In-situ precipitation measurements

This chapter describes the measuring principles and technological solutions available for in-situ measurements of liquid (rain) and solid (snow) atmospheric precipitation. They can be classified into catching and non-catching precipitation gauges. Instruments belonging to the first family are generally based on gravity-related measuring principles (weighing, tipping buckets, floating devices), while the second group includes instruments based on optical, acoustic, and microwave principles (e. g., disdrometers). All instruments are subject to both systematic (often unknown) biases and measurement uncertainties, depending on the design, the measuring principle, the algorithms used for data interpretation and correction, etc. Moreover, environmental factors affect the measurement accuracy as well, depending on the atmospheric conditions at the collector, the siting characteristics, etc. Typical environmental factors include the gradients of atmospheric temperature, wind speed, and solar radiation and may result in a significant underestimation of accumulated precipitation. The present chapter addresses the achievable accuracy of instruments for in-situ measurement of liquid and solid precipitation, based on both the outcomes of the recent WMO intercomparison initiatives and the accurate laboratoy and field tests presently ongoing within the activities of the WMO/CIMO Lead Centre on Precipitation Intensity (Italy).