The QME AERI LBLRTM: A Closure Experiment for Downwelling High Spectral Resolution Infrared Radiance

Abstract

Research funded by the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) program has led to significant improvements in longwave radiative transfer modeling over the last decade. These improvements, which have generally come in small incremental changes, were made primarily in the water vapor self- and foreign-broadened continuum and the water vapor absorption line parameters. These changes, when taken as a whole, result in up to a 6 W m−2 improvement in the modeled clear-sky downwelling longwave radiative flux at the surface and significantly better agreement with spectral observations. This paper provides an overview of the history of ARM with regard to clear-sky longwave radiative transfer, and analyzes remaining related uncertainties in the ARM state-of-the-art Line-by-Line Radiative Transfer Model (LBLRTM). A quality measurement experiment (QME) for the downwelling infrared radiance at the ARM Southern Great Plains site has been ongoing since 1994. This experiment has three objectives: 1) to validate and improve the absorption models and spectral line parameters used in line-by-line radiative transfer models, 2) to assess the ability to define the atmospheric state, and 3) to assess the quality of the radiance observations that serve as ground truth for the model. Analysis of data from 1994 to 1997 made significant contributions to optimizing the QME, but is limited by small but significant uncertainties and deficiencies in the atmospheric state and radiance observations. This paper concentrates on the analysis of QME data from 1998 to 2001, wherein the data have been carefully selected to address the uncertainties in the 1994–97 dataset. Analysis of this newer dataset suggests that the representation of self-broadened water vapor continuum absorption is 3%–8% too strong in the 750–1000 cm−1 region. The dataset also provides information on the accuracy of the self- and foreign-broadened continuum absorption in the 1100–1300 cm−1 region. After accounting for these changes, remaining differences in modeled and observed downwelling clear-sky fluxes are less than 1.5 W m−2 over a wide range of atmospheric states.

Far-infrared surface emissivity and climate

“…1B shows band-averaged transmission between 100 μm and 15.9 μm (100 cm −1 and 630 cm −1 ) rising to 0.41 at 0.1 mm PWV, while transmission between 200 cm −1 and 630 cm −1 , a spectral range that covers ∼46% of total thermal emission, rises to 0.5. This is consistent with previous findings (42,43) and has significant implications for the relationship between surface emissivity and total infrared energy budget. Fig.…”

Evaluation of two Vaisala RS92 radiosonde solar radiative dry bias correction algorithms

“…ARM has used the MWR-retrieved PWV as a "standard" for correcting for first-order radiosonde biases CadyPereira et al, 2008), calibrating its Raman lidar (Turner and Goldsmith, 1999), and evaluating infrared radiative transfer models (e.g., Turner et al, 2004).…”

Atmospheric and surface retrievals in the Mars polar regions from the Thermal Emission Spectrometer measurements

“…In our work with OSS, the LBLRTM model [Clough et al, 1992[Clough et al, , 2005 serves as the line-by-line reference. The choice of LBLRTM gives direct access to ongoing radiance closure studies [Clough et al, 1999;Turner et al, 2004] and, together with the monochromatic nature of OSS, enables the OSS model to be quickly and rigorously updated as our knowledge of the fundamental spectroscopic parameters improves. A recent example involves the implementation of new P and R branch line coupling coefficients for CO 2 [Niro et al, 2005;Clough et al, 2008].…”