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ATM 10 Severe and Unusual Weather Prof. Richard Grotjahn L7 http://canvas.ucdavis.edu/ Rotor & wave clouds, Boulder CO © R Grotjahn Lecture topics: • Topographic winds and T variations 1. Mountain ridge convergence lines 2. Mountain-valley drainage flows 3. Katabatic winds • Topographic winds from P gradient 1. Severe downslope winds 2. Santa Ana Rotor & wall clouds, Independence CA © V. Grubisic Topographic Circulations driven mainly by T variations Morning convection over 14,309 ft Uncompagre Peak, CO. © R. Grotjahn 1. Mountain-Valley winds: Ridgeline convection • Thermal circulations when topography generates T differences • Sun heats hillside, air contacting hill warmed, so less dense than air at same elevation over valley; creates wind up valley. At same elevation: • More heating here (on mountainside) • Than here (in clear air) Figure 10.27 (Ahrens) Daytime Convection Valley breezes fed by sunlight heating mountain sides lead to… Convergence over higher ridge lines so convection develops first over ridges. Collegiate pks, CO, © R. Grotjahn Thunderstorm forming over 14,073 ft Mt. Columbia T-lapse-Uncompag-smmt.mov Video of the day Convection building over mountains over time. Eventually showers Finally thunderstorms Time lapse hunderstorms forming over San Juan Mtns Uncompagre Pk CO, © R. Grotjahn T-lapse-Uncompag-smmt.mov Ridgeline Convection: Satellite view Valley breezes fed by sunlight heating mountain sides lead to… Convergence over higher ridge lines so convection develops first over ridges. Convection forming over Cascades NOAA photos courtesy PSU 2. Mountain-Valley winds: Valley fog • Thermal circulations when topography generates T differences • Nocturnal radiational cooling of mountain slope creates relatively denser air that sinks down sides of mountain. At same elevation: • More cooling here (on mountainside) • Than here (in clear air) Figure 10.27 (Ahrens) Nightime Valley fog • Colder air forms along ridges • Since its more dense it drains down sides of valleys. • Colder air can ‘pond’ in the valley bottom • In valleys that colder air can mix with warmer, moister air to form fog • Peaks & ridges stay clear. 3. Katabatic Winds • Strong “drainage” flow off of a high cold plateau. • Severe katabatic winds occur at the margins of large, high altitude ice sheets: Antarctica, Greenland, Patagonia. Speeds can exceed 100 mph. • “gap winds” are different though cold air on one side of topography creates pressure force through gap. Patagonian Ice sheet drainage © R. Grotjahn Katabatic Wind Fig. 16.15 L H H -21C L 0 C © Hannes Grobe • Strong radiative cooling over ice sheet creates very cold air layer. • Cold air very dense, causes thin layer of high pressure. • A pressure gradient is set up that draws the cold air down slope. • The very dense air accelerates, due to gravity, as it descends. Antarctic Katabatic Winds • The severest winds blow off Antarctica, where speeds have exceeded 175 knots! 50 m Antarctic Ice sheet katabatic flow © Samuel Blanc Topographic Circulations driven mainly by P variations Wave clouds over Continental Divide (30km west of) Boulder CO. © R. Grotjahn 1. Chinook, Foehn, Downslope winds • Goes by various names around the world. • Stable flow blocked by topography descends adiabatically on the down wind side. • Descent brings fast-moving high altitude winds down to the surface • Found several places in west (often east of N-S mountains) • Not driven by T changes, but affects T on lee side of topographic barrier. • Santa Ana and ‘California Norther’ are a separate category Severe Chinook Elements • (1) upstream winds blocked, (2) severe winds, (3) lee side warming, (4) wall cloud, (5) rotor, (6) wave clouds, (7) hydraulic jump, (8) rotor cloud Chinook elements: winds blocked upstream Stable air 1. Strong Chinooks occur when flow is blocked by mountain. 2. Low level flow from left does not cross over the mountain. 3. Winds are stronger aloft, these high winds are brought to surface on lee side of the mountain. 4. After losing much elevation, the air adiabatically warms. Chinook elements: Severe winds • Example: Boulder Colorado windstorm damage • Can occur in any month, but most common in winter (notice the snow) Chinook elements: Severe winds • Chinook elements: fluctuating winds • Strongest surface winds are at base of the ‘wave’. • Wave base location moves back & forth over course of a few minutes • Location at the circle has strongest winds in panels B & D, much weaker winds in A & C. • Consequently, – the winds vary rapidly from near 0 to 130+ mph in a matter of a minute! Figs. 16.02 & 16.B Chinook elements: warm lee side Figure 16.06 Warming occurs for chinook and for Santa Ana winds. Why is lee side surface air warmer? Adiabatic Compression Even without latent heat release, stable air upwind of mountain means ELR much less than adiabatic lapse rate, Sinking high altitude air warms much faster change of ELR. Example of Adiabatic Compression Figure 10.30 (Ahrens) Here air on lee side is warmed 8 oC by latent heat release compared to the 10C air on the windward side. Figure not quite relevant for strong chinooks since those occur when low-level flow (on left) is blocked by the mountain. In the figure, flow is not blocked but goes over mountain Chinook elements: wall cloud • Wall cloud along mountain ridgeline indicates a Chinook. • Photo is example west of Boulder CO. Chinook Elements: Wave Clouds • These clouds are actually some distance east of Mt. Shasta. • 2nd example: Mt. Rainier Chinook Elements: Wave Clouds • Wave clouds over Owens valley Carson River valley © R. Grotjahn Chinook Elements Wave Clouds • Wall cloud in diagram is marked with solid arrow. • Rotor and wave clouds marked by dashed arrows Wave clouds Wind direction from the air • Looking SW towards Wasatch front, UT. Chinook elements: Pressure patterns • Severest have strongest winds at upper levels of troposphere: “jet stream” (colored yellow) overhead. • At surface: – low pressure on lee side (@D = 1004 mb) – high pressure upstream of mountain, (@G=1016mb) • Creates PGF that drives air from west to east side. PGF is perpendicular to P .D contours • High pressure area has stable air – mtns block flow on upstream side Chinook elements: Wave clouds from space Looking towards NE at clouds downstream of the Colorado Rockies Chinook Elements - wave clouds • High pressure to the west suppresses clouds to west • So underside of wave cloud is lit when sun is low • Result: beautiful sunsets! • Location: Test your understanding: • Owens valley & southern Sierra Nevada mtns. What elements do you see? Sierra Nevada Mtns Chinook • Dust showing the high winds rise up into rotor cloud 2. Santa Ana winds • Similar to the chinook (Driven by P changes across topography) – But has a larger scale of strong winds – And topographic funneling of flow more important Santa Ana Winds L Figure 10.32 (Ahrens) Easterly winds descending from southern California's elevated desert plateau have: 1) heating by adiabatic compression and 2) lowering relative humidity. PGF is parallel to the air motion (red arrows) towards southwest Santa Ana: channeled by topography • Winds can exceed 100 mph. • Like Chinook, occurs more often in winter. • Often preceded by strong Northerlies here in CA central valley. Fig. 16.11 Santa Ana windflow • Static image and movie loop (~4 days) Colors: relative humidity Contours: elevation Colors: Fire weather index Arrows: longer is faster wind Arrows: longer is faster wind Santa Ana winds & fire • 26 October 2003 – $2.5 B losses • Cedar fire: – Worst in CA history (at that time) – 15 deaths – 2,200 homes lost • Fig. 16.13 (below) Santa Ana winds & fire • 20-24 October 2007 – Worse than 2003 disaster 071020-22a-latest.mov – 1/2 to 1M people evacuated Largest in US history – 500,000 ac. burned – >1,500 structures burned – Pt Magu NAS 110kt gust – Cost estimate >$ 2B – 9 deaths ‘Diablo’ North winds & fire • 8-10 October 2017 – $1.3+ B losses • Tubbs fire: – 22 deaths – ~5,600 structures lost, ~3,000 homes (5% of Santa Rosa) – 2,000 more structures lost in adjacent fires • http://www.latimes.com/proje cts/la-me-northern-california- fires-structures/ Dashed: Fire at 11:30pm 8 October. Red shading: extent 4 hours later. Vectors: estimated surface winds during those 4 hours. Burned area as of 10 October Green shading: wildland-urban interface. Burned area 10 October Maps and image from New York Times, 12 October 2017 Video(s) of the Day • September 2005 – Agnew pass area (View towards Mono Lake) • Chile (Villarica) Villarica-t-lapse-20s.mov © Honza Rejmanek Chinook-ovr-mono-lk.mov Lecture Summary: • Topographic winds mainly from temperature variation – Ridgeline convection: During daylight: higher topography warms up air more than air over valley – air rises up topographic slopes – Nocturnal drainage flows: At night: colder air forms near higher topography. Colder is more dense. More dense air sinks down topographic slopes into valley. – Katabatic winds: cold air over a high altitude glacier is more dense, flows down slopes. • Topographic winds mainly from pressure variation – Need PGF in same direction as wind. Adiabatic compression warms that air. – Severe downslope winds: brings high speed air down from high elevations. – Elements: (1) upstream winds blocked, (2) severe winds, (3) lee side warming, (4) wall cloud, (5) rotor, and wave clouds – Santa Ana: variation on severe downslope winds but covers larger region. Central valley “north wind events” are similar. • Topography channels & concentrates flow making high winds End of lecture 7 Next time: heat waves .
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