A Close Look at the
Aerosol-Cloud Interaction in
Tropical Deep Convection
Chien Wang
Massachusetts Institute of Technology
Why Does Aerosol Matter to Clouds
Saturation requirement to form new
particles through homogeneous
homo-molecular nucleation in the
atmosphere: S > 3.5
Note: Typically, in-cloud S < 1.01
Lowering the required S
1)
2)
Mixed vapor of 2 (binary) or 3 (ternary) species – hetero-molecular
homogeneous nucleation: mainly aerosol nucleation
Existing surface – heterogeneous nucleation on insoluble (with small
contact angle) or soluble aerosols (ion factor as well)
Heterogeneous nucleation of droplets and ice crystals
Water droplet nucleation:
Hygroscopic aerosols acting as nuclei. Note that it is
existing aerosol NOT molecular collision efficiency that
determines the nucleation rate
Four ice nucleation modes:
IN
H2O(g)
Heterogeneous deposition
cloud droplet
Condensation-freezing
Immerse
Contact
Ice
crystal
Aerosol and Cloud microphysical processes
water vapor
CCN
IN
cloud
droplet
nucleation
ice
crystal
condensation/
evaporation
riming/
freezing
snowflake
rain
drop
graupel
hailstone
melting
collection/
coagulation/
conversion
precipitation
Scavenging:
nucleation & impaction
Production:
evaporation (recycling)
Aerosol-Cloud Interaction
and
Its Impact on Radiation
Radiation
Aerosol Indirect Effect
Clouds
Aerosols
Cloud Indirect Effect(?!)
A Three-Dimensional Cloud-Resolving Model
Aerosols:
N of CCN, IN
or Multiple mode
multi-moment model
1b
1a
Radiation:
2
-four-stream
including ice cloud
Cloud Properties:
6b 6a
1
winds, T, P, Qv, lightning
7 Hydrometeors (Q & N)
40+ microphysical
conversions
Chemistry:
Species: 25g+16c,r+7i
Reactions:
35g + 21eq + 32aq + 7h
5
4a
4b
2
3b 3a
Environment:
large-scale forcings
and input fluxes
References: Wang and Chang, 1993; Wang et al., 1995; Wang and Prinn, 2000;
Wang 2005; Ekman et al., 2004; 2006
A 3-D CRM Study
(Wang 2005a&b; JGR)
Research Issues
 How does tropical deep convection respond to the 1) increase of
CCN concentration; 2) change of aerosol chemical composition;
and 3) modified aerosol properties at different altitudes?
 What are the chemical and physical consequences of aerosol
effect on convection?
Model and Simulations
 CEPEX March 8 soundings; 200  100  50 grids with 2  2  0.5 km
resolution; 4 hours simulation; supporting runs with 1.0 – 0.25 km
horizontal and 250 – 50 m vertical resolutions.
 Prognostic CCN (hygroscopic, Aitken or accumulation mode) and
IN (water-insoluble); activation of CCN: N = CSk
 No “external sources”
 90 runs with 30 initial concentrations of CCN, CCN0 from 100 to
5500/cc with a increment of 200/cc; also 50 and 6000/cc; different
autoconversion.
(a)
1000
3
Cloud Droplet Concentration (1/cm )
Cloud Droplet
100
10
1
0
1000
2000
3000
4000
5000
6000
3
Initial CCN Concentration (1/cm )
The Response of Cloud
Particle Number
Concentrations to the
Increase of Initial CCN
Concentration
(c)
1.5
3
Ice Crystal Concentration (1/cm )
Ice Crystal
1.2
Note: All runs use the same
initial IN profile.
0.9
0.6
0.3
0.0
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
Effective Radius of Hydrometeors vs. Initial CCN Concentration
(c)
(a)
120
35
Ice Crystal
110
Cloud Droplet
30
Effective Size (m)
Effective Radius (m)
100
25
20
15
10
5
90
80
70
60
50
40
0
0
1000
2000
3000
4000
5000
0
6000
1000
2000
3000
4000
5000
6000
3
Initial CCN Concentration (1/cm )
3
Initial CCN Concentration (1/cm )
(d)
(b)
600
Graupel
600
Raindrop
Effective Radius (m)
Effective Radius (m)
500
400
300
500
400
300
200
200
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
Maximum Coverage of Cloud
vs.
Initial CCN Concentration
Total Precipitation
vs.
Initial CCN Concentration
(a)
0.5
Berry scheme
Kessler scheme
NOAC
60
50
40
30
Berry scheme
Kessler scheme
NOAC
20
10
Total Domain Precip (mm)
Maximum Cloud Cover (% of domain area)
70
0.4
0.3
0.2
0.1
0.0
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
(b)
2
Column Loading of Condensed Water (g/m )
20
Budget of Water:
• Supply and consumption
of water vapor
increases with CCN0;
• precipitation efficiency
varies little with CCN0
15
10
5
Berry scheme
Kessler scheme
NOAC
0
0
1000
2000
3000
4000
5000
6000
3
50
Precip (mm)
Estimated "Precip Effeciency" (%)
60
40
30
Berry scheme
Kessler scheme
NOAC
0.8
0.4
0.7
0.3
0.6
0.2
0.5
0.1
0.4
0.0
0.3
-0.1
0.2
-0.2
0.1
-0.3
20
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
0.0
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
qvflux (mm)
Initial CCN Concentration (1/cm )
0.5
Surfaces of Updraft  5m/s (Brown) or Downdraft  2 m/s (Blue)
60 min.
90 min.
100/cc
60 min.
100/cc
90 min.
700/cc
60 min.
700/cc
90 min.
3100/cc
3100/cc
Contribution of Ice-Phase Processes to
Rain Production (%)
100
90
80
70
60
Berry scheme
Kessler scheme
NOAC runs
50
0
1000
2000
3000
mass
number
mass
number
mass; number = 100
4000
5000
3
Initial CCN Concentration (1/cm )
6000
Correlations of microphysical
conversions and precipitation:
The importance of riming
Relative Changes of Microphysical Conversions
Budget of rain:
The dominant role of
Ice-phase microphysics
160
crim
rrim
frg
depi
migr
140
120
100
80
60
40
20
0
0
20
40
60
80
Relative Change of Total Precipitation
100
Hr2 mean
0
Hr3 mean
Hr4 mean
2
Domain-mean Forcing (W/m )
Radiative Effects
Solar TOA
-30
Domain-Mean
Cloud Shortwave
Forcing
vs. Initial CCN
Concentration
-60
-90
-120
-150
2
Cloud-mean Forcing (W/m )
-200
-250
Cloud-Area-Mean
Cloud Shortwave
Forcing
vs. Initial CCN
Concentration
-300
-350
-400
-450
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
Water vapor redistribution by the modeled convection
Cloud-Area Mean
4.2
11.6
4.0
11.4
3.8
11.2
3.6
12-16 km
11.0
3.4
10.8
3.2
3.0
10.6
650
920
3
Water Vapor Concentration (mg/m )
3
Water Vapor Concentration (mg/m )
Ambient Mean
640
900
630
880
620
860
6-12 km
610
840
600
590
820
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
0
1000
2000
3000
4000
5000
3
Initial CCN Concentration (1/cm )
6000
Ratio of 4th Hour Mean Cloud Mole Fractions in 12-16km
and Initial Mean Ambient Mole Fractions in 0-6km
0.68
CO
0.67
0.66
0.65
0.64
0.65
Efficiency of
vertical transport
of gaseous
species from the
lower to upper
troposphere
SO2
0.64
0.63
0.62
0.61
0.08
CH2O
0.06
0.04
0
1000 2000 3000 4000 5000 6000
3
Initial CCN Concentration (1/cm )
(Cohan et al., 1999):
0.90
0.89
(H2O2)
Scavenging efficiency
of a fast soluble gas Y
0.87
Yconv  YUT
  1
(1   )YBL
X BL  X conv

X BL  X UT
CO is used as the reference
species in the modeled case
with β = 82-88%.
0.86
(CH2O)
0.75
0.70
0.65
0.60
0.40
0.30
(SO2)
Here, the dilution
factor of a species X:
0.88
0.20
0.10
0.00
0
1000
2000
3000
4000
5000
6000
3
Initial CCN Concentration (1/cm )
Influence of the Modeled Cloud on Gaseous Chemistry:
Total condensed water (0.1 g/kg surfaces in yellow color)
and NO2(g)/NO(g) ratio (2.0 surfaces in blue color), all from CCN0 = 100/cm3 run
1.3
Cloud-Ambient Ratio of OH Concentration (Hour 4)
1.2
1.1
12 - 16 km
1.0
0.6
Redistribution of
OH Radicals
6 - 12 km
0.5
0.2
0.1
0 - 6 km
0
1000
2000
3000
4000
5000
6000
3
Initial CCN Concentration (1/cm )
Influence of the Modeled Cloud on Heterogeneous Chemistry
O3(s)  2 pptm (blue), CH3OOH(s)  2 pptm (green),
and HNO3(s)  1 pptm (brown); all from CCN0=100/cm3 run.
Summary
Tracers

Soluble

CCN
Radiative
UT
LT
forcing reactions photolysis
Freezing
Level & Q
Qi
Cld area
W
Precip QI
LWC
Warm
QR
CDNC
CD re
Total
QR
Subl.
Likely
changing
color for
continental
cases
Important Points:
 Many properties of modeled tropical deep convection
DOES NOT respond MONOTONICALLY to the change
in CCN0.
 Aerosol effect could be more substantial in clean
environment (low CCN0 cases).
 Dynamics AND microphysics are equally important in
determining the response of convective cloud to CCN0.
 Ice-phase microphysics plays an important role in
precipitation formation and development of modeled cloud.
 Some conclusions drawn from this study perhaps can be
only applied to the specific cloud type.
Importance of Including Prognostic Aerosol Properties in the Model:
Results of a Size-Resolving Aerosol Model (Ekman et al., 2004)
Note: Observed. max. value (>7nm) in anvil: 2.5·104 Modeled max. value (>6nm) in anvil: 5.5 ·104
Aitken mode (6nm<d<30nm)
20
10
0
50
100.0
250
Horizontal distance (km)
177.5
316.2
Concentration (100 cm-3)
562.5
1000.0
Acc. mode aerosol (d>30nm)
20
10
0
50
0.1
250
Horizontal distance (km)
1.0
10.0
Concentration (cm-3)
100.0
1000.0