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NOAA Technical Memorandum GLERL-135

____________________________________________________________________________

Great Lakes Ice Cover Climatology 

Update: Winters 2003, 2004, and 2005

Raymond A. Assel

NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105

Great Lakes Environmental Research Laboratory

Ann Arbor, Michigan

July 2005

____________________________________________________________________________

UNITED STATES NATIONAL OCEANIC AND

DEPARTMENT OF COMMERCE ATMOSPHERIC ADMINISTRATION

Carlos M. Gutierrez Conrad C. Lautenbacher, Jr.

Secretary Administrator
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NOTICE

Mention of a commercial company or product does not constitute an endorsement

by the NOAA. Use of information from this publication concerning proprietary

products or the tests of such products for publicity or advertising purposes is not

authorized. This is GLERL Contribution No. 1363.

This publication is available as a PDF file and can be downloaded from GLERL’s

web site: www.glerl.noaa.gov. Hard copies can be requested from GLERL,

Information Services, 2205 Commonwealth Blvd., Ann Arbor, MI 48105.

pubs.glerl@noaa.gov.
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TABLE OF CONTENTS

ABSTRACT.............................................................................................................................. 4

INTRODUCTION...................................................................................................................... 4

DATA ........................................................................................................................................ 4

ANALYSIS METHODS ............................................................................................................ 5

PRODUCTS AND DISCUSSION.............................................................................................. 6

DATES OF FIRST (LAST) ICE AND ICE DURATION............................................................ 7

SEASONAL PROGRESSION OF ICE COVER....................................................................... 10

The 2003 Great Lakes Ice Cycle ....................................................................................... 10

The 2004 Great Lakes Ice Cycle ....................................................................................... 12

The 2005 Great Lakes Ice Cycle ....................................................................................... 14

CONCLUDING OBSERVATIONS ......................................................................................... 16

REFERENCES......................................................................................................................... 17

APPENDIX 1. DATES OF FIRST ICE, LAST ICE, AND ICE DURATION ........................... 18

APPENDIX 2. DAILY ICE COVER TIME SERIES ............................................................... 19

APPENDIX 3. DAILY ICE COVER ANOMALIES ................................................................ 20

LIST OF FIGURES

1. Dates of first reported ice ≥ 10% (left) and ≥ 90% (right) for 2003, 2004, and 2005................ 7

2. Dates of last reported ice ≥ 10% (left) and ≥ 90% (right) for 2003, 2004, and 2005................. 8

3. Duration of ice cover ≥ 10% (left) and ≥ 90% (right) for 2003, 2004, and 2005 .................... 9

4. Daily lake-averaged ice cover, winter 2003........................................................................... 10

5. Winter 2003 ice cover and anomaly charts for Jan. 16, Feb. 17, Mar. 24, and Apr 14........... 11

6. Daily lake-averaged ice cover winter 2004............................................................................ 12

7. Winter 2004 ice cover and anomaly charts for Jan. 15, Feb. 5, Feb. 19, and Mar. 15.. ........... 13

8. Daily lake-averaged ice cover, winter 2005........................................................................... 14

9. Winter 2005 ice cover and anomaly charts for Jan. 17, Feb. 3, Mar. 3, and Apr. 4................ 15

LIST OF TABLES

1. Summary of dates of original ice charts.................................................................5
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Great Lakes Ice Cover Climatology Update:

Winters 2003, 2004, and 2005

Raymond A. Assel

ABSTRACT

A 30-winter ice concentration climatology (Assel 2003a) is updated 

for winters: 2003, 2004, and 2005. Original ice charts are from the

National Ice Center and the Canadian Ice Service. These data are

quality controlled for over-water grid cell location and ice

concentration codes to be consistent with Assel (2003a). Data are

available in the form of ASCII grids and graphic files. The 2003, 2004,

and 2005 ice cycles are described and discussed briefly within the

context of dates of first (last) ice, ice duration, daily lake-averages, and 

spatial and temporal distribution patterns of ice cover and anomalies.

INTRODUCTION

The annual formation and loss of ice cover on the Great Lakes each winter, i.e. the annual ice

cycle, affects the lake’s ecosystem and the regional economy of the Great Lakes. In 2003, a 30-

winter climatology of Great Lakes ice cover was published (Assel 2003a). The information 

presented here updates the total ice concentration grids of original ice charts digitized by other

government agencies during the winters 2003, 2004, and 2005 and also updates the following

analysis products given in Assel (2003a): dates of first ice, dates of last ice, ice cover duration,

daily ice cover grids, daily lake-averaged ice cover, and computer animations of the seasonal

progression of ice cover. Anomaly ice charts for 2003, 2004, and 2005 similar to those in Assel

(2005) for winters 1973-2003 are also included in this update. Line plots and ice charts portray

the seasonal (temporal) and spatial patterns and trends of ice cover over the winters of the update

period, and ASCII files make it practical to use these data in numerical analysis applications.

These data, analysis methods, and products are described and discussed briefly. The primarily

purpose of this report is to make access and use of these data feasible to others in need of such

information.

DATA

There are 143 digitized original ice charts in this update, 51, 46, and 46 for winters 2003, 2004,

and 2005, respectively (Table 1). In general, two ice charts are produced for each week, one by

the National Ice Center and one by the Canadian Ice Service. Ice charts from both sources are on

the National Ice Center Web site, http://www.natice.noaa.gov/products/gl-ches/index.htm. The

data consists of grids of over-water cells and land cells in a 510 row by 516 column matrix.

These data were downloaded to GLERL and quality controlled for location of over-water grid 

cells and ice codes in order to be consistent with the 30-winter GLERL ice cover climatology.

The GLERL, NIC, and CIS ice codes are given in Assel et al. (2002). The center of each GLERL

grid cell is cross referenced with latitude and longitude in Assel (2003a), see

http://www.glerl.noaa.gov/data/ice/atlas/daily_ice_cover/daily_grids/dailygrids.html for latitude
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and longitude files. This facilitates application of these data in a Graphic Information System

environment and for other applications in which geographic location information is needed.

ANALYSIS METHODS

An analysis for threshold ice concentration

values of dates of first (last) ice each winter,

and ice duration (the difference between 

these dates) was made for each winter

season. The original ice charts had ice

concentration recorded to the nearest 10%.

The analysis was made on nine overlapping

threshold ice concentration values in 10%

increments starting from ice concentration

≥10%, and ending at ice concentrations

≥90% (i.e., ≥10%, ≥20%, ≥30%, ≥40%,

≥50%, ≥60%, ≥70%, ≥80%, ≥90%). A grid 

search of over-water cells, cell by cell, was

made on the original ice chart grids for each 

winter (Table 1) to determine the first (last)

observed date for each of the nine threshold

values of ice concentration at each grid cell.

These dates of observed first (last) ice may

not be the actual date that ice first formed

and actual last date of ice because this

analysis is limited by the dates of the first

and last ice charts of each winter and the fact

that daily observations were not made.

However, it is likely that for most of the

Great Lakes surface, the actual date of first

ice occurred somewhere between the date of

the first ice chart, and the ice chart that has

the observed threshold value of ice

concentration. This is true because on the

date of the first ice chart, most of the Great

Lakes are usually open water. A similar

comment is true for the date of last ice cover.

Additional information and details on dates

of first (last) ice and ice duration for the 30-

winter base period are given in Assel

(2003b).

One hundred and eighty-two daily Great Lakes ice cover grids are created for each winter season

between December 1 and May 31 (May 30 for leap years). A temporal linear interpolating of ice

concentration between consecutive observed ice chart grids, cell by cell, for a given winter

Table 1. Summary of Dates of Original Ice Charts

Chart 2003 2004 2005

No. Month Day Year Month Day Year Month Day Year

1 12 2 2002 12 1 2003 12 2 2004

2 12 5 2002 12 3 2003 12 6 2004

3 12 9 2002 12 8 2003 12 9 2004

4 12 12 2002 12 11 2003 12 13 2004

5 12 16 2002 12 15 2003 12 16 2004

6 12 19 2002 12 18 2003 12 20 2004

7 12 23 2002 12 22 2003 12 23 2004

8 12 26 2002 12 25 2003 12 27 2004

9 12 30 2002 12 29 2003 12 30 2004

10 1 2 2003 1 1 2004 1 3 2005

11 1 6 2003 1 5 2004 1 6 2005

12 1 9 2003 1 8 2004 1 10 2005

13 1 13 2003 1 12 2004 1 13 2005

14 1 16 2003 1 15 2004 1 17 2005

15 1 20 2003 1 19 2004 1 20 2005

16 1 23 2003 1 22 2004 1 24 2005

17 1 27 2003 1 26 2004 1 27 2005

18 1 30 2003 1 29 2004 1 31 2005

19 2 3 2003 2 2 2004 2 3 2005

20 2 6 2003 2 5 2004 2 7 2005

21 2 10 2003 2 9 2004 2 10 2005

22 2 13 2003 2 12 2004 2 14 2005

23 2 17 2003 2 16 2004 2 17 2005

24 2 20 2003 2 19 2004 2 21 2005

25 2 24 2003 2 23 2004 2 24 2005

26 2 27 2003 2 26 2004 2 28 2005

27 3 3 2003 3 1 2004 3 3 2005

28 3 10 2003 3 4 2004 3 7 2005

29 3 13 2003 3 8 2004 3 10 2005

30 3 17 2003 3 11 2004 3 14 2005

31 3 20 2003 3 15 2004 3 17 2005

32 3 24 2003 3 18 2004 3 21 2005

33 3 27 2003 3 22 2004 3 24 2005

34 3 31 2003 3 25 2004 3 28 2005

35 4 3 2003 3 29 2004 3 31 2005

36 4 7 2003 4 1 2004 4 4 2005

37 4 10 2003 4 5 2004 4 7 2005

38 4 14 2003 4 8 2004 4 11 2005

39 4 21 2003 4 12 2004 4 14 2005

40 4 24 2003 4 15 2004 4 18 2005

41 4 28 2003 4 19 2004 4 21 2005

42 5 1 2003 4 22 2004 4 25 2005

43 5 5 2003 4 26 2004 4 28 2005

44 5 8 2003 4 29 2004 5 2 2005

45 5 12 2003 5 3 2004 5 5 2005

46 5 15 2003 5 6 2004 5 9 2005

47 5 19 2003

48 5 22 2003

49 5 26 2003

50 5 29 2003

51 6 2 2003
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season was used to create the daily grids. Ice cover grids prior to the first observed ice chart and

after the last observed ice chart were set to zero for over water grid cells. The daily average ice

cover grids are considered to be reasonable estimates of actual ice cover concentration because:

(I) the average number of days between observed ice charts (Table 1) is 3 days (2003), 2 days

(2004), and 3 days (2005), (II) the date of first and last ice chart are December 2 and June 2 

(2003), December 1 and May 6 (2004), and December 2 and May 9 (2005), and (III) the lake

averaged ice cover on each Great Lake for the first (last) ice chart was < 1.2% (0%). Assel and 

Norton (2001) provide further description of the creation of the daily time series and of the

graphic files used to create computer animations that portray the seasonal progression of ice

cover. Once the graphic files for the animation were produced, any number of different

animation programs could produce the actual animations.

Daily lake average ice cover is calculated from the daily grids. A grid encoding the grid cells for

each lake and river is given in a fixed formatted text file (GLmask2.txt) in Assel (2003a). Each 

record in that file has a format of 516 I1. Land grid cells have a code of 0 or 1. The grid cell

codes for each lake and river are: 4 for Lake Superior, 5 for Lake Michigan, 6 for Lake Huron, 7 

for Lake Erie, 8 for Lake Ontario, 2 for Lake St. Clair, 3 for the Detroit River, and 9 for the St.

Marys River. All grid cell ice concentrations for a given lake or river for a given date are

summed, and a lake (river) average is then calculated by dividing the sum by the total number of

grid cells for that lake (river). The total number of grids cells for each Great Lake are: 13,850

(Lake Superior), 8545 (Lake Michigan), 9161 (Lake Huron), 3574 (Lake Erie), and 2801 (Lake

Ontario).

Assel (2005) produced anomaly ice charts for each winter from 1973 to 2002. Ice anomaly charts

were produced for each day of the winter by calculating the difference (for each over water grid

cell) between the 30-winter weekly median ice concentration grid and the ice concentration grid 

for a date that corresponds to the mid-week day of the weekly median (e.g., the 30-winter weekly 

median ice chart for January 12-18 would be used to calculate the ice anomaly chart for an ice

chart dated January 15). In a similar manner, ice anomaly charts were produced from the daily

ice grids for the three winters of the update period. Ice anomaly charts provide a metric of the

trends in ice cover over any date in a given winter season relative to the 30-winter base period.

Positive (negative) differences occur when the ice chart for a given winter has ice cover

concentration greater (less) than the 30-winter median.

PRODUCTS AND DISCUSSION

Analysis products are summarized in a series of line plots (for lake averaged ice cover) and

charts for: (1) dates of first (last) ice, ice duration, (2) daily ice concentration, and (3) daily ice

concentration anomalies. ASCII text files containing integer codes for ice are also produced for

all the data given in the graphic plot file products. Data and file documentation for dates of first

(last) ice and ice duration are given in Appendix 1; daily ice cover and its associated products

(daily grids, daily images, computer animations, lake-average plots, and table), are given in

Appendix 2; daily ice cover anomalies (animations, images, ASCII grids) are given in Appendix

3. Seasonal and spatial trends are discussed below using selected data from these appendices.
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DATES OF FIRST (LAST) ICE AND ICE DURATION

The two extremes, ice ≥ 10% (ice < 10% is considered to be the boundary between ice and open

water) and ice cover ≥ 90%, virtually a complete ice cover are shown in Figures 1-3 for dates of

first (last) ice and ice duration, respectively. More mid-lake areas of Lakes Superior, Michigan,

and Huron formed ice later or were virtually ice free in 2004 and 2005 (Figure 1) relative to 

2003. Some shore areas formed ice earlier (December) in 2005 compared to 2003.

Figure 1. Dates of first reported ice ≥ 10% (left) and ≥ 90% (right) for 2003, 2004, and 2005.
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The dates of last ice cover in areas with ice on Lakes Superior, northern Michigan, Huron, and 

the eastern half of Lake Erie were in general later in 2003 relative to 2004 and 2005 (Fig. 2).

Figure 2. Dates of last reported ice ≥ 10% (left) and ≥ 90% (right) for 2003, 2004, and 2005.
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The duration of ice cover (Fig. 3) was in general longer in 2003.

Figure 3. Duration of ice cover ≥ 10% (left) and ≥ 90% (right) for 2003, 2004, and 2005.
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SEASONAL PROGRESSION OF ICE COVER

Seven-day moving averages of the daily lake–averaged ice cover and selected charts of ice cover

and ice cover anomalies are used to portray the seasonal progression of the 2003, 2004, and 2005

ice cycles. More data and computer animations are given in Appendix 2 and Appendix 3.

The 2003 Great Lakes Ice Cycle

The Great Lakes lake-averaged ice cover in 2003 was above the Long-Term-Median (LTM)

most of that winter. In 2003, Lake Erie was near its maximum ice extent (≥ 90%) from the end of

the third week of January to the middle of the second week of March (Figure 4). Lakes Superior

Huron, and Michigan were near their maximum ice cover (≥ 90%, ≥ 90%, and ≥ 40%,

respectively) from the end of February to

middle of the second week of March. Lake

Ontario was near its maximum ice cover (≥

40%) the third week of February and again

from the end of February to middle of the

second week of March. Lake-averaged ice

cover was less than 10% on Lake Ontario by

the middle of the fourth week of March, on 

Lake Erie by the end of the first week of

April, on Lake Michigan by the end of the

second week of April, on Lake Superior by

the end of the third week of April, and on

Lake Huron by the end of April.

Figure 4. Daily lake-averaged ice cover, winter 2003.

The spatial patterns of ice cover for 2003 for early winter (January 16), mid winter (February

17), early spring (March 24), and mid spring (April 14) along with the spatial patterns of the

difference between the 30-winter weekly median ice concentration and the ice chart for a given

date, i.e., the ice anomaly charts are shown as Figure 5. In mid-January the ice cover was limited

to the shore regions of the Great Lakes, primarily in bays, with the exception of Lake Erie, where

it covered the west basin and about half of the center basin of that lake. The corresponding

anomaly ice chart for mid January shows that shore ice cover was below the 30-winter median 

along the shores of Lake Superior and along selected northern areas in Lakes Michigan and

Huron. Ice cover was above the 30-winter median in much of the center Lake Erie basin. By the

middle of February, ice cover extended over most of Lakes Superior, Huron, and Erie, over the

northern portion of Michigan, and over much of the eastern half of Lake Ontario. The ice cover

extent in mid-February (February 17) was greater than the 30-winter median over Lakes

Superior, Michigan, Huron, and Ontario but not over Lake Erie. Ice cover anomaly chart in early

spring (March 24) was similar to the February 17 anomaly chart, i.e. ice cover was still

anomalously high over most of Lakes Superior, northern Michigan, and all of Huron. However,

Lake Erie now also has anomalously high ice cover. Ice cover was lost over the next three

weeks. By April 14 (with the exception of several bays in Lakes Superior, Huron, and Michigan)

there were only relatively small areas of ice in the lakes proper; these areas show up in the

corresponding anomaly ice chart as above-median ice cover, primarily along and lakeward of the

south shore of Lake Superior, in Green Bay and along the north shore of Lake Michigan, and in 

Georgian Bay in Lake Huron.
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Figure 5. Winter 2003 ice cover and anomaly charts for Jan. 16, Feb. 17, Mar. 24, and Apr 14.
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The 2004 Great Lakes Ice Cycle

Seasonal maximum ice cover in 2004 occurred earlier than it did in 2003, did not last as long,

and with the exception of Lake Erie, was less than it was in 2003. In 2004, Lake Erie ice cover

was ≥ 90% and near its seasonal maximum ice extent (95%) the last week of January through the

end of the first week of February and again during the second half of the third week of February 

(Figure 6). Lake Ontario ice cover was ≥ 30% and near its maximum ice extent (38%) the last

week of January. Lake Huron ice cover was ≥ 55% and was near its seasonal maximum extent

(65%) from the end of January to the middle of the first week of February and again from the

middle of the second week of February to the end of the third week of that month. Lake

Michigan ice cover was ≥ 30% and near its seasonal maximum ice extent (36%) from the middle

of the first week to the middle of the second week of February, and briefly near the end of the

third week of February. Lake Superior ice cover was ≥ 40% and near its seasonal maximum ice

extent (52%) from near the end of the first

week of February to near the end of the

second week of February and again briefly

near the end of the third week of that month.

The occurrence of lake-averaged ice cover

<10% was earlier in 2004 than it was in

2003. Lake-averaged ice cover <10%

occurred near the end of the second week of

March for Lake Ontario, the end of the third

week of March for Lake Superior, the middle

of the last week of March for Lake

Michigan, the middle of the first week of

April for Lake Erie, and at the end of the last

week of April for Lake Huron.

Figure 6. Daily lake-averaged ice cover, winter 2004.

Early winter (January 15) ice cover was a little more extensive in 2004 (Figure 7) relative to 

2003 (Figure 5) on Lakes Superior, the north end of Lake Michigan, and the perimeter of Lake

Huron. The opposite is true of Lake Erie. By early February (Figure 7), extensive ice cover

formed lakeward of the shore. Ice cover was greater than the LTM along the northwest shore and 

lakeward of other near shore regions of Lake Superior, along the west shore and extending in

narrow band along the southeast coast of Lake Michigan, and lakeward of the west coast and the

south end of Lake Huron. Ice cover on February 19 was near its seasonal maximum, and the

pattern of ice cover anomalies were similar to those of early February. There were still positive

anomalies along the southern end of Lake Michigan, mid-lake area of southern Lake Huron, and 

mid-lake areas of Lake Superior. However, there were now negative anomalies in western Lake

Superior (and along portions of its south-central and northeast shore), and a new area of negative

anomalies along the northwest shores of Lake Huron and Georgian Bay. By the middle of March

2004, ice cover was limited primarily to portions of the shore and in large bays of the Great

Lakes. The ice anomaly chart for March 15 had large negative anomalies in Lake Superior,

western Georgian Bay, and central Lake Erie. The March 15, 2004 ice cover was similar to the

April 14, 2003 ice cover, and less than March 24, 2003 (Figure 5). Thus, the ice cycle in many

areas of the Great Lakes had ended earlier in 2004 than it did in 2003.
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Figure 7. Winter 2004 ice cover and anomaly charts for Jan. 15, Feb. 5, Feb. 19, and Mar. 15.
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The 2005 Great Lakes Ice Cycle

The magnitude of the seasonal maximum ice cover was about the same as it was in 2004, and it

was bimodal for Lakes Michigan, Huron, and Erie (Figure 8) as it was in 2004 (Figure 6) for

Lakes Superior, Michigan, Huron, and Erie. Lake Erie ice cover was ≥ 85% and near its seasonal

maximum (93%) from the last week of January to the middle of the second week of February

and again during the first week of March. Lake Ontario ice cover was ≥ 30% and near its

seasonal maximum extent (38%) from the middle of the last week of January to the middle of the

first week of February. Lake Huron ice cover was ≥ 50% and near its seasonal maximum (59%)

from the middle of the last week of January to the middle of the first week of February and again 

from the middle of the first week to the middle of the third week of March. Lake Michigan was

near its seasonal maximum (29%) and had ice cover ≥ 20% from the end of the third week of

January to the middle of the first week of

February and again during the first week of

March. Lake Superior had ice cover ≥ 40%

and was near its seasonal maximum ice

cover (54%) from the middle of the second

to the middle of the third week in March.

The occurrence of lake-averaged ice cover <

10% in spring 2005 was later than it was in

2004, but earlier than it was in 2003. Lakeaveraged ice cover <10% occurred the

middle of March for Lake Ontario, the first

day of April for Lake Michigan, the third

day of April for Lakes Superior and Erie,

and near the end of the second week of April

for Lake Huron.

Figure 8. Daily lake-averaged ice cover, winter 2005.

The spatial patterns of ice cover for 2005 for early winter (January 17), near its maximum

(February 3), near its maximum for a second time (March 3), and for early spring (April 4) are

portrayed in Figure 9. The mid-January ice cover was restricted to large bays and shoal areas of

the Great Lakes. A large area of negative ice anomaly was located primarily along the north 

central shore of Lake Erie, indicating less ice than the LTM for that area. Smaller areas of both 

positive and negative ice cover anomalies lined the shores of the other four Great Lakes.

The ice cover in early February was near its seasonal maximum. It was still restricted to the

shore and shoal areas of Lakes Superior and Michigan. It extended out many miles into the lake

from the west shore of Lakes Huron, covered most of Georgian Bay, the perimeter of Ontario,

and virtually all of Lake Erie. In lake areas with ice this early, February ice cover was in general

greater than the LTM in Lakes Huron, Erie, and Ontario, it was less than the LTM in Lake

Superior, and near the LTM in the deeper open lake areas of Lakes Michigan, Superior, Huron,

and Ontario.
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Figure 9. Winter 2005 ice cover and anomaly charts for Jan. 17, Feb. 3, Mar. 3, and Apr. 4.
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A month later, March 3, ice cover is again near its seasonal maximum. Ice cover had increased

and now extends farther lakeward from the shore and shoal areas of Lake Superior, but for this

time of year, it is below the LTM for the western mid lake area and in an offshore band lining the

mid-lake eastern lake basin. There is now an extensive ice cover in northern Lake Michigan that

is similar to the LTM. However it extends farther southward than the LTM (indicated by positive

values on the ice anomaly chart). The ice cover in Lake Huron is now less than it was a month

earlier along the west shore of that lake, but it is still greater than the LTM along the western 

lake shore. Georgian Bay ice cover has also decreased from its early February value. However, it

is near the LTM with the exception of an area in the southwest quadrant that is below the LTM.

Lake Ontario’s ice cover has also receded from its early February extent; the bulk of the

remaining ice cover is located in the northeast quadrant. Most of the lake is open water and is

near its LTM. High ice concentration still extends over most of Lake Erie with the exception of a

linear band along the north shore and off of Long Point. The bulk of Lake Erie’s ice cover is near

the LTM, it is less than the LTM in the boundary between the west and central lake basin with

small patches of positive and negative anomalies elsewhere.

The lakes were virtually free of ice cover by the middle of the first week of April with the

exception of small discrete areas of ice near shore and in bays. Anomaly ice charts for April 4

show that there were small patches with both positive and negative ice cover anomalies: positive

for the south shore Lake Superior, the north shore of Lake Michigan, Saginaw bay in Lake

Huron; negative for the Apostle Islands and Whitefish Bay in Lake Superior, the southwest shore

for the Straits of Mackinac on Lake Michigan, the north tip of Georgian Bay in Lake Huron, and 

the east end of Lake Erie.

CONCLUDING OBSERVATIONS

The differences in the timing of first (last) ice and ice duration for the ice cover in 2003 relative

to 2004 and 2005 brings up the following question: are there any corresponding signals in the

biological, geochemical, or physical process in the winter and early spring lake ecosystem that

relate to the differences in timing of ice cover during these three winters? While beyond the

scope of this study, this question need to be addressed.

The description of the ice cover for 2003, 2004, and 2005 was meant to highlight the data and 

one application of these data, i.e., a very limited descriptive analysis of the annual ice cycle for

each winter and a limited comparison with a the 30-winter climatology. These data are also

useful for developing ice cover models (statistical and conceptual), assessing the impact of

spatial and temporal variations in ice cover on the Great Lakes fauna and flora, assessing the

impact of ice cover on the physical processes within the lake and between the lake and

atmospheric boundary layer, and for marine engineering, winter navigation, and educational

applications.
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REFERENCES

Assel, R.A. 2005. Great Lakes weekly ice cover statistics. NOAA Technical Memorandum

GLERL-133. NOAA Great Lakes Environmental Research Laboratory, Ann Arbor, MI,

27 pp. ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-133/

Assel, R.A. 2003a. NOAA Atlas. An Electronic Atlas of Great Lakes Ice Cover, Winters: 1973-

2002. NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI.

http://www.glerl.noaa.gov/data/ice/atlas/

Assel, R.A. 2003b. Great Lakes ice cover, first ice, last ice, and ice duration: Winters 1973-2002.

NOAA Technical Memorandum GLERL-125. NOAA, Great Lakes Environmental

Research Laboratory, Ann Arbor, MI.

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-125/tm-125.pdf

Assel, R. A., D. C. Norton, and K. C. Cronk. 2002. A Great Lakes ice cover digital data set for

winters 1973-2000. NOAA Technical Memorandum GLERL-121. NOAA, Great Lakes

Environmental Research Laboratory, Ann Arbor, MI, 45 pp.

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-121

Assel, R.A. and D. C. Norton. 2001. Visualization of Great Lakes Ice Cycles. 2001. EOS,

Transations, American Geophysical Union, 82(7) February 13, 2001.

http://www.agu.org/eos_elec/00259e.html
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APPENDIX 1. DATES OF FIRST ICE, LAST ICE, AND ICE DURATION

The table below contains the file names for the date of first reported ice (FST), the dates of last reported ice (LST),

and for ice duration (DUR). Click on ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-135/Appendix1/ to

download these data.

COMPRESSED UNCOMPRESSED Names No. files

FSTYYGRID.ZIP FSTYYMM.DAT 9

FSTYYPLOT.ZIP FSTYYMM.GIF 9

LSTYYGRID.ZIP LSTYYMM.DAT 9

LSTYYPLOT.ZIP LSTYYMM.GIF 9

DURYYGRID.ZIP DURYYMM.DAT 9

DURYYPLOT.ZIP DURYYMM.GIF 9

Where: FST (LST) = first (last) reported ice file, DUR= ice cover duration in days. GRID = ASCII data file,

PLOT = ice chart plot file, YY = year (03, 04, 05), MM = threshold, for first (last) reported date and ice duration.

First Ice Cover Plots (ice concentration ≥: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%)

Last Ice Cover Plots (ice concentration ≥: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%)

Ice Duration Plots (ice concentration ≥: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%)

GRID File Structure

There are 510 records for each GRID file. Each record has 516 "cells" in a I3 format, where I3 = a integer.

GRID Data Codes

code = -1 (land)

code = 0 (ice cover concentration is less then the threshold value)

code = 1 to 182 (date that the ice cover concentration threshold value was first (or last)

observed or for ice duration this is the number of days between first and last ice reported ice cover. If the first

and last date are the same the duration is 1, if there was no ice cover for the given ice concentration value the

duration is 0. For example if the date code for a single grid cell on a file of "first date ice cover greater or equal

to 90%" was 63, that corresponds to a date of February 1; if the date code for a single grid cell on a file of "last

date ice cover greater or equal to 90%" was 122, that corresponds to a date of April 1. See Table 1 below for a

complete list of the first / last date codes and associated dates.
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APPENDIX 2. DAILY ICE COVER TIME SERIES

Daily Great Lakes ice cover grid time series were created for each winter season by linear interpolation between

observed ice chart grids for each winter season for 2003, 2004, and 2005. Click on the icons (in blue below) to

go to an ftp site where you can download: 1) computer animations, 2) daily grids, 3) daily images, and 4) daily lake

averages. The time series for each winter always begins Dec 1, and is 182 days, so it ends on May 31 for non-leap

years and May 30 for leap years.

1. Computer Animations- portray temporal changes in spatial patterns of ice cover over a winter season.

There are two types of animation files (.AVI and .FLC). File Names YYYYnic.FLC and YYYYnic.AVI, where

YYYY = 2003, 2004, or 2005. Click on

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-135/Appendix2/ComputerAnimations to download the

computer animation files.

2. Daily Grids- ASCII files, 510 x 516 grid cells of daily ice concentrations for each winter season. Click on

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-135/Appendix2/DailyGrids to download the daily grids.

Zip File Name: YYYYCT.zip

YYYY = year (2003, 2004, or 2005), CT = in an indicator for total ice concentration. Each .zip file has 182 Grid

files. The Grid file name encodes the date and type of data: no data, interpolated data, or observed data. Grids prior

to the date of the first and after the last ice chart each winter are coded zero.

Grid File Names: YYYdddI.CT

YYY = winter season (103 is for 2003, 104 is for 2004, and 105 is for 2005)

ddd = day of the winter season (101=Dec 1, ...132=Jan 1, ...282=May 31, 282 = May 30 for leap years such as

2004)

I = data indicator: I = 0 (no data), I = 1 (interpolated data), I = 2 (the original ice chart data, i.e., data that was

downloaded from the National Ice Center and quality controlled for GLERL’s ice codes and format).

Grid File Format: 510 records, each record has 516 "grid cells" in I2 format.

Ice Concentration Codes: -1 = land, 0 to 99 = fraction of grid cell covered by ice, expressed as a percent, 100% ice

cover was coded as 99 to save room.

Coordinates: The latitude and longitude coordinate of the center of each grid cell is given in the following two fixed

formatted text files: Latgrid.zip and Longrid.zip. There are 510 records in each file. The format of each record is:

516 (f 9.5,1x).

3. Daily Images- individual images of ice concentration spatial patterns for each day of each winter. Daily image

files of color-coded ice concentrations over the Great Lakes were produced from the Daily Grid files. Image files for

a given winter (2003, 2004, 2005) are compressed in a .zip file. Click on

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-135/Appendix2/DailyImages to download these data.

Zip File Name: YYYYGIF.ZIP, YYYY = year (2003, 2004, 2005), Each .zip file has 182 image files

Image File Names: YYYdddI.gif, YYY = winter season (103=2003, 104=2004, 105=2005)

ddd = day of the winter season (101=Dec 1, ...132=Jan 1, ...282=May 31), I = data indicator: I = 0 (no data), I = 1

(interpolated data), I = 2 (observed data), GIF. = Image file format.

4. Daily Lake Averages- spatial averages of ice cover for each Great Lake. Click on

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-135/Appendix2/DailyLakeAverages to download these data.

Plots of each lake & winter are named LakeYY.JPG where Lake is Sup, Mic, Hur, Eri, Ont, and YY is 03, 04, or

05. Plots are 7-day moving averages of the daily data given in the ASCII file Lake Avg.txt.

The text file Lake Avg.txt is a tab delimited ASCII file with 183 records. The first record is a header the remaining

records are the daily lake averaged ice cover for each Great Lake from December 1 to May 31 (May 30 for 2004).
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APPENDIX 3. DAILY ICE COVER ANOMALIES

Introduction. Ice cover anomaly grids are the difference between the weekly median ice concentration grid and

the ice concentration grid for the daily ice chart in a given year with the date (month and day) that corresponds to the

center day (fourth day) of the weekly median. Differences were calculated for each over-water grid cell. The first

weekly median ice chart is for December 1-7. The last median ice chart is for the week of May 22-28. Thus,

anomaly ice charts can be calculated for all days between December 4 and May 25. The actual number of anomaly

ice charts for a given winter depends upon the date of the first and the date of the last ice chart. It is not possible to

calculate an ice anomaly chart prior to December 4 or prior to the date of the first observed ice chart if it is later than

December 4, and after May 25, or after the date of the last observed ice chart if it is earlier than May 25. The daily

ice charts used to calculate the median are described in Appendix 2 of this report under “Daily Grids”.

Anomaly Ice Charts Grids. Anomaly ice chart grids are in compressed files. The file name encodes the

winter of the anomaly charts (AnomYY.zip) where YY is the winter (03, 04, or 05). The anomaly grid files

contained in the compressed files have names that encode the date and type of data, i.e., interpolated data, or

observed data.

Grid File Names: YYYdddI.ANO

YYY = winter season (103 for 2003, 104 for 2004, and 105 for 2005)

ddd = day of the winter season (101=Dec 1, ...132=Jan 1, ...273=May 22)

I = data indicator: I = 1 (interpolated data), I = 2 (observed data)

ANOM is an indicator for ice anomaly data.

Grid File Format:

510 records, each record has 516 "grid cells" in integer (I3) format.

Ice and Land Grid Cell Codes: Land grid cells have a value of 999. Ice cover anomalies can vary from –99

to +99. Over water cells with missing data have a code of 888.

Anomaly Ice Chart Images and Animations. Daily Image files and computer animations of the daily

image files of ice concentration anomalies over the Great Lakes for a given winter were produced from the daily

anomaly ice chart grid files. Image files for a given winter are contained in a compressed image file. Computer

animations are available in two formats: “.AVI” and “.FLC”. Image files are arranged by winter season in

compressed files.

Compressed Image File Names: YYYYanogif.zip, where YYYY is the winter (2003, 2004, or 2005), “ano” is an

indicator for ice anomaly image data, and “gif” defines the image format.

Image File Names: YYYddd.GIF, where “YYY”, “ddd”, and “gif” are as defined above.

Animation file Names: YYYYano.AVI, and YYYYano.FLC, where “YYYY” and “ano” are as defined above.

Click on ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-135/Appendix3/grids to download anomaly ice

chart grid files.

Click on ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-135/Appendix3/animations to download computer

animations for each winter. There are two animations for winters 1989, 1990, 1991, 1992, 1993, 1994, and 1995

because there were separate NIC and CIS ice chart anomaly files those winters.

Click on ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-135/Appendix3/images to download images used to

create the computer animations.