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Lapse rate- decrease of temperature with height:
G = - dT/dz |
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Environmental lapse rate (G) order 6C/km in free
atmosphere |
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Gd-
dry adiabatic lapse rate- rate at which an unsaturated parcel cools when
lifted= 9.8 C/km |
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Gs-
saturated adiabatic lapse rate- rate at which a saturated parcel cools when
lifted= 4-9.8 C/km |
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Vertical momentum equation |
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vertical accelerations due to imbalance between
downward directed gravitational force and upward directed pressure gradient
force |
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Stable- adiabatic parcel displaced from original
altitude accelerated back towards original altitude |
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Neutral- adiabatic parcel displaced from
original altitude. continues to move at a constant speed |
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Unstable- adiabatic parcel displaced from
original altitude continues to accelerate away from original altitude |
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Absolutely Stable: G< Gd |
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Absolutely Unstable: G> Gd |
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Conditionally Unstable: Gs < G<
Gd |
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Plot vertical profile of temperature, moisture,
wind as a function of elevation |
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Skewed to draw attention to vertical variations
in temperature that deviate from typical 6C/km decrease with height |
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Dew point temperature- absolute measure of water
vapor = f(e) |
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Adiabatic parcel conserves potential temperature
q as it rises or sinks |
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Stable atmosphere: d q /dz > 0 |
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Neutral atmosphere: d q /dz = 0 |
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Unstable atmosphere d q /dz < 0 |
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PBL-Layer in atmosphere affected by interaction
with the surface |
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Free atmosphere- atmospheric layer above the PBL
in which state variables largely unaffected by the surface |
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Daytime convective boundary layer |
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Neutral lapse rate above surface |
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Parcels move freely vertically |
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Strong mixing |
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Can be several thousand meters deep over western
U.S. |
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Nocturnal stable layer |
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Temperature usually increases with height away
from the surface – inversion |
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Parcels flow horizontally |
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Little mixing |
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Usually few hundred meters deep |
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Terrain controls wind speed and direction |
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However, some general characteristics of wind
speed vs. altitude |
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Mid-latitudes: |
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Wind speed increases with height |
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Mt. Washington 1915 m: 23m/s in winter;12m/s in
summer averages |
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Tropics |
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Wind speed decreases with height |
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New Guinea 4250 m: 2 m/s DJF average |
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El Misti Peru 4760 m 5 m/s average |
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Vertical compression of airflow over mountain
accelerates air |
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Friction retards flow |
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Small scale roughness effects (<10 m
dimension) |
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Form drag (10m<topography<1km) |
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Dynamical pressure perturbations created |
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Proportional to slope2 |
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Influences atmosphere through considerable depth |
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Consider case first of steady state,
incompressible fluid flowing through constriction: Bernoulli effect |
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For well-mixed conditions (near neutral lapse
rate) |
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U2 = u1 ln (z2/zo)/ln(z1/z0) |
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Roughness length zo=.5 h A/S where h
height of obstacle, A- silhouette area, S surface area A/S< .1 |
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Zo- height where wind approaches 0 |
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Notes