Variable influence of terrain on precipitation patterns: Delineation and Use of Effective Terrain Height in PRISM

An issue encountered when mapping precipitation is the importance of terrain in defining the patterns of precipitation. In the mountainous western U.S., terrain dominates the spatial patterns of precipitation. In flat or gently rolling areas such as the Great Plains, the role of terrain is more subdued, although precipitation variations have been documented over low hills in Illinois (Changnon et al., 1975) and Sweden (Bergeron, 1968), and in narrow valleys in Canada (Longley, 1975). Conceptually, the effectiveness of a terrain feature in amplifying precipitation depends partially on its ability to block and uplift moisture-bearing air. This ability is determined mainly by the profile the feature presents to the oncoming air flow. Steep, bulky features with continuous ridge lines oriented normal to the flow can generally be expected to produce greater precipitation/elevation (P/E) regression slopes than low, gently rising, features with discontinuous ridge lines oriented parallel to the flow. One might imagine a spectrum of “effective” terrain heights, ranging from large features that produce highly three-dimensional (3D) precipitation patterns, to a nearly flat condition which exhibits two-dimensional (2D) patterns only. Between these extremes would be a transition between 2D and 3D patterns, for which P/E slopes would range from zero to values typical of mountainous areas. Ideally, the effectiveness of the terrain would be reflected in the station data, and thus in the empirical P/E regression slopes. In reality, the station data are rarely of sufficient density and reliability to provide such a detailed and accurate picture. If spatial estimates of the 2D/3D nature of the terrain were available a priori, the range of allowable P/E slopes could be varied to the appropriate degree, providing an independent check and constraint (if necessary) on the empirically-derived P/E slopes.