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| Titel |
Improved scattering radiative transfer for frozen hydrometeors at microwave frequencies |
| VerfasserIn |
A. J. Geer, F. Baordo |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 7, no. 6 ; Nr. 7, no. 6 (2014-06-25), S.1839-1860 |
| Datensatznummer |
250115828
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| Publikation (Nr.) |
 copernicus.org/amt-7-1839-2014.pdf |
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| Zusammenfassung |
| To simulate passive microwave radiances in all-sky conditions requires better
knowledge of the scattering properties of frozen hydrometeors. Typically,
snow particles are represented as spheres and their scattering properties are
calculated using Mie theory, but this is unrealistic and, particularly in
deep-convective areas, it produces too much scattering in mid-frequencies
(e.g. 30–50 GHz) and too little scattering at high frequencies (e.g.
150–183 GHz). These problems make it hard to assimilate microwave
observations in numerical weather prediction (NWP) models, particularly in
situations where scattering effects are most important, such as over land
surfaces or in moisture sounding channels. Using the discrete dipole
approximation to compute scattering properties, more accurate results can be
generated by modelling frozen particles as ice rosettes or simplified
snowflakes, though hexagonal plates and columns often give worse results than
Mie spheres. To objectively decide on the best particle shape (and size
distribution) this study uses global forecast departures from an NWP system
(e.g. observation minus forecast differences) to indicate the quality of
agreement between model and observations. It is easy to improve results in
one situation but worsen them in others, so a rigorous method is needed: four
different statistics are checked; these statistics are required to stay the
same or improve in all channels between 10 GHz and 183 GHz and in all
weather situations globally. The optimal choice of snow particle shape and
size distribution is better across all frequencies and all weather
conditions, giving confidence in its physical realism. Compared to the Mie
sphere, most of the systematic error is removed and departure statistics are
improved by 10 to 60%. However, this improvement is achieved with a simple
"one-size-fits-all" shape for snow; there is little additional benefit in
choosing the particle shape according to the precipitation type. These
developments have improved the accuracy of scattering radiative transfer
sufficiently that microwave all-sky assimilation is being extended to land
surfaces, to higher frequencies and to sounding channels. |
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