Radomes:

Do they Matter?

Rob Thompson

Effects on Polarisation

• I have previously shown that the Thurnham radome effects Zdr

Happens everywhere!

recently submitted paper Tabary, Boumahmoud, Andrieu, Thompson, Illingworth, Le Bouar & Testud

Evaluation of two "integrated" polarimetric Quantitative Precipitation Estimation (QPE) algorithms at C-band TRAPPES RADAR (near Paris)

Average differential reflectivity along azimuth. The colour represents the elevation

Italian radar

Radome effects Z

• The radome will also affect Z

• Generally radomes must attenuate the beam

• Water on the radome (when raining) will act as an attenuator (and therefore emitter) – Dropping the apparent Z on the ray

Radome Wetting

• Villarini & Krajewski (2010) Review of the Different Sources of Uncertainty in Single Polarization Radar-Based Estimates of Rainfall • According to Austin (1987), the radome attenuation is significant only for wavelengths smaller than or equal to 5 cm and negligible for wavelengths as long as 10 cm (e.g., WSR-88D radars).

• Germann (1999) found that the two-way radome attenuation for a C-band radar can be up to 5.4 dB in moderate rainfall, resulting in the measurement of only 43% of the rain.

• Generally radome wetting is ignored!

How big is it?

• Kurri & Huuskonen (2008) Measurements of the Transmission Loss of a Radome at Different Rain Intensities

– Lab experiments with a radome panel

– Artificial rain (a hose) – Consider wet and dry effects – Scale results to a full radome

Kurri & Huuskonen

• Use C-band wavelengths • Consider attenuation a function of rainfall intensity

• 2 methods to calculate radome loss – Calculation from permittivity –Measured via C-band horns

• Examined “maintenance” effects

Kurri & Huuskonen

• Calculated from permittivity

• DRY LOSS 0.34 ± 0.26 dB Remember this is ONE-WAY

Calculated one-way transmission loss of a 6.7m diameter

radome as function of the rain intensity. Transmission loss is

calculated at 5.65 GHz

Kurri & Huuskonen

• Measurements: • Don’t plot errorbars, but state them in the paper (~0.7dB)

• SO for radar attenuation, suggesting 2dB at 5mm/hr

• Cleaning makes marginal difference

• Waxing made 1.5dB difference at 22.1mm/hr (but not scalable to full radome)

• Anderson (1975) 15 months at 20 GHz. Anderson concluded that the hydrophobic property of the radome surface will degrade with time. After 6 months, the transmission loss at a rain intensity of 10 mm h-1 had risen by about 6 dB.

Wet radome measurements performed with the dirty and cleaned radome scaled to be valid for a 6.7-m radome. The solid line presents the theoretical transmission loss based on Gibble’s formula. Measurements and calculations are carried out at a water temperature of 30.5°C

Measurement in the field

• Manz, Hardwerker, Loffler-Mang, Hannesen and Gysi – Radome influence on weather radar systems with emphasis to rain effects

• Microwave source at a few km range

• Radar receiver signal monitored.

Manz et al

• Remember this is ONE-WAY

• Lines show manufacturers claim

Comparison

Shameless graphics

• They pretty much agree • Radome wetting is probably fairly important

Comparison

Even Wetting?

• These papers all assume the wetting to be even across the radome

• Is that realistic? –Would there be more wetting on the side facing the wind?

10:00

17:00

RAIN PASSING PREDANNACK

Jan 10th

EMISSION at 0.5o

Lowest (smoothed) emission

This is the emission caused by

the radome Assuming

even wetting

Wind is from the South-West

RAIN PASSING PREDANNACK

Jan 10th

EMISSION at 3.9o

1.5dB

~8mm/hr

~5mm/hr

Is this wetting on the side facing the wind?

Rain isn’t the only problematic

weather!

Summary

• Radome effects on polarisation parameters are well known

• Wet radome attenuation is not well understood and (until recently?!) immeasurable – But may be a large effect – 2dB at 5mm/hr – Cleaning radomes doesn’t help

• Waxing does, but needs to be regularly repeated

• Is radome attenuation even over the radome?