Robert Wood University of Washington W ith contributions from Eowyn Baughman Dan Grosvenor Daniel McCoy Chris Terai Matt Lebsock Dave Leon Jeff Snider Tony Clarke Duli ID: 240687
Download Presentation The PPT/PDF document "Re-examining links between aerosols, clo..." is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Re-examining links between aerosols, cloud droplet concentration, cloud cover and precipitation using satellite observations
Robert Wood
University of Washington
W
ith contributions
from
Eowyn
Baughman, Dan
Grosvenor, Daniel McCoy, Chris
Terai
, Matt
Lebsock
, Dave Leon, Jeff Snider, Tony
Clarke,
Duli
ChandSlide2
MotivationCorrelations between aerosols and clouds in the current climate are used to infer aerosol indirect effects (e.g. Lebsock, Quaas, IPCC….)But aerosols and clouds both respond to meteorological variability
In this presentation, we explore three ways in which meteorology complicates the relationship between aerosols and cloudsSlide3
Message 1
:
Precipitation variations drive changes in cloud droplet
concentration
in marine low cloud
Photograph: Tony Clarke, VOCALS
REx
flight RF07
Image: Pablo
Saide
High droplet concentration
Low
droplet
concentrationSlide4
What controls CCN variability in the marine boundary layer?A simple CCN budget for the PBL
Entrainment
Surf. Source
Precip
. Sink
Assume nucleation/secondary processes unimportant
Dry deposition is negligible (
Georgi
1990)
Sea-spray formulation (e.g. Clarke et al. 2006)
Ignore advection
Precipitation sink primarily from accretion process
Equivalency of CCN and cloud drop conc.
N
d
Wood et al. (
J.
Geophys
. Res.
2012)Slide5
Steady-state CCN budgetConcentration relaxes to FT concentration NFT + wind speed dependent surface contribution dependent upon subsidence rate (D z
i)Precipitation sink controlled by precipitation rate at cloud base PCB
. Use expression from Wood (2006).Free Tropospheric CCN
Precip
. Sink
Sea-Spray Production
Wood et al. (
J.
Geophys
. Res.
2012)Slide6
Precipitation is primary control of Nd
away from coastal zones
Wood et al. (J. Geophys. Res. 2012)
Without aerosol source variations
, model
reproduces significant amount of variance in
N
d
over oceans
implications for interpretation of AOD vs
precipitation relationshipsSlide7
Consequences
CCN
Cloud LWP
CCN
Cloud LWP
Aerosol-driven precipitation suppression
or
precipitation-driven aerosol suppression?
Possible
causal
relationship
Observable relationshipSlide8
Message 2: Hygroscopic growth of aerosols in the vicinity of clouds leads to correlations between AOD and cloud cover that have little to do with aerosol indirect effects
Chand, D., R. Wood, S. J. Ghan
, M. Wang, M. Ovchinnikov, P. J. Rasch, S. Miller, B. Schichtel, and T. Moore (2012
), Aerosol
optical depth increase in partly cloudy conditions, J.
Geophys
. Res., 117,
D17207
.Slide9
Relative AOD enhancement (
=/
CF=0.15) shows no clear pattern with aerosol sources and is almost hemispherically symmetrical
Retrieved AOD is greater when it is cloudierSlide10
Enhancement of AOD captured with simple statistical modelRelate cloud fraction to RH pdfRelate hygroscopic aerosol growth in clear portion of box to RH pdf
Make assumptions about vertical structure
Can explain correlation between AOD and cloud cover without recourse to AIEs Slide11
Consequences
AOD
Cloud cover, albedo
AOD
Cloud cover, albedo
Aerosol effects on clouds
o
r cloud effects on aerosols?
Possible
causal
relationship
Observable relationshipSlide12
Message 3: Using aerosol-cloud correlations in the current climate to infer aerosol-driven cloud changes can be misleadingWRF-Chem, indirect effect of biomass burning aerosols on cloud
Two simulations: one with BB aerosols on; one without
Baughman et al. [2014]
BBon
-
BBoffSlide13
CCN composited on an offshore flow index
t=-5 day
t=0
t=+5 day
CCN anomaly
0 50 100 [cm
-3
]
timeSlide14
LWP composited as a function of offshore flowLWP lower when high CCN loading offshore….
BBon
LWP anomaly
-50 0 +50 [g m
-2
]Slide15
LWP composited as a function of offshore flowResponse very similar even without BB aerosols
BBoff
LWP anomaly
-50 0 +50 [g m
-2
]Slide16
Take-home messagesPrecipitation variations drive variations in cloud droplet concentration on a range of timescalesHygroscopic growth of aerosols in the vicinity of clouds leads to correlations between AOD and cloud cover that have little to do with aerosol indirect effectsUsing aerosol-cloud correlations in the current climate to infer aerosol-driven cloud changes can be misleadingSlide17Slide18
What controls CCN variability in the marine boundary layer?A simple CCN budget for the PBLAssume nucleation/secondary processes unimportant
Dry deposition is negligible (Georgi 1990)Sea-spray formulation (e.g. Clarke et al. 2006)
Ignore advectionPrecipitation sink primarily from accretion process Equivalency of CCN and cloud drop conc. Nd
Entrainment Nucleation/Secondary Dry Dep.
Surf. Source
Precip
. Sink Advection
Wood et al. (
J.
Geophys
. Res.
2012)Slide19
Difference () in AOD (MODIS, 550 nm) between 1x1o boxes with 85% cloud cover and those with 15% cloud cover
Retrieved AOD is greater when it is cloudierSlide20
Consequences
Observable ‘sensitivity’ to CCN
True sensitivity to CCN
Using aerosol-cloud correlations in the current climate to infer aerosol-driven cloud changes can be misleading