Characteristics of the co-occurrence of wind and rain and the driving-rain index

Abstract

The relationship between the amount of wind-driven rain or driving-rain and the driving-rain index (product of annual rainfall and mean wind speed) is studied. An analytical approach is proposed to obtain the relationship between these two quantities. It is observed that they are related by two factors, (a) the driving-rain coefficient and (b) the co-occurrence factor. The driving-rain coefficient is shown in the paper to be a function of drop size distribution which depends on the rainfall intensity and the type of rain. The co-occurrence factor describes the amount of correlation between wind and rain. The characteristics of wind and rain at four meteorological stations across Sydney, Australia are studied. The driving-rain characteristics for these sites are observed to be different. The amount of driving-rain for a given site is also found to be very different compass directions. They can differ by a factor of 10 and more. It is believed that for the purpose of weather resistance design of building facade, the directional driving-rain index should be used. Using the proposed method, the co-occurrence factor for the dominant driving-rain direction in Sydney is found to be 1.12 and the driving rain coefficient is calculated to be 0.204. With these two factors the amount of driving rain can be calculated from the driving-rain index.