National Ice Center 7-Day Sea Ice Charts Introduction

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National Ice Center 7-Day Sea Ice Charts
The National Ice Center’s (NIC) mission is to provide global sea ice analyses and forecasts to US military and civilian government customers. Manpower and fiscal resources for the NIC are provided through a cooperative agreement between the Department of the Navy (DoN), the National Oceanic and Atmospheric Administration (NOAA) and the United States Coast Guard (USCG). Customers of NIC, federal and otherwise, use analog and digital ice guidance products in mission planning, scientific research and in support of real-time vessel operations in polar regions. NIC was formerly known as the Navy/NOAA Joint Ice Center (JIC).
In 1996, NIC agreed to undertake a data re-analysis and conversion project which would make its historical or legacy data more useful for environmental research under the auspices of the Gore-Primakov Commission Environmental Working Group (EWG). In undertaking this legacy data digitization project, NIC intended to review and correct all its historical weekly ice analyses in order to provide the most accurate data possible for the production of an ice climatology in the Arctic.
Description of NIC Sea Ice Charts:
Since 1972, NIC has produced routine weekly analyses of global sea ice conditions. Until 1996, NIC produced all of these charts using traditional hardcopy cartography techniques. NIC represents sea ice on its charts as unique polygonal areas ("polygons"), each containing somewhat homogeneous sea ice conditions. That NIC analysts determined that the sea ice entities falling within the boundaries of a polygon shared common attributes such as overall concentration or stage of development of individual ice entities. NIC has traditionally portrayed sea ice attributes associated with each unique polygon in the form of the WMO ice code label (egg code) which serves to encapsulate all of the pertinent sea ice information into a concise form for reference on a paper chart. These attributes would eventually form the basis for exporting this information into digital gridded fields or GIS data sets, in the form of tabular information relationally linked to the spatial polygon features in a vector data set or a group of grid cells in a raster data set.
The complexity of information contained in the unclassified sea ice charts varied from year to year depending on the availability of remotely sensed and in-situ oceanographic data. Ice analyses produced during the early years of this database often lacked detail and generalized actual ice extent and coverage. Early analysis shortfalls resulted from: 1) poor resolution of early hardcopy (analog) satellite imagery, 2) the absence of verifiable in-situ data and 3) the degradation of image quality due to the high frequency of obscuring meteorological (clouds) and oceanographic (suspended sediment) conditions. With the recent introduction of digital Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) fine (0.55 km resolution) imagery, NIC analysts acknowledged they often under-reported the extent of the sea ice in early years.
NIC originally produced its ice charts using all available satellite imagery, in-situ reports and meteorological/oceanographic guidance data. Appendix A summarizes the annual availability of these data. Note that these tables list data available for the analysis of ice in all Arctic regions. Analysts typically produced charts using any data up to 72 hours old. Data older than this were also analyzed but then adjusted with ancillary meteorological and oceanographic model guidance (specifically, air temperature, surface winds, ice drift and theoretical ice thickness). Therefore, the ice charts are composite charts rather than snapshots of the ice coverage on a certain day or time. Ice parameters of interest include: ice edge, ice concentration, stage of development (or thickness) and in some cases form of ice (e.g. landfast ice). Thus, depicted areas (polygons) or boundaries (lines) may be analyzed or estimated (from model output) based on changes in any of the above listed parameters.
It is important to realize that the relative accuracy and level of analysis detail varies considerably over the 23 year record of the SIGRID data set. Early Arctic ice charts (1972-1979) described ice conditions based on total concentration alone. The 1980-1991 Arctic analyses described total ice concentration as well as the predominant stages of development or ice type. Analysis done during these years was performed primarily on imagery in a hardcopy format. In 1992, improved sea ice prediction models and the increasing use of digital satellite data allowed analysts to break down the amount (or partial concentration) of each ice type within each defined area in the Arctic. The only forms of ice described throughout all analyses of the data set are landfast and belts/strips. It was standard analysis procedure (in all years) to use remotely sensed and/or in- situ data that was up to (but not exceeding) 72 hours old from the date of the analysis. Therefore, ice analyses were often derived from more than one data source and those sources may have been acquired at different times.
In most cases, areas of similar ice concentration are identified through the subjective interpretation of percent ice cover evident on visible and infrared imagery. An exception to this procedure would be the use of concentration products derived from microwave satellite sensors and direct observations by airborne or ship/shore based observers. Thus, concentration estimates are directly affected by the quality and resolution of the data. Stages of development are identified primarily by in-situ observations (including aerial reconnaissance and ship/shore reports) and theoretical ice thickness estimates based on freezing degree day accumulations. In some cases, new and young ice may be identified by the color or appearance on visible/infrared imagery. In general, the most recent ice analyses (in the data set) do the best job of characterizing the sea ice cover. Particular emphasis is given to defining the sea ice in the marginal ice zone. Areas of sea ice may be differentiated based on total concentration, ice type, form or a combination of these parameters.
The recent improvement in NIC analysis capabilities can be attributed to three factors: 1) a progressive increase in volume of incoming satellite data, 2) an improvement in the resolution of data used in each analysis and 3) the ability to process and enhance remotely sensed data in digital format. Prior to obtaining the capability to enhance digital satellite imagery on NIC workstations, hardcopy imagery was enhanced using look-up tables designed to meet the requirement for cloud detection as well as sea ice. The NIC began to process and display TIROS AVHRR and DMSP OLS Fine digital data in 1988 and 1993, respectively.
Data Sources:
The global sea ice analysis effort at the NIC requires the fusion of many data sources having widely varying scales, capabilities and resolutions. Theses operational data sources have evolved over the years and can be grouped into the following categories: satellite data, ship and shore station reports, aerial ice reconnaissance observations, drifting buoy reports, data and analyses from other national and international ice centers, and climatology. During the 1950s and 1960s, sea ice information was gathered almost exclusively from visual observations from US Navy aerial reconnaissance missions and from observations reported by ships at sea or shore stations. At that time, ice analyses were typically done only in direct support of ships operating in the ice. In 1972, newly acquired visual and infrared imagery from NOAA weather satellites were combined with aerial reconnaissance data to give the NIC the capability to produce weekly Arctic sea ice analyses year ‘round. Today, approximately 85 percent of the data used for sea ice analysis are satellite-derived products. The two largest sources of satellite data are AVHRR aboard the TIROS series of satellites and OLS aboard the US Air Force DMSP satellite series. Both provide imagery in the visible and infrared portions of the spectrum. AVHRR data has a 1.1 km resolution and is received in both the HRPT and LAC transmission modes on a daily basis. DMSP OLS Fine has a 0.55 km resolution and is received daily at the NIC. ERS-1 SAR data also provided a small and infrequent contribution to the overall composited global analyses from 1992 – 1994.
Although the NOAA AVHRR and DMSP imagery are of a resolution suitable for NIC global and regional scale products, the sensors are not all-weather since clouds limit their capability for ice detection. These data are augmented with SSM/I concentration products in cloud covered regions. Although the resolution of these data are too coarse to allow for tactical-scale ship support, SSM/I is an all-weather sensor, which is useful in identifying the ice edge and major concentration areas. When SSM/I data is used in conjunction with AVHRR and OLS, the overall ice conditions can be effectively characterized.
Aerial ice reconnaissance data was also available from the Navy component of the NIC, the Canadian Ice Service and the Danish Meteorological Institute’s ice reconnaissance programs. This data contains detailed information on ice edge, concentration of ice, stage of development and ice topography and serves to provide excellent ground truth data.
Appendix A contains tables that summarize the annual availability of each satellite platform and sensor used to produce the global sea ice analyses. It is important to note that the percent utilization of each data type varies both temporally and spatially in the weekly analysis files which make up the complete 1972 – 1994 unclassified sea ice data set.
Data Revision and Conversion:
In 1995, NIC began digitizing its entire unclassified hardcopy sea ice chart archive (1972-1994) through services provided by the National Climatic Data Center (NCDC) in Asheville, NC. NCDC digitized these charts as vector (point, line and polygon) data, then converted the resulting data to 25km cell-size ASCII gridded fields in the World Meteorological Organization’s Sea Ice in Gridded Format (WMO SIGRID). Attributes, derived from the egg code used to describe each area, were entered via a keyboard. Translation of the resulting digital (x-y coordinate) file into a grid cell format was accomplished through the use of a master grid table. Once the sea ice attributes were linked to each grid cell, the program generated the SIGRID data group records. These gridded data sets include SIGRID ice codes on total ice concentration, partial ice concentrations associated with various ice types or stages of development, and in some cases, the form of ice. These data were further converted into ARC/INFO polar stereographic grids and tables of ice code values with SIGRID ice codes. Contiguous groups of grid cells which had all ice attributes in common in the final ARC/INFO format grids were assigned unique values based on the total number of ice polygons in the original sea ice chart. This unique value was then linked to the actual sea ice attribute information within ARC/INFO, creating gridded fields which were relationally linked to tabular attribute information. These latter ARC/INFO grids were provided to ERIM International for conversion into EASE formatted and SIGRID formatted data as described in the section on EASE ice chart methods.
Data Coverage:
One of the significant shortfalls in the data set is the arbitrary exclusion of ice south of 45 N in the Arctic. In some years, sea ice may extend south of 45N in the waters south of Newfoundland, Sea of Okhotsk and the Sea of Japan. Weekly maps of sea ice in the Yellow Sea are currently being digitized at the NIC and are not included in the SIGRID database. Without re-digitizing the original hardcopy charts, data south of 45N can no longer be reconstructed due to the inadvertent disposal of the x-y coordinate files. Another potential weakness in the SIGRID data is that there is no metadata describing the source of information for each weekly analysis. Analyses may be based on DMSP OLS fine data, SSMI passive microwave or in some cases an estimate of ice drift.
In January 1997, National Ice Center became aware that many SIGRID files contained errors in ice code attribute information and in some cases, depiction of ice region boundaries. Types of errors which quality controllers corrected included: (1) incorrectly labeled sea ice polygons (regions) and (2) inconsistencies in the egg code attributes assigned to the sea ice areas along the seams (90W and 90E meridians) of the weekly global Arctic ice analyses. The latter inconsistency was found because both the Arctic East and Arctic West data files often did not contain the identical information along common boundaries. In some cases, the joining of the data files revealed minor differences in the described ice attributes.
During the original 1995 effort to correct the SIGRID data set, quality controllers discovered five weekly SIGRID files could not be recovered due to excessive landmask inconsistencies and other data problems. These corrupted fields are not included in the final SIGRID data set. The omitted gridded fields include (g197705qcf; g198505qcf; g198603qcf; g199113qcf, and g199140qcf).
The following NIC staff are acknowledged for their contributions to sea ice database: LCDR Don Taube, USN; LT Sean Memmen, USN; LT Kyle Dedrick, USN; LT Keith Rohwer, USN; Mr. Eric Tran; Ms. Ellen Haas; ET2(SW) Mark Segall, USN; AG3 Jennifer Werner, USN; AG2 Donald Werner, USN; MST2 Richard Hernandez, USCG; AG1 Robert Dorsett, USN; AG1 Joseph Chastain, USN; AG1 Paul McKenna, USN; Mr. Paul Seymour; Mr. David Benner; Ms. Selina Nauman; Mr. Frank Kniskern; Mr. Don Barnett; Mr. John Breckenridge; Mr. Martin Doody; Mr. Ralph Perniciaro. A very special thank you to all of the current and former sea ice analyst, production team members and Information Technology personnel of the National/Naval Ice Center, Washington DC, since 1972.

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