Lecture 4: The Urban Atmosphere: problems and basics
Transcript of Lecture 4: The Urban Atmosphere: problems and basics
The Urban Atmosphere: problems and basics
Elie Bou-‐Zeid Princeton University
Civil & Environmental Engineering Lecture 4
Meted: Weather and the built environment
h3p://www.meted.ucar.edu/ Meted: Great resource for interac<ve educa<onal modules in meteorology from the US Na<onal Center for Atmospheric Research
Free, but you have to register We will now see the “impacts on the atmosphere” part of one module on “Weather an the built environment” which can be viewed in interac<ve mode at h3p://www.meted.ucar.edu/broadcastmet/wxbuiltenv/
Slide 2
What is the Atmospheric Boundary Layer?
Mixing height = ABL height
Pollutant, heat and humidity are trapped in the ABL& their fluxes into the upper atmosphere are controlled by its dynamics
Slide 3
Why is it important: Surface Drag & Atmospheric Dynamics
50 % of the kinetic energy dissipation in the atmosphere occurs in the ABL
Surface drag is one of the main drivers of ocean currents
Slide 7
Why is it important: Main source of heat & moisture
wateraerosols
heat momentumkineticenergy
Slide 8
© 2011 Elie Bou-‐Zeid
Why do we study it here: atmospheric layer engulfing our cities ©
Slide 11
From Fernando. Fluid Dynamics of Urban Atmospheres in Complex Terrain. Annual Review of Fluid Mechanics (2010) vol. 42 (1) pp. 365-‐389
Structure and layers of the ABL Slide 13
∂U
∂t+U.∇U= − 1
ρ∇p + 1+ ′ρ
ρ⎛⎝⎜
⎞⎠⎟g+υ∇2U
− fc k×U © 2011 Elie Bou-‐Zeid
Internal equilibrium layer ~ (1/100) x ~ 10% of IBL
Internal Boundary layer ~ (1/10) x
Outer flow
surface 2 surface 1
x = distance downstream of the surface change
Changes in surface condi<ons
Slide 14
ABL layers over an urban area
Slide 18
Outer layer
Surface layer
Fernando. Fluid Dynamics of Urban Atmospheres in Complex Terrain. Annual Review of Fluid Mechanics (2010) vol. 42 (1) pp. 365-‐389
Problem
ABL over a city at night ~ 300 m
Incoming air U = 1 m/s T = 28 C
Ci< is 5 km long
Outgoing air U = 1 m/s T = ??
Hcity= 75 W/m2
Slide 19
Above the sublayer: the urban ABL
The urban ABL is driven by the large scale mean horizontal pressure gradients
Like all environmental flows it, is highly turbulent We can use simplified analy<cal models for it (e.g. simple problem we just solved)
We can use more sophis<cated numerical models that capture many physical phenomena (e.g. urban canopy models used in Weather or Climate models, we’ll see one later)
We can go all the way and used coupled surface-‐flow models using turbulence resolving models
∂p / ∂x,∂p / ∂y
Slide 27
Stability of the ABL
0 5 10 150
50
100
150
200
Hei
ght (
m)
0 5 10 150
200
400
600
800
1000
Hei
ght
(m)
ColderHotter
weak, intermittent turbulence
HotterColder
Thermals
Intense mixing
θv (°C) θv (°C)
Inversion
Slide 28
© 2011 Elie Bou-‐Zeid
ABL Turbulence, important in all models : Buoyancy Vs. Shear
Urban ABL modeling heavily involves turbulence modeling See flash anima<ons on the COMET website of UCAR
Slide 29
Bowen Ratio and its effect on the ABL Bowen Ratio = Bo=
e
HL E Rn=H+LE+G
H creates much more buoyancy than LE Humid surface:
Low ABL Weaker turbulence
Dry Surface (Urban):
Higher ABL Stronger turbulence
Slide 30