Lecture 18 Mountain Snowstorms Orographic Cloud viewed from rom lee side

16.2 Average Annual Snowfall Number of Days with 1 inch or more snowfall

16.5

Orographic Clouds in the Rockies They produce the snow that fills the reservoirs and rivers and facilitates many recreational activities (skiing, x-country skiing, snowmobiling, etc) Can lead to flash flooding that threatens commerce and lives Have the potential to be “ improved ” in their yield, through weather modification (or does that just rob some other region of their water?).

Orographic Clouds in the Rockies The snow pack of the western mountains of North America melts to provide precious water for the West. – Domestic use has first priority for water – Industry has second priority – Irrigation has third priority, but uses 95% of all of the water

Water Rights in the West There simply is not enough water in the West Water rights are a property in the West, more valuable than the land itself in many cases! The Colorado River, the life-blood of the West, fed by the snow melt of the Rockies, never reaches the Gulf of California! Major fight between Northern California and Southern California is over water – Southern California diverts water from Northern California drainage by aqueducts taking away potential commerce from the North.

Water Rights in the West The Front Range Cities and agriculture receive a large portion of their water from the West slopes via a large siphon (Big Thompson Project) built during the depression. They fill reservoirs in the spring runoff and empty them during the growing season.

Water Rights in the West Big fight on the Front Range of Colorado, Thornton (a suburb of Denver) secretly bought up the water rights from some farmers in Northern Front Range, near Ft. Collins and and then announced a plan to build an irrigation ditch to bring their water south, but Ft. Collins is taking it to court because that will limit the ability of Ft. Collins to grow.

Water Rights in the West The Colorado River water rights were based on the flow of the river during a 10 year period, I think, in the 1880 ’ s and that period was above normal! Colorado is responsible for delivering a certain number of acre feet of water to Utah and Utah to Arizona and so on. Municipal, Industrial and agricultural interests OWN the water and so the snow that falls! Any additional snow that one could make is worth $$$ and represents power. Water is everything in the West!

Major Issues Cloud Precipitation efficiency – Short time scale of parcels passing over mountain – Ice crystal concentration (a function of temperature) may not get high enough to make precipitation through Bergeron-Findeisen growth – Cloud top temperature an issue for how many crystals are formed

Therefore, it is important that somewhere in the cloud, the temperature be at least as cold as –20 C to get crystals that are: – Numerous enough to consume and so convert the cloud droplets by Bergeron-Findeisen process to ice crystals – Sparse enough to be large enough (once all of the cloud droplets are converted) to precipitate themselves

View of Western Mountains Important flow regimes that maximize upslope precipitation for various regions Upstream precipitation can deplete an airmass of moisture necessary to form upslope precipitation at a particular location. Must consider: – Elevation of upstream barriers – Moisture source region – Synoptic lifting by airmass

16.3 Topography cross section across western US

Rocky Mountain Eastern Slope Upslope Storms Upslope usually shallow, ie easterly flow confined to below 700 mb Gulf Moisture from east necessary Surface (850 mb) and 700 mb flow important

Figure

Cold Air Damming Major upslope storms associated with cold air damming Artic flow from north moves southward along mountain barrier Flow wants to turn right (into the barrier) from coriolis, forming a trapped Kelvin wave effect, ie the cold air warps up against the barrier Flow from the east rides up over the cold air, producing snow band over the plains

16.10

Cold air Damming on East Coast East coast storms are famous for the cold air damming there Similar form to the damming in the Rockies