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- Miao, J., et al.
(författare)
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The potential of polarization measurements from space at mm and sub-mm wavelengths for determining cirrus cloud parameters
- 2003
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 3:1, s. 39-48
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Tidskriftsartikel (refereegranskat)abstract
- The millimeter and sub-millimeter waves have been attracting a lot of attention recently in the cloud remote sensing community. This is largely because of their potential use in measuring cirrus cloud parameters with airborne or space-borne radiometers. In this study, we examine the possibility of using polarization measurements in this frequency range to get information on the microphysical properties of cirrus clouds. By using a simple radiative transfer model, we calculated the brightness temperature differences at the vertical and horizontal polarization channels for the following seven frequencies: 90, 157, 220, 340, 463, 683, and 874 GHz. The ice crystals in cirrus clouds are modeled with nearly spherical particles, circular cylinder, and circular plate, as well as with mixtures of these types. We found that the polarization difference signal shows a unique "resonance'' feature with the change of ice particle characteristic size: it has a strong response only in a certain range of ice particle size, beyond that range it approaches zero. The size range where this resonance happens depends to a large extent on particle shape and aspect ratio, but to a much less extent on particle orientation. This resonance feature appears even when ice clouds are composed of a mixture of ice crystals in different shapes, although the magnitude and the position of the resonance peak may change, depending on how the mixture is made. Oriented particles generally show larger polarization difference than randomly oriented ones, and plates have larger polarization difference than cylinders. However, the state of particle orientation has a significantly stronger effect on the polarization difference than the particle shape (cylinder or plate). This makes it difficult to distinguish particle shapes using millimeter and sub-millimeter radiometric measurements, if there is no information available on particle orientations. However, if the state of particle shape mixture can be predetermined by other approaches, polarization measurements can help to determine ice particle characteristic size and orientation. This information, in turn, will benefit our retrieval of the ice water path of cirrus clouds.
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- Verdes, C., et al.
(författare)
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Pointing and temperature retrieval from millimeter-submillimeter limb soundings
- 2002
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:16, s. 4299-
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Tidskriftsartikel (refereegranskat)abstract
- Passive microwave limb sounding instruments like the Millimeter-Wave Atmospheric Sounder (MAS) or the Microwave Limb Sounder (MLS) observe dedicated oxygen lines for the derivation of temperature and pointing information, since these quantities are essential for the quality of the retrieval of the trace gas mixing ratio. Emission lines of oxygen are chosen because the volume mixing ratio (VMR) profile is known. In this paper, we demonstrate the capabilities of a new and innovative method by means of which accurate temperature and pointing information can be gathered from other strong spectral features like ozone lines, without including accurate knowledge of the VMR profile of these species. For this purpose, retrievals from two observation bands with a bandwidth of about 10 GHz each, one including an oxygen line, have been compared. A full error analysis was performed with respect to critical instrument and model parameters, such as uncertainties in the antenna pattern, calibration uncertainties, random pointing error, baseline ripples, baseline discontinuities, and spectroscopic parameters. The applied inversion algorithm was the optimal estimation method. For the selected scenario and instrumental specifications we find that the retrieval of a pointing offset and the atmospheric temperature profile can be achieved with a good accuracy. The retrieval precision of the pointing offset is better than 24 m. The retrieval precision of the temperature profile is better than 2 K for altitudes ranging from 10 to 40 km. Systematic errors (due to model parameter uncertainties) are somewhat larger than these purely statistical errors. Investigations carried out for different atmospheric states or different instrumental specifications show similar results.
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