The emergent flux

The particular light flux that is of the greatest interest in the present context is the upwelling light flux just below the surface. However, the flux that, after due correction, is remotely sensed, is the emergent flux -that part of the upwelling flux that succeeds in passing up through the surface. How are the two related?

Although about half the total upwelling light flux is reflected downwards again at the water-air interface, this does not represent a serious loss, since it is mainly the flux at larger angles which undergoes reflection. Remote sensing normally involves measurement of radiance at a specific angle rather than total upward irradiance, this angle being not more than 58° from the nadir. Thus remote sensing is normally concerned with that upwelling flux which, below the surface, is at angles of 0 to 40° to the vertical, and under calm conditions only to 6% of this is reflected downwards at the water-air boundary. Reflection in this angular range is

Table 7.1 Scheduled Ocean-Colour Sensors.

Scheduled

Swath

Resolution

#of

Spectral

Sensor Agency Satellite

launch

(km)

(m)

bands

coverage (nm)

Orbit

HSI DLR EnMAP

2013

30

30

228

420-2450

Polar

(Germany)

GOCI KARI/ COMS-1

Early 2010

2500

500

8

400-865

Geostationary

KORDI (South Korea)

OLCI ESA (Europe) GMES-Sentinel 3A

2013

1270

300/1200

21

400-1020

Polar

(ESA/EUMETSAT)

OLCI ESA (Europe) GMES-Sentinel

2017

1265

260

21

390-1040

Polar

3B (ESA/EUMETSAT)

S-GLI J AXA (Japan) GCOM-C (Japan)

2014

1150-1400

250/1000

19

375-12 500

Polar

VIIRS NOAA/NASA NPP (USA)

2011

3000

370/740

22

402-11800

Polar

VIIRS NOAA/NASA JPSS-1 (USA)

2015

3000

370/740

22

402-11800

Polar

From the IOCCG website (www.ioccg.org/sensors/scheduled.html) 2010.

From the IOCCG website (www.ioccg.org/sensors/scheduled.html) 2010.

somewhat increased under rough conditions: at the smaller angles losses remain insignificant but can rise to 16 to 27% for angles at the upper end of this range.41

As the light passes through the water-air boundary it undergoes refraction, which in accordance with Snell's Law (§2.5) increases its angle to the vertical. A further consequence of refraction is that the flux contained within a small solid angle, do, below the surface spreads out to a larger solid angle, n2 dm (where n is the refractive index), above the surface. Because of this effect, the value of emergent radiance at any given angle is about 55% of the corresponding subsurface radiance from which it is derived. Combining this effect with the much smaller effect of internal reflection, Austin (1980) proposed a factor of 0.544 for relating radiance just above the surface, Lw(0,f) (the water-leaving radiance), to the corresponding radiance just below the surface, Lu(0',f)

0' being the nadir angle within the water, 0 the angle in air after refraction at the surface, and f the azimuth angle of the vertical plane containing Lw(0,f) and Lu(0',f) relative to the plane of the Sun. If Lw(0,f) can be determined from remote sensing measurements, then it can be multiplied by 1.84 to give Lu(0',f), and thus provide information about the underwater light field. For water-leaving radiance values at the higher zenith angles, Lu(0',f) can be calculated more accurately using eqn 6.16 (see §6.4)

Gordon (2005) has shown, by modelling the light field, that even at wind speeds as high as 20ms-1, the transmittance of the roughened water-air interface (away from whitecaps) for upward radiance is nearly identical to that for a flat surface.

As we saw earlier (§6.4), it is not the absolute value of upward radiance, but the ratio of upward radiance to downward irradiance, which contains information about the composition of the water, and this can be calculated using eqn 6.20

rs Ed(0+)(1 - p)[1 - p(6', f)] from the experimentally determined Lw(0,f) and Ed(0+) values.

0 0

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