-equivalent winds)¶

Transition: 2018-09

On 2018-09-25 the KNMI processor changed (AWDP 3_2_01): the model function became CMOD7, the winds became 10 m stress-equivalent winds, and the ECMWF background winds became stress-equivalent as well. The near-real-time record was not reprocessed, so values with different definitions sit on either side of this date.

Sources: KNMI — OSI SAF ASCAT-B 25 km product history · KNMI — CMOD7 description · ASCAT Wind Product User Manual v1.17

Metop ASCAT → Metop-SG-B SCA (Scatterometer) Planned¶

Transition: 2026

SCA, a C-band scatterometer on the B-type satellites of EUMETSAT's second-generation polar system Metop-SG, will succeed ASCAT. The first, B1, is due to launch in October 2026 according to ESA and November 2026 according to WMO OSCAR; when checked on 2026-10-13 there was no official schedule for SCA data release. Wider swaths and cross-polarised measurements are meant to improve resolution and extend the range of measurable high winds.

Sources: ESA — First MetOp Second Generation-B satellite spreads its wing (2026-02-25) · WMO OSCAR — Metop-SG-B1 · WMO OSCAR — SCA scatterometer · ESA/EUMETSAT SCA Science Plan (2019) · ESA — MetOp-SG facts and figures

Available variables¶

L2 variable names follow Appendix B (NetCDF) of the ASCAT Wind Product User Manual and match the PO.DAAC variable list. NetCDF files hold only the selected wind solution (no ambiguities or sigma0), in swath-shaped (NUMROWS, NUMCELLS) arrays. L3 variable names follow the Copernicus Marine product user manual.

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

L2 swath (OSI SAF 25 km and 12.5 km coastal NetCDF, same at PO.DAAC)¶

L3 daily grids (Copernicus Marine, per satellite and pass direction)¶

The full variable list is in the official documentation.

How to get the data¶

• NASA PO.DAAC — Metop-C ASCAT L2 25 km (ASCATC-L2-25km) — Earthdata Search / earthaccess / OPeNDAP / Harmony subsetting / podaac-data-subscriber
  • Metop-B is ASCATB-L2-25km (from 2012-10-29), the coastal products are ASCATB-L2-Coastal and ASCATC-L2-Coastal, the ended Metop-A records are ASCATA-L2-25km and ASCATA-L2-Coastal (to 2021-11-15), and the Metop-A climate data record is ASCATA_L2_25KM_CDR. Each NetCDF file holds one orbit. A free NASA Earthdata Login is required.
• EUMETSAT Data Store (original OSI SAF archive, full record) — Web search and download, EUMETSAT Data Access API (BUFR and NetCDF)
  • For example, OSI-102-c is collection EO:EUM:DAT:0760, OSI-104-b is EO:EUM:DAT:0192, OSI-104-c is EO:EUM:DAT:0202 and the Metop-A climate record OSI-150-a is EO:EUM:DAT:METOP:OSI-150-A. A free EUMETSAT account is required. The data are also broadcast in real time via EUMETCast.
• OSI SAF / KNMI FTP (last 3 days) — FTP (ftppro.knmi.nl; directories ascat_b_osi and ascat_c_osi, 6 km demonstration product in ascat_b_osi_057)
  • Credentials are provided on request after registration. Only the last 3 days are kept, so get older data from the Data Store or PO.DAAC.
• Copernicus Marine — L3 daily grids NRT (WIND_GLO_PHY_L3_NRT_012_002) — copernicusmarine toolbox (Python and command line) / web subsetting
  • Metop-B and Metop-C (0.125° and 0.25°) and the HY-2, Oceansat-3 and FY-3E scatterometers, as separate datasets per satellite and pass direction (from 2016-01-01; e.g. cmems_obs-wind_glo_phy_nrt_l3-metopc-ascat-des-0.125deg_P1D-i). A reprocessed product, WIND_GLO_PHY_L3_MY_012_005 (1991-08 to 2026-06; Metop-A/B, QuikSCAT, ERS, Oceansat-2), is also available. A free Copernicus Marine account is required.
• RSS ASCAT V2.1 (C-2015 model function; inter-calibrated A/B/C climate record) — Direct HTTPS download (metopa, metopb and metopc folders; daily 0.25° grids in bmaps_v02.1, swaths in ascat*_wind_vectors_v02.1) No login needed
  • RSS's own processing, separate from OSI SAF. Daily grids hold separate morning (descending) and evening (ascending) maps.

Example code¶

Downloads Metop-C 25 km L2 files from PO.DAAC with earthaccess, applies the quality flags recommended in the user manual (VarQC, KNMI QC, monitoring) around the Korean Peninsula and prints the mean wind speed. Run earthaccess.login() once with your NASA Earthdata account first. Files from the KNMI FTP site (.nc.gz) need to be decompressed first.

# pip install earthaccess xarray netCDF4
import earthaccess
import xarray as xr

earthaccess.login()
results = earthaccess.search_data(
    short_name="ASCATC-L2-25km",            # Metop-B: ASCATB-L2-25km, coastal: ASCATC-L2-Coastal
    temporal=("2025-08-01", "2025-08-01T23:59:59"),
    bounding_box=(120, 30, 135, 40),        # west, south, east, north (around Korea)
)
files = earthaccess.download(results, "./ascat")

REJECT = (1 << 16) | (1 << 17) | (1 << 18)  # VarQC failed, KNMI QC failed, monitoring flag
for f in files:
    ds = xr.open_dataset(f)
    lon = ds["lon"] % 360                    # works whether longitudes are 0–360 or ±180
    flag = ds["wvc_quality_flag"].fillna(0).astype("int64")
    box = (ds["lat"] >= 30) & (ds["lat"] <= 40) & (lon >= 120) & (lon <= 135)
    ws = ds["wind_speed"].where(box & ((flag & REJECT) == 0))
    if ws.count() > 0:
        print(f, int(ws.count()), float(ws.mean()))  # valid WVCs and mean wind speed (m/s) per orbit

Practical tips¶

• The user manual stresses that WVCs with the KNMI QC flag (wvc_quality_flag bit 17) set must be rejected, and recommends not using values with the variational QC flag (bit 16) or the monitoring flag (bit 18) set either.
• C-band is fairly insensitive to rain, but in heavy rain the measurements no longer fit the model function (large inversion residual) and are rejected by KNMI QC. RSS notes that ASCAT can be less accurate in heavy rain, especially at low winds. Check flags and gaps around monsoon fronts and typhoon rain bands.
• Wind solutions are also present over sea ice (bogus winds), marked by both the KNMI QC flag and the ice flag (bit 14). The Bayesian ice screening sometimes mistakes rainy WVCs for ice, and cells with model SST above 5 °C are always treated as open water. In seas that freeze in winter, such as the Sea of Okhotsk and the Bohai Sea, check the ice flag together with ice_prob.
• No winds are retrieved when the land fraction of a WVC exceeds 0.02. The 25 km and standard 12.5 km products miss about 35 km nearest the coast because of land contamination, so along complex coastlines with many islands, such as Korea's west and south coasts, use the coastal product (OSI-104, PO.DAAC *-L2-Coastal), which reaches about 15 km from the coast.
• The user manual notes that wind speeds above 25 m/s are generally less reliable, and winds above 30 m/s are flagged (bit 12). Do not use them as they are for typhoon peak winds.
• Although WVCs are spaced 25 km apart, PO.DAAC gives the effective resolution as 50 km (25 km for the coastal product). Curl and divergence at grid-spacing scales are noisy, so interpret them at the effective resolution.
• There are gaps between the two swaths (below the satellite) and between orbits, so one satellite cannot cover all oceans in a day. Combining Metop-B and Metop-C reduces the gaps, but for daily means a gridded analysis such as CCMP is better.
• Processing changed during the near-real-time record (a +0.2 m/s step with the switch to neutral winds in November 2008; CMOD7 and stress-equivalent winds from 2018-09-25). Use reprocessed data (Copernicus Marine L3 MY, OSI-150, RSS V2.1) for long-term work.
• According to the RSS inter-calibration study, ASCAT-C is about 0.05 m/s lower than ASCAT-B, and ASCAT-A had calibration steps in mid-2016 and late 2019 (corrected in RSS V2.1.1). Check for these when mixing satellites.
• The PO.DAAC description of the coastal products still mentions CMOD5.n, but according to the KNMI product history CMOD7 has been used since 2018-09-25. Rely on the KNMI and OSI SAF product pages for version information.
• Gaps of several hours due to orbit manoeuvres or instrument anomalies are common (e.g. the Metop-C out-of-plane manoeuvre on 2026-09-16). Missing periods are listed in the anomaly history on the KNMI product pages and in OSI SAF service messages.

Notices and change history¶

References for your paper¶

Dataset citation¶

Metop-B ASCAT 25 km Winds (OSI-102-b) — DOI 10.15770/EUM_SAF_OSI_NRT_2021

@misc{osisaf_osi102b,
  author    = {{EUMETSAT OSI SAF}},
  title     = {Metop-B {ASCAT} 25 km Winds ({OSI-102-b})},
  publisher = {EUMETSAT Ocean and Sea Ice Satellite Application Facility},
  doi       = {10.15770/EUM_SAF_OSI_NRT_2021}
}

Metop-C ASCAT 25 km Winds (OSI-102-c) — DOI 10.15770/EUM_SAF_OSI_NRT_2022

@misc{osisaf_osi102c,
  author    = {{EUMETSAT OSI SAF}},
  title     = {Metop-C {ASCAT} 25 km Winds ({OSI-102-c})},
  publisher = {EUMETSAT Ocean and Sea Ice Satellite Application Facility},
  doi       = {10.15770/EUM_SAF_OSI_NRT_2022}
}

Metop-B ASCAT coastal Winds (OSI-104-b) — DOI 10.15770/EUM_SAF_OSI_NRT_2018

@misc{osisaf_osi104b,
  author    = {{EUMETSAT OSI SAF}},
  title     = {Metop-B {ASCAT} coastal Winds ({OSI-104-b})},
  publisher = {EUMETSAT Ocean and Sea Ice Satellite Application Facility},
  doi       = {10.15770/EUM_SAF_OSI_NRT_2018}
}

Metop-C ASCAT coastal Winds (OSI-104-c) — DOI 10.15770/EUM_SAF_OSI_NRT_2019

@misc{osisaf_osi104c,
  author    = {{EUMETSAT OSI SAF}},
  title     = {Metop-C {ASCAT} coastal Winds ({OSI-104-c})},
  publisher = {EUMETSAT Ocean and Sea Ice Satellite Application Facility},
  doi       = {10.15770/EUM_SAF_OSI_NRT_2019}
}

Metop-A ASCAT L2 25 km winds data record (OSI-150-a) — DOI 10.15770/EUM_SAF_OSI_0006

@misc{osisaf_osi150a,
  author    = {{EUMETSAT OSI SAF}},
  title     = {Metop-A {ASCAT} {L2} 25 km winds data record 2007-2014},
  year      = {2016},
  version   = {1.0},
  publisher = {EUMETSAT Ocean and Sea Ice Satellite Application Facility},
  doi       = {10.15770/EUM_SAF_OSI_0006}
}

Reference papers¶

CMOD7 model function (OSI SAF winds since 2018-09) — DOI 10.1109/JSTARS.2017.2681806

@article{stoffelen2017cmod7,
  author  = {Stoffelen, A. and Verspeek, J. and Vogelzang, J. and Verhoef, A.},
  title   = {The {CMOD7} Geophysical Model Function for {ASCAT} and {ERS} Wind Retrievals},
  journal = {IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing},
  volume  = {10},
  number  = {5},
  pages   = {2123--2134},
  year    = {2017},
  doi     = {10.1109/JSTARS.2017.2681806}
}

Scatterometer wind climate data records (OSI SAF CDR) — DOI 10.1109/JSTARS.2016.2615873

@article{verhoef2017cdr,
  author  = {Verhoef, A. and Vogelzang, J. and Verspeek, J. and Stoffelen, A.},
  title   = {Long-Term Scatterometer Wind Climate Data Records},
  journal = {IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing},
  volume  = {10},
  number  = {5},
  pages   = {2186--2194},
  year    = {2017},
  doi     = {10.1109/JSTARS.2016.2615873}
}

ASCAT-A/B/C inter-calibration (RSS V2.1 climate record) — DOI 10.3390/rs13183678

@article{ricciardulli2021ascat,
  author  = {Ricciardulli, Lucrezia and Manaster, Andrew},
  title   = {Intercalibration of {ASCAT} Scatterometer Winds from {MetOp-A}, -{B}, and -{C}, for a Stable Climate Data Record},
  journal = {Remote Sensing},
  volume  = {13},
  number  = {18},
  pages   = {3678},
  year    = {2021},
  doi     = {10.3390/rs13183678}
}

Acknowledgements¶

OSI SAF products are CC BY 4.0, so credit EUMETSAT OSI SAF and cite the product DOI. The KNMI product pages ask for "copyright (year) EUMETSAT" on each product used. If you used RSS data, RSS asks for the statement "C-2015 ASCAT data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team."

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¶

• OSI SAF — wind products list
• OSI SAF — Metop-B ASCAT 25 km Winds (OSI-102-b)
• OSI SAF — Metop-B ASCAT coastal Winds (OSI-104-b)
• OSI SAF — Metop-A ASCAT climate data record (OSI-150-a)
• ASCAT Wind Product User Manual (PDF)
• KNMI — ASCAT-B 25 km product page (anomaly and change history)
• KNMI — wind processing status
• OSI SAF — service messages
• Copernicus Marine — L3 scatterometer winds NRT
• Copernicus Marine — L3 scatterometer winds reprocessed
• RSS — ASCAT page
• This site — CCMP V3.1 (6-hourly gridded analysis combining ASCAT and other satellite winds)