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In most cases GSFC used all daily data from a particular instrument until it was no longer available. For example the SMMR data was used until it ran out on 87232. Then they picked up with the next instrument's data (SSM/I F8).
The only exception to this occurred in December 1991 when the SSM/I F8 became only a "single data swath" after Julian day 352, 1991 (91352). In this case they immediately picked up with SSM/I F11 (91353).
When dealing with a special monthly average that was derived from daily data from 2 instruments GSFC had to choose one of the 2 possible file extensions. With the flip of a coin, 198708.N07 for August 1987 and 199112.N08 for December 1991 were chosen. To maintain the MSDOS file nomenclature 3 digit extension one or the other extension had to be selected. The decision was made to use the extension of the instrument which contributed the most daily files for a given month. Thus August 1987 was 198708.N07 because there were 10 SMMR dailies (N07) and only 5 SSM/I F8 dailies(N08).
Below the (YYDDD YEARday) are listed that were used for these 2 special monthly averages (August 1987/December 1991):
There are 2 months which were averaged using daily files from two instruments:
YYDDD where YY is year DDD is 3 digit Julian day example 87232 is Julian day 232 of 1987.
For the SMMR & SSM/I F8 overlap, use all of SMMR through 87232 then use SSM/I F8 daily data for rest of August 1987. Since the SMMR collected data every other even numbered day in 1987 only use these even numbered F8 days: 87234, 87236, 87238, 87240, 87242. Thus for August 1987:
SMMR : 87214, 87216, 87218, 87220, 87222, 87224, 87226, 87228, 87230, 87232
SSM/I F8: 87234, 87236, 87238, 87240, 87242
For the SSM/I F8 and SSM/I F11 overlap in December 1991:
From day 91353 through 91365 the F8 data are only single data swath. Use the F11 from 91353 through 91365. Use F8 from 91335 through 91352. Use F11 from 91353 through 91365. Thus for December 1991:
SSM/I F8: 91343, 91344, 91345, 91346, 91347, 91348, 91349, 91350, 91351, 91352
SSM/I F11: 91353, 91354, 91355, 91356, 91357, 91358, 91359, 91360, 91361, 91362, 91363, 91364, 91365.
In the NSIDC Document NSIDC-0051 (where the passive microwave derived polar sea ice concentration time series is described), it is stated under Section 6 (Note of Caution):
Computations of ice extents and ice areas should not be calculated from monthly-averaged ice concentration maps. It is recommended that they be computed from daily maps of ice concentrations that are then used to compute monthly averages of those parameters.
Could you explain why this is preferable?
It is recommended that daily data be used to compute time series of ice extent and area because using monthly-averaged data may result in a biased time series. Monthly-averaged ice extents and areas should be obtained from daily extents and areas. Documentation of the differences in sea ice extent and area between the daily concentrations and monthly-averaged concentrations is currently under way.
NSIDC has created animations of mean monthly sea ice concentrations for both the Arctic and Antarctic regions for the time period October 1978 through December 1996. QuickTime movies have also been created for each year in the time series. Users may find these particularly useful as browse tools, or as a quick means of examining seasonal patterns of sea ice change. Both the animations and the movies were created from monthly sea ice concentration GIF files provided by the Goddard Space Flight Center (GSFC). Included with the images is GSFC's standard color bar, which describes sea ice concentrations in four percent intervals. Ice concentrations less than 12 percent are considered open water.
The animations include data from four different satellites. Data from the Nimbus-7 satellite were used for the period October 1978 to July 1987. DMSP F8 data were used from August 1987 through December 1991. Data from the DMSP F11 satellite were used from January 1992 until September 1995. Data for the remaining 16 months are from the DMSP F13 satellite.
Several differences exist between the two tools. The animations show ice concentration variation for the entire study period. The animations were built using a free program called gifmerge. The Arctic animation contains 4145189 bytes and the Antarctic animation contains 4607857 bytes. They cover the period October 1978 to December 1996, giving them a length of 219 frames. The animations can be displayed on nearly all web browsers and are constructed to loop infinitely with a dwell time of 0.5 seconds. However, early versions of some browsers might only display the first frame when the animation is loaded, or might not recognize the instruction for the 0.5-second dwell time.
A number of free and commercial tools are available for both workstations and PCs that allow viewers to speed up and slow down loops and step through the animation frames individually. Two useful free tools for users with X-windows are ImageMagick, and xanim. Both tools utilize large amounts of swap space and allow users to construct their own animations from lists of single-frame GIF files. A commercial package called Animagic, available from Right to Left Software, Inc., allows users to speed up and slow down loops and step through frames individually.
The QuickTime movies show annual change for each year from 1979 to 1996. The movies were created from the GIF files using the SGI utility dmconvert. Each movie file is approximately 8.31 MBytes in size and can be displayed by most web browsers using a QuickTime viewer which is compatible with a majority of computers and can be downloaded free of charge. The viewers operate as stand-alone executables, providing users the ability to step forward and backward through individual frames. Unix users wishing to view the QuickTime movies can download xanim to manipulate individual movie frames.
