Download Basic Meteorology - Northern Arizona University

Basic Meteorology
Diana Anderson
Jeff Herring
Center for Environmental
Sciences and Education
Northern Arizona University
Environmental Studies Program
Prescott College
Contact Information
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Dr. Diana Anderson
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Center for Environmental Sciences and
Education, NAU
E-mail: [email protected]
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Outline of Topics
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Energy Balance at the Earth’s Surface
Why Do We Have Seasons?
Why Does Air Move?
What Are Clouds?
Atmospheric Stability
Air Pollution Dispersion
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Energy Balance at the Earth’s Surface
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Must consider energy from sun (solar
radiation), energy emitted by ground,
clouds, greenhouse gases (infrared or IR
radiation)
Some solar radiation reflected by clouds or
ground; fraction reflected is “albedo”
Heat redistributed by air, water motion
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Average Energy Balance
in the Atmosphere
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Energy Balance
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On average, areas of
Earth closer to equator
(0 deg. latitude) get more
incoming solar radiation
than they lose in IR
radiation
Areas near poles lose
much more IR than they
gain in solar radiation
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Why Do We Have Seasons?
What do you think?
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Why Do We Have Seasons? (cont.)
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Common misconception
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Seasons caused by changes in distance
between Earth and Sun
But what about Australia?
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Why Do We Have Seasons? (cont.)
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Primary cause of
seasons is change in
incoming solar
radiation due to tilt of
Earth’s axis
Let’s act it out using a
globe and a light
source
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Why Does Air Move?
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Consider horizontal motion of air
separately from vertical motion
Most wind is horizontal; vertical wind
speeds typically much less than horizontal
wind speed
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Why Does Air Move? (cont.)
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To understand air motions, consider
concept of pressure
Air pressure: measure of total force of
collisions with air molecules
 Another conceptualization: weight of column
of air overhead
If air pressure different at two locations at same
elevation, air will “blow” from higher pressure to
lower pressure
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Why Does Air Move? (cont.)
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Vertical motions of air also important to
formation of clouds, dispersion of air pollution,
etc.
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Factors causing vertical motion very different than
those causing horizontal motion
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Pressure decreases rapidly as altitude increases, but pressure
difference balanced by gravity
Vertical air motions can be caused by turbulence,
wind flowing over mountains, larger scale weather
patterns, local heating and cooling
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Why Does Air Move? (cont.)
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Localized solar
heating causes air
near ground to
expand
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Air may then be less
dense than
surrounding air; so
buoyant (“floats”
upward like hot air
balloon)
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What Are Clouds?
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What Are Clouds? (cont.)
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Droplets of liquid
water or solid ice
crystals suspended in
air
We see reflection of
sunlight off cloud
droplets or crystals
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What Are Clouds? (cont.)
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In atmosphere,
anything that causes
air to cool can cause a
cloud to form
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Most common way:
air expands and cools
as it rises
Unsaturated air
parcels cool by about
10°C for every 1 km
they rise (“adiabatic
lapse rate”)
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What Are Clouds? (cont.)
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Fog usually due to
cooling of air at
night or as it
moves over a
cooler surface
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Atmospheric Stability
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Key factor in
determining severity
of air pollution is
stability of
atmosphere
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Atmospheric Stability (cont.)
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Stable atmosphere: vertical air motion
inhibited
Unstable atmosphere: vertical motion
triggered by local heating or turbulence is
enhanced
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Atmospheric Stability (cont.)
Best way to
determine stability of
atmosphere: launch
weather balloon
Temperature Sounding, Flagstaff, April 27, 5 am.
6000
5500
5000
Altitude (m)
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4500
4000
3500
3000
2500
2000
-20
-15
-10
-5
0
5
Temperature (Celcius)
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Atmospheric Stability (cont.)
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Using this data, make
plot of vertical
temperature profile
How does this
compare to the April
27 profile?
Balloon Launch at Flagstaff
January 21, 2002, 5 am
Temperature, Celcius
Altitude (m)
-11.1
2192
0.8
2362
2
2657
1.6
2743
0.2
3075
-2.9
3658
-3.7
3800
-5.1
3962
-7.7
4267
-12.9
4877
-13.9
4990
-17.7
5576
-17.3
5680
-17.9
5791
-18.1
5831
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Atmospheric Stability (cont.)
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To understand, compare
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rate of atmospheric temperature change
with
height (environmental lapse rate or ELR)
with
rate at which temperature of rising parcel of
air changes (adiabatic lapse rate or ALR)
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Atmospheric Stability (cont.)
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Given that ALR is –10ºC per km, estimate
what would happen to parcel of air
pollution
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At ground that is 8ºC warmer than
surrounding air
At 5 a.m. on January 21
A parcel of air will rise if warmer than
surrounding air at same altitude
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Atmospheric Stability (cont.)
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Temperature inversion
occurs when temperature
increases with height
Temperature inversions
tend to trap air pollution
by limiting maximum
mixing depth (MMD)
into which pollution can
be dispersed
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Atmospheric Stability (cont.)
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Inversions often form
near ground at night
due to radiation
cooling
Higher altitude
inversions can be
caused by large-scale
sinking of air
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Air Pollution Dispersion
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Effects of pollution on
human and ecosystem
health depend on
pollution concentration
Rate of dispersion of
non-reactive pollutant
determines concentration
 “The solution to
pollution is dilution”
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Air Pollution Dispersion (cont.)
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Worst episodes of air
pollution have occurred
during winter
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Ground level radiative
cooling causes fog to
form
Fog reflects solar
radiation, preventing
ground from rewarming
Resulting inversion can
be very low and
persistent
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Air Pollution Dispersion (cont.)
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October 25-31, 1948, Donora, Pennsylvania:
strong inversion caused pollution to accumulate
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20 people died from exposure, including 17 in
14-hour period
December 5-9, 1952, London, England: fog
inversion caused SO2 concentrations up to 1.3
ppm (almost 10 times EPA limit).
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Several thousand people died, probably from
combination of exposure to fine particles and SO2
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Air Pollution Dispersion (cont.)
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Ventilation describes
degree to which
atmospheric motion
dilutes air pollution
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Ventilation high in
periods of wind or when
atmospheric stability is
low
Stagnation will occur
when wind speed is low
and stability is high
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Air Pollution Dispersion (cont.)
Pasquill Stability Categories
Surface wind
speed at 10 m
(m/s)
Day
Night
Incoming Solar radiation
Cloud Cover
Strong
Moderate
Slight
Thinly Overcast
Mostly Cloudy
<2
A
A-B
B
2-3
A-B
B
C
E
F
3-5
B
B-C
C
D
E
5-6
C
C-D
D
D
D
>6
C
D
D
D
D
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Air Pollution Dispersion (cont.)
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Our goal: predict air
pollution
concentrations given
knowledge of sources
and meteorology
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Air Pollution Dispersion (cont.)
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One approach: assume each individual
plume behaves in Gaussian manner
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Results in concentration profile with bellshaped curve
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Air Pollution Dispersion (cont.)
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Gaussian behavior extends in two
dimensions
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Air Pollution Dispersion (cont.)
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This assumption allows us to calculate
concentrations downwind of source using this
equation
where
c(x,y,z) = contaminant concentration at the specified coordinate [ML-3],
x = downwind distance [L],
y = crosswind distance [L],
z = vertical distance above ground [L],
Q = contaminant emission rate [MT-1],
sy = lateral dispersion coefficient function [L],
sz = vertical dispersion coefficient function [L],
u = wind velocity in downwind direction [L T-1],
H = effective stack height [L].
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Air Pollution Dispersion (cont.)
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The Bad News: This equation would get
pretty ugly to solve, especially with
multiple sources to consider
The Good News: Computers can do the
work for us!
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Other Meteorology Resources
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www.wrh.noaa.gov/Flagstaff/ Access to local National Weather
Service Office for local forecasts, satellite imagery, and radar.
Provides link to main NWS homepage and thereby to other regional
forecasts
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www.unisys.com: Almost all of information available to any weather
forecaster. Previous sites more user-friendly, but this one provides
nitty-gritty weather details. Best feature is access to output from all
major computer forecast models. Great satellite photos, too
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www.rap.ucar.edu/weather/upper Link to view most recent
skewT/log-P diagrams from the U.S. networ.
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Other Meteorology Resources (cont.)
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www.wrcc.dri.edu/: Western Region Climate Center at the Desert
Research Institute. Huge amount of information on climate in the
Western U.S. Note especially their pages on El Nino and their
historical summaries (http://www.wrcc.dri.edu/climsum.html) of
western U.S. climate
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www.intellicast.com: Another good, all-purpose weather web
site. Best part of site excellent quality of radar data, which
differentiates between snow, rain and mixed precipitation
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wunderground.com Good site for quick forecasts and simple maps
of weather conditions in the US. They also have some information
for other countries
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For more links, see the NAU weather station web page at:
http://art.artsci.nau.edu/nauws
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