Investigation of orographic rainfall in south coastal mountains of British Columbia

Storm rainfall in the south coastal mountains north of Vancouver, British Columbia was investigated with the central goal of elucidating and quantifying orographic rainfall production and distribution using available meteorological information. Storm size ranged from 10 to 130 mm with all but one storm showing an orographic effect. Correlation analysis indicated significant relationships between the amount of orographic rainfall and the following storm parameters: wind speed component normal to the mountain barrier, moisture content of the lower atmosphere, freezing level and air mass stability. Attempts to classify storm types in relation to orographic rainfall, using analysis of variance, met with limited success. A physical model for estimating rainfall intensities over windward mountain slopes, based on concepts of precipitation physics and wind flow over mountain barriers and incorporating the storm parameters noted above, was adapted and tested on 4 stable and 4 unstable storms. Procedures are given for applying this model. For stable storms, the model can give consistent estimates of rainfall amount and distribution over the mountain slopes. A standard deviation of ±11% for optimum orographic lifting conditions was found. For unstable storms, model estimates are unreliable due to uncertainty in estimating convective vertical wind velocities. Eigenvector analysis of areal rainfall patterns reveals one basic pattern inherent in most storms, with 98% explained variance. This rainfall pattern conforms closely to topographic configuration. This thesis thus illustrates the extent and nature of the dominant influence of topography on rainfall in the study area and shows the value of available weather data in explaining and estimating orographic rainfall.Forestry, Faculty ofGraduat