Radiative Heating in Underexplored Bands Campaign (RHUBC)
Now available: RHUBC-II website
Between February and March 2007 at the ACRF North Slope of Alaska site in Barrow, high-spectral-resolution observations will be collected by two state-of-the-art infrared Fourier transform spectrometers that sample in the far-infrared (wavelengths longer than 15 microns). During RHUBC (pronounced "roobik"), detailed observations in the 17 - 100 micron (100 - 600 cm-1) and 6 - 7 micron (1400 - 1700 cm-1) spectral regions will be obtained. These spectral bands are largely underexplored because they are normally opaque from the surface due to the strong absorption by water vapor; however, these bands do become semi-transparent in very low water vapor conditions. In particular, the so-called "Arctic window" opens up during the very dry conditions that are experienced during winter in the high latitudes.
This chart shows the spectral and height dependence of the infrared cooling rates for a mid-latitude summer profile. Note that the majority of the infrared cooling in the middle and upper troposphere occurs in spectral regions that RHUBC will investigate. (Larger image.)
Radiative cooling and heating in the mid-to-upper troposphere contribute significantly to the dynamical processes and radiative balance that regulate Earth's climate. In the longwave, the dominant agent of this radiative cooling is water vapor. The dominant role of water vapor in upper tropospheric processes is shown below the spectral cooling rate profile for a mid-latitude summer (MLS) atmosphere containing water vapor, carbon dioxide, and ozone. Absorption in the 100-600 cm-1 (17-100 microns) and 1400-1700 cm-1 (6-7 microns) bands are responsible for the majority of the infrared cooling in the middle and upper troposphere, and thus to properly improve and validate radiative transfer models good observations are needed in these spectral regions.
However, accurate radiance observations in the far-infrared to evaluate the adequacy of the radiative transfer models (and improve them) have been limited by two things. First, due to the strength of the absorption of water vapor in this region, the atmosphere is largely opaque at the surface for the majority of locations around the globe. Second, until recently there have been a lack of well-characterized and accurate radiometers that measure spectral radiation in this band. The relatively recent development of new state-of-the-art infrared interferometers which measure downwelling radiance in this spectral region allow the uncertainties in the radiative transfer models in the far-infrared to be evaluated and addressed.