Precipitation amount and intensity measurements using a windscreen

Measurements of precipitation amounts with precipitation gauges are affected by various error sources. An overview of the errors is given in WMO (1994). One of the errors is the so-called wind effect that causes loss of precipitation due to wind field deformations near the gauge rim. The precipitation amount measurements are performed automatically by KNMI using an electronic precipitation gauge in a so-called English setup* . This setup is used to create a horizontal airflow above the orifice of the precipitation gauge in order to reduce the errors induced by wind field deformations. The setup is rather costly to build and requires additional maintenance. At some locations in the Netherlands the setup is particularly impractical as a result of high ground water level and/or poor drainage. In addition, the setup with the precipitation gauge orifice at the level of the measurement field is sensitive to debris, since leafs, grass and sand are easily blown into the precipitation gauge and cause instrument failures. Therefore, KNMI performed tests with the precipitation gauge simply placed on the measurement field. In such a setup the precipitation amount measurements are affected by errors induced by a deformation of the wind field by the precipitation gauge itself. The wind field deformation will blow part of the precipitation that would otherwise fall into the gauge over the rim and will thus cause a reduction of the measured amount of precipitation (see e.g. Sevruk and Nešpor, 1998 and Nešpor and Sevruk, 1999).