Areal homogeneity of ZRrelations

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Areal homogeneity of Z-R-relations

• Gerhard Peters, Bernd Fischer, Marco Clemens
• Meteorological Institute
• University of Hamburg

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• 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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• 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.

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• 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.

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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.
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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.
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• 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.

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Scanning X-Band Radar
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• Radar calibration procedure

rain gauge
MRR1
raw dsd1
raw rain rate1
cal. dsd1
cal. Z1
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MRR-XMWR comparison after calibration, 30 s
averages
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• Evidence of modes of Z-R-relations

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

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

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

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• Space correlation of adapted Z-R-relations

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

Remote impact of adapted Z-R-relations (Z-R-relati
on, adapted for station 2 and applied to station
15 in 6km distance)
Events
15 Sept. 25 Oct. 2005
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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.