METR 2413 HOMEWORK ASSIGNMENT #4

DUE Wednesday February 11, 2004

This homework assignment is designed to familiarize you with radar technology.


1. What is the Doppler effect, who was it named after, and why?

The Doppler effect is an apparent change in the frequency or wavelength of a wave, such as sound or EM radiation, when the source of the wave and the observer are moving relative to one another. The wavelength decreases when the source and observer are approaching each other, and increases when they are moving away from each other. The Doppler effect is named for the Austrian scientist Christian Doppler (1803-1853) who used sound waves to demonstrate this effect.

2. What percentage of time does a radar spend transmitting, and what percentage of time does a radar spend ‘listening’?

For NEXRAD radars in normal operation, the PRT is 1 millisec and the transmit time is 1.57 microsec, so the time transmitting is 0.157% and the time listening is 99.843%.

3. If the reflectivity (Z) determined by a WSR-88D is 100,000 mm^6 m^-3, what is the reflectivity in terms of dBZ values?

dbZ = 50

4. Use the dBZ value computed in (3) to estimate the rainfall over a 2 hour period. If you were a forecaster for the National Weather Service, and the estimated rainfall that you computed fell in Norman over a 2-hour period, would you be worried? Why or why not?

This dbZ value corresponds to a rainfall rate of about 2.5 inches/hour or a 5 in accumulation in 2 hours. This rainfall rate over two hours is likely to lead to flooding, so warnings may be required.

5. What is the wavelength of the SMART-R mobile Doppler radar? Why is the wavelength (longer/shorter) on the SMART-R radar than the wavelength of the WSR-88D?

The wavelength of the SMART-R radar is about 5 cm, whereas for the WSR-88D radars it is about 10.7cm. The wavelength is shorter because this allows a smaller receiving antenna to be used to receive the reflected signals.