Development Of A 10cm -1 Resolution 6.3 µm H 2 O Band Model

AbstractA simple model for developing band model parameters at moderate spectral resolution (~10 cm-1) is applied to the 6.3 ym H20 band. The underlying assumption of the approach is that the shapes of bands of the same symmetry can be superimposed to within ~10 cm"1. The band centers of the hot bands for H20 in the 6.3 urn are known, and the reference bandshapes can be generated from the AFGL HITRAN line atlas. A preliminary set of band model param­ eters is presented and compared to the NASA parameters.IntroductionThe objective of this study is to demonstrate that a 10 cm""1 band model for the 6.3 ym H20 band can be developed through a combination of currently available high resolution data at low temperatures and simple theoretical considerations for scaling the low temperature data to high temperatures (up to 3000K) . At present, the Standard Infrared Radiation Model (SIRRM) , (1) uses the NASA^2) parameters for the 6.3 ym band. Even though these parameters are tabulated every 5 cm"1 in the SIRRM code, their actual spectral resolution is only 25 cm"1.^2' 3) Although the parameters are given at various temperatures over the range of 200-3000K, the experimental data upon which these parameters are based does not adequately cover this temperature range. That is, some of the temperature points represent either relatively large interpolations or extrapolations from measurements.ApproachThere are several approaches available for developing an H20 band model. One approach is completely experimental. One measures the emission and/or absorption at sufficient resolution and at various temperatures from 300-3000K and then uses the data to define the band model parameters. This approach has been utilized previously. ' 4' These are not easy measurements as they invoove measuring a very small absorption in the wings of the 6.3 ym band even for a long path (of order 10m). The 5.4-4.5 ym window region is particularly difficult to measure at lower temperatures (below 1200K) . A second approach is completely theoretical. One computes the location and intensity of every vibration-rotation transi­ tion using some approximation method to solve the Schroedinger equation for a vibrating - rotating asymmetric top. The basic trade-off is computational efficiency (i.e., cost) versus accuracy. In principle, this approach could currently be adopted and give good results, however the computing requirements could be restrictive. A third approach, and the one to be discussed in detail, involves using the relatively accurate high resolution low temperature data in conjunction with theoretically justifiable scaling procedures, to create a 10 cm"1 resolution H2O band model from 200 to 3000K which covers the 10-5.4 ym region (1000-2200 cm""1).At low to modest temperatures, typically below TOOK, adequate band model parameters at any spectral resolution can be generated directly from the Air Force Geophysics Laboratory HITRAN ( 5) line atlas. Above 700K this approach becomes worse with increasing temperature. This is because the HITRAN compilation is missing both the higher temperature lines corre­ sponding to hot bands and the higher energy rotational lines from the ground vibrational state. Since we require that line locations need to be known to within only 10 cm"1 reso­ lution, instead of the ~0.01 cm"1 accuracy of the HITRAN compilation, it is possible to create an augmented version of the HITRAN atlas which contains the necessary hot band lines needed to get to 3000K. The underlying assumption of the approach is that the shapes of bands of the same symmetry (water has A and B type bands) can be superimposed to within 10 cm-1. since the band centers of the hot band transitions are known and the integrated band strengths can be estimated reasonably well, then only one A and one B type H2O refer­ ence band is needed to generate the missing lines. These reference bands are taken from the HITRAN atlas.Part of this project was performed while the author was at Aerodyne Research, Inc.