Accurately accounting for radiative energy balance between the incoming solar and the outgoing infrared radiative fluxes is very important in modeling the Earth's climate. Water vapor absorption plays a critical role in the radiative heating rate profile in the midtroposphere by strongly absorbing both infrared and solar radiation in several absorption bands throughout the electromagnetic spectrum. One of the most important of these absorption bands is in the far-infrared portion of the spectrum, where the far-infrared is defined here to be wavelengths longer than 15 microns. A large fraction (~40%) of the outgoing infrared flux is emitted by water vapor in the far-infrared. Errors in the radiative transfer models associated with the far-infrared and other strong water vapor absorption bands will therefore affect the calculation of the planet's total outgoing radiative flux and its vertical distribution of the radiant energy; these errors may result in inaccurate modeling of the general circulation of the planet.
A set of field experiments, called the Radiative Heating in Underexplored Bands Campaigns (RHUBC), has been conducted as part of the Atmospheric Radiation Measurement (ARM) program. The RHUBC campaigns deployed spectrally resolved far-infrared spectrometers alongside other ARM observations in extremely dry environments to provide a robust and complete dataset that allows radiative transfer models to be evaluated in the far-infrared and other spectral regions where water vapor absorbs strongly. RHUBC I was conducted in February–March 2007 in Barrow, Alaska, and RHUBC II was conducted in August–October 2009 in the Atacama Desert region of Chile at an altitude of 5.3 km. The motivation for and initial results from these experiments are described, as well as the implications for global climate models.
University of Wisconsin—Madison, Madison, Wisconsin
Atmospheric and Environmental Research, Inc., Lexington, Massachusetts