Submillimeter atmospheric transmission measurements on Mauna Kea during extremely dry El Nino conditions: implications for broadband opacity contributions

We present broadband atmospheric transmission spectra obtained on Mauna Kea, Hawaii (4100 m. above sea level) on UT April Ist, 1998 and July Ist, 1999 under very similar pressure and temperature conditions. The 1998 measurements occurred under conditions of extremely low atmospheric water vapor, with a ground-level relative humidity of approximate to2%. As a result of its dryness the Mauna Kea site allows access to a partially transparent atmosphere up to frequencies exceeding 1000 GHz where the relative importance of atmospheric continuum-like absorption is much larger than at millimeter wavelengths, and hence easier to measure. As shown in this paper, these conditions have allowed us to measure and separate the submillimeter absorption spectrum into three terms: resonant lines, non-resonant absorption of the dry atmosphere due to collision-induced mechanisms involving electric quadrupoles, and continuum-like water vapor opacity. The spectra presented here were obtained with a Fourier transform spectrometer (FTS) at the Caltech Submillimeter Observatory and cover a continuous frequency range from 350 to 1100 GHz, with a finest spectral resolution of 200 MHz. The 1998 conditions were so exceptionally dry that an atmospheric window centered at 1035 GHz showed up to 35% zenith transmission. The calibration of our data is especially careful and includes corrections for differences between the ground atmospheric temperature and the calibrator temperature, as well as for the tropospheric temperature lapse rate, and the water vapor scale height. This procedure is able to yield transmission spectra calibrated to within 1-2%, A multilayer atmospheric radiative transfer model has been used for data analysis. (C) 2001 Published by Elsevier Science Ltd.