# Presentation: 097237

## Impact of biomass burning aerosols on clouds and cloud property retrievals

- Eric M. Wilcox, NASA/GSFC
- Harshvardhan, Purdue U.
- Steve Platnick, NASA/GSFC

- Calipso 532nm attenuated backscatter (km-1 sr-1)

- Altitude (km)

## Biomass burning aerosol

- over cloud
- (mean aerosol ssa0.87m=0.86)

- Dust aerosol over cloud
- (mean aerosol ssa0.87m=0.96)

- Haywood et al. (2004)

**Notes:**
Haywood et al., perform a theoretical calculation of the MODIS liquid cloud optical thickness and effective radius retrieval with and without an overlaying layer of aerosol using aerosol layer thicknesses and optical properties based on SAFARI aircraft observations.  Here I am showing their results for the two channels used in the MODIS over-ocean cloud retrieval for both biomass burning and dust aerosols.  The reduction in 0.86um reflectance owing to absorption yields up to a 30% underestimate in cloud optical depth.  This is based on a mean measured single scatter albedo of 0.86.
Our goal is to determine if this bias can be identified in the MODIS observations and get some kind of handle on how frequent it may lead to a substantial bias in the retrieval.

## LWPMODIS = 2/3re

- LWPMODIS = 2/3re

- Bennartz (2007)

- MODIS LWP from level 2 1km pixels.
- AMSR-E LWP from Wentz et al. 0.25 deg. gridded.
- Grid cells are only used if all 1km MODIS pixels within the grid cell have a valid  and re retrieval - confident overcast (~40% of cloud cover).
- OMI aerosol index is used to indicate presence of biomass burning aerosol.

- July, Aug. Sept. 2005 and 2006:

**Notes:**
The basic premise of the study is that absorbing aerosol above cloud should yield a bias in the LWP product reported from MODIS owing to the effect of aerosol absorption on the optical depth retrieval.  To diagnose the bias we use the Wentz et al. AMSR-E microwave LWP retrieval which is not impacted by the presence of aerosol.  We compare the two LWP retrievals for overcast samples on a quarter-degree grid, which is sufficient to capture the AMSR-E footprints.  Since the AMSR-E algorithm averages clear-sky and cloudy skies within the same scene, we must limit ourselves to scenes that we are confident are overcast, so we take only quarter-degree gridcells where 100% of MODIS 1 km pixels have a confident optical thickness and effective radius retrieval.

## AI>1.5 minus AI<=1_x0003_

**AI>1.5 minus AI<=1_x0003_**

**AI>1.5 minus AI<=1_x0003_**

**Notes:**
Scatter plots of AMSR LWP minus MODIS LWP:
-For reasons we do not quite understand there appears to be a systematic bias in favor of MODIS which increases with cloud optical depth, and would be nice to resolve.
-But for the purposes of our study, the significant point is that this bias vanishes when we limit the analysis to samples with OMI AI greater than 1.5

## Southeast Pacific Ocean

**Southeast Pacific Ocean**

- Note:virtually all data over S. Pac. are AI<=1

## Slide 6

**Notes:**
If we assume published values of LWP uncertainty ~15-20 g m^-2, then we find that the bias attributable to absorbing aerosol only exceeds that for cases of OMI AI>2.5 or 3.  This is a relatively small fraction of samples.

## Overcast scenes from CIFEX

- April 2004 N. Pacific Ocean

- clean

- polluted

- polluted is aerosol # conc. > 50 cm-3
- for particles 0.1 - 3.0 m

- Overcast scenes over S. Atlantic
- JAS 2005 - 2006

- -1 < AI < 1 (clean)

- 1 < AI < 2

- AI > 2  (polluted)

## AI <= 1 (clean)

**850 hPa**

**700 hPa**

- AI > 2 (polluted)

**850 hPa**

**700 hPa**

## AI <= 1 (clean)

- AI > 2 (polluted)

**Hypothesis: Absorbing aerosols above the boundary layer strengthens the inversion and reduces cloud-top entrainment (Johnson et al. 2004).**
**This leads to:**
**greater LWP**
**Lower cloud top**

## Summary

- A low bias in the MODIS cloud optical thickness retrieval is expected for cases of biomass burning aerosol over low clouds (low bias up to 30%) which increases with cloud optical thickness.
- A comparison of MODIS and AMSR-E LWP retrievals indicates that there is a systematic bias for high aerosol cases that increases with cloud optical thickness.
- The bias only exceeds uncertainties in LWP retrievals for cases of OMI AI > 0.25 to 3.
- Little evidence is found of microphysical interaction of aerosols with cloud.
- For cases of similar lower-tropospheric stability, high OMI AI scenes have a higher 850 hPa temperature and higher LWP (by ~20 g m-2).  This supports the hypothesis that heating above the cloud by aerosol absorption can increase LWP through a reduction in cloud-top entrainment.