Clouds and dust: CALIPSO observations of transatlantic
dust transport
Alexander Marshak (GSFC) Weidong Yang (USRA), Tamas Varnai (UMBC), Olga Kalashnikova
(JPL), Alex Kostinski (Michigan Tech)
Fraction of cloud-free vertical profiles
Far from clouds (> 5km)
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20
MODIS, all cloudsCALIPSO, clouds below 3 km
Cum
ulat
ive
hist
ogra
m v
alue
Distance to nearest cloud (km)
CALIPSO (ColorRatio vs. Backscat close to and far from clouds)
Global night data over ocean
Fraction of cloud-free vertical profiles
Close to clouds (< 5km)
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20
MODIS, all cloudsCALIPSO, clouds below 3 km
Cum
ulat
ive
hist
ogra
m v
alue
Distance to nearest cloud (km)
CALIPSO (ColorRatio vs. Backscat close to and far from clouds)
Global night data over ocean
Main Question
How does the hygroscopicity of dust affect its optical properties near clouds?
Hygroscopicity is the particles’ ability of taking up water from humid air
Dust is mostly composed of water-insoluble minerals and shows nearly complete hydrophobicity or at least poor hygroscopicity
Area of Interest
Colors indicate the average nighttime CALIOP Dust Optical Depth (DOD) over oceans for the June 7-July 7, 2007 period.
Area of Interest
Colors indicate the average nighttime CALIOP Dust Optical Depth (DOD) over oceans for the June 7-July 7, 2007 period.
AOD DOD
Data
attenuated backscatter coefficient: β’532; attenuated color ratio: χ’ (ratio between β’1064 and β’532); depolarization ratio: δ’ (ratio between the perpendicular and parallel components of β’532)
Vertical distribution of dust
backscatter
color ratio depol. ratio
0.001 0.0015 0.002 0.0025 0.0030
1
2
3
4
5
EMW
β'532
, km-1 sr-1
Alti
tude
, km
0.5 0.55 0.6 0.650
1
2
3
4
5
χ'
Alti
tude
, km
0.05 0.1 0.15 0.2 0.250
1
2
3
4
5
δ'
Alti
tude
, km
0 0.02 0.04 0.06 0.08 0.1 0.120
1
2
3
4
5EMW
Normalized number of dust samples
Alti
tude
, km
norm. # of samples
EMW
Color ratio vs. depolarization ratio
0.17
0.18
0.19
0.2
0.21
0.22
0.23
0.55 0.56 0.57 0.58 0.59 0.6 0.61 0.62
EM
δ'
χ'
(a)
- decrease with altitude in the concentration of non-dust particles mixed in from below
- different fall speeds vertically separating the relatively more spherical dust particles from the least spherical ones (Ginoux, 2003).
- backscatter from larger dust particles can be more depolarized (Mishchenko and Hovenier, 1995 ).
color ratio
depo
l. ra
tio
Depolarization ratio vs. longitude
0
0.005
0.01
0.015
0.02
0.025
0.03
-70 -60 -50 -40 -30 -20 -10 0
Dep
olar
izat
ion
Rat
io
vert
ical
incr
ease
rat
e, k
m-1
Longitude, deg
from 3 to 4 km - more and less spherical dust particles getting vertically separated because of their different sedimentation speeds
- particles with irregular shapes have greater cross-sectional areas and drag-coefficients, thus more irregular particles fall slower than more spherical ones
- the upward increase gets stronger westward simply because the sedimentation process has more time to work
Dust near clouds
0.001
0.0015
0.002
0.0025
0 2 4 6 8 10
β'53
2, k
m-1
sr-1
Distance to cloud, km
(b)
0
0.075
0.15
0.225
0.3
0
10
20
30
40
0 2 4 6 8 10
Dust Alt. > 2 km Dust Alt. < 2 km
All Aerosols
Rat
io o
f num
ber o
f dus
t pro
files
ove
r all
aero
sol p
rofil
es
Num
ber of all aerosol profiles, 103
Distance to cloud, km
(a)
0.4
0.5
0.6
0.7
0 2 4 6 8 10
χ'
Distance to cloud, km
(c)
0
0.05
0.1
0.15
0.2
0.25
0 2 4 6 8 10
δ'
Distance to cloud, km
(d)
backscatter
color ratio depol. ratio
- black: high dust - green: low dust - orange: all aerosols
Cloudy vs Clear
MODIS CALIPSO
Schematic illustration of Cloud Fraction (CF) definitions for 5 km resolution dust pixels. CF is the fraction of cloudy 0.3 km-resolution
pixels in 5 km size areas containing dust
Vertical profile of dust backscatter coeff. @ 532 nm
0 0.0015 0.003 0.0045 0.0060
1
2
3
4
5
CF=00<CF<0.60.6<CF<1CF=1
β'532
, km-1sr-1
Alti
tude
,km
β'
532
0 0.0015 0.003 0.0045 0.0060
1
2
3
4
5
β'532
, km -1 sr-1
Alti
tude
, km
0 0.0015 0.003 0.0045 0.0060
1
2
3
4
5
β'532
, km-1 sr-1
Alti
tude
, km
CF=00<CF<0.60.6<CF<1CF=1
Vertical profile of dust color ratio
0.4 0.5 0.6 0.7 0.8 0.90
1
2
3
4
5
χ'
Alti
tude
, km
χ'
0.4 0.5 0.6 0.7 0.8 0.90
1
2
3
4
5
χ'
Alti
tude
, km
0.4 0.5 0.6 0.7 0.8 0.90
1
2
3
4
5
χ'
Alti
tude
, km
CF=00<CF<0.60.6<CF<1CF=1
Vertical profile of dust depolarization ratio
0.05 0.1 0.15 0.2 0.250
1
2
3
4
5
δ'
Alti
tude
, km
δ'
0.05 0.1 0.15 0.2 0.250
1
2
3
4
5
δ'
Alti
tude
, km
0.05 0.1 0.15 0.2 0.250
1
2
3
4
5
δ'
Alti
tude
, km
CF=00<CF<0.60.6<CF<1CF=1
Summary
(1) backscatter doesn’t change much with altitude in the SAL, while dep. ratio and color ratio increase;
(2) the vertical increase rate of dust depolarization ratio in the SAL
increases as plumes move westward; (3) the amount of nearby clouds barely affect the backscatter and
color ratio of dust volumes inside of SAL but affect them significantly below the SAL.
(4) an apparent increase in depol. ratio near clouds indicates that
particles in some dusty volumes become more spherical in the humid air near clouds, and cannot be identified by CALIPSO as dust.
Color ratio vs. depolarization ratio
May 25 - June 25, 2008 June 7- July 7, 2007
0.15
0.16
0.17
0.18
0.19
0.2
0.21
0.22
0.55 0.56 0.57 0.58 0.59 0.6 0.61
EM
δ'
χ'
(a)
0.15
0.16
0.17
0.18
0.19
0.2
0.21
0.22
0.55 0.56 0.57 0.58 0.59 0.6 0.61δ'
χ'
(c)
Vertical profile of dust # of samples
0 500 1000 1500 20000
1
2
3
4
5
Number of Samples
Alti
tude
, km
E
0 500 1000 1500 20000
1
2
3
4
5
Number of Samples
Alti
tude
, km
W
0 500 1000 1500 20000
1
2
3
4
5
Number of Samples
Alti
tude
, km
M
CF=00<CF<0.60.6<CF<1CF=1
