Description and analysis of a 20-year (1960-79) digital ice- concentration database for the Great Lakes of North America

A digital ice-concentration database spanning 20 years (1960 to 1979) was established for the Great Lakes of North America. Data on ice concentration, i.e. the percentage of a unit surface area of the lake that is ice-covered, were abstracted from over 2 800 historic ice charts produced by United States and Canadian government agencies. The database consists of ice concentrations ranging from zero to 100% in 10% increments for individual grid cells of size 5 x 5 km constituting the surface area of each Great Lake. The data set for each of the Great Lakes was div i ded into half-month periods for statistical analysis. Ma ximum, minimum, median, mode, and average ice-concentrations statistics were calculated for each grid cell and half-month period. A lakewide average value was then calculated for each of the half-month i ce-concentration statistics for all grid cells for a given lake. Ice-cover variability and the normal extent and progression of the ice cover is discussed within the context of the lakewide averaged value of the minimum and maximum ice concentrations and the lakewide averaged value of the median ice concentrations, respectively. Differences in ice-cover variability among the five Great Lakes are related to mean lake depth and accumulated freezing degree-days. A Great Lakes ice atlas presenting a series of ice charts which depict the maximum, mi nimum, and median icecover concentrations for each of the Great Lakes for nine half-monthly periods, , starting the last half of December and continuing through the last half of April will be published in 1933 by the National Oceanic and Atmospheric Administration (NOAA). The database will be archived at the National Snow and Ice Data Center of the National Environmental Satellite Data and Information Service (NESOIS) in Boulder, Colorado, USA, also in 19B3. INTRODUCTION The Laurentian Great Lakes, shown in Figure I, are one of the major water resources of North America. Systematic observations of the ice cover of the Great Lakes have been made for over 20 years by United States and Canadian government agencies in support of activities in the fields of research, operations, Fig.l. Map showing the relative positions of the Great Lakes and Lake St Clair (C). and planning. The ice cover is important because it has a major impact on the economy of the Great Lakes region through its effects on navigation, hydropower generation, lake levels, flooding, and damage to shore structures. Reports describing the annual ice cycle for individual winters have been published during the past 20 years and many of them are included in the selected bibliography for this paper. An initial analysis of the ice-cover data was made by Rondy (1969). He produced an ice atlas of normal ice cover on the Great Lakes for early winter. midwinter, and for the periods of maximum ice extent and early ice decay based on a subjective analysis of data from six winters. The Great Lakes-St Lawrence Seaway Winter Navigation Demonstration Program, authorized by the United States Congress in 1970 and completed in 1979, provided the major impetus to revise Rondy's ice atlas because of the need for greater detail in ice-cover data to support increasing operational and research activities. This paper describes a database containing * Great Lakes Environmental Research Laboratory (GLERL) Contribution No.314.