# Presentation: 632612

## BRDF correction and IOP retrieval from angular Rrs: A non-Case1 approach

**ZhongPing Lee****1****, Deric Gray****2****, Bertrand Lubac****2****, Alan Weidemann****2****, **
**Robert Arnone****2****, Paul Martinolich****2**
**1****Northern Gulf Institute, Mississippi State University, **
**Stennis Space Center, MS 39529; zplee@ngi.msstate.edu**
**2****Naval Research Lab, Stennis Space Center, MS 39529**

## BRDF background:

_**BRDF background:**_

- _Bottom line_: Water-leaving radiance changes with angle.

## BRDF correction data flow:

**BRDF correction data flow:**

- Ω: {θSun,θv,φ}

**Advantage****:**
**  ****[Chl] is the only in-water property required.**

**Conditions****:**
** **Require waters to follow the Case1 bio-optical relationships, e.g.
- fixed CDOM:Chl and bp:Chl dependences.

**Minor: Interpolate among ****7**** wavelengths and ****6**** [Chl].**

## Non-Case1 approach:

_**Objective**_**: ****Obtain BRDF correction and IOP retrieval without the Case-1 **
**         ****assumption,  i.e., not limited by CDOM:Chl ratio or bp:Chl ratio.**

**a. Most (> 90%) of the G (~ f/Q) variation is resulted from change of b****b****/(a+b****b****)  (e.g. Gordon et al 1988);**
**b. Minor (<50%) variation is due to ****Ω**** (****θ****Sun****,****θ****v****,****φ****), especially in the remote-sensing domain;**
**c. Minor (<30%) variation is due to phase function.**

## Critical information required for IOP retrieval and BRDF Correction:

- Angular geometry (Ω)
- Particle phase function

- Need G values to analytically invert Rrs.

- G depends on phase function

## G {Gw,g0,g1} Table

| wind | (θSun,θ,φ) | bb/b | Gw | g0 | g1 |
| --- | --- | --- | --- | --- | --- |
| 0 | (0,0,0) | 0.005 | 0.060 | 0.055 | 0.203 |
| ..... | ..... | ..... | ..... | ..... | ..... |
| 5 | (15,20,135) | 0.010 | 0.060 | 0.044 | 0.158 |
| 5 | (45,30,30) | 0.010 | 0.050 | 0.042 | 0.243 |
| ..... | ..... | ..... | ..... | ..... | ..... |
| 10 | (15,20,135) | 0.010 | 0.060 | 0.044 | 0.156 |
| ..... | ..... | ..... | ..... | ..... | ..... |

** ****functions of wind, ****Ω****, and b****b****/b.**

**G {G****w****,g****0****,g****1****} Table**

## IOP retrieval and BRDF correction, & examples:

_**Remote-sensing domain**_

**Azimuth range**

**Nadir range**

**Strong Sun glint  region**

## Rrs comparison

**R****rs****(****Ω****) compared with R****rs****(****Ω****0****)**

**R****rs****(****Ω********Ω****0****) compared with R****rs****(****Ω****0****)**

**Rrs comparison**

**Example 1****: **
**Knowing exactly the phase function**

## Rrs comparison

**R****rs****(****Ω****) compared with R****rs****(****Ω****0****)**

**R****rs****(****Ω********Ω****0****) compared with R****rs****(****Ω****0****)**

**Rrs comparison**

**Example 1****: **
**Knowing exactly the phase function**

## Rrs comparison

**R****rs****(****Ω****) compared with R****rs****(****Ω****0****)**

**R****rs****(****Ω********Ω****0****) compared with R****rs****(****Ω****0****)**

**Rrs comparison**

**Example 1****: **
**Knowing exactly the phase function**

## IOP comparison

**Comparison of total absorption**

***a*****tot****  [m****-1****]**

***a*****tot****  [m****-1****]**

_**IOP comparison**_

**Comparison of backscattering coefficient**

***b*****bp****  [m****-1****]**

***b*****bp****  [m****-1****]**

**Example 1****: **
**Knowing exactly the phase function**

## IOP comparison

**Example 1****: **
**Knowing exactly the phase function**

_**IOP comparison**_

**Comparison of total absorption**

***a*****tot****  [m****-1****]**

***a*****tot****  [m****-1****]**

**Comparison of backscattering coefficient**

***b*****bp****  [m****-1****]**

***b*****bp****  [m****-1****]**

## Rrs comparison

**Incorrect phase function**

**R****rs****(****Ω********Ω****0****) compared with R****rs****(****Ω****0****)**

**R****rs****(****Ω****) ******** R****rs****(****Ω****0****)**

**R****rs****(****Ω****0****) of b****b****/b = 0.5%.**

_**Rrs comparison**_

**R****rs****(****Ω****) is generated with b****b****/b = ****0.5%****;**
**R****rs****(****Ω****) inversion used G of b****b****/b = ****1.5%****.**

## Rrs comparison

**Example 2****: **
**Incorrect phase function**

**R****rs****(****Ω********Ω****0****) compared with R****rs****(****Ω****0****)**

**R****rs****(****Ω****) ******** R****rs****(****Ω****0****)**

**R****rs****(****Ω****0****) of b****b****/b = 1.0%.**

_**Rrs comparison**_

**R****rs****(****Ω****) is generated with b****b****/b = ****1.0%****;**
**R****rs****(****Ω****) inversion used G of b****b****/b = ****2.5%****.**

## IOP comparison

**Incorrect phase function**

_**IOP comparison**_

**R****rs****(****Ω****) is generated with b****b****/b = ****1.0%****;**
**R****rs****(****Ω****) inversion used G of b****b****/b = ****2.5%****.**

**Comparison of total absorption**

***a*****tot****  [m****-1****]**

***a*****tot****  [m****-1****]**

**Comparison of backscattering coefficient**

***b*****bp****  [m****-1****]**

***b*****bp****  [m****-1****]**

## Rrs comparison

**Example 2****: **
**Incorrect phase function**

**R****rs****(****Ω********Ω****0****) compared with R****rs****(****Ω****0****)**

**R****rs****(****Ω****) ******** R****rs****(****Ω****0****)**

**R****rs****(****Ω****0****) of b****b****/b = 0.5%.**

_**Rrs comparison**_

**R****rs****(****Ω****) is generated with b****b****/b = ****0.5%****;**
**R****rs****(****Ω****) ****inversion**** used G of b****b****/b = ****1.0%****.**

## IOP comparison

**Example 2****: **
**Incorrect phase function**

_**IOP comparison**_

**Comparison of total absorption**

***a*****tot****  [m****-1****]**

***a*****tot****  [m****-1****]**

**Comparison of backscattering coefficient**

***b*****bp****  [m****-1****]**

***b*****bp****  [m****-1****]**

**R****rs****(****Ω****) is generated with b****b****/b = ****0.5%****;**
**R****rs****(****Ω****) ****inversion**** used G of b****b****/b = ****1.0%****.**

## Phase function selection:

- Rrs(Ω)

- bb/b;
- Phase function

**b****b****/b = 0.1 b****bp****(555) + 0.005 Y + 0.05 b****bp****(555)/*****a*****(490); *****Lee***** et al (2004)**

- Phase function is an input, then how to determine it?

- 2. From water-mass classification: assign different bb/b for different water mass

- 3. Iteration:

**{*****a*****,*****b*****bp****, Y}**

- w/ default bb/b

- 1. Average (default) particle phase function

## Conclusions:

- b. These model parameters are tabulated for
-         5 phase functions, 6 sun angles, (7x12+1) view angles,
-        and 4 wind speed; phase function is the _*only*_ required input from below the surface;

_**Conclusions**__:_

- c. When phase function is well estimated, both IOP retrieval and BRDF correction can be well achieved from Rrs(Ω);
- d. For incorrectly estimated phase function, less influence to BRDF correction; slightly more influence to derived absorption; much larger impact to particle backscattering coefficient;
- f.  It is not necessary to have a Case1 assumption for the above objectives.

## Slide 20