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| Titel |
The microphysical information content of polarimetric radar measurements in the melting layer |
| VerfasserIn |
Silke Troemel, Alexander V. Ryzhkov, Pengfei Zhang, Clemens Simmer |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093894
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| Publikation (Nr.) |
 EGU/EGU2014-9069.pdf |
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| Zusammenfassung |
| The practical utilization of the backscatter differential phase δ, measured by polarimetric
weather radars, is not well explored yet. δ is defined as the difference between the phases of
horizontally and vertically polarized components of the wave caused by backscattering from
objects within the radar resolution volume. δ bears important information about the
dominant size of raindrops and wet snowflakes in the melting layer. The backscatter
differential phase, which is immune to attenuation, partial beam blockage, and
radar miscalibration, would complement the information routinely available from
reflectivity ZH, differential reflectivity ZDR, and cross-correlation coefficient Ïhv which
are traditionally used for characterizing microphysical properties of the melting
layer.
Actual measurements of δ have been performed with a number of polarimetric WSR-88D
radars operated at S band in US. Similar observations of δ were made in Germany using
research X band radars in Bonn (BoXPol) and Jülich (JüXPol). Contrary to our expectations
δgbservations at S band showed much higher magnitudes than the δ observations at X band.
Maximal observed δ at X band is 8.5°, whereas maximal observed δ at S band is 40°.
Model simulations which assume spheroidal shapes for melting snowflakes in the absence of
aggregation within the melting layer yield much lower values of δ than observed, especially
at S band. According to simulations of δ the simulated values of δ are relatively
small and barely exceed 4° at X, C, and S bands. Indeed, the simulations assume
that mixed-phase particles do not interact with each other and wet snowflakes do
not aggregate. Taking aggregation into account in the model the magnitude of δ
can be significantly higher. The huge observed δ magnitudes at S band ranging
from 18 to 40°, however, are impressive and unexpected at first. Since all X band
observations are from Germany and all S band observations taken into account
are from the U.S., part of this effect may be attributed to the climate difference
between the U.S. and Germany. Thus, dual frequency observations of δ in the same
storm have been included to verify the unexpected high δ observations at larger
wavelengths. Measurements from C band radars from the German Weather Service
network show again δ of 30° and more, while the overlapping research X band
radars provide δ values around 5°. Similar dual frequency observations will be
performed with C band scanning ARM precipitation radars and WSR-88D S band
radars.
Theoretical simulations using a two-layer T-matrix code are used to examine conditions
which may favor more intense aggregation within the melting layer and explain the origin of
observed pronounced signatures at S and C bands. To simulate these δ magnitudes the
presence of very large water-coated snowflakes with diameters exceeding 1 cm has to be
assumed.
The important information about microphysical properties, aggregation processes and
growth of snowflakes within the melting layer in all polarimetric radar variables will be
elaborated and presented. |
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