Snow midterm - snow module D PDF

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snow module D...

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Cold Air Pooling This downslope movement of air that occurs in the absence of larger-scale wind flows is known as a katabatic wind , or drainage flow/wind (Learning Goal 6b), and leads to cold air pooling within mountain valleys. The downslope flows from smaller valleys (called subdrainages) converge in the main valleys, where the cold air accumulates. Cold air pooling can strengthen an already surface temperature inversion, where temperature increases with height (Learning Goal 6c). So if this happens at nighttime, then why do skiers care? 1. The effects can last beyond sunrise. Drainage flows continue all night, building cold pools until the sun starts hitting the valleys and slopes the next morning. Even then, it can take several hours for the cold pool to dissipate. So if you start your ski day early in the morning when there is high pressure, you may be misled into dressing for colder temperatures than necessary. Often mid-mountain temperatures will be warmer than the valley bottom, due to the inversion. As you continue up towards the peak, however, temperatures may begin to cool again as you travel above the inversion, and temperatures decrease with height. 2. The effects can persist for multiple days. Cold air pooling strengthens inversions. As we discuss further in Learning Goal 6c, inversion episodes can persist for days or even weeks under persistent high pressure in the dead of winter. 3. A lot of people ski at night! Many ski resorts have night skiing, with floodlit ski runs. The local mountains on the north shore of Vancouver all offer this: Seymour, Grouse, and Cypress Mountains. While you can often get above the fog, sometimes the layer is so deep that it might affect you on these lower elevation mountains. 4. Backcountry trips may start or end in the dark (Fig. 6a.2), and may include camping overnight. Campsite selection should be done with an awareness of these processes. Setting up a tent or bivouac sack in a valley bottom under cold, clear conditions, will expose your camp to the coldest temperatures and to the drainage flow. It's often better to set up camp a little way up the mountainside, so that you're still sheltered from potential large-scale winds, but also avoid the cold drainage flows of the valley bottom.

Learning Goal 6b: Describe the diurnal evolution of slope flows A diurnal slope flow is induced by thermal (temperature) changes over the course of a day, as a result of the sun heating the earth's surface (which in turn heats the air above it), and then the earth's surface cooling after the sun sets (which in turn cools the air). (high pressure).

Daytime slope flows During the day, the incoming shortwave radiation (sunlight) from the sun heats up the ground surface. This is called radiative heating . The warm air rises and travels up the mountainside (as opposed to rising vertically). This upslope thermally-induced motion is called an upslope flow , or anabatic wind (10-20km/h) As the warm air reaches the ridgetop or mountaintop it rises vertically, often combined with the air from the next valley over. If there is enough moisture present, cumulus clouds (called anabatic cumulus clouds) can form over the mountaintop When the sunshine is strong and sustained, for example over the course of a whole afternoon or day, a return flow can form in the valley. As a skier or snowboarder, you should not be concerned about anabatic winds producing hazardous wind speeds. However, you should be aware of the warmer temperatures it can bring to the mountain slopes, on top of the regular daytime heating, in fair weather. It can also help to dissipate valley fog/cloud, and cold pools/inversion

Nighttime slope flows nce the sun sets and the ground is in the shade, the ground is no longer being heated by the sun. Instead, it cools, losing energy by radiative cooling . In turn, the air next to the ground surface also cools down. This colder, denser air drains down the mountainside into the valley, due to gravity Katabatic wind speeds reach 10 to 30 km/h and typical depths of the sinking layer are 10 to 100 metres. As the shallow layer of cold air moves downslope, it converges into small valleys and gullies. The air in these gullies converge into larger river valleys, and eventually into a valley cold pool of air

One of the main concerns with katabatic winds for skiers is the cold temperatures in these drainage flows. These can make your first and last ski runs of the day pretty cold. Also, the resulting cold air pooling in valley bottoms (Learning Goal 6a) and valley cloud/fog formation present additional concerns

Learning Goal 6d: Identify conditions that favour valley cloud/fog formation and dissipation...