1 Accurate Rainfall Measurement : The Neglected Achilles Heel of HydroMeteorology

Rainfall measurement has an extensive historical precedent. Attempts have been made to standardise measurement procedures. This has never been successfully achieved. There are many sources of measurement error, some of which are compounded by poor rain gauge siting and a variation in gauge height. By far the worst cause of measurement inaccuracy is due to windinduced undercatching. Some solutions have been proposed to tackle this problem but none have been fully implemented, and little has been done on the topic for several decades. Rain gauge intercomparisons performed for different types of rain gauges situated at a standard height showed that the design shape of the gauge is significant, in terms of measured rainfall catch. The type of rainfall event was also shown to be significant, with events typical of west-coast UK uplands shown to be more susceptible to wind-effects than a large convective event on the UK’s east coast. Different types of instruments were also demonstrated to have varying degrees of sensitivity to rainfall, further complicating the measurement process. A number of key findings are presented which pertain to; the aerodynamics of the gauge itself, the type of rainfall event and wind conditions, the siting of the rain gauge, and why there is a pressing need for standardisation of rainfall measurement. Implications of the findings describe an inconvenient truth in hydro-meteorology which transcends a variety of applications of rainfall data, from real-time flood forecasting to Numerical Weather Prediction models. Having discussed the problem and the importance of solving it, the experimental design of a multisite field experiment is discussed, supplemented by the use of Computational Fluid Dynamics (CFD) simulations.