Precipitation Patterns and topographyAlison Anders
University of Illinois
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Mountains have a strong influence on the atmosphere: they alter the flow of air and respond to solar radiation differently than the surrounding atmosphere. As a consequence, in mountainous environments, precipitation is enhanced in some regions and decreased in others. One familiar example is the "rain-shadow": a region of low precipitation in the lee of topography. However, the interactions between topography and the atmosphere can produce other patterns of precipitation as well, and the spatial scales of these patterns vary from the size of entire orogens to individual valley and ridges. These spatial differences in precipitation can influence geomorphology directly by changing the rates of various erosional processes, or indirectly through their influence on mountain ecosystems. Understanding the impact of precipitation patterns on mountain geomorphology is an area of ongoing research. Knowledge of the atmospheric processes that produce mountain precipitation patterns is crucial for this research.
Air flowing toward mountains can either flow up and over them or slow down, and turn to flow around them: a phenomena called blocking. Which scenario occurs is dependent on the height of the topography and the resistance of air to rising. The vertical profile of temperature and humidity in the air determines its resistance to flowing over topography - in general, warmer and wetter air is less resistant to rising. In the case of air flowing over the mountains, precipitation is concentrated on the windward facing side and a rain-shadow occurs on the lee side. As moist air is forced up the windward slope it expands and cools, eventually causing water droplets to condense when the air is saturated. These droplets form clouds and grow to produce rain or snow that typically falls out on the windward side of the range. After reaching the crest, air flows down the lee side, contracting and warming, which causes water droplets to evaporate, suppressing precipitation. This precipitation-topography relationship is dominant in mountain ranges where there is a consistent wind direction providing moist air and where elevations are moderate: perhaps less than 2500 meters or so. The island of Molokai in the Hawaiian chain has a climatological (long-term) precipitation pattern reflecting this process. Tradewinds from the NE produce precipitation on the NE corner and a rain-shadow to the SW that can be seen in the vegetation distribution of this satellite photo.
The geomorphic impact of an asymmetric precipitation distribution, like that described above, is a tendency for an asymmetric topography with the drainage divide migrating away from the high precipitation side and large-scale slopes higher on the lee side. These differences are seen in the Andes Mountains and reflect the changing dominant wind direction in the north versus the south. If air cannot flow over the mountains, more complicated flow patterns and precipitation distributions can result. As air approaches the topography, it slows down. The Coriolis effect causes the air to turn when it slows (toward the left in the Northern Hemisphere). The precipitation pattern associated with these flow conditions will still be increased on the windward side of the range and decreased in the lee, but there may also be along-range differences in precipitation. The Himalaya show this effect as precipitation decreases along the range front from East to West, reflecting the decreasing moisture supply as winds, turned left along the Himalayan front, deliver monsoon precipitation.
Notice that there is another scale of variability in precipitation along the Himalaya: high precipitation totals track the major valleys to the north toward the Tibetan Plateau. In these large valleys, air can flow farther north and brings moisture into this dry region. These storms have the potential to mobilize sediment and carry off the products of mass wasting from high-elevation regions.
Another precipitation pattern that shows the impact of blocking is found along the south side of the European Alps. Here, storms coming from the south are partially blocked and turn to the left to flow along the south side of the Alps. However, the Alps bend sharply to the south at the southern end, and the strong convergence of air in this corner contributes to a precipitation bulls-eye in the Ticino and Maggia river valleys. The impact of this precipitation pattern on geomorphology can be seen in the decreased peak elevations and depression of cirque-floors in the precipitation bulls-eye. Looking at smaller spatial scales, the scales of individual ridges and valleys, precipitation is enhanced on ridges and decreased on valleys along the windward side of the Olympic Mountain Range. The wind comes from the southeast and, at a large scale, the precipitation decreases sharply from the SW to NE side of the range (precipitation in grey contours of 750 mm/yr; topography in black contours of 250 m). Measurements and climate models suggest that the enhanced precipitation on ridges relative to valleys is a persistent feature of the climate. Geomorphic effects of this precipitation pattern remain undocumented, but landscape evolution modeling indicates that they have the potential to influence hypsometry, slopes, peak elevations and channel concavities. At the spatial scale of a few kilometers, precipitation patterns remain poorly constrained in most mountain ranges. New technology, such as satellite precipitation radar, provides the ability to document such patterns for the first time. The possible geomorphic impact of these patterns remains unknown. However, precipitation is a fundamental driver of erosional processes and exherts a strong control on ecosystem distributions, suggesting that these precipitation patterns may be important in understanding mountain geomorphology.