Test and Evaluation of an Updated Orographic Precipitation Model for Operational Use

Test and Evaluation of an Updated Orographic Precipitation Model for Operational Use

Eric R. Thaler NOAA/National Weather Service Denver/Boulder, Colorado

Orographically generated precipitation is a distinctive component of the overall precipitation accumulation in areas of varied topography. Accurately predicting this type of precipitation, especially when it falls as snow, is an important operational forecast problem. These orographic snowfall events ofttimes have more significant impacts on transportation, recreation, avalanches, reservoir storage and river flows than do events associated with synoptic-scale migratory troughs and cyclones.

For several decades, operational forecasters in Colorado have been using an orographic precipitation model based on work done by Rhea in the late 1970s. This model has performed well throughout its long history, despite several shortcomings. Among these are an overly simplified depiction of mountain wave dynamics, the assumption of pseudo-adiabatic lapse rates, flow confined to two dimensions, and the inability to easily incorporate synoptic-scale processes. The effects of departures of atmospheric lapse rates from the pseudo-adiabatic were added during the 1990s. This enhanced the accuracy of the forecasts considerably but still left room for improvement. In addition to these deficiencies in the model physics, the output is currently only available in a non-digital format and solely for Colorado. This severely restricts its usefulness in the current era of grid point and digital data.

To overcome some of the aforementioned limitations, a new version of the model is being developed for use during the 2013-2014 cold season. Based on research and theoretical advances that have occurred over the last several years, this new rendition of the model will take advantage of computational resources presently available in local forecast offices. These factors will provide what is anticipated to be more accurate and useful quantitative snowfall forecasts for mountainous areas. Very high-resolution output will be in a digital format, making it possible to more directly incorporate the forecasts into the National Weather Service's National Digital Forecast Database. Furthermore, output in this form could also be provided to users via geographical information systems and other graphical and text-based data outlets.

Ensemble runs of the model are also envisioned. These could serve as the basis for probabilistic snowfall forecasts, having potential utility for a variety of users. Finally, the model will not be landlocked to Colorado, instead having the capability of being applied to other geographic regions.