Sea ice product samples
Revision as of 12:43, 10 May 2011 by Siri
The EuroCryoClim Pilot Project demonstrator gives access to monthly sea ice concentration products for the northern hemisphere, covering the period from 1988 to 2007, and to two climate change indicator products showing the sea ice extent for March and September of the same period.
The monthly products are based on daily products from the ongoing EUMETSAT OSI SAF reprocessing project for historical SSMR and SSM/I data. The reanalysis produce a consistent data set for sea ice concentration, ice edge and ice type, both swath data and daily accumulated data, going back to 1978. The resolution is 10 km.
Sea ice concentration (SIC)
The sea ice product files contain average sea ice concentration and the corresponding standard deviation per pixel during the period. In addition the product files contain fields for minimum and maximum concentration. These two fields are compiled of extreme values for each pixel and are thus not showing a global minimum/maximum situation.
The ice concentration fields take values from 0 % to 100 % according to the percentage of ice per pixel. Pixels over land, close to the coast or in an area of no satellite coverage are set to fill value. The figure below illustrates the appearance of sea ice products in the demonstrator. Should more than two days of satellite data be missing from the input files, no average field will be computed for this month. Some months may therefore contain a field of fill value only.
The above figure shows sample of average sea ice concentration (upper left), standard deviation in sea ice concentration (upper right), second minimum (lower left) and second maximum (lower right) – all shown in % – taken from the October 2000 monthly aggregated sea ice concentration file.
Sea ice extent (SIE)
It is common to use a concentration of 15% sea ice to mark the location of the sea ice edge, and this limit is therefore chosen in CryoClim as well. The monthly sea ice edge product is currently based on reprocessed daily sea ice concentration data. There are plans to reprocess and release a consistent data set of global, daily sea ice edge products based on SMMR and SSM/I data within the EUMETSAT OSI SAF Continuous Development and Operation Phase 2 (CDOP 2, starting summer of 2012). Following a future release of a reprocessed sea ice edge data set for daily gridded data, the monthly sea ice edge products in CryoClim could be updated to be based on the daily sea ice edge products.
For now, the monthly sea ice edge product is generated as follows: for each daily SIC field a daily sea ice edge is found by marking the pixels with 15% or higher ice concentration. The monthly ice edge is then found by averaging this daily “sea ice edge”. Should the ice concentration be 15% or more for more than half of the days of available data, the pixel is set to be within the ice edge and given the value 1, otherwise the pixel gets the value 0. The below figure shows the sea ice edge products for March and September 2007. The SIE monthly averaged field is stored on the same file as the monthly averaged SIC field.
The above figure shows examples of the monthly sea ice edge product. The left panel shows the average sea ice edge for March 2007, while the right panel shows September 2007.
Climate indicators for sea ice
Sea ice extent (SIE) and sea ice area (SIA) are integrated measures of the amount of sea ice in a region of interest, in our case the Northern Hemisphere. Both SIE and SIA have unit km2 and are traditionally computed from daily or monthly maps of Sea Ice Concentration (SIC) as available from CryoClim, among others. Traditionally, the time series of March (September) sea ice extent are featured, as they correspond to yearly maximum (minimum) extent in the Northern Hemisphere.
The time-series of daily sea ice concentration values from the EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI SAF, http://osisaf.met.no) was used to compute daily sea ice extent values. These daily sea ice extent values were then averaged over time to yield monthly sea ice extent.
The OSI SAF delivers two SIC time series, and both of them are combined in the CryoClim sea ice extent climate indicator:
1. The OSI SAF SIC re-processing dataset (OSI-409, Oct 1978 to Dec 2007)
2. The OSI SAF SIC operational product (OSI-401, available from 2005)
Note that the OSI-409 SIC time-series is also the baseline for the maps of CryoClim monthly sea ice concentration and edge.
For the purpose of studying climate variability, the OSI-409 dataset has higher quality than OSI-401 product. It was indeed recently processed (March 2010), with state-of-the-art algorithms, including adaptative sea ice tie-point, correction of land spill-over, and interpolation of missing data (Eastwood et al. 2010). However, this dataset currently only covers the period from October 1978 (SMMR instrument) to December 2007. Until it is extended in the OSI SAF projects, the climate indicator time-series has to be extended to recent years with other SIC data sources, such as the OSI-401 product.
A specific research effort was thus conducted by the CryoClim team at met.no, to ensure that the two SIE time-series (before and after December 2007) are consistent. In practice, a series of corrective steps were applied to OSI-401 for years 2008 to 2011, that are documented by Lavergne et al. 2010.
Illustration 1 and Illustration 2 plot such time series for the CryoClim project. In both figures, the negative trend in sea ice extent is clearly documented. The full SIC time series was processed to produce these graphs, from 1979 to September 2010. Illustration 3 plots the relative change in ice extent when compared to the 1979-2000 base period. The later figure documents a negative trend for both the September and March values, but more pronounced for September. The data used to plot these graphs will shortly be available from the CryoClim web portal.
Eastwood, S. (editor), The OSI SAF global reprocessed sea ice concentration dataset - Product User's Manual, 2010. Available online at http://osisaf.met.no.
Lavergne, T., Killie, M. A., Eastwood, S., and Breivik, L.-A., Extending the CryoClim Arctic sea ice extent time series with operational OSI SAF products from 2008 onwards - 07/2010