the influence of urban heat island circulation in

The influence of urban heat island

circulation in idealized city -- from building scale to city scale

Xiaoxue Wang, Yuguo Li

The University of Hong Kong

9th International Conference on Urban Climate July 20th-24th 2015 Toulouse, France

2

Why do we consider the urban heat island circulation ?

When UHIC happened, the worse pollution and warmer scenarios

generally happened compared with the windy day .

Preconditions for urban heat island circulation (UHIC)

Thermal inversion layer

Zero background wind

Horizontal temperature gradient Horizontal pressure gradient UHIC

ICUC9, Toulouse, France

(Falasca,2013)

ICUC9, Toulouse, France 3

Models

It is not easy to anlyze the air flow in the canopy or around a single

building by coupling meothd.

Coupling method?

The boundary condition changes with time during the UHIC

evolution.

CSCFD compared with traditional CFD

Porous model

Meso

-scale

(~ 105 m)

City

-scale

(~ 103 m)

Micro

-scale

(~ 101 m)

Porous model

Building details resolved

4

Modeling the mesoscale phenomena

Coordinate transformation

Modifying basic equations and turbulence by

comparing with mesoscale atmospheric

governing equation

Coriolis terms

Modified buoyancy term

Compressibility term for energy equation

Coordinate transformed terms

Modeling the overall city scale effect

Adding porous turbulence model

Modeling the 24-hour variation

Using daily surface temperature for ground

Improving numerical stability

Using an absorbing layer at the top


Modified CFD model -- City-Scal CFD (CSCFD)

eff 1l tJ

1/ 1 ξznJ

CSCFD - basic governing equations

0

eff 0 0

n

n n n n n n C n

d Vp V T T g F F F

dt

2 2

0 0 0

0

0

11

n

n c n n n

F

J T T g F w J p J z

Coordinate transformed terms

0 0

0

0

n

C

fv lw J

F fu

luJ

Coriolis terms

2 3

0

fl Fn n

CF V Q V

K K

Porous model terms

0 0nT T g

Modified buoyancy term

2

t

kC

0 p n

eff n T

d C Tk T S S

dt

0 ΓT p n s

p

gS JC w

C

A transformed compressibility term

not existing in the conventional CFD

0 sen

1S Q

H

Modified heat source

considering porosity 5

6

CSCFD - turbulence governing equations

0 0n n n

tn n n n k b n k k

k

dk k G G S S

dt

2

0 1 3 2 0n n

t n nn n n n k n b

n n

dC G C G C S S

dt k k

1 3n

n ts

n t p

gS C C g

k Pr C

2 3 3

0

82 2

3

l F Fk n n n k

C CS k Q k F

K K K

2 3 3

0

8 82

3 3

f

l nF Fn n n

r r

kC C QS Q

K x xK K

n

tk s

t p

gS g

Pr C

Coordinate transformed terms Porous turbulence model terms

22/07/2015 ICUC9, Toulouse, France

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.50

500

1000

1500

2000

2500

3000

x/D

Hei

ght

(m)

0 0.2 0.4 0.6 0.8 10

20

40

60

80

100

x/D

Hei

ght

(m)

7

Quasi-steady UHIC – domain

55 km×2.95km 5 km×100m

Not to scale

x

z

29

50

m

10 km

Symmetry

Wall

Pressure

outlet

Constant heat flux (city)

50 m

110 km

Table 1 Mesh and other model related setting

City

area

Other

area

Time step 1 second

Simulation

time

10 hours to achieve

quasi-steady state

Porosity 0.75 1.0


Constant heat flux (rural, 50W/m2)

8

Comparing with others’ data

-3 -2 -1 0 1

-3

-2

-1

0

Yoshikado (1992)

Richiardone and Brusasca (1989)

Kurbatskii (2001)

Kristof et al. (2009)

Hidalgo et al. (2009)

Our simulation

Godowitch et al. (1987)

Clarke and McElroy (1974)

Lu et al. (1997)

Cenedese and Monti (2003)

Catalano et al. (2012)

Falasca (2013)

zi/D=2.86Fr

z i/D

Fr

0.001 0.01 0.1 1 10

0.001

0.01

0.1

1

Simulation

Field experiment (St. Louis, Missouri)

Water tank

experiment

Our predicted mixing heights agree reasonably well with the field

data, scaled lab data and computations in literature.


Mixing height(zi): the height where the maximum difference between the plume

centerline and ambient density profiles occurs (Lu and Arya, 1997)

301

301

301

30

2

302

301.5

-1 -0.5 0 0.5 10

100

200

300

400300 301 302 303 304 305 306 307 308

302

30

2301

303

301

-1 -0.5 0 0.5 10

100

200

300

400300 301 302 303 304 305 306 307 308

Potential temperature

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

500

1000

1500

2000300 301 302 303 304 305 306 307 308 309 310 311 312


-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

500

1000

1500

2000300 301 302 303 304 305 306 307 308 309 310 311 312

Potential temperature (K)

Hei

ght

(m)

x/D

Quasi-steady UHIC– comparing a flat city and a porous city

(Porosity= 0.75 𝜆𝑝 = 0.25) Porous city Flat city

Wide neck Narrow neck

Hei

ght

(m)

x/D

Hei

ght

(m)

Hei

ght

(m)

x/D x/D




9

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

500

1000

1500

2000-5 -4 -3 -2 -1 0 1 2 3 4 5

Horizontal velocity

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

500

1000

1500

2000-5 -4 -3 -2 -1 0 1 2 3 4 5


1 m/s 4 m/s


Hei

gh

t (m

)

Horizontal velocity (m s-1)

x/D

10

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

500

1000

1500

2000-1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

Hei

ght

(m)

x/D

Vertical velocity (m s-1)

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

500

1000

1500

2000-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Vertical velocity

x/D


Hei

ght

(m)

0.0 0 .0

0.0

0.0

0.0

0.0

0.4

0.4

0.80.8

0.8

1. 2

1.2

-1 -0.5 0 0.5 10

100

200

300

400-0.4 0.0 0.4 0.8 1.2

Hei

gh

t (m

)

x/D


0.0

0.00.0

0.0

0.00.0

0.0

0.0

0.0

0.0

0.00.0

0.00.0

0.0

0.4

0.40.4

0.4

0.8

1.2

1.2

-1 -0.5 0 0.5 10

100

200

300

400-0.4 0.0 0.4 0.8 1.2

Vertical velocity

Hei

ght

(m)

x/D



Multiple upward flows Single upward flow


11

• When the sensible heat flux is larger, the mixing height is higher,

vertical velocity is larger, the height of the maximum vertical velocity is

higher, and the UHIC is stronger.

Quasi-steady UHIC – effect of heat flux

12

301 302 303 304 305 306 307 308

0

200

400

600

800

1000

1200

1400

Heig

ht

(m)


Q =60 (W/m2)

Q =100(W/m2)

Q =150(W/m2)

Q =200(W/m2)

Small heat flux

Large heat flux

Sm

all

Lar

ge

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

200

400

600

800

1000

1200

1400

Heig

ht

(m)

Vertical velocity (m/s)

Q =60 (W/m2)

Q =100 (W/m2)

Q =150 (W/m2)

Q =200 (W/m2)

Large

heat flux

Small

heat flux


-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

1

2

3

z/z i

u/UD

Flat city

porosity=0.75

porosity=0.5

water tank experiment

(our group)

Theory

13

Comparing with water tank experiment

Case Relative reverse

height

ϕ=1.0

(Theory)

39.5%

ϕ=1.0

(Flat city)

42.6%

ϕ=0.75 50.3%

ϕ=0.5 51.3%

ϕ=0.8 ∗ 55.6% Flow reverse height

Reference height

Increase

* water tank data from Yan Yifan

Our water tank model produced a relatively lower scaled flow, which agreed with

other similar studies in the water tank, needs to be further studied.

Relative reverse height

=Reverse height

Reference height


(Lu, 1997)

14

The proposed model could simultaneously simulate the wind and

thermal environment around several specific buildings and the

basic dynamics in the whole meso-scale area.

In the future, the details around the building will be analyzed.

Preliminary result: simultaneously consider the

environment around buildings and city climate


Summary

• When the city is introduced by porous media, the

velocity in the city decreases, the neck of the plume

increases, and multi-upward flows are observed.

• Even when the background wind is zero, the velocity

in the city is not zero due to the UHIC.

• The mixing height simulated by CSCFD agrees well

with other data in literature.

• Dense cities also increase the relative reverse height

at the edge of the city.

• Further data will be needed, and validation of the

code for synoptic wind conditions is also needed.

22/07/2015 ICUC9, Toulouse, France 15

THANK YOU

x (km)

Hei

ght(

m)

0 25 50 75 100 125 150 175 200 2250

500

1000

1500

2000

2500

Absorbing layer

x (km) x/D

Hei

ght(

m)

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

500

1000

1500298 299 300 301 302 303 304 305 306 307 308

x/D


X

Heig

ht

(m

)

90000100000110000120000130000140000150000160000170000180000190000

0

500

1000

1500

2000

9 10 11 12 13 14 15 16 17 18 19

x (km)

520

The general plume features are well predicted.

Comparing with meso-scale model

Prediction of CSCFD Prediction of Yoshikado (1992)

17 kmKz


-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

200

400

600

800

1000300 300.5 301 301.5 302 302.5 303 303.5 304

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

200

400

600

800

1000300 300.5 301 301.5 302 302.5 303 303.5 304

• The city height does not affect the

mixing height, but affects the

entrainment at the lower part.

Quasi-steady UHIC – effect of city

18

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

200

400

600

800

1000300 300.5 301 301.5 302 302.5 303 303.5 304

Hei

gh

t (m

)


x/D

Hei

gh

t (m

)

x/D

x/D

Hei

gh

t (m

)

302 303 304 305 306 307 308

0

200

400

600

800

1000

1200

1400

Heig

ht

(m)


H=100m

H=50m

H=10m

100 m

10 m

19

Application 2 – 12-hour evolution of daytime UHIC

The sensible heat flux

is partitioned with

25% in the porous air

and 75% at the ground

surface (assuming

porosity is 0.75).

7 8 9 10 11 12 13 14 15 16 17 18

-100

0

100

200

300

400

500

Sen

sibl

e he

at f

lux

(W m

-2)

Time (Hour)

city

rural

city - rural

anthropogenic heat

Anthropogenic

heat

Net urban-rural heat

Not to scale The same cities in Application

1 is used here.

The 12-hour profiles for the sensible heat flux

(calculated from the new 24-hour boundary condition)

295 300 305 310 315

0

500

1000

1500

2000

2500

295 300 305 310 315

Heig

ht

(m)


at x/D=0 (city center)

in clear case

8:00

10:00

12:00

14:00

16:00

18:00


at x/D=0 (city center)

in porous case

8:00

10:00

12:00

14:00

16:00

18:00

8 9 10 11 12 13 14 15 16 17 18

0

500

1000

1500

2000

2500

Hig

ht

(m)

Time (Hour)

city center (porous)

city center (clear)

city edge (porous)

city edge (clear)

rural area (porous)

rural area (clear)

20

Application 2 – 12-hour evolution of daytime UHIC

– Comparing mixing height for a flat city and a porous city.

The porous city has a lower mixing

height due to flow resistance, which

leads to multiple smaller plumes.

x/D

z 1

2

0-0.5-1 0.5 1

Flat city Porous city

Flat city

Porous city


21

The mixed porous-resolved approach

0

Wind

Wind

x

y

W B

B

Other “city” area Area of interest

(fully resolved)

Other “city” area

(as porous in case P&B,

as flat in case N&B,

and resolved in case B&B.)

B=W=H

x/H

z/H

7H 6.6H 20.3H

Wind


Porosity = 0.75

Similar

Different

Only interested area is modelled

All areas are CFD modelled

Mixed porous-resolved approach

The mixed porous-

resolved approach can

predict well the airflow

pattern in the other

“city” areas and its

impact on micro-scale

flows in the area of

interest

The mixed porous-resolved approach

Other

“city” area

Other

“city” area Area of interest

22

Verification - Comparing the mixed

porous-resolved approach with full

resolution in an ideal city

-1 0 1 20

1

2

3

4

5

6

7

8

u (m s-1

) at line1

z/H

P&B

B&B

-1 0 1 20

1

2

3

4

5

6

7

8

u (m s-1

) at line2

P&B

B&B

-1 0 1 20

1

2

3

4

5

6

7

8

u (m s-1

) at line3

P&B

B&B

-1 0 1 20

1

2

3

4

5

6

7

8

u (m s-1

) at line4

P&B

B&B

-1 0 1 20

1

2

3

4

5

6

7

8

u (m s-1

) at line5

P&B

B&B

-1 0 1 20

1

2

3

4

5

6

7

8

u (m s-1

) at line6

P&B

B&B

Not to scale

The mixed approach

produces nearly

identical u-velocities

with the fully

resolved approach.

23


is the temperature lapse rate

24

CSCFD Coordinate transformation

0

11 ln 1

g z

z R T

Based on the pressure equation,

The transformation coefficient is

Kristóf et al. (2009)

1

ln 1 nz z

z

nw e w

z

p np e p

After the transformation:

;nT T z

0 1 z

n e ;nu u

;nv v


Including all buildings in a CFD model is not possible

25

Methodology -- City-Scale CFD (CSCFD)

Resolved

Only modelling neighborhood (such

as AVA) is not accurate I propose -

Treat the other urban area using a porous

turbulence model approach

Porous media


How to analyze the UHIC’s effect in the city/around the

buildings ?

by modeling the environment around several buildings in isolation?

or by modeling the whole city with simple parameterizations?

or by a modified CFD model -- City-Scal CFD (CSCFD)?

The urban heat island

circulation is a multi-scale

flow problem, which includes


Introduction

the mesoscale (~103 km)

the city scale (101~103 km)

the local scale (10-3~101 km)

22/07/2015

the influence of urban heat island circulation in

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