Lysbilde 1

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Beam Propagation modelling and blockage
correction in the Nordic Weather Radar Network
(NORDRAD) Uta Gjertsen, met.no, Norway
Günther Haase, SMHI, Sweden Joan Bech,
Meteocat, Spain Heikki Pohjola, FMI,
Finland ERAD, Barcelona, 18.9.-
22.9.2006

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Structure

• The NORDMET/NORA Beam Propagation Project
• Examples from Korpoo, Hemse, Røst
• Anaprop modelling
• Beam blockage correction for standard conditions,
  example Bømlo
• Summary blockage correction
• Conclusions

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NORDMET NORA
Beam Propagation Project
Background Issues concerning the beam
propagation are of great importance for the
quality of radar products. Objectives
Coordinate the work carried out in the Nordic
countries to define common algorithms. Address
typical cold climate challenges like beam
propagation in inversion situations, shallow
precipitation, snow Deliverables beam
blockage correction, anaprop information
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Radars Hemse, Sweden and
Korpoo, Finland 4.7.2006
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Radar Røst, 6 July 2005 00 UTC
Simulated height of the lower beam limit
and beam blockage HIRLAM11 analysis at the
radarsite
Simulated height of the lower beam limit
and beam blockage radiosonde Bodo
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Questions

• What is normal beam propagation in the Nordic
  countries?
• Is the BPM a useful tool? And what are the
  limitations?
• Radiosonde or NWP model data for refractivity
  profiles?
• Should anaprop be considered when correcting
  precipitation estimates for beam blockages?
• When and where is anaprop most likely?

Refractivity
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ANAPROP variability

• Data from 2003 and 2004 Norwegian Radio Soundings
• Shows only surface ducts

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Summary ANAPROP

• BPM produces reasonable output with radiosondes
  and HIRLAM
• Radiosondes are not always representative for the
  conditions at the radar site, HIRLAM might be
  better
• HIRLAM 6-hour forecast is good for the cases
  investigated
• Potential for forecasting sea clutter, 1-dim.
  HIRLAM
• Refractivity shows a latitude-dependent pattern

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Correction field for precipitation
(BBbeam blockage, ydistance center beam -
topography, aradius radar beam cross section)
(bcorrcorrection factor for precipitation,
bMarshall-Palmer b coefficient)
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Radar Bømlo


Correction filed
Corrected precipitation
Uncorrected precipitation

-
Corrected precipitation
Uncorrected precipitation
Corrected areas
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Blockage correction and VPR
G/R bias as a function of distance for 3 winter
months at radar Bømlo
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Summary blockage correction

• The blockage correction reduces bias and scatter
• Average bias reduction is 2.6 dB at ranges 40 to
  160 km and blockage between 50-70
• Correction fields for standard conditions are
  good enough
• Considering a decrease of reflectivity with
  height within the measurement volume improves the
  results

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Conclusions
The BPM gives realistic output for normal
conditions and in cases with anaprop What is the
most realistic profile of temperature, humidity
and pressure at the radar site? Correction
fields produced by BPM reduce the gauge/radar
bias and scatter The improvement is dependent on
the distance from the radar and the degree of
blockage -gt consider to correct for inhomogenous
beam filling? The blockage correction for
standard conditions should be implemented in the
Nordic countries There is also a potential for
sea clutter forecasting and radar site selection
assessment (presentation by G. Haase)
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Thank you for your attention!

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