Best-guess water vapor profiles for the RHUBC-II campaign

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Description

Emission and absorption of far-infrared radiation by water vapor in the mid-to-upper troposphere are important drivers of that region’s dynamics. Surface radiation measurements 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. The ARM Climate Research Facility recently conducted a set of field experiments, the Radiative Heating in Underexplored Bands Campaigns (RHUBC), targeted at lowering these uncertainties. RHUBC-II was held from August–October 2009 at a site at 5400 m in the Atacama Desert of Chile, during which the precipitable water vapor during clear periods was as low as 0.2 mm. Under these conditions, the increased transparency allows ground-based far-IR measurements to contain useful information about these radiative processes. The RHUBC-II 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. Improving far-IR spectroscopic parameters using 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 will utilize an optimal estimation approach to refine the sonde profiles using observations from the RHUBC-II GVRP instrument, which has 14 channels on the 183.3 GHz H2O line. Different retrieval approaches will be evaluated, as will the accuracy of the methodology specified by Miloshevich et al. (2009) for removing biases in sonde H2O profiles. These methods for obtaining RHUBC-II H2O profiles will be evaluated via comparisons to measurements in the sub-mm region by the collocated Smithsonian FTS. This study will result in a best-guess water vapor profile for each RHUBC-II clear-sky case; the complete set will be placed in the campaign archive.