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ATMS 373- Mountain Meteorology

ATMS 373- Mountain Meteorology. Packet#4 Precipitation. http://www.ils.unc.edu/parkproject/webcam/webcam.html. ATMS 373- Mountain Meteorology. Outline Precipitation (Chap. 8; Sections 8.1-8.6) Cool season storms (MetEd Link 1) Impact on fronts (Article 2) NW flow snowfall (Article 3).

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ATMS 373- Mountain Meteorology

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  1. ATMS 373- Mountain Meteorology • Packet#4 • Precipitation http://www.ils.unc.edu/parkproject/webcam/webcam.html

  2. ATMS 373- Mountain Meteorology • Outline • Precipitation (Chap. 8; Sections 8.1-8.6) • Cool season storms (MetEd Link 1) • Impact on fronts (Article 2) • NW flow snowfall (Article 3) http://www.ils.unc.edu/parkproject/webcam/webcam.html

  3. ATMS 373- Chapter 8 • Precipitation • Primary danger is when it occurs with an unexpected • type • intensity • time • frequency http://appalachianhistory.blogspot.com/2007/08/north-carolina-ghost-town.html

  4. ATMS 373- Chapter 8 • Precipitation • (8.1) Types of Precipitation • Hydrometeors • Types of precipitation particles

  5. ATMS 373- Chapter 8 • Precipitation; (8.1) Types of Precipitation • Sleet • (U.S.) ice pellets that form when rain or partially melted snowflakes refreeze before reaching the ground • (U.K.) mixture or rain and snow

  6. ATMS 373- Chapter 8 • Precipitation; (8.1) Types of Precipitation • Snow pellets (a.k.a. graupel) • Common in high mountain areas in summer • Low density particles • Formed when a small ice particle falls through a cloud of supercooled water droplets • Tiny droplets freeze on impact • Coating is called rime

  7. ATMS 373- Chapter 8 • Precipitation; (8.1) Types of Precipitation • Snowpack density • Water content of snow • Expressed as specific gravity • Numerically equivalent to actual snow density if units are g cm-3 • Divide water-depth equivalent of snow by actual snow depth • 10 inch snowfall, if melted, might produce 1 inch of liquid water, specific gravity = 0.10 • Densest (wettest) snow has a specific gravity near 0.40 • Most snow densities in U.S. ~ 0.04 to 0.10 g cm-3 http://radio.weblogs.com/0101170/images/water/measuringsnowpack.jpg

  8. ATMS 373- Chapter 8 • Precipitation; (8.1) Types of Precipitation • Snowpack density factors • Temperatures of cloud and surface • Cloud humidity • Wind speed between cloud and surface • High snow density combinations; “warm” sfc temperatures, small crystals, strong winds • Low snow density combinations; low sfc temperatures, highly branched crystals, weak winds http://www.digitalfieldguide.com/blog/date/2007/03/page/2/

  9. ATMS 373- Chapter 8 • Precipitation • (8.2) Intensity of Precipitation • Light • Moderate • Heavy

  10. ATMS 373- Chapter 8 • Precipitation • (8.3) Measuring Precipitation • Rain • Depth • Weight • “tips” • Snow • Depth • Weight • Liquid equivalent http://facstaff.unca.edu/dmiller/site01a.JPG

  11. ATMS 373- Chapter 8 • Precipitation • (8.4) Formation of Precipitation • Unsaturated parcel cools (9.8oC km-1) through lift (adiabatic expansion) • Reaches its lifted condensation level (LCL) • Continued lift of parcel cools it at the moist adiabatic lapse rate (slower cooling rate due to latent heat release)

  12. ATMS 373- Chapter 8 • Precipitation; (8.4) Formation of Precipitation • Vapor condenses onto cloud condensation nuclei (prefer largest and most hygroscopic particles) • Air parcel may be lifted above the freezing level • Supercooled liquid water; liquid water droplets at temperatures below freezing http://www.flame.org/~cdoswell/wxmod/drops.JPG

  13. ATMS 373- Chapter 8 • Precipitation; (8.4) Formation of Precipitation • Hundreds of cloud droplets per cubic centimeter (cc) • Cloud droplet diameter ~ 20 micrometers (mm) • Follow air motions within cloud • Need particles much larger than cloud droplets to form precipitation http://www.flame.org/~cdoswell/wxmod/drops.JPG

  14. ATMS 373- Chapter 8 • Precipitation; (8.4) Formation of Precipitation • Precipitation formation mechanisms • Warm rain processes (T > -15oC) • Requires cloud droplets of different sizes (different fall speeds) • Initially requires broad spectrum of CCN sizes • Common over coastal regions • Ice crystal process • Requires coexistence of water droplets and ice crystals • Common over temperate continental regions http://physics.uwstout.edu/WX/u5/U5_04a.gif

  15. ATMS 373- Chapter 8 • Precipitation; (8.4) Formation of Precipitation • Precipitation formation mechanisms • Ice crystal process • In mixed phase clouds, ice crystals form at the expense of supercooled liquid droplets • As particles fall, growth occurs through collisions • Riming • Aggregation http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/graphics/natural_seed_schem.jpg

  16. ATMS 373- Chapter 8 • Precipitation; (8.4) Formation of Precipitation • Precipitation formation mechanisms • Unglaciated clouds • Water clouds • Clearly defined edges • Glaciated clouds • Composed of ice particles only • Diffuse or filmy appearance • Optical effects; halos, sundogs

  17. ATMS 373- Chapter 8 • Precipitation; (8.4) Formation of Precipitation • Precipitation formation mechanisms • Liquid precipitation • Dark appearance when falling from the cloud base • Ice precipitation • Whitish, filmy appearance when falling from cloud base (right)

  18. ATMS 373- Chapter 8 • Precipitation; (8.4) Formation of Precipitation • Precipitation formation mechanisms • Precipitation can evaporate or sublimate before reaching the ground (virga)

  19. ATMS 373- Chapter 8 • Precipitation • (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.1) Effects of Terrain Height • (8.5.2) Day to Day and Diurnal Variations • (8.5.3) Seasonal Variation • (8.5.4) Year to Year Variations http://www.fao.org/ag/agl/swlwpnr/reports/y_ea/z_mn/mn_map/mnmp131.png

  20. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.1) Effects of Terrain Height • Note influence of terrain on mean annual precipitation (1931-1960) • Olympic Mntns, Coast Range, Cascade Range, Sierra Nevada, Rockies, Black Hills, Appalachians

  21. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.1) Effects of Terrain Height • Focus on Washington state • Olympic Mntns • Cascade Range • Recall Sequim, WA http://www.northolympic.com/webcams.php

  22. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • Affected by • Terrain height • Proximity to moisture sources • Terrain relief • Terrain aspect (direction a slope is facing) relative to direction of approaching wind

  23. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.1) Effects of Terrain Height • Prevailing westerly winds lift moist Pacific air masses up over the Olympic and Cascade ranges • Annual precipitation amounts • 150” Olympics • 110” Cascades • 14” Sequim, WA • 6” east of Cascades

  24. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • Maps of precipitation distribution in mountainous regions are of limited value • Limited accuracy (wind, frequent freezing and thawing, heavy precipitation) • Maps based on interpolations between widely dispersed measurements http://www.cropscience.org.au/icsc2004/symposia/1/5/2085_deng.htm

  25. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.1) Effects of Terrain Height • Valleys typically have the driest climates • Rhone Valley (Switzerland) • Central Valley (CA) • Great Basin (western U.S.) • North and South Parks (CO) • Jackson Hole (WY) http://www.fws.gov/stillwater/Virttour/Virttour1.htm

  26. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.2) Day to Day and Diurnal Variations • Precipitation forecasts* are one of the most difficult forecasts in meteorology • Synoptic-scale weather • Moisture availability • Cloudiness • Wind speed and direction • Exposure and degree of insolation http://www.hpc.ncep.noaa.gov/ *especially difficult in summer (isolated T-storms)

  27. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • Diurnal Variations • Often reflect diurnal variations in thunderstorm activity • Rockies; highest in early to late afternoon • Midwest; highest at night, in conjunction with the low-level jet (LLJ) • LLJ develops just above sfc of sloping Great Plains • LLJ brings moist air northward from Gulf of Mexico

  28. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.3) Seasonal Variation • West Coast; winter storms bring moisture from the ocean • Intermountain West; late spring or summer convection provide most precipitation • Quite variable along Appalachians, depending on latitude

  29. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.3) Seasonal Variation • Winter; land is cold, high pressure common, precipitation less frequent • Summer; land is warm, low pressure is common, moisture flows from ocean/gulf to land • Potential for heavy rainfall if marine air is lifted up steep mountainsides http://gateway.cd.gov.ab.ca/flooding.aspx

  30. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.3) Seasonal Variation; monsoons • Himalayan Monsoon of Southeast Asia, most familiar example http://www.belfasttelegraph.co.uk/news/world-news/article2838887.ece

  31. Winter Summer Changing annual wind flow patterns associated with the winter Asian monsoon. Clear skies and winds blow from land to sea Changing annual wind flow patterns associated with the summer Asian monsoon. Warm humid air blows up from equator bringing rainy weather.

  32. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.3) Seasonal Variation; monsoons • Mexican Monsoon (a.k.a. North American Monsoon) of Southwest U.S. • Shift in winds in mid-summer from the south brings moisture from the Gulf of California

  33. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.3) Seasonal Variation; monsoons • Mexican Monsoon (a.k.a. North American Monsoon) of Southwest U.S. • Feeds afternoon thunderstorms on east side of Great Basin (Arizona’s Mogollon Rim)

  34. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.3) Seasonal Variation; monsoons • Mexican Monsoon (a.k.a. North American Monsoon) of Southwest U.S. • Responsible for summertime precipitation maxima in AZ, NM, and southern UT, CO • Sheets of thick cirrus observed in afternoon or evening indicate the presence of low-level monsoonal moisture http://www.nasa.gov/centers/goddard/news/topstory/2006/rainfall_name.html

  35. ATMS 373- Chapter 8 • Precipitation; (8.5) Spatial and Temporal Distribution of Precipitation • (8.5.4) Year to Year Variations • Can be very large in continental interiors • Where rain comes predominantly from isolated T-storms (highly variable) • Leads to occasional droughts and floods http://www.cpc.ncep.noaa.gov/products/expert_assessment/seasonal_drought.html

  36. ATMS 373- Chapter 8 • Precipitation • (8.6) Icing • Large supercooled liquid droplets • Water spreads before freezing, forming a layer of clear ice • Small supercooled liquid droplets • Droplets freeze before they have time to spread across a surface (white rime)

  37. ATMS 373- Chapter 8 • Precipitation; (8.6) Icing • Rime • Common in areas having clouds with supercooled liquid with bases below the mntn summit • Augments annual precipitation in eastern Cascades by 2-5 inches • Causes significant problems for power lines and instrumentation

  38. ATMS 373- Chapter 8 • Precipitation • MetEd Link#1 • Dynamics and microphysics of cool season orographic storms

  39. ATMS 373- Scaling Parameters • Froude number (Fr) • U = wind speed normal to mountain • N = Brunt-Väisälä frequency • S = vertical (for some applications, horizontal) scale of the mountain Wallace & Hobbs (2006), p. 407, 408

  40. ATMS 373- Scaling Parameters • Froude number (Fr) • A measure of whether flow will go over (surmount) a mountain range • Small Fr; low-level airflow is forced to go around the mountain and/or through gaps • Larger Fr; more airflow goes over the mountain crest Wallace & Hobbs (2006), p. 407, 408

  41. ATMS 373- Background • Quasi-geostrophic theory strong PVA p2 weak PVA p1 +Z final time initial time

  42. ATMS 373- Background • Quasi-geostrophic theory (continued) weak PVA p2 strong PVA p1 +Z final time initial time

  43. ATMS 373- Background • Quasi-geostrophic theory (continued) without compensating vertical motions p2 cold air advection p1 +Z final time initial time

  44. ATMS 373- Background • Quasi-geostrophic theory (continued) with compensating vertical motions p2 cold air advection p1 +Z final time initial time

  45. ATMS 373- Background • Quasi-geostrophic theory (continued) • Strong PVA aloft/ weak PVA low  rising motions • Weak PVA aloft/ strong PVA low  sinking motions • Cold air advection  sinking motions • Warm air advection  rising motions

  46. ATMS 373- Impact on Fronts • Frontal Interaction with the Appalachian Mountains. Part I: A Climatology • Philip Schumacher, David Knight, and Lance Bosart • Monthly Weather Review, November 1996 • p. 2453-2468

  47. ATMS 373- Impact on Fronts • Introduction • Purpose: (1) document the synoptic-scale signature of retarded and unretarded fronts, and (2) identify which atmospheric parameters are most important in determining when a front will be retarded in its interaction with the Appalachians

  48. ATMS 373- Impact on Fronts • Introduction • Three to five cold fronts interact with Appalachians per month on average • Geometry of mountains provides an excellent source for comparison to idealized studies • Reasonably good data coverage

  49. ATMS 373- Impact on Fronts • Introduction • Bergeron (1937) • “mountain ranges retard all fronts” • O’Handley and Bosart (1996) • 80% of fronts crossing Appalachians were slowed by the mountains • No studies have compared the evolution of retarded and unretarded fronts

  50. ATMS 373- Impact on Fronts • Introduction • Numerous idealized studies have shown the importance of Froude number in determining the subsequent retardation of the front • Direct correlation between size of Froude number and degree of retardation

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