# Presentation: 202942

## C. K. Gatebe
Goddard Earth  Sciences and Technological Center University of Maryland Baltimore County

Outline
Introduction
Overview of the Cloud Absorption Radiometer (CAR) & BRDF Measurements
Atmospheric correction scheme
Results of Selected CAR Measurements (CLAMS)
Summary

**C. K. Gatebe**
**Goddard Earth  Sciences and Technological Center ****University of Maryland Baltimore County**
**Outline**
**Introduction**
**Overview of the Cloud Absorption Radiometer (CAR) & BRDF Measurements**
**Atmospheric correction scheme**
**Results of Selected CAR Measurements (CLAMS)**
**Summary**

**More information**** -->**

**http://car.gsfc.nasa.gov**

## Historical Overview

***Historical Overview***

**Developed 1982-1983**

**?**

## Personnel 

| Peter Hobbs | Ward Meyer | UW team: Pilots, Engineers, Technicians |
| --- | --- | --- |
| Si-Chee Tsay | Jason Li | NASA GSFC Engineers (Peter Shu, Rusty, etc.) |
| Steve Platnick | Tom Arnold |  |
| Peter Soulen | Paul Hubanks |  |
| Charles Gatebe |  |  |
| Oleg Dubovik |  |  |
| Alex Lyapustin |  |  |
| Eric Vermote |  |  |
|  |  |  |
|  |  |  |

**Michael King**

## Location of BRDF Measurements during CLAMS 

***Location of BRDF Measurements during CLAMS****** ***

## UW CV-580 Aircraft

**SSFR**

**AATS**

**Cloud Absorption Radiometer (CAR)**

***UW CV-580 Aircraft***

## Airplane AOD with AATS-14 

***Airplane AOD with AATS-14****** ***

## CAR: Cutaway 

***CAR: Cutaway ***

## CAR: Spectral bands

***CAR: Spectral bands***

**(0.340-2.301 μm)**

## CAR Timing Wheel

**Science Data**

**Eng. Data**

**Dark current Data**

**Auto Zero**
**samples**

**START**

**42°**

**64°**

**48°**

**190°**

**Zenith**

**Nadir**

***CAR Timing Wheel***

## Signal-to-noise ratio:  Dark Ocean Scene

## BRDF Measurements

***BRDF Measurements***

## Ocean Facet

***Ocean Facet***

## Mathematical model

- I(μ, )=

- +

**Lower boundary condition for RT for ocean-atmosphere system:**

**Azimuthally independent Cox-Munk model (N&T-1988):**

**Nb. Total BRDF of ocean, R=Rcm+Rw**

## CAR Atmos. Corr. Model 

**Rewrite the RT equation at the flight level z as:**

**Atmospheric correction is done using:**

## Atmospheric Correction Steps

**Step 1:**

**Initialize U = 1 m/s; Aw =0; R=Rcox-munk**

**Step 2:**

**Compute diffuse radiance, I*****sky***** and path radiance, I*****path***

**Step 3:**

**Compute the full BRDF for all view angles of the CAR:**

## Atmospheric Correction Steps

**Step 4:**

**In the glint region, defined as R > max(Rw,0.02), find the**
**-best-fit wind speed for the difference R-Rw. **
**-Update Rcox-munk.**

**Step 5:**

**Update Rw and compute Aw for zenith angles <60°**

**Step 6:**

**Repeat 2-6 until U and Aw stabilize**

**Nb. Processing is done per channel. Dispersion of U a good**
**quality indicator. **

## Notation

**Sunglint**

**Forward direction (+ve)**

**Backward direction (-ve)**

**Nadir: 0°**

## BRDF at 0.472 μm

## Reflectance in the Principal Plane

**10 July; 20°; 2; 278°**

**17 July; 16°; 6; 7°**

**23 July; 30°; 1; 227**

**26 July; 21°; 9; 350°**

**30 July; 33°; 11; 136°**

**31 July; 21°; 8; 197°**

**02 Aug.; 44°; 3; 217°**

**Scan Angle (°)**

**Reflectance**

## Reflectance (Perpendicular Plane)

**10 July; 20°; 2; 278°**

**17 July; 16°; 6; 7°**

**23 July; 30°; 1; 227**

**26 July; 21°; 9; 350°**

**30 July; 33°; 11; 136°**

**31 July; 21°; 8; 197°**

**02 Aug.; 44°; 3; 217°**

## Water-leaving Reflectance

**0.472 μm 0°/90° Planes**

**0.682 μm 0°/90° Planes**

**0.870 μm 0°/90° Planes**

## Water-leaving Reflectance

## BRDF (water-leaving) at 0.472 μm

**0.000**

**0.004**

**0.008**

**0.012**

**0.016**

**0.020**

**July 10**

**July 23**

**Aug 02**

**July 10**

**July 23**

**Aug 02**

**0.000**

**0.006**

**0.013**

**0.019**

**0.025**

**-80**

**-40**

**0**

**40**

**80**

**July 10: 20°; u=2 m/s**

**July 23: 30°; u=1 m/s**

**Aug. 02: 44°; u=3 m/s**

**Scan Angle (°)**

## BRDF at 0.682 μm

***BRDF at 0.682 μm***

**0.000**

**0.004**

**0.008**

**0.012**

**0.016**

**0.020**

**July 10**

**July 23**

**Aug 02**

**0.000**

**0.006**

**0.013**

**0.019**

**0.025**

**-80**

**-40**

**0**

**40**

**80**

**July 10: 20°; u=2 m/s**

**July 23: 30°; u=1 m/s**

**Aug. 02: 44°; u=3 m/s**

**July 10**

**July 23**

**Aug 02**

**Scan Angle (°)**

## BRDF at 0.870 μm

***BRDF at 0.870 μm***

**0.000**

**0.004**

**0.008**

**0.012**

**0.016**

**0.020**

**July 10**

**July 23**

**Aug 02**

**0.000**

**0.006**

**0.013**

**0.019**

**0.025**

**-80**

**-40**

**0**

**40**

**80**

**July 10: 20°; u=2 m/s**

**July 23: 30°; u=1 m/s**

**Aug. 02: 44°; u=3 m/s**

**Note:**

**July 10**

**July 23**

**Aug 02**

**Scan Angle (°)**

## Message for Remote Sensing

**These results are consistent with calculations made by e.g. Plass et al. (1975)**
**Applied Optics, Vol. 14, No. 8, 1924-1936**

**Water-leaving radiance seems to be only slightly sensitive to nadir angles (******** ≤ 60°) regardless of the sun angle - particularly those sun angles in the range observed in this study.**** **

**Water-leaving radiance is a major contributor of the total radiance at nadir angles where the total radiance is small.**** **

## Ocean Albedo

## Ocean Albedo (Black Sky)

***Ocean Albedo (Black Sky)***

**A(0.472 μm)=0.046±0.004**

**A(0.682 μm)=0.029±0.003**

**A(0.870 μm)=0.026±0.002**

## Ocean Albedo (White Sky)

***Ocean Albedo (White Sky)***

**A(0.472 μm)=0.016±0.005 (0.014±0.002)**

**A(0.682 μm)=0.004±0.002**

**A(0.870 μm)=0.002±0.001**

## Comparison of Albedo Values

**Observations/Calculations**

**Payne (1972)****	****    0.28****	**** 0.035****	****    0.061**

**Clear Sky**

**Overcast sky**

**SZA****	****10****	****          70****	****70**

**Kondratyev (1969)   0.27-032****	**** 0.05****	****     -**

**Hollman (1968)****	****    -****	****           0.03      0.05**

**Grischenko (1959)   0.33-0.47****	**** 0.05****	****     -**

**Anderson (1952)****	****    0.21****	**** 0.04-0.05 0.04-0.05**

**Burt (1954)****	****	****   0.24****	****	**** 0.05****	****    0.06**

**Ter-Markariants (1959) 0.26   0.04****	****   0.07**

**Table from Payne(1972), JAS, 29, 1924-1936**

## Validation of Cox-Munk Model

**0.015**

**0.00**

**-0.015**

**High Glint**

**Low Glint**

**Isotropic**

**anisotropic**

**Anisotropic-GC**

**Difference: R(measurements-CoxMunk)/Rmax**

**Difference: R(measurements-CoxMunk)**

## Cox-Munk Validation cont.

**50**

**40**

**30**

**10**

**20**

**0.60**

**0.40**

**0.20**

**0.00**

**0.20**

**0.40**

**0.60**

**0.60**

**0.40**

**0.20**

**0.00**

**0.20**

**0.40**

**0.60**

**70**

**50**

**30**

**10**

**-10**

**-25**

**-30**

**-35**

**-40**

**-45**

**-50**

**0.012**

**0.008**

**0.004**

**0.000**

**-0.004**

**-0.008**

**-0.012**

**Scan Angle (°)**

**Scan Angle (°)**

**Scan Angle (°)**

**High Glint: July 31**

**High Glint: July 23**

**Low Glint: July 31**

**R (Meas. - CM)/Rmax**

**R(Meas - CM)**

## Slide 33

## Summary 

**Unique BRDF measurements**
**Full Angular coverage**
**Fine angular resolution**
**High SNR even over dark scenes**
**Auxiliary data**
**AATS**
**Buoy data**
**Chemistry**