High-Resolution Image

The Department of Energy's ARM (Atmospheric Radiation Measurements) Program is preparing for the highest land-based field campaign in history in order to study a fundamental issue in global warming: the radiative cooling of the upper troposphere in the far-infrared. Previously, no field campaign has been able to linger at such altitudes long enough to incisively validate the radiative cooling calculations central both to greenhouse theory and to climate models used for global warming predictions. (Upper figure) The site selected for the deployment of ARM instruments in Chajnantor, Chile, a large volcanic plain at 5 km altitude. Gargantuan astronomical facilities including the new Atacama Large Millimeter Array will be located in the plain below the site. The site itself is at 5.4 km altitude and was selected to have the lowest possible water vapor column overhead; values below 1 mm are routine at this site. Only with such low water columns can meaningful spectral longwave measurements in the 20-600 cm-1 spectral region (the far-infrared) be made; otherwise the atmosphere would merely appear black. Longwave spectrometers from three groups (Univ. of Wisconsin, NASA Langley, and the UK) will participate for the two-month campaign in the Fall of 2009. A variety of ARM instruments will be fielded in order to close the radiation problem, that is, to allow model calculations to be compared with the spectral observations. These instruments include microwave radiometers sensitive to very small amounts of water vapor.

(Lower figure) Spectral longwave radiative cooling profile for H2O, CO2 (355 ppm), and O3 as a linear function of pressure for a Midlatitude Summer atmosphere. The results are spectrally averaged over 25 cm-1. Color scale x 10-3 is in units of K/day/(cm-1). The key point is the yellow-orange blob around altitude 300 mb and wavenumber 300 cm-1; this is where most of the longwave cooling of the clear troposphere takes place. This longwave cooling has never been measured spectrally before, since it cannot be measured from lower altitudes. (adapted from Clough and Iacono, 1995, Fig 1)

(Submitted by Warren Wiscombe, NASA/GSFC & Chief Scientist, DOE's ARM Program)