Correlation Coefficient (CC) Dual Polarization Pre-Deployment Operational Assessment Warning Decision Training Branch (WDTB) Radar Operations Center (ROC)
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Correlation Coefficient (CC)
Dual Polarization Pre-Deployment Operational Assessment
Warning Decision Training Branch (WDTB)Radar Operations Center (ROC)
Objectives
• After completion of this module, you should be able to identify specific characteristics of Correlation Coefficient (CC) including:
– Definition– Applications (CC implies….)– Limitations (Watch out for….)
Correlation Coefficient (CC)
2/122/12
*
vvhh
hhvv
SS
SSCC
Definition Possible Range of
Values
Units Abbreviated Name
Measure of how similarly the horizontally and vertically
polarized pulses are behaving within a pulse volume
0.2 to 1.05 None CC (AWIPS)RHO (GR Analyst)
ρHV (Literature)
Physical Interpretation
Non-Meteorological (birds, insects, etc.)
Metr (Non-Uniform)(hail, melting snow, etc.)
Metr (Uniform)(rain, snow, etc.)
Shapes are complex and highly variable. Horizontal and vertical pulses will behave very differently with these objects
Shapes can be complex and are mixed phase. Horizontal and vertical pulses behave somewhat differently with these objects
Shapes are fairly simple and do not vary much. Horizontal and vertical pulses behave very similarly with these objects
Low CC (< 0.9) Moderate CC (0.85 to 0.95) High CC (> 0.97)
AWIPS Characteristics
Reflectivity Correlation Coefficient
8-bit (256 levels): 1 deg x 0.25 km4-bit (16 levels): 1 deg x 1.0 km
Operational Applications
• Identification of:
1. Meteorological vs Non-meteorological Echoes
2. Melting Layer
3. Rain vs. Snow
4. Large Hail
5. Irregular Hydrometeor Shapes
6. Tornadic Debris
7. Quality of other polarimetric variables
1. Meteorological vs. Non-Meteorological Echoes
Reflectivity Correlation Coefficient
• Meteorological echoes: > 0.9• Non-meteorological echoes :<< 0.85
2. Melting Layer
Reflectivity Correlation Coefficient
• Bright band not always visible• Shows up as a ring of low correlation coefficient
3. Rain vs. Snow
Reflectivity Correlation Coefficient
Transition from high to low CC marks the rain/snow transition line
4. Giant Hail
Reflectivity Correlation Coefficient
• Large hail = > 2.0 inches in diameter• Reduction in CC due to Mie scattering effects (0.8 to 0.93)
5. Irregularly Shaped Hail
• Complex shape results in complex scattering
– Reduction in CC
Photo courtesy: NWS WFO Hastings
6. Tornadic Debris
Reflectivity / SRM Correlation Coefficient
• CC drops significantly in debris field seen on radar• Collocated with a velocity couplet
7. Quality of Other Polarimetric Variables
• CC < 0.95– Other variables
affected• CC behaving well?
– CC > 0.98 in pure rain
10.99
0.98
0.970000000000001
0.960000000000001
0.950000000000001
0.9400000000000010.93
0.920.910.9
0.890000000000001
0.880000000000001
0.870000000000004
0.860000000000001
0.8500000000000010
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Correlation Coefficient (CC)
Stan
dard
Dev
iatio
n of
ZDR
(dB)
Factors to consider when looking at CC
1. Degradation with range
2. Mixture of hydrometeors
3. Non-uniform Beam Filling (NBF)
4. Range folding in batch cuts
2. Mixture of Hydrometeors
• CC decreases– Lowest when relative contributions to the signal are
equal (e.g. equal contributions from rain and snow)
Higher CC Lower CC
Rain/Hail Rain/SnowRain/Hail Rain/Snow
3. Non-Uniform Beam Filling (NBF)
Line of Storms Melting Layer
Significant gradients in reflectivity in vertical or horizontal
4. Range Folding in Batch Cuts
Velocity Correlation Coefficient
Range folding may obscure features in CC in the batch cuts
Summary
• Definition
– Measure of how similarly the horizontally and vertically polarized pulses are behaving in a pulse volume
• Operationally
– Can provide additional clues to help identify features more easily (i.e. meteorological vs. non-meteorological echoes)
• Limitations
– Certain situations can degrade the quality of CC (i.e. non-uniform beam filling)
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