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).

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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.

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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)

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287 288 289 290 291 292 293 294Virtual potential temperature (K)

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287 288 289 290 291 292 293 294Virtual potential temperature (K)

287 288 289 290 291 292 293 294Virtual potential temperature (K)

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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

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14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33Julian Days, 1998

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14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33Julian Days, 1998

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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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