Numerical models of the raingauge exposure problem, field experiments and an improved collector design

A new design of conventional raingauge is proposed that minimizes the most important source of systematic error in point rainfall measurements: that due to the interaction of the wind with the gauge (the ‘raingauge exposure’ problem). Basic physical principles, numerical simulations and extensive field trials in England and Denmark are also used to provide a consistent quantitative description of the wind‐induced errors of conventional cylindrical gauges. It is shown that an inverted conical collector with a large semi‐vertical angle may provide the basis of an adequate solution of the raingauge exposure problem. The need to minimize outsplash demands that the collector should have a fairly large diameter and take the form of a flat champagne glass. A small diameter flat champagne glass collector has been tested for over 31/2 years against a standard 5′’ cylindrical gauge, a 5′ pit gauge and an inverted conical gauge at the Institute of Hydrology, Wallingford. The insensitivity of the losses of the ‘flat champagne glass’‐shaped collector to wind speed at low rates of rainfall is confirmed, as well as the need for a larger collector diameter to minimize outsplash. A combination of a ‘steep champagne glass’ gauge and a ‘flat champagne glass’ gauge might lead to even better estimates of rainfall in high winds, notably on ships, and could provide a low‐cost approach to improving snowfall measurements on land.