1. 1 CIRA/NOAA/ESRL, Boulder, CO
Aerosol-cloud-surface flux Interactions in warm cumulus clouds over land
Hongli Jiang1
Graham Feingold2
1 CIRA/NOAA/ESRL, Boulder, CO
2 NOAA/ESRL, Boulder, CO
RICO workshop, Sept. 21, 2006

2. The “First Aerosol Indirect Effect”
More aerosol  more drops while LWC remains constant (Twomey 1974)
The “Second Aerosol Indirect Effect”
More aerosol  more drops  suppressed coalescence  less rain  larger LWP  longer lifetime (Warner ’68, Albrecht 1989)

3. Prior Work Local effects on clouds Ackerman et al. (2000)
Johnson et al. (2004)
Koren et al. (2004)
Feingold et al. (2005)
Menon et al. 2002
Regional Effects:
Disruption in precipitation
patterns in China:
Drought in north; floods in south
Cloud Fraction
Smoke Optical Depth

4. Objectives:
1. Study the second aerosol indirect effect on warm cumulus clouds over land
- Aerosol induced changes in LWP, cloud fraction, precipitation, etc….
2. Examine the semi-direct effect
- Evaluate the importance of coupling aerosol radiative properties to microphysics, dynamics, surface soil and vegetation model
Consider counteracting effects of the 2nd aerosol
indirect effect and the semi-direct effect

5. S1 Simulations: Aerosol-Cloud Interactions + Land Surface Model
Incoming
solar radiation
Surface sensible and
latent heat fluxes
balance

6. S2 Simulations: Aerosol-Cloud Interactions + Aerosol Radiation +
Land Surface Model
Incoming
solar radiation
Aerosol
scattering &
absorption
Incoming
solar radiation
diminished by
aerosol
Surface sensible and latent
heat fluxes reduced
balance

7. Table 1. Description of Experiments
Na , cm-3 ta
Aerosol Heating
S1-100
100
0.04 No
S1-500
500
0.20
S1-1000
1000
0.40
S1-2000
2000
0.80
S2-100
Yes
S2-500
S2-1000
S2-2000

8. Simulation of case from Amazon SMOCC experiment
Large Eddy Model (LES ~ Dx ~100m)
Resolves aerosol and drop sizes + dissolved aerosol
Resolves large eddy dynamics
Radiation model (Harrington et al., 2000)
Radiatively-active aerosol – absorbing aerosol heats atmosphere locally
Soil and vegetation model (Walko et al., 2000)
Domain size: x=y=6.4 km; z= 5.0 km
Grid size: Dx=Dy=100 m; Dz=50 m
Dt = 2 sec
Smoke:
ωo ~ 0.9 (dry)
Optical properties calculated in 8 λ bands (SW and LW)
Effects of uptake of water vapor on size and composition
Various values of concentration Na, but constant with height

9. S1: No Aerosol Heating CF Zdepth Unexpected:
LWP
S1: No Aerosol Heating
Na=100
100/cc
500/cc
2000/cc CF
Zdepth
Unexpected:
No clear separation in LWP, cloud fraction, and cloud depth as Na increases.
Zbase
Expected:
More aerosol  more drops
 less rain
Rain rate Nd
Reduction in LWP and CF not evident here
I would use this as a “wrap-up slide”

10. S1: No Aerosol Heating: 5-h averages vs Na
When raindrops are excluded in the LWP calculation, second aerosol indirect effect is simulated
Dynamic variability is much larger than aerosol effects on LWP, CF, cloud depth
Standard deviation

11. S2: With Aerosol-Radiative Coupling
LWP CF
Zdepth
Zbase
Distinct decrease in LWP, cf, cloud depth, and cloud base especially for Na> 500/cc. Line types are the same as in previous figures.
rain rate
w’w’

12. τ S2: With Aerosol-Radiative Coupling: 5-h average vs Na LWP CF Tsfc
Non-monotonic behavior
LWP τ CF
Tsfc
Rnet
Nd,int
Don’t know how to duplicate the labeling second time after picture frame 12.
Fsen+lat
Zdepth

13. τ S2: With Aerosol-Radiative Coupling LWP LWP 64% CF 58% Zdepth 62%
(S2(2000)-S2(100))/S2(100), %
LWP
64% CF
58%
Zdepth
62%
Tsfc
-1.31oC
Rnet
31%
Rsw
26% CF
Tsfc CF
Tsfc
Rnet
Nd,int
Nd,int
Rnet
Zdepth
Fsen+lat
Zdepth
Fsen+lat

14. Summary S1 simulations (2nd indirect effect only):
Increase in Na leads to
increase in Nd, cloud optical depth t,
decrease in reff,
reduction in surface precip
Aerosol effects on LWP, cloud fraction are small and well within the dynamical variability at a given Na
S2 simulations (2nd indirect + semi-direct effects):
The aerosol blocks up to 26 % of incoming solar radiation from reaching the surface;
Reduced surface radiative fluxes  reduction in surface heat fluxes  strong decrease in LWP, cloud fraction, cloud depth, and weaker convection;
Possible non-monotonic response of cloud properties to increases in aerosol

15. Final Comments
Current work focused on determining the effects of poor representation of mixing in LES
Damkohler No. = teddy/tevap (homogeneous/inhomogeneous)
Evaporation limiters: (C. Jeffery, J. Reisner, JAS 2006)
W. Grabowski (J. Climate 2006)
S. Krueger: EMPM

16. BOMEX SMOCC Cloud Fraction LWP (cloud ave.) LWP (domain ave.)
Note large std deviations!
LWP (cloud ave.)
100
1000 10
500
1000
2000
Aerosol Conc., cm-3
Aerosol Conc., cm-3
Xue and Feingold 2006
Jiang and Feingold 2006
Excluding drizzle