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

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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)

Typical Values

AWIPS Characteristics

Reflectivity Correlation Coefficient

8-bit (256 levels): 1 deg x 0.25 km4-bit (16 levels): 1 deg x 1.0 km

Menu Location of CC

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

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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

1. Degradation with Range / Low SNR

Reflectivity Correlation Coefficient

Noted by CC > 1.0

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)

Conclusion

• Contact Information

– dualpol_list@wdtb.noaa.gov