Radar Refractivity Measurements How and Why

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Radar Refractivity Measurements How and Why?

• Jeff Keeler Frédéric Fabry
• NCAR/ATD McGill University
• Boulder, Co Montréal, Canada

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IHOP Objectives

• Science How much does knowing 4-D H2O
  distribution assist in QPF and modeling?
• Instrumentation How can we best measure H2O,
  especially vapor?
• S-Pol How accurately can we estimate moisture
  from refractivity measurements?

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Refractive Index and Meteorology
N Refractivity near the surface N (n 1)
106 where n is the index of refraction N
f(e, T, P) N hence changes as air mass
properties (moisture and temperature) change.
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Radar Measurements of Refractive Index
4pf r n c
Target phase ?(r) 2pf ttravel(r)
For fixed targets, phase will change as n changes.
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Convection Regeneration Example
Convective line forms, decays, and reforms on
radar-detected moisture gradient
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S-Pol at IHOP near Liberal, KS
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IHOP Ground Targets at S-Pol
Liberal
KS OK

• Ground targets were plentiful -- many power
  phone lines, elevators, farms,

Hooker
Beaver
Beaver R. valley
Homestead
OK TX
Booker
Perryton
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Real Time Display Example
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Diurnal cycle (mostly)
Wet
Dry
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IHOP Examples Dry-Line Genesis
7º
11º
P3
Broad Td gradient Hints of convergence
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IHOP Examples Dry-Line Genesis
2051
P3
Tight gradient moving
Tightening gradient
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IHOP Examples Dry-Line Evolution
As dry line moves west
To join back after
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On Fine Lines and N-Field Gradients

• Not all fine lines are visible in refractivity
• Some convergence regions have little N
  gradients
• Some convergence lines occur above nocturnal
  stable layer.

Cold front
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On N-Field Gradients Without Echoes
Action expected from the west of S-Pol nothing
in the south/east.
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On N-Field Gradients Without Echoes
Insect highway no special meaning
Broad N gradient
South winds as in weather map
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Examples Boundary Layer Processes
Rain accumulation two days before next event
foggy next day
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Examples Boundary Layer Processes
Sunny, cool, calm morning will warm quickly
Anomalous propagation echoes
No wind (!) Maximizes local effects
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IHOP Examples Night-Time Bore
Gust front
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Final Example Initiation Null Case
Moisture gradient on fine line
Three lines of Cu clouds
Moist band
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The evening moisture mismatch mystery

• Evaluate the representativeness and accuracy of
  refractivity-derived moisture measurements.

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What Next?

• Evaluate refractivity-derived moisture
  representativeness, accuracy, utility
• Analysis of the many IHOP events
• Data assimilation at meso-?/? scale
• Consider for WSR-88D ORDA product