Dear all,               
                                                                
Shouldn't we vary the water IOPs for Salinity and Temperature? bbw is                    
strongly salinity dependent (30% between zero and 39psu) its value                       
will not change much between open ocean environment but since we know                    
it we may as well get used to do it (and thus will not forget to                         
change it when we do Great Lakes work). Similarly modulating                             
absorption with temperature is important in the red and NIR (we can                      
also salinity correct).                                                                  
                                                                                         
Note that salinity and temperature corrections are often applied to                      
the ac-9 simply so their effect on water is not translated to the                        
signal relative to water (due to dissolved and particulate materials).                   
When water is added back to a_pg and c_pg (e.g. when total a and total                   
c are reported), most often the water added is fresh and at a specific                   
temperature (e.g. Pope and Fry's) and thus is not representative of                      
the conditions where the water was measured (e.g. arctic ocean). It is                   
important that any ac-9 data used here should be those of the                            
particulate and/or dissolved components only (w/o the added water).                      
                                                                                         
In coastal waters this may not be an issue but in mid-ocean gyre it                      
will be. In addition this introduces a bias we can easily do without.                    
                                                                    
Another issue: the value you report below have probably been evaluated                   
using a 1 or 2nm bandwidth spectrophotometer. The satellite bandwidth                    
is larger (10-20nm) which means it averages over a certain band range.                   
We should do our best to perform that average on all optical                             
properties we use (validation set and water values) to insure that                       
areas where these properties may be changing non-linearly are not                        
introducing biases. in-situ ac and bb instrument have typically 10 or                    
20nm bandwidth (though some c meters have laser sources with less than                   
1nm bandwidth).                                                                          
                                                                                         
Cheers,                                                                                  
  Emmanuel

Hubert and all:

The (0.52 & 1.7) coefficients were developed for nadir view, as an overall average for various Sun angles. As Hubert showes here, apparently not that bad (within 5%). The 0.52 will change a little for view angle away from nadir, but to what extent will depend on angles and VSF, etc. We have an ongoing project looking into that, and would be happy to report our findings later this year or next year.

On the other hand, these coefficients affect more to derived bb or bbp, less to atot. If all measurements are made within a 30 deg view angle and Sun are within 70 deg from zenith, the angle-introduced error to retrieved IOPs will be small (<10%).

As to Hubert's question of IOCCG data set, yes, it is there, but only nadir-viewed Rrs(0+) and Rrs(0-).

zhongping

Hi Hubert,

Thank you for interesting information! I am also interested by this issue for my ocean color modeling. I did my own attempts to find Rrs(0+)/Rrs(0-) ratio. Actually, a form of this ratio was established by Saunderson in 1942 (so called "Saunderson's correction"), and it is just in the form (hyperbolic) that use you or Lee at al. I also used this form, attracting some theoretical (Fresnel & Snell's laws; Quan and Fry, 1995; Morel and Gentili, 1996; Molenaar et al., 1999), observational (Aas and HØjerslev, 1999; Toole et al., 2000) and radiative transfer modeling (Mobley, 1999; Aas, 2010; Kokhanovsky & my own, unpubl.) stuff. I have results now for any wavelength and solar zenith and viewing angles, but for the better comparing with your results, I may give "a" & "b" for your lambda (490 nm),  3 sun angles (0, 30, 60) and nadir viewing:

0 deg: a = 0.515, b = 1.642    
30 deg: a = 0.515, b = 1.744   
60 deg: a = 0.515, b = 2.057

Thus, my results are extremely close to Zhongping's results, and, of course, close to yours.

Interestingly to analyze an impact of temperature and salinity. My model allows to do this. As an example, I can show coefficients for the normal temperature (25 deg), extremal high salinity (280 per mil) and previous wavelength and angles:

0 deg: a = 0.479, b = 1.768    
30 deg: a = 0.479, b = 1.877   
60 deg: a = 0.479, b = 2.214

Best wishes,

Leonid