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| Titel |
Greenhouse Gas Profiles Retrieval in Cloudy Air from Combined IR-Laser and Microwave Occultation Measurements |
| VerfasserIn |
Veronika Proschek, Gottfried Kirchengast, Susanne Schweitzer |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056542
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| Zusammenfassung |
| ACCURATE–Climate Benchmark Profiling of Greenhouse Gases and Thermodynamic
Variables and Wind from Space is a new climate satellite concept, which enables
simultaneous measurement of greenhouse gases (GHGs), 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
LEO-LEO microwave occultation (LMO) technique.
The LIO uses near-monochromatic signals in the short-wave infrared range (~2-2.5 μm).
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. Accurate temperature, pressure, and humidity profiles, including accurate
altitude levels, are derived from simultaneously measured LMO signals and serve as essential
pre-information for the retrieval of the GHG profiles. These LMO signals reside at
frequencies within 13-23 GHz and, optionally, 175-195 GHz. The current mission design is
arranged for the measurement of six GHGs (H2O, CO2, CH4, N2O, O3, CO) and four
isotopes (13CO2, C18OO, HDO, H218O, the latter two optional), with focus on
the upper troposphere/lower stratosphere (UTLS, 5-35Â km). The GHG retrieval
can be performed in clear and cloudy air, retrieving complete GHG profiles for
no and weak cloudiness (like sub-visible cirrus) and cloud-gaps-interpolated or
cloud-top-limited GHG profiles for broken clouds or vertically extended clouds,
respectively.
In this presentation we introduce the algorithm to retrieve GHG profiles under cloudy
air conditions from quasi-realistic forward-simulated intensities of LIO signals
and thermodynamic profiles and altitude retrieved in a preceding step from LMO
signals. At the core of the GHG retrieval methodology is the differencing of two
LIO transmission signals, one being GHG-sensitive at a target absorption line and
one being a close-by reference outside of any absorption lines (reference channel).
The reference signal is used to remove atmospheric “broadband” effects by the
“differential transmission” approach. The key preparatory step of the GHG retrieval in
cloudy air is to produce a cloud layering profile, flagging level by level the degree of
cloud influence from zero (no relevant cloudiness) to unity (opaque cloudiness).
For this purpose a reference channel, where transmission is essentially unity, is
employed to estimate such a profile from using the channel’s transmission profile to
assign cloud flags from zero (< 3 dB loss) to unity (> 15 dB loss, cloud blocking).
More than one reference channel can be used for cross-checks and robustness of
this estimation. The differential transmission profiles are then interpolated over
gaps of limited extend (typically < 1 km) with non-zero flagged levels (broken
cloudiness) and cut off at the top of vertically extended non-zero flagged levels (thick
cloudiness), respectively. Subsequently the retrieval proceeds as in clear air, i.e., an Abel
transform converts differential transmission profiles to absorption coefficient profiles,
followed by retrieving GHG volume mixing ratios taking into account the absorption
cross sections under the temperature-pressure conditions provided by the LMO
profiles.
The accuracy of retrieved GHG profiles is found better than 1% to 4% for single profiles
in the UTLS region outside clouds, and the profiles are essentially unbiased. The associated
cloud layering profile provides information on potential gap-interpolations and on cloud-top
altitude, respectively. The methodology shows promising prospects for accurate GHG
monitoring based on the combined LIO and LMO technique, including through broken
cloudiness. |
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