Optimised method for estimating directional driving rain from synoptic observation data

In this article, the annual directional exposure to driving rain and its characteristics are analysed and discussed at four Spanish sites that are characterised by different rainfall levels and topographical and wind conditions. For this study, the present weather observation method is used, which is based on average annual rainfall data and synoptic observations of the present weather. The results of this study are compared with those obtained by applying the ISO 15927-3:2009 standard, which is based on the semi-empirical analysis of hourly wind and rainfall data. This study identifies the intrinsic dependence of the aforementioned synoptic method on the weather conditions that exist at each site, which affect the reliability and accuracy of the estimates. Thus, corrective changes that would enable the synoptic method to generate more reliable approximations are proposed, and a new optimised methodology is presented; the precision of the new method relies on synoptic observations but is independent of weather conditions. The results, validated at four Spanish sites, suggest that in the absence of hourly data for implementing the ISO standard, this optimised synoptic method is able to generate reasonably accurate estimates of the annual directional exposure to driving rain, regardless of the particular site conditions.