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Medium Resolution Sea Ice Drift product (OSI-407)

In the context of the first Continuous Development and Operation Phase (CDOP-1), a sea ice drift product was introduced in the Ocean and Sea Ice SAF product's portofolio. It is based on visible and infrareded data from the Advanced Very High Resolution Radiometer on board the Metop satellite.

The spatial resolution is 20 km on a Polar Stereographic Grid similar to the one used for the other OSISAF sea ice products. It is an approximately 24 hours ice drift dataset processed twice daily basis, which means that a product file measuring sea ice motion between e.g. midday on the 16th and 17th of February is available before 18:00 hours on the 17th. The product is based on a Maximum Cross Correlation technique on swath data.

  1. Introduction and FAQ
  2. Algorithms
  3. Validation
  4. Documentation and links

1. Introduction and FAQ

How should I cite this dataset?

This dataset shall be referred to as the medium resolution sea ice drift product of the EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI SAF, www.osi-saf.org).

Which satellite sensors are processed?

The sensors and channels used are AVHRR VISible and InfraRed (channel 2 and channel 4) on board EUMETSAT platform Metop-A.

What is the spatial resolution of this product?

The medium resolution sea ice drift product is a gridded dataset. The grid has 20 km spacing on a Polar Stereographic projection mapping. Definitions for the projection parameters can be found in the NetCDF files as well as in the Product User's Manual.

What is the time-span of this product?

One day (24 hours). This is the time delay between the start and the stop time of the motion described by one vector. As the product is based on swath data the valid time between 2 swath, and thus the ice drift period, can vary up to 1 hour.

Two or four datasets are distributed every day, which one should I use?

The medium resolution ice drift product is based on single sensors, either on VIS (channel 2) or on IR (channel 4) data. The IR data are used during teh cold period where the temperature difference between water and ice surface typically is large and the VIS data are used during the warm period where the spectral difference between water and ice surface is large. VIS and IR data production cover the month of MJJAS and SONDJFMAMJ, respectively. During the 3 overlapping month of May, June and September both IR and VIS ice drift production is running twice per day and else only VIS or IR production is run twice per day.

Today's product file is empty or has large data gaps, what happened?

Three possible answers to missing ice drift data:

  • 1: In case of interrupted data link with the satellite operating centres, no input swath data are available for processing of sea ice drift data inside the time constraints, resulting in an empty grid.
  • 2: Clouds and high atmospheric water in general, cause no surface data avaliable for feature tracking.
  • 3: The ice drift vector filtering routine has excluded drift vectors.
In general - the number of successfully processed ice drift vectors from this product is strongly varying with the time of year, such that the cold winter month is the period of the by far highest number of successful ice drift vectors. This is due to frequent atmospheric opacity in the warmer period. In all cases, the status_flag dataset in the product file indicates the reasons for missing data.

What is the legend used for the quicklooks?

Quicklooks for the product can be viewed from this page.

Scaling of vectors

For easier visualization, the magnitude of the vectors was scaled by XX. Since the spacing between the base point of two arrows is 20.0 km, vectors that are drawn with a length of 20.0 km are really corresponding to a drift of XX.X km (distance between the start point and stop point after 24 hours).

Dates associated to the displacements

Quicklooks are indexed on the stop date of the displacement. A quicklook displayed for date January, 10th 2010 is really corresponding to the displacement from the 9th to the 10th of January.

Colour codes for the Quicklooks

ColourBackground
landOperational Sea Ice Edge product
open water"
unclassified (coastal areas)"
open ice"
closed ice"

Where can I find the ice motion for Antarctic regions?

The processing chain has currently not been implemented for computing sea ice motion in the Southern Hemisphere as the atmospheric conditions there are very limited suited for ice tracking using optical data. However ice drift in Antarctic will be monitored and evaluated later.

I do not fancy NetCDF, do you have other formats?

The processing chain is currently only producing NetCDF files. Please let us know (contact) if you need another format.

2. Algorithms

2.1 Daily maps

The bi-daily medium resolution ice drift production covers is running on swath data with a width off approximately 3000km. This setup ensures a full coverage of the Arctic on a daily basis. The ice mask used is the OSI-402 ice edge product.

2.2 Motion tracking algorithm

The algorithm used for the medium resolution ice drift product is an ordinary Maximum Cross Correlation (MCC) Technique. A commonly known draw back for this technique is quantification effects, but this effect is minimized for this product as the input data are of relative high resolution, namely ~1 km at nadir.

3. Validation

Validation of OSISAF medium resolution sea ice drift dataset was conducted in the Arctic against in situ drifters. Trajectories were usually made available as hourly (and sub-hourly) GPS records for an optimum temporal collocation. The validation exercise extended over 1 Arctic winter and summer: IR drift data from 1st September 2008 to 30th June 2009 and VIS data from 1st March 2009 to 30st of September 2009. Statistical results and graphs document an excellent agreement between the OSISAF medium resolution sea ice drift datasets and the reference data. The validation results presented here are taken from the visiting scientist report referred to below. Other validation data can be found in the product specific validation report, also referred to in Documentation and links, below.

Trajectories from the Ice Tethered Profilers (ITP), the Tara schooner and the Russian manned stations NP-35 and NP-36 (AARI) were processed during this validation exercise.

Matchup statistics are summarized in the following table and graph:

Datasetavg_Ex [km]avg_Ey [km]std_Ex [km]std_Ey [km]corr_ExEyN
AVHRR IR (Metop-A)-0.080.041.000.750.10583
AVHRR VIS (Metop-A)0.120.040.870.740.4663

Validation Graph
Validation Graph
Validation statistics for the 2 medium resolution datasets in the Arctic (left IR data and right VIS data).

4. Documentation and links

The following documentation is available, further describing the OSI-407 ice drift product.
Quicklooks for this dataset can be accessed from this page.
   
       
     

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