Integration of models and observations of aerosol-cloud interactions

kayakjokeMechanics

Feb 22, 2014 (3 years and 4 months ago)

52 views

Integration of models and observations of
aerosol
-
cloud interactions

Robert Wood

University of Washington

Radiative

“forcing”
components

Cloud effects

Direct
Twomey

Mixed ph.


Semi
-
dir 2
nd

AIE

t
cld



N
d
1/3

LWP
5/6


(First AIE/
Twomey
)


N
d





Precip
.


(Second AIE/Albrecht)

Isaksen

et al.
(
Atmos.
Env
. 2009)

State of play

Isaksen

et al. (
Atmos.
Env
.
, 2009)

IPCC 2007

Model estimates of the two major aerosol
indirect effects (AIEs)


Pincus and Baker (1994)



1
st

and 2
nd

AIEs comparable



GCMs (Lohmann and
Feichter 2005)


1
st

AIE:
-
0.5 to
-
1.9 W m
-
2


2
nd

AIE:
-
0.3 to
-
1.4 W m
-
2



Aerosol particles induce changes

in cloud

macrophysical

properties.

The
Twomey

effect is insufficient

Shiptrack

surprises!


Liquid water content in
shiptracks is

typically

reduced
compared with
surrounding cloud


Clear refutation of
Albrecht’s hypothesis

courtesy Jim Coakley, see Coakley and Walsh (2002)

3.7

m

LES results

Cloud droplet concentration [cm
-
3
]

LWP
[g m
-
2
]






P
0
[mm d
-
1
]



w
e
[cm s
-
1
]


Impact of aerosols
simulated by varying
N
d


Increased
N
d



Reduced
precipitation


increased
TKE


increased
entrainment
w
e


Changes in
w
e

can
sometimes result in cloud
thinning (reduced
LWP
)


Also noted by Jiang et al.
(2002)

Ackerman et al. (2004)

Transient response of an equilibrated mixed
layer PBL model to
N
d

increases



Ratio of Albrecht to
Twomey

effect R
IE

(right)
is a
strong function of cloud base
height



More elevated cloud base
heights
z
cb

lead to Albrecht
effects which partly cancel
those due to
Twomey

effect



Elevated
z
cb

associated with
dry FT and less surface
drizzle, consistent with LES
results, but with a far less
sophisticated model


hope
for the representation in
climate models

Wood (
J. Atmos. Sci.,
2007)

Sedimentation of cloud droplets

Bretherton
,
Blossey

and Uchida, GRL, 2007

Cloud droplet sedimentation removes water from the (

10 m thick)
entrainment interface, lowers LWC there, reduces evaporative cooling, and
suppresses entrainment, resulting in thicker clouds

Since increased
N
d

reduces

sedimentation


pollution can lead to
thinner

clouds

Effects of drizzle
vs

effects of sedimentation of cloud
droplets


GCSS DYCOMS
-
2 RF02 drizzling Sc case study

Ackerman et al. (MWR, 2009)

With drizzle, without sedimentation

With drizzle and sedimentation

Effect of drizzle Effect of sedimentation

.....in this case, sedimentation dominates over drizzle impact


on cloud LWP

Microphysically
-
driven
supersaturation

differences,
can drive LWC differences

Kogan

and Martin,
Kogan

et al. (JAS, 1994, 1995)

Height [km]



Steady
-
state
supersaturation

inversely proportional to
N

and mean radius



More polluted clouds have
more active turbulence and
(in this case) more cloud
water



Also
microphysically
-
limited
evaporation rate (Feingold
inter alia.)

Necessary conditions for AIEs in warm
clouds


Aerosols must result in increases in cloud
droplet concentration


Present day geographical variability of cloud
droplet concentration should be simulated by
GCMs

January

MODIS

CAM
-
5



Use method of Boers
and Mitchell (1996),
applied by
Bennartz

(2007)



Screen to remove
heterogeneous clouds
by insisting on
CF
liq
>0.6
in daily L3



Cloud top droplet
concentration in warm
clouds from CAM
-
5

MODIS

CAM
-
5

July



CAM
-
5
broadly

captures land
-
ocean
contrasts in
N
d





Opposite sign of
seasonal cycle

over NH land
(MODIS>CAM in
winter; MODIS<CAM
in summer)




Clear evidence of S.
African and S.
American biomass
burning in MODIS
and CAM

Determine weak
-
link parameterizations

Chuang et al. (2011)



Effect of varying
autoconversion

schemes (in CAM 5)
on second AIE




Second AIE varies
by a factor of 5 or
more

Autoconversion

in the real world

z
*

z
*

cloud top

cloud base



Accretion
, not
autoconversion

is the
dominant precipitation
production mechanism
.....even in weakly
-
precipitating clouds

Composite of aircraft data in stratocumulus from Wood (
JAS
, 2005)

Precipitation susceptibility


Construct from Feingold and Siebert (2009) can be used to
examine aerosol influences on precipitation in both models and
observations



S

=
-
(
dln
R
CB
/
dln
N
a
)
LWP,h



Data from stratocumulus over the SE Pacific,
Terai

and Wood (
Geophys
. Res.
Lett
.
, 2011)



S
decreases strongly with
cloud thickness


Consistent with increasing
importance of accretion in
thicker clouds


Consistent with results
from A
-
Train (
Kubar

et al.
2009, Wood et al. 2009)



What controls
N
d
?




Simple

budget model for
CCN/
N
d

in the MBL:










Assume aerosol sources constant
(here represented by FT
concentration “buffer”)



Model pattern almost entirely
driven by precipitation sinks



Can reproduce significant amount
of variance in
N
d

over oceans


implications for significance of AOD
vs

r
e

relation ships

Wood (2011)

Conclusions


Most ways (and these are numerous) in which aerosols
impact warm clouds are mediated via cloud droplet
concentration


Singling out
Twomey

effect is introducing a biased effect, and
is pointless



Cloud droplet concentration can be estimated from
space but need to establish credibility of estimates,
especially away from Sc regions



CloudSat

and A
-
Train providing ways of establishing
sensitivity of warm rain to aerosols...and vice versa

A proposal


A limited area
perturbation
experiment to critically
test hypotheses related
to aerosol indirect
effects



Cost

$30M



Stevens and Feingold (
Nature
, 2009)