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Air Pollution Meteorology, Exam 2
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Gravity
Key Concepts:
Terms in this set (95)
What is the planetary boundary layer?
The layer of air closet to the Earth's surface
How thick is the planetary boundary layer?
300 to 3000 m
How do pollutants move in the atmosphere?
Most are trapped in the planetary boundary layer; those that don't are able to travel extraordinary distances
What forces affect the planetary boundary layer?
Surface Forces
How long does it take for surface to affect the planetary boundary layer?
approximately an hour
At what depth does the temperature stabilize in the soil.
2.5 cm - still steadily fluctuates
15 cm - significantly stabilized
What is the surface layer and its properties?
The lowest 10 percent of the planetary boundary layer; strong wind shear; unstable during the day; very stable at night
What does strong wind shear physically translate to?
a change in wind speed with height, from 5 to 6 m/s
What is a mixed/convective layer?
Strong thermals mix warm air near the air which has cooler air above.
When does the mixer/convective layer occur?
Only during the day
What is the stability of the mixed/convective layer?
surface: unstable
middle: neutral
top: stable (inversion or entertainment)
What are mixed/convective layer smoke plume layers like?
Mostly looping (otherwise fumigating/coning)
When does the residual layer occur?
It is what is left of the mixed/convective layer after sunset (after there are no more thermals)
What is a residual layer?
A turbulent layer with:
Neutral stability, eroded by a surface stable layer
What are residual layer smoke plume layers like?
Coning plumes
What causes the stable layer to form?
The bottom part of the residual layer becomes stable due to the ground cooling. Thus the turbulence is suppressed
What are the smoke plumes like in the stable layer?
The smoke plumes are fanning
What is the structure of the PBL during the day (after sunrise to sunset)
Surface Layer and Up: Convective/Mixed Layer
Capped by cloud layer and Entertainment zoned
What is the structure of the PBL during the night (after sunset to sunrise)
-The Stable Layer develops (gets bigger over time)
-residual layer above the stable layer and capping inversion
-capping inversion at top
What are the effects of friction in the PBL?
Significant drag against the Earth's surface, high energy dissipation
What is turbulence like in the PBL?
It is continuous through the layer
How is the PBL mixed?
It is mixed rapidly vertically and horizontally
Are there diurnal variations in the PBL?
There are strong diurnal variations in the PBL
What are the effects of friction in the free atmosphere?
There is insignificant friction, little energy dissipation
What is turbulence like in the free atmosphere?
Only in convective clouds and in jet streams
What are wind patterns like in the PBL?
They are logarithmic in the surface later, commonly cross-isobaric
What are wind patterns like in the free atmospheric?
Nearly geostrophic (parallel to isobars)
What is the thickness of the free atmosphere?
8-18 km
What is the mixing of the free atmosphere?
rapid in the horizontal, weak vertically
What are the diurnal variations in the free atmosphere?
weak
What forces are in the moment equation?
The body, surface, and apparent forces.
What is a body force?
A force that affects the entire bulk of a fluid parcel but acts as a distance
What is a surface force?
Affect the surface of a fluid parcel while in contact with other parcels.
What are examples of body forces?
Gravity and Magnetism
What are examples of surface forces?
Normal stresses, Shear and Tangential Forces
What kind of forces is pressure?
A Normal Stress -> Surface Force
What kind of force is a viscous force?
Shear and Tangential Force
What is an Apparent Force?
Due to non-inertial earth forces
What is the difference between true and effective/apparent gravity?
Effective/apparent gravity includes the centrifugal acceleration due to the Earth's rotation
Where does the true gravity point?
Towards the center of the Earth
Where does the effective gravity point?
It is aligned with the local center, perpendicular to the tangent plane on the actual Earth's surface
What causes the pressure gradient force?
Only caused by the differences in the hydrostatic pressure on opposite sides on an air parcel
What does the pressure gradient force cause an air parcel to do?
move from regions of high pressure to low pressure
What is the relationship between isolines and the magnitude of the pressure gradient force?
The closer together the isolines, the larger the PGF
What is the relationship between isolines and the direction of the pressure gradient force?
-Perpendicular to the isobars (it is a gradient)
-always points towards lower pressure
What direction does the Coriolis effect push wind in the northern hemisphere?
To the right
What direction does the Coriolis effect push wind in the southern hemisphere?
To the left
What are the factors that affect the magnitude of the Coriolis effect?
Latitude and wind speed
What is the Coriolis effect at the equator?
Zero
What is the Coriolis effect at the the poles?
Maximum
What is the effect of the wind speeds on the Coriolis effect?
The faster the wind speed the faster the deviation
What effect does the Coriolis effect have on the wind?
It effects the wind direction, not the speed
What types of stresses causes Friction?
The normal and tangential stresses that arise due to mountains and buildings because the air parcel is moving
What type of force is friction?
It is a surface retarding force, it opposes the air parcel's motion.
What causes Friction?
The 9 component turbulence tensor, the divergence of the turbulent tensor
How does friction affect wind?
It retards wind speed near the surface
How does friction affect the Coriolis force?
It lowers the Coriolis force
When does the wind travel parallel to the isobars?
When the pressure gradient is equal to the Coriolis effect, no friction present
In the upper levels
How do we treat viscous force?
In-compressible flow, non-divergent uniform and constant kinematic viscosity of air v
What does the continuity equation tell us?
In a fixed volume, changes in mass are due to the net mass flux through the volume
Mathematically, what is divergence?
Del dot Veolcity =
du/dx + dv/dy + dw/dz
Physically what is divergence?
More is leaving the then is coming out, the vectors are increasing in length away from the volume of interest
What is a retarding force?
A force that moves in the exact opposite direction
What is surface roughness?
The height above the ground at which wind speed is zero; a measure of the vertical turbulence that occurs when a horizontal wind flows over a rough surface
What variable is surface roughness represented by?
zo
What does a high surface roughness indicate?
a high magnitude of turbulence
What is the surface roughness and turbulence for a perfectly smooth surface?
The surface roughness is zero and the mechanical turbulence is minimized
What is the general estimate for surface roughness?
1/30 of the average height of the elements protruding from the surface
What is the relationship between the wind speed and surface roughness?
The lower the surface roughness the faster the wind speed
What is the conversion between knots to m/s?
knots/2 = m/2
How does Reynolds averaging relate the flux and the mean.
U = Ubar +U'
What is the mean flux equal to
0
What causes the variance in the wind?
The turbulence
What is the covariance?
The level of interdependence; non linear of mixed turbulent terms indicate the degree of common relationships between the two variables
What does a covariance of ) indicate?
unrelated turbulence
What is a flux?
A quantity of something (q) that goes through an area per unit time
What is the variable that represents flux
q
What is kinematic flux?
Flux divided by density
What are the units of kinematic flux?
The variable of interest times velocity
What are the units of flux?
The variable of interest/(area times time)
What is does TKE stand for?
Turbulent Kinetic Energy
What is Turbulent Kinetic Energy per unit mass?
combines all linear turbulent terms into one scalar
What are ix, iy, and iz?
Turbulent intensities, the normalized standard deviations
What are the assumptions for turbulence?
Stationary, Homogeneous, and Isotropic
Which direction does the pollutant concentration gradient point?
From low to high
Which direction does the pollutant concentration flux point?
From high to low
What is Fick's Law?
The flux is proportional to minus the gradient of the concentration F(vector) = −D∇c
What are the units of F(vector)?
kg/(m^2*s)
it is a flux so this makes since
What are the units of D?
m^2/s
What is D?
molecular diffusivity
What factors affect molecular diffusivity?
The pollutant, viscosity of the fluid you are in, the temperature
-constant in time and space
How does temperature affect molecular diffusivity?
The higher the temperature, the higher the number of molecular collisions
What are the general assumptions that allow us to solve the molecular diffusivity equation?
The pollutants are inert, there are no sources or sinks
What does the momentum equation relate?
Pressure gradient force and Coriolis effect
What does it mean for winds to be geostrophic?
The wind Pressure Gradient Forces and Coriolis Effects are balanced
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