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Areal homogeneity of Z-R-relations

Areal homogeneity of Z-R-relations. Gerhard Peters, Bernd Fischer, Marco Clemens Meteorological Institute University of Hamburg. Introduction Approach Experimental setup Radar calibration procedure Evidence of modes of Z-R-relations Space correlation of adapted Z-R-relations Conclusions.

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Areal homogeneity of Z-R-relations

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  1. Areal homogeneity of Z-R-relations Gerhard Peters, Bernd Fischer, Marco Clemens Meteorological Institute University of Hamburg

  2. Introduction • Approach • Experimental setup • Radar calibration procedure • Evidence of modes of Z-R-relations • Space correlation of adapted Z-R-relations • Conclusions

  3. Introduction There is an unquestioned need for adapted Z-R-relations for improved Radar precipitation estimation. The success of using gage networks for adapting Z-R-relation parameters (pZR) depends on the pZR time-space correlation patterns, about which only little is known. pZR, as retrieved from in-situ/radar comparisons, are uncer-tain due to distance between in-situ sensor and radar volume and due to the small sampling volume of in-situ sensors. The averaging time required, to reduce the sampling errors below the real pZR variability, may be longer than the pZR decorrelation time.

  4. Approach: During the AQUARADAR-field-campaign-2005 vertically pointing Doppler radars were used to study decorrelation properties of pZR. Due to the sampling properties of these instruments some of the aforementioned difficulties are mitigated: The pDSD can be measured • within the weather radar sampling volume and • with the same time resolution as the weather radar.

  5. Space correlation: In this preliminary study only the space correlation aspect of pZR was considered. With respect to the time domain, we believed to begin with in the existence of “modes” of pZR. The term “mode” shall express that the pZR apparently show extended periods of persistence and a tendency of more or less sudden jumps. No attempt was made here to relate pZR to some physical pro-perties of rain processes, which is the objective of other AQUA-RADAR sub-projects, and will be considered in a later stage. Here sub-periods of the rain time-series were assigned subjec-tively to modes, which were characterized by the application of constant adapted values of pZR.

  6. Choice of mode length: As we had no precise definition of “modes” for the purpose of this study, the division into periods of constant “modes” was arbitrary to some extent. The qualitaive guidance was as follows: Very short mode-periods, would lead to perfectly adapted but nevertheless useless Z-R-relations, as only short space correlation can be expected. In the extreme case of one sample the adapted Z-R-method becomes identical with the DSD-based results. Very long mode-periods do not yield much improvement over the use of a fixed Z-R-relation, but this small improvement is expected to show a longer space correlation. We decided intuitively between “very long” and “very short” without having yet a criterion for the optimum choice.

  7. Experimental Setup: Close to the DWD observatory Lindenberg in NE-Germany a chain of 13 Micro Rain Radars (MRR) was set up spanning a distance of about 6 km in SW-NE-direction. The MRRs provided DSDs with 20 s time resolution and 100 m vertical resolution on vertical profiles extending up to 3 km altitude. The MRR chain was in the range of a X-band mini weather radar (X-MWR), a modified navigation radar with pencil beam antenna and digital data acquisition. The X-MWR elevation angle was fixed at 11° leading to altitudes of common MRR and X-MWR measuring volumes between 600 and 1000 m above ground, which was safely below the melting layer.

  8. Berlin 30 km Scanning x-band radar Line with 15 MRRs

  9. Scanning X-Band Radar

  10. Installation of one of 13 MRRs Rain Gauge

  11. XMWR raw ZX cal. ZX MRR2 cal. Z2 cal. dsd2 raw dsd2 raw Z2 MRR3 cal. Z3 cal. dsd3 raw dsd3 raw Z2 cal. Z15 cal. dsd15 raw dsd15 raw Z15 MRR15 • Radar calibration procedure: rain gauge MRR1 raw dsd1 raw rain rate1 cal. dsd1 cal. Z1

  12. MRR-XMWR comparison after calibration, 30 s averages

  13. Evidence of modes of Z-R-relations

  14. Evidence of modes of Z-R-relations cont.

  15. Evidence of modes of Z-R-relations cont.

  16. Evidence of modes of Z-R-relations cont.

  17. Space correlation of adapted Z-R-relations Local impact of adapted Z-R-relations Events 15 Sept. – 25 Oct. 2005

  18. Space correlation of adapted Z-R-relations cont. Remote impact of adapted Z-R-relations (Z-R-relation, adapted for station #2 and applied to station #15 in 6km distance) Events 15 Sept. – 25 Oct. 2005

  19. Conclusions • Z-R-relations, after piecewise adaptation in time ( modes), were demonstrated to show reduced scatter at 6 km distance from the reference point. • Far excursions from a fixed Z-R-relation were removed particularly efficiently. • A concept for optimum and automatic choice of modes needs still to be developed.

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