4.2. Algorithms

The only changes in the bio-optical algorithms, as compared with the previous issue [6], were made due to changes in the spectral channels of MODIS-Aqua. The SeaWiFS algorithms remained unchanged.

Chlorophyll concentration

From SeaWiFS data, chlorophyll concentration in the sub-regions 1-5 was calculated with the algorithm developed by specialists from MHI NANY [29-31]:



Chl= 1.13 [LWN(510)/LWN(555)] -3.33.    (7)



For the sub-regions 6-8 the algorithm developed by specialists from SIO RAS [32] was used:

Chl= 0.88 [LWN(510)/LWN(555)] -2.24. (8)

Comparison between the two algorithms has shown that the Chl–SIO values (derived with the algorithm (8)) are higher than the Chl–MHI values (derived with the algorithm (7)) if Chl–SIO < 0.4 mg·m-3. The Chl–SIO values are lower than the Chl–MHI values if Chl–SIO > 0.7 mg·m-3. Within the Chl–SIO range from 0.4 to 0.7 mg·m-3 the Chl values derived by the two algorithms differ by less than 15%.

The both algorithms are assumed to be valid in warm season (May-September). They can result in considerable errors in absolute values of hl in cold season. However, it can be assumed that relative changes of chlorophyll concentration are reproduced adequately with the both algorithms, and for this purpose our computations were also carried in cold season.

From MODIS-Aqua data, the algorithm to derive chlorophyll concentration in the sub-regions 1-5 was complicated:

Chl = 0.5 (Chl1 + Chl2), (9)


where Chl1 = 1.13 [0.66 Rrs(488)/Rrs(547) + 0.40]–3.33;

Chl2 = 1.13 [2.35 Rrs(531)/Rrs(547) – 1.44]–3..33.

For the sub-regions 6-8 the Chl algorithm with MODIS data was similar to (8):

Chl = 0.83 [0.996 Rrs(531)/ Rrs(547)]-4.36. (10)

Particle backscattering

For calculation of the particle backscattering coefficient bbp in the Black Sea from SeaWiFS data, the simplified algorithm described in Section 2.2 was used, but Kd(555) was calculated through the ratio LWN(490)/ LWN(555) instead of LWN(510)/ LWN(555) in the Barents Sea.

Computations of the particle backscattering coefficient bbp in the Black Sea from MODIS-Aqua data were performed by the same algorithm as with SeaWiFS data but the ratio of LWN (488)/LWN (547) was used instead of LWN (490)/LWN (555) (our calculations showed that those ratios differ little from each other).

Yellow substance absorption

In the Black Sea it is possible to retrieve the values of the yellow substance absorption coefficient ag(440). The version of a semianalytic algorithm described in [12] was used. It is difficult to estimate errors of the derived values of ag because they depend strongly on errors of the atmospheric correction. Of course, the cases with negative values of LWN were rejected out, but we could not check the errors in LWN which were not so pronounced.