AERI downwelling infrared radiance observations and calculations: case studies from the 1997 DOE ARM CART Water Vapor Intensive Operations Period

The 1997 Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program Water Vapor Intensive Operations Period (WVIOP) was conducted September 15-October 5, 1997 at the ARM Southern Great Plains Cloud and Radiation Testbed (SGP-CART) site near Lamont, Oklahoma. The goals of the WVIOP were to optimize the capabilities of CART instrumentation for characterizing atmospheric water vapor and construction of an accurate water vapor dataset for validation purposes. Downwelling radiance measurements by the Atmospheric Emitted Radiance Interferometer (AERI) at the CART site can be used to identify uncertainties in both the water vapor spectral line parameters and continuum used in line-by-line radiative transfer calculations. This identification is necessary to test the hypothesis of using selected water vapor lines in the AERI infrared spectrum as a transfer standard for absolute water vapor calibration. Two case studies are presented, 16 September 0400-0500 UTC and 26 September 0500-0700 UTC, selected due to the relative stability of the atmospheric water vapor profile as determined by the on-site Raman lidars. The authors present comparisons of AERI observations and line-by-line calculations to assess uncertainties in knowledge of the atmospheric state and forward model accuracy.