Temperature
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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 |
Stability
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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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Stability
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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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Lapse Rate
Parcel Theory
Skew-T log P diagrams
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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) |
Stability
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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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Planetary Boundary Layer
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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 |
PBL
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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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Diurnal PBL Evolution
Diurnal Change in
Temperature
Surface based temperature
inversion
Elevated Inversion
Diurnal changes in
stability
Mountain/Valley PBL
Mountain PBL
Free Air vs. Mountain
Valley vs. Summit
Influence of Wind Speed
Influence of cloud cover
Diurnal Temperature Range
Diurnal Temperature
Range: Western U.S.
Wind Speed
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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 |
Wind Speed over Summit
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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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Vertical compression
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Consider case first of steady state,
incompressible fluid flowing through constriction: Bernoulli effect |
Slide 26
Slide 27
Wind over Hill
Free Air vs. Summit
Roughness Effects
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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 |
Terrain Roughness