RHUBC

Radiative Heating in Underexplored Bands Campaign (RHUBC)

22 February 2007 - 14 March 2007

Lead Scientist: David Turner

Observatory: NSA

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. Due to the much greater abundances of this gas at lower levels of the atmosphere, the spectral regions in which the mid-to-upper tropospheric cooling occurs are opaque when viewed from the vast majority of surface locations. The opacity of the lower atmosphere is a formidable obstacle in evaluating radiative processes important in the mid-to-upper troposphere from the surface; however, an even more substantial obstacle has been the lack of radiometric instrumentation in the most critical spectral region for these processes, the far-infrared. The development of a new generation of instruments for the measurement of spectral radiation in the far-infrared provided the capability to rectify this state of affairs. These instruments allowed the evaluation of radiatively important processes in the mid-to-upper troposphere. This presented ARM with a terrific opportunity to contribute substantially to the improvement of the parameterization of these crucial radiative processes in climate simulations. We conducted the Radiative Heating in Underexplored Bands Campaign (RHUBC, pronounced “roobik”) from 22 February to 14 March 2007 at the NSA site in Barrow. This experiment made detailed observations of the downwelling infrared radiation in the 17- 100 ìm (100-600 cm-1) rotational and 6.7 ìm (1350-1850 cm-1) õ2 water vapor bands. Both of these spectral bands are underexplored because they are normally opaque at the surface due to strong absorption by water vapor, and hence the radiative heating in these bands is uncertain. High-spectral-resolution observations were collected by three state-of-the-art Fourier Transform Spectrometers (FTS): the ARM AERI-ER (400 - 3000 cm-1), the NASA/LaRC FIRST (20 - 1600 cm-1), and the Imperial College TAFTS (80 - 650 cm-1). During the IOP period, the precipitable water vapor (PWV) was small (typically less than 3 mm) and thus important parts of the rotational and õ2 water vapor bands were semitransparent, and the incidence of low stratus clouds was at a minimum (~ 40-50%). Specifically, the primary goals of RHUBC were:

• To conduct clear-sky radiative closure studies in order to reduce the key uncertainties in the water vapor spectroscopy, including the foreign-broadened water vapor continuum and water vapor absorption line parameters. This campaign allowed a robust set of measurements corresponding to low PWV and cold temperatures to be collected; this is unobtainable in the laboratory.
• Instrument cross-calibration and validation. FIRST, TAFTS, and the AERI-ER are stateof- the-art instruments that operate in far-IR for the purpose of atmospheric radiative transfer studies. None of these instruments have been validated in an operational environment against a complementary interferometer from a different manufacturer. The inter-comparison allowed a higher confidence in the results from all three instruments.
• The investigation of the radiative properties of sub-arctic cirrus. The combination of the AERI-ER, FIRST, and TAFTS allowed simultaneous high-resolution measurements of Arctic cirrus emission in the far-IR for the first time. The additional instrumentation (MPL, MMCR) at the ARM site provided a comprehensive array of auxiliary data, maximizing the scientific value of this data set.

The ultimate impact of RHUBC was increased knowledge of mid-to-upper tropospheric radiative processes and, therefore, improved simulations of future climate.