Back to basics: Measuring rainfall at sea: Part 1 - In situ sensors
It has previously been shown that the scintillations produced by raindrops falling through a collimated laser beam can be used to measure the drop-size distribution and the rainfall rate, both averaged over the path. We now present a theoretical analysis, verified by observation, showing that the variance of the scintillations detected by a line-detector measured at frequencies near 1 kHz is closely related to rain rate and is nearly independent of drop-size distribution. If only rain rate is desired, the variance type of optical rain gauge has several advantages over the earlier model. It could use a diverging beam, thus eliminating the practical difficulties of maintaining adjustment and pointing of a collimated beam. Furthermore, it is less sensitive to the presence of updrafts and downdrafts along the beam and can thus be used over rough terrain.