Copernicus Marine Service Arctic MFC: Validation of sea level anomaly and geostrophic velocity

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Validation results: Altimetry data from Jason-3

Results by bulletin date

January 2024 01-04 01-11 01-18 01-25	February 2024 02-01 02-08 02-15 02-22 02-29	March 2024 03-07 03-14 03-21 03-28	April 2024 04-04 04-11 04-18 04-25	May 2024 05-02 05-09 05-16 05-23 05-30	June 2024 06-06 06-13 06-20 06-27	July 2024 07-04 07-11 07-18 07-25	August 2024 08-01 08-08 08-15 08-22 08-29	September 2024 09-05 09-12 09-19 09-26	October 2024 10-03 10-10 10-17 10-24 10-31
September 2023 09-07 09-14 09-21 09-28	October 2023 10-05 10-12 10-19 10-26	November 2023 11-02 11-09 11-16 11-23 11-30	December 2023 12-07 12-14 12-21 12-28

Validation of the Copernicus Marine Arctic MFC results for sea level anomaly and geostrophic velocity is performed using altimeter data from the Jason-3 satellite. These data have been corrected for a number of processes, including removal of the tidal signal and correction for the inverse barometer effect. Note that altimeter data from Jason-3 are assimilated in the EnKF used in the ARCMFC (TOPAZ) model. However, we use an updated product for this validation, which may have improvements due to an updated computation of the inverse barometer, and more precise knowledge of the satellite orbit.

Additional information is provided under Processing below.

Validation Data: Jason-3

Each Jason cycle is diveded into 254 tracks, half of which are ascending and half are descending (more below). The instrument's sampling frequency is approximately 0.927 Hz, corresponding to a horizontal separation of about 18.5 km along the track. The mean measurement noise of the Jason-3 data is 2.6 cm.

Jason-3 data are available from the Copernicus Marine product SEALEVEL_GLO_PHY_L3_NRT_OBSERVATIONS_008_044.

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Validation results: Altimetry data from CryoSat-2

Results by bulletin date

January 2024 01-04 01-11 01-18 01-25	February 2024 02-01 02-08 02-15 02-22 02-29	March 2024 03-07 03-14 03-21 03-28	April 2024 04-04 04-11 04-18 04-25	May 2024 05-02 05-09 05-16 05-23 05-30	June 2024 06-06 06-13 06-20 06-27	July 2024 07-04 07-11 07-18 07-25	August 2024 08-01 08-08 08-15 08-22 08-29	September 2024 09-05 09-12 09-19 09-26	October 2024 10-03 10-10 10-17 10-24 10-31
September 2023 09-07 09-14 09-21 09-28	October 2023 10-05 10-12 10-19 10-26	November 2023 11-02 11-09 11-16 11-23 11-30	December 2023 12-07 12-14 12-21 12-28

Validation Data: CryoSat-2

Validation of the Copernicus Marine Arctic MFC results for sea level anomaly and geostrophic velocity is also performed using altimeter data from the the CryoSat-2 satellite. These data have been corrected similarly to those from the Jason missions (see details above).

The CryoSat repeat cycle is diveded into 5344 tracks, covered during 369 days of operations. This repeat cycle is made up of sub-cycles from approximately 30 days. The orbit period is 99.16 minutes. The mean measurement noise of the CryoSat-2 data is 2.6 cm. More information about the CryoSat mission is available from ESA.

CryoSat-2 data are available from the Copernicus Marine product SEALEVEL_GLO_PHY_L3_NRT_OBSERVATIONS_008_044.

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Validation results: Altimetry data from Sentinel-6a

Results by bulletin date

January 2024 01-04 01-11 01-18 01-25	February 2024 02-01 02-08 02-15 02-22 02-29	March 2024 03-07 03-14 03-21 03-28	April 2024 04-04 04-11 04-18 04-25	May 2024 05-02 05-09 05-16 05-23 05-30	June 2024 06-06 06-13 06-20 06-27	July 2024 07-04 07-11 07-18 07-25	August 2024 08-01 08-08 08-15 08-22 08-29	September 2024 09-05 09-12 09-19 09-26	October 2024 10-03 10-10 10-17 10-24 10-31
September 2023 09-07 09-14 09-21 09-28	October 2023 10-05 10-12 10-19 10-26	November 2023 11-02 11-09 11-16 11-23 11-30	December 2023 12-07 12-14 12-21 12-28

Validation Data: Sentinel-6a

Validation of the Copernicus Marine Arctic MFC results for sea level anomaly and geostrophic velocity is additionally performed using altimeter data from the the Sentinel-6 satellite. These data have been corrected similarly to the other missions (see details above).

Sentinel-6a was placed in the Jason-3 orbit, from which it has provided observations of sea level anomalies from March 2022 onwards. At the same time Sentinel-6a have also substituted Jason-3 as the reference altimeter mission. The repeat cycle and track data are thus identical to the information provided for Jason-2/-3 above. The mean measurement noise of the Sentinel-6a data is 1.6 cm. See also ESA's analysis of the data accuracy.

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Processing

Sea level anomaly

The RMS difference between model sea level and observations is calculated track-by-track. Before the root-mean-square differences and correlations are computed, the following processing is performed: data are extracted for a calendar day data that are either outside of the validation domain or outside of the wet part of the ARCMFC PHYS domain, are discarded (tracks with no data inside the domain are disregarded) model results (daily mean values) corresponding to the remaining data are extracted from the ARCMFC PHYS results along-track model anomalies are derived by subtraction of the local model sea level climatology
	The full set of Jason ground tracks north of 50°N.
	CryoSat ground tracks north of 50°N. The figure shows an example of the coverage during the time period covered during one of the weekly validation bulletins (17 days).

Geostrophic velocity component

Here, we take advantage of the relationship between the sea surface slope and geostrophic velocity. Since sea level data are only available along the satellites' ground tracks, we are restricted to assessing the component of velocity normal to these tracks.

The relationship between sea surface slope along the track, h_s and speed of current component normal to the track, v^gstr, is
   v^gstr = g|h_s/f|
where g is the gravitational acceleration and f is the local Coriolis parameter.

In order to compare the geostrophic velocity component derived from observations, we adopt the same approach for the model results (and not model velocity), i.e., we compute this quantity based on model results for sea level. Note that to comply with the algorithm for computation of anomalies for sea level as described above, we here add the model climatology to the observed sea level anomalies (we choose to compare speed, not speed anomalies).

Finally, in order to account for directional differences in the derived results for speed of the geostrophic velocity component, the model results are set to a negative value in cases were the slope sign is opposite in model and observations.

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Regions

Jason, Sentinel The ocean circulation model used in Copernicus Marine Arctic MFC covers the Arctic Ocean, the North Atlantic Ocean and adjacent ocean regions. The northern part is depicted in the figure to the left. Validation of sea level anomalies from model results are performed for two domains: an extended domain indicated by the blue and green regions in the figure the Nordic Seas, shown as the green region The validation is performed using processed data from the altimeter onboard the Jason-3 and Sentinel-6a satellites. The region covered by the satellites is limited by the 66° 8' parallell, as indicated in the figure below. This limit in the poleward extent of data significantly reduces the extent of the key domain for Copernicus Marine Arctic Monitoring and Forecasting Centre. However, using Jason-3 and Sentinel-6a data almost eliminates possible problems due to the presence of sea ice at higher latitudes. Like its predecessors TOPEX/Poseidon, Jason-1 and Jason-2, Jason-3 and Sentinel-6a have orbital periods of about 6750 s (1:52:30). The full surface trace is constituted by 254 tracks, with one ascending track and one descending track in each orbit. Tracks in the region relevant for the Arctic Monitoring and Forecasting Centre are shown by the black curves below. The repeat period then becomes approximately 10 days. Using tide gauge data from around the world as reference, validation of Jason-2 altimetry data has revealed a standard deviation of the sea level anomaly of approxiately 4 cm.

CryoSat Similarly to the validation that uses data from the Jason missions, we consider two domains: an extended domain indicated by the blue and green regions in the figure the Nordic Seas, shown as the green region Note that the CryoSat and Sentinel missions extend much further north than Jason's extent. Data from CryoSat are available as far north as approximately 82°N.

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ARCMFC PHYS (TOPAZ)

The model results are produced with the TOPAZ ocean data assimilation model system. Presently, TOPAZ is run weekly with data assimillation one week prior to the bulletin date, then applying a one-week 100 member ensemble simulation ending on the bulletin date. Then, for each day a 10 day deterministic forecast is produced. TOPAZ was developed and is maintained by the Nansen Center.

ARCMFC PHYS (TOPAZ) results are available as Copernicus Marine product ARCTIC_ANALYSISFORECAST_PHY_002_001 from the Copernicus Marine Data Store. An archive of historical forecasts are available from a thredds server.