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| Titel |
Profiling Wind and Greenhouse Gases by Infrared-laser Occultation: Algorithm and Results from Simulations in Windy Air |
| VerfasserIn |
Andreas Plach, Veronika Proschek, Gottfried Kirchengast |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250086573
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| Publikation (Nr.) |
 EGU/EGU2014-465.pdf |
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| Zusammenfassung |
| We employ the Low Earth Orbit (LEO-LEO) microwave and infrared-laser occultation
(LMIO) method to derive a full set of thermodynamic state variables from microwave signals
and climate benchmark profiling of greenhouse gases (GHGs) and line-of-sight (l.o.s.) wind
using infrared-laser signals. The focus lies on the upper troposphere/lower stratosphere region
(UTLS - 5 km to 35 km). The GHG retrieval errors are generally smaller than 1% to 3%
r.m.s., at a vertical resolution of about 1km.
In this study we focus on the infrared-laser part of LMIO, where we introduce a new,
advanced wind retrieval algorithm to derive accurate l.o.s. wind profiles. The wind retrieval
uses the reasonable assumption of the wind blowing along spherical shells (horizontal
winds) and therefore the l.o.s. wind speed can be retrieved by using an Abel integral
transform.
A “delta-differential transmission” principle is applied to two thoroughly selected
infrared-laser signals placed at the wings of the highly symmetric C18OO absorption line
(nominally ±0.004 cm-1 from the line center near 4767 cm-1) plus a related “off-line”
reference signal. The delta-differential transmission obtained by differencing these signals is
clear from atmospheric broadband effects and is proportional to the wind-induced Doppler
shift; it serves as the integrand of the Abel transform.
The Doppler frequency shift calculated along with the wind retrieval is in turn also used
in the GHG retrieval to correct the frequency of GHG-sensitive infrared-laser signals for the
wind-induced Doppler shift, which enables improved GHG estimation. This step therefore
provides the capability to correct potential wind-induced residual errors of the GHG retrieval
in case of strong winds.
We performed end-to-end simulations to test the performance of the new retrieval in
windy air. The simulations used realistic atmospheric conditions (thermodynamic state
variables and wind profiles) from an analysis field of the European Centre for Medium-Range
Weather Forecasts (ECMWF). GHG profiles were taken from the Fast Atmospheric Signature
Code (FASCODE) model. Three geographic regions were investigated, representing three
different atmospheric conditions: Tropics (TRO) - a warm and moist atmosphere, Standard
(STD) - an intermediate atmosphere at mid-latitudes, and Sub-Arctic Winter (SAW) - a cold
and dry atmosphere. We will discuss the results in windy air, which show an encouraging
performance both for the wind retrieval throughout the stratosphere (essentially unbiased
l.o.s. winds with rms errors within 2 m/s over about 15 to 35 km) and for the GHG
estimation. |
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