In this Issue



Issue 7, Fall 1993


The Fifth meeting of the Polar DAACs User Working Group (PoDAG) was held at the University of Alaska's SAR Facility on 1-2 November. Despite cold temperatures outside the discussion was "hot" amongst the PoDAG members and DAAC staff. The following report summarizes the high points, as viewed from an NSIDC perspective.

In general the PoDAG has become more interested in the priorities assigned to individual data sets and products by each DAAC.

Provision of Data and Data Sets

Dr. Robert Thomas set the stage for the two day meeting by stating his view of NASA's research funding and the roles of data centers and the science community in near-term snow and ice research. He said in part that snow and ice data systems must produce products which are more widely applicable to environmental research. The numbers of scientists who will directly analyze remotely sensed data will remain constant or slowly decline, but those who can use derived parameters and products will grow. The snow and ice remote sensing community (both scientists and data centers) must concentrate on providing energy flux estimates, ice motion and/or concentration fields, surface albedos, etc. - parameters which can be plugged into models or process studies.

The message to the data centers is clear. We must balance the provision of the basic observations (e.g., SSM/I, AVHRR, ERS-1 SAR imagery) with development of higher level products. This will have to be accomplished in collaboration with the snow and ice and other research communities. PoDAG's role will be in helping the DAACs prioritize their data management and product generation efforts.

Passive Microwave Data Intercomparisons

Results of several comparisons between SSM/I sensors and gridding methods were presented. V. Troisi and K. Steffen showed the results of the SSM/I F-8 - F-11 comparison of gridded data. In general, the two data sets had small but significant differences in gridded, averaged TBs. NSIDC is unsure why this has occurred and is investigating its processing software to be certain that any differences are due to sensor differences and not processing software.

R. Weaver presented an intercomparison of gridded TBs derived from the SSM/I polar stereographic format and the Equal Area SSM/I Grid (EASE) format. The polar stereographic grid is that used with the current NSIDC CD-ROMs; the EASE-Grid will be used by the SSM/I Land and Snow Processes Pathfinder products. The results of this study showed that the gridded TB values are essentially the same, except where rapid ice movement was not averaged out due to the EASE-Grid temporal sampling. Sea ice concentrations calculated from each data stream were essentially identical, again excepting areas where rapid temporal changes occurred.

Recommendations Common Between ASF and NSIDC

ASF and NSIDC have been requested to provide one page descriptions of their respective V0 data holdings. The purpose of this exercise is to allow PoDAG to understand the nature of each data set and to start the process of prioritization of these data.

PoDAG will review the user statistics provided by the DAACs to ESDIS to see if these data meet PoDAG's need to understand the user communities that each DAAC serves.

PoDAG requested both DAACs to provide staff structural diagrams. These diagrams will be utilized in an effort to understand how each DAAC has been staffed to meet user and data set distribution demands.

Recommendations Specific to NSIDC

NSIDC will make an intercomparison of gridded passive microwave brightness temperatures as derived from the following sources: The National Geophysical Data Center DMSP Digital Archive (proposed source) and Remote Sensing Systems Inc. (the current source). This study will be for a duration of at least 3 months, but more likely, one year.

NSIDC will make an intercomparison of gridded sea ice concentration products derived from the current SSM/I polar stereographic gridded data set and the Equal Area SSM/I Earth Grid (EASE). This study will be for a duration of at least 3 months, but may extend over most of the SSM/I Pathfinder time period of 18 months.

NSIDC is preparing a 2-3 month duration AVHRR 1-km data set for use by PoDAG members as well as other scientists. This data set will be utilized to determine the optimum temporal collection frequency for 1-km data from the various receiving sites.

A catalog of archived scenes will be sent to several PoDAG members by January 1994. It will be distributed more widely later in the spring.


Data from the DMSP Digital Data Archive at the NOAA/National Geophysical Data Center (NGDC) will be available soon. Operational Linescan System (OLS visible and infrared images) will be available in hard-copy and digital form from NGDC and NSIDC beginning about February 1994.

The archive of DMSP data is a cooperative effort between NOAA, NSIDC, DOD and NASA. Data will be archived at NGDC and distributed to users by NGDC and NSIDC.

Data from the Air Force Global Weather Central (AFGWC) has been copied to 8 mm Exabyte tapes on an experimental basis since March 1992. The recording systems at AFGWC will be replaced in late January 1994, which will result in a continuous flow of data to Boulder. This will include all DMSP sensors and OLS data in both 'smooth' (2.7 km resolution) and 'fine' (0.55 km resolution) modes.

The software to process the 'smooth' mode OLS data is now operational. Some test data products have been generated. Software to create an inventory and browse data is being developed. NSIDC has been designing and writing much of the user services software including an X-window based graphical user interface which includes a data base search module, orbit mapping tool, and image display and enhancement applications. Prototype software modules for several parts of this system are now being tested, and are available via anonymous ftp at

For more information, contact Greg Scharfen at Internet: or (303) 492-6197.


Application of Special Sensor Microwave Imager (SSM/I) Data for Snow Cover


NSIDC, in collaboration with the SSM/I Products Working Team (SPWT), has been developing a structure for earth-gridded SSM/I data which provides global coverage. The prototype resulting from this project, the Equal Area SSM/I Earth Grid (EASE- Grid Version 0.0), was distributed for evaluation to a selected group of researchers during 1993. Based on information gained from these evaluations, NSIDC has now completed Version 1.0 of EASE-Grid. Guidance on the format of Version 1.0 was also received from the Polar DAAC User Working Group (PoDAG) and the NASA/NOAA Pathfinder SSM/I Science Working Group which also has adopted EASE-Grid as the format for the SSM/I Pathfinder Level 3 Products (earth gridded).

Version 1.0 will be distributed on CD-ROM during January, 1994. The CD will contain data for the full Northern Hemisphere for the period January 13 to February 29, 1988, as well as a limited sample of data in the full global Mercator projection and the Southern Hemisphere projection.

The purpose of the EASE-Grid is to provide a data structure/format (binning, gridding and projection) to the general user which is easier to use than swath format, yet maximizes the radiometric integrity of the original TB values, maintains high spatial and temporal precision, and involves no averaging of original swath data. Version 1.0 is designed to provide data in two earth projections:

  1. Cylindrical (modified-Mercator) Equal Area Projection - Full Global Coverage, (useful for mid- to low-latitudes studies); Aspect Ratio 1.0 at 30 x and 0.75 at 0 x.
  2. Azimuthal Equal Area Projection - Full Northern and Southern Hemisphere in separate projections (useful for high-latitude and polar studies); Aspect Ratio 1.0 at 90 x and 1.17 at 45 x.

The cell size is 25.0 km for all channels and 12.5 km for 85 GHz. The binning method involves an interpolation which, (a) artificially increases swath sampling density (16 times) using coefficients derived from the actual antenna pattern, and (b) assigns a TB to an earth grid cell by applying the value of nearest neighbor within the over-sampled array. There are two files (arrays) for each day (ascending and descending orbits) for each projection and for each frequency and polarization. At higher latitudes (>55 x) where more than two passes per day are possible, the pass that is closest in local time to the equator crossing time is selected. Data are in HDF and data compression is applied.

The following precision is used: Brightness Temperature, 0.1K; Time, 0.1 hour; Lat./Lon. 0.01x. No special software tools are required to perform coordinate or grid transformations. Array Files are both I,J and Lat./Lon. thus compatible with all image processing software and associated tools using array format input.

For more information, contact: Richard Armstrong at Internet: or (303) 492-1828.

Reduction of Weather Effects in the Calculation of Sea Ice Concentration with the DMSP SSM/I

A serious problem in mapping the polar sea ice covers in both hemispheres has been the false indication of sea ice over the open ocean and at the ice edge. These spurious sea ice concentrations result from the presence of atmospheric water vapor, nonprecipitating cloud liquid water, rain, and sea surface roughening by surface winds. While these effects are relatively minor at polar latitudes in winter, they result in serious weather contamination problems at all latitudes in summer (Cavalieri et al., 1992).

This problem was addressed for sea ice concentrations derived from the Nimbus 7 SMMR data through the development of a weather filter (Gloersen and Cavalieri, 1986). The filter is based on the polarization (PR18) and spectral gradient ratio (GR37/18) distribution of ice-free and ice-covered seas. If GR(37/18) is greater than 0.08, then the sea ice concentration is set to zero. While this eliminates most of the unwanted weather effects, it also eliminates sea ice concentrations less than 15%. This is not considered a serious problem for most applications, because the ice-edge has previously been defined as the 15% ice concentration contour as determined from satellite radiometers.

Application of a GR(37/19) filter for use with sea ice concentration maps derived with the SSM/I sensor is less successful, because the 19.35 GHz SSM/I channels are significantly closer to the center of the 22.2 GHz atmospheric water-vapor line and thus are more sensitive to changes in atmospheric water vapor resulting in greater contamination problems.

A new weather filter has been developed (Cavalieri and St. Germain, 1994) and delivered to the National Snow and Ice Data Center in Boulder, Colorado, for routine processing of the SSM/I data for generating sea ice concentration maps. The new filter is a combination of the original SSM/I GR(37/19), which effectively eliminates most of the spurious concentration resulting from wind-roughening of the ocean surface and from cloud liquid water, with another GR filter based on the 22.2 GHz and 19.35 GHz channels. The rationale for using GR(22/19) is based partly on the sensitivity of the 22.2 GHz to water vapor and partly on the need to minimize the effect of ice temperature variations at the ice edge.

This new weather filter works as follows. If GR(37/19) is greater than 0.05 and/or GR(22/19) is greater than 0.045, the sea ice concentration is set to zero. These GR thresholds effectively eliminate most of the weather contamination, except for winds greater than about 30 m/s, cloud liquid water more than 24 cm, water vapor greater than 0.2 cm, and rain rates greater than 12 mm/hr. Except for a few case studies completed during the development of this filter, the extent to which it eliminates ice-edge concentrations in different regions of the Arctic and Antarctic for different seasons is unknown. Work is currently underway to determine the overall effectiveness of the new SSM/I weather filter.

Cavalieri, D.J., et al. (1992) NASA Sea Ice Validation Program for the DMSP SSM/I: Final Report. NASA Technical Memorandum 104559, National Aeronautics and Space Administration, Washington, DC, 126p.

Cavalieri, D.J.; St. Germain, K.M. (1994) Reduction of weather effects in the calculation of sea ice concentration with the DMSP SSM/I, in preparation.

Gloersen, P.; Cavalieri, D.J. (1986) Reduction of weather effects in the calculation of sea ice concentration from microwave radiances. J. Geophys. Res., 91: p.3,913-3,919.

For further information contact: Don Cavalieri, NASA/Goddard, (301) 282-2444 or D. Cavalieri/OMNET.


1-km Polar AVHRR Archive

Since August of 1993, NSIDC has been collecting and archiving a bi-polar data set of 1-km HRPT and LAC AVHRR imagery. The archived data is received by HRPT stations at McMurdo, Palmer, Fairbanks, Prince Albert, and Tromsø, and includes LAC images of both polar regions as well. NSIDC gets most of the data as a polar subset of the Global Land 1-km AVHRR Data Set, currently collected by the Eros Data Center (EDC). Antarctic HRPT data is received from the Scripps Institution. Combined, these two data sets provide nearly complete coverage of both poles' sea ice, land ice, and polar lands at nearly 1-km resolution (Figure 1), for all 5 bands of the AVHRR sensor (visible, near-IR, and thermal-IR).

Figure 1. Coverage map for Polar 1-km AVHRR data set

Figure 1. Coverage map for the Polar 1-km AVHRR data set. Not shown in LAC coverage, which is somewhat variable; in general, the daily LAC imagery in the data set covers the Siberian arctic coast and the Weddell Sea.

Although the image data is not yet available for general distribution, a catalog of archived scenes, with information on location, time of acquisition, image size, and data quality, may now be distributed to any interested party upon request. This will allow potential users of the data set to see what areas are covered, and how frequently they are covered. A browse data set for each image is currently being installed on the NSIDC optical jukebox; this will be made accessible over the Internet in the near future. A test data set, consisting of one week's worth of archived 1- km AVHRR data for the northern hemisphere, is being assembled and will be available on 4 mm DAT or 8 mm Exabyte tapes. The specific data format has not yet been determined, but the data will be uncalibrated and not geolocated. This data set is intended to allow the Polar DAAC User Working Group, and any other interested parties, to have an early look at the archive and evaluate the usefulness of the collected data for various applications. Parties interested in receiving a copy of the catalog or the test data set, or interested in access to the browse data over the network, may contact Chris McNeave, User Services, NSIDC, Campus Box 449, University of Colorado, Boulder, CO 80309-0449, or NSIDC hopes to make the general archive of 1-km AVHRR data available in the first half of 1994.

For more information contact: Ted Scambos at or (303) 286-1113.


ARCSS Grant Opportunities

The National Science Foundation's Office of Polar Programs has continuing research opportunities within the Arctic System Science Program (ARCSS). Investigations that have as their objective the integration of results from previous or current ARCSS projects are particularly invited, as are projects that would link the main components of ARCSS. Arctic System Science: A Plan for Integration, the report of the ARCSS Executive Committee to the NSF (Arctic Research Consortium of the United States, September 1993) presents the framework of ARCSS in terms of key scientific questions, and results to date from each of the ARCSS component programs. Copies of this document are available from NSF/OPP/ARCSS on request.

Consideration for FY94 funding requires that proposals be received at NSF by 30 May 1994. Contact Dr. Patrick J. Webber, ARCSS Program Director at NSF, for further information or to obtain a copy of the ARCSS document.

Contact: National Science Foundation, Office of Polar Programs, Arctic System Science Program, 4201 Wilson Boulevard, Arlington VA 22230, Internet:, or (703) 306-1030.

OAII Science Steering Committee Meeting

Claire Hanson attended a meeting of the Arctic System Science (ARCSS) Ocean/Atmosphere/Ice Interactions (OAII) Science Steering Committee in San Francisco on 11 December. Discussions included ongoing and planned field programs in the Arctic Ocean, status of the Arctic research vessel, possible closer collaboration with the DOE Atmospheric Radiation Measurement Program, and introduction of a new initiative on Arctic Shelf-Basin Biology, to be the subject of an open discussion at the February 1994 AGU/Ocean Sciences meeting in San Diego. Hanson updated the Committee on ARCSS data management activities at NSIDC, including forthcoming Alaska land data, Northeast Water Polynya (ARCSS/OAII /NEW) 1992 and 1993 field data, and Arctic solar radiation data, all scheduled to be released to the ARCSS community on CD-ROM or other appropriate media during 1994. The ARCSS/ OAII Newsletter no. 3 (early 1994), published by the OAII Science Management Office, University of Washington/Polar Science Center, will contain additional details concerning ARCSS/OAII science issues and activities.


Claire Hanson and Roger Barry attended the Arctic Environmental Data Directory (AEDD) Working Group meeting jointly with the International AEDD (IAEDD) Steering Committee in San Francisco, 9-10 December. The purpose of the meeting was to continue development of an action plan for 1994 for the IAEDD "directory of Arctic data directories," an initiative put forward by USGS and UNEP/GRID-Arendal at a September workshop in Arendal, Norway. A main issue of discussion is standardization of directory information, with emphasis on existing DIF-based and DIF-compatible efforts. The Steering Committee will meet again in March 1994 (probably in Tromsø) to develop a plan for a workshop in Russia in fall 1994 at which the Russian environmental institutes (an estimated 800 institutes) will be invited to active participation in development of the directory. The main focus of the directory of directories will be environmental data. Iceland, Sweden and Denmark will be invited to join the Steering Committee, as they have not so far been represented. The Ministry of Environment Protection of Russia is actively involved in the group.


Snow Ecology: A Report on a New Initiative

Snow plays a key role in the ecology of much of Earth's surface, especially in circumpolar and high-altitude regions where ecosystems are under increasing stress from global changes in climate and local human development. Until recently, our knowledge of snow has been restricted to areas of study associated with specific physical, chemical and biological disciplines. Although this research has resulted in a significant progress in understanding snowpack dynamics, there has been no concerted attempt to integrate the results of these studies to further our knowledge of snow as a life-support milieu and as a component of larger terrestrial ecosystems. In order to develop a better understanding of snow ecology through interdisciplinary studies, a nucleus of research workers from both the physical and biological sciences recently formed the Snow Ecology Working Group (SEWG). The SEWG was approved as a contribution of the International Commission on Snow and Ice (ICSI) to the International Geosphere-Biosphere Program (IGBP) at the ICSI Bureau Meeting in Vienna in August 1991. ICSI is a commission of the International Association of Hydrological Sciences (IAHS).

The objectives of the Working Group are to:

  1. facilitate exchange of knowledge and expertise on snow and snow-covered systems between researchers in the physical, chemical and biological sciences;
  2. develop a conceptual framework for snow ecology as a science and an experimental methodology for the study of snow and snow-covered systems;
  3. develop conceptual and applied comprehensive models for the processes, states, evolution and stability of snow ecosystems;
  4. produce documents which outline the conceptual framework of snow ecology, the state of the science and appropriate experimental methods;
  5. organize an international conference with emphasis on the methodology and application of the conceptual framework to the development of models which explain the evolution of snow and snow-related ecosystems.

To achieve these objectives, a Snow Ecology Workshop was convened by SEWG in Québec City, June 3-7, 1993. The Workshop was sponsored by the Natural Sciences and Engineering Research Council of Canada, Hydro-Québec, the Canadian Polar Commission, the Institut National de la Recherche Scientifique (Université du Québec) and the Department of the Environment of the Government of Québec (Environment Québec). The Workshop was attended by university and government researchers and graduate students; the 20 participants included climatologists, physicists, chemists, microbiologists, plant ecologists, and invertebrate and large mammal ecologists.

Copies of a brief report on the Workshop are available now. The complete Proceedings will be published in book form in 1994. For a copy of the report or further information, please contact: Professor H. Gerald Jones, Institut National de la Recherche Scientifique (Université du Québec), Ste.-Foy, Quëbec, Canada, G1V 4C7, or fax (418) 654-2562.


An Assimilated Dataset for Earth Science Applications

A recent paper in the Bulletin of the American Meteorological Society (Schubert, S.D. et al., vol. 74, no. 12, December 1993, pp. 2331-2341) describes a "...multiyear gridded global atmospheric dataset for use in climate research, including tropospheric chemistry applications." The assimilated product "...incorporates rawinsonde reports, satellite retrievals of geopotential thickness, cloud-motion winds, and aircraft, ship, and rocketsonde reports." Parameters are given at 6-hourly intervals " the full resolution of the assimilating general circulation model." Some surface parameters are given at 3-hourly intervals.

This data set will be available from the Goddard DAAC.

For further information, see the BAMS paper or contact Head, Data Assimilation Office, Code 910.3, NASA/GSFC, Greenbelt MD 20771; telephone: (301) 286-8203; or on Internet:


The Third Circumpolar Symposium on Remote Sensing of Arctic Environments, 16-20 May 1994, Fairbanks, Alaska. Contact: Dr. Martin O. Jeffries, Program Chair, at (907) 474- 5257 or Internet: Deadline: 1 February for abstracts.

International Symposium on the Role of the Cryosphere in Global Change, 7-12 August 1994, Byrd Polar Research Center, the Ohio State University, Columbus, Ohio. Contact: Secretary General, International Glaciological Society, Lensfield Road, Cambridge CB2 1ER, U.K.

International Conference on the Arctic and North Pacific: "Bridges of Science between North America and the Russian Far East - Past, Present, and Future," 25-27 August, Anchorage, Alaska, 29 August to September, Vladivostok. Contact: Dr. Gunter Weller, Internet: or fax: (907) 474-7290.