Download Chinook or Foehn Winds Chinook, cont

Chinook or Foehn Winds
•  The true chinook wind (vs. any warm wind
blowing out of the mtns) is a warm wind resulting
from latent heating.
•  Recall that wind forced over a mountain barrier
will expand and cool as it ascends, but on the
down hill side will compress and warm as it
descends.
•  If no heat is added during this process, the process
is adiabatic and if the air parcel returns to the same
pressure level it started out at, it will be the same
temperature. (reversible process).
•  Now, suppose the parcel is almost saturated BEFORE it
ascends (say, on the seaward side of a coastal mountain
range).
•  As it ascends the mountain ridge, it cools at the moist
adiabatic rate 6 C/km (right?) it produces condensate and
releases latent heat. IF the timescale is long enough for
precipitation to form, the ascending air mass precipitates
out much of its condensate.
•  But, on the descending side, little or no cloud water is left
to re-evaporate and… since the condensate has been lost
(precipitated out), no heat has to be used by the parcel to
re-evaporate the condensate (irreversible process).
•  Hence as it descends it warms as a dry parcel at the dry
adiabatic lapse rate (10 C/km)
•  If the parcel returns to its original elevation, it has gained
in temperature. Latent heat has been converted to sensible
heat, and the parcel is dry and warm.
Chinook, cont
•  The only problem with this model is that the air is now so
much warmer that it is buoyant and does not want to return
to the surface!
•  In reality, in Chinook situations there is usually lower
pressure on the lee (downslope) side of the mountain
barrier and this helps to “draw” the air back down...
•  NOTE: The chinook (a thermodynamic phenomenon) is
distinct from the mountain wave (a hydrodynamic or
hydraulic phenomenon.)
•  The mountain wave also results in warming (for different
reasons), but usually comes with VERY HIGH damaging
winds.
•  In reality both Mtn Waves and the Chinook arise from the
same large-scale pressure patterns and can occur at the
same time. Most forecasters do not distinguish between the
two.
Santa Ana (desert) winds
Another warm dry wind is typified by
the Santa Ana winds. These are not
true chinook winds, but also are the
result of compressional warming, as
the relatively cool air over the high
Great Basin (7000 ft) flows through
passes towards lower pressures. Along
the coast.
The true chinook produces a distinct wall of cloud, visible on the
lee (downwind) side as a cap with rough edges (much different
from the lenticular cloud). This cloud formation is referred to as a
Chinook or Foehn wall. The warm air on the lee side is so dry it
can melt a foot of snow a day, depending on conditions.
This air already starts out dry, but
relative humidity is lowered even
further as the air warms on descent to
near sea level. These hot, dry winds
are responsible for many of the bad
forest fires in the Los Angeles area.
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Mountain and Valley Circulations
•  Remember how the sea/land breeze was driven by
localized heating/cooling of the land vs. water?
•  In COMPLEX TERRAIN (i.e. mountains) a similar
situation exists, since the terrain that faces the sun most
directly gets the strongest heating.
•  For example, east-facing slopes get heated first thing in the
morning. As they are warmed, they warm the boundary
layer (BL) just above them, making this air buoyant, so
that it causes an up-slope current to form.
•  As this air is removed it is replaced by air from lower
down on the slope. This eventually causes an “upstream”
current to flow IN THE VALLEY.
FLYING TIPS and mountain/valley
circulations
•  Warmed slopes have rising air, eventually causing updrafts
at/along ridge lines
•  Upstream valley flows also develop, but their “response
time scale” is slower than slopes
•  Slopes that are in a cooling mode usually have downslope
winds, causing downdrafts along ridge lines.
•  Again, down slope or drainage flows develop due to
cooling (diurnal katabatic wind?)
•  depending on terrain configuration these effects can be
quite strong.
•  large-scale (synoptic) pressure gradients can mask or even
reverse these diurnal flows.
•  These effects are most evident on otherwise calm and
cloud-free days.
ACSL or Lenticular Clouds, cont.
•  ACSL are typically found when strong flow
interacts with the terrain. THESE ARE CLOUDS
TO AVOID, as they are often associated with
strong up and down drafts.
•  ACSL often sit on top of, or just to the lee of, the
terrain that forces them.
•  ACSL result from waves, and a decaying wave
train probably continues downstream from the
forcing object (i.e. the mountain ridge) even
though no clouds are apparent!
Just the opposite happens once the hillside is shaded. The surface
cools (how?) and the BL cools also, becomes negatively buoyant
and sinks, causing a downward motion, which eventually causes a
down-valley flow in the valley bottom
Atmospheric and mountain waves
•  Waves, and therefore wave clouds, are usually forced
(caused) by localized phenomena (such as thunderstorms,
topography, etc.).
•  Since waves tend to be localized, they are harder to
forecast than other meteorological phenomena
•  Therefore, a pilot needs to 1) identify wave clouds, and 2)
be aware of what they imply about the clear air
surrounding them.
•  One of the most important wave clouds are known as
Altocumulus Standing Lenticular clouds, often abbreviated
in METARS as ACSL (as in: ACSL ALL QDS)
ROTORS
Occasionally a ROTOR will develop in the same environment that
spawns ACSL, especially if the ridge is steep and quite tall. Surviving a
rotor encounter in an aircraft is rare.
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How to fly smart in the presence of
mountain waves and wave clouds
•  AVOID THEM!
•  use ACSL as an indicator— remember that they suggest
strong winds (and up/downdrafts) at ridge top.
•  The dangerous conditions that cause ACSL along one
ridge probably exist along all similarly-oriented ridges,
EVEN IF THEY SHOW NO ACSL THEMSELVES.
•  keep a good distance above the terrain! Mtn waves can
cause you to lose hundreds to thousands of feet in a
manner of several seconds.
•  expect strong downdrafts/severe turbulence (and possibly
rotors) to the lee of all tall ridges that are more or less
perpendicular to the ridge top flow.
MOUNTAIN FLYING cont.
•  remember that flow (wind direction) in valleys
between mountains is often a poor indicator of
flow direction at ridge top.
•  flow in valleys is “channeled” and can often be
90° different than the ambient (large-scale) flow.
•  in the presence of mtn waves, flow in valleys is
always gusty, often turbulent, and occasionally
changes direction 180° almost instantly.
•  in addition to wave clouds, blowing snow off
ridge tops (and blowing sand on river bottoms)
can present a visibility hazard. (Watch for signs of
these.)
•  use your brain and what you know about fluid
flow to avoid these hazards.
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