# Presentation: 483244

## Bluing of Aerosols near Clouds: Results from a Simple Model and MODIS Observations

- Alexander Marshak (GSFC)
- Tamás Várnai and Guoyong Wen (UMBC/GSFC)
- Lorraine Remer and Bob Cahalan (GSFC)
- Jim Coakley (OSU) and Norman Loeb (LRC)

## What happens to aerosol in the vicinity of clouds?

- All observations show that aerosols seem to grow near clouds
- or
- (to be safer) “most satellite observations show a positive correlation between retrieved AOT and cloud cover”, e.g.:

- from Loeb and Manalo-Smith, 2005

- from Ignatov et al., 2005

**Cloud Fraction (%)**

- from Zhang et al., 2005

- Alexander Marshak

- May 14, 2008

**Notes:**
Ignatov et al. note that the correlation can be either real or artificial
Loeb and Manalo-Smith mentioned different meteorological conditions and RH
Zhang et al. claim that artifacts like cloud contamination or adjacency effect result in 10-20% overestimation in monthly AOT

## What happens to aerosol in the vicinity of clouds?

- All observations show that aerosols seem to grow near clouds.
- However, it is not clear yet how much grows comes from
-  “real” microphysics, e.g.
-  increased hydroscopic aerosol particles,
-  new particle production or
-  other in-cloud processes.
-  (“artificial”) the 3D cloud effects in the retrievals:
-  cloud contamination,
-  extra illumination from clouds

- Alexander Marshak

- May 14, 2008

## How do clouds affect aerosol retrieval?

- Both
-  cloud contamination (sub-pixel clouds)
-  cloud adjacency effect (a clear pixel with in the vicinity of clouds)
- may significantly overestimate AOT.
- But they have different effects on the retrieved AOT: while cloud contamination increases “coarse” mode, cloud adjacency effect increases “fine” mode.

- Alexander Marshak

- May 14, 2008

## The Ångström exponent and the cloud fraction vs. AOT

- from Kaufman et al., IEEE 2005

- Atlantic ocean, June-Aug. 2002; each point is aver. on 50 daily values with similar AOT in 1o res.;
- for AOT < 0.3, as AOT increases CF and the Ångström exponent also increase;
- the increase is due to transition from pure marine aerosol to smoke (or pollution);
- the increase in AOT cannot be explained by cloud contamination but rather aerosol growth.

- Alexander Marshak

- May 14, 2008

## More clouds go with larger AOT and larger (not smaller!) Ångström exponent

- from Loeb and Schuster (JGR, 2008)

-  one month of data: 25 1ox1o in each 5ox5o region over ocean (off the cost of Africa) are subdivided into two groups with
- a  <a>
-    and
- a  <a>
- meteorology has been checked as similar for two groups

- Alexander Marshak

- May 14, 2008

**Notes:**
In contrast to the Kaufman et al. (2005) results, the aerosol populations are restricted spatially (to 5x5 degree regions) and have the same large-scale meteorology.

## AOT and Ångström exponent vs. distance from the nearest cloud (AERONET data)

- from Koren et al., GRL, 2007

- Time passed from the last cloud (min)

- AOT (0.47 m)

- Ångström exponent

- The Ångström exponent increases with distance to the nearest cloud while the AOT increases

- Alexander Marshak

- May 14, 2008

## Airborne aerosol observations in the vicinity of clouds 

- Courtesy of Jens Redemann

- From airborne extinction rather than scattering observations 3D effects decrease AOT rather than increase it

- Alexander Marshak

- May 14, 2008

## Aerosol-cloud radiative interaction (a case study)

- Collocated MODIS and ASTER image of Cu cloud field in biomass-burning region in Brazil at 53o W on the equator, acquired on Jan 25, 2003

- Wen et al., 2006

## ASTER image and MODIS AOT

- from Wen et al., JGR, 2007

- ASTER image

- MODIS AOT

- Alexander Marshak

- May 14, 2008

## Cloud effect at 90-m resolution

- Thin clouds, <>=7

- Thick clouds, <>=14

- AOT0.66=0.1

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

- Alexander Marshak

- May 14, 2008

## Conceptual model to account for the cloud enhancement (at 0.47 m)

- aerosol or molecule

- from Wen et al., JGR 2008:
- molecule (**82%**)
- + aerosol (**15%**)

- MODIS sensor

**Conceptual model to account for the cloud enhancement (at 0.47 ********m)**

- surface (**3%**)

- Alexander Marshak

- May 14, 2008

## Assumption for a simple model

- Molecular scattering is the main source for the enhancement in the vicinity of clouds
*thus*
*we retrieve larger AOT and fine mode fraction*

- Alexander Marshak

- May 14, 2008

## How to account for the 3D cloud effect on aerosols?

- The *enhancement* is defined as the difference between the two radiances:
-  one is reflected from a broken cloud field with the scattering Rayleigh layer above it
-  and one is reflected from the same broken cloud field but with the Rayleigh layer having extinction but no scattering

- from Marshak et al., JGR, 2008

- Alexander Marshak

- May 14, 2008

## Stochastic model of a broken cloud field

- AR = 2

- Ac = 0.3

- Clouds follow the Poisson distr. and are defined by
-  average optical depth, <>
-  cloud fraction, Ac
-  aspect ratio, AR = hor./vert.

- AR = 1

- Alexander Marshak

- May 14, 2008

## Stochastic model of a broken cloud field

- AR = 2

- Ac = 0.3

- Clouds follow the Poisson distr. and are defined by
-  average optical depth, <>
-  cloud fraction, Ac
-  aspect ratio, AR = hor./vert.

- AR = 1

- Alexander Marshak

- May 14, 2008

## Cloud-induced enhancement at 0.47 m

- LUT:
- The enhancement vs <> for AR = 1.  Ac=1 corresponds to the pp approximation.

- Alexander Marshak

- May 14, 2008

## Cloud-induced enhancement:our simple model and 3D RT calculations

- The enhancement vs <> for Ac= 0.6 and 3 cloud AR = 0.5, 1 and 2.  Different dots are from Wen et al. (2007) MC calculations for the thin and thick clouds.

- Alexander Marshak

- May 14, 2008

## Ångström exponent

- Ångström exponent vs <> for Ac= 0.5 and AR = 2.
- Three cases: clean, polluted and very polluted.

**0.47 ********m vs. 0.65 ********m**

**0.65 ********m vs. 0.84 ********m**

- clean

- clean

- polluted

- polluted

- very polluted

- very polluted

- The cloud adjacency effect increases the Ångström exponent

- from Marshak et al., JGR, 2008

- Alexander Marshak

- May 14, 2008

## MODIS observations (Várnai and Marshak, 2008, in preparation)

- Collection 5 MOD02, MOD06, MOD35 products
- 2 weeks in Sep. and March in 2000-2007 (2x2 weeks in 8 years)
- North-East Atlantic (45°-50°N, 5°-25°W), south-west from UK
- Viewing zenith angle < 10°
- Pixels included in plots:
- Ocean surface with no glint or sea ice
- MOD35 says “confident clear”, all 250 m subpixels clear
- Highest cloud top pressure nearby > 700 hPa (near low clouds)
- Nearby  pixels are considered cloudy if MOD35 says definitely cloud.

## Example of the region: Sep 22, 2005

## Average reflectance vs. dist. to clouds for 0.45, 0.65, 0.87, and 2.1 m

- mean and std

## Reflect. Diff from Values at 10 km vs Cloud Optical Depth

- 2.10 m

## Reflect. Diff from Values at 10 km vs Cloud Optical Depth

- 0.87 m

## Reflect. Diff from Values at 10 km vs Cloud Optical Depth

- 0.65 m

## Reflect. Diff from Values at 10 km vs Cloud Optical Depth

- 0.47 m

## Asymmetry with respect to the sun

- 0.47 m

## Latency effect for 2.1 um

## Point spread function effect for 0.53 m(preliminary results)

*with Jack Xiong*

## Cloud contamination in 0.47 m(preliminary results)

*with Jack Xiong*

- latency effect removed (**red** curve);
-  assumed that at 2.1 m the increase is due to undetected subpixel clouds (**blue** curve)
-  assumed that 2.1 m has the same point spread function as 0.53 m (**green** curve)

## Work in progress

-  select a few MODIS subscenes with
-  broken low Cu;
-  retrieved AOT;
-  over ocean with no glint, etc;
-  analyze AOT, CF, average COT over many 10 x 10 km areas;
-  use a simple stochastic model and RT to estimate upward flux;
-  use CERES fluxes to convert BB to spectral fluxes;
-  use ADM to determine spectral fluxes from MODIS radiances;
-  estimate cloud enhancement and compare the results;
-  use a simple linearization model.

*with Norman Loeb and Lorraine Remer*

## Conclusions

-  No clear understanding from satellites alone of what happens to aerosols at the vicinity of clouds.
-  Accounting for the 3D cloud-induced enhancement helps.
-  For certain conditions, 3D cloud enhancement 3D1D only weakly depends on AOT.  Molecular scatt. is the key source for the enhancement.
- The enhancement increases the “apparent” fraction of fine aerosol mode (“bluing of the aerosols”).
-  MODIS observations confirm that the cloud induced enhancement increases with cloud optical depth.
-  Retrieved AOT *can* be corrected for the 3D radiative effects.

- Alexander Marshak

- May 14, 2008

## More clouds go with larger AOT and larger (not smaller!) Ångström exponent

-  25 1ox1o in each 5ox5o region over ocean (over the entire globe) are subdivided into two groups with
- a  <a>
-    and
- a  <a>
- meteorology has been checked as similar for two groups

- Difference in cloud fraction

- Difference in fine-mode fraction

- from Loeb and Schuster (JGR, 2008)

- Alexander Marshak

- May 14, 2008

**Notes:**
Differences between high and low aerosol areas for 5by5 degree over ocean