Understanding and evaluating the changing characteristics of wind-driven rain loads in a warmer climate for Canada

The amount of rainfall that passes through a vertical plane, during the co-occurrence of rain and wind, is defined as Wind-driven rain (WDR). WDR is the most important moisture source affecting the performance of building façades, and can lead to several undesired results for buildings. In future, higher climate variability and more extreme events are expected. Therefore, Hygrothermal and durability analysis of façades require quantification of future WDR loads for a changing climate. This study evaluates the changing characteristics of WDR loads across Canada for the end of century using an ensemble of regional climate model simulations for the Representative Concentration Pathway 8.5 emissions scenario. The regional climate model, i.e. the Global Environmental Multiscale model, is validated by comparing model-simulated WDR-related climate variables with observations and reanalysis products. The validation results provide confidence in using the model to assess the projected changes to WDR loads for Canada, albeit some biases. Three types of WDR loads, based on semi-empirical equations, are considered in this study: (1) omnidirectional WDR, (2) directional WDR and (3) WDR spells. Omnidirectional and directional WDR amounts are calculated over periods of interest. The former indicates WDR exposure of a specific region, while the latter represents the potential moisture content of absorbent surfaces since it takes into account the façade orientation and wind direction. The WDR spell amounts are representative of the probability of rain penetration through the façade and more critical for design purposes. Furthermore, 3-year return levels of annual maximum WDR spell loads are also used to develop WDR risk category maps for Canada and specifically for 16 Canadian cities. Future projections suggest large increases in WDR loads for some regions of Canada. The increases in these loads are mainly due to increases in both rainfall and wind magnitudes for Arctic Canada, while for other regions it is mostly due to changes in rainfall. Results indicate a shift in the timing of the highest monthly WDR loads from summer to fall. This suggests higher WDR penetration through wall systems, given the relatively low evaporation rate in fall compared to summer even in a warmer climate. Detailed city-level analysis of directional WDR loads suggests large increases for the most critical façade orientations for most of the sixteen cities considered, with the largest increases for those along the east and west coast. The projected changes to the characteristics of future WDR spells imply more severe extreme WDR events and higher deterioration risk. Furthermore, the developed WDR risk category maps help identify façade orientations with elevated risk in future climate which is crucial for the development of detailed guidelines to ensure climate-resilient buildings

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