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
Motivation
• Cloud condensation nuclei (CCN) measurements are essential for understanding the
role of aerosols in modifying cloud properties (aerosol indirect effect)
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.
Andreae, 2011 (ACP)
Can we use vertically-resolved CALIOP aerosol measurements for a better
quantification of the aerosol indirect effect?
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)
Dataset
• 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
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.
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
Lat
Lon
Aerosol
Cloud height
Cloud
Height
30 km
5 km
2 km: Free tropospheric aerosol layer
300 m: Below-cloud aerosol
layer
Aerosol
Boundary layer
Cloud height
Aerosol extinction coeff.
=cloudy
=clear-sky
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.
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˚
AOD in the boundary layer and free troposphere
Boundary layer AOD
Free tropospheric 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)
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
Mean aerosol extinction coeff. in the boundary layer
Boundary Layer
300 m layer below cloud top
(1/km)
(1/km)
•
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.
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
✓
MODIS pixels
✓
=cloudy
=clear-sky
Cloud droplet number concentration (Nd)
MODIS Nd
(cm-3)
•
•
Mean Extinction 300 m below cloud top
✓
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)
Total AOD
✘
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).
Extra
Development of a new CALIOP aerosol extinction dataset
– Aircraft data NASA LaRC high spectral resolution lidar
(HSRL), “ground truth”
– CALIOP-SODA 1 and 2-layer algorithms
– CALIPSO standard aerosol product
• CALIOP-SODA
Altitude (m)
• Retrievals Validation: CALIPSO Caribbean
Campaings 2008 and 2010
– 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.
Aerosol extinction (532 nm) (km-1)
Do CALIOP-SODA aerosol retrievals spatially co-vary with the
cloud microphysics in marine low clouds?
Dust frequency of occurrence (Omar et al., 2009).