# Presentation: 193761

## How 3D Science Can Help to Correctly Interpret MODIS Data for	Better Understanding of Aerosol-Cloud Interactions

**Alexander Marshak (GSFC)**
**Robert Cahalan (GSFC) **
**Támas Várnai (UMBC) **
**Guoyong Wen (UMBC) **
**Brian Vant-Hull (UMD)**

- Alexander Marshak

- Nov 1, 2006

## Instead of Intro

- clouds are complex and “satellite analysis may be affected by potential cloud artifacts” (Kaufman and Koren, 2006);
-  for **aerosol** retrievals:
-  cloud contamination
-  adjacency effect
- may significantly overestimate AOT
-  for **cloud** retrievals:
-  clear sky contamination (with dark surface)
-  cloud 3D structure (e.g., shadowing)
- may significantly overestimate droplet size
- To study **cloud-aerosol interaction** we need to be sure that retrieved **correlations** between AOT and *r*e reflect real physics rather than remote sensing problems.

- Alexander Marshak

- Nov 1, 2006

## A striking example

****

***r*****e**

- Brazil, Jan. 25, 2003
- centered at 0o, 53.78oW

**ASTER image**

**Aerosol**

**Cloud**

- Alexander Marshak

- Nov 1, 2006

## A striking example: const AOT

**Modeled (with *****const***** AOT but MODIS 3D cloud structure)**
** ****vs. Observed Reflectance. **
**Cor. coef. = 0.77**

- Alexander Marshak

- Nov 1, 2006

## A striking example: 3D corrected vs. 3D uncorrected 

***r*****e**** vs. AOT**

- ‘before’ means 3D-uncorrected

- Alexander Marshak

- Nov 1, 2006

## A striking example: 3D corrected vs. 3D uncorrected

***r*****e**** vs. AOT**

- ‘after’ means 3D-corrected

- Alexander Marshak

- Nov 1, 2006

## Relationship between sun and sensor locations

- The division of the data set into *backscatter* and *side-scatter* geometries.

**Relationship between sun and sensor locations**

- Alexander Marshak

- Nov 1, 2006

## Cumulus clouds(MODIS, Aug.-Oct. 2002)

- from Vant-Hull et al. (2006)

**Retrieved *****r*****e**** for back (illum)- and **
**side (shad)- scattered geometries**
**(col. 4 and “5”)  **

- Alexander Marshak

- Nov 1, 2006

## Collection 4    vs.    Collection 5 

- Cloud optical depth, 

- Droplet effective radius,* r*e

- Alexander Marshak

- Nov 1, 2006

## Aerosol-cloud photon interaction

- Alexander Marshak

- Nov 1, 2006

## Aerosol-cloud radiative interaction

- Thin clouds, <>=7

- Thick clouds, <>=15

- AOT0.66=0.1

- enhancement: ******3D****-********1D **

- Alexander Marshak

- Nov 1, 2006

## Contributors to 3D cloud effects

-  Rayleigh scattering
-  Aerosol amount
-  Surface reflectance

**What is the relationship between these three factors?**

**and, of course,**
-  Distance from cloudy pixels
-  Cloud optical thickness

- Alexander Marshak

- Nov 1, 2006

## Enhancement of reflectance vs. AOT

- AOT=0.1

- AOT=0.5

- AOT=1.0

- enhancement: ******3D****-********1D**** **

- Alexander Marshak

- Nov 1, 2006

## Enhancement of reflectance vs. Rayleigh scattering

**Cloud-molecule scattering is the key process**

-  3% from surface
-  15% from aerosol
-  82% from molecule

- Alexander Marshak

- Nov 1, 2006

## Conceptual model to account for 3D cloud effects

- aerosol or molecule

- molecule
- + aerosol

- MODIS sensor

**Conceptual model to account for 3D cloud effects**

- surface

- Alexander Marshak

- Nov 1, 2006

## Effect of distance to a cloudy pixel

- 3D enhancement (red)

- Cumulative distr. (blue)

- Nearest cloud dist. (km)

- 0.66 m

- Alexander Marshak

- Nov 1, 2006

## 3D radiative effect in Cu clouds.Viewing angle dependence

- Rel. azimuth > 90o

- Rel. azimuth < 90o

- Alexander Marshak

- Nov 1, 2006

## MODIS data used

-  The whole year: Sept. 2004 - Aug. 2005
-  Collection 4
-  Liquid phase; high confidence;  ≥ 2
-  SZA between 55o and 80o
- Based on local temp. gradient T, all cloudy pixels are divided into 3 **equally populated** categories with thresholds:
-  most homogeneous: 0.3-0.5 C/km (50-80 m)
-  most inhomogeneous: 1.1-1.5 C/km (180-250 m)

- Alexander Marshak

- Nov 1, 2006

## Cloud optical thickness

**The most homogeneous (0.3-0.5 C/km) and the most inhomogeneous (1.1-1.5 C/km) cloudy pixels**

- =c22+c1+c0

- Alexander Marshak

- Nov 1, 2006

## Cloud optical thickness: col. 4 vs col. 5

**Collection 4**

**“****Collection 5”**

- =c22+c1+c0
- c2=1.8 10-3 (col.4)
- c2=1.2 10-3 (“col.5”)

**“****Col. 5” shows some improvements for inhomog. clouds; **
**no change for homog. clouds**

- Alexander Marshak

- Nov 1, 2006

## Cloud effective radius, re

**The most homogeneous (0.3-0.5 C/km) and the most inhomogeneous (1.1-1.5 C/km) cloudy pixels**

- F

- B

- Alexander Marshak

- Nov 1, 2006

## Cloud effective radius: col. 4 vs col. 5

**Collection 4**

**“****Collection 5”**

**The homog. clouds show no changes (mean±1.5 ********m) while there are some improvements for the inhomog. clouds**

- Alexander Marshak

- Nov 1, 2006

## Conclusions

-   For study aerosol-cloud interaction in Cu environment, 3D cloud effects cannot be ignored;
-  3D cloud enhancement only weakly depends on AOT; molecular scattering is the key source for the enhancement;
-   Retrieved AOT can be corrected for the 3D rad. effects;
-   Both cloud  and *r*e show strong dependence on VZA;
-   For study aerosol-cloud interaction, a difference of 2-3 m in *r*e doesn’t necessarily mean the effect of aerosols on clouds but rather can be a remote sensing problem.

- Alexander Marshak

- Nov 1, 2006

## How 3D Science Can Help to Correctly Interpret MODIS Data for	Better Understanding of Aerosol-Cloud Interactions

**Alexander Marshak (GSFC)**
**Robert Cahalan (GSFC) **
**Támas Várnai (UMBC) **
**Guoyong Wen (UMBC) **
**Brian Vant-Hull (UMD)**

- Alexander Marshak

- Nov 1, 2006

## Example of corrected AOT

|  | True | MODIS |  |
| --- | --- | --- | --- |
| 0.47 m | 0.21 | 0.40 | 48% |
| 0.66 m | 0.08 | 0.22 | 64% |

|  | True | MODIS |  |
| --- | --- | --- | --- |
| 0.47 m | 0.18 | 0.30 | 40% |
| 0.66 m | 0.08 | 0.12 | 33% |

**Thin clouds**

**Thick clouds**

- Alexander Marshak

- Nov 1, 2006

## Cloud optical thickness: land and ocean

**The most inhomog. (1/3) fraction of cloudy pixels over land and ocean; Liquid clouds over ocean are more homogeneous**

- =c22+c1+c0
- c2=2.4 10-3 (land)
- c2=1.4 10-3 (ocean)

- Alexander Marshak

- Nov 1, 2006

## Cloud effective radius: land and ocean

**Droplet sizes retrieved over ocean are less sensitive to VZA (for all clouds)**

- Alexander Marshak

- Nov 1, 2006