temperature lapse rate- decrease of temperature with height: = - dt/dz environmental lapse rate ( ...
Post on 21-Dec-2015
231 views
TRANSCRIPT
Temperature• Lapse rate- decrease of temperature with height:
= - dT/dz• Environmental lapse rate () order 6C/km in free
atmosphere
• d- dry adiabatic lapse rate- rate at which an unsaturated parcel cools when lifted= 9.8 C/km
• s- saturated adiabatic lapse rate- rate at which a saturated parcel cools when lifted= 4-9.8 C/km
Stability
• Vertical momentum equation– vertical accelerations due to imbalance between downward
directed gravitational force and upward directed pressure gradient force
• Stable- adiabatic parcel displaced from original altitude accelerated back towards original altitude
• Neutral- adiabatic parcel displaced from original altitude. continues to move at a constant speed
• Unstable- adiabatic parcel displaced from original altitude continues to accelerate away from original altitude
Skew-T log P diagrams
• Plot vertical profile of temperature, moisture, wind as a function of elevation
• Skewed to draw attention to vertical variations in temperature that deviate from typical 6C/km decrease with height
• Dew point temperature- absolute measure of water vapor = f(e)
Stability
• Adiabatic parcel conserves potential temperature as it rises or sinks
• Stable atmosphere: d /dz > 0
• Neutral atmosphere: d /dz = 0
• Unstable atmosphere d /dz < 0
z Stableatm
Planetary Boundary Layer
• PBL-Layer in atmosphere affected by interaction with the surface
• Free atmosphere- atmospheric layer above the PBL in which state variables largely unaffected by the surface
PBL
• Daytime convective boundary layer– Neutral lapse rate above surface– Parcels move freely vertically– Strong mixing – Can be several thousand meters deep over western U.S.
• Nocturnal stable layer– Temperature usually increases with height away from the
surface – inversion– Parcels flow horizontally– Little mixing– Usually few hundred meters deep
Wind Speed
• Terrain controls wind speed and direction• However, some general characteristics of wind speed
vs. altitude• Mid-latitudes:
– Wind speed increases with height– Mt. Washington 1915 m: 23m/s in winter;12m/s in summer
averages
• Tropics– Wind speed decreases with height– New Guinea 4250 m: 2 m/s DJF average– El Misti Peru 4760 m 5 m/s average
Wind Speed over Summit
• Vertical compression of airflow over mountain accelerates air
• Friction retards flow– Small scale roughness effects (<10 m
dimension)– Form drag (10m<topography<1km)
• Dynamical pressure perturbations created• Proportional to slope2
• Influences atmosphere through considerable depth
Vertical compression
• Consider case first of steady state, incompressible fluid flowing through constriction: Bernoulli effect
Conservation of energy:Kinetic Energy + work down by pressure force + potential energy = 0
Vertical compression
• (U22 – U1
2)/2 + (p2 – p1)/ + g(z2 – z1) = 0
U2 > U1
z2 = z1 sop2 < p1
Lower pressure in constriction
12
Vertical compression
• (U22 – U1
2)/2 + (p2 – p1)/ + g(z2 – z1) = 0
U2 > U1
z2 = z1 sop2 < p1
Lower pressure over summit
12
Roughness Effects
• For well-mixed conditions (near neutral lapse rate)
• U2 = u1 ln (z2/zo)/ln(z1/z0)
• Roughness length zo=.5 h A/S where h height of obstacle, A- silhouette area, S surface area A/S< .1
• Zo- height where wind approaches 0