Contents preface (VII) introduction 1—37

Radar Measurement of Precipitation

In regions of difficult and inaccessible terrains, precipitation can be measured (within about 10% accuracy of the rain gauge measurements) with the help of a radar (radio detecting and ranging). A radar transmits a pulse of electromagnetic waves as a beam in a direction depending upon the position of the movable antenna. The wave travelling at a speed of light is partially reflected by cloud or precipitation particles and returns to the radar where it is received by the same antenna. The display of the magnitude of the energy of the returned wave on the radarscope (i.e., radar screen) is called an echo and its brightness is termed echo intensity. The duration between the transmission of the pulse and appearance of the echo on the radarscope is a measure of the distance (i.e., range) of the target from the radar. Direction of the target with respect to the radar is decided by the orientation of the antenna at the time the target signal is received. The echo is seen in polar coordinates. If there is no target (i.e., cloud or precipitation particles), the screen is dimly illuminated. A small target would appear as a bright point whereas an extended target (such as a rain shower) would appear as a bright patch. The radarscope being divided as per the coordinate system, the position of the target can be estimated. By having a proper calibration between the echo intensity and rainfall (or its intensity), one can estimate the rainfall (or rainfall intensity). The Indian Meteorological Department has a well-established radar network for the purpose of detecting thunderstorms besides a few cyclone-warning radars along the eastern coast of the country.

The wavelength of the electromagnetic waves transmitted by the meterological radars is in the range of 3 to 10 cm; the usual operating range being 5 cm (for light rains) to 10 cm (for heavy rains). The relationship among the characteristics of the waves and the rainfall intensity is represented by
P_r = CZ/r²

where, P_r is the average echo power, r is the distance from radar to target and C is a suitable constant. The radar echo factor Z is related to the intensity of rainfall I (in mm/hr) as

Z = aI^b
in which, a and b are numerical constants that are determined by calibrating the radar. One may, thus, obtain

I = [r²P_r /(aC)]^1/b

Present day developments in radar measurements of precipitation include on-line processing of the radar data and Doppler type radars for measuring the velocity and distribution of raindrops.