National Snow and Ice Data Centre
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The AVHRR Polar Pathfinder 1.25 km data set provides information about polar climate and geophysical processes at the highest practical spatial and temporal resolution. For the first time, daily composites of AVHRR-collected cryospheric variables are compiled at a resolution suitable for regional and local monitoring, and process studies, of changing surface conditions such as sea ice formation, sea ice motion and ice sheet extent. The higher resolution provides the closest link between field data and the greater perspective afforded by a satellite view. Products include daily sea-ice motion vector fields and twice-daily composited images of albedo, skin surface temperature, and the calibrated and geolocated AVHRR channel data used to generate them. Products are mapped to the NSIDC Equal-Area Scalable Earth-Grid (EASE-Grid, Armstrong and Brodzik 1995). Spatial coverage includes both hemispheres, extending poleward of 50 degrees N and 53 degrees S latitudes. Temporal coverage begins September 1993 and extends through the present (1998). Plans are to continue acquisition and processing for 18 months after the Moderate Resolution Imaging Spectrometer (MODIS) becomes operational. For this AVHRR Pathfinder demonstration, three regions have been selected from 1997 Northern Hemisphere composites to correspond to areas of active field programs and ongoing research. These areas, the Surface Heat Budget of the Arctic Ocean (SHEBA) study region in the Beaufort Sea, the ETH/CU Camp in western Greenland, and Point Barrow, Alaska, cover the main polar surface types: sea ice cover, permanent ice sheets, and polar lands. Time-series visualizations are provided rather than a sample of the data product, to conserve space on the CD-ROM.
The AVHRR Polar Pathfinder 5 km data set provides a time-series overview of each pole. It contains a high level of detail, yet still shows the entirety of each polar region. For the first time, daily composites of AVHRR-collected variables are compiled at a resolution suitable for process studies on a hemispheric scale. Temporal coverage begins in July 1981 and continues through the present, thereby extending the temporal coverage of upcoming MODIS and and other AM-1 products. Calibrated channel data, surface broadband albedo, surface (skin) temperature, and ancillary data, including solar zenith angles, relative azimuth angles, satellite elevation angles, time of pixel, cloud mask, orbit masks, and surface-type masks, are provided. These ancillary data allow users to apply algorithms of their choice to calculate albedo and surface temperature. Additionally, albedo and surface (skin) temperature have been compiled along with the other data sets noted above into composites at two selected local times for each day. These products are mapped to the EASE-Grid. Also included are ice motion products which include displacement vectors that provide an historical record of ice motion prior to the availability of ERS-1, ERS-2, and RADARSAT-derived ice products, and offer greater spatial coverage than is currently available from buoys or radar data for the Arctic and Antarctic. The 5 km resolution AVHRR sample data on this CD-ROM covers both poles and comprises the same products as does the full data set, with the exception of ice motion. The CD-ROM contains data for September 8, 1997. Additionally, five movies for various periods in 1988 and 1989 illustrate some of the characteristics of the 5 km data when viewed as time series. One movie illustrates a novel "background" clear-sky skin temperature application. Five km AVHRR Polar Pathfinder products were averaged into 25 km grid cells, providing browse products for the Northern and Southern Hemispheres. The 25 km browse products also provide means of analyzing conditions and trends at a more manageable data volume. Please note that only portions of the complete time series are available for distribution at the time of release of this CD-ROM.
Derived from the global TIROS Operational Vertical Sounder (TOVS) data set, the TOVS Polar Pathfinder provides users with gridded daily and monthly arctic atmospheric soundings. Since 1979, TOVS has been a vital tool for weather forecasting, supplying users with information on the vertical distribution of temperature and moisture in the global atmosphere. TOVS has been particularly useful in providing atmospheric soundings over regions where in situ measurements are sparse. TOVS Polar Pathfinder data were obtained using a modified version of the Improved Initialization Inversion Algorithm to identify geophysical parameters in snow- and ice-covered areas. The data set has been designed to address the particular needs of the polar research community, and includes quantities used to compute surface turbulent fluxes and drive ocean models. Variables retrieved from satellite observed radiances for this product include atmospheric temperature profiles, water vapor, surface (skin) temperature, total effective cloud fraction, cloud top pressure, cloud top temperature, turning angle between geostrophic wind and surface stress over ice, emissivity, boundary layer stratification and geostrophic drag coefficient. TOVS data coverage spans July 7, 1979 through December 31, 1996 at a resolution of 100 km. Sample products for four selected dates in 1988 are included on this CD-ROM.
The SMMR and SSM/I Pathfinder brightness temperatures provide scientists with more than 20 years of consistently processed, passive microwave data for use in climatological and earth science research. Each file represents gridded data for a single sensor channel and polarization, derived from either ascending or descending orbits for one day, thereby allowing discrimination of diurnal changes in earth surface processes. Each data sets consists of three series: the Northern and Southern Hemispheres and a global projection. Grids represent spatially interpolated data. The interpolation techniques maximize the radiometric integrity of original brightness temperature values, maintain high spatial and temporal precision, and involve no averaging of original swath data. SMMR coverage began October 1978 and continued through July 1987. Grid resolution is 25 km for all channels. SSM/I coverage began August 1987 and continues through the present. Processing is ongoing with current data available within three to six months of sensor data acquisition. Resolution is 25 km for all channels and 12.5 km for the 85 GHz channels. A sample data set comprising selected SSM/I EASE-Grid brightness temperature data and time files for all three projections from September 8 (day 251), 1997 is included on this CD-ROM.
The P-Cube data set is a 100 km resolution subset of many important variables from three Polar Pathfinder Projects in common projection (Lambert azimuthal equal-area) and common grid (EASE-Grid). For the first time, data sets from multiple sensors measuring the Arctic region are being distributed in a manner that facilitates study of polar processes and interactions between them. A "data cube" of the Polar Pathfinder Products, hence the name "P-Cube," allows users to access variables derived from three satellite sensors and use them together. The common spatial resolution for P-Cube is dictated by the lowest resolution product (TOVS data). The P-Cube allows browsing of the multi-sensor data and exploring the relationships among the variables. For AVHRR and SSM/I data, the user may turn to the full resolution, single instrument Polar Pathfinder data sets for greater details. A wide range of polar climate research applications is possible using this multi-dimensional suite of variables from AVHRR, TOVS and passive microwave instruments. The prototype version of the P-Cube included on this CD-ROM has a temporal coverage of January 1, 1988 to December 31, 1989, and a spatial coverage poleward of 60 degrees N latitude. Future versions of the P-Cube will have expanded temporal and spatial coverage, as well as additional variables. The current prototype version of the P-Cube includes the following: atmospheric temperature (10 levels), precipitable water (5 layers), boundary layer stratification parameter, geostrophic drag coefficient, turning angle, and microwave surface emissivity (from TOVS); surface albedo, surface (skin) temperature, solar zenith angle, cloud fraction, standard deviation of surface albedo, and standard deviation of surface temperature (from AVHRR); and total ice concentration, surface type and ice velocity (from AVHRR and SSM/I). From the National Centers for Environmental Prediction (NCEP) surface pressure is provided. Please note that the P-Cube data set is a prototype for a potential long term data set consisting of integrated Polar Pathfinder Products. At the time of the production of the Polar Pathfinder Sampler CD-ROM, the algorithms for a number of variables are still undergoing development and validation.
To address the needs of the cryospheric science community, the SMMR-SSM/I, AVHRR and TOVS research teams collaborated to make it as easy as possible for the Polar Pathfinders and related data sets to be used together. The teams employed a common projection, the NSIDC Equal-Area Scalable Earth-Grid (EASE-Grid), file naming conventions and validation methods to develop consistently processed data sets that are easy to combine and contrast. The Polar Pathfinder Sampler CD-ROM is an innovative assemblage of atmospheric and surface measurements from all three Pathfinders. Included is a two-year period of daily merged Pathfinder data sets at 100 km-resolution, placed in a multidimensional structure known as the "P-Cube." In addition, samples of full-resolution Special Sensor Microwave Imager(SSM/I), TIROS Operational Vertical Sounder (TOVS) and Advanced Very High Resolution Radiometer (AVHRR) (both the 1.25 km and 5 km products) Polar Pathfinder data sets are included, also in Equal-Area Scalable Earth-Grid (EASE-Grid), making it easy to compare parameters at multiple resolutions. The new product provides data for a wide range of polar climate research applications, but is especially keyed to the needs of investigators dealing with large-scale atmospheric changes, surface heat and mass balance studies, and sea ice modelling. This dataset is public