Uncertainties Associated with Theoretically Calculated N2 Broadened Half-Widths of H2O Lines
| Main Authors: | Ma, Q., Tipping, R. H., Gamachec, R. R. |
|---|---|
| Format: | info Proceeding Journal |
| Terbitan: |
, 2010
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| Subjects: | |
| Online Access: |
https://zenodo.org/record/18027 |
Daftar Isi:
- For many accurate atmospheric applications involving the water vapor molecule, it is desirable to know the half-widths and their temperature dependences better than a 3 % uncertainty for strong lines and 10 % for weaker lines1. Unfortunately, of all spectroscopic parameters in HITRAN for H2O, the self and air half-widths and their temperature dependences contain the largest uncertainties. Recently, in order to improve this situation, the H2O database in HITRAN has been updated. In developing this updated version, in cases where there are no half-width values to the desired accuracy available in the vast experimental literature, theoretically calculated values have been adopted. For many years researchers have used the Robert-Bonamy (RB) formalism and adopted a realistic potential model to derive these values. Because of the large number of potential matrix elements appearing in these calculations, one is forced to introduce cut-offs that may limit the accuracy attainable. To obviate the necessity of lower cut-offs, we recently reformulated the theory using the coordinate representation2. With different choices of the cut-offs used in the theoretical calculations, we have carried out extensive numerical calculations of the N2-broadend Lorentzian half-widths of the H2O lines using the modified RB formalism3. Based on these results, we are able to thoroughly check the convergence. We find that with lower cut-offs commonly used in the literature, one is able to obtain converged values only for lines with large halfwidths. Conversely, for lines with small half-widths much higher cut-offs are necessary to guarantee the convergence. We also analyze uncertainties associated with the calculated half-widths, and these are correlated as above. In general, the smaller the half-widths are, the poorer the convergence and the larger the uncertainty associated with them. For convenience, one can divide all H2O lines into three categories: large, intermediate, and small according to their half-width values. One can use this division to judge whether the calculated half-widths are converged or not, based on the cut-offs used, and also to estimate how large their uncertainties are. We conclude that with the current RB formalism for lines in category 1, one can achieve the accuracy requirement set by HITRAN while for lines in category 3, it is absolutely impossible to meet this goal.
- Session V-6.