KØBENHAVNS UNIVERSITET

Transcription

1 KØBENHAVNS UNIVERSITET INSTITUT FOR FYSISK OCEANOGRAFI THE USE OF THE COLOUR INDEX FOR DETERMINING QUANTA IRRADIANCE IN THE SEA By N. Højerslev and N. Jerlov REPORT No. 35 COPENHAGEN JULY 1977

2 KØBENHAVNS UNIVERSITET INSTITUT FOR FYSISK OCEANOGRAFI THE USE OF THE COLOUR INDEX FOR DETERMINING QUANTA IRRADIANCE IN THE SEA By N. Højerslev and N. Jerlov All right reserved No part of this work may be reproduced in any form whatsoever without the permission of the Institute of Physical Oceanography University of Copenhagen REPORT No. 35 COPENHAGEN JULY 1977

3 J. i. TRYKTEKNIK A/S

4 THE USE OF THE COLOUR INDEX FOR DETERMINING QUANTA IRRADIANCE IN THE SEA by N. H^jerslev and N. Jerlov Measurements of the colour index F(lm ) = 1,(180, U50 nm, 1m)/, 525 nm, 1m) have demonstrated its usefulness in the practical work. The meter recording the index is a simple instrument which can "be lowered manually from shipboard preferably to one meter. An observation of the index is accomplished in a few minutes. Systematic field studies have proved that the index most likely is a linear function of the depth at which the percentage of surface quanta irradiance ( nm) is 10% (Jerlov, 197^; Hjzijerslev et al., 1977)- A recent investigation (Jerlov, 1977) has given the result that constant relationships exist between different quanta percent levels. In consequence the index yields full information about the quanta distribution in the whole euphotic zone. Additional experimental material is supplied below in order to strengthen the comparison between index and quanta irradiance. It seems also pertinent to look into the theory with a view to assessing the significance of the empirical relationships established. Results The index is measured as a ratio of radiances through 180 which makes observations close to the ship feasible. Since underwater radiance does not show large variations between l80 and 90 the index can as well be defined F(lm) = Eu(U50, lm)/eu(525 nm,lm). Previously, a straight line is presented which describes the relation of index versus 10$ quanta level (Hjzfjerslev et al., 1977)* Thanks to the introduction of colour index pertaining to irradiances it has been possible to prolong the line at both ends; some data of low index from the Belts and the Baltic Sea as well as a point found for the Sargasso Sea are added (Fig. l). The indices defined as irradiance ratio (black dots) fit well in the scheme and confirm the straightness of the line. The representation in Fig. 1 com- prises all water masses except very turbid waters in coastal zones. A strong increase of the index occurs from 3.7 for Type I in the optical classification towards no less than J+.57 for Sargasso water which is obviously quite unique as to the extreme clarity of its surface water. In accordance with the finding that constant relationships exist between different quanta levels we can now proceed to represent these levels

5 4 Z (30%), Z (10%), Z (3%), and Z (l%) as functions of the colour index, F (lm)(fig. 2, Table 2). The equations are as follows: Z (30%) = *F(lm) 'F 2 (lm) Q. Z (10$) = k.o *F(1m) Z ( 3%) = F(.1m) Z ( 1%) = F(im) The simplicity of the equations is not due to chance "but has its root in the optical classifications established. F Theory From the definition of the colour index E^ (U50 run), i m ) u (1m) = E /Toe C (525 nm), S % 1m) u It follows that the colour index can be written with sufficiently accuracy as E u (i.?0 nm.om) /EJ525 nm.om) _ K (u ^ + R ^ ^ F (lm) = E d (lt 5 0 mn.om)/ E d ( nm, o m)' 6 U since the downwelling irradiances E^ (U50 nm,o m) and E^ (525 nm, o m), respectively, are of the same magnitude. For the oceanic water types I - III the exponential term above containing vertical attenuation coefficients for the upwelling irradiance varies only slightly around unity so it will be omitted. The reflet ance ratio at the surface R(A,o) defined as E (X, o m)/ u E^(A, o m) is proportional to the ratio of the backscattering coefficient b^ and the absorption coefficient a, i.e. b b (A, o m) R(X, o m) ~ rr r a (X, o m) (for instance Joseph, 1950, Gordon et al., 1975 s and Prieur, 1976). Accordingly, b (U50 nm) a(525 nm) F ( 1m) = b (525 nm) a(u50 nm) b The backscattering coefficient b^ can be devided into two parts b, = Cb, ) + (b ) b bp b m where the first term expresses backscattering solely due to particles and the second one the molecular backscattering. It is now assumed that (b ) P

6 5 is virtually independent of the wavelength À in the range 1+5Q nm and _ 3 that (\) m is proportional to X (Morel, 197*0. This implies that the possible maximum variation of the ratio nm) ^(525 nm) = A runs from unity to an upper limit of 1.9^ for all natural waters. However, for the oceanic water masses belonging to the optical classes I - III (Jerlov, 1976) the actual variation of A is within the range of i.e. "by selecting a fixed value for A equal to 1.3 the maximum variation of A could attain ± 25%. This is obviously a too crude approximation. An improved assumption would he proportionality between A and Z {10%) A = a Z (10%) + $ 0. since a high value for A involves dominant molecular backscattering occurring in clear water and thus high values in Z (10%). The coefficients a and 3 are estimated below from measurements performed in turbid and very clear water. The central Baltic Sea: A - 1 since (b )» (b, ) bp b m Z (10%) ~ 9 m (H^jerslev, 197^) Correspondingly for the Sargasso Sea: A = 1.6 (Kullenberg, 1968) Z (10%) readily found a = 0.011*6 6 " 0.87 Consequently, ~ 50 m (Lundgren and H^jerslev, 1971)* The coefficients a and 3 are F(im) - (0.011*6 Z q (l0*) ) ffilo ) The aim is now to express the absorption coefficient in terms of vertical attenuation coefficients for downwelling irradiance. attempts is reported by Aas (1976). (la) A review of such different For the present purpose it suffices to apply the rather simple near-surface approximation given by Lundgren and Hjzijerslev ( 1971 ) instead of the more elaborated one given by Aas (1976): a(a) = K,(A) d sec 9 + UR(x) s where 8 is the polar zenith distance of the direct sun rays in water. 3

7 V X ) Expressing R(X) in terms of (Prieur, 1976) a(x) * (K,(X) h. U)) cos6 d b s Accordingly, K (525nm) b (525nm) F( 1m ) = (O.OXM Z (10«+ 0.87) K (1> } _ b (1> } = d b K (525nm) (0.01U6. Zq(lOjS) ). Kd(1 50nm) (ID) Jerlov's optical classification of water masses (1976) for the classes I - III gives empirical the following relations: K d (525mn) = 0.65 K d (^50nm) (2) K,(^50nm) - K J (U65nm) (3) d d For the oceanic optical classes I - III equation (l"b) becomes F( 1m) - (0.01^6 Z (10%) ) ( K ( 5^) From Jerlov's classification for the optical classes I - III the following empirical relation between K^(^65nm) for the upper 10 meters and Z (10%) is valid: Z a (l0» = ^ih^) meters (5) Inserting equation (5) into ( 0 gives F(lm) = 0.000b^ Z {10%) Z (10%) + O.lU (6) q. 1 This theoretical expression for F(lm) is deduced from the most simple assumptions. Although the theoretical F(1m) contains a quadratic term in Z (10%) the overall fit to the empirical linear F( lm) is surprisingly good as depicted in Fig. 3*

8 7 References AAS, E., The vertical attenuation coefficient of submarine irradiance. Rep. Inst. Geophysics, Univ. Oslo, 19:20pp. GORDON, H. R., 0. B. Brown, and M. M. Jacobs, Computed relationships between the inherent and apparent optical properties of a flat homogeneous ocean. Appl. Optics, Vol. lu, No.2:pi*17-^27. H0JERSLEV, N. K., 197^. Baltic. Inherent and apparent optical properties of the Rep. Inst. Fys. Oceanogr., Univ. Copenhagen, 23:1-Spp. H0JERSLEV, N. K., N. Jerlov, and G. Kullenberg. Colour of the ocean as an indicator in photosynthetic studies. J.Cons.int.Explor.Mer. In press. JERLOV, N. G., 197^ocean. A simple method for measuring quanta irradiance in the Rep. Inst. Fys. Oceanogr., Univ. Copenhagen, 2U:5pp. JERLOV, N. G., 197Ô. Marine Optics. Elsevier Oceanogr. Ser.s Amsterdam, 2nd Ed.: 203pp. JERLOV, N. G., 1977irradiance. JOSEPH, J., Classification of sea waters in terms of quanta J. Cons. int. Explor. Mer. In press. Untersuchungen iiber Ober - und Unterlichtmessungen im Meere und iiber ihren Zusammenhang mit Durchsichtigkeitsmessungen. Deutsch.Hydrogr.Z., 3:p32U-335. KULLENBERG, G., Scattering of light by Sargasso Sea water. DSR, Vol 15: pil23-^32. LUNDGREN, B. and N. K. H^jerslev, 1971Sea. Daylight measurements in the Sargasso Results from the "DANA" Expedition January - April Rep. Inst. Fys. Oceanogr., Univ. Copenhagen, li+:21pp. MOREL, A., Data Rep. SCOR Discoverer exp. May 1970 I. SIO Ref MOREL, A., 197^. Optical properties of pure water and pure sea water. In: N. Jerlov and E. Steemann Nielsen (editors), Optical Aspects of Oceanography. PRIEUR, L., Academic Press, New York, N.Y. pl-2u. Transfert radiatif dans les eaux de mer. Application a la determination de paramétrés optiques caractérisant leur teneur en substance dissoute et leur contenu en particules. d'etat es Sciences Physiques. A :300pp. These de doctorat No. d'enregistrement au C. N. R. S.,

9 8 Table I. Stations where colour index F(lm) was measured. Notation Region Number of Observations 1 The Belts 10 2 Baltic Sea lu 3 North Sea 28 k North Sea 18 5 off West Africa 9 6 North Sea 18 7 North Sea 23 8 Gibraltar Strait 2 9 North Sea North Sea 8 11 East of Gibraltar 1 12 East of Sardinia 2 13 Drake Passage 2 Ih South of Sardinia 5 Radiance measurements F = L(l80 s U50 run)/ L(l80, 520 nm) 15 Baltic Sea k 16 Gibraltar Strait 1 17 East of Sardinia 3 18 South of Sardinia 1 19 North of Haiti* 1 20 Sargasso Sea 1 Irradiance measurements F = E u (U50 nm)/e^(520 s Morel (1973)

10 9 Table II. Relation "between colour index and different quanta percent levels. Colour index Quanta percent level 30 % 10 % 3 % 1 % o k l Ho k kk k Ik l4.6 3U , Ho hk continued

11 Table II. continued Colour index Quanta percent level 30 % 10 % 3 % 1 % kj U h.o k.l k b * k.k U ^ k I

12 11 COLOUR INDEX F(1m) Fig. 1. Experimental values of the colour index versus experimental values of the 10%-level for the downwelling quanta irradiance ( nm) in different sea waters, COLOUR INDEX F (1m) Fig. 2. Empirical relations between the colour index and selected percentage-levels for the downwelling quanta irradiance ( nm). Included are the optical water types I, IA, IB, II, III and 1 (Jerlov, 1977).

13 12 COLOUR INDEX F(lm) Zq (10%) m Fig. 3. Colour index versus the depth of the 10%-level for the downwelling quanta irradiance for the experimental and the theoretical case, respectively.