Radar Issues

Francis J. Merceret

NASA/YA-D

12 November 2002

Issues and Instruments

• Beam Filling• Scan Strategy• Attenuation

– Wet Radome

– Intervening Precipitation

WSR-88D (NEXRAD)

• 10 cm

• Doppler

• NWS/MLB

WSR-74C

• 5 cm

• PAFB

Beam Filling – Radar Characteristics

WSR-88D WSR-74C

Beam Width (degrees)

0.95 1.6

Pulse Length (Km)

0.47 0.9

Beam Filling – Equivalent Attenuation, WSR-88D

0.5 1 2 4

10 0 0 0 0

20 0 0 0 0

50 -2.2 0 0 0

100 -5.2 -2.2 0 0

150 -7 -4 -0.9 0

Eff. Attn. as function of feature size (Km) and range (Km)

SizeRange

Beam Filling – Equivalent Attenuation, WSR-74C

0.5 1 2 4

10 0 0 0 0

20 -0.5 0 0 0

50 -4.5 -1.4 0 0

100 -7.5 -4.5 -1.4 0

150 -9.2 -6.2 -3.2 -0.2

Eff. Attn. as function of feature size (Km) and range (Km)

SizeRange

Scan Strategy – WSR-88D

ENSCO, Inc.

Applied Meteorology Unit

USAFNOAA

NASA

®

3/28/94

ENSCO, Inc.

Slide 9F

NEXRAD / McGill Inter-Evaluation

0 20 40 60 80 100 120 140 160 180 2000

5000

10000

15000

20000

25000

Altitu

de

(m)

Horizontal Range(km)

Radar Beam CoverageWSR-88D VCP 11

Number of Scans: 14

Beam Width: 0.95 degrees

Elevation Angles

0.48°

1.45°

2.42°

3.34°

4.31°

5.23°

6.20°

7.51°

8.70°

10.02°

12.00°

14.02°

16.70°

19.51°

Scan Strategy – WSR-74C (old)

0

5000

10000

15000

20000

25000

30000

35000

0

5000

10000

15000

20000

25000

30000

35000

Radar Beam Coverage: WSR-74C (present scan)

26 20 16 13 10 7.5 5.0Elevation Angles

4.0

3.0

2.0

0.4

1.0

0 10 20 30 40 50 60

Horizontal Range (nm)

SLC 17A SLC 39B

Scan Strategy – WSR-74C (new)

0

5000

10000

15000

20000

25000

30000

35000

0

5000

10000

15000

20000

25000

30000

35000

Radar Beam Coverage: WSR-74C (modified scan #2)

26 22.4 19.1 16.1 13.4 10.9 8.6 6.6

4 .8

3.2

1.8

0.4

Elevation Angles

0 10 20 30 40 50 60

Horizontal Range (nm)

SLC 17A SLC 39B

Wet Radome Attenuation - Methodology

• Literature search (print and electronic)

• Compile into single database

• Fit empirical formula: L=C*R*tanh2(F/10)

R = rainfall rate (mm/hr)

F = frequency (GHz)

C = 0.165 (standard), 0.0575 (hydrophobic)

L = two-way loss (dB)

Wet Radome Attenuation - Results

Rain Rate (mm/Hr)

S-BandHydrophobic

S-BandStandard

C-BandHydrophobic

C-BandStandard

1 0.01 0.03 0.03 0.10

2 0.02 0.06 0.07 0.19

5 0.05 0.14 0.17 0.48

10 0.1 0.28 0.33 0.95

20 0.2 0.56 0.66 1.9

50 0.49 1.4 1.66 4.8

100 0.98 2.8 3.32 9.5

200 1.95 5.6 6.63 19

Wet Radome Attenuation – Example: 24 June 2001

WSR-88D

WSR-74C

Precipitation Attenuation - Methodology

• Literature search for relationships among R(mm/hr), Z(mm6/m3) and M(g/m3)

• Stratiform rain, convective rain, snow, and Marshall-Palmer precipitation types

• Select worst case relationship and worst case type (worst case = highest predicted attenuation for given measured Z)

• Empirical Model: A(dB/Km)=a*10^(b*dBZ) where a and b are wavelength-dependent constants

Precipitation Attenuation - Results

Worst Case Two Way Attenuation vs Reflectivity

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

0 10 20 30 40 50 60 70

Reflectivity (dBZ)

Att

enua

tion

(dB

/Km

)

10 cm 5 cm 10 cm model 5 cm model

Conclusions

• Beam filling is rarely a concern• Each radar has scan pattern gaps that might

be significant in a particular location, but use of both radars together can mitigate this

• Wet radome attenuation is a major problem for the WSR-74C, but not for the WSR-88D

• Precipitation attenuation can be a major problem for quantitative measurements