The WMO Solid Precipitation Intercomparison and the determination of universal wind speed corrections

Wind greatly reduces the amount of precipitation intercepted by a precipitation gauge, with the lost snowfall from an unshielded gauge exceeding the magnitude of the measurement itself in windy conditions. The World Meteorological Organization’s Solid Precipitation Intercomparison Experiment (WMO-SPICE) was initiated in 2010 to facilitate: 1) the development of a universal standard for precipitation measurement, 2) the evaluation of automated precipitation gauges, and 3) the creation of transfer functions to determine the actual amount of precipitation for different wind shields and gauges. The development of transfer functions is of particular interest to hydrological research, as these transfer functions can greatly increase the accuracy of winter precipitation measurements. Transfer functions have been developed and evaluated using the WMO-SPICE results for several different types of unshielded weighing gauges and for gauges within different types of wind shields. For all-weather weighing gauges, the resultant measurement biases were more strongly related to wind shielding than the type of gauge.  Because WMO-SPICE measurements are available from sites with varying climates, they also provide an unprecedented opportunity to develop and test ‘universal’ transfer functions that are more generally appropriate for all climates. The biases associated with the use of a single transfer function at different sites were assessed using these unique data, and were much smaller than other still-unresolvable errors in the measurements.