FIRST Measurements of Downwelling Far-IR Radiance at Table Mountain, California

We present observations of far-infrared (far-IR) emission from Earth’s atmosphere measured by the Far-Infrared Spectroscopy of the Troposphere (FIRST) instrument at Table Mountain, California during September and October 2012. The FIRST observations are made simultaneously with radiosonde launches providing vertical profiles of temperature and water vapor. In addition, total precipitable water is measured onsite at Table Mountain via GPS every 30 minutes. Temporally coincident observations from the AIRS instrument and CALIPSO satellite are occasionally available as are measurements from the TMF water vapor profiling lidar. The radiosonde, GPS, and satellite data are used with a radiative transfer model to compute the expected clear-sky downwelling radiance for comparison with the FIRST observations. Assessment of measured and modeled atmospheric radiances requires consideration of a number of factors including: atmospheric variability, instrument stability, instrument calibration, and uncertainty in the inputs to the radiative transfer model. Of particular interest is the knowledge of instrument calibration, which must be known (for observations at Table Mountain) for brightness temperatures ranging from 180 K to 290 K. Uncertainty in knowledge of atmospheric radiative transfer calculation inputs also limits the extent to which accurate comparisons can be made. Comparison of observations and model calculations are critical for understanding the extent to which “radiative closure” is obtained – that is, the degree to which we can model and predict the far-IR part of the spectrum. The far-IR emission is driven essentially by water vapor, a primary greenhouse gas in Earth’s climate system