Dirk Offermann

Trace gas densities and dynamics at and above the tropopause as derived from CRISTA data

The CRISTA system is highly flexible as regards the location where the measurements are taken. High data densities can be obtained by controlling the view directions of the three IR telescopes. This is used for detailed validation of CRISTA H2O measurements at 12 km by means of an airplane experiment (FISH). The data are also used to determine water vapor variability at this altitude at midlatitudes. High data density allows detailed analysis of trace gas and temperature fields in the Indonesian region (Hawk Eye measurement mode). Pronounced small and medium-scale structures are found here at various altitudes (12 - 45 km). Considerable coupling of these structures is indicated and deserves further analysis.

Infrared limb sounding measurements of middle-atmosphere gravity waves by CRISTA

We consider the example of the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment to deduce the sensitivity of an infrared emission limb sounder to gravity waves of different horizontal and vertical wavelength. The sensitivity studies show that gravity waves with horizontal wavelengths of the order of 100-200 km or longer can be detected. The deduced sensitivity factors are validated by comparing CRISTA and data sonde temperature profiles. Analysis of CRISTA temperature data reveals large gravity wave amplitudes in the stratosphere over southernmost South America. The horizontal structure is compared to model calculations. Global distributions are discussed with respect to convective sources, wind modulation, and Coriolis force modulation. It is shown that even the very dense spatial sampling of the CRISTA instrument is insufficient to fully resolve the horizontal structure of the waves which are seen in the vertical. Hence, increased spatial resolution of about 50 X 50 km or better is required to obtain all information the limb sounding technique is capable to provide.

Calibration procedures and correction of detector signal relaxations for the CRISTA infrared satellite instrument.

Remote sensing from space has become a common method for deriving geophysical parameters such as atmospheric temperature and composition. The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument was designed to sound the middle and the upper atmosphere (10-180 km) with high spatial resolution. Atmospheric IR emissions were measured with Si:Ga bulk or Si:As blocked impurity band detectors for a wavelength interval of 4-17 microm and Ge:Ga bulk detectors for 56-71 microm. An overview of the calibration of the instrument and the correction of detector signal relaxations for the Si:Ga detectors are given, both of which are necessary to provide high-quality IR radiance data as input for the retrieval of atmospheric temperature and trace gas mixing ratios. Laboratory and flight data are shown to demonstrate the quality of the results.

Detector signal relaxations and their correction: the Si:Ga bulk detectors of the CRISTA instrument

The CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument measured atmospheric trace gas emissions in the infrared using the limb scanning technique. For the first time three viewing directions were used by a satellite instrument in near-earth orbit (300 km) to obtain an unprecedented spatial density of the daily global measurement net. The high measurement speed needed for an enhanced horizontal resolution was achieved by cooling the instrument with supercritical and subcooled helium and by using Si:Ga bulk or Si:As blocked impurity band (BIB) detectors for the wavelength range 4-17 micrometers and Ge:Ga bulk detectors for longer wavelengths. The detectors were operated at temperatures between 2.5 and 13 Kelvin. Under these conditions the signal of the detectors shows non-stationary effects (relaxation effects) degrading measured spectra to some extent.These effects are difficult to account for as they can only be described by using at least 6 parameters depending on signal height and illumination history. In this paper an empirical model to correct the non-stationary effects of the Si:Ga detectors is presented. The model is based on measured signal responses after step-like illumination changes. Several tests using different data sets show that the model works well under various conditions.