Aircraft Measurements of the Earth's Albedo over India

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Indian Journal of Radio &Space Physies Vol. 4,December 1975,pp, 304-309

Aircraft Measurements of the Earth's Albedo over India *

A. MANI

India Meteorological Department, New Delhi

&

O. CHACKO, V. DESIKAN &G. VIJAYAN India Meteorological Department, Poona

Received 30 April 1975

Satellite observations of the earth's albedo consistently indicate that the earth is a warmer and darker planet than was previously helieved and that more solar energy is absorbed by the earth-atmosphere system than hitherto accepted, primarily in the tropics. In order to verify these observations, direct measurments of the albedo of various surfaces were made on the ground at a number of stations in India and from aircraft over extensive land and sea areas. Two thermoelectric pyranometers were used to measure the global and reflected solar radiation, the data]being .::ontinuously recorded in flight. These data were supplemented by observations: over the Arabian sea from INS Darshak, an oceanographic ship during its cruises in 1974. The measurements show that the albedo over land varies from 0'10 to 0'25, depending on the n'lture of the surface and that over the sea from 0'04 to0'13 depending on the state of the sea. The albedo of clouds varies from 0'40 to0'60 for active cumulus to 0'25 to0'35 for altostratus. This compares with the value of 0'29 for the planetary albedo obtained from satellite observations.

1. Introduction

THE MEANVALUEof the earth's albedo or the ratio

of the reflected to incident radiation, integrated over

the whole solar spectrum from 0'2 to 4'0 p.m was generally accepted to be about 0'35, till very recently, when satellite observations indicated much lower values of the order of 0'28 (ref. 1). The tropical regions particularly were observed to have a lower albedo than believed in pre-satellite days. A better knowledge of the reflection and absorption properties of the clouds and the atmosph(~re in the short wavelengths, especi- ally over the tropical and equatorial regions, from independent measurements is, therefore, essential if the remotely sensed satellite data are to be understood and interpreted correctly. The results of aircraft measurements of the albedo ofthe earth and of clouds over India and the adjoining seas made during 1972 from a Canberra research aircraft and during 1973 from a Dakota aircraft have been published2•

The pre~ent paper summarizes the results of recent aircraft observations of albedo made from a DC 3 aircraft over the Bombay-Poona area and the Rihand Dam catchment area and also presents the results of measurements of albedo made over the Arabian Sea on-board the oceanographic ship INS Darshak during its 1974 cruises. These are supplemented by surface observations at Delhi.

*Paper presented at the Symposium on Earth's Near Space Environment, 18-21 February 1975, held at the National Physical Laboratory, New Delhi 110012.

304

2. Aircraft Measurements

An instrumented DC-3 aircraft was used by the Indian Institute of TlOpical Meteorology, Poona, for its weather modification experiments during 1974 over the Bombay-Poona region (18°N; 74°E) and the catchment area of Rihand river in Uttar Pradesh!

Madhya Pradesh (24°N; 83°E). This opportunity was availed of to supplement the earlier aircraft observations of albedo made in 1972 from a Canberra aircraft.

The altitude of the DC-3 aircraft during the flights was between 600 and 2400 m above ground-level. The instruments used for the measurement of global and reflected solar radiation \\ere the same and have been described in the earlier paper2• One thermoelectric pyranometer was mounted at the centre on the top of the fuselage and the other one on the lower surface of the fuselage, a little behind the Pilot's cabin. The locations were so chosen that the instruments were horizontal when the flight was level. The outputs of the instruments were connected to Honeywell potentiometric four-channel recorder, with a full scale deflec- tion of 0-15 mV and a chart speed of 300 em per hr. An inverter was used to convert the 12 V de supply available on board the aircraft to the 230^V ac required for the recorder. The aircraft flew over a region about 50 km to the east of Poona and on several occasions around Poona and later confined its flights to the Rihand catchment area during the

1974 monsoon season.

Wj I" Ii I~1 I 1111'1 ~Ilt,ill

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MANI et al,: EARTH'S ALBEDO OVER INDIA

Table I-Albedo of Earth's Surface and of Clouds around Poona Using DC-3 Aircraft Solar radiation

Flight TimeSolar

SurfaceAlbedoRemarks GloDal

Reflected level hrsEleva- type (calfem'fmin)

amsl tion m

deg 21 June 1974

1'53-1'62

0'10-0'30 2440 1330-134669 Sea surface

W~131

1'07-1'41

0'12-0'18650 1422-142664 Sea surface 0'10-0'13 1'39

0'21650

1427 61

Coastal line0'15

1'06-1'28 Cu below1526-152719500'39-0'4950

0'37-0'38

{)'94-1'19 Vegetated land1542-15462470O'}9-0'220'17-0'2446

23 June 1974

No clOud above

I

0"21-0'41

0'08-0'21 2140 1555-155843 Cu in different

0'23-0'76

I

stages

0'22 Cu just before1559-160021400'130'5942

raining

I 0'14-0'22

0'08-0'10 2140

~601-160433 Cu, raining 0'45-0'57

J

8 July 1974 0'48-0,62

0'11-0'15 2500 1549-155345

Sc thin 0'20-0'28

1

0'48-0'57

0'26-0'35 1675 1641-164233

Cu top just below 0'70-0'76 I Cloudy

15 July ]974 }-

I

0'50-0'87 Sc 1521-152721300'12-0'280'22-0'3851 J

1'17

213D0'16 1528 37 Bare soil€:lear0'14

Cu patch in between reduced global by 24%

1

I

}- Cloudy

I l

J

Jr 0'18-0'19 0'49-0'71 0'13

Sc

Barren jungle Ac in patches above Cu Hillock

Obra thermal 0'19 power station

Water reservoir 0'16-0-19 Forest, Gundi ().}3

Land 0'11-0,15

Stratus 0'20

Thick forest 0'05 Land seen through 0'17-0'29 patches of stratus

51'9 42'1 36'3 26'3 20'0 ]5'8

6 Oct. 1974 19'5 20 Sep, 1974

1238-1239 65'8

1635

19 Sep, 1974 1428-1431 46'8

1625-1627 20'4

1402 1415-1417 1500-1501 1525-1528 ]611 1638 1655-1702 1100

0'17

Table 2-Albedo of Earth's Surface and of Clouds over Varanasi-Rihand Valley Using DC-3 Aircraft

Solar radiation Flight Time Solar Surface Albedo Remarks

Reflected level hrs eleva- type

(calfem2fmin) amsl tion

m deg

10 Sep, 1974 55'0 Global

0'62-1'12 15500'12-0'20

0'91 16500'13

0'76-0'89 17000'09-0'13

0'46 15800'09

(l'22

0'01 1525

0'35-0'49 14900'09-0'10

0'50-0'74

0'09-0'14 1530

0'45-0'65 27800'22-0'46

0'23

2]600'03

1'37-0'98

0'34-0'23 1280 '0'89

1'20-126 22250954-10140'19-0'2248'2-51'7 Natural vegeta- 0'15-0'118

1

1'32-1'39 tion

0'15-0'19

22251032-105454'5-57'6 Natural vegeta-0'12-0'14 tion, rocky sur-

I

face in between

0"12-0"14

J

^Ck."ky

bushes 1'44-1'49

0'17-0'20 22251212-124061'4 Natural vegeta-

tion, Mainly

I

bare soil rocky

I

in character

I

l'41-1'47

O'19-0'3{)

22251302-132160'5-58'5 Hilly terrain of(}'17-0'21

I

Western Ghats,

I

. Vindy~sSatpura rangesand

j

305

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INDIAN J.RADIO SPACE PHYS., VOL. 4, DECEMBER 1975 3. Results

The values of the global and reflected solar radiation and the albedo of the earth's surface obtained during the flights over the Bombay-Poona area and over the Varanasi-Rihand Dam area are summarized in Tables 1 and 2.

was of the order of ^0'16 and^0'20 and that over barren land (black cotton soil) interspersed with vegetation, was between 0'10 and O·lS. There were small clouds throughout the latter half of the flight, cumulus clouds below giving an albedo value between 0'30 and 0·3S.

3.1 Flights o,er Bombay-Poona Area

During regular cloud-seeding operations, the flights were mainly over the south-eastern and eastern sectors of Poona over Baramati. Most of the results unfortunately could not be used, because the aircraft was generally not level in flight. On 23 June the albedo over a large cumulus cloud in its various stages of development and dissipation was measured.

Albedo increased rapidly from 0'23 to 0'76 during the growth of the cloud, decreased to 0'59 just before precipitation occurred and dropped to 0'45 during actual rain.

Fig. 1 shows th(: results of observations on 21 June 1974 when the aircraft flew from Bombay over the Arabian sea before turning towards Poona.

Over the sea the global radiation was of the order of 1'40-1'56 cal/cm2jmin and the reflected radiation 0'01-0'30 caljcm2jmin. The albedo values were generally from 0'03 to 0'10 for the sea surface. The skies were clear throughout the period. The albedo showed a sudden increase to 0'15 as the coast was crossed. Over the hills and valleys of the Western Ghats the albedo varied between 0'19 and 0'27.

Albedo over cultivated land, mainly grass vegetation,

3.2 Flights from Varanasi over Rihand Valley

The aircraft was based at Varanasi (25°N: S3°E) for this series of cloud seeding operations and observations taken during the flight from Varanasi to Rihand Dam and back. On 10 Sep. 1974 the sky was covered generally by cumulus clouds and the global radiation recorded was low. There was no cloud below when the aircraft was over th~ Obra thermal station at a height of 1100 m and the albedo obtained was 0'19. The value over the adjoining water reservoir was between 0'16 and 0'19 and 0.13 over the forest Gundi. Barren land gave a value of 0'11-0'13 and cultivated land from 0'13 to 0'19.

Albedo over stratus cloud ranged from 0'20 to 0'25 while over large cumulus it was of the order of 0'4S- 0'59 and 0'27-0'59 over stratocumulus-cumulus in patches. A dense forest gave a value for albedo 0'05.

The sky was generally cloudy again on 19 Sep.

1974. The albedo recorded ranged from O'lS to 0'19 for sparse jungle without many trees, and 0'13-0'lS for barren hillocks devoid of vegetation. Over stratocumu- Ius it was 0'36 becoming 0'48 when cumulus also occur-

red with stratocumulus. Albedo over small cumulus was 0'33 and increased to 0'49-0'71 when altocumulus clouds appeared in patches over cumulus clouds.

.~

N .

E Eo

•.•.•.

Cio_•..

z

Q

0·5 ~ Ci

<{

.4

lr

o

w

.3 t>

_wO a

-' w

l!.CD

lr<{w-'

\..

t·.1i

~~.

^\

.. \

--- REFLECTED RADIATION ,_.-- GLOBAL RADIAT ION

r·~6EDO

II_I

I

• '•.,1

• I

·

^.,

I ••

oJ ••

.:

',

' .

'/'-

...

....

-'

<{ 1·0

a

Il)

_-'

~ 0'9

~

E 1-4

"'^{...••.•}

E

~a_o 1·3

"'

ZQ ',2

~

o

<{ 1-1

a::

Fig.I--Values of global and reflected solar radiation and albedo on 21June 1974during flight from Bombay to Poona

306

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--- ..__ ~ ~ L.

MANI et al.: EARTH'S ALBEDO OVER INDIA

0'5

0·1

I ^,IIIIIIIIIItI^,

0 0 0000'"000

.., ^N '"N'";;;'"^M M

~ o ~

^:: -

~

::

^:::? ^~

o

5

'"

0>

o

n -- REFLECTED RADIATION

-- GLOBAL RADIATION

... ALBEDO

<:>

a;

C>

1·5

c

E N-- I'~

~E -0u Z•.

o

;: 1·3

~ • !{\

o

^-'u ¹

<l: ' II

a:: •• ''1

....J ')/-I~

r'

<l: ~ .. \loo wI ^f 10. ••

m1·2 .' ;:'~" \ /

o " 'j ..

,vv/' ,

^-L--_o

M0>

o

Fig. 2-Values of global and reflected solar radiation and albedo on 6 Oct. 1974 during flight from Varanasi to Poona

Records were obtained during the return flight of the aircraft on 6 Oct. 1974 from Varanasi to Bombay where it was grounded after the programme. The aircraft was generally flying at 2225 m. The skies were clear throughout the period and the ground also was generally visible clearly. The global radiation gradually increased with time from 1'00 at 0858 hrs 1ST to 1'49 around 1208 hrs 1ST (Fig. 2). The reflected radiation \vas more or less steady varying betwten 0'15 and 0'23, occasionally increasing to 0'28.

The value of albedo was steady over the vegetated fields varying between 0'13 and 0'20. Over the bare ground and rocks the values were between 0'11 and 0'14.

4. Surface Albedo Measurements

4.1 Measurements over Land

Surface albedo measurements over a bare soil surface were first started at Poona in 1963, and have been continue9 on a regular basis. Since then addi- tional observations have been made at New Delhi from 1972 and at a number of stations in the country2 (1975). Fig. 3 gives the isopleths of albedo of the natural underlying surface (grass) at Delhi.

The albedo of a particular kind of soil is subject to variations, depending on its colour, structure and humidity. Thus, with the establishment of the monsoon at Delhi surface albedo shows a marked fall from 0.25 to d·19. The mean value of albedo is 0'23 compared to the much lower value of 0'14 at Poona over black cotton soil.

The diurnal variation of albedo is influenced by surface roughness and irradiation conditions, such as the solar elevation, the relation between scattered and global radiation as well as variations in the spectral composition of the incident radiation. Fig. 3 shows that duHng the day the albedo at Delhi is a minimum generally at about noon. During the year the minimum is during Aug. - Sep. soon after the establishment of the monsoon .•.

4.2 Measurements over Sea

A series of measurements of .global solar radiation and reflected solar radiation Were made on board INS Darshak, an oceanographic ship, during its cruises in the North Arabian Sea from 17°N; 73°Eto 25°N;

600E from Dec. 1973 to May 1974. The global radiation pyranometer was mounted on gimbals on a boom 0'6 m long at the highest point over the ~ridge. The reflected radiation pyranometer was installed on a 5 m long boom with gimbal mountings, forward of the quarter deck facing the surface of the sea. The outputs were recorded on a Honeywell 4-channel potentiometric recorder. The results for selected days with cloudless and cloudy skies are given in Table 3.

The average value of the albedo of the sea from aircraft measurements over the Arabian sea, the Bay of Bengal and the Indian Ocean immediately· to the south of the tip of the peninsula measured from the Canberra aircraft was about 0'02 to 0'16 for solar elevation angles of 22° to 81° (ref. 2). Tke values obtained from the DC-3 aircraft range between.Q·01

307

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1 I I

,

Fig. 3-Isopleths of albedo at Delhi showing hourly variation in different months

1

:. ,111

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i

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INDIAN J. RADIO SPACE PHYS., VOL, 4, DECEMBER 1975

and 0'13 over the Arabian sea. INS Darshak measurements give similar values of the order of 0'04 to 0'15 (Table 3).

The diurnal variation of albedo is, as expected, very striking. The values for higher solar angles range from 0'04 to 0'09, the lowest values 0'04 and 0'05 being recorded around noon. These values correspond to calm seas, The albedo increases with the increasingly rough seas and with decreasing angles of solar elevation. This is evident on 26 Jan. when the waves were larger than usual. Cloud cover slightly increases the albedo of water surfaces as can be seen for example from Table 3, On 15 Feb, albedo was low with mainly Ac clouds present upto 1200 LAT and it increased thereafter when Sc clouds covered larger portions of the sky.

The importance of solar elevation and sea surface conditions such as wavelength, bubbles both at and

A M

M

'f! j /'J

~~1rcc6

_N _'20~

o

Table 3-Albedo over Sea Surface for Measurements on Board INS Darshak for Whole Cruises and for Selected Days

Waves Weather

Values of albedo at hrs LAT

5-6 6-7 7-88-9 9-10 10-11 11-12 12-13 13-14 14~1515-1616-1717-18 18-19 Cruise I (21-27 Dee, 1973)

Small

i-I

^m

Small

i-i

^m

Mainly clear

Mainly clear with occasional Sc clouds

0'40 0'23 0'13 0'10 0'08 0'07 0'07 0'08 0'09 0'13 0-19 0'29 - Cruise II (4-28 Jan. 1974)

0'37 0'23 0'14 0'09 0'07 0'06 0'07 0'08 0'10 0'14 0'24 0'25 -

Sc, Cu clouds Smalll m

Calm/small

i

^m

Cruise III (5 Feb.-l Mar. 1974)

- 0'29 0'21 O'll 0'07 0'06 0'06 0'06 0'06 0'08 0'13 0'21 0'33 - Cruise IV (16 Mar.-14 Apr. 1974)

Mainly clear with 0'35 0'30 0'19 0'11 0'07 0'06 0'06 0'05 0'06 0'07 0']0 0'16 0'25 0'33 occasional Sc&Ci

douds

Small

1-1

m Mainly Sc clouds

Cruise V (3-12 May 1974)

0'50 0'26 0'19 0'13 0'08 0'06 0'06 0'06 0'07 0'07 0'07 0'12 0'22 0'35

Small

t

^{m (course} ^{4/8 Sc}

north-eastward)

Small

1

^{m (course} ^Clear

earstward and then westward)

2/8 Sc, Ac, Cs Small

i

^m

(course eastward)

Small

!

^{m (course}

south-eastward) Calm

(course eastward)

Clear

13 Jan. 1974(Iat. : 21'6-21'8° N; long. : 61'7-63'0° E)

0'33 0'27 0'13 0'08 0'07 0'07 0']0 0'10 0'09 0'12 0'16 0'14- 26 Jan, 1974(lat, : 19'5-20'0° N; long, : 68'9-69'5° E)

0'50 0'25 0'15 0'11 0'16 0'07 0'06 0'09 0'11 0'08 0'17 0'33- 15 Feb, 1974(lat. : 23'6-24'0° N; long, : 63° E)

0'17 0'10 0'08 0'07 0'07 0'10 0'11 0'09 0'10 0'13 0'21 0'22- 13 Apr. 1974(lat. : 22'4 ON; long, : 60'6-62'9° E)

0'18 0'20 0'13 0'10 0'09 0'08 0'07 0'06 0'06 0'06 0'07 0'10 0'16 0'20 8 May 1974 (lat. : 17'5° N; long, : 72'6-73'2° E)

0'78 0'30 0'26 0'13 0'07 0'05 0'05 0'08 0'08 0'07 0'15 0'19 0'24 0'60

•....~

'.

308

I· ,I I" h~' I'l"rl" 1'11'1 "'I' " ' ¹¹¹¹¹ ^'I 'III .~, fill,1III!lIHIlli I : Iii I' I I I

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MANI et al. :EARTH'S ALBEDO OVER INDIA

below the surface, water turbidity, etc. on albedo are well known. The albedo of a water layer either smooth or roughened by wind (but not to the point of having breaking waves, white caps, large quantities of bubbles, etc.), however, has a remarkably constant value for any given solar elevation and clear sky conditions.

5. Conclusions

The albedo of sea and land surfaces and the albedo of clouds for solar radiation measured from aircraft flying at levels from 1SOto 2400 m over the

Indian subcontinent and the adjoining seas during the mons,ooil season July to Sep. 1974 give reflectance data over extensive areas of the Indian -subcontinent oversoil, vegetation, water basins and the sea. The results are in good agreement with surface observations over land and sea and with satellite measure- - ments of the planetary albedo.

References

1. VONDER HAAR, T.H., RASCHKE. E., BANDBBN, W &

PASTERNAK, M.,Solar Energy, 14(No.2) (1973),175.

2. MANI, A., CHACKO, 0., DElllKAN. V. '" AHBYANKAR, V. V., Indian J.Met. Geophys., ~ (No.1) (1975),29.

~I

1 1

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