Birger Schimpf

Robust and Efficient Inversion of Vertical Sounding Atmospheric High Resolution Spectra by Means of Regularization

Retrieval of atmospheric temperature or constituent profiles from vertical sounding spectroscopic measurements is an ill-posed problem, and additional information has to be introduced in the inversion process in order to compute meaningful solutions. Modern mathematical techniques are shown to be suitable for an analysis of these problems and the actual inversion to retrieve atmospheric profiles. A stable and efficient numerical implementation of Phillips-Tikhonov regularization techniques is discussed; generalized singular value decomposition is the appropriate tool to compute the formal solution of the modified minimization problem, and the L-curve permits determination of the optimum balance between information from measurement and side constraints. It is also shown that these techniques can provide further insight in the basic ill-posed nature of the inverse problem, give tools for the diagnostics of the retrieved profiles, and allow a discussion of the relation to other standard retrieval techniques, especially optimal estimation. The methods are demonstrated on examples for retrieval of ozone and hydroxyl profiles from simulated far infrared high-resolution spectra. A comparison with optimal estimation retrieval is performed by a preliminary analysis of a Spitsbergen millimeter-wave spectrum of ozone.

The 2.5 THz heterodyne spectrometer THOMAS: Measurement of OH in the middle atmosphere and comparison with photochemical model results

The interpretation of recent odd hydrogen measurements in the stratosphere from balloons and in the mesosphere from space indicates a serious lack of understanding in atmospheric HOx chemistry. In order to resolve these persisting problems, coincident measurements of HOx molecules and/or measurements that cover both altitude regions are desirable. In this work, the airborne 2.5THz heterodyne spectrometer Terahertz OH Measurement Airborne Sounder (THOMAS) is introduced. Since the first THOMAS measurements in 1994/1995, the spectrometer was significantly improved by modification or replacement of individual components. The THOMAS instrumental setup and properties are presented together with a retrieval algorithm for atmospheric parameters based on a Phillips-Tikhonov regularization scheme. Furthermore, the results of a complete error assessment are given. In August 1997, during the second CRISTA/MAHRSI campaign (Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere / Middle Atmosphere High Resolution Spectrograph Investigation), OH observations were performed by THOMAS covering altitudes between about 30 and 90 km over a full diurnal cycle. Hydroxyl column densities derived from THOMAS measurements are presented and compared to photochemical model results. The model calculations using the standard HOx chemistry systematically show higher values by about 15% for the 40–90 km and 50–90 km OH columns. Moreover, a recently proposed change of an HOx chemistry reaction rate is included into the comparison which, for the same altitude intervals, yields OH column densities that are about 10% lower than the THOMAS measurements. A detailed comparison of the THOMAS and MAHRSI measurements is presented in a seperate publication [Englert et al., 2000].

Extension of a Fourier-Transform-Spectrometer for Atmospheric Absorption and Emission Measurements

The German Aerospace Center (DLR) Oberpaffenhofen/Germany operates a high resolution Fourier-Transform-Spectrometer (Bruker IFS 120 HR). The photomectric characterzied instrument was primarily used for precision measurements of trace gases in the laboratory (spectral resolution ≥ 0.001cm-1). The presented extension allows emission and absorption measurements in the atmosphere. Compared to absorption measurements emission measurements are independent of extraterrestrial sources (sun, moon), thus, making the instrument more versatile. Our instrument is not intended to be used for operational atmospheric measurements. It will mainly be used for developing new data products and methods, exploiting the synergy from absorption/emission measurements, laboratory spectroscopy an FT instrumentation expertise. In the paper the instrumental setup is shown and first measurements are presented.