Cryosphere Team Gridding Assessment Summary Round 2

1. Cryosphere Team Gridding Assessment Summary Round 2, 13 Dec 2012 Contributors: Jim Maslanik (CU), Dan Baldwin (CU), Robert Mahoney (NGAS), Yinghui Liu (CIMSS), Mark Tschudi (CU), Peter Romanov (CREST), Igor Appel (IMSG), Jeff Key (NOAA)

2. Recommendations • Gridding of VIIRS Snow/Ice data should not be activated until the following issues are resolved: • Significant regions of missing sea ice and false ice have been observed in the gridded snow/ice cover rolling tiles evaluated by the Cryo Team • Gran2Grid routine is incorrectly allowing poor quality Sea Ice Concentration IP to be gridded due to a potentially inadequate Gran2Grid quality threshold test based on sea ice concentration quality weights. A significant amount of the false ice observed in the snow ice rolling tiles is near cloud edges and in regions of reduced VCM and Sea Ice Concentration IP performance (low light conditions). • Since the current manual monthly seed update process is too slow and infrequent to capture the rapid snow/ice coverage changes, automated daily updates of the GridIP-VIIRS-Snow-Ice-Cover-Rolling-Tile with NOAA Global Automated Multi-sensor Snow/Ice data product is recommended to replace the current manual process until the gridding issues are resolved. • Recommend determination of the feasibility, optimal technical approach and time required to implement the automated daily updates as an interim solution. • The gridding evaluation should continue when additional data are available, as not all orbits that have views of the regions of interest were included in the last test data set.

3. Gran_to_gridding Assessment Approach • Factory Test result data were provided to the CryoTeam with VIIRS Gran2Grid activated for 17 days and ending on Nov. 30, 2012 • Test data provided for 10 sub-regions for Nov. 28, 29 and 30, 2012 snapshots of VIIRS-GridIP-VIIRS-Snow-Ice-Cover-Mod-Gran (IVSIC) and GridIP-VIIRS-Snow-Ice-Cover-Rolling-Tile (IVGSC) • Compositing tests could not be performed due to missing data (not all orbits that have views of the regions of interest were included in the test data set) • Products were examined for October and November 2012, with comparisons to ancillary data. Aspects focused on include: • Quality of the updated IVSIC snow/ice mask - Did the mask accurately represent true conditions? Were there any gridding artifacts present? • Characteristics of supporting fields such as “time of observation” and “geo error” data that provide insight into gridding process - Were the update times realistic? Are there non-physical patterns apparent in the fields? • Investigation of possible explanations of, and solutions for, observed inclusion of “false” sea ice in the IVSIC mask.

4. Key Findings in the Analysis of Global Mosaic of Gridded Snow/Ice Cover • Global mosaic of IVGSC Snow/Ice, ObsTime and GeoError fields for Nov. 28 and 29 test data examined and Nov. 28 analysis are shown. Results are simiar to Oct. 13-17 analysis. • Days since last ObsTime update of the global IVGSC tiles and the GeoError field - Update times look good but updates are shown over regions of ocean where the Sea Ice Conc. IP does not retrieve by design. • Anomalies that can be seen in the global mosaic of rolling tiles are: • Large region of missing sea ice near Antarctic and significant amounts of false sea ice are showing up in the grid for regions known to not have Sea Ice • Large region of missing snow over Eurasia and North Am. most like due to 60 deg solar zenith angle exclusion threshold still implemented in this test data set • Unexplained update of the ObsTime and GeoErr fields in the IVGSC tiles over regions of the ocean that the VIIRS Sea Ice Conc. IP does not retrieve by design (between lat 50º S and 36º N) but the ObsTime showing expected updates • GeoError has pattern that is not completely understood.

5. Key Findings in the Analysis of Global Mosaic of Gridded Snow/Ice Cover, cont. • Granulated tiles do not properly reproduce the existing state and changes both in the snow and ice cover. • Performance of the gridding/regridding algorithm with respect to the snow cover is questionable. In some cases the system clearly fails to update information on the snow cover in snow/ice tiles. The extent of snow cover mapped in granulated tiles in Alaska in the end of November looks as if it has been never updated at least since the end of September. • The ice cover is updated in IVSIC granulated snow/ice product. However due to frequent ice commission errors updates to the IVSIC product result in accumulation of a substantial amount (up to 10-15% area fraction) of false ice in areas that never experience ice cover. • Poor quality of granulated snow and ice tiles (IVSIC) may adversely affect the quality of other VIIRS products (e.g., cloud mask).

6. Supporting Illustrations

7. Updating of snow/ice mask with VIIRS sea ice fractions yields accumulation of “false” ice over time. False ice retrievals are mostly associated with cloud masking problems, so potential exists to further degrade the VCM performance as false ice is added to the snow/ice mask.

8. VIIRS sea ice fraction/concentration IP is retrieving false sea ice over lower-latitude ocean areas (red in image on right below). Occasional false retrievals are expected but may (or will) accumulate in the snow/ice mask over time with current gridding approach.

9. NOAA Multi-sensor Snow/Ice Map for November 28, 2012 Robert Mahoney, NGAS Sinusoidal projection of the NOAA Global Automated Snow/Ice Map product for Nov. 28, 2012. Red circles correspond to missing snow/ice and yellow circles correspond to false snow/ice in the IVGSC tiles for the Nov. 28, 2012 test data.

10. GridIP-VIIRS-Snow-Ice-Cover-Rolling-Tile Set For End Of Day November 28, 2012 Snow not updated due to solar zenith angle exclusion threshold set 60 deg. False Sea Ice Missing Sea Ice Robert Mahoney, NGAS Global mosaic of the GridIP-VIIRS-Snow-Ice-Cover-Rolling-Tile files for the Nov 28, 2012 snapshot of the global tiles. Regions with missing sea ice (and comment on snow) and false sea ice the rolling tiles are indicated by the red and yellow circles respectively

11. ObsTimeDiff Computed from IVGSC ObsTime Field For End Of Day Nov. 28, 2012 No snow updates over land where Snow Cover Binary Map EDR > 60 SZA poor quality threshold still implemented (need to confirm) Unexpected updates over ocean between 50º S and 36º N Days Robert Mahoney, NGAS Global mosaic of days since last update from end of the day on Nov. 28, 2012 snapshot of the global tiles based on the ObsTime field in the IVGSC tiles. Unexpected updates of the ObsTime field (but not sea ice) are performed over oceans in regions where Sea Ice Concentration IP does not retrieve by design between latitudes 36 N and 50 S.

12. GeoError Field From IVGSC For End of Day November 28, 2012 km Robert Mahoney, NGAS Global mosaic of GeoError field for the Nov. 28, 2012 snapshot of the global IVGSC tiles. Unexpected updates performed over oceans in regions where Sea Ice Concentration IP does not retrieve by design between latitudes 36 N and 50 S. Also, pattern needs to be further understood. GeoError is a relative measure of the geolocation error of off nadir pixels compared to nadir (GeoError is 0 km for nadir pixels and 50 km for edge of scan pixels).

13. “Geo error” and “observation time” fields show some non-physical patterns which may indicate errors in gridding or non-optimal use of weights or quality flags in the snow/ice grid updating process.

14. Ice identification errors cause accumulation of large amounts of spurious ice in granulated snow/ice tiles (IVSIC) Nov 28, 2012 Granulated snow/ice tile White: snow Yellow:ice Area fraction of spurious ice on water in Pacific North-East is ~15% Large amount of spurious ice is retained in the granulated snow/ice product (IVSIC) Ice is identified only north of ~35N On Nov 28-29 the area fraction of ice cover in IVSIC granules was ~8% in West Atlantic off the coast of US and ~3% in Mediterranean Sea.

15. Ice identification errors cause accumulation of large amounts of spurious ice in granulated snow/ice tiles (IVSIC) Nov 28, 2012 Nov 29, 2012 Spurious Ice area fraction: ~3% Spurious ice area fraction: ~8%

16. Daily snow cover, VSCMO Snow: white, land: green clouds: light gray Nov 28, 2012 Nov 29, 2012 Granulated snow/ice cover, IVSIC Snow: white, land: green, ice: yellow, water: blue Nov 28, 2012 Nov 29, 2012

17. Snow cover in snow/ice tiles is not updated (at least in North America) Nov 28, 2012 Snow map VSCMO White: snow Gray: clouds Only “confidently clear” pixels are mapped as “clear” Snow is identified in the VIIRS snow product (VSCMO) but is not incorporated in the granulated snow/ice product (IVSIC) Granulated snow/ice tile IVSIC White: snow Yellow:ice

18. Another example of an apparent failure of the algorithm to update snow/ice tiles IVSIC, Nov 28 Snow is mapped in IVSIC on Nov 28 VSCMO, Nov 28 Snow is not identified on Nov 28 and therefore has to be removed from IVSIC on the next day IVSIC, Nov 29 White: snow Green: land Gray: clouds Blue: water However the next day (Nov 29) IVSIC is almost exactly the same as Nov 28.

19. Specifying a higher ice-fraction weight threshold in the snow/ice mask updating would help reduce inclusion of false sea ice: