J. Oberheide

The CRISTA‐2 mission

[1]xa0The second mission of the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment took place in August 1997. The experiment was flown aboard the ASTROnomical Shuttle PAllet Satellite (ASTRO-SPAS) free-flying platform launched by the NASA space shuttle. CRISTA analyzes the infrared radiation emitted by trace gases from the Earth limb in the altitude regime from the upper troposphere to the lower thermosphere. The main aim of CRISTA is to detect small-scale dynamically induced structures in the distribution of trace constituents in the middle atmosphere. The instrument is therefore equipped with three telescopes that simultaneously collect the infrared radiation from three different air volumes. The high spatial density of the measurement grid obtained during the first CRISTA mission in November 1994, as well as the latitudinal coverage, was considerably improved by making use of newly developed satellite pointing and maneuvering capabilities. The altitude coverage was extended to include the upper troposphere where water vapor distributions are analyzed. Dynamically induced features are observed in practically all trace gases and at various spatial scales. The smallest scales that could be analyzed on the basis of the CRISTA data set are well below 100 km. Compared to the first mission, much more emphasis was laid on measurements in the upper mesosphere and lower thermosphere—this was possible because of higher radiometric sensitivities in some channels. Atomic oxygen, carbon dioxide, and ozone densities are derived in the upper mesosphere and lower thermosphere. The mission conditions allowed the study of polar stratospheric clouds (PSC) over the Antarctic and of polar mesospheric clouds (PMC) at high northern latitudes. For the first time, summer high latitude mesopause temperatures were retrieved from CO2 15-μm spectra using a nonlocal thermodynamic equilibrium model. The derived temperatures compare well with a temperature climatology based on rocket soundings.

Tidal signatures in temperature data from CRISTA 1 mission

Temperature measurements in the stratosphere and mesosphere were taken during the first Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA 1) mission using CO2 emissions. These measurements range from 13 to close to 100 km, and individual temperature measurements have a precision of 1 K. The CRISTA orbit was circular at an inclination of 57°, so local time variations during the 7 day mission were small for a given latitude and orbit leg. Zonal averages of the data show significant structure in the vertical and as a function of latitude. Temperature differences between the zonal mean data from the ascending and descending portions of the orbit are of the form expected from the diurnal tide. The maximum zonal mean difference is ≈20 K and occurs over the equator at an altitude of 75 km. Zonal variations in the temperature difference indicate that the tidal amplitude is not uniform at all longitudes. At the equator the maximum amplitude (30 K) appears over the African sector and the minimum (10 K) over the Pacific sector. This variation is most likely a nonmigrating wavenumber 2 diurnal tide although other less plausible possibilities exist. To facilitate comparisons with model results the temperature variations are converted to equivalent vertical displacements, assuming the tidal motions are adiabatic. Such an approach is appropriate in the mesosphere where diurnal variations due to radiative effects are small and reduces the dependence of the measured temperature amplitude on the background temperature profile. Equivalent vertical displacements are also calculated using results from the global scale wave model. For the most part the model results and observations are in excellent qualitative and good quantitative agreement from the tropopause to the mesopause. However, the observed vertical wavelength is smaller, and its amplitude in terms of equivalent displacement is smaller in the upper mesosphere than that in the model. The decrease in amplitude is consistent with that expected from the transition from equinox to solstice conditions.

Stratospheric transport by planetary wave mixing as observed during CRISTA‐2

[1]xa0Planetary waves drive the mean meridional circulation of the stratosphere and at the same time facilitate quasi-horizontal mixing of trace gases. This paper presents significant day-to-day variability of stratospheric trace gas fields associated with large planetary wave activity observed during the second mission of the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment. Geopotential height data of the UK Met Office show that the CRISTA-2 observations in the Southern Hemisphere winter were made during a period of extremely large amplitudes of both wave-1 and wave-2. The planetary wave-1, usually a quasi-stationary feature, moved eastward with the traveling planetary wave-2. The large amplitudes of both wave-1 and wave-2 led to a significant displacement of the edge of the polar vortex toward the tropics (down to 30°S). As a result of the large wave amplitudes and favorable phase alignment, the anticyclone drawing up tropical air was unusually strong, and thus considerable wave-induced trace gas flux from the tropics toward midlatitudes was observed, mainly in the form of a pronounced planetary-scale tongue advected out of the tropics around the vortex and into the anticyclone. Quantitative transport calculations based on a sequential data assimilation system highlight the importance of such transport events for trace gas eddy-flux in the Southern Hemisphere winter stratosphere.

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.

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.

New results from CRISTA

Trace gas distributions and temperatures in the mesosphere and lower thermosphere were derived from infrared spectra measured by the two CRISTA experiments flown in November 1994 and in August 1997. CRISTA (CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere) is a triple telescope cryogenically cooled infrared spectrometer which senses the Earth limb from a Shuttle orbit. The geographical coverage was -57°/+68° and -74°/+74° during the two missions, respectively. Each mission lasted slightly more than one week. The mesospheric set of trace gases include ozone, carbon dioxide and carbon monoxide, methane, water vapor, and atomic oxygen. In addition temperatures and pressures are obtained from the CO2 15 μm band. The temperature/pressure results are used to derive geostrophic wind fields. Most of the data reduction required non-LTE modelling of the radiation properties of the species. Practically all data exhibit considerable large scale structures in both latitude and longitude due to planetary waves or interhemispheric transport.