[1] We compare a number of radiative transfer models for atmospheric sounding in the millimeter and submillimeter wavelength range, check their consistency, and investigate their deviations from each other. This intercomparison deals with three different aspects of radiative transfer models: (1) the inherent physics of gaseous absorption lines and how they are modeled, (2) the calculation of absorption coefficients, and (3) the full calculation of radiative transfer for different geometries, i.e., up-looking, down-looking, and limblooking. The correctness and consistency of the implementations are tested by comparing calculations with predefined input such as spectroscopic data, line shape, continuum absorption model, and frequency grid. The absorption coefficients and brightness temperatures calculated by the different models are generally within about 1% of each other. Furthermore, the variability or uncertainty of the model results is estimated if (except for the atmospheric scenario) the input such as spectroscopic data, line shape, and continuum absorption model could be chosen freely. Here the models deviate from each other by about 10% around the center of major absorption lines. The main cause of such discrepancies is the variability of reported spectroscopic data for line absorption and of the continuum absorption model. Further possible causes of discrepancies are different frequency and pressure grids and differences in the corresponding interpolation routines, as well as differences in the line shape functions used, namely a prefactor of (n/n 0 ) or (n/n 0 ) 2 of the Van-Vleck-Weisskopf line shape function. Whether or not the discrepancies affect retrieval results remains to be investigated for each application individually.Citation: Melsheimer, C., et al. (2005), Intercomparison of general purpose clear sky atmospheric radiative transfer models for the millimeter/submillimeter spectral range, Radio Sci., 40, RS1007,

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[1] The simulated retrieval performance of a submillimeter wave limb sounder was compared with that of an up-looking instrument with identical observation frequency bands and comparable noise temperature. The frequency bands were 624.32-626.32 and 649.12-650.32 GHz, and the retrieval simulations focused on the key trace gas species O 3 , HCl, and ClO. As expected, the limb geometry leads to a better altitude resolution and larger measurement altitude range. The same retrieval setup was applied to measured spectra, taken by the up-looking Airborne Submillimeter Radiometer (ASUR) instrument on 4 September 2002 at 19.11°E, 71.90°N and on 19 September 2002 at 44.10°E, 4.10°S. The observed structures in the fit residual near the HCl spectral lines at 625.9 GHz lead to the conclusion that the pressure shift parameter of HCl is likely to be higher than the value in the HITRAN spectroscopic database. Depending on the assumed temperature dependence of the shift, the HCl pressure shift value consistent with the ASUR data is 0.090-0.117 MHz/hPa instead of the 0.030 MHz/hPa reported in HITRAN. This result is in good agreement with very recent independent laboratory work which suggests a value of 0.110 MHz/hPa for the shift.

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