Microclimates in tropical urban complexes

Energy and Buildings, 15 - 16 (1990/91) 83 - 92

83

Microclimates in Tropical Urban Complexes B. PADMANABHAMURTY

School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067 (India)

ABSTRACT

Irrespective of the location, heat islands are formed either at highly commercial places or densely populated areas. Utilizing the traditional airport meterological data and city centre data, diurnal, seasonal and annual variations of temperature, dew point, winds, visibility, stability, mixing height and ventilation coefficient and the diffusion climatology of urban and rural complexes are studied. At street level, temperature, humidity and wind on the exposed side are quite different from shaded areas. These are also influenced not only by size, shape and orientation of buildings but also by their location in urban, suburban or rural surroundings. Temperature decreases up to 3 °C and dew point increases by the same amount in shade. Vegetated areas reduced air temperature by 1.5°C and increased dew point even by 5 °C. Discomfort prevailed on the exposed side as well as on the shaded side at different times of the day and was found to be a function of dimension, orientation and location of the building.

1. INTRODUCTION M i c r o c l i m a t e is the p a t t e r n of s e a s o n a l a n d geographical distribution, both horizontally a n d v e r t i c a l l y , of t h e v a r i o u s p r o p e r t i e s in the l o w e s t l a y e r s of air. A r e a s o n a b l e c o n s e n s u s of t h e h o r i z o n t a l a n d v e r t i c a l scales a n d t h e i r limits is 1 0 - 2 m to 10am a n d 1 0 - 2 m to 10m, respectively. With increasing industrialization and urbanization, a term "local climate" has c o m e into vogue: 102 m to 104 m a n d 10 -1 m to 10am on the h o r i z o n t a l a n d v e r t i c a l scales r e s p e c t i v e l y . B o t h t h e s e scales of c l i m a t e are i n t e r l i n k e d a n d c a n n o t be s t u d i e d in isolation.

M i c r o c l i m a t i c differences in u r b a n a r e a s o c c u r due to the t h e r m a l c h a r a c t e r i s t i c s of u n d e r l y i n g surfaces, a m o u n t s of n e t r a d i a t i v e flux received, r o u g h n e s s , colour, s t r u c t u r e , t e x t u r e a n d c o v e r of the surface. Besides these, t h e u r b a n c e n t r e s e x h i b i t e n t i r e l y different m e t e o r o l o g i c a l c h a r a c t e r i s t i c s comp a r e d to t h e i r r u r a l c o u n t e r p a r t s w h i c h f o r m the " u r b a n c l i m a t o l o g i c a l studies". In the p r e s e n t p a p e r the m i c r o c l i m a t e s of t r o p i c a l u r b a n c o m p l e x e s are p r e s e n t e d a n d discussed.

2. MATERIALS AND METHODS Conventional meteorological data from a city c e n t r e a n d a i r p o r t in r e s p e c t of t e m p e r a ture, wind, visibility a n d t u r b i d i t y at the r u r a l l o c a t i o n w e r e utilized. U r b a n h e a t i s l a n d a n d m i c r o c l i m a t i c d a t a w e r e c o l l e c t e d at the sites on specific d a t e s t h r o u g h o u t t h e day.

3. RESULTS

3.1. Urban heat island U r b a n h e a t islands at a t r o p i c a l i n l a n d station, V i j a y a w a d a (16°32 ' N, 80°48 ' E) a n d a coastal station, Visakhapatnam (17°43 ' N, 83°14 ' E), d u r i n g t w o w i n t e r m o n t h s are s h o w n in Figs. 1 - 4 . T h e h e a t i s l a n d i n t e n s i t y is a b o u t 2 °C at t h e i n l a n d s t a t i o n c o m p a r e d to 0.4- 0.8 °C a t the c o a s t a l station. T h e u r b a n r u r a l t e m p e r a t u r e difference o v e r a n u m b e r of y e a r s a t the c o a s t a l s t a t i o n s h o w e d t h a t the p e r c e n t a g e f r e q u e n c y of o c c u r r e n c e in t h e r a n g e 0 - 2 °C is 90% while in t h e r a n g e 2- 4 °C it is only 10% (Fig. 5). T h i s s h o w s t h a t in t h e t r o p i c s as one a p p r o a c h e s t h e e q u a t o r t h e r e a p p e a r s to be some t e n d e n c y of a d e c r e a s e in t h e h e a t i s l a n d i n t e n s i t y [ 1 - 4]. H o w e v e r , this n e e d s i n t e n s i v e i n v e s t i g a t i o n a t a n u m b e r of Elsevier Sequoia/Printed in The Netherlands

84

/ c~

i,

o

~

2.

J~

Fig. 1. Surface temperature distribution on January 3, 1987, Vijayawada.

7

2~

Fig. 2. Surface temperature distribution on February 24, 1988, Vijayawada. (a) coastal tropical, (b) inland tropical, (c) inland highland tropical, and (d) island tropical stations at various latitudes. For comparison the heat island intensities at different stations in India during winter nights are shown in Table 1. The heat island is a function of the size of a city and population is a surrogate of city size. Figure 6 shows the relation between population density and temperature. It can be ob-

served that in tropical cities the temperature increases only slightly with population density unlike in mid-latitude urban complexes [5]. At urban (Safdarjung) and rural (Palam) locations in Delhi, the percentage frequencies of occurrence of minimum temperature, winds at 03 & 12 Z and visibilities at 00 7, in winter are presented in Figs. 6-9. At rural Delhi there is a higher tendency to record lower

85

Fig. 3. Surface temperature distribution on December 30, 1986, Visakhapatnam.

%

~.~.~____-

. ~ - . . . _ ~ - o ' ~ - - ~ U ~~ -

""

~ 2 6 . 0

/

~ 2 5 . 8 ~

h~ N'~ 2s8 V ~

260

f ,ooo, Fig. 4. Surface temperature distribution on February 26, 1988, Visakhapatnam.

temperatures. The wind roses indicate fewer calm periods at the urban location compared to the rural area. Winds are from all directions at the rural location but at the urban centre there are preferred directions because of the alignment of buildings, roads, etc. Percentage frequencies of visibility at both locations show that the rural location has poor visibility. This might be due to the entrainment of soil dust into the atmosphere by stronger winds. Secondly, lower temperatures at Palam also cause condensation of water vapour thereby reducing visibility.

3.2. Turbidity Monthly variation of turbidity at the rural location is shown in Fig. 10. The increase of turbidity from J a n u a r y to June is due to: (a) advection of dust from Rajasthan; (b) re-entrainment of dust into the lower atmosphere by surface winds, and (c) transport of this dust by convection and turbulence into the higher levels [6]. Monsoonal rains till September wash the dust resulting in lowering of the turbidity [7]. Subsequently it increases slightly due to calm winds, inversions and suspension of particles.

86

TABLE 1 H e a t i s l a n d i n t e n s i t i e s at some I n d i a n c i t i e s Statio~

Latitude

Longitude

Location

Heat island intensity

(c) New D e l h i Bhopal Calcutta Bombay Pune Visakhapatnam Vijayawada Madras

28+35 ' N 23+17 ' N 2232' N 1854' N 18~'32' N 17 4 1 ' N 16 3 2 ' N 1300' N

77412 ' 7T21' 78+27 ' 72¢'49' 73°51 ' 8314' 80°48' 80°11 '

E E E E E E E E

Inland Inland Inland Coastal Inland Coastal Inland Coastal

I00

22 I

ou21 • 20~- °

9O e-

g

70 6O

• •

.

•

•

18r-

~. r71-

+ ,61I

~'-

a.

•

8O

tl_

e-

. .

6.0 6.5 4.0 9.5 10.0 0.6 2.0 4.0

i

I

+

+

l

i

I

d

l

I

+

+

I

+

I

t

40

L,0 80 120 160 200 240 280 320 Population density (persons per ocre)

30

Fig. 6. Variation of temperature with population density

2o

in V i s a k h a p a t n a m .

I0

IllllllllllJl o

2

f

I

4

6

Heat Island Intensity'c

] ]

- . .

Fig. 5. P e r c e n t a g e f r e q u e n c y of h e a t i s l a n d i n t e n s i t i e s of ( T u -TR) of d i f f e r e n t r a n g e s a t V i s a k h a p a t n a m d u r i n g m i n i m u m t e m p e r a t u r e p e r i o d in w i n t e r .

The annual average turbidity is steadily increasing showing that Delhi's atmosphere has become more turbid over the years.

3.3. Microclimates 3.3.1. Urban complexes Air temperature, dew point and wind are measured at a height of 1.5 m above ground on the side exposed to direct sun (exposed environment) and on the hind side of the building where it is under the shadow of the building (shaded environment) to assess the impact on the microclimate. Temperatures in the shaded and exposed environments are shown in Figs. 11 - 13. The maximum relief on the shaded side is 2.5 °C at midday. Wind in the exposed environment is much higher. Dew point was also

+ALAM

E0

TEMPERATURE

"C

Fig. 7. P e r c e n t a g e f r e q u e n c y of o c c u r r e n c e of m i n i m u m t e m p e r a t u r e in J a n u a r y .

higher by about 2 °C on the shaded side (Figs. 14 and 15). Similar observations under channetling wind conditions (Fig. 16) relieved the shaded environment by 2°C more or less t h r o u g h o u t the day. Winds, unlike the previous case, are fairly close t h r o u g h o u t the day in both environments. Dew point under these conditions (Fig. 17) was only 1 °C higher in the shade. Thus channelling wind conditions reduced the relief of temperature and dew point, and hence reduced the comfort.

87

08 30 HRS IST

STATION : PALAM

STATION : SAFDURJUNG

% 17 30 HRS ISI

"

~

lcm = 50% " ~

,--~ ~-b

7-10

11-16

Fig. 8. Wind roses. B0 [] [] [] []

JanuQry April August October

• [] • []

60

Jonuory Apri; August October

z

u. 30

1

2

3

~

5

6

7

8

9

VISIBILITY CLASSES F i g . 9(a), P e r c e n t a g e f r e q u e n c i e s o f v i s i b i l i t i e s

0

1

2

3

l.

5

6

7

8

9

VISIBILITYCLASSES at Palam

(05:30 IST).

Fig. 9(b). Percentage frequencies of visibilities at Safdurjung (05:30 IST).

3.3.2. Suburban complexes Temperature, wind and dew point on the exposed and shaded sides of buildings are shown in Figs. 18 and 19. Maximum temperature relief of 2 °C and a dewpoint increase of 0.4°C were observed on the shaded side. Winds were always lower on the shaded side. Changing weather conditions like cloudiness and wind resulted in irregular reliefs.

3.3.3. Rural complexes Temperature, wind and dew point on the exposed side and shaded side of buildings are

shown in Figs. 20-23. A maximum temperature relief of 1.5 °C and a dew point increase of 0.7 °C was achieved. Winds were always lower on the shaded side compared to the exposed environment.

3.3.4. Urban forests Forest cover reduced temperatures by 2.5°C, and increased dew point by 5.1°C. Winds inside the forest were calm while outside they were high (Figs. 24 and 25).

88 0.4 40 c2

35 0.3

3O LJ

°--25, L~

t

2O

~2 la.J CO

u.J

o-15

~, ~- SHADED o----o EXPOSED

10 @;1

I

I

L

P

I

P

I

I

8.00 9.00 10.00 1100 12.00 13.00 14.00 15.00 16.00 TIHE(HourslST)

]

F

J M

A

L

MI

i J

J

J

A

S

0

£ N D

Fig. 10. M o n t h l y v a r i a t i o n o f t u r b i d i t y in P a l a m .

Fig. 12. T e m p e r a t u r e i n t h e e x p o s e d a n d s h a d e d e n v i r o n m e n t s at a n u r b a n c e n t r e , b e h i n d L a l k i l a .

~ _ ~ } EXPOSED

35

35

30

130

o~25

~25 20'

~-20 o+-1 1[z F--

-~°---~-} SHADED

CL

:15

C3_

c--'--oEXPOSED = = SHADED

t~

Z

'-- 10 i

'~

5~ I J I i I I I i 8.00 900 1000 11.00 12.00 13.00 lh00 15.00 1600 TIME (Hours ]ST) Fig. 11. T e m p e r a t u r e i n t h e e x p o s e d a n d s h a d e d e n v i r o n ments under calm conditions in an urban complex, Ashok Vihar.

3.3.5. Cropped farmland The surface of a crop farm reduced the temperature to a maximum of 1 °C and the dew point considerably by 3.2 °C. Winds over bare soil were always higher (Figs. 26 and 27).

4. S U M M A R Y

This paper discusses the urban climatological and microclimatological studies in Delhi. Heat island intensities at various Indian cities were found to be irregular, defying any pattern. Minimum temperatures at rural Delhi

0 830

]1 I I I I I L i 0 930 10.30 11.30 12.30 13.30 I~.30 1530 1630 TIME (Hours IST.

Fig. 13. Temperature and wind in exposed and shaded e n v i r o n m e n t s u n d e r v a r i a b l e s k i e s in a n u r b a n area, Delhi university.

show a higher tendency to record lower temperatures. Fewer calm periods occur at urban complexes compared to rural areas. In rural areas winds blow from all directions, in urban complexes they have preferred directions following the alignment of buildings and streets. Rural areas have poor visibility compared to urban complexes owing to lower temperatures causing condensation and re-entrainment of soil dust due to stronger winds. Turbidity over Delhi increased from January to July due to an advection of dust from the desert area in the west and transport of the dust into the upper air by convection and turbulence.

89

8 14 12

6 ~I0

~5-

o 4

= : SHADED o---o EXPOSE D

o

o..

~3 z. 2

; ~

= SHADED EXPOSED

2 I

800 9.10010100II.0012100131.0014100IS.mOO16!00 TIME(HoursIST) Fig. 14. Dew point in exposed and shaded environments under calm conditions in an urban complex, Ashok Vihar.

0

I

I

i

I

I

[

I

i

800 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 TIME (Hours IS.T}

Fig. 17. Dew point in exposed and shaded environments under channelling winds at an urban complex, the N.D.M.C. building.

12

4 0 - z~__~}EXPOSED

8

35- ~}SHADED

i7

~I0

"- 30 ~ g ~ 6 4,

~6

LJ

:

= SHADED EXPOSED

~25,

s

<~20

4~

•- 15 ua uJ ~

2

~3

2Q o_

k

10

0

,

;

,

,

I

,

]:

'0

8.00 9.,0 I0'.00 11. 0 12.00 13.00 14.00 15,00 16. 0 TIME (Hours I ST)

Fig. 15. Dew point in exposed and s h a d e d environments in an urban centre behind Lalkila.

I

I

I

I

I

I

]

~)0 g.00 10.00 11.00 12,00 13.00 14.00 I~S.00 16.00 TIME (Hours IST)

Fig. 18. Temperature and wind in exposed and shaded environments in a suburban area, Rohini fiats.

v ~'~ 2C

e

z

~15,

~-

10' 5 0

~

o

=

-'~

~ I

2 o

EXPOSED

_z

1

_-j SHADED

I

I

I

I

[

I

0

8.00 9.00 10.00 11.00 12.00 13.00 14,00 15.00 16.00 TIME IHours ].S.I]

Fig. 16. Temperature and wind in shaded and exposed environments under channelling winds at an urban complex, the N.D.M.C. building.

Monsoonal rains wash out the pollutants until September, reducing turbidity. Calm winds, inversions and suspension of small-sized particles increased visibility later. However, it was found that the annual average turbidity has steadily increased indicating the atmosphere over Delhi is gradually becoming turbid. Microclimatic parameters, namely, air temperature, dew point and wind at 1.5 m above the ground on the exposed side of a building and on the shadow side of the building (referred to as shade), inside and outside a forest, over a crop and bare soil were observed. Shades in the urban, suburban, and rural complexes gave a temperature relief of 2.5 °C,

90

10'

18

q

9

16~ 14

?

o~12 o~

z

~_ 6 z

~10

5 o- 5

~8 D

: : SHADED o---oEXPOSED

LLJ

C~

h 3 2~ -=

2

ii 0 I ] I I I I I I 8,00 9.00 10.00 11.00 12.00 13.00 1/,.00 15.00 16.00 TIME (Hours ].ST)

Fig. 19. Dew point in exposed and shaded environments in a suburban area, Rohini flats.

"- SHADED

o - - - o EXPOSED

0L ~ i I I 8.30 9.30 10.30 1130 12.30 131.30 I/.130 15!30 161.30 TIME (Hours ]ST)

Fig. 21. Dew point in exposed and shaded environments in a rural area, J.N.U. Club building.

/*0

35

~7 20

o'30

6 4

v

,.~ c~c

EXPOSED

20

S "~ E

.}SHADED

>-15 LU

o Z

15

~i0

10 ~J F-

5~ I

I

I

I

1

I ..

o---o EXPOSED : : SHADED

I

8.30 930 1030 1130 1230 13.30 I~.30 15.30 16.30 TIME (Hours IS.T)

Fig. 9.0. T e m p e r a t u r e environments

in a rural

and wind in exposed and shaded area,

J.N.U.

Club

building.

2.0 °C and 1.5 °C, a dew p o i n t i n c r e a s e of 2.0 °C, 0.4 °C and 0.7 °C and l o w e r winds, respectively. U n d e r c h a n n e l l i n g w i n d s in urban areas, t h e r e w a s a t e m p e r a t u r e relief of 2 °C m o r e or less t h r o u g h o u t the day on the shaded side, t h e s a m e w i n d s on either side of t h e building, and o n l y 1 °C i n c r e a s e in d e w p o i n t on the s h a d e d side. U r b a n forests and cropped f a r m l a n d r e l i e v e the t e m p e r a t u r e by 2.5 °C and 1 °C, and i n c r e a s e the d e w p o i n t by 5.1 °C and 3.2 °C respectively. Winds under forest c o v e r are light but s t r o n g e r outside. Over

8.00 9,00 10.00 11.00 12.00 13.00 1/*.00 15.00 16.00 TIME (Hours [SZ)

Fig. 22. Temperature in exposed and shaded environments in mid-winter in a rural area, J.N.U. Library building. cropped farmland, w i n d s are retarded compared to bare soil.

ACKNOWLEDGEMENTS The a u t h o r is e x t r e m e l y t h a n k f u l to Prof. M. M. Y o s h i n o and Prof. Y. N a k a m u r a for the i n v i t a t i o n to present this paper and for p r o v i d i n g financial a s s i s t a n c e .

91

7

35

030 ~25

o

o_

E

Y_ ~20

~u 5

4~ t~

L~J

41

3D _z

a. 15 L=J

:~ uJ 3

"-I

D

5

"- -" SHADED o---o EXPOSED

1

Fig. 23. Dew point in shaded and exposed environments in mid-winter in a rural area, J.N.U. Library.

•

}

OUTSIDE FOREST

over crop

I

0 I I I I I I I 0 8.30 9.30 10.30 11.30 12.30 13.30 14.30 15.30 16.30 TIME (Hours (.ST.)

I I L I I ~ I I 8.00 9.00 1000 11.00 12.00 13.00 14.00 15.00 16.00 TIME (Hours(ST)

40

z~Wind

Fig. 26. Temperature and wind over bare soil at a height of 1.5 m and over a crop.

8

9

35

"/

8

=30

6,

~25 q

5 E

6

u.J

~Q_2 0

43 t/1 o_

0-- 5

~I_________~}INTHE FOREST

~"

7

v

24

~15

OA

o3

10 5 0

2 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 TIME {Hours I.S.T)

Fig. 24. Temperature and wind outside and inside a forest near D.L.T.A.

I 0

e : Cropped surface o - - o Bare soil I

I

~

I

8.30 9.30 10.30 11.30 1230

I

1

L

I

13.30 14.30 1530 1630

TIME (Hours ]ST.}

Fig. 27. Dew point over bare soil and over a cropped surface at a height of 1.5 m.

14 12 10(

REFERENCES

C o._. 8

1 B. Padmanabhamurty, Some aspects of the urban climates of India, Proc. Conf. Urban Climatology and its

~- 6 3~

Applications with Special Regard to Tropical Areas, Publ. No. 652, WMO, Geneva, 1986, pp. 136- 165.

~4

o---o EXPOSED 0

I

I

I

I

I

I

(

[

800 9.00 10.00 11.00 12.00 13.00 I/,.00 15.00 16.00 TIME (Hours ]ST)

Fig. 25. Dew point inside and outside the forest near D.L.T.A.

2 S. Sachi Devi, Urban heat island studies at Visakhapatnam, Paper presented at IFHP/CIB/WMO/IGU Int. Conf. Urban Climate Planning and Building, Kyoto, Japan, November 6-11, 1989. 3 S. K. Pradhan and P. Menon, A study of urban heat island over Bhopal, Mausam, 37 (3) (1986) 407. 4 S. D. Sundersingh, Effect of heat islands over urban Madras and measures for its mitigation, Proc. Int. Conf.

92

Urban Climate, Planning and Building, Kyoto, Japan, November 6- 11, 1989, Energy Build., 15 - 16 (1990/91) 245 - 252 (these Proceedings). 5 T. R. Oke, City size and the u r b a n heat island, Atmos. Environ., 7 (1973) 769- 779,

6 S. Rangarajan, Studies on atmospheric ozone and solar radiation, Ph.D. Thesis, Univ. of Poona, Pune, 1966. 7 A. Mani, O. Chacko and S. Hariharan, A study of' Angstrom's turbidity parameters from solar radiation measurements in India, Tellus, X X I (6) (1969).