II' Local Mesoscale Circulation Systems

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II. Local Mesoscale Circulation Systems

• Upslope Precipitation
• Orographically enhanced Convection
• Downslope Wind Storms

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Upslope Precipitation

• Simple Orographic Storms
• Cold-Air Damming Upslope Systems
• Orogenic Convection

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Neutral Atmosphere

• Consider Flow over a cylinder mountain in a
  neutrally stratified atmosphere
• Formation of an evanescent wave, i.e. a wave
  having decreasing amplitude with height

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Stable Atmosphere

• Consider Flow over a cylinder mountain in a
  stably stratified atmosphere
• Formation of vertically propagating or trapped
  mountain wave

Vertically propagating wave
Trapped lee wave
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Orographic Lifting

• Consider the following sounding

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Orographic Lifting

• Now superimpose the lifting profile for the
  cylinder

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Blocking Effects
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Blocking by Hawaii
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Flow Blocking by Hawaii
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Orographic Destabilization by Differential
Advection
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Orographic Cloud Formation

• Besides orographic lifting, mountains can create
  clouds directly by their
• Thermal forcing
• Obstacle effects
• Mountain waves

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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

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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.

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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.

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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.

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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 1880s 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!

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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?).

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Orographic Cloud From Lee Side
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Early Studies of Orographic Clouds at Colorado
State University

• Professor Lou Grant ran a pioneering study of the
  potential for Weather modification in the Rockies
  during the 1960s and early 1970s in the
  mountains near Climax Colorado (also near
  Leadville and Summit County)
• Ran a long series of blind seed/no seed tests,
  measuring resulting snowfall
• Concluded that there was a statistically
  significant 15 increase in snowfall from seeding
  the clouds with silver iodide
• That result was contested later

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COSE(Colorado Orographic Seeding Experiment)

• 1981/1982 return to field to perform a physical
  mechanism study of orographic clouds and the
  effects of seeding
• Study set up in the Park Range, base operations
  at the Storm Peak Laboratory at the sumit of
  Steamboat Springs ski area.

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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

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Fletcher Ice Crystal Concentration

• Cloud droplets initiate almost 100 from
  available cloud condensation nuclei (CCN) within
  100-101 saturation. Typical values of CCN are
  100-1000 cm-3, depending on how dirty the air.
• Ice nuclei are activated much less easily,
  because a crystal must begin growing on an
  imperfect base, i.e. a non water crystal.
• At 0C, ice is very particular on what it will
  start to grow on, and almost no substance is
  close enough to actual ice

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Fletcher Ice Crystal Concentration

• As temperature gets colder, ice becomes less
  particular about how close the base aerosol is to
  actual ice, allowing increasing proportions of
  the available aerosols to act as ice nuclei.
• An empirical study by Fletcher, yielded the
  relationship for ice nuclei

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• Where is the Celsius temperature and IN is in
  cm-3.
• Note that
• IN is at 0C and increases by about 1 order
  of magnitude for every 4C drop in temperature.
• IN reaches (a typical CCN value) at 40 C,
  but cloud droplets of that concentration (fog)
  are so small they dont fall, neither would ice.
• Ice crystals big enough to fall, tend to have
  concentrations of cm-3, or the
  Fletcher concentration at 20C.

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• 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

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Fate of Orographic Clouds
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Evolution of an Orographic Cloud

• Begins as shallow cloud with only cloud water
  droplets
• Grows deeper to reach 20 at cloud top and may
  form precipitation
• Becomes more shallow and precipitation ceases
• Can seeding be used early and late to get more
  precipitation out?

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Other Issues for Precipitation

• Crystal Types Some crystal types fall faster
  than others
• Aggregation Aggregation of Crystals produces
  larger faster falling crystals
• Multiplication There are mechanisms for
  increasing crystal concentration by
  multiplication
• Hallett-Mossop Mechanism
• Crystal fragmentation.

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Crystal Fragmentation

• Delicate crystals can become broken
• Broken pieces become fragments that can act as
  new ice nuclei

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Hallett-Mossop Multiplication

• Near 4 to 6 Celsius, cloud droplets greater
  than 12 micros radius riming onto an ice crystal
  form a shell of ice before freezing into their
  interior.
• When the ice formation spreads to the interior,
  the interior of the droplet expands and explodes
  through the ice shell producing small crystal
  splinters, as many as 360 splinters per droplet.
• These splinters are perfect ice nuclei that
  dramatically increases the IN content.
• The splinters grow immediartely to form numerous
  needle shaped crystals

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Needle Aggregates

• Ice has a propensity to stick to other ice in the
  range of 0 to 5 Celsius. So needle crystals,
  especially when numerous from Hallett-Mossop
  multiplication, are likely to aggregate when the
  fall from the 6 C zone down to lower levels
  where temperatures approach 0C. Not often in
  Rockies, but likely in marmer orographic clouds
  of the Sierras.
• Snow produced by needle aggregates is quite
  sticky and good packing. Good for snowmen, bad
  for skiing. mashed Potato snow, Cascade
  Concrete.

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Dendrite Aggregates

• Crystals growing in the range of 12 to 15 C
  will be either hexagonal plates if unsaturated
  with respect to liquid, or dendritic if
  saturated. Saturation is likely if
• Too few crystals to drive humidity below water
  saturation
• Active updraft producing saturation
• Because of their complex structure, dendrites
  tend to get tangled up forming dnedrite
  aggregates.
• Dendrite aggregates tend to be airy and low
  density, forming a light low density dry snow
  pack.
• Good for skiing Champaign powder, bad for
  snowmen.

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Prefrontal Orographic Storms

• Inverse relationship between precipitation and
  liquid water
• A direct relationship between cloud top
  temperature and liquid water content
• Magnitude of liquid water considerably higher
  over mountain slopes

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22 January, 1982
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15 December, 1982
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Postfrontal Storms

• Liquid water content has little variability
  upwind, but varies considerably in vicinity of
  mountain
• Magnitude of liquid water content inversely
  related to precipitation at mountain base
• Liquid water production near ridgeline associated
  with both
• Orographic lifting
• Convective forcing

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21 December, 1981
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Pure Orographic Storms

• Shallow, tops warmer than 22C and limited
  horizontal extent
• Changes in liquid water content inversely
  associated with precipitation rate
• One case- decrease in liquid water content
  associated with a decrease in cloud top

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14 January, 1982
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COSE systems

• High LWC ? Low precipitation rate at mountain
  base
• High LWC ? warm cloud top temperature

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Locations of Supercolled Water
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Observed Crystal Habits
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Observed Relationship of IN to T
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Sierra Cooperative Pilot Project(SCPP skippy)

• To study physical processes in Sierra mountain
  induced storms to verify and improve the weather
  modification technology

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Barrier Jet
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Conceptual Model of Sierra Storm
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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

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Diurnal Variation of Upslope Storms

• Seemingly a night time maximum of precipitation
• Apparent reason may be the radiationally cooling
  makes clouds colder , hence
• More efficient
• More likely to cool below dew point

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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

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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

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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

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