1. Vertically resolved CALIPSO-CloudSat aerosol extinction coefficient in the marine boundary layer and its co-variability with MODIS cloud retrievals
David Painemal1,2, Richard Ferrare2, Marian Clayton1,2, Fu-Lung Chang1,2, Sharon Burton2, Damien Josset3, Patrick Minnis1,2 and Yan Feng4
1Science Systems and Applications, Inc
2NASA Langley Research Center
3Naval Research Laboratory
4Argonne National Laboratory
A-Train Symposium 2017

2. Motivation
Cloud condensation nuclei (CCN) measurements are essential for understanding the role of aerosols in modifying cloud properties (aerosol indirect effect)
Andreae, 2011 (ACP)
AOD 500 nm
CCN0.4%
Satellite-based studies mostly rely on aerosol optical depth (AOD) as a proxy for CCN. But:
The AOD-CCN relationship is scattered (e.g. Andreae, 2011 ACP).
The aerosol layer close to the cloud cannot be isolated from the rest of the atmospheric column.
3D radiative transfer effects in broken scenes and cloud contamination impact passive AOD.
Can we use vertically-resolved CALIOP aerosol measurements for a better quantification of the aerosol indirect effect?

3. Development of a new CALIOP-CloudSat aerosol extinction dataset
Aerosol retrievals from a new CALIOP-CloudSat dataset developed in NASA-LaRC.
The lidar equation is iteratively solved using CALIOP attenuated backscatter at 532 nm and 1064 nm, constrained with an independent AOD retrieval (e.g. Fernald et al. 1984).
AOD is taken from the Synergized Optical Depth of Aerosols (SODA) project, which derives AOD from CALIOP and the CloudSat radar ocean surface scattering cross-section (Josset et al., 2008 GRL).
Two methods:
1-layer method: one homogeneous aerosol layer with a constant lidar ratio with height (determined during the retrieving process).
2-layer method: the lidar ratio is prescribed = 25 sr in the boundary layer, and the lidar ratio for the upper layer is estimated during the iteration process.
Both methods compare better with aircraft data over the Caribbean than CALIPSO aerosol standard product (Ferrare et al., 2014)

4. Dataset Period of study: August-September 2013 CALIOP-SODA:
Retrievals derived using 1 km averaged CALIOP attenuated backscatter and SODA AOD
Nearly global oceanic coverage (45˚N-45˚S)
532 nm retrievals using 1-layer method
CALIOP cloud height
MODIS cloud retrievals (CERES Ed 4 algorithm)
1km pixel resolution
Cloud optical depth (t), 3.8 mm effective radius (re), and droplet number concentration (Nd, f(t,re))
Satellite cloud retrievals collocated with CALIOP
Period of study: August-September 2013

5. Total aerosol optical depth AOD
SODA AOD
MODIS AOD (level 3)
Biomass burning aerosols over the southeast Atlantic dominates the AOD pattern.
SODA AOD consistent with other satellite datasets.

6. AOD and extinction in the boundary layer and free troposphere
We used the cloud top height from CALIOP as a proxy for boundary layer height
CALIOP-SODA aerosol and CALIOP cloud properties averaged to a 5km resolution.
Aerosol and clouds matched within a 30 km segment. Z
2 km: Free tropospheric aerosol layer
Boundary layer
Aerosol extinction coeff.
Cloud Height
300 m: Below-cloud aerosol layer
Lon
Lat
30 km
5 km
Cloud height
Aerosol
=cloudy
=clear-sky

7. Aerosol extinction for different cloud heights
(m)
Aerosol extinction is composited as a function of cloud height.
Higher extinction in the boundary layer (BL)
Shallow BL located near the continents
Relative humidity appears to modulate the extinction.

8. CALIOP-SODA maps
1-layer aerosol and cloud height retrievals averaged to a 5km spatial resolution.
Data were further averaged to a 30 km spatial resolution
Mean maps were created with a spatial grid of 10˚x8˚

9. AOD in the boundary layer and free troposphere
Free tropospheric AOD
Boundary layer AOD
Free troposphere: important contributor to the total AOD
Boundary layer:
Aerosol burden in coastal regions is evident.
High AOD near 40˚S, likely connected to sea salt production
Da Silva et al. (2012)

10. Cloud top height
(m)
Cloud height modulates the magnitude of AOD in the boundary layer
Mean aerosol extinction coefficient is a more relevant parameter for the investigation of the aerosol indirect effect

11. Mean aerosol extinction coeff. in the boundary layer
(1/km)
Boundary Layer
Boundary Layer: High values in coastal regions
300 m layer below the cloud top:
The coastal pattern is enhanced
Relative decrease of extinction for the extra-tropics and the Indian Ocean.
(1/km)
300 m layer below cloud top

12. MODIS cloud properties: Preliminary results
1km MODIS pixels matched with CALIPSO ground track.
5 pixels east and west of nadir are also stored.
MODIS cloudy areas ≥ 2kmx5km are preserved only (useful for reducing clear sky contamination and 3D radiative effects in MODIS)
CALIPSO ground track
CALIPSO 1km pixels
MODIS pixels ✓ x
MODIS pixels
=cloudy
=clear-sky

13. Cloud droplet number concentration (Nd)
The spatial co-variability between Nd and aerosol extinction in the boundary layer starts to emerge…
Disagreement in the eastern Atlantic and Arabian Sea might be attributed to biomass burning aerosols and dust (less efficient CCN)
(cm-3)
MODIS Nd
Mean Extinction 300 m below cloud top
Total AOD ✘ ✓

14. Future Work Conclusions
The new CALIOP-SODA dataset provides unique vertical information of aerosol distribution for aerosol-cloud interactions studies.
Aerosol extinction in the boundary layer is consistent with continental aerosols being transported offshore
MODIS cloud properties
Remarkable spatial co-variability between CALIOP-SODA aerosol extinction in the boundary layer and MODIS cloud droplet number concentration.
Vertically resolved CALIOP data are key for advancing our understanding of the aerosol indirect effect beyond the “AOD framework”
Future Work
Produce one year of daytime CALIOP-SODA aerosol retrievals
Use 2-layer method for regions with high AOD in the free troposphere.
Develop screening methods for combining daily MODIS and CALIOP retrievals.
Explore the use of CloudSat for aerosol-precipitation interactions.
Acknowledgements: NASA CCST program (award # NNH16CY04C).

15. Extra

16. Development of a new CALIOP aerosol extinction dataset
Aerosol extinction (532 nm) (km-1)
Altitude (m)
Retrievals Validation: CALIPSO Caribbean Campaings 2008 and 2010
Aircraft data NASA LaRC high spectral resolution lidar (HSRL), “ground truth”
CALIOP-SODA 1 and 2-layer algorithms
CALIPSO standard aerosol product
CALIOP-SODA
Both 1-layer and 2-layer methods agree well with HSRL observations in the NW Atlantic.
CALIPSO standard product
Profiles overestimation
Challenges associated with layer detection, classification, and selection of an appropriate lidar ratio.
Do CALIOP-SODA aerosol retrievals spatially co-vary with the cloud microphysics in marine low clouds?

17. Dust frequency of occurrence (Omar et al., 2009).