C. Verdes

Intercomparison of general purpose clear sky atmospheric radiative transfer models for the millimeter/submillimeter spectral range

[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/n0 )o r (n/n0) 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.

Pointing and temperature retrieval from millimeter-submillimeter limb soundings

[1] Passive microwave limb sounding instruments like the Millimeter-Wave Atmospheric Sounder (MAS) or the Microwave Limb Sounder (MLS) observe dedicated oxygen lines for the derivation of temperature and pointing information, since these quantities are essential for the quality of the retrieval of the trace gas mixing ratio. Emission lines of oxygen are chosen because the volume mixing ratio (VMR) profile is known. In this paper, we demonstrate the capabilities of a new and innovative method by means of which accurate temperature and pointing information can be gathered from other strong spectral features like ozone lines, without including accurate knowledge of the VMR profile of these species. For this purpose, retrievals from two observation bands with a bandwidth of about 10 GHz each, one including an oxygen line, have been compared. A full error analysis was performed with respect to critical instrument and model parameters, such as uncertainties in the antenna pattern, calibration uncertainties, random pointing error, baseline ripples, baseline discontinuities, and spectroscopic parameters. The applied inversion algorithm was the optimal estimation method. For the selected scenario and instrumental specifications we find that the retrieval of a pointing offset and the atmospheric temperature profile can be achieved with a good accuracy. The retrieval precision of the pointing offset is better than 24 m. The retrieval precision of the temperature profile is better than 2 K for altitudes ranging from 10 to 40 km. Systematic errors (due to model parameter uncertainties) are somewhat larger than these purely statistical errors. Investigations carried out for different atmospheric states or different instrumental specifications show similar results. INDEX TERMS: 1640 Global Change: Remote sensing; 3260 Mathematical Geophysics: Inverse theory; 0350 Atmospheric Composition and Structure: Pressure, density, and temperature; 0394 Atmospheric Composition and Structure: Instruments and techniques;

Expected performance of the Superconducting Submillimeter-Wave Limb Emission Sounder compared with aircraft data

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. Copyright 2005 by the American Geophysical Union.

Superconducting sub-millimeter wave limb emission sounder SMILES

The main aims of the SMILES project are the development and use of superconducting mixers (superconductor-insulator-superconductor SIS technology) in a heterodyne sub-millimeter receiver for remote sensing of the atmosphere from space. The two frequency bands to be observed, 624.2-628.6 GHz and 649.3-653.1 GHz, are chosen such that the chemistry of the upper troposphere and the stratosphere, together with some climatological aspects, can be investigated. These frequency ranges allow the observation of a number of interesting species, such as ClO, BrO, HO/sub 2/, NO, SO/sub 2/, O/sub 3/, HCl, HOCl, HN0/sub 3/, N/sub 2/0, H/sub 2/0/sub 2/ and 0/sup 18/0. A simulated spectrum is shown.

Derivation of molecular species profiles, atmospheric temperature profile, and instrumental pointing from SMILES instrument

Recent progress in sub-millimeter wave receiver technology gives the possibility to drastically improve the quality of limb sounding data by use of a superconductor-insulator-superconductor (SIS) mixer element. This receiver will detect molecular spectra with a signal-to-noise ratio one order of magnitude better than the conventional ambient--temperature Schottky receiver. SMILES (Superconductor Submillimeter-wave Limb-emission Sounder) is proposed by the Communications Research Laboratory and the National Space Agency of Japan, with technical support from the National Astronomical Observatory, and with scientific support from the University of Bremen, in order to demonstrate the new sub-millimeter wave technology in space, and to conduct the measurements of limb-emission sounding for a group of molecular species profiles. In order to anticipate the performance of the instrument, retrieval simulations are carried out. Synthetic measurements, as will be recorded by the SMILES instrument, are generated by the use of a forward model. These are then inverted, using an inversion model, in order to derive the variables of interest, such as molecular species profiles (e.g., O3, ClO, HCl), atmospheric temperature profile, or a first order instrumental pointing correction (i.e., a pointing offset). The applied inversion algorithm is the Optimal Estimation Method (OEM). The advantage of the OEM is that it allows a formal error analysis needed for a general error characterization of retrieval performance. The error analysis takes into consideration the total statistical error, the measurement error, the vertical altitude resolution, and the correlation between the retrieved quantities. The altitude domain of a good measurement sensibility is defined by the measurement response.