Overview

The intensity and distribution of precipitation and snowfall in mountainous regions can depend strongly on the characteristics of the flow and shape, size, and orientation of the terrain. At the end of this learning activity, you should be able to identify the windward and leeward sides of a mountain barrier and identify favored flow directions for precipitation and snowfall enhancement.

Instructions

Utilize the information located in the Scenario and Key Concepts to answer the questions located in this activity. You may refer to your textbook and notes to help you work through the assigned questions if desired. You will have two attempts to complete this activity. The highest grade will automatically be recorded. Once you have submitted your activity for a second time, you will be able to see the correct answers. Discuss with your instructor if you have questions about your answers or feedback.

Activity is due Sunday, by 11:59 p.m. MT

Submission and Assessment Guidelines

You will complete the quiz associated with this scenario within Canvas. For help on how to take a quiz in Canvas, review How do I take a quiz.
This learning activity is worth 6 points toward your final grade.

Scenario

You are planning a ski trip to Kirkwood, a ski resort in the Sierra Nevada of California south of Lake Tahoe that averages 463 inches of snow per season. You are wondering what flow patterns favor heavy snowfall at the resort so that you can recognize when a major storm may be coming.

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.

Windward and Leeward effects 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.

Three dimensional depiction of flow exposure in the central Wasatch 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.

Schematic of period with upslope flow over Wasatch Back 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.

Color-filled topographic map of the Sierra Nevada region 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.

Citations

Scenario:

Kirkwood

Citation: Kirkwood. (n.d.).

Images:

Key Concepts

- Figure 1. Schematics of orographic precipitation enhancement in blocked and unblocked flow
  - Citation: Steenburgh, J. (2014). Secrets of the greatest snow on earth: Weather, climate change, and finding deep powder in Utah’s Wasatch mountains and around the world. Utah State University Press.
- Figure 2. Sixteen direction wind rose
  - Citation: Geographyrealm.com. (n.d.). Sixteen direction wind rose. https://www.geographyrealm.com
- 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.
  - Citation: Steenburgh, J. (2014). Secrets of the greatest snow on earth: Weather, climate change, and finding deep powder in Utah’s Wasatch mountains and around the world. Utah State University Press.
- 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.
  - Citation: Steenburgh, J. (2014). Secrets of the greatest snow on earth: Weather, climate change, and finding deep powder in Utah’s Wasatch mountains and around the world. Utah State University Press.
- Figure 5. Topography of the Sierra Nevada region. Color fill based on elevation in meters following scale at lower right. Kirkwood indicated by triangle.
  - Citation: Maps for Free (n.d.). Topography of the Sierra Nevada region. https://maps-for-free.com

Quiz:

- Question 3
  - Citation: Department of Atmospheric Sciences. (n.d.). Distribution of wind speed and direction during major storms at Kirkwood. University of Utah.
- Question 4
  - Citation: Department of Atmospheric Sciences. (n.d.). Distribution of wind speed and direction during major storms at Snowbird. University of Utah.
- Question 5
  - Citation: Maps for Free (n.d.). Topographic map of northeastern Utah and southeastern Wyoming, including the Uinta Mountains. https://maps-for-free.com