11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Friday, 7 June 2002
The water-vapor continuum at 220-250 K retrieved from atmospheric transmission and emission measurements
Penny Rowe, University of Washington, Seattle, WA; and V. P. Walden and S. G. Warren
The water-vapor continuum spectrum has previously been measured in the laboratory at 296 and 353 K. Those measurements have been used (e.g. in LBLRTM) for atmospheric radiance calculations, even though temperatures in the earth's troposphere are often considerably lower then 296K. To measure the continuum absorption at low temperatures requires long absorption pathlengths, on the order of a kilometer, because of the low water-vapor density at low temperatures. Long paths are difficult to achieve in a laboratory; however, the clear atmosphere of the Antarctic Plateau is suitable as an outdoor "laboratory" because of the great horizontal homogeneity of the surface and the near-surface atmosphere.

To obtain the continuum in the 6.3-micron band and the rotation band, transmission was measured through horizontal atmospheric paths of up to 1000m using a Fourier transform infrared (FTIR) transmissometer during two summer seasons at South Pole Station. Downwelling radiance was also measured using an atmospheric emitted radiance interferometer (AERI) during two summer seasons and one winter season. The AERI measurements cover the spectral range 420-3000 cm-1; the transmission measurements cover 400-5600 cm-1. The water-vapor density along the horizontal path was measured by frostpoint hygrometers. The vertical profiles of temperature and water vapor were measured by radiosondes launched daily, as well as by special-purpose radiosondes and frostpoint hygrometers flown on a tethered balloon or kite.

Preliminary results suggest that the continuum at -30 C in the shoulder of the 6.3-micron band (1400-1475 cm-1) is lower than the room-temperature continuum, whereas in the band center (1575-1675 cm-1) it is higher than the room-temperature continuum.

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