Rain rate calculations are presented in meteorological books and manuals.
For applications in microwave propagation, real calculations are quite difficult due to the following:
- accumulated precipitation water is measured by rain gauges of different designs. Trying to use time derivatives never works, and "our water (causing additional loss)" never falls into a particular rain gauge.
- trying to use radar is very uncertain as rain rate causing certain reflectivity is an exponent in an uncertain Z function.
- The only real way to obtain loss over time must be measured over one year to get good statistical distribution. FRom this distribution curve one can estimate short-time rain effect.
- Real precipitations are very local; rain cells vary in size and form, terrain takes part as well as wind.
To conclude: rain rate is difficult to obtain, mostly for shorter time intervals. Using it to get microwave loss makes not much sense. Best is to use a radio link for long-time observation, then estimate short-time effects if needed.