Analysis of spectra from the RHUBC-II campaign
Authors
Eli Jay Mlawer — Atmospheric and Environmental Research, Inc.
David D. Turner — NOAA Earth System Research Laboratory
Karen E. Cady-Pereira — Atmospheric and Environmental Research, Inc.
Daniel Gombos — Atmospheric and Environmental Research, Inc.
Dharshani (Nimali) Bopege — CIMMS/University of Oklahoma
Alison Chase — AER Inc
Vivienne Helen Payne — Jet Propulsion Laboratory/California Institute of Technology
Maria Paola Cadeddu — Argonne National Laboratory
Category
Radiation
Description
The mid-to-upper troposphere is an important driver of that region’s dynamics. Surface measurements of thermal and solar radiation in typical conditions contain no pertinent information about these radiative processes due to absorption by water vapor in the intervening lower atmosphere. The resulting relatively high uncertainty in our knowledge of these processes is reflected in a corresponding uncertainty in climate models predictions for the mid-to-upper troposphere. In an initiative targeted at targeted at lowering these uncertainties, the ARM Climate Research Facility recently conducted the second Radiative Heating in Underexplored Bands Campaigns (RHUBC-II) from August–October 2009 at a site at 5400 meters elevation in the Atacama Desert of Chile. During RHUBC-II, precipitable water vapor values as low as 0.2 mm were observed during clear periods. This campaign included a number of instruments that provided spectrally resolved measurements in strong H2O absorption bands, including two instruments that measure throughout the far-IR and one in the near-IR.Analysis of RHUBC-II measurements requires accurate specification of the water vapor profiles in the radiating column above the site. Vaisala RS-92 radiosondes were regularly launched during operational periods of RHUBC-II, but these sondes have well-known accuracy issues in conditions of low humidity and during daytime. Previous ARM radiative closure studies have demonstrated that measurements associated with well-characterized H2O absorption lines in the microwave can provide information that improves the accuracy of water vapor profiles measured by sondes. This study has utilized an optimal estimation approach to refine the sonde profiles using observations from the RHUBC-II G-band vapor radiometer profiler (GVRP) instrument, which has 14 channels on the 183.3 GHz H2O line, resulting in a best-guess water vapor profile for each RHUBC-II clear-sky case. These profiles allow analysis of near-IR spectrally resolved measurements of solar radiation by the absolute solar transmittance interferometer (ASTI), from which calibrated observations were obtained for two days at the end of the campaign.