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| Titel |
ACCURATE: Greenhouse Gas Profiles Retrieval from Combined IR-Laser and Microwave Occultation Measurements |
| VerfasserIn |
Veronika Proschek, Gottfried Kirchengast, Susanne Schweitzer, Johannes Fritzer |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044349
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| Zusammenfassung |
| The new climate satellite concept ACCURATE (Atmospheric Climate and Chemistry in the
UTLS Region And climate Trends Explorer) enables simultaneous measurement of profiles
of greenhouse gases, isotopes, wind and thermodynamic variables from Low Earth Orbit
(LEO) satellites. The measurement principle applied is a combination of the novel LEO-LEO
infrared laser occultation (LIO) technique and the already better studied LEO-LEO
microwave occultation (LMO) technique. Resulting occultation events are evenly distributed
around the world, have high vertical resolution and accuracy and are stable over long time
periods.
The LIO uses near-monochromatic signals in the short-wave infrared range (~2-2.5 μm
for ACCURATE). These signals are absorbed by various trace species in the Earth’s
atmosphere. Profiles of the concentration of the absorbing species can be derived from signal
transmission measurements. Accurately known temperature, pressure and humidity profiles
derived from simultaneously measured LMO signals are essential pre-information for the
retrieval of the trace species profiles. These LMO signals lie in the microwave band region
from 17-23 GHz and, optionally, 178-195 GHz. The current ACCURATE mission design is
arranged for the measurement of six greenhouse gases (GHG) (H2O, CO2, CH4, N2O, O3,
CO) and four isotopes (13CO2, C18OO, HDO, H218O), with focus on the upper
troposphere/lower stratosphere region (UTLS, 5-35 km). Wind speed in line-of-sight can be
derived from a line-symmetric transmission difference which is caused by wind-induced
Doppler shift. By-products are information on cloud layering, aerosol extinction, and
scintillation strength.
We introduce the methodology to retrieve GHG profiles from quasi-realistic
forward-simulated intensities of LIO signals and thermodynamic profiles retrieved
in a preceding step from LMO signals. Key of the retrieval methodology is the
differencing of two LIO transmission signals, one being GHG sensitive on a target
absorption line and one being a close-by reference outside of any absorption lines. The
reference signal is used to remove atmospheric broadband" effects by this differential
absorption" approach. Refractivity and impact parameter of the LIO signals, needed for
the retrieval, can be computed from the LMO-derived thermodynamic profiles.
An Abel Transform converts the differential LIO log-transmission profile to the
absorption coefficient. Based on the absorption coefficient and the absorption cross
section of the GHG under investigation, that can as well be computed from the
LMO-derived profiles, the number density profile or volume mixing ratio of the desired
GHG can be finally derived. When using several LIO signals, best sensitive to the
same GHG at different heights, a joint optimal GHG profile can be constructed by
combining the individual profiles in an inverse-variance-weighted manner (practically
used for H2O, obtained from 3-4 signals, and for CO2, obtained from 2 isotope
signals).
The thermodynamic parameters (temperature, pressure and humidity) derived
from LMO as basis for the LIO retrieval are found to be accurate to better than
0.5 K for temperature, 0.2% for pressure, and 10% for humidity. The accuracy of
retrieved trace species profiles is found better than 1% to 4% for single profiles in the
UTLS region (outside clouds which block infrared) and the profiles are essentially
unbiased (biases |
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