Problems in interpreting radar images

Echoes not related to hydrometeors:

• Reflection of microwaves from ground or water surface, radar mirage
• Birds, insects, chaff from airplanes
• Solid targets, such as masts and buildings
• Other microwave sources, such as the sun and other radars
• Second trip echoes

Echoes from solid targets, birds, and such is called clutter. The weather radar software performs clutter cancellation that removes clutter without destroying hydrometeor data.

Echo intensity differences not related to differences in precipitation intensity:

• Calibration issues:
• Beam blocking: buildings near the antenna can partially or totally shade echoes from a certain direction.
• Attenuation in rain: echoes behind heavy rain can disappear, especially if the wavelength is small. Attenuation correction can help in situations where the echo is too weak, but it cannot correct a situation where there is no echo.
• Water phase: ice, melting ice, and water scatter microwaves differently. Bright band can be 7 dBZ.
• Drop size: especially melting hail. 2 mm (0.08 in) droplet = 64 of 1 mm (0.04 in) droplets.

Even if the measurement is correct, there is not a 100 % correlation with a gage:

• Far from the antenna, the beam is high above the earth and can overshoot a precipitating cloud. The rain may evaporate below the measurement height. Orographical enhancement can occur.
• In cold winters, the difference between measurement height and surface can be 10 dB. Vertical reflectivity profile correction can help to fix quantitative errors, but if there is no measurement the correction does not help.