Wednesday, April 16, 2014

Orographic Precipitation

Precipitation enhanced by air lifting over mountain barriers is known as orographic precipitation. Smith (2006) describes mountains as a mechanism that modifies airflow to enhance or suppress precipitation events. Thus, orographic effects can contribute to a significant portion of total precipitation during a storm or can result in rain shadows. Understanding orographic effects will help improve Utah’s winter forecasts which are important to road safety, avalanche forecasts, flood warning and mitigation, and water management.
Orographic effects creating clouds along the mountains in near Spanish Fork. Notice the absence of clouds over Spanish Fork Canyon.
BKB (c) 2012
Orographic precipitation for a mountain barrier perpendicular to the prevailing winds can be explained with a simple model. As a moist air mass ascends the windward side of the mountain the water condenses and is removed by precipitation. Orographic precipitation usually stays close to the barrier where it is lifted and cooled. The removal of water from the air increases the potential temperature. As the air descends the lee side of the mountain, temperatures warm adiabatically and the air becomes warmer than it was before its windward ascent (Roe, 2005). In general, windward sides of mountains and higher elevations tend to receive more precipitation than the lee side of the mountain and adjacent valleys. This classical model, however, is overgeneralized. Orographic effects become complicated when additional factors are considered. For instance, precipitation patterns in the Intermountain West vary spatially and in intensity because of synoptic features, atmospheric stability, terrain geometry, and the Great Salt Lake-effect.
Classic orographic precipitation model.
Orographic enhanced precipitation events were observed during the IPEX study (Schultz, et al., 2002). During IOP 3 heavy and widespread orographic precipitation accompanied upper-level winds perpendicular to the mountain barrier. This event resulted in almost two feet of snow at Atla ski resort in about twelve hours. Areas in the Salt Lake Valley also received exceptional snow accumulation, suggesting that stable air blocked at the base of the mountain lifted the air before reaching the mountain barrier. Other snow events during IPEX showed different precipitation patterns because of differences in atmosphere and terrain characteristics.
Radar Image from P-3 aircraft during IPEX. Highest radar reflectivities are over the Wasatch showing the effects of orography on precipitation. Also notice the higher reflectivities just upstream the mountains, suggesting there is blocking at the base of the mountain causing increased precipitation in the valley.

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 Research
Since 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.

Reading List
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.

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