Polar Ice Team :: 2005-2006
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Estimating Firn Emissivity, from 1994 to1998, at the Ski Hi Automatic Weather Station on the West Antarctic Ice Sheet Using Passive Microwave Data

Firn is the compacted snow layer that has remained at or near the surface of an ice sheet for longer than one season but has not yet compressed into glacial ice. Knowledge of firn surface temperature trends across the Antarctic Ice Sheet is useful for documenting and quantifying change and for providing a temporal and spatial context for Antarctic research performed during the upcoming International Polar Year (IPY). The spatial and temporal variability of firn emissivity and the factors that control it are not currently well known although satellite passive microwave radiometer data has been proven to be useful to obtain reasonable surface temperature trend estimates across limited temporal and spatial gaps in AWS coverage. Over the last decade, techniques using passive microwave data have been pioneered by a number of investigators; including Jezek et al., (1993) and. Shuman et al., (1995).

In collaboration with Dr. Christopher Shuman, at NASA Goddard Space Flight Center’s Cryospheric Sciences Branch, the 2005-2006 Polar Science Research Team compared archived surface temperature data from an Automatic Weather Station (AWS) on the West Antarctic Ice Sheet with brightness temperature data collected by the Special Sensor Microwave Imager (SSM/I) aboard the Defense Meteorology Satellite Program (DMSP) polar orbiting meteorology satellite series. The ratio of passive microwave brightness temperature and AWS in-situ near surface temperature provides the firn emissivity estimate necessary to extrapolate surface temperature trends across temporal and spatial gaps in AWS coverage. This relationship is generally known as the ‘Rayleigh-Jeans Approximation’ (Hall and Martinec 1985)

As ‘ground truth’ data for our study, AWS temperatures at 3 hourly intervals for the “Ski Hi” AWS site (75º South Latitude, 71 º West Longitude) in West Antarctica were obtained via internet file transfer from the AWS Project data archive at the Space Science and Engineering Center (SSEC) at the University of Wisconsin in Madison. The passive microwave time series of daily DMSP SSM/I brightness temperatures, geographically and temporally overlapping the Ski Hi site were obtained via Internet ftp file transfer from the National Snow and Ice Data Center (NSIDC) at the University of Colorado in Boulder. These 25x25 km remote sensing data were tabulated in a Microsoft EXCEL spread sheet to derive daily average surface temperatures at Ski Hi AWS location. The daily ratio of the SSM/I brightness temperature to the AWS surface temperature provided an emissivity trend from which to extrapolate surface temperatures The Ski Hi AWS operated from late February 1994 until late November 1998. The team developed mathematical/statistical techniques to robustly estimate the surface emissivity trend over this time period, and use it to obtain estimates of surface temperature during data gaps in the AWS archive longer than one day. This work was the first step to deriving a surface temperature trend across the Antarctic ice sheet from 1987 through the present. Additional efforts may include assessing the previous passive microwave sensor (SMMR) that operated from 1978 to 1987 and also temperature retrievals from IR sensors such as AVHRR and MODIS.