Download Composition of the Atmosphere

Composition of the Atmosphere
Average molecular weight = 28.97 g/mole
Global Circulation of the Atm. (Fig 4.9, and
4.3a 4.3b)
:
ITCZ (intertropospheric convergence zone) near
equator
band of atm ~ 100’s km wide
Equatorial low surf. pressure belt (doldrums:
weak P gradients, light winds)
persistent convergence => associated clouds
(can reach to tropopause) and rain.
Moves N Jan. to July
N and S of ITCZ to about 20-30 ° lat (tropical region)
Easterly trade winds (blow east to west),
(easterlies or easterly winds)
~ 30°N and 30°S
Regions of prevailing high P centers, mainly
over oceans => subtropical anticyclones
H => fair weather and dry => most deserts at
this latitude
Mid-Latitudes (30 - 60 °)
Winds shift to prevailing westerlies (W to E)
This is more consistent in S. Hem. (roaring
forties, screaming fifties.)
How to explain this
Hadley Circulation Model (~1700’s)
Circulation similar to land-sea breeze
Driven by hot equator - cold poles
Explains ITCZ near equator and movement to N.H. Jan. to July since region of max.
heating follows sun
Does not account for coriolis, which causes
• upper flow from eq. to pole to turn E
• low level flow to turn W
Hadley cell breaks down at about 30°N and 30°S
air subsides -> high P belt at 30° Latitude (clockwise rotation NH, counter CW, SH)
0 - 30°: easterly surface winds
Net result: coriolis acc. prevents air movement pole-ward, air must lose angular momentum (thru
frictional force at surface)
NH: more land mass + mountains = rough
SH: mainly oceans = smooth
results: stronger prevailing westerlies in SH
less pole-ward flow in SH, cold Antarctic atm
3-Cell Model (Hadley, Ferrel, and Polar Cells):
Circulation in 3 cells controlled by heating at Eq. cooling at poles
• Hadley Cell Winds Aloft (N.H.)
deflected more to E as move N,
westerly winds aloft increase in speed further from Eq.
meet equator-ward winds from Ferrel cell at ~30°
region of sharp T contrast -> subtropical fronts (at surface)
strongest winds at tropopause -> subtropical jet stream
• Subtropical High Pressure belt (30°)
descending air at subtropical fronts -> subtropical high-pressure belts (surface)
P-gradients weak near H: light winds (horse latitudes), clear, little rain (deserts)
• Subpolar low pressure belt (60°): region characterized by stormy weather
• Polar Front: forms at intersection of Ferrel and Polar cells (air masses of large T differences)
west-east winds highest at tropopause -> polar front jet streams
these jet streams tend to meander N - S (subtropical jets meander much less)
• Westerly Winds aloft at midlatitudes: winds traveling around H and L’s aloft are connected, results
in westerly winds aloft in Ferrel cell
Semipermanent pressure systems (over oceans, exist all year)
Subtropical H-pressure belts in NH and SH dominated by surface H-pressure systems over oceans
(semipermanent surface high-pressure centers)
Move N in summer, S in winter (N.H.)
N.H. Pacific: Pacific High
N.H. Atlantic: Bermuda-Azores high
Subpolar Low Pressure belts dominated by semipermanent surface L-pressure centers
Move N in summer, S in winter (N.H.)
N.H. Pacific: Aleution low
N.H. Atlantic: Icelandic low
Thermal Pressure Systems (over land, seasonal)
form due to surface heating/cooling (depends on surface properties, soil/water)
cp’s
air
1.0 kJ/kg/K
water
4.2
clay
1.4
dry sand 0.8
Low pressure systems form over regions of heating, eg, deserts, sunny areas, (called: thermal low
pressure systems)
eg: Majove Desert, plateau of Iran, north of India (summer monsoon)
Winter; thermal high pressure systems due to cool land surface (air density increases, air
descends), tend to be shallow, e.g., Siberian high, Canadian high (Rocky Mnts, US/Canada)
Large-Scale Systems Effects on A.Q.
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General circulation, semipermanent H & L, jet streams all effect long range transport of
pollutants.
By the time macro-scale systems play a role, pollutants have been dispersed by mesoscale and synoptic systems, plumes have lost identity and merged into regional haze and
polluted air masses (models are good at predicting the transport by these systems).
Role of Surface Pressure Systems
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Low pressure systems reduce pollutant concentrations due to precipitation
scavenging
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High pressure systems increase pollutant concentrations due to
•
clear skies (no precip. scavenging, enhanced photochemical activity)
•
sinking air (pollutants trapped near surface)
•
low wind speeds, and recirculation around H increases concentrations over
time
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Systems tend to stagnate (persist over a region for a number of days)
•
Recall AP disasters, most occur when H pressure system stall over
urban/industrial regions.
Synoptic Weather Systems
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Air mass: large body of air with homogeneous properties over 1,000’s of km (e.g., T,
moisture content, etc; acquire pollutants over source region)
Air masses separated by fronts (sharp boundary separating air masses)
•
Cold and warm fronts tend to form around L pressure systems, especially
cyclones near 60°, cold fronts rotate c. clockwise
Air masses acquire properties by sitting or moving slowly over a source region where
acquire surface properties (source region: large, flat, uniform properties, dominated
by H pressure system)
Air masses moved by the general circulation of the atm.
Air masses modified at surface by new surface characteristics,
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surface air modified rapidly
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can eventually be modified up to FT
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modified by mainly turbulent mixing in PBL, vertical convection, instability at
fronts
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modified layer depth/fetch
= 0.1 for neutral, unstable PBL;
= 0.01 for stable PBL
Frontal systems and pollution:
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Can produce stable conditions (warm air over cold) leadingto less dispersion
Fronts typically result in little pollution build up due to associated frontal
lifting causing clouds and precipitation.