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| Titel |
Cloud top height retrieval using the imaging polarimeter (3MI) top-of-atmosphere reflectance measurements in the oxygen absorption band |
| VerfasserIn |
Alexander Kokhanovsky, Rose Munro |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250123384
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| Publikation (Nr.) |
 EGU/EGU2016-2625.pdf |
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| Zusammenfassung |
| The determination of cloud top height from a satellite has a number of applications both for
climate studies and aviation safety. A great variety of methods are applied using both active
and passive observation systems in the optical and microwave spectral regions. One of the
most popular methods with good spatial coverage is based on the measurement of outgoing
radiation in the spectral range where oxygen strongly absorbs incoming solar light. Clouds
shield tropospheric oxygen reducing the depth of the corresponding absorption line as
detected by a satellite instrument. Radiative transfer models are used to connect the solar light
reflectance, e.g., in the oxygen A-band located around 761nm, and the cloud top
height. The inverse problem is then solved e.g. using look-up tables, to determine the
cloud top height. In this paper we propose a new fast and robust oxygen A-band
method for the retrieval of cloud altitude using the Multi-viewing Multi-channel
Multi-polarization Imaging instrument (3MI) on board the EUMETSAT Polar System Second
Generation (EPS-SG). The 3MI measures the intensity at the wavelengths of 410,
443, 490, 555, 670, 763, 765, 865, 910, 1370, 1650, and 2130nm, and (for selected
channels) the second and third Stokes vector components which allows the degree of
linear polarization and the polarization orientation angle of reflected solar light to be
derived at up to 14 observation angles. The instrument response function (to a first
approximation) can be modelled by a Gaussian distribution with the full width at half
maximum (FWHM) equal to 20nm for all channels except 765nm, 865nm, 1370nm,
1650nm, and 2130nm, where it is equal to 40nm. The FWHM at 763nm (the oxygen
A-band location) is equal to 10nm. The following 3MI channels are used in the
retrieval procedure: 670, 763, and 865nm. The channels at 670 and 865 nm are
not affected by the oxygen absorption. The channel at 763nm is affected by the
oxygen concentration vertical profile. The higher clouds will screen the oxygen below
them and have larger value of ratios x=R(763nm)/r(763nm) as compared to the
lower clouds. Here r(763nm) is the value of reflectance at 763nm for the artificial
atmosphere free of oxygen. This value can be derived by the linear extrapolation of
reflectances at 670 and 865nm as measured by 3MI over ocean. Over land this technique
requires a modification.We assume that the top of atmosphere reflectance in the
oxygen A-band can be presented in the following way: R(763nm)=r(763nm)T,
where the transmittance term can be derived using the radiative transfer theory.
The algorithm is validated using synthetic 3MI measurements. It has been found
that the retrieval technique can be used for clouds with cloud top heights in the
range 0.5-10km and cloud optical thicknesses above 5.0. For lower cloud optical
thicknesses, the cloud is inhomogeneous on a scale of the 3MI pixel (4x4km) and
the assumption used in the calculation of the transmittance term T (a horizontally
homogeneous cloud layer) is no longer valid. The technique can also be applied
to heavy aerosol events such as dust outbreaks, smoke, and volcanic eruptions. |
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