QUALITY INFORMATION DOCUMENT For Arctic Ice Extent Indicator. ARC_SEAICE_INDEX_002

Transcription

1 QUALITY INFORMATION DOCUMENT For Arctic Ice Extent Indicator. Issue: 1.2 Contributors: Steinar Eastwood, Lars-Anders Breivik, Bruce Hackett, Thomas Lavergne, Gorm Dybkjær, Cecilie Wettre Approval Date by Quality Assurance Review Group :

2 CHANGE RECORD Issue Date Description of Change Author Validated By December, 2014 All First draft version extracted from MYOS-OSI-QUID-SEAICE v1.8 B. Hackett L.-A. Breivik 1.1 May all Change format to fit CMEMS graphical rules L. Crosnier June 2016 all Updates to add product to CMEMS catalogue C. Wettre, T. Lavergne EU Copernicus Marine Service Public Page 2/ 12

3 Table of contents I Executive summary... 4 I.1 Products covered by this document... 4 I.2 Summary of the results... 5 I.3 Estimated Accuracy Numbers... 5 II Production Subsystem description... 6 III Validation framework... 7 IV Validation Results... 8 IV.1 Validation procedure... 8 IV.2 Validation results... 8 V Quality changes since previous version VI References EU Copernicus Marine Service Public Page 3/ 12

4 I EXECUTIVE SUMMARY I.1 Products covered by this document This document provides to MyOcean users information on the quality of the OSI monthly Sea Ice Extent (SIE) index for the Arctic. It is a Targeted Product that was developed at the request from European Environmental Agency (EEA). The product is named: The product is derived from daily sea ice concentration products from the EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI SAF), developed and delivered by the Norwegian Meteorological Institute (MET Norway) and the Danish Meteorological Institute (DMI). The sea ice edge data are produced in the EUMETSAT OSI SAF project, and were developed in cooperation with CryoClim and the MyOcean projects. The data are monthly averaged SIE values which are found by computing daily SIE values from the OSI SAF daily Sea Ice Concentration (SIC) files, and then averaging at the end of the month. Figure I.1.1: Sample plot of the sea ice extent index time series. EU Copernicus Marine Service Public Page 4/ 12

5 I.2 Summary of the results The accuracy of the index is directly dependent on the accuracy of the OSI SAF SIC data (products SEAICE_GLO_SEAICE_L4_NRT_OBSERVATIONS_011_001 and SEAICE_GLO_SEAICE_REP_OBSERVATIONS_011_009) from which it is calculated. The OSI SAF global sea ice concentration near-real-time and reanalysis products are compared to the NIC ice charts. There are no ground-truths available for validating a sea ice extent index, but exhibits similar trends and inter-annual variations as a similar index delivered by US National Snow and Ice Data Center (NSIDC). I.3 Estimated Accuracy Numbers It is not possible to assign an accuracy number to this product. EU Copernicus Marine Service Public Page 5/ 12

6 II PRODUCTION SUBSYSTEM DESCRIPTION The monthly sea ice extent values contained in the product are computed from daily maps of sea ice concentration produced by the EUMETSAT OSISAF. The methodology to compute ice extent from ice concentration maps is explained in the PUM. The extent is calculated and stored as ASCII and NetCDF files. EU Copernicus Marine Service Public Page 6/ 12

7 III VALIDATION FRAMEWORK The SIC maps (both climate data records and near-real-time products) delivered by the OSISAF are thoroughly validated, mainly against navigational ice charts. The relevant validation framework for the basis data - SEAICE_GLO_SEAICE_REP_OBSERVATIONS_011_009 - is described in the QuID for that product (ref MYOF-OSI-QUID-SEAICE). The index itself is not validated since there are no alternative observations to serve as groundtruth. It is inter-compared here to the Sea Ice Index (SII), a similar index from the NSIDC. EU Copernicus Marine Service Public Page 7/ 12

8 IV VALIDATION RESULTS IV.1 Validation procedure We performed an inter-comparison between; The new version of the OSISAF SIE product that is redistributed in CMEMS as since Q An older version of the OSISAF SIE index, which was phased out in June The SIE time-series from NSIDC's Sea Ice Index ( The time series for March (maximum seasonal Arctic ice extent) and September (minimum seasonal Arctic ice extent) are plotted on the same graph, and linear trends are computed and compared. IV.2 Validation results The validation results for the SIC maps are documented in a series of OSISAF documents, all available from The inter-comparison of SIE shows that the new OSISAF values (redistributed in CMEMS as ) are slightly lower than the old ones, since 2010 for March, and since 2013 for September. The new SIE has a higher level of agreement with NSIDC SIE than the old time-series. This is illustrated in Figures IV.1 and IV.2. EU Copernicus Marine Service Public Page 8/ 12

9 Figure IV.1: inter-comparison of SIE for March EU Copernicus Marine Service Public Page 9/ 12

10 Figure IV.2: inter-comparison of SIE for September All the curves agree in the declining trends of Northern Hemisphere sea ice extent, more pronounced in September month. The offset between OSISAF and NSIDC values is known from other studies and is mostly due to the use of different land masks. The trends in SIE decline for March and September for the new, old and nsidc are given in Table 1. ARC_SEAICE_INDEX_00 2 Old OSISAF NSIDC SII September March Table 1: Northern Hemisphere Sea Ice Extent trend in mill. km2 /decade The exhibits a slightly faster trend of decline than the NSIDC SII. It is not assessed here if the differences in the trends are statistically significant given the (unknown) uncertainties of the SIE values. EU Copernicus Marine Service Public Page 10/ 12

11 V QUALITY CHANGES SINCE PREVIOUS VERSION This is the second version for the MYOF-OSI-QUID-INDEX-002. No significant change in quality is documented since previous version, but the improved accuracies of the OSISAF SIC products used as input (see PUM) are expected to lead to a better index. The new SIE has a higher level of agreement with NSIDC SIE than the old time-series. EU Copernicus Marine Service Public Page 11/ 12

12 VI REFERENCES The NSIDC Sea Ice Index is accessed and documented here: ( The OSISAF SIC products upon which are documented (algorithm baseline, validation reports, and product user manuals) here: EU Copernicus Marine Service Public Page 12/ 12