Orographic precipitation and Oregon's climate transition

Oregon's sharp east-west climate transition was investigated using a linear model of orographic precipitation and four datasets: (a) interpolated annual rain gauge data, (b) satellite-derived precipitation proxies (vegetation and brightness temperature), (c) streamflow data for a small catchment, and (d) stable isotope analysis of water samples from streams. The success of the linear model against these datasets suggests that the main elements of the model (i.e., airflow dynamics, cloud time delays, condensed water advection, and leeside evaporation) are behaving reasonably, although the high Oregon terrain may push the linear theory beyond its range of applicability. A key parameter in the linear model is the cloud delay time (tau), encapsulating the action of orographic cloud processes. Each dataset was examined to see if it can constrain the tau values. The statewide precipitation patterns from rain gauge and satellite constrain the tau values only within a broad range from about 500 to 5000 s. A focus on the sharp gradient on the lee slopes of the Cascades suggests that tau values in the range of 1800-2400 s are preferred. The study of the small Alsea watershed constrains tau little, as it receives a mixture of upslope and spillover precipitation. Stable isotope ratios in stream water indicate an atmospheric drying ratio of about 43%, requiring an average cloud physics delay time greater than T = 600 s.