Raingauges are the gold standard of precipitation measurement. They are not about to be made redundant by radar, satellites, optical raingauges or disdrometers. They are the key instruments, the principal source of data. However, while not being a substitute for raingauges, satellite passive and active microwave sensors are extremely valuable additional tools, which, when combined with raingauge measurements, offer the most promising way forward. This chapter looks at what needs to be done to get the best out of both.
In considering the future, I will concentrate on the measurements required to obtain accurate global and regional means, trends and variability of precipitation through the twenty-first century along with an estimate of extremes. These measurements are needed not just in order to follow climate change for the knowledge itself, in the spirit of the ancient Greeks, but for the practical tasks of validating climate models, for adjusting satellite and radar algorithms and for calibrating these remote sensor readings. The data are also needed to allow plans to be made to cope with climate change and will be of great practical value, in the spirit of the Roman Empire, to agriculture, water management and flood warning. Trenberth et al. (2004) give a good précis of future needs.
Limitations of the existing raingauge network
While there is already a worldwide network of raingauges in operation, on land, it is unsatisfactory, as explained in Chapter 8, because of the wide variety of gauges used and the different methods of exposing them, which introduce a whole train of errors.