I want to convert kd490 unit

[doaa_naguib]

Hi everyone,
I've downloaded MODIS aqua kd490 product, concerning the turbidity. the unit I found is (1/m), but this unit makes no sense in the presentation it ranges between 0.002 and 0.3 it can never represent the transparency level. How can I convert its values to understood units, real transparency levels in meters, or even to FNU.

thank you in advance

[OB.DAAC - SeanBailey]

Well, Kd490 is not a measure of turbidity, it's a measure of diffuse attenuation. Turbidity is effectively a measure of scattering. While scattering is part of attenuation, it is not everything, as attenuation also includes absorption. So there is a relationship between turbidity and Kd490, but it is not necessarily a simple one (although it *could* be, if scattering is the dominant contributor to attenuation).

1/Kd is generally considered the 1st optical depth (roughly the 37% light level), so you might be able to use that...

Sean

[doaa_naguib]

OB.DAAC - SeanBailey wrote:
> Well, Kd490 is not a measure of turbidity, it's a measure of diffuse
> attenuation. Turbidity is effectively a measure of scattering. While
> scattering is part of attenuation, it is not everything, as attenuation
> also includes absorption. So there is a relationship between turbidity and
> Kd490, but it is not necessarily a simple one (although it *could* be, if
> scattering is the dominant contributor to attenuation).
>
> 1/Kd is generally considered the 1st optical depth (roughly the 37% light
> level), so you might be able to use that...
>
> Sean

[doaa_naguib]

thank you for your response,
I've got the point, But I think I still could use the KD490 as a general trend to the turbidity, couldn't I?


OB.DAAC - SeanBailey wrote:
> Well, Kd490 is not a measure of turbidity, it's a measure of diffuse
> attenuation. Turbidity is effectively a measure of scattering. While
> scattering is part of attenuation, it is not everything, as attenuation
> also includes absorption. So there is a relationship between turbidity and
> Kd490, but it is not necessarily a simple one (although it *could* be, if
> scattering is the dominant contributor to attenuation).
>
> 1/Kd is generally considered the 1st optical depth (roughly the 37% light
> level), so you might be able to use that...
>
> Sean

[GES DISC - jimacker]

Euphotic depth is also an optical quantity related to turbidity.

[OB.DAAC - SeanBailey]

Sure, Kd490 can be used as a proxy for turbidity, with the caveats I mentioned earlier. As Jim points out that euphotic depth can also be used as a proxy for turbidity - euphotic depth is the 1% light level. In my earlier post I suggested you may consider deriving the first optical depth - the 37% light level - from Kd490. In fact, there is are a couple of algorithms available in the l2gen software that compute a Secchi depth (roughly the 10% light level). As long as you keep in mind that these proxy quantities should not be directly compared to a nephelometric measurement (NTU), you can certainly use them.

Sean