File 1: aricecon.dat - Arctic Sea Ice Concentrations
This file contains rectangular, equal area grids of monthly sea ice concentrations. The grids depict ice conditions at the END of each of 1136 months. The monthly values are interpolated to the last day of each month, rather than being values for the last week of each month. [Note that this data set differs from most other climatological data sets which contain monthly AVERAGES, not end of the month values.]
Ice concentration values are ASCII characters representing the number of tenths of ice coverage in each grid area. The grid interval is 1 degree latitude (60 n mi).
The file contains 1136 grids, arranged sequentially as follows:
1) January 1901
2) February 1901
. .
. .
. .
. .
1135) July 1995
1136) August 1995
Possible grid point values for the ice concentration grids are:
'0','1',...,'9' tenths of grid square area covered by ice.
'*' ten/tenths sea ice coverage
'.' grid point lies over land.
The following formatted FORTRAN READ statement will place an 80 x 58 ice concentration grid into the array ACON(80,58).
READ(NU,1000) MONTH, YEAR
DO 10 J=1,58
READ(NU,1010) (ACON(i,j),i=1,80)
10 CONTINUE
1000 FORMAT(I5,I5)
1010 FORMAT (80A1)
Similarly the following C fscanf statement will place an ice concentration grid into the character array acon[80][58].
fscanf(I_file,"%d %d",&month, &year);
for(i=0;i<58;i++)
{ for(j=0;j<80;j++)
{ fscanf(I_file,"%1s ", icon[i][j]);
}
}
where month = (1,2,...,12) and
year = (1901,1902,...,1995)
Note that the file arctic.template is a FORTRAN template for the extraction of a particular month/year. This code was complied on an SGI running IRIX and is intended as a template only.
File 2: aricesrc.dat - Data source grids
The data sources for the ice concentrations vary spatially and temporally. There are seven basic data sources for the ice concentrations:
- Danish Meteorological Institute
- Japan Meteorological Agency
- Naval Oceanographic Office (NAVOCEANO)
- Kelly ice extent grids (based upon Danish Ice Charts)
- Walsh and Johnson/Navy-NOAA Joint Ice Center
- Navy-NOAA Joint Ice Center Climatology
- Temporal extension of Kelly data (see note below)
- Nimbus-7 SMMR Arctic Sea Ice Concentrations or
DMSP SSM/I Sea Ice Concentrations using the NASA Team Algorithm
Each ocean grid point will be assigned a value (1-8) indicating the source of the data for each corresponding ice concentration grid.
** TEMPORAL EXTENSION OF KELLY GRIDS:
Sea ice extent data is provided by Kelly, et. al. 1988. The ice extent data is compiled for the months April-August for the majority of the period 1901-1956. In this data set, we utilize the Kelly data to create an ice concentration data source for the early period of record. This data is given very low priority in the hierarchy of available data so that if there are data from any sources (except climatology), we replace the extended Kelly data with this new source data. The modification of the Kelly data is done in two parts: (1) conversion from ice extent to ice concentrations, and (2) temporal extension of the available data.
(1) We add a marginal sea ice zone to the Kelly ice extent data by computing average ice concentration drop-off rates for the period during which there are satellite observations. These drop-off rates indicate the rate at which ice concentrations decrease as a function of distance from open water and distance from 10/10 ice concentrations. The drop off rates vary with season; the summer melt season drop-off rate is about 0.5 that of the freeze-up season. We apply these drop-off rates to the Kelly ice extent data to create a marginal sea ice zone.
(2) Regional sea ice anomalies have been shown to persist for many months and even seasons (Chapman and Walsh, 1991). We attempt to capitalize on this persistence by extending the ice anomaly data from (1) forward and backward in time to fill in the months September-March for each year in the 1901-1956 period. We compute lagged autocorrelations for the period of satellite observations and use the autocorrelations as weighting functions in the temporally extended data. For example, an anomaly for November is made by summing the anomalies of the preceding August and the subsequent April weighted by the -3 month lag autocorrelation and the +5 month lag autocorrelations, respectively.
We have attempted to stretch the useful information included in the Kelly ice extent data to extract as much information as possible from the data. We feel that the addition of these extensions make a more complete and detailed data set useful for most applications.
** Satellite derived data updates to Walsh sea ice database
During October, 1996, updates were made to the Walsh sea ice database. The database previously contained data through December, 1990. Updates to this data set are, and will continue to be made using ice concentrations obtained via the SSMI sources using the NASA Team algorithm. Ice conditions derived from these sources are flagged as "8" in the aricesrc.dat data set description file.
In order to maintain a consistent data source for the last part of the period, all data from October, 1978 through August, 1995 are from the SMMR/ SSMI sources. This means that data from previous versions of this data set were replaced by SMMR and SSM/I data from Oct. 1978 - Dec. 1990.
It appears that the SMMR and SSM/I data contains significant differences poleward of the ice edge for most months. Ice concentrations are generally lower in the central Arctic for the these data than for other data sources. Ice extents appear to be consistent across data sets, ice areas derived from pre-1978 data may be significantly higher than those calculated from the satellite period. The figure contained in icearea.ps provided with this data illustrates the rather abrupt jump in total northern hemisphere ice area around October 1978. The figure contained in icextnt.ps, ice extents calculated assuming 100% coverage everywhere ice was observed, illustrates that the extent data is more consistent between data sources.
We urge you to pay special attention to the data sources and their limitations when using this data.
The following formatted FORTRAN READ statement will place an 80 x 58 ice source grid into the array ACON(80,58).
READ(NU,1000) MONTH, YEAR
DO 10 J=1,58
READ(NU,1010) (ACON(i,j),i=1,80)
10 CONTINUE
1000 FORMAT(I5,I5)
1010 FORMAT (80A1)
Similarly, the following C fscanf statement will place an ice concentration grid into the array character array acon[58][80].
fscanf(I_file,"%d %d",&month, &year);
for(i=0;i<58;i++)
{ for(j=0;j<80;j++)
{ fscanf(I_file,"%1s ", icon[i][j]);
}
}
where month = (1,2,...,12) and
year = (1901,1902,...,1995)
File 3: arctic.grd Arctic Sea Ice Area Grid
The following formatted FORTRAN READ statement will place an 80 x 58 ice concentration grid into the array IAREA(58,80).
READ(NU,1000) MONTH, YEAR
DO 10 J=1,58
READ(NU,1010) (IAREA(i,j),i=1,80)
10 CONTINUE
1000 FORMAT(I5,I5)
1010 FORMAT (80(I2,1x))
Similarly the following C fscanf statement will place an ice concentration grid into the array character array iarea[80][58].
fscanf(I_file,"%d %d", &month, &year);
for(i=0;i<58;i++)
{ for(j=0;j<80;j++)
{ fscanf(I_file,"%d ", iarea[i][j]);
}
}
The Area grid contains the ocean areas corresponding to each grid point. Possible area grid point values are:
1,2,...,19,20 Number of tenths of 1 degree latitude square
covered by ocean (e.g., 10 = 60x60 n mi2); values
exceeding 10 are found near land areas where
adjacent grid point(s) lie over land.
0 Point lies over ocean but not within ice
grid area Figure 1.
-1 Grid point lies over land.
See the supplementary Documentation Section at the end of this document for further description of the ice area grid.
File 4: arctic.crd - Latitude/Longitude Coordinates for Arctic Grids.
This file contains the latitude (degrees N) and longitude (degrees E) coordinates for all points in the 80 x 58 Arctic grid.
The positive x axis is along 340E, the positive y axis is along 70E, and these axes intersect at point (35,24) which is the North Pole. The (1,1) grid point is considered to be the upper left hand corner of the grid rectangle. Some reference grid points are:
latitude and longitude grid coordinate samples:
( 1, 1) = 48.9512 N 125.923 E
( 1, 2) = 49.5031 N 127.095 E
(35,24) = 90.0000 N 250.000 E
( 1,58) = 41.9167 N 205.000 E
(80, 1) = 39.4629 N 7.07210 E
(80,58) = 33.5996 N 302.927 E
** Thanks to Steve Worley of NCAR for the above grid coordinate samples
** and documentation clarifications.
The following FORTRAN READ statement will put the latitudes and longitudes into REAL arrays xlat(80,58) and xlon(80,58):
READ (NU,1000) ((xlat(i,j),i=1,80),j=1,58)
READ (NU,1000) ((xlon(i,j),i=1,80),j=1,58)
1000 FORMAT(80(f8.4,1x))
Similarly in C:
for(i=0;i<58;i++)
{ for(j=0;j<80;j++)
{ fscanf(L_file,"%8f ", &lat[i][j]);
}
}
for(i=0;i<58;i++)
{ for(j=0;j<80;j++)
{
fscanf(L_file,"%8f ", &lon[i][j]);
}
}
}
Supplementary Documentation for arctic.grd
Grid Cell Value Coding Scheme
W. Chapman (Univ. Illinois) and C. Hanson (NSIDC)
92/04/13
Grid cell values:
The area grid assigns values to grid cells based on whether they represent land or ocean areas. Grid cells are defined by the center point of the cell. If the center point lies over land the cell value is -1 in the area grid and in the ice concentration grids. The ocean area of the cell is "given" to an adjacent cell.
A grid cell has a value of -1 in the Arctic if the cell lies entirely over land.
A grid cell has a value of 0 if the cell lies over ocean but is outside the ice grid.
A grid cell has a value of 10 if the cell lies entirely over ocean.
A grid cell partly over land and partly over ocean has a value of -1, if the center point of the cell is over land. The cell adjacent (oceanwards) has a value of 10 plus its neighbor's ocean area value. See Figure 1. A grid cell partly over land and partly over ocean with its center point over ocean is given the value (in tenths) of the area of the grid cell covered by ocean. (i.e. a number between 0 and 10).
Figure 1:
-\-------------------- Shown are three adjacent grid cells,
| \ | | | with the center point of each indicated
| .\ | . | . | by the dots. The slashes through cell 1
| / | | | are intended to indicate a continental
----/----------------- outline. Land is to the left of the line,
cell 1 cell 2 cell 3 ocean is to the right. The cells in Figure 1
would be coded as follows in the area grid:
Cell 1 = -1 (land)
Cell 2 = 16 (cell 2 is entirely ocean so value is 10, cell 1
is 6 tenths ocean so value is 6; 6 + 10 = 16)
Cell 3 = 10 (ocean)
This coding scheme excludes land cells from calculations of ice coverage, without excluding the area of ocean within a land cell that contains part land and part ocean.
The following explanation was contributed by John Weatherly, University of Illinois, Department of Atmospheric Sciences:
Given an Arctic grid point at : A degrees E longitude
B degrees N latitude
In the Arctic grid, this point is at:
YJ = 24 + (90 - B) cos(2(3.14)*(A+110)/360)
XI = 35 + (90 - B) sin(2(3.14)*(A+110)/360)
70 E
YJ
(1,1) _____________________|____________________(1,80)
| | |
| | |
| | |
| | |
| | |
| | |
| | |
| NP |____________________|____ XI
| (24,35) | 340 E
| |
| |
| |
| |
| |
| |
|_________________________________________|
(58,1) (58,80)
Arctic Monthly Sea Ice Concentration Grids: References
Chapman, W.L. and J.E. Walsh (1991) Long-Range Prediction of Regional Sea Ice Anomalies in the Arctic. "Weather and Forecasting" 6(2), pp. 271-288.
Walsh, J.E. (1978) A data set on Northern Hemisphere sea ice extent. National Snow and Ice Data Center, "Glaciological Data, Report GD-2", part 1, pp. 49 - 51.
Walsh, J.E. and C.M. Johnson (1978) Analysis of Arctic sea ice fluctuations 1953-77. "Journal of Physical Oceanography", 9(3), p. 580-591.
Technical Report of the Japan Meteorological Agency (JMA) No. 109 (ISSN 0447-3868) titled "Report on 30 years of Observation of Sea Ice" by Marine Department, JMA. Published by JMA, Tokyo, February 1988.
Kelly, P. M. 1979. An arctic sea ice data set, 1901-1956. Glaciological Data, Report GD-5: Workshop on Snow Cover and Sea Ice Data. National Snow and Ice Data Center, 101-106.
Sample Ice Concentration Grid:
12 1990
.................................*....................00..0.000............00000
................................................*......00.00000............00000
...................................*..**...*.00588.......00000000..........00000
.................................*..*.*****880..8........00000000..........00000
.................................*.*****888800..8.......00......00.........00000
................................**.****.*80000......000000.....00..........00000
...............................********.*800000.....00.00.....0............00000
..............................********.88000000...............000000000000000000
..............................*******..00000000................00000000000000000
.88...........................*****..88000000000...............00000000000000000
.88..........................*****8***000000000000............000000000000000000
5677...................*....******00*8000000000000000000000000000000000000000000
4567.................***....*********0**0000000000000000000000000000000000000000
24667................*****************0***0*000000000000000000000000000000000000
12467.............*.*********..*****.**0*****00000000000000000000000000000000000
01257.............*********0***********0***.**0000000000000000000000000000000000
0137.............**********************0*....00000000000000000000000000000000000
01377.............**********************800.000000000000000000000000000000000000
0137...............*.********************000000000000000000000000000000000000000
0237.............***************************600000000000000000000000000000000000
0258............********************************66000000000000000000000000000000
..5889..........***9*******************************6000000..00000000000000000000
...889.........****************************.*********00000...0000000000000000000
....89........*****************************.....*.******000..0000000000000000000
.....9........**************************0.............***0...0000000000000000000
000...........***************************..............***0000000000000000000000
00000.........***************************...............**0000000000000000000000
00000..........**************************...............**0000000000000000000000
000000.........**************************...............**0000000000000000000000
000000.........***********************...................*0000000000000000000000
0000000........*****0*****************....................0000000000000000000000
0000000........**********************....*...............00000000000000000000000
00000000.***..************************...*...*............*800000000000000000000
000000000688..************************...**.*****...........80000000000000000000
000000000**...*********************..**..**********.*........8000000000000000000
0000000008*****************************...***********.........000000000000000000
0000000000******....************.*****.*..***********6.......0000000000000000000
0000000008***..*.....************...***..**.**********60000000000000000000000000
000000000*****........***********.*******.......*******0000000000000000000000000
000000000**..*.........*******...****.*.*...........***6000000000000000000000000
000000000**.............******..***.*.***....*......****600000000000000000000000
000000000*..............******.*....***.**..****...******00000000000000000000000
0000000008................**.***....***..**..****.....***40000000000000000000000
000.000000....................**......**..**.****.....******0555**00000000000000
00000.00000.....................*..*****.....***.********88****5*660000000000000
0000000.000.....................***..**......***********...*8***.630000000000000
0000000000..00..............................*..***....**..........00000000000000
00000000000..0000...........................*.***................0.0...000000000
00000000000000000..........................********...............0...0000000000
000000000000000000.........................********...............0..00000000000
0000000000000000000........................*********.............00.000000000000
0000000000000000000.......................***********............000000000000000
00000000000000000000.......................**********.........0..000.00000000000
00000000000000000000.......................**********................00000000000
000000000000000000000.......................**********...............00000000000
000000000000000000000...............................*0..............000000000000
0000000000000000000000..............................***.............000000000000
0000000000000000000000............................................00000000000000
Sample Ice Data source grid:
12 1990
.................................5....................66..5.555............55555
................................................5......66.55555............55555
...................................5..55...5.55555.......55555555..........55555
.................................5..5.55555555..5........55555555..........55555
.................................5.55555555555..5.......55......55.........55555
................................55.5555.555555......555555.....55..........55555
...............................55555555.5555555.....55.55.....5............55555
..............................55555555.55555555...............555555555555555555
..............................5555555..55555555................55555555555555555
.66...........................55555..55555555555...............55555555555555555
.66..........................555555555555555555555............555555555555555555
6666...................5....5555555555555555555555555555555555555555555555555555
6666.................555....5555555555555555555555555555555555555555555555555555
66666................55555555555555555555555555555555555555555555555555555555555
66666.............5.555555555..55555.5555555555555555555555555555555555555555555
66666.............5555555555555555555555555.555555555555555555555555555555555555
6666.............555555555555555555555555....55555555555555555555555555555555555
66666.............5555555555555555555555555.555555555555555555555555555555555555
6666...............5.55555555555555555555555555555555555555555555555555555555555
6666.............555555555555555555555555555555555555555555555555555555555555555
6666............5555555555555555555555555555555555555555555555555555555555555555
..6666..........555555555555555555555555555555555555555555..55555555555555555555
...666.........5555555555555555555555555555.55555555555555...5555555555555555555
....66........55555555555555555555555555555.....5.555555555..5555555555555555555
.....6........555555555555555555555555555.............5555...5555555555555555555
555...........555555555555555555555555555..............5555555555555555555555555
55555.........555555555555555555555555555...............555555555555555555555555
55555..........55555555555555555555555555...............555555555555555555555555
555555.........55555555555555555555555555...............555555555555555555555555
555555.........55555555555555555555555...................55555555555555555555555
5555555........55555555555555555555555....................5555555555555555555555
5555555........5555555555555555555555....5...............55555555555555555555555
55555555.555..555555555555555555555555...5...5............5555555555555555555555
555555555555..555555555555555555555555...55.55555...........55555555555555555555
55555555555...555555555555555555555..55..5555555555.5........5555555555555555555
555555555555555555555555555555555555555...55555555555.........555555555555555555
5555555555555555....555555555555.55555.5..555555555555.......5555555555555555555
5555555555555..5.....555555555555...555..55.555555555555555555555555555555555555
55555555555555........55555555555.5555555.......55555555555555555555555555555555
55555555555..5.........5555555...5555.5.5...........5555555555555555555555555555
55555555555.............555555..555.5.555....5......5555555555555555555555555555
5555555555..............555555.5....555.55..5555...55555555555555555555555555555
5555555555................55.555....555..55..5555.....55555555555555555555555555
555.555555....................55......55..55.5555.....55555555555555555555555555
55555.55555.....................5..55555.....555.5555555555555555555555555555555
5555555.555.....................555..55......55555555555...55555.555555555555555
5555555555..55..............................5..555....55..........55555555555555
55555555555..5555...........................5.555................5.5...555555555
55555555555555555..........................55555555...............5...5555555555
555555555555555555.........................55555555...............5..55555555555
5555555555555555555........................555555555.............55.555555555555
5555555555555555555.......................55555555555............555555555555555
55555555555555555555.......................5555555555.........5..555.55555555555
55555555555555555555.......................5555555555................55555555555
555555555555555555555.......................5555555555...............55555555555
555555555555555555555...............................55..............555555555555
5555555555555555555555..............................555.............555555555555
5555555555555555555555............................................55555555555555