MEASUREMENTS OF THE DOWNWARD LONGWAVE RADIATION SPECTRUM OVER THE ANTARCTIC PLATEAU AND COMPARISONS WITH A LINE-BY-LINE RADIATIVE-TRANSFER MODEL FOR CLEAR SKIES

A I-year field program was conducted at South Pole Station in 1992 to measure the downward infrared radiance spectrum at a resolution of 1 c m(-1) over the spectral range 550-1667 cm(-1). The atmosphere over the Antarctic Plateau is the coldest and dries? on Earth, where in winter , surface temperatures average about -60 degrees C, the total column w ater vapor is as low as 300 mu m of precipitable water, and the clear- sky downward longwave flux is usually less than 80 W m(-2). Three clea r-sky test cases are selected, one each for summer, winter, and spring , for which high-quality radiance data are available as well as ancill ary data to construct model atmospheres from radiosondes, ozonesondes, and other measurements. The model atmospheres are used in conjunction with the line-by-line radiative transfer model (LBLRTM) to compare mo del calculations with the spectral radiance measurements. The high-res olution calculations of LBLRTM (approximate to 0.001 cm(-1)) are match ed to the lower-resolution measurements (1 cm(-1)) by adjusting their spectral resolution and by applying a correction for the finite field of view of the interferometer. In summer the uncertainties in temperat ure and water vapor profiles dominate the radiance error in the LBLRTM calculations. In winter the uncertainty in viewing zenith angle becom es important as well as the choice of atmospheric levels in the strong near-surface temperature inversion. The spectral radiance calculated for each of the three test cases generally agrees with that measured, to within twice the total estimated radiance error, thus validating LB LRTM to this level of accuracy for Antarctic conditions. However, the discrepancy exceeds twice the estimated error in the gaps between spec tral lines in the region 1250-1500 cm(-1), where emission is dominated by the foreign-broadened water vapor continuum.