Analysis of the Downward Directed Far-infrared Radiances Measured by the FIRST Instrument at Table Mountain

Abstract:

The Far-Infrared Spectroscopy of the Troposphere (FIRST) is a Fourier Transform Spectrometer, which has been deployed several times to measure the far to middle infrared atmospheric spectra within the 200 to 800 cm^-1 range. A detailed laboratory calibration of FIRST was conducted before the most recent field campaign to NASA’s JPL Table Mountain Facility. Radiosonde data were taken concurrently with the FIRST measurements during this deployment to provide temperature and water vapor profiles for model simulations of the FIRST spectra. Results are presented for the night of October 19, 2012, the driest time during this deployment when the total column water vapor amount was less than 0.3cm. The FIRST data showed significant spectral development between 400 and 600 cm^-1, wherein over 90% arose within 2.8 km of the surface. In contrast, the spectra between both 200 and 400 cm^-1 and 600 and 750 cm^-1 were characteristic of blackbody radiation with near-surface temperatures. This result can be ascribed to very high opacities wherein the mean free paths were frequently less than 40m. This, in combination with near-surface temperature inversions, required layer thicknesses as fine as 10m for the radiative transfer calculations. Overall, we found that when all of the measurement and modeling uncertainties were considered that the FIRST measurements agreed with the model calculations to within the combined uncertainties. Most notably, we found that the uncertainties in the model calculations exceeded those in the measurements, with the uncertainty in the radiosonde measured water vapor abundances being the largest contributor. Newly proposed instruments, such as those for the CLARREO mission, will have much greater accuracies than FIRST, and consequently will have the capability to measure the far-infrared spectra to much higher accuracies than can be computed. Direct measurements of the far-infrared are essential to the assessment of future climate change.