Updated Water Vapor Retrievals and AERI Radiative Closure Analysis from the RHUBC-II Campaign

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Description

Dynamics in the mid-to-upper troposphere are driven by the emission and absorption of radiation by water vapor in that region. The representation of these radiative processes in models has a high uncertainty since the underlying spectroscopy has not been validated with respect to spectrally resolved surface radiation measurements in the relevant spectral regions, a result of the high opacity of the atmosphere in these regions under typical atmospheric conditions. This leads to a correspondingly high uncertainty in climate models’ predictions for the mid-to-upper troposphere. The ARM Program conducted the second Radiative Heating in Underexplored Bands Campaigns (RHUBC-II) to improve knowledge of these radiative processes. RHUBC-II occurred between Aug-Oct 2009 at a site at 5400m in the Atacama Desert of Chile where precipitable water vapor (PWV) values as low as 0.2 mm were observed during clear periods. This campaign included the collection of spectrally resolved measurements in strong H2O absorption bands from the Atmospheric Emitted Radiance Interferometer (AERI), which measures in the thermal infrared, and two instruments that measure throughout the far-IR. The analysis of RHUBC-II measurements requires accurate knowledge of the water vapor field above the observing instruments, but the Vaisala RS-92 radiosondes launched during RHUBC-II have well-known accuracy issues in conditions of low humidity and during daytime. We retrieve accurate water vapor profiles through an optimal estimation approach that uses the radiosonde measurements and observations from the RHUBC-II GVRP instrument, which has 14 channels on the 183.3 GHz H2O line. More accurate values of this line’s self-broadened width and temperature dependence parameter have recently become available, leading to an updated derivation of the foreign-broadened width. These parameters were included in the recent release (v5.0) of AER’s microwave radiation code, MonoRTM. We have rerun all water vapor retrievals for the campaign and, in addition, now performing a retrieval for each GVRP measurement, leading to a dataset of more than 3000 retrieved profiles. We will present these new retrievals, as well as the analysis of spectral residuals between AERI measurements and LBLRTM calculations using these retrieved profiles. The AERI - LBLRTM biases will be shown for different PWV categories, and will allow the identification of spectroscopic issues.