Rainfall estimation by BMRC CPol radar

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Rainfall estimation by BMRC C-Pol radar

• 1Lei Feng and 1,2Ben Jong-Dao Jou
• (??) (???) .
• 1National ST Center for Disaster Reduction
• 2National Taiwan University

ICMCS-V 2006.11.03
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Objectives

• To illustrate the ability of rainfall estimation
  using Areal R(FDP) and R(KDP) by BMRC C-Pol
  radar. Radar-Raingauge comparisons in three
  different sizes of area
• Multi-beam (Area 100 km2) Areal R(FDP)
• Single-beam (Area 25 km2) Areal R(FDP)
• Point (Area 2 km2) R(KDP)
• Try to correct the wind drift effect when
  comparing with single raingauge.

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Two Areal Rainfall schemes
Notice the difference Gate area ? with range ?
but NSSL scheme without area weighting
Keep the area weighting, but need FDP information
at each range gate
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BMRC C-Pol
rain gauge network
5 single-beams, the area of each beam is 25 km2
1 multi-beam, the area of each beam is 100 km2
18 raingauges, the radar coverage of each gauge
is 2 km2 (radius 0.8 km)
RD-69
In Darwin 18 rain gauges in the 10 x 10 km2 area
C-Pol radar at (0,0)
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Case A - 15 Jan 1999
Case A, Time series plot (100 km2)
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Case B - 01 Mar 1999
Case B, Time series plot (100 km2)
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Case C - 17 Mar 1999
Case C, Time series plot (100 km2)
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Multi-beam results

• Area size 100 km2
• Very high correlation coefficient 0.97
• Small standard deviation 1.99 mm/hr
• Little underestimate
• Sample number 108

case (ABC)
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Single-beam results

• Area size 25 km2
• High correlation coefficient 0.94
• Small standard deviation 3.43 mm/hr
• Little underestimate
• Sample number 590

case (ABC)
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Point results

• Area size 2 km2
• Low correlation coefficient 0.86
• Large standard deviation 6.38 mm/hr
• Under estimation
• Sample number 1091

case (ABC)

• The result is getting worse as the verification
  area getting smaller. Why ?

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Point Comparison Problems

• Inherence difference of the measurements Rain
  gauge accumulates continuously rainfall on a
  point while radar samples almost instantaneously
  a volume averaged rainfall rate.

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Can we correct the wind drift effect ?
Strong horizontal wind Overestimate or
Underestimate ?
from DLOC
How about the wind drift effect ?
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Optimal offset vector

• An area of radar data which covering all surface
  rain gauges is moved around the original point in
  a square window (8km x 8km) with 200 m interval
  in X and Y direction.
• The cross-correlation coefficient is calculated
  between the time lagged (1.5 minute) surface rain
  rates of the gauges and the space shifted radar
  rain rates.
• A two-dimensional correlation field is produced.
  The distance from the point of the maximum
  correlation to the original point was defined as
  optimal offset of the horizontal displacement.

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Optimal offset vector
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2 km
Only 39/89 volumes can be easily found out the
offset vectors, most of them are convective type
rain.
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Checking the optimal vector far from system
moving velocity case
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No wind drift correction
After wind drift correction
Case B
Case A
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after wind drift correctionBut significant
underestimation
No wind drift correction
If the coefficient of R(KDP) estimator increase
50, it look better. Can we do this change for
this case ?
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Rainfall with smaller raindrops need to use
higher coefficient in R(KDP) estimator
small Zdr small Do
big Zdr big Do
Adopted from L. D. Carey ATMO 689
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In case C, D0 is significant lower than case A
and B
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Storm motion
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Disdrometer observation
Volume median diameter D0 estimation
Radar estimation
Note the comparison here is not the same case,
but are similar squall line type precipitation in
Darwin.
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No wind drift correction
After wind drift correction
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summary (1)

• Its very important to consider the wind drift
  effect when doing single point radar-gauge
  comparison. In this study, the normalized error
  has 17 improvement.

Correlation Coefficient of radar-raingauge
comparison
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summary (2)

• Use the BMRC C-Pol radar phase base estimator to
  estimate rain rate is very accurate , especially
  on convective rainfall.
• For accurate rain rate estimation, it needs to
  consider the DSD variability such as stratiform
  rainfall, orographic rainfall, shallow convective
  warm rain and so on when using R(KDP) estimator.

Thanks !
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Optimal vector Finding, 0640 15-Jan-1999
(C-Pol at Darwin)
Lag 0 min
Lag 1 min
Lag 2 min
Lag 3 min
Lag 4 min
Lag 5 min