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| Titel |
On the microwave optical properties of randomly oriented ice hydrometeors |
| VerfasserIn |
P. Eriksson, M. Jamali, J. Mendrok, S. A. Buehler |
| 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 ; 8, no. 5 ; Nr. 8, no. 5 (2015-05-05), S.1913-1933 |
| Datensatznummer |
250116352
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| Publikation (Nr.) |
 copernicus.org/amt-8-1913-2015.pdf |
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| Zusammenfassung |
| Microwave remote sensing is important for observing the mass of ice
hydrometeors. One of the main error sources of microwave ice mass retrievals
is that approximations around the shape of the particles are unavoidable. One
common approach to represent particles of irregular shape is the soft
particle approximation (SPA). We show that it is possible to define a SPA
that mimics mean optical particles of available reference data over narrow
frequency ranges, considering a single observation technique at the time,
but that
SPA does not work in a broader context. Most critically, the required air
fraction varies with frequency and application, as well as with particle
size. In addition, the air fraction matching established density
parameterisations results in far too soft particles, at least for frequencies
above 90 GHz. That is, alternatives to SPA must be found.
One alternative was recently presented by Geer and Baordo (2014). They used a
subset of the same reference data and simply selected as "shape model" the
particle type giving the best overall agreement with observations. We present
a way to perform the same selection of a representative particle shape but
without involving assumptions on particle size distribution and actual ice
mass contents. Only an assumption on the occurrence frequency of different
particle shapes is still required. Our analysis leads to the same selection
of representative shape as found by Geer and Baordo (2014). In addition, we
show that the selected particle shape has the desired properties at
higher frequencies as well as for radar applications.
Finally, we demonstrate that in this context the assumption on particle
shape is likely less critical when using mass equivalent diameter to
characterise particle size compared to using maximum dimension, but a better
understanding of the variability of size distributions is required to fully
characterise the advantage.
Further advancements on these subjects are presently difficult to achieve
due to a lack of reference data. One main problem is that most available
databases of precalculated optical properties assume completely random
particle orientation, while for certain conditions a horizontal alignment is
expected. In addition, the only database covering frequencies above
340 GHz has a poor representation of absorption as it is based on outdated
refractive index data as well as only covering particles having a maximum
dimension below 2 mm and a single temperature. |
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