Orographic precipitation and Oregon's climate transition

We examine Oregon's sharp east-west climate transition using three approaches: a) a comparison of a linear theory of orographic precipitation with interpolated rain-gauge data; b) a spatial analysis of satellite-derived precipitation proxies (vegetation and brightness temperature); c) stable isotope analysis of water samples from streams along an east-west cross-mountain transect. The success of the linear model against both rain-gauge and proxy data suggests that the key elements in the model (airflow dynamics, cloud time delays and lee side evaporation) are behaving reasonably. The satellite proxy data agree with model and raingauge data, illustrating the impact of precipitation on landscape, but not providing an improved verification field. A key parameter in the linear model is the cloud physics delay time. A larger cloud delay time increases the downslope evaporation and decreases the atmospheric drying ratio and the isotope fractionation. We deduce this time from measured stable isotope ratios in stream water. River water samples collected during the dry season show a strong eastward decrease in deuterium and oxygen-18 concentrations; from the coast to central Oregon. This decrease indicates an atmospheric drying ratio of about 43%. This ratio is consistent with an average cloud physics time delay of about = 1200 seconds with an uncertainty of about 50%. The amount of small scale spatial structure in the model fields and the sensitivity of the drying ratio to cloud delay suggest that the small scale elements of Oregon's complex terrain are controlling some of the important processes.