Tải bản đầy đủ - 0 (trang)
CHAPTER 5. REFLECTION AT THE SEA SURFACE

# CHAPTER 5. REFLECTION AT THE SEA SURFACE

Tải bản đầy đủ - 0trang

70

R E F L E C T I O N AT T H E SEA S U R F A C E

reflected. This occurs at a water surface (n = +) for i = 539, and

in this case:

1 na-1 a

ps = 2 na+l

(-)

Values of the Fresnel reflectance according to the above equations

are tabulated as functions of the solar elevation (Table XVI).

Linearly polarized light which is completely internally reflected

suffers a phase change between the parallel and perpendicular com(1954)

ponents giving rise to elliptically polarized light. WATERMAN

and IVANOPP

and WATERMAN

(1958a) have proved that elliptically

polarized light occurs just below the water surface in lines of sight

differing from the vertical by less than the critical angle (see Chapter 6).

T AB L E XVI

REFLECTANCE OF RADIATION AGAINST A CALM SURFACE

Reflectance

Anglc of

incidence

(Yo)

0")

P II

PI

PS

0

5

10

15

20

25

30

35

2.0

2.0

1.9

1.8

1.7

1.4

1.2

0.9

0.6

0.3

0.1

0.2

0.4

1.7

4.7

11.0

24.0

49.3

100

2.0

2.1

2.1

2.3

2.5

2.7

3.1

3.6

4.3

5.3

6.7

8.6

11.5

15.8

21.9

31.3

45.9

67.4

100

2.0

2.0

2.0

2.0

2.1

2.1

2.1

2.3

2.4

2.8

3.4

4.4

5.9

8.7

13.3

21.2

34.9

58.3

100

40

45

50

55

60

65

70

75

80

85

90

71

THEORY

Reflectance for dzfuse radiation

The reflection of the diffuse component is more di&cult to express

quantitatively. A first approximation considers diffuse light of equal

radiance from all directions L of the sky. The reflectance p(i) for

the angle of incidence i is taken from the Fresnel equations. It follows

from the definition of reflectance and from eq.5 that :

2n

Pd =

2n

Jr

Jr

p(i)~

sin i cos idi

= ]rp(i) sin 2idi

(29)

L sin i cos idi

Several workers have evaluated this integral for a smooth water

surface using Fresnel's reflectance. The consistent value is found

to be 6.6 yo (see BURT,1954a).

For the reflectance in the case of a cardional distribution of sky

radiation (eq.24), PREISENDORFER

(1957) has given an exact solution

which yields 5.2 yo.

Reflectance for global radiation

When dealing with the reflectance of global radiation we have to

treat separately the reflectance of direct energy or solar radiation,

ps, and that of sky radiation, pa.

The total reflectance is expressed as the sum:

where E and E, are the incident and reflected irradiance respectively,

and n is the significant ratio of sky radiation to global radiation.

The result of computations of the reflected energy from eq.30

using p,-values from eq.27 and pd = 0.066 and assuming the

incoming radiation to be a given function of the solar elevation, is

illustrated, for example, by the set of curves for several n-values in

Fig.23 (NEUMANN

and HOLLMAN,

1961). The curve for solar radiation

only (s = 0) develops a prominent maximum at 20" as well as a slight

minimum at 50". The intersection point of the curves at 29" represents

the case of p, = pd = 0.066.

The assumption of uniform sky radiation is inadequate, and more

realistic approaches have been made. Cox and MUNK(1956) have

72

REFLECTION AT T H E SEA SURFACE

10

-

n=l

I

Er

0-

8-

7-

n = 0.5

c

.-

'C

6-

E

Solar elevation

Fig.23. Amount of reflected energy for different portions n o f diffuse radiation

in the global radiation. (After NEUMANN

and HOLLMAN,

1961.)

0'

30'

60'

Solar zenith diStcnet3

1'

Fig.24. Reflected radiance L(i) divided by the sky radiance at zenith for calm

weather and for a Beaufort 4 wind. (After Cox and MUNK,1956.)

73

THEORY

also employed a semi-empirical model and considered a clear tropical

sky on the basis of observations at Bocaiuva, Brazil, for the sun at a

zenith angle of GO". They made the simple assumption that the

reflectance is a function of the zenith angle only, ignoring the

increased radiance near the sun (Fig.21). Their results for clear and

overcast sky conditions are presented in Fig.24.

Effectof

waves

The complete investigation of the interaction of light with the sea

surface confronts us with the additional problem of reflection from

a wind-roughened surface. The following changes in reflection are

immediately realized. With high solar elevations the angle of incidence

will be increased on an average, whereas it will be decreased for low

elevations. The former effect is not important, as the reflectance does

not vary much with the solar elevation ifthis is high. In contrast, the

reflectance for a low sun is drastically reduced by wave action. This

principal feature was first formulated and investigated by LE GRAND

(1939). It appears that the reflectance is independent of the presence

of waves in some intermediate elevation interval. BURT (1954a),

employing a semi-theoretical model, actually found that such independence occurs somewhere between solar elevations of 10" and

30".

The thorough interpretation of the wind effect given by Cox and

MUNK(1956) also takes into account shadowing and multiple reflections for a low sun. Their radiance curves in Fig.25 demonstrate in

essence that the wave action becomes a factor for solar elevations

TABLE XVII

REFLECTANCE

(yo)OF THE SEA FOR SKYLIGHT

SkY

Sea

Smooth

Uniform

Overcast

6.6

5.2

Rough

BURT

(19544

Cox and MUNK

5.7

4.8

5.0-5.5

4.3-4.7

(1956)

74

REFLECTION AT T H E SEA S U R F A C E

l

0’

L

.

30.

~

~

*

60‘

*

90”

l

Solar zenith distance

Fig.25. Reflectance of solar radiation from a flat surface and from a surface

roughened by a Beaufort 4 wind. (After Cox and MUNK,1956.)

below 20”. GRISEENKO

(1959) and HISHIDAand KISHINO(1965)

have demonstrated similar wave effects at low angles. As expected,

the reflection of sky light is less affected by rough sea (Table XVII).

Cox and Munk point out that with an absolutely flat sea the horizon

would not be visible. Actually the sea always contrasts with the sky

and becomes darker when the wind increases. The shadowing is also

examined by LAUSCHER

(1955), who computed data which amply

illustrate the irregular reflection pattern from a wave of maximal

steepness (Michell wave).

Glitter of the sea

Glitter is a phenomenon bearing upon reflection of solar radiation.

This special aspect of reflection has given rise to much speculation.

The glitter pattern being open to everyone’s observation, it has in

popular speech been given poetic names such as “the road to

happiness” (SHOULEIKIN,

1941) or “the golden bridge” (STELENAU,

1961). The glitter arises when a flat surface is roughened by wind and

the image of the sun formed by specular reflection explodes into

glittering points. This is because water facets occur with an orientation so as to reflect sunlight to the observer. Increasing roughness

will enlarge the width of the glittering band. The phenomenon is

most spectacular at solar elevations of 30-35” (cf. Fig.23). The

pattern becomes narrower when the sun sets.

CONCEPT O F ALBEDO

75

The distribution of radiance of glitter has been the subject of

theoretical treatment with a view to estimating the slopes of the sea

surface. The interested reader may consult the original papers by

HULBURT(1934), DUNTLEY

(1952), Cox and MUNK(1956), SCHOOLEY

(1961), and MULLAMAA

(1964b).

The latter has also considered the polarization of the reflected

glitter, which is shown to be dependent both on solar elevation and

on the direction of reflection. The polarization at the maximum

radiance is near zero at i = 0" and increases with zenith distance to

100 yo at i = 35-45", subsequently decreasing to 20 % at i = 90".

CONCEPT O F ALBEDO

In order to scrutinize reflection events and secure adequate definitions, the following symbols are pertinent:

Ead= downward irradiance in air.

Eau= upward irradiance in air.

Ewd= downward irradiance in water.

Ew = upward irradiance in water.

Since reflection against the sea surface takes place from above as

well as from below, two kinds of reflectance are distinguished: ( 7 )

p a = reflectance in air; and (2)pw = reflectance in water.

The albedo A of the sea is defined as the ratio of the energy leaving

the sea to that falling on it:

The above definitions yield the following identities :

Ead-Eau = Ewd-Ewu

Eau = paEad+Ew-pwEw

From eq.31 the albedo may therefore be written:

Some confusion in reflection studies has resulted from failure to

distinguish the reflection (at a surface) in a strict sense, namely p a ,

and the albedo which is the sum of p a and the percentage of light

76

REFLECTION AT T H E SEA SURFACE

back-scattered from the sea. Furthermore, the factor pw must be

accounted for. Because of the relatively small variation of the scattering function in the back-scatter field, we can assume in a first approximation that the upwelling light is completely diffuse and unpolarized.

Because Fresnel reflection beneath the surface involves total reflection

in the angle interval 48.6-90", integration of eq.29 yields pw = 48%.

Hence it is not permissible to neglect this factor as most workers

have done.

EXPERIMENTAL VALUES OF REFLECTANCE

A selection among the multitude of reflection observations is called

for. The systematic studies of albedo as a function of various parameters made by ANDERSON

(1954) and FORS(1954) will be our

primary references. The Lake Hefner results suggest that the albedo

under clear skies is only weakly dependent on wind speed and air

mass turbidity, including the effects of clouds. We may infer that the

albedo is primarily a function of solar altitude. Even with a low

stratus cloud cover, the dependence on the position of the sun

(1948),

persists. This is consistent with the findings of NEIBURGER

but conflicts with earlier statements (Chapter 4).

Table XVIIl shows the true reflectance for a flat level surface and

clear sky. These values have been drawn chiefly from Anderson's

results with due correction for the existing low shortwave backscattering from the water of Lake Hefner.

In rough weather it is difficult to distinguish between the true

reflectance and the shortwave light back-scattered from the sea. White

caps and air bubbles in the surface layer contribute greatly to the

albedo. Experiments have verified the reduction of reflectance at

low solar elevations owing to wave action. The present author found

TABLE X V I I I

REFLECTANCE OF UNPOLARIZED RADIANT ENERGY (SUN+ SKY) FROM A HORIZONTAL

WATER SURFACE

Solar altitude(")

Reflectance(yo)

90

3

60

3

50

3

40

4

30

6

20

12

10

27

5

42

DISPERSION O F REFLECTION

77

:

p

+

1

10

0400

500

-'05

600

700 nrn

Fig.26. Reflectance (yo) of global radiation for different solar elevations as a

function of wavelength. Corrected for cosine error of the collector.

(After SAUBERER

and RUTTNER,

1941.)

that a flat sea surface is a barrier to the light from a distant lighthouse

tower, whereas the reflection of this light from a rough surface is

remarkably low.

DISPERSION OF REFLECTION

It is established by observations that for solar elevation below 30"

a dependence of reflection of irradiance on wavelength is developed

(Fig.26). SAUBERER

and RUTTNER(1941) have given the right explanation of this effect which - since the dispersion of light reflection is

only 6% -must be associated with the amount of dif€use light in the

global light. This is 90 % in the violet and 22 yoin the red for a solar

elevation of 10"(Fig.22). The average angle of incidence is thus much

less for the violet than for the red, which accounts for the stated

difference in reflection.

CHAPTER

6

R E F R A C T I O N A T THE S E A S U R F A C E

REFRACTION LAW

The interface between air and sea is a boundary between two media

of different optical density. An electromagnetic wave falling on the

surface decomposes into two waves ; one is refracted and proceeds

into the sea, the other is reflected and propagates back into the air

(Fig.27).

The law of refraction is:

sin i

-=n

sinj

(33)

a relation known as Snell's law. The angles i a n d j are defined so as to

describe a refracted wave as being in the same plane as the incident

wave. For definiteness we may take the refractive index n of sea water

relative to air to be 8. From a geophysical point of view there is no

need to account for the variation of the index with temperature,

salinity and wavelength (Chapter 2, p.25).

For the case of grazing incidence (i = 90') a limiting angle of

refractionj = 48.5" is obtained. As seen from the sea, the sky dome

is compressed into a cone of half-angle 48.5'. If they form an angle of

more than 48.5" with the vertical, upward travelling rays are totally

reflected at the surface (Fig.27).

WATER

SURFACE

Fig.27. Refraction and total internal reflection at the sea surface.

EFFECT OF WAVES O N REFRACTION

79

Refraction is associated with polarization according to Fresnel's

formulae. Direct sunlight penetrating into the water becomes partly

polarized. Calculations by MULLAMAA

(1964a) testify that the degree

of polarization increases with the sun's zenith distance and reaches

magnitudes as great as 27 yo during sunset.

The linearity of skylight polarization remains unchanged by refraction though there is a rotation of the plane of oscillation.

C H A N G E OF RADIANCE A N D I R R A D I A N C E AT T H E SURFACE

It follows from simple geometrical considerations that the radiance

L, below water of refractive index n is na times the radiance Lain air

reduced by reflection losses at the surface (GERSHUN,

1939) :

L, = #a( 1-&La

(34)

The downward irradiance in air and in water are respectively:

d E a = La cos i d o = Lacos i sin id+&

dE, = L, cosj d w = L, cosj sinjdr\$dj

From eq.33 and 34 it follows that:

dE, = (1 -p)dEa

(35)

which verifies the obvious fact that the interface between air and

water changes irradiance on a plane parallel t o the interface by

reflection only.

EFFECT OF WAVES O N REFRACTION

Refraction according to Fig.27 is described by Snell's law only in

the ideal case of a flat surface. Waves cause fluctuations of the direction of refracted rays. Deviations of the direction of maximum

radiance can amount to as much as f 1 5 yo according to theoretical

deductions of MULLAMAA

(1964a). Because of wave action, the image

of the sun as seen from beneath the surface disintegrates into a glitter

pattern with features different from those of the reflected glitter. The

refracted glitter subtends a smaller angle and is of the order of 1,000

times more intense than the reflected glitter. Due to lens action of

### Tài liệu bạn tìm kiếm đã sẵn sàng tải về

CHAPTER 5. REFLECTION AT THE SEA SURFACE

Tải bản đầy đủ ngay(0 tr)

×