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Lecture 1:
Introduction to Urban Atmospheric Flows
By
H.J.S. Fernando
The Planet Earth• One of the 1011 planets and stars in our
galaxy (Milky Way), and there are 1011
galaxies in the universe, 4.6 billion years old
• Unique in the sense that it is the only planet so far known to sustain life
• Quite vulnerable as it maintains conditions in narrow bands of properties.
– e.g., water is maintained as a liquid, has a sufficient amount of oxygen, temperature is conducive for life.
– The mass (6 x 1024 kg) and its radius (6400 km) are such that it can keep an atmosphere via gravitation.
• Climate (average atmospheric state over at least a score of years, modulated by seasonal cycles) is fragile
Earth as seen from the TOPEX-POSEIDON satellite (orbits Earth 4700 times/yr and has continuously surveyed ocean currents with radar altimeters since 1992).
World population ~ 6 billion- expanding at a rate ~ 1.5%
Figure 1: Urban and rural population prospects (from the population division of the department of Economic and Social Affairs of the United Nations Secretariat, World Population Prospectus: The 2003 Revision of the World Urbanization Prospects)
Urban Share of World’s Population
Brown Revolution
•Megacities ( >10 M people)
Figure 3: The emergence of Supercities (Source: United Nations, World
Urbanization Prospects, The 1999 Revision)
Supercities ( >5 M people)
Urban Airflow (Meteorology) Studies
Why?• human comfort• energy usage• air quality• security
Governing Factors:
• city’s meteorology
• pollutant emissions and where
• topography
• landuse
A Rapidly Urbanizing Region: Phoenix
Urban (Latin meaning – City)Definition for Urban - vary
- Sweden and Denmark > 2000- Japan > 2000- USA and Mexico > 2500
Los Angeles, California
Climate System
Urban Ecosystems (Environment)
• Ecosystem - Community of living things interacting with non-living things.
Prediction of Environmental Motions
Mouse Moral: Seeing a part makes a fine tale. Wisdom comes, however, from seeing the whole.(From Nature, Nov. 2000).
(5/5)
Atmospheric Motions
• Thermally driven motions are dominant
Idealized global atmospheric circulation
Global ScaleL ~ 103 - 104 kmT ~ 50 - 100 yrs
Synoptic (Regional Scale)L ~ 1000 km
T ~ days, weeks
Urban ScaleL ~ 10 - 100 km
T ~ 5 - 10 yr; matured~ 1 - 3 yr; rapidly expanding
Rural
NeighborhoodL ~ 0.3 - 10 km
Global ↔ Local• Climate variability• Land cover change• Economic development• Technology & diffusion• Population dynamics
Long range flow
& transport
Regional B.C.
Solutions• Command & control• Market-driven
Air Quality (CO, O3, PM)• Sources (bio & anthro.)• Meteorology (T, v, q)• Solar insolation• B.C.
MesoscaleL ~ 100 km
T ~ hours - day
Urbansecurity
Street CanyonL ~ 10 - 100 m
CBDL ~ 1 - 2 km
PersonalL ~ 1 m
System Horizontal Scale (km)
Vertical Scale (km)
Time Scale
Global Synoptic
>1000 3 - 10 1-6 months
Regional Macro
500-1000 1-10 1-6 months
Local Climate
1-10 1/100 – 1/10
1 to 24 hrs
Micro-climate
<1/10 <1/100 <24 hrs
Prediction of Environmental Motions
• Schematic diagram of global climate system, to illustrate the way in which the Earth’s atmosphere - ocean system, and land surface area - is divided into thousands of boxes with sides typically extending several hundred kilometers in latitude and longitude, and with heights of a few kilometers in altitude.
• In a general circulation model (GCM), the computer treats each box as a single element as it calculates the evolving global climate.
– The GCM imposes seasonal and latitudinal changes of incoming solar radiation, the height and shape of the continents, and other external conditions which affect the behavior of the atmosphere.
– In GCMs, for example, the equations may be solved in hourly increments over at least 20 years of simulated time to generate an output which is statistically ‘accurate’.
– Such large and time-consuming calculations require the use of best super-computers.
(1/5)
From Synoptic-scale to Personal-scale
WRF, MM5
(i) Interpolation(ii) CFD
- Full scale (TKE, dissipation…)- Diagnostic way (sparse
observations + conservations laws)
Phoenix Terrain
Diffusion Coefficients
ihi
i
j
j
imji
x
bKbu
x
U
x
UKuu
Horizontal Wind & Topography of Domain 2: 0500 LST Jan 30 – 0500 LST Feb 1
Every hour
48-hour Urban Simulation
Simulation for Oklahoma City
Urban Meteorological Issues
• Terrain (70% are in complex terrain)
• Landuse changes (rapidly growing!)
• Urban heat island
• Transport and Dispersion of Pollutants (Air Quality)
Atmospheric Boundary Layer
ABL - The layer near the ground affected by the presence of the ground
• Drag is important
• Stratification is important
(Stable, Unstable and Neutral)
• Rotation – depending on the view
• Terrain (complex or flat)
Flat Terrain BL
Phoenix ABL
287 288 289 290 291 292 293 294Virtual potential temperature (K)
0
20
40
60
80
100
120
140
160
180
200
220
1641-1644
1657-1701
1721-1724
1738-1741
1756-1800
1814-1817
1835-1838
1856-1900
z(m
)
287 288 289 290 291 292 293 294Virtual potential temperature (K)
0
20
40
60
80
100
120
140
160
180
200
220
1641-1644
1657-1701
1721-1724
1738-1741
1756-1800
1814-1817
1835-1838
1856-1900
287 288 289 290 291 292 293 294Virtual potential temperature (K)
287 288 289 290 291 292 293 294Virtual potential temperature (K)
0
20
40
60
80
100
120
140
160
180
200
220
1641-1644
1657-1701
1721-1724
1738-1741
1756-1800
1814-1817
1835-1838
1856-1900
0
20
40
60
80
100
120
140
160
180
200
220
1641-1644
1657-1701
1721-1724
1738-1741
1756-1800
1814-1817
1835-1838
1856-1900
z(m
)
Mexico City, MexicoComplex Terrain ABL
Image source: NASA Goddard Space Flight Center Scientific Visualization Studio
An Ideal Complex Terrain
(katabatic)
(anabatic)
Ufp ~1
(synoptic)
Slope Flows
Whiteman 2000
Flows in CT Urban AirshedsSynoptic Flow
Thermal circulation
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
0
2
4
6
8
10
Win
d s
pee
d,
m/s
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
0
90
180
270
360
Win
d d
irec
tio
n,
deg
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33Julian Days, 1998
5
10
15
20
25
Air
tem
per
atu
re, o
CW
ind
sp
ee
d (
m s
-1)
Win
d D
ire
ctio
n (
o )T
em
pe
ratu
re (
o C)
(a)
(b)
(c)
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
0
2
4
6
8
10
Win
d s
pee
d,
m/s
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
0
90
180
270
360
Win
d d
irec
tio
n,
deg
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33Julian Days, 1998
5
10
15
20
25
Air
tem
per
atu
re, o
CW
ind
sp
ee
d (
m s
-1)
Win
d D
ire
ctio
n (
o )T
em
pe
ratu
re (
o C)
(a)
(b)
(c)
Winds in Phoenix – Little synoptic, sloshing
Pollution in Complex Terrain
Phoenix
Los Angeles
Salt Lake City
Hong Kong
Chemical Spills
From the Arizona Republic
Experimental Modeling of the Cold Pool Destruction
Cold Pool BreakupLow B
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