The Operational Benefits of having Zero Degree Elevation and RHI Scans for the Washington Coastal Radar

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The Operational Benefits of having Zero Degree
Elevation and RHI Scans for the Washington
Coastal Radar
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The need to optimize the radar for a coastal
region with terrain

• Although a one-size fits all approach to radar
  acquisition and use has certain administrative
  advantages, to do so for the new Washington coast
  radar would fail to best meet the forecast
  challenges of the region
• An area ravaged by intense and rapidly evolving
  cyclones (as they approach and interact with the
  coast and mountains). These storms approach from
  the data sparse Pacific Ocean and bring deadly
  hurricane force winds, heavy precipitation, and
  devastating floods.
• Critical storm structures (coastal jets,
  low-level jets, frontal-waves, etc.) are
  typically in the lowest levels with many of them
  occurring below 10kft.
• Critical precipitation production levels and
  structures are also typically below 10kft. Rare
  severe convection is also low-topped.
• Much of the winter is spent with the melting
  level between 2 and 5 kft and highly variable in
  space and time. Determining its height and
  evolution is critical to precipitation type and
  flood forecasting. A shift up or down of a 1 kft
  can make the difference between no flooding and
  major flooding.

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To meet these special regional challenges it is
critical that the radar include

• a zero degree elevation angle scan as part of the
  scanning strategy
• to extend the useable data coverage as far
  offshore as possible
• to provide the longest possible warning time of
  approaching storms and information regarding
  internal low-level structures and features
• to get detailed low-level information regarding
  precipitation structures and flow over/around the
  coastal terrain.
• the ability to complete RHI scans (interleafed
  between required PPI scans) along key azimuths
• to accurately place the rain/snow level in the
  areas of complex terrain
• to get detailed vertical cross sections of
  precipitation structures and flow over the
  terrain and to show the structure of approaching
  weather systems
• To accurately define the bright band/melting
  level height for input into the storm and river
  forecast processes
• To view low-level convective storm structure for
  areas of rotation, wind and heavy precipitation.
  The typical, albeit rare, severe thunderstorm in
  the PNW is very shallow and rarely sampled by
  more than one or two PPI scans with most beams
  overshooting the cells.

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Zero Degree Scan Strategy

• A zero degree scan angle allows far greater
  horizontal range, in this case allowing the radar
  to view the lower atmosphere much farther
  offshore.
• Concerns about sea clutter were not found to be
  serious when zero-degree scans were used at
  Westport during the IMPROVE-1 project, using a
  radar very similar to the proposed (S-band,
  dual-polarization).

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Zero Degree Scans Proved Effective and Useful
During the IMPROVE Experiment

• During Improve 1 (Jan-Feb 2001), the zero degree
  angle scan provided valuable information far out
  over the Pacific Ocean.

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Distance-Height Diagram for Zero Degree and
Standard 88-D Elevation Angles
VCP 21 example Beamwidth 0.93 deg The legend
indicates the elevation angles Solid (dashed)
lines indicate the center (edges) of the beam
No coverage at .5 degree
Note the absence of low-level coverage of the
standard .5 degree elevation angle at greater
distances (more than 100 km out). The zero
degree angle provides a major improvement in
coverage at long ranges in the critical lower
atmosphere.
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Most of the proposed sites have a good view
seaward at zero degrees
Example Saddle Hill
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A 0 degree scan allows viewing of significant
precipitation features earlier (farther offshore)
and can show shallow features overshot by the 0.5
degree scansThis has substantial operational
implications
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An example of the substantial superiority of 0.0
degree elevation angle over 0.5 degree for seeing
important features approaching from the
Pacific(Following images are from the SPOL
radar at Westport, WA, during the IMPROVE field
experiment)
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Zero Degree Heavy Rain Band Obvious
Offshore0.5 Degree Not there.
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Even close in major differences0.5 cant see
heavy low level rain
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Why RHI scans are important in the western U.S.
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Necessity of RHI scans

• Provides far more vertical resolution because
• The radar continuously scans through elevation
  angles.
• There is no need for interpolation steps from
  horizontal or conical planes to elevations in the
  cross section.
• Does not have the problem of gaps between
  scanning levels that is inherent in synthetic
  PPI-based approaches.

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There are a number of reasons why RHI scans are
of great value in the western U.S. and why the
new WA coastal radar should make use of them

• The vertical resolution with which the bright
  band (BB) and other structures can be determined
  from PPIs (even with the use of dual
  polarization) is limited compared to true RHIs.
• Determining the level of the bright band
  (BB)/melting level is critical for hydrologic
  diagnosis and prediction in the orographic west.
• In this region of relatively low freezing levels,
  BB contamination is a problem for QPE. Detailed
  BB identification with the best possible
  resolution i.e., that provided by RHI scans, will
  facilitate improving the QPE quality.

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RHI Scans Have Been Used to Great Advantage in
Field Programs Such as IMPROVE and MAP to
determine microphysical processes over terrain
and bright band levels.
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Synthetic RHIs do not allow forecasters to gain a
clear picture of the vertical structures of
important weather features over terrain and
elsewhere
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An Example of a Synthetic RHI scan Toward the
Cascades using the Camano Island WSR-88D

• Note the coarse, broken nature of the cross
  section, particularly aloftand this is for a
  close-in section!

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The Contrast with A True RHI is Substantial
More Structure and Detail, No Gaps
Example from IMPROVE 1 SPOL at Westport looking
to the NE.
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More examples of True RHIs
Example from IMPROVE 1 SPOL at Westport looking
to the W
Example from IMPROVE 1 SPOL at Westport looking
to the E
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A Future Hydrometeorological Testbed

• There has been substantial discussion of the
  Olympic Mountains and western Washington serving
  as a future hydrometeorological testbed for the
  NWS and the research community.
• A radar capable of operational RHI scans is
  required for such application.
• The proposed new scanning approaches can be
  evaluated in Washington for use throughout the
  NWS Western Region.

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In addition to the clear operational advantages,
there are scientific benefits

• Such capabilities will help provide the backbone
  of a hydrometeorological research testbed that
  could be centered on the Olympic Peninsula.
• This testbed could play an important role in
  improving precipitation estimates and forecasts
  over the region.
• The fine scale vertical structure of the
  precipitation field revealed by RHI scans will be
  essential for validating and calibrating
  high-resolution models that explicitly represent
  the microphysics.

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Bottom Line

• Zero degree scans allow forecasters to see
  considerably farther offshore and to view
  low-level structures missed by 0.5 degree scans.
• RHI scans greatly enhance the ability of
  forecasts to diagnose precipitation over complex
  terrainwhich is critical in this region, and to
  see detailed structures of incoming weather
  systems.
• Both operations and operations-oriented research
  will greatly benefit from both capabilities.

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Radar Specifications

• It is essential that the specifications for the
  new coastal radar include both zero degree
  elevation scans and RHI as operational
  capabilities. I.e., the design metrics should
  allow for repeated stopping/starting PPI scans to
  complete interleaved RHI scans with acceptable
  wear.
• A zero degree scanning strategy should be
  included from the beginning.
• A hybrid PPI/RHI scanning strategy should be
  developed by a joint NWS/University team, using
  data from previous research experiments (IMPROVE)
  as a testbed.
• This strategy can be perfected when the new radar
  is installed.