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| Titel |
Development of a cloud-screening method for MAX-DOAS measurements |
| VerfasserIn |
Clio Gielen, Michel Van Roozendael, Francois Hendrik, Caroline Fayt, Christian Hermans, Gaia Pinardi, Tim Vlemmix |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078329
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| Zusammenfassung |
| In recent years, ground-based multi-axis differential absorption spectroscopy (MAX-DOAS)
has shown to be ideally suited for the retrieval of tropospheric trace gases and deriving
information on the aerosol properties. These measurements are invaluable to our
understanding of the physics and chemistry of the atmospheric system, and the impact on the
Earth’s climate.
Unfortunately, MAX-DOAS measurements are often performed under (partially) cloudy
conditions, causing data quality degradation and higher uncertainties on the retrievals. A high
aerosol load and/or a strong cloud cover can introduce additional photon absorption or
multiple scattering. The first effect strongly impacts the retrieved differential slant columns
(DSCDs) of the trace gases, leading to an underestimation of the atmospheric column
density. Multiple scattering, on the other hand, becomes important for low clouds
with a high optical depth, and cause a strong increase in the retrieved trace gas
DSCDs.
The presence of thin clouds can furthermore introduce a degeneracy in the retrieved
aerosol optical depth, since they will have similar effect on the MAX-DOAS measurements.
In this case, only information on the trace gas DSCDs can be successfully retrieved. If the
cloud cover consists of broken or scattered clouds, the MAX-DOAS method becomes very
unstable, since the different elevation angels will probe regions of the sky with strongly
deviating properties.
Here we present a method to qualify the sky and cloud conditions, using the colour index
and O4 DSCDs, as derived from the MAX-DOAS measurements. The colour index is defined
as the ratio of the intensities at the short- and long-wavelength part of the visible spectral
range, typically at 400 nm and 670 nm. For increasing optical thickness due to clouds or
aerosols, the colour index values decrease and values for different elevation angles converge.
In the case of broken clouds, the colour index shows a strong and rapid temporal variation,
which is easily detectable.
Additional information is derived from the O4 DSCD measurements, since they are quite
sensitive to the change of the light paths due to scattering at different altitudes. For
example, thick clouds at low altitude show a very strong increase in the DSCD
values due to scattering, combined with a low colour index value due to the intensity
screening.
In general, our method shows promising results to qualify the sky and cloud conditions of
MAX- DOAS measurements, without the need for other external cloud-detection systems
such as Brewer instruments or pyrheliometers. |
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