Evaluating the accuracy of the far-infrared spectral observations, water vapor measurements, and the radiative transfer model in extremely dry, clear-sky periods during RHUBC-II
Jennifer S. Delamere — University of Alaska, Fairbanks
Robert O. Knuteson — University of Wisconsin
David D. Turner — NOAA- Global Systems Laboratory
David C. Tobin — University of Wisconsin
Eli Jay Mlawer — Atmospheric and Environmental Research, Inc.
Maria Paola Cadeddu — Argonne National Laboratory
Martin G Mlynczak — NASA - Langley Research Center
Dave Johnson — NASA - Langley Research Center
Richard Philip Cageao — NASA - Langley Research Center
Luca Palchetti — Istituto Nazionale di Ottica, INO-CNR
Giovanni Bianchini — INO-CNR, Area della Ricerca CNR
Aronne J Merrelli — University of Wisconsin
Category
Radiation
Description
The longwave net flux profile, in a wide variety of atmospheric conditions, shows a marked difference when computed with the pre-RHUBC-I water vapor continuum model (CKD v2.4.1) versus the post-RHUBC-I model (MT_CKD v2.4). The accuracy of the two continuum models is being evaluated with RHUBC-II data, and GCM simulations are being used to gauge the impact of the difference in the LW flux.
The far-infrared (wavelengths between 15 and 100 µm) is an extremely important spectral region, as nearly 40% of the outgoing longwave radiation and a significant portion of the infrared radiative cooling in the middle-to-upper troposphere are directly attributable to radiative processing in this spectral band. Radiative transfer models in this spectral region have significant uncertainties due to the relative lack of data to use in developing and validating models. ARM/ASR, in collaboration with other national and international agencies, recently conducted two field experiments called the Radiative Heating in Underexplored Bands Campaign (RHUBC), with RHUBC-I held at the ARM site in Barrow, Alaska, in February–March 2007 and RHUBC-II held in the Atacama desert in Chile at over 5000 m MSL in August–October 2009.
Analysis of data from RHUBC-I suggested that the water vapor continuum absorption model used in infrared radiative transfer calculations needed adjustment in the far-infrared. The changes made to the continuum model have a significant impact in the atmospheric cooling in clear skies, resulting in negative net flux differences approaching -0.8 W/m2 for atmospheres below approximately 500 mb and net flux increases approaching 0.9 W/m2 above 500 mb. We are evaluating the impact of these differences in the net longwave radiative flux profile on the global circulation using the NCAR Community Earth System Model (CESM). Initial results will be shown from decadal simulations with and without this modification to the water vapor continuum absorption model at the meeting.
The minimum amount of precipitable water vapor (PWV) observed during RHUBC-I was 0.95 mm, resulting in the atmosphere being semitransparent in the 17 to 26 µm region; thus, the radiative transfer model could only be evaluated in this spectral interval. The minimum amount of PWV observed during RHUBC-II was nearly five times drier (0.2 mm), allowing the spectral region from 17 to 45 µm to be evaluated. The RHUBC-II data set provides a more complete test of the accuracy of the adjustment made to the continuum model; however, the analysis must also investigate the accuracy of the PWV retrievals and water vapor profiles as well as the radiometric accuracy of the spectrally resolved radiance observations. Initial results from this analysis will also be shown.
