On the accuracy of wind-driven rain measurements on buildings

Wind-driven rain (WDR) measurements on buildings are being conducted for many decades. They provide an indication of the WDR falling onto different parts of a building facade and are an essential tool for WDR model development and model validation. However, up to now, very few investigations concerning the accuracy of WDR measurements have been performed. No publication of WDR measurements has been found that provides an indication of the errors involved. Availability of error estimates is essential for the interpretation and the use of WDR measurement data. In this paper, the main errors associated with WDR measurements are identified and investigated. It is shown that especially the evaporation of adhesion water from the gauge catch area can be important and a method to estimate this error will be proposed. It is shown that this error can be very large (up to 100%) and that it depends not only on the gauge type but also on the type of rain event. Finally, guidelines for the design of WDR gauges and for the selection of WDR measurement data that are suitable for model development and model validation are given.