Far-Infrared Spectroscopy of the Troposphere

The far-infrared, defined herein as the spectral region between 15 and 100 micrometers, is a virtually unobserved component of the infrared emission spectrum of the Earth and its atmosphere. The radiative balance of the troposphere is influenced strongly by radiative cooling associated with emission by water vapor at far-infrared wavelengths extending out beyond 50 micrometers. Up to half of the outgoing longwave radiation from the Earth occurs in the far-infrared, depending on atmospheric and surface conditions. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are well recognized as major uncertainties in understanding and predicting future climate. Despite this fundamental importance, far-infrared emission (spectra or band-integrated) has rarely been directly measured from space, airborne, or ground-based platforms. Current and planned operational and research satellites typically observe the mid-infrared only to about 15.4 micrometers. NASA has recently selected the Far-Infrared Spectroscopy of the Troposphere (FIRST) project for development under its Instrument Incubator Program (IIP). FIRST will observe the infrared emission spectrum between 10 and 100 micrometers. FIRST is envisioned as the next-generation atmospheric sounder, combining both radiation budget measurements with atmospheric profiling capability. In this talk we will review the importance of the far-infrared, discuss the FIRST instrument, and highlight the role of far-infrared observations in the overall climate observing system of the future.