1. ASCAT sensors onboard MetOps : application for sea ice
Fanny GIRARD-ARDHUIN
IFREMER, Laboratory of Oceanography from Space, Plouzané, France
©ESA/EUMETSAT
Backscatter values from scatterometers are commonly used to estimate wind field over oceans, their ability to monitor sea ice coverage, age and drift has been also demonstrated. Thanks to the work on previous scatterometers, the operational ASCAT scatterometer sensor onboard MetOp is used for sea ice applications with very good results. Incidence-adjusted backscatter coefficient maps are routinely processed over the polar areas for sea ice geophysical interpretation in order to increase the long time series of ice parameters already available through the previous scatterometers since the 1990's. The backscatter maps enable the discrimination between multi year ice and first year ice, these maps can also be used as a basic product for Arctic sea ice drift estimation. The synergy between ASCATs sensors onboard MetOp-A and -B enables to have an almost daily full coverage of the sea ice areas at both hemispheres. These routinely produced data are available at CERSAT/Ifremer for research and models/operational systems.
Spatial coverage improved using both ASCATs on MetOp-A and -B
Sea ice extent
ASCAT-B
Contour of the radiometer (SSM/I) sea ice edge (concentration=15%)
Data gap patches
ASCAT-A
ASCAT A+B
Homogeneous values over sea ice
High values over open ocean areas
ASCAT-A
ASCAT-A
ASCAT- A&B
Sea ice search area
Standard deviation of the backscatter for one day for each pixel, ASCAT/MetOp-A data. March 10, 2010.
ASCAT A+B
ASCAT-A 60
100 80 40 20
Antarctic daily sea ice 40° incidence-adjusted backscatter maps from ASCATs. Sept 15, 2014
The sea ice edge can be detected from scatterometers, in particular C-band frequency scatterometer (ASCAT) detects the first steps of freeze of the sea ice at fall.
Arctic daily number of data for a pixel in the sea ice search area.
ASCAT-B
Using both ASCATs onboard MetOp-A & -B is very useful to improve the spatial coverage, needed in particular in the Antarctic area
Sea ice roughness
ERS, NSCAT, QuikSCAT, ASCAT scatterometers time series
Following the ice type evolution during winter
Russia
Pixel resolution : 12,5 km (25 km for ERS)
Since 1992
NOV
NOV
DEC
FEB
APR
Daily (weekly for ERS)
Multi year ice has survived at least a summer melt, it is thicker than first year ice.
FY MY
FY : first year ice
MY : multi year ice
Unique time series 1992-present available at CERSAT, ongoing with the ASCAT sensors
Canada
Greenland
Sea ice roughness evolution during winter using ASCAT/MetOp-A data
Daily mean Arctic sea ice roughness at adjusted constant 40° incidence angle. April 7, From ASCAT/MetOp-A data.
Sea ice displacement estimate at low resolution
From daily backscatter maps, sea ice displacement can be inferred (method and validation in Girard-Ardhuin & Ezraty, IEEE TGRS 2012)
Monthly drift maps
3 day-lag
6 day-lag
Drift grid spacing: 62,5 km
Daily except melt season
SSMI
Merged ASCAT/SSMI
ASCAT
IFREMER algorithm
Examples of monthly merged ASCAT&SSMI sea ice displacement map for October and March, made from the 6 and 3 day-lags map estimate.
Drift data density for Individual sensor (black, blue, green), Merged ASCAT&SSMI (red) and interpolated map (brown) for winter.
Individual sensor sea ice motion maps and Merged ASCAT&SSMI map, 3 day-lag (Oct. 5–8, 2007). Drift vectors less than one pixel are marked with a cross.
Better data density for the Merged and filled product, in particular for early fall and early spring
The merging of multi-sensors enables to have more vectors, sharply increases the number of data of the monthly drift maps also during freeze period, worst period of estimation  enables to estimate sea ice drift from September until May
More than 80% data density
Combination of the radiometer and scatterometer data increases the density by 20 to 30%
Several applications
ASCATs in tandem increase the vectors density up to 8 to 12%
Estimation of sea ice fluxes (Laptev seas, Fram strait…)
Assimilation of sea ice drift → greatly improves results on drift but also on concentration and thickness (Rollenhagen et al., JGR 2009)
Estimation of the backtrajectory to find pollution origin
Unique time series of sea ice displacement maps 1992-present, with SSM/I, SSM/I merged with QuikSCAT, ongoing with the ASCATs sensors merging with SSM/Is sensors
Ifremer/CERSAT products available at
ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/psi-drift/
ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/psi-backscatter/
and
Products used in EU, GMES, space agencies projects : SWARP, SPICES, Climate Change Initiative, Copernicus...