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| Titel |
Raindrop size distribution variability estimated using ensemble statistics |
| VerfasserIn |
C. R. Williams, K. S. Gage |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 2 ; Nr. 27, no. 2 (2009-02-04), S.555-567 |
| Datensatznummer |
250016395
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| Publikation (Nr.) |
 copernicus.org/angeo-27-555-2009.pdf |
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| Zusammenfassung |
Before radar estimates of the raindrop size distribution
(DSD) can be assimilated into numerical weather prediction models, the DSD
estimate must also include an uncertainty estimate. Ensemble statistics are
based on using the same observations as inputs into several different models
with the spread in the outputs providing an uncertainty estimate. In this
study, Doppler velocity spectra from collocated vertically pointing
profiling radars operating at 50 and 920 MHz were the input data for 42
different DSD retrieval models. The DSD retrieval models were perturbations
of seven different DSD models (including exponential and gamma functions),
two different inverse modeling methodologies (convolution or deconvolution),
and three different cost functions (two spectral and one moment cost
functions).
Two rain events near Darwin, Australia, were analyzed in this study
producing 26 725 independent ensembles of mass-weighted mean raindrop
diameter Dm and rain rate R. The mean and the standard deviation
(indicated by the symbols <x> and σx) of
Dm and R were estimated for each ensemble. For small ranges of <Dm>
or <R>, histograms of σDm and
σR were found to be asymmetric, which prevented Gaussian
statistics from being used to describe the uncertainties. Therefore, 10, 50,
and 90 percentiles of σDm and σR were used to
describe the uncertainties for small intervals of <Dm> or
<R>. The smallest Dm uncertainty occurred for <Dm> between 0.8 and 1.8 mm with the 90th and 50th
percentiles being less than 0.15 and 0.11 mm, which correspond to relative
errors of less than 20% and 15%, respectively. The uncertainty
increased for smaller and larger <Dm> values. The
uncertainty of R increased with <R>. While the 90th
percentile uncertainty approached 0.6 mm h−1 for a 2 mm h−1 rain
rate (30% relative error), the median uncertainty was less than 0.15 mm h−1
at the same rain rate (less than 8% relative error). This study
addresses retrieval error and does not attempt to quantify absolute or
representativeness errors. |
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