Aerosol-cloud-surface flux Interactions in warm cumulus clouds over land

Hongli Jiang1

Graham Feingold2

1 CIRA/NOAA/ESRL, Boulder, CO2 NOAA/ESRL, Boulder, CO

RICO workshop, Sept. 21, 2006

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)

Prior Work

• Local effects on clouds

– Ackerman et al. (2000)

– Johnson et al. (2004)

– Koren et al. (2004)

– Feingold et al. (2005)

Clo

ud

Fra

ctio

n

Smoke Optical Depth

Menon et al. 2002

Regional Effects:Disruption in precipitation patterns in China: Drought in north; floods in south

2. Examine the semi-direct effect - Evaluate the importance of coupling aerosol radiative properties to microphysics, dynamics, surface soil and vegetation model

Objectives:

1. Study the second aerosol indirect effect on warm cumulus clouds over land

- Aerosol induced changes in LWP, cloud fraction, precipitation, etc….

3. Consider counteracting effects of the 2nd aerosol indirect effect and the semi-direct effect

Incoming solar radiation

Surface sensible and latent heat fluxes

S1 Simulations: Aerosol-Cloud Interactions + Land Surface Model

balance

Incoming solar radiation

Incoming solar radiationdiminished by aerosol

Surface sensible and latentheat fluxes reduced

Aerosolscattering &absorption

balance

S2 Simulations: Aerosol-Cloud Interactions + Aerosol Radiation +Land Surface Model

Table 1. Description of Experiments

EXP Na , cm-3 a Aerosol Heating

S1-100 100 0.04 No

S1-500 500 0.20 No

S1-1000 1000 0.40 No

S1-2000 2000 0.80 No

S2-100 100 Yes

S2-500 500 Yes

S2-1000 1000 Yes

S2-2000 2000 Yes

Simulation of case from Amazon SMOCC experiment

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

• Large Eddy Model (LES ~ x ~100m) – Resolves aerosol and drop sizes + dissolved aerosol– Resolves large eddy dynamics (rams@noaa)– 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: x=y=100 m; z=50 m• t = 2 sec

Expected: More aerosol more drops less rain

Rain rate

Nd

Unexpected: No clear separation in LWP, cloud fraction, and cloud depth as Na increases.

Na=100

LWP

CF

Zdepth

Zbase

S1: No Aerosol Heating

100/cc

500/cc

2000/cc

• Dynamic variability is much larger than aerosol effects on LWP, CF, cloud depth

• When raindrops are excluded in the LWP calculation, second aerosol indirect effect is simulated

S1: No Aerosol Heating: 5-h averages vs Na

Standard deviation

rain rate

w’w’

S2: With Aerosol-Radiative Coupling

LWP

CF

Zdepth

Zbase

S2: With Aerosol-Radiative Coupling: 5-h average vs Na

LWP

CF

Nd,int

Zdepth

τ

Tsfc

Rnet

Fsen+lat

Non-monotonic behavior

(S2(2000)-S2(100))/S2(100), %

S2: With Aerosol-Radiative Coupling

CF

Nd,int

Zdepth

Tsfc

Rnet

Fsen+lat

LWP τ

CF

Tsfc

Nd,int Rnet

Zdepth

Fsen+lat

LWP 64%

CF 58%

Zdepth 62%

Tsfc -1.31oC

Rnet 31%

Rsw 26%

LWP

SummaryS1 simulations (2nd indirect effect only):

• Increase in Na leads to increase in Nd, cloud optical depth 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

Final Comments

• Current work focused on determining the effects of poor representation of mixing in LES

– Damkohler No. = eddy/evap

(homogeneous/inhomogeneous)

– Evaporation limiters: (C. Jeffery, J. Reisner, JAS 2006)

– W. Grabowski (J. Climate 2006)

– S. Krueger: EMPM

Aerosol Conc., cm-3

Cloud Fraction

LWP (domain ave.)

LWP (cloud ave.)

BOMEX

10 100 1000

LWP (cloud ave.)

Aerosol Conc., cm-3

500 1000 2000

SMOCC

Excluding drizzle

Note large std deviations!

Xue and Feingold 2006 Jiang and Feingold 2006