PRISM

PRISM (Parameter-elevation Regressions on Independent Slopes Model) monthly precipitation is the gridded dataset used by Mountain Mapper software to distribute rainfall from a single QPF point to a gridded QPF field. A raster of 30-year average annual PRISM-derived precipitation is shown in Figures 1 and 2 below for California and Nevada, respectively. In these examples, it is easy to see the strong influence that topography exerts on the distribution of average annual precipitation.

Figure 1. PRISM Annual Average Precipitation - California

Figure 2: PRISM Annual Average Precipitation - Nevada

PRISM is an expert system that uses point data and a digital elevation model (DEM) to generate gridded estimates of climate parameters. PRISM is well-suited to mountainous regions, because the effects of terrain on climate play a central role in the model's conceptual framework. It is called an expert system, because it attempts to mimic the process an expert would use to map climate parameters.

PRISM uses digital elevation model (DEM) data on the 4 km x 4km grid interval and climatological precipitation records. The goal of PRISM is to derive a more complete and realistic climatological record of precipitation in mountainous terrain. The basic steps in the PRISM process include:

• Determine the "orographic elevation" of each available precipitation station by noting on which 4km x 4km DEM grid element it resides.
• Compute the aspect (direction of maximum slope) of each 4km grid element.
• Derive a regression equation between observed precipitation and orographic elevations for contiguous areas having the same aspect.
• Use the regression equation to compute precipitation for each 4km grid element over the contiguous area.
• Repeat the above steps for all other areas selected on the basis of contiguity and terrain aspect, thus deriving and using many "local" regression relationships between precipitation and "orographic elevation". Sparse coverage of observed precipitation requires some compromises and smoothing in some areas for the operation of this process, but the basic principles still apply.

The precipitation data PRISM used to distribute precipitation consists of three basic climate datasets:

• The National Climatic Data Center 1961-1990 normals dataset (CLIM-81) observed by the National Weather Service Cooperative Climate Network; The NRCS SNOTEL (SNOwpack TELemetry) network dataset;
• Supplemental datasets submitted by the individual State Climatologists or Regional Climate Centers.
• A panel of State Climatologists from several western states, plus additional experts, critically reviewed PRISM methods and maps of precipitation in their areas of interest. The panel concluded that PRISM produced precipitation maps that equaled or exceeded the quality of the best manually-prepared (hand-drawn) maps available.

That's probably more than you ever wanted to know about PRISM, but if you want to know more, please refer to the Oregon Climate Service web site.

Page last modified: July 31, 2006