Line‐by‐line calculations of atmospheric fluxes and cooling rates: Application to water vapor

Abstract

A model for the accelerated calculation of clear sky fluxes based on the line‐by‐line radiance model FASCODE has been developed and applied to the calculation of cooling rates for atmospheric water vapor. The model achieves computational accuracies for the longwave upwelling and downwelling fluxes of the order of 0.2%, an accuracy well within current limitations imposed by uncertainties in the spectral parameters, the line shape, and the associated continua. For the same treatment of line shape, the Voigt profile with a 10 cm−1 cutoff and no continuum, the results from the present model are in acceptable agreement with those from two other line‐by‐line models reported as part of the intercomparison of radiation codes used in climate models (ICRCCM). For this line profile and the mid‐latitude summer atmosphere, the largest difference between the results from our model and the Goddard Laboratory for Atmospheres (GLA) model occurs for the downwelling flux at the surface, with the present model providing a value greater than that from GLA. The differences are generally consistent with greater atmospheric opacity from the present model, attributable to the inclusion of a self‐broadening component for the half width for water and to finer spectral sampling in the lower‐pressure regime. Utilization of the line shape and associated continuum model included in FASCODE gives results that are significantly different from those provided by the other two models. This radiance model, including contributions from the foreign continuum as well as from a modified self‐continuum, has received extensive validation against measured radiance spectra, an example of which is provided. For the mid‐latitude summer atmosphere the principal contribution from the foreign continuum occurs in the upper troposphere in the 250–350 cm−1 spectral region, whereas the contribution from the self‐continuum, dependent on the square of the water vapor density, is greatest in the lower troposphere. For the mid‐latitude summer atmosphere the foreign continuum contributes 0.4 Kd−1 or 20% to the cooling in the upper troposphere and the self‐continuum contributes 1.9 K d−1 to the cooling rate at the surface due to water vapor. The latter is 0.17 K d−1 less than the cooling rate from the GLA model which is principally due to a modification of the self‐continuum. A significant result that has developed from the present work is the insight into atmospheric radiative processes provided by spectral profiles of the cooling rate. In the spectral domain there exists a mapping between the altitude and the molecular absorption strength as weighted by the Planck function. The extremely high correlation between the outgoing spectral radiance at the top of the atmosphere and the spectral cooling rate profile suggests that measurement of the outgoing spectral radiance can provide important information about atmospheric state that is not available from spectrally integrated quantities. Our results also indicate the critical importance of the spectral region from 100 to 60...

Clear-sky far-infrared measurements observed with TAFTS during the EAQUATE campaign, September 2004

“…In particular, in the FIR, water vapour has been shown to have a dominant effect on the radiative cooling of the planet to space (King, 1993;Sinha and Harries, 1995). It is known that emission in the FIR, from the pure rotation band of water vapour, is very important; a significant fraction of the atmosphere's cooling to space takes place via the water vapour pure rotation band (100-500 cm −1 ) from the mid and upper troposphere (Rodgers and Walshaw, 1966;Clough et al, 1992;Slingo and Webb, 1997;Brindley and Harries, 1998). Indeed, radiative transfer modelling suggests that between 20% and 50% of this cooling takes place in the FIR.…”

Satellite observations of the water vapor greenhouse effect and column longwave cooling rates: Relative roles of the continuum and vibration‐rotation to pure rotation bands

“…The weighting with p represents the fact that the rotational lines are strong (i.e., opacity is very large at the line centers) and thus increase in absorption is mainly through the wings of the Lorentzian lines, which are pressure-broadened [Cess, 1974]. Alternately, collisions between unlike molecules (foreign broadening) predominate [Clough et al, 1992] in broadening the rotational lines, and the line width depends on the total molecular population (i.e., on p). In the window region, the continuum optical depth is determined largely by the water vapor amount weighted by the partial pressure of H 2 O, e w , and expressed as e w e ¼ 1=g ð Þ Z qe w dp;…”

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

“…The training profiles are chosen to be representative of the expected variability, including atmospheric temperature and composition, surface pressure, surface emissivity and reflectivity, and viewing and solar angles. 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.…”