Precipitation Processes and Prediction in Complex Terrain: Perspectives from the Great Basin and Lake- and Sea-Effect Regions (Invited Presentation)

A fundamental problem in mountain meteorology concerns the generation and enhancement of precipitation produced by the interaction of moist flow with hills, mountains, and other forms of complex terrain. The remarkable diversity of precipitation systems and terrain features makes generalizations difficult, however, with each region posing unique challenges for weather and climate prediction.

This talk examines two such challenges. The first is the relatively poor skill of quantitative and probabilistic numerical precipitation forecasts over the Great Basin of western North America, which covers much of the interior western United States. The Great Basin features narrow, steeply-sloped mountain ranges that lack a sustained high-mountain mass and are separated by broad valleys and basins. Historically, numerical model skill in this region has been much lower than found over the broader Cascade–Sierra Mountains upstream. Validation of contemporary operational and experimental numerical forecast systems over the region show that skill is improving as grid spacings shrink to 3-km or less, but still lags that found in the Cascade–Sierra Mountains. Possible contributors to this skill discrepancy include inadequate representation of the region’s fine-scale topography, a lack of spatial coherence of precipitation systems within the region, complex multi-ridge effects on moist airflow and precipitation processes, and difficulties resolving shallow, orographic convection. Examples of complex multiscale effects on precipitation that are not well understood and pose a challenge for numerical prediction will be presented.

The second is the interaction of lake- and sea-effect precipitation systems with complex terrain. Our work in this area concentrates on the Tug Hill Plateau, which rises at a very graduate slope to 500-m above the elongated Lake Ontario of eastern North America, and the more formidable and irregular orography near the western coasts of Japan’s Honshu and Hokkaido Islands. These regions feature dramatically different lake and topographic scales, but in both cases, the characteristics of the incident flow, shape of the coastal geometry, mode of the lake- or sea- effect precipitation, and shape and size of the terrain contribute to the inland and orographic enhancement of precipitation, which can vary dramatically during and between storms. Although contemporary numerical forecast systems provide reasonable guidance concerning the likelihood of lake- or sea-effect precipitation, and in some cases magnitude, these multiscale effects serve as a challenge for specific local prediction.

In 1956, John Sawyer wrote that “inter-relations between rainfall, height, aspect, wind-direction, etc., are so complicated that a satisfactory estimation of orographic rainfall…can only be obtained from the consideration of rainfall over short periods during which the physical and dynamical processes are reasonably constant. It cannot be adequately obtained from a study of charts of a month or longer. A similar approach is also necessary if we are to tackle the forecast problem.” The challenges described above illustrate that such a perspective remains essential today if we are to advance predictive skill on local scales in areas of complex terrain.