Aerosol properties in partly cloudy
conditions
A. Marshak (NASA GSFC),
T. Varnai (JCET),
T. Eck (GESTAR),
W. Yang (GESTAR)
From Chapter 7 of IPCC AR5 report
“… aerosol measured in the vicinity of
clouds is significantly different than it
would be were the cloud field … not
present”.
Thus
“ascribing changes in cloud properties to
changes in the aerosol remains a
fundamental challenge.”
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., Ignatov et al., 2005, Loeb and Manalo-Smith, 2005,
Zhang et al., 2005, Chand et al., 2012).
However, it is not clear yet how much grows comes from (e.g., Quaas et al., 2010;
Loeb and Schuster, 2008; Su et al., 2008)
(i) aerosols swelling in the humid air surrounding clouds;
(ii) cloud processing;
(iii) new particle formation;
(i) undetected cloud particles (Zhang et al., 2005);
(ii) clouds scattering sunlight into nearby clear areas
2008);
(i)
instrument issues.
(Marshak et al.,
MODIS
(South of UK)
AOT
- AOT(3km) > AOT(6km)
- AOT(CF=0.5) > AOT(CF=0.1)
Várnai and Marshak (2015)
CALIPSO
(around the Azores)
Backscatter
- Backsct(3km) > Backsct (6km)
- Backsct(CF=0.5) > Backsct(CF=0.1)
Yang et al., 2016
CF-AOD correlation
Varnai et al., 2017
Two options for color scale
Though there are some variations, CF-AOD correlation is positive globally.
Over much of the globe AOD is 30-50% higher on days that were cloudier than
average than on days that were less cloudy than average.
CF-AOD correlation
is strong for all aerosol types
a
b
Sulfate
Carbon
d
c
Dust
Correlation of CF & AOD
Sea salt
Varnai et al., 2017
Maps of local CF-AOD correlation considering for each location only those days
in JJA when the MERRA-2 reanalysis indicated that a single aerosol type
dominated in 1° by 1° area
What’s about correlation
between CF and particle size?
Aerosol size vs. CF in
MODIS and CALIPSO products
CALIPSO
MODIS (‘ocean color’ product)
Total color ratio
1 km
3 km
5 km
Várnai and Marshak (2015)
Yang et al., 2014
Why does particle size decrease with CF for MODIS even though it increases for CALIOP?
Aerosol size vs. CF in
MODIS and CALIPSO products
MODIS (‘ocean color’ product)
Duality in behaviors:
-
aerosol particle size does
not increase with CF
-
it still increases in the
proximity of clouds.
Fig. 4c in Várnai and Marshak (2015)
Why does particle size decrease with CF for MODIS even though it increases for CALIOP?
MODIS Level 3 data
Correlation between CF and AE
(‘dark target’ over ocean and ‘deep blue’ over land)
a
JJA
Correlation of CF & AE
b
DJF
Positive correlation between CF & AE is widespread though not universal
Coarse & fine mode AOD vs. CF in Level 3 data
Fine mode
Coarse mode
Correlation of CF & AOD
The AOD increase with CF is stronger for fine mode than for coarse mode
Why?
•
•
•
•
small mode is more hygroscopic,
coarse mode aerosol is at altitudes less affected by cloud-related humidity increase,
new particle formation creates small aerosols,
3D radiative interactions (bluing)
Implication:
The primary reason for CF-AOD correlations is NOT cloud contamination
What’s about ground-based
measurements?
AERONET during DRAGON
Eck et al., 2014
AOD vs time
Eck et al., 2014
ESSEX
Aerosol Optical
Depth (AOD)
Large jump in AOD (~0.3 at 440 nm) at the
DRAGON Essex site occurred just after solar
noon on July 5.
Essex
However, the AE (440-870 nm) remains
very high (>1.9) suggesting possible new
particle formation in the cloud environment
since a particularly dense cluster of clouds
is seen in the vicinity of the Essex site.
Relationship between CF-AE correlation and particle size
Map of mean AE for JJA
Correlation CF-AE for JJA
a
Small mode
Coarse mode
JJA
Correlation of CF & AE
Correlation CF-AE vs. AE
Change in AE of bimodal distr. when the size of
FM particles increases through hygroscopic swelling
Take home messages
• Both MODIS and CALIOP most likely show a positive
correlation between AOD (or BKS) and CF;
• The relationship between particle size and CF is much
more complex and is not dominated by a single factor;
• MODIS shows that in many large regions AE increases
with increasing CF; on contrast, CALIOP shows a
positive correlation between CF and aerosol particle
size;
• In bimodal particle populations, the sign of AE
variations does not necessarily indicate the direction
of particle size changes.
Thank you
CALIPSO
Where do we see particle increase?
Varnai et al., 2011
Yang et al., 2014
- ColorRatio(1km) > ColorRatio(5km)
- ColorRatio(CF=0.5) > ColorRatio (CF=0.1)
CALIPSO & MODIS
(relative enhancement)
Varnai et al., ACP 2013
A yearlong dataset
based on co-located
MODIS and CALIOP
data analysis over
oceans: 60°S - 60°N.
Likely due to the 3D radiative effects of clouds (25-30%)
Aerosol properties vs. CF in MODIS DT products
all = all DT 1 km pixels (DT QA≥1)
best = all ocean color QAs met
contaminated = sstwn QA
near clouds = strlwn QA
both strlwn and sstwn QA
3 km
from low cloud
Aerosol properties vs. CF in MODIS DT products
all = all DT 1 km pixels (DT QA≥1)
best = all ocean color QAs met
contaminated = sstwn QA
near clouds = strlwn QA
both strlwn and sstwn QA
6 km
from low cloud
Duality in behaviors:
- aerosol particle size does not increase with CF,
- it still increases in the proximity of clouds.
Aerosol behaviors in partly cloudy regions are affected by
several factors:
- hygroscopic swelling,
- meteorological co-variation of cloudiness and aerosols,
- new particle formation,
- cloud processing.
- 3D radiative interactions (bluing),
- undetected cloud droplets,