|Orographic effects creating clouds along the mountains in near Spanish Fork. Notice the absence of clouds over Spanish Fork Canyon.|
BKB (c) 2012
|Classic orographic precipitation model.|
Atmospheric stability determines how likely an air mass will rise over a mountain barrier. Under stable conditions low-level flow is blocked (Steenburgh, 2003) or is diverted around the barrier (Roe, 2005). For the stable case, the cold air blocked at the mountain base acts as a slope for the air mass to ascend resulting in condensation and precipitation ahead of the mountain barrier. For this reason, large mountain barriers with lots of blocking tend to have more orographically influenced precipitation on the windward side and ahead of the mountain while lower mountain barriers with less blocking receive more precipitation at their peaks (Roe, 2005). When the atmosphere is neutral, flow approaching the mountain barrier is allowed to ascent the entire mountain without being blocked. For unstable conditions, convection along the mountain barrier can cause heavy precipitation (Steenburgh, 2003).
Forecasting orographic precipitation becomes more complex when lake-enhanced precipitation is considered. Sensitivity studies by Alcott and Steenburgh show that a combination of lake-effect and orographic enhancement can result in significant precipitation events such as the “Hundred-Inch Storm” of 2001. Their study also investigated the influence of upstream mountain ranges and orographically forced convergence by concave terrain.
Like all numerical weather prediction, model based orographic predictions are poor due to insufficient representation of complex topography as well as imperfect model physics. In addition, snowfall isn’t directly predicted in the models. Instead, snow accumulation is estimated by assuming a snow-to-water ratio (Schultz, et al., 2002). A ratio of 10:1 is traditionally used, but that is not necessarily a good estimate for all cases as ratios can spread from 25:1 to 5:1. Forecasters should also be cautious when analyzing radar data. Since the radar beam is aimed over the mountain peaks, low elevation orographic precipitation is not measured by radar (Schultz, et al., 2002).
Future ResearchSince much of the validation of orographic precipitation models are done only during large field campaigns it would be beneficial to install or obtain additional and more accurate precipitation observations at higher elevations to verify and improve numerical weather forecasts.
Alcott, T. I., & Steenburgh, W. J. (2013). Orographic Influences on a Great Salt Lake-Effect
Snowstorm. Monthly Weather Review, 2432-2450.
Roe, G. H. (2005). Orographic Precipitation. Annual Review of Earth and Planetary Science, 645-671.
Schultz, D. M., Steenburgh, W. J., Trapp, R. J., Horel, J., Kingsmill, D. E., Dunn, L. B., . . . Trainor, M. (2002). The Intermountain Percipitation Experiment (IPEX). Bulliten of the American Meteorological Society, ES1-ES30.
Smith, R. B. (2006). Progress on the theory of orographic precipitation. Geological Society of America Special Paper 398, 1-16.
Steenburgh, W. J. (2003). One Hundred Inches in One Hundred Hours: Evolution of a Wasatch Mountain Winter Storm Cycle. Weather and Forecasting, 1018-1036.