Module 3 | Scenario 3: Flow Direction and Orographic Precipitation

Key Concepts

a. Mountain effects on precipitation

The intensity of precipitation in mountainous regions often depends on the interaction of the flow with the topography. Precipitation enhancement often occurs on the windward side of a mountain range like the Sierra Nevada. The windward side is the side of the mountain range that faces the flow and is where air is forced upwards, resulting in cloud and precipitation development. This is sometimes referred to as orographic lift. Sometimes, the low-level flow near the base of the mountains is blocked and the ascent and precipitation enhancement begin upstream of the initial mountain slope. In contrast, if the flow is unblocked, the ascent begins near the initial mountain slope and the precipitation enhancement occurs near and over the mountain. Each of these two scenarios is illustrated in Figure 1.

Figure 1. Schematics of orographic precipitation enhancement in blocked and unblocked flow.  From Steenburgh (2014).

In situations with strong blocking, the precipitation is sometimes greater over the windward lowlands or initial mountain slope. More commonly, precipitation increases with elevation and is greatest over the windward slopes or near the crest. 

Precipitation often spills over the crest and into the lee (known as spillover), but typically decreases as one moves down the leeward slopes, which face away from the flow. Often there is a precipitation shadow downstream of the mountain, with little to no precipitation even as a major storm rages on the mountain crest. 

b. Wind/flow direction

Meteorologists define wind direction (also referred to as flow direction) based on the compass direction from which the wind originates. For example, a west wind comes from the west and blows toward the east. It is common for wind directions to be reported based on a 16-direction wind rose such as the one depicted in Figure 2. For instance, as one moves clockwise 90 degrees from west to north the directions are west (W), west-northwest (WNW), northwest (NW), north-northwest (NNW), and north (N).  

Figure 2. Sixteen direction wind rose. From geographyrealm.com. 

The wind direction can change with altitude. For example, the wind direction can be southerly in the Salt Lake Valley and north-westerly on the highest Wasatch peaks. Meteorologists diagnosing or forecasting mountain effects on precipitation typically look at the wind direction at an observing site or pressure level near mid-mountain or crest level.  

c. Diagnosing upslope and downslope flow

Using the mid-mountain or crest-level wind direction, one can infer if the flow is moving upslope or downslope. Typically when doing this, one is not too worried about the fine-scale terrain details, but instead the "bulk" topography. One reason why the Cottonwood Canyons and Alta, Snowbird, Brighton, and Solitude ski resorts are so snowy is that it is an "island" of high terrain that experiences orographic lift during flow from several directions, as depicted in Figure 3. 

Figure 3. Three-dimensional depiction of upslope flow in the central Wasatch Mountains from the northwest, southwest, and southeast.  Perspective is looking from the northwest toward the southeast. 
From Steenburgh (2014).

Conversely, one reason why it snows less on the Wasatch Back where Park City Mountain Resort and Deer Valley are located is that it is often on the leeward side of the Wasatch. Every now and then, however, there is a storm period during which there is moist flow from the southeast or east, and the Wasatch Back temporarily becomes the windward side and receives as much as or more snow than the Cottonwoods.

Figure 4. Three-dimensional depiction of upslope flow over the Wasatch Back due to flow from the southeast or east.  Perspective is looking from the northeast toward the southwest. From Steenburgh (2014).

d. Kirkwood

As illustrated in the color-filled topographic map in Figure 5, Kirkwood ski area (identified with a triangle) is located in the Sierra Nevada of eastern California, just west of the Sierra crest. The Sierra Nevada form an elongated mountain barrier about 600 kilometers long and 100 kilometers wide. They rise gradually from near sea level in the Central Valley to 2,500 to 4,000 meters at the Sierra Crest. The highest lifts and peaks at Kirkwood reach almost 3,000 meters.  

Figure 5. Topography of the Sierra Nevada region. Color fill based on elevation in meters following scale at lower right provided by maps-for-free.com. Kirkwood indicated by triangle.