Detection of NearSurface RainSnow Boundaries with a Polarimetric Radar

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• Detection of Near-Surface Rain-Snow Boundaries
  with a Polarimetric Radar
• by
• Aldo Bellon and Isztar Zawadzki

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• There is considerable overlap in the polarimetric
  characteristics of rain and of
• snow in (Z-Zdr) and (Z-?hv) planes.
• A pixel-by-pixel identification is therefore not
  possible.
• The polarimetric characteristics of the melting
  layer, whether horizontally
• stratified as in a regular bright band or
  vertically elongated across a
• synoptic front, are distinct from stratiform rain
  and from snow.
• (Lower ?hv and higher Zdr than in rain and snow)
• Therefore, rain-snow delineation is contingent
  upon the reliable identification
• of the melting layer (Ryshkov et al. 2005).
• - Unlike a regular BB, frontal rain-snow
  boundaries are advected across
• the radar coverage. A 3-D examination (vertical
  profiles) should
• distinguish the rain from the snow region.

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X-band Vertically Pointing Radar (Collocated with
the scanning radar)
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UHF PROFILER (30 km east of the scanning radar)
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Reflectivity VPR (30 to 50 km) from scanning
radar superimposed with VAD winds
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530
830
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730
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730
830
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430 to 1030
710 to 910
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809 Z 14-Jan-05
0.7 Refl PPI
0.7 Zdr PPI
0.7 Pid PPI
0.7 RHOhv PPI
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809 Z 14-Jan-05
Refl
Zdr
Pid
RHOhv
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818 Z 14-Jan-05
Refl
Zdr
RHOhv
Pid
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Melting Index (MIX) Function (Zdr, ?hv)
Thresholds for identifying wet-snow pixels
Zdr thresh gt (0.7 to 1.0 db) ?hv thresh lt
(0.90 to 0.95)
Limits Zdr up (2.0 to 3.0) db ?hv low
(0.60 to 0.80)
Zdr range (Zdr up - Zdr thresh) ?hv range
( ?hv thresh ?hv low)
Zdr (contr) (Zdr-Zdr low)/Zdr range(1/p)
?hv (contr) (?hv up -?hv)/?hv range(1/p)
MIX100.0Zdr(contr) ?hv(contr) If measured
Zdr and ?hv are each half way their range,
then MIX25, 50, 63 and 71 for p1, 2, 3 and 4
respectively
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0.7 Zdr PPI
0.7 Refl PPI
0.7 Mix PPI
0.7 ?hv PPI
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Zdr RHOhv 0.5 km CAPPIs 0809 Z
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Zdr RHOhv 0.8 km CAPPIs 0809 Z
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Zdr RHOhv 1.1 km CAPPIs 0809 Z
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Zdr RHOhv 1.5 km CAPPIs 0809 Z
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Zdr RHOhv 2.0 km CAPPIs 0809 Z
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Zdr RHOhv 2.5 km CAPPIs 0809 Z
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Zdr RHOhv 3.0 km CAPPIs 0809 Z
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Zdr RHOhv 3.5 km CAPPIs 0809 Z
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Rain-Snow Boundary Map Indicator
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Local VPRs (20 km by 20 deg) every 10 km in
range (Within 60 km, azimuth spacing depends on
range)
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BB aloft
BB at ground
Zoom of LVPR map
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Rain and Snow Region Identification from
Reflectivity LVPRs
Rain Detection of a BB signature Reflectivity
peak above the lowest trusted layer followed by
a significant drop both in the snow above and
in the rain below it (Peak and drop
user-selectable and range and intensity
adjusted) Snow a) Reflectivity peak in lowest
(or two lowest) layer (or just above a ground
echo, but lt 1 km) with no BB signature from LVPRs
or from polarimetry. But very strong gradient
declares low BB OR b) Gradient in the
lowest layer must exceed at least 1 or 2 dBZ/km.
(discards constant profiles associated with
rain). Detection of a significant ive
gradient nullifies (a)
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Rain-Snow Boundary Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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BB Bottom
BB Top
BB Thickness
BB characteristics from 3-D polarization
measurements
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BB Thickness
BB Bottom
704 Z
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BB Thickness
BB Bottom
724 Z
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BB Thickness
BB Bottom
744 Z
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BB Thickness
BB Bottom
804 Z
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BB Thickness
BB Bottom
824 Z
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BB Thickness
BB Bottom
844 Z
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BB Thickness
BB Bottom
904 Z
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BB Thickness
BB Bottom
924 Z
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BB Thickness
BB Bottom
944 Z
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Zdr LVPR
RHOhv LVPR
704 Z
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Zdr LVPR
RHOhv LVPR
754 Z
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Zdr LVPR
RHOhv LVPR
844 Z
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Zdr LVPR
RHOhv LVPR
929 Z
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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UHF DATA for 23-DEC-2004 NOT AVAILABLE
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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Rain-Snow Map Indicator
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2030
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Rain-Snow Map Indicator
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CONCLUSIONS

• Near-surface rain-snow boundaries have been
  detected and seen to advect
• over considerable distances ( gt 100 km) and/or
  over several hours.
• The identification of the rain/snow regions can
  be made with moderate success
• - the rain region is easier to confirm because
  of the presence of a BB
• aloft (from polarimetry and/or LVPRs)
• - the confirmation of a snow region from the
  reflectivity gradient
• typically associated with snow profiles is
  more problematical.
• - General trends are observed in the polarimetric
  variables before and after a
• frontal passage (ie., in the rain and snow
  regions), but a more robust
• and automatic distinction is limited by
  considerable overlap.
• The numerous parameters for identification of a
  melting region from
• polarimetry, or from reflectivity LVPRs, and of
  the snow region
• from reflectivity gradients are to be optimized.
• -Validation with surface observations/disdrometic
  data needs to be done.