Data Description

Yearly maps of early melt (earliest observed melt conditions), melt (melt conditions observed from this point until freeze), early freeze (earliest observed freeze conditions), and freeze (freeze conditions observed from this point on) for the surface of sea ice derived from SSM/I data. Reference:

Markus, T., J. C. Stroeve, and J. Miller (2009), Recent changes in Arctic sea ice melt onset, freezeup, and melt season length, J. Geophys. Res., 114, C12024, doi:10.1029/2009JC005436

Product Updated, 4/18/2017

This update adds melt maps for 2015 and 2016. 2016 uses F17 for the first part of the year and then switches to F18. The intercalibration between F17 and F18 looks good so at the moment no bias adjustment is done. We will continue to monitor the performance of F18.

Fixed a bug that didn't reinitialize the default freeze value between pixels which meant that in a low number of pixels there was no new freeze value determined and the algorithm used the prior result. This fix made the version 730s across all sensors.

 

Product Updated, 7/2/2015

In early October 2011, the AMSR-E instrument developed a fault and further data was unavailable on the archive. The previous source data for this routine, SSM/I F13, stopped providing data at the end of 2007 and at that time the routine had been adapted to AMSR-E and maps were processed from 2003 through 2010. The routine has now been adapted to use SSM/IS F17 as a data source for microwave brightness temperatures with the version “701s”, and the 2011 maps were processed.

In February 2014, we investigated anomalous behavior of the routine in coastal areas on the early freeze and freeze maps. We discovered that the bias correction used for F17 was misreporting ice concentration values during the melt period, and that was resulting in the poor freeze-up values. The bias was adjusted and the datset was reprocessed for 2007 on with version "702s".

The following figure shows average melt onset, freeze onset, and average melt duration in days for the entire Arctic.  

 

figure displaying trends in melt onset, freeze onset, and melt season length

 

Data Format

Readme for melt / freeze products

4 files:

YYYY - year
VVVV - version, current set is 730s.

Integer arrays, 304 x 448, big endian (windows and mac will need to byteswap). Values are the day of year where the event described in the filename occurs (in 'melt', a value of 175 means melt conditions are present on June 24 in a non-leap year).

 

Data Flags:

-3
land
0
open water
75-410
valid data

All negative values represent pixels where melt or freeze could not be determined; the different negative values correspond to different criteria in the algorithm and should not be considered for analysis. The earliest melt possible due to the algorithm is day 75. Data is no longer masked for the polar "hole" in the coverage, and users should note that the hole changes size with satellite transitions.

Updated on 7/02/2015

Removed the AMSR-E products from this distribution in favor of SSM/I / SMMR trend only, since AMSR-E stopped providing data. Updated 2007+ to improved bias adjustment, version 702s.

Updated on 4/18/2017

Updated through 2016. Version changed to 730s across all the instruments as a bug in the default ice freeze value for some cases was corrected.

 

Contact info:

README last updated 4/18/2017

Data Files

  • melt_update_2017.tar - tar, 39.71 MB: Last Updated: April 18, 2017 Artic sea ice melt/freeze datafiles (tar'ed)
  • melt_update_2017.zip - zip, 3.2 MB: Last Updated: April 18, 2017 Artic sea ice melt/freeze datafiles (zipped)