Accuracy Improvement and Uncertainty Reduction of Spectroscopic Parameters of Water Vapor Lines in the HITRAN Database

The primary objective of this project is to provide theoretical support for updating and improving parameters for the water vapor molecule in the HITRAN database. The latest changes made to the pressure broadened half-width, induced line shift, and temperature exponent for the principal oxygen isotopologue of water vapor in HITRAN come mostly from theoretical calculations that rely on the Robert-Bonamy formalism, which was developed more than three decades ago and contains several approximations. Direct calculation of these spectroscopic parameters from first principles is characterized by theoretical complexity as well as numerical difficulty. The proposed approach offers several new features and advantages. First, re-formulating the Robert-Bonamy formalism shows that the removal of one debilitating approximation actually reduces the computational burden. Second, introducing the coordinate representation in Hilbert space enables the use of sufficiently high cut-offs to guarantee full convergence for all the calculations. Third, contribution analysis from different collisional processes establishes a new guideline for selecting benchmark lines in optimizing the potential model. Furthermore, the adoption of the causal function concept from signal processing to obtain the Hilbert transforms, crucial in calculating line shifts, allows the calculations to go forward without even having to perform the Cauchy principal integrations. With these powerful tools in hand, the broadened half-widths, line shifts, and temperature exponents for the absorption lines in the pure rotational band will be recalculated ? all with full convergence and from the 'exact' trajectory model. These results will significantly reduce the numerical uncertainties, and are thus more reliable than the existing calculated values in HITRAN 2008.

Reliable information retrieval using space and ground-based instruments, and reliable climate and atmospheric modeling are directly dependent on accurate water vapor spectroscopic parameters provided by HITRAN. Research scientists in many different disciplines have come to expect the high quality of HITRAN as their indispensable data resource that simply must be correct. The updating of the HITRAN water vapor spectroscopic parameters will improve the accuracy of radiative transfer modeling, such as the 20 micrometer 'dirty window' spectral region that controls the local energy balance in the cold and dry polar atmospheres. The computational approach described herein is applicable not only to the water vapor lines in the pure rotation band, but to other bands as well. Moreover, it can also be used to predict line parameters that involve very high j states in the development of HITEMP, a project designed to address high-temperature modeling of the spectra of molecular gases needed for astrophysics, planetary and stellar atmospheres, as well as industrial, environmental, and surveillance applications.