South Pole submillimeter sky opacity and correlations with radiosonde observations

Submillimeter wave-length observations of the South Pole wintertime atmospheric opacity were performed on a regular basis starting in the year 1992. The absorption spectrum of pressure broadened water vapor lines is known to dominate submillimeter atmospheric transparency, and changes in opacity should be correlated with changes in the precipitable water vapor column (PWV). Indications of PWV were derived from radiosonde observations and the expected correlation with opacity was observed. Thus submillimeter radiometry can be used to predict radiosonde derived PWV and vice versa. These comparisons can be used to help interpret radiosonde PWVs. For example, we show that the absolute calibration of one instrument type, the A.I.R. Model 5A, in use at the South Pole since February 22, 1997, is doubtful and indicates wintertime PWVs about a factor of 2 smaller than expected when compared to opacity and PWV relationships established previously. Comparisons were also made to PWVsat, the precipitable water vapor column derived by assuming saturation over ice. Radiosonde data going back to 1961 were available, and we examined long-term trends in the wintertime PWV and PWVsat. The primary emphasis here was to reveal the expected long-term site quality of the South Pole for performing ground based submillimeter astronomy. Thus we focused on wintertime conditions. Our comparisons of submillimeter opacity to PWV also provide robust determinations of the magnitude of dry air opacity, important for improving submillimeter radiative transfer models.