The  Urban  Atmosphere:  problems  and  basics

Elie  Bou-­‐Zeid Princeton  University

   Civil  &  Environmental  Engineering Lecture  4

Meted:  Weather  and  the  built  environment  

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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  

Cloud topped ABL

<Mixing height> = <ABL height>

Slide  4  

Why is it important: Atmospheric dispersion

Slide  5  

Why is it important: Atmospheric dispersion Slide  6  

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 is it important: Hydrology + Hydrometeorology Slide  9  

Why is it important: Energy

Slide  10  

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 of the ABL  

Slide  12  

θ v                                  q v  ,  ρ

©  2011    Elie  Bou-­‐Zeid    

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  

Slide  15  Slide  15  Slide  15  

From  :  h(p://www.epa.gov/hea1sland/

Slide  16  

From  :  h(p://www.epa.gov/hea1sland/

Slide  17  

From  :  h(p://www.epa.gov/hea1sland/

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  

q  is  total  sensible  heat  /  internal  energy  in  our  control  volume  NOT  specific  humidity  

Assuming  steady  state,  later  in  the  night  for  example,  not  always  true  

Urban  sublayer  

Slide  22  

Types  of  flow  

Slide  23  

Types  of  flow  

Slide  24  

Turbulence over 1 building

Slide  25  

Turbulence over several buildings

Slide  26  

(Bou-­‐Zeid  et  al.,  BLM,  2009)  

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  

Recommended  Readings  

 Oke:  Boundary  Layer  Climates   Fernando  :  Annual  Review  of  Fluid  Mechanics  (2010)  vol.  42  (1)  pp.  365-­‐389,  (references  therin  are  also  very  good)  

 Belcher:  Mixing  and  Transport  in  Urban  Areas,  Phil.  Trans.  R.  Soc.  A  (2005)  363,  2947–2968