Transcript No Slide Title

EGCE 583 Meteorology for Air
Pollution Control
Engineers
Jeff Kuo, Ph.D., P.E.
[email protected]
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Content
The Atmosphere
Horizontal
Atmospheric Motion
Vertical Motion in
the Atmosphere
Winds
Temperature
Inversion
Fumigations,
Stagnations
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Atmosphere
Our interests in meteorology: wind speed and
directions, atmospheric stability.
78% nitrogen, 21% oxygen, 1% argon, 0.03% CO2,
and other traces gases: change little.
Moisture content varies. 20 oC, 50% RH = 1.15 mol
(or volume) percent.
Water content at saturation  rapidly with T.
Atmosphere becomes thinner with increasing
height. ½ of mass < 3.4 mi, 99% <20 mi.
Vertical motions in the atm are one or two order of
magnitude smaller than horizontal.
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Horizontal Atmospheric
Motion
The horizontal movement of
atmosphere is driven mostly
by uneven heating of the
earth surface and modified
by the earth rotation
(Coriolis force) and the
influence of ground and sea.
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Horizontal Atmospheric Motion –
Equatorial heating, polar cooling
The prevailing winds along
the Earth's surface should be
Poles-to-equator.
The flow is mechanically
unstable and breaks into
three major convection zones
in each hemisphere.
The Coriolis force deflects
these wind flows to the right
in the Northern hemisphere
and to the left in the
Southern hemisphere.
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Horizontal Atmospheric Motion –
Equatorial heating, polar cooling
For example, between 30
degrees and 60 degrees
North latitude the solar
convection pattern would
produce a prevailing surface
wind from the South.
However, the Coriolis force
deflects this flow to the right
and the prevailing winds at
these latitudes are more
from the West and
Southwest. They are called
the prevailing Westerlies.
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Vertical Motion in the
Atmosphere –
 change with T
MP

RT
d dM dP dT




M
P
T
At constant M and P, @ 20 oC for each 1oC
increase in temperature:
d
dT
1


 0.0034

T
293.15
d / 
 0.0034 [in ( oC ) 1 ] @ 20o C
dT
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Vertical Motion in the Atmosphere –
 change with RH
M avg  y water M water  (1  y water ) M air
 M air  y water ( M water  M air ); y water  0.023RH (@20o C )
M avg  29  0.253RH
d

dM avg
M avg
 0.253dRH
 0.253dRH


29  0.253RH
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d / 
 8.7 10 5
dRH
[in (% RH ) 1 ]
A 40% increase in RH is required to produce similar
effect as 1 oC increase in T.
It explains that changes in T mainly drive the
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vertical motion of the atmosphere.
Vertical Motion in the Atmosphere –
 change with P
dP
  g (Barametri c Eq.)
dz
dP
gM

dz
P
RT
For a perfect gas undergoing a reversible, adiabatic
process (isentropic process) and assume Cp, air = 3.5R:
dP C P dT

P
R T
dT
gM
(9.8m / s 2 )( 29 g / mol )
kg
( )


dz
C P 3.5  8.314m 3  Pa / mol  K 1000 g
 5.37 oF / 1000 ft  10o C / km
 dry adiabatic lapse rate
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Vertical Motion in the Atmosphere –
 change with P
The “standard atmosphere” represents the average
observed T over the U.S. has a lapse rate of 6.49
oC/km (3.56 oF/1000 ft)
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Vertical Motion in the Atmosphere –
Atmospheric Stability
Super-adiabatic: the actual lapse rate > dry
adiabatic lapse rate (10 oC/km, 5.4 oF/1000 ft)
If an air parcel
starts at some point
where T = Tsurrounding
and it moves along
the adiabatic curve,
its T will be greater
than surrounding at
the new location,
buoyancy will force
it to continue up.
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Vertical Motion in the Atmosphere –
Atmospheric Stability
Sub-adiabatic: the actual lapse rate < dry
adiabatic lapse rate (10 oC/km, 5.4 oF/1000 ft)
If an air parcel
moves along the
adiabatic curve,
its T will be
colder than
surrounding at
the new location,
negative
buoyancy will
force it back
downward.
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Vertical Motion in the Atmosphere –
Atmospheric Stability
Neutral: the actual lapse rate = dry adiabatic
lapse rate (10 oC/km, 5.4 oF/1000 ft)
If an air parcel
moves along the
adiabatic curve,
its T will be the
same as
surrounding at
the new location,
buoyancy will
force it neither up
or downward.
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Vertical Motion in
the Atmosphere –
Atmospheric
Stability
Temperature Inversion: very
stable (vertical atmospheric
movement is damped).
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Vertical Motion in the Atmosphere –
Atmospheric Stability
Stable, neutral, and unstable atmosphere can occur
at the same place at different times of day on any
clear, dry sunny day.
Standard
At dawn:
Atmosphere
<1000’: Inversion, occurs
6000’
every clear night with low
Adiabatic
winds on most land
Atmosphere
surface (extremely stable)
Z
> 1000’ in the region with
midstandard lapse rate
afternoon
1000’
(mildly stable) – most
Dawn Dawn + 4h
T common one and called
radiation inversion. 15
Vertical Motion in the Atmosphere –
Atmospheric Stability
When the sun comes up, it heats the ground. There
will be a layer of warm air near ground (practically at
the adiabatic lapse rate).
By mid-afternoon,
Standard
inversion is gone. The
Atmosphere
heated air, having an
6000’
adiabatic lapse rate
Adiabatic
extends to 6000’. In the
Atmosphere
few hundred feet close to
ground is super-adiabatic,
midthe air is unstable.
afternoon
1000’
Before sunset, the ground
Dawn Dawn + 4h
T begins to cool and forms a
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weak inversion.
Vertical Motion in the Atmosphere –
Atmospheric Stability
Birds can soar for a long time by
riding on the vertical updraft
from ground heating. Birds and
human gliders cannot soar at
dawn, but in the afternoon (Hot
air balloon ride mostly at dawn).
Dust devil occurs in deserts on
sunny afternoon. The ground is
much hotter than the air above,
a layer of hot air forms next to
ground. When a disturbance
allows some of this air to rise, it
does so as a column.
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Vertical Motion in the Atmosphere –
Mixing height
Above certain altitude the ambient
temperature begins to increase with
increasing height. The height at which
this slope inversion occurs is called the
inversion height or Mixing height.
Mixing height grows during the day and
larger in summer (summer morning ~
450 m, afternoon ~2100 m, winter
afternoon ~970 m).
Stratosphere is practically isothermal.
The troposphere-stratosphere boundary
is a mixing height. Airlines often fly
above this boundary, ~ 11,021m.
Mixing
height
Height at
which
moisture
begins to
condense
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Vertical Motion
in the
Atmosphere –
Mixing height
The rising air saturates at a given level above
the surface. Hence, the base of a cloud is flat.
Once saturation is reached, the condensation of
water releases latent heat. This adds buoyancy
to the air making it rise even faster.
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Vertical Motion in the
Atmosphere – Moisture
Most of discussions so far are for dry air.
Most of moisture in the atmosphere is evaporated
from tropical oceans.
The average residence time of a water molecule in the
world atmosphere is ~ 9 days.
Moist Air is LESS Dense Than Dry Air!!! So if the air
near the surface becomes moist, the density is
lowered. This means that adding humidity to the
lowest layers of air makes the atmosphere unstable.
As the air parcel rises, RH increases (total pressure
decreases, but vapor pressure of pure water also
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decreases at a faster rate).
Vertical Motion in the
Atmosphere – Moisture
When air is forced to
rise over a mountain,
the process is called
orographic lifting.
Air initially encounters the mountain with T = 10 oC.
As it is forced to rise, it cools at the dry adiabatic
rate until saturation is reached. Water vapor was
condensed on the windward side to form clouds and
perhaps precipitation. During the descent on the
other side, the air immediately becomes
unsaturated. The air on the lee side will become
warmer and drier. Such an effect is often termed a
rain shadow (Sierra Nevada mountains)  deserts.
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Vertical Motion in the
Atmosphere – Moisture
If condensation occurs with increasing elevation
hcondensation dX
dT
 dT 
 


dz
C P ,air
dz
 dz  dryadiabatic
X is molar humidity
and dX/dz <0 due to
condensation.
Moist adiabatic lapse
rate < dry adiabatic
lapse rate
Moist adiabatic lapse ~
6.5 oC/km ~ standard
atmosphere.
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Wind Velocities
U2  z 2 
  
U1  z1 
p
The average wind velocity = 10 mph (4.5 m/s).
Calm (< 2 mph) – anemometers become
unreliable.
Wind speed  with elevation. Why? (for the
equation, z < 200 m and p = surface
roughness)
Typically geostrophic or gradient velocity,
frictionless velocity is reached at ~500 m.
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Wind Velocities
U2  z 2 
  
U1  z1 
p
Below this elevation: planetary boundary layer.
Inversions and stable atmospheres are normally
associated with low ground-level wind velocities.
When planetary boundary layer is unstable (midafternoon), a great deal of momentum transfer
between two layers  greater ground-level V.
Sailboat races are always scheduled for early
p.m.
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Wind - Directions
Wind direction is of concern (source  receptor).
Major storms normally associate with low-P
systems.
Mountains, valleys, and shorelines all influence wind
direction and magnitude.
On a clear night the ground is cooled. If the
ground is not flat, the more dense layer will tend to
flow downhill.
Mountains can act as barriers to low-level wind 
trap the air pollutants in the LA Basin (the problem
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is compounded by the near-by ocean).
Wind – Direction
(sea/land breezes)
During the day, the land surface heats up warmer
than the water. The result is rising motions over the
land. The air over the nearby water then flow
towards the land to replace the rising air. This is the
sea breeze.
At night the situation reverses. Now the land cools
rapidly, and becomes colder than the water. The flow
towards the water is called a land breeze.
The cold sea breeze make the lower-level air mass in
LA cooler, hence more stable  also it works with
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mountains to trap the air.
Wind – Direction
(Santa Ana wind)
When a strong high pressure system forms in the
central Rocky mountain, the resulting pressure
gradient sometimes is oriented so that winds are
from higher elevation regions, over the Sierra
Nevada into southern and central California.
When the air reaches sea level near the
California coast, it is warm and dry. The large
pressure gradient results in strong wind speeds Santa Ana winds.
They generally create very excellent air quality,
as the pollution is blown out to sea. However,
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they also greatly increase the fire danger.
wind
Temperature
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Wind – Direction
(wind rose)
Wind roses summarize frequency of winds of
varying V and directions.
A west wind blows from west to east.
Average V of LA downtown= 6.2 mph, prevailing
wind direction = west.
Prevailing wind direction may not be the direction
of the strongest wind.
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Temp.
Inversion
Inversion lessens vertical/horizontal air movement and
pollutant dispersions. 4 ways to produce:
 Cool a layer of air from below
 Heat a layer of air from above (often occurs in a
high-P region – summer between storms  slow
net downward air flow. The sinking air will  in T
and become warmer than air below it: elevated
inversion (subsidence inversion, inversion aloft).
Common in sunny, low-wind situations: LA in
summer).
 Flow a layer of cold air in under a layer of warm air
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(nighttime flow of cold air down valleys  causes
Temperature Inversion….
drainage inversions in the winter. If condensation
results, forming a fog, the sun cannot get to the
ground, the inversion persists for days – a cold fog
for several days in central California)..
 Flow a layer of hot air above a layer of cold air (air
flowing down the lee side of a mountain range is
warmed by adiabatic compression. It may be
warmed to T higher than that of air at the foot of
the mountains).
Cool sea breezes, colder winter sun, snow covering
(sunlight reflector and good emitter of IR), fog layer
(sunlight reflector) will lengthen inversion.
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Temperature
Inversion….
Warm air on top of cooler air
Marine Inversion
Subsidence Inversion
Radiation Inversion
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Inversions aren’t all bad!!
Bright lights, big city
Marine inversion layer
The frequent marine inversion layers in the Los Angeles basin
are GREAT for the Mt. Wilson Observatory. The light and
pollution from the city get blocked/stuck in the inversion and
the observatory is above it all.
But mostly they are:
Noon in Donora PA
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Fumigations,
Stagnations
If a source is in a strong inversion region, its plume
will be trapped and travel with local wind with little
dilution.
The inversion is slowly destroyed by solar heating.
When the unstable mass of air from the heated
ground reaches the plume, the plume will mixes
down to the ground, often at a high C, producing a
short-term fumigation.
In most eastern US, there is regular alternation of air
masses from Gulf of Mexico (warm, humid) and from
central Canada (cold, dry). In autumn, one of these
air masses may remain in place for >4 days 
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atmospheric pollutant concentration rise (stagnation).
Dry adiabatic lapse rate
Atmospheric stability and
dynamics can have some
interesting and noticeable
effects on air pollution in
the atmosphere.
Important to
consider
meteorology when
dealing with pollution
Other meteorological
considerations: wind
speed and direction
Topography (valley,
mountains…)
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