Attenuation of the radar signal is a concern for the Patrick Air Force based WSR74C. There are 2 causes of attenuation.

1. Wet Radome: If the radome of the radar is in a precipitation storm, the water on the radome can attenuate the signal. descripton
2. Intervening Precip: If a strong strom exists in the space between the radar and a distant target the signal can be attenuated. descripton

Wet Radome

Frank Merceret wrote 2 reports on the subject of wet radome attenuation. The first ( Wet Radome Attenuation F. Merceret) discusses the results of a literature search. He then developed an empirical formula (from teh data in the literature) to determine the 2-way transmission loss as a function of rainrate and radara frequency. He concluded that the effects are small (< 1 dB) until 10 mm/hr is reached. For the 88D, which is a 10 cm radar with a hydrophobic radome, the effects are < 1 dB untill 100mm/hr and do not exceed 2 dB up to 200 mm/hr. The 74C, a 5 cm radar with a standard radome, will se 5 dB attenuation at 50 mm/hr.

The second report ( Wet Radome Recovery Time of PAFB WSR-74C F. Merceret, M. Brooks, J. Ward ) deals with recovery time once the 74C radome is wet. They compared teh 74C (which is subject to wet radome attenuation) with the 88D (attenuation effects due to wet radome are much smaller). The attenuation was plotted as a function of time and then modeled by an exponential function. Unfortunately they only had one case they could do quantitativly, but a qualitative look at the ABFM data showed agreement with the conclusions. The time constant of the exponential function was found to be < 9 minutes.
return

Attenuation due to Intervening Precipitation

Frank Merceret also prepared a spreadsheet of calculations to determine the attenuation due to intervening precipitation for the 74C (5cm) and 88D (10cm) radars. ( Attenuation by Intervening Precip by F. Merceret )

In summary:

• For reflectivities up to 30 dBZ the attenuation for a 5 cm radar is about 2x10^-2 dB/km. So at 50 km the attenuation would be 1 dB. The attenuation for a 10 cm radar at 30 dBZ is more like 3x10^-3 dB/km.
• at 50 dBZ teh 5 cm radar attenuation is 1 dB/km so at 50 km the signal if virtually all attenuated. The 10 cm, on the other hand has a 3x10^-2 dB/km attenuation or 1.5 dB at 50 km.

Fig 1: CAPPI showing "notch" of intervening precipitation attenuation for 74C.

By comparison, the 88D (10 cm radar) is only attenuated by 1.5dB under the same 50 dBZ and 50 km conditions. Fig 2 shows the CAPPI of the 88D at a comparable time.

Fig 2: CAPPI showing no "notch" of intervening precipitation attenuation for the 88D.

return

Cases of attenuation specific to ABFM.
Attenuation Periods of WSR74C
Examples of WSR74C Attenuation
Case Types and attenuation issues