An Analysis of Far-Infrared Radiances Obtained By the First Instrument at Table Mountain through the Use of Radiative Transfer Calculations

Abstract:

The Far-Infrared Spectroscopy of the Troposphere (FIRST) instrument is a Fourier Transform Spectrometer with a moderately high spectral resolution (0.643 cm-1 unapodized). Designed to measure the most energetically important range (100 to 2000 cm-1) of Earth’s emitted infrared spectrum, the FIRST instrument was specifically engineered to include the often overlooked far-infrared (100 to 650 cm-1). To date, the FIRST instrument has been deployed on several field missions, both balloon-borne and ground-based, with the most recent deployment occurring at NASA’s Jet Propulsion Laboratory Table Mountain Facility in California during the months of September and October 2012. This deployment, located 2,285 meters above the Mojave Desert, provided an opportunity to observe down-welling radiances under low water vapor conditions, with some cases having total column water vapor amounts of approximately 2 to 3 millimeters. Such low water vapor conditions allow for stringent testing of both the FIRST instrument and the radiative transfer methods used to analyze these measurements. This study is focused on the analysis of the FIRST measurements in the far-infrared obtained during clear-sky conditions, and requires an accounting of the uncertainties in both the measured and calculated radiances. The former involves the manner in which calibration and full-sky conditions affect the radiances measured by the FIRST instrument. The latter involves not only differences in the model formulations and the uncertainties in the water vapor and temperature data provided by the radio-sonde measurements, but also the critical differences and uncertainties contained within the input line parameter databases. This study specifically explores the significant differences that can arise in the calculated radiances that are associated with the choice of a line parameter database, and how this choice affects the analysis of the FIRST measurements.