Fabien Solmon Laboratoire d'Aérologie Toulouse (LA) solf@aero ...

Interactions between climate and atmopheric chemistry over West Africa
Fabien Solmon
Laboratoire d’Aérologie Toulouse (LA)
solf@aero.obs-mip.fr
PART 1 : Dust / African monsoon interactions
PART 2 : Challenging scientific issues involving west african climate and atmospheric chemistry
Thanks to …
ICTP
Abidjan and Niamey
University
LA
A. S. Zakey, F. Giorgi … A. Konaré , S. Ibrah
M.Mallet , E.M Assamoi, F.
Malavelle, C. Liousse, B.
Guillaume …

Dust / African monsoon interactions
Key question : Role of dust aerosol direct (and semi-direct) radiative effect on
monsoon dynamic and regional precipitations.
Weather system scale
Meso-scale models can show contrasted responses to regional dust radiative forcing, depending
on regions of interactions, system dynamical features, dust load and vertical distribution ..( e.g
Grini et al., 2006; Chaboureau et al. 2007, Mallet et al. 2009 …)
Climate time scale (GCM studies)
Monsoon and precip weakening by dust
Yoshioka et al.,2007
Monsoon and precip strenghtening by dust
Lau et al., 2009
Regional climate approach

Incoming
solar flux
F F 0 .R
0
absorption
scattering
F 0 .e -AOD
surface
Aerosol optical depth AOD describes the
aerosol extinction due to the sum of absorption
and scattering effects.
Scattered fraction R : depends on surface
albedo, incident radiation angle , AOD,
absorption and scattering properties of
aerosols
Dust Short Wave radiative forcing
Outgoing flux
TOA SW Radiative forcing : difference of
outgoing fluxes without and with aerosol
All other atmospheric and surface variables
being fixed.
> 0. = warming of the system
< 0. = cooling of the system
Dust
Refractive
index
Mie theory
SRF SW Radiative forcing : difference of
net flux at the surface
Always < 0. = cooling of the surface
Extinction cross section (Qext, m 2. g -1 )
Single scattering albedo (SSA)
Asymetry parameter (g)

Dust Long Wave radiative forcing
Atmospheric layers absorb and emit (grey body) in thermal radiation range.
Radiative equilibrium between layers
TOA LW Radiative forcing : difference of
outgoing fluxes without and with aerosol
All other atmospheric and surface variables
being fixed
ABS
EM
surface
SRF LW Radiative forcing : difference of
net flux at the surface
Always > 0. = relative warming of the
surface …

A modelling approach …
Regional Climate Model
High resolution limited area models
adapted to climatic simulations.
Forced by analysis or GCM outputs.
RegCM (ICTP/UNESCO, Trieste, it)
Giorgi and Mearns (1999), RegCM special issue of JGR (1999)
…
RegCNET
Special Issue of Theor., Apl., Clim., sep 2006

Aerosols in RegCM
Tracer model / RegCM3 (Solmon et al., 2006; Zakey et al., 2006)
∂χ
= −V
∂t
⋅∇χ
+
F
H
+
F
V
+ T
CUM
+
S
χ
−
R
w,
ls
−
R
w,
cum
−
D
dep
+ ∑ Q − Q
p
l
Transport
Primary
Emissions
Removal
terms
Physico – chemical
transformations
Particles and chemical species considered
SO --
4
BC(soot)
OC(total organic carbon)
DUST(4 bins)
SO 2
Presribed emission inventories On line emissions
SO 2
0.01-1
Aqueous and gazeous
conversion
(Qian et al., 2001)
Hydrophilic
(20% at
emission)
Hydrophobic
(80%at
emission)
Hydrophilic
(50%at
emission)
Hydrophobic(5
0%at
emission)
0.01-1
µm
1-2.5
µm
2.5-5
µm
5-20
µm

dust aerosol module in the ICTP RegCM3 model
Dust module in RegCM3
Marticorena and Bergametti, 1995.
Alfaro et al., 1998.
Saltation
wind
Soil granulometry
10 µm – 10000 µm
0.01 – 20 µm
Transport and
removal
Sand-blasting
Surface properties
(roughness, humidity, vegetation)
Radiation scheme
absorption
diffusion
emission
(SW+LW)
SVAT scheme
⎡∂T
⎤
δ ⎢ t ⎥
⎣ ∂ ⎦
rad
δF
LE , H
RegCM3 regional
climate model

dust animation (February 2000)

Validations …
MODIS
‘Deep blue’
AOD
MAR 2006
RegCM
AOD

Seasonal scale …
1.5
MISR AOD
JJA
(2000-2006)
RegCM AOD
Vertical extinction
profiles …
RegCM
Lidar
M’Bour
Senegal

Dust impact on African Monsoon
Control simulation : NODUST
Dust simulation : DUST radiative impact (SW +LW)
Experiments
Simulation Period : 1996 to 2006, 60 km resolution
Boundary conditions : NCEP 2 reanalysis
(BATS, Grell)
Sensitivity studies:
Dust single scattering albedo : SSA
(absorption properties)
Sea surface temperature feedback

Dust radiative forcing (SW+LW), JJA 1996-2006
MISR AOD
RegCM AOD
Consistent with Balkanski et al., 2007; Li et al., 2004

Average dynamical and precipitation response to dust over Sahel
Konaré et al., 2008; Solmon et al., 2008
( NODUST, JJA 1996-2006)
Mean circulation at 865 hpa
m.s -1
( DUST -NODUST, JJA 1996-2006)
Differential circulation at 865 hpa m.s -1
mm/day
15
mm/day

Cloud water, meridional circulation and precip. difference (DUST-NODUST)
15W-15E
JJA 96-06
average
B :‘Elevated heat
pump’ effect
( Lau et al. 2009)
A :Reduction of
‘monsoon pump’
Precip difference
(DUST-NODUST)

A : Reduction of ‘monsoon pump’ intensity …
Sensible heat flux difference (DUST-NODUST)
W/m2
Measured Impact of dust on sensible
heat flux during the March 2006 SOP
Cooling of the Lower troposphere and reduction of the moist
static energy gradient.
Moist Static Energy gradient

B: Elevated heat pump …
15W-15E
JJA 96-06
average
B :‘Elevated heat
pump’ effect
( Lau et al. 2009)
Dust SW
radiative
heating ….
Can create positive
anomaly of CAPE

NODUST
1996-2006 average precip
mm/day
DUST
longitude
June
July
Aug.
… Signature of both effects on weather systems (easterly waves)…

Response to dust forcing vs. Precipitation bias.
mm/day
R egio n
(1 5W -15E a vera g e)
5 N – 10 N
10 N – 17 N
1 7-N 2 0 N
O B S
C R U
T R M M
C R U
T R M M
C R U
T R M M
B ias
(m m /d ay a n d % )
(D U S T –N O D U S T )
(m m /d ay a n d % )
-1.23
(-2 0.3 % )
-0.18
(-3.0 % )
-1 .2 3
(-2 4% )
-0 .1 9
(-3 .8 % )
+ 1 .0 9
(+ 2 0% )
-0 .42
(-7.9% )
+ 0.06
(+ 1.4% )
-0.2 8
(-6 .1 % )
-0.23
(+ 41% )
-
+ 0.04
(+ 7.4 % )
-0.41
(-92% )
+ 0.05
(+ 1 1.3% )
Im p rov m en t ?
n o
n o
yes
yes
yes
yes

Climate sensitivity to dust absorption properties
Variability of measured values of dust single scattering albedo values (mineral
composition, coating, aerosol size distribution ..) : impacts on the climatic response ?
Standard case
More absorbing dust
Very diffusive dust
SSA ~0.95 (fine dust) SSA ~0.90 SSA ~0.99

Simulated precipitation anomaly over the ocean ?
JJA 96-06
mm/day
Strong elevated heat pump effect …
Over ocean only the diabatic heating
contribution is efficient since SST are
forced in standard simulations …
PSEA (dust – nodust )+ dif. circulation
cbar
kg/kg

Simple SST experiment : SST* = SST - 0.8 x AOD
as a result of downward SW attenuation at the surface
(consistent with Evan et al., 2009, Yoshioka et al.,2007)
T2m dif. (DUST –NODUST)
STD case
K
T2m dif. (DUST –NODUST)
SST* case
K

STD case
SST* case
PSEA dif (dust –nodust)
cbar
PSEA dif (dust –nodust)
cbar
mm.d-1
mm.d-1
With SST* correction :
Same kind of response over land ..
a weaker dust induced cyclonic anomaly and less precipitation are obtained over ocean …

STD case
SST* case
Evap dif (dust –nodust)
mm.d-1
Evap dif (dust –nodust)
mm.d-1
Decrease of local humidity source
available for deep convection
… Need for regional Ocean coupled model to fully asses the regional climate
response to aerosol forcing over Ocean …

Conclusion part 1
•Regional precipitation responses depend on coexisting differential circulations patterns induced by the
dust radiative forcing at different tropospheric levels.
•Surface and lower troposphere cooling induces a decrease of the monsoon pump intensity whereas
atmospheric diabatic warming over the source areas trigger an elevated heat pump effect resulting in
enhanced convection and cloud formation in the higher troposphere over the Sahel.
•The net regional impact of dust on average precipitation results from these coexisting effects. Drying is
dominant over Sahelian region except for a limited band over northern Sahel which sees enhanced
precipitations. Model precipitation bias is positively impacted when dust are accounted for.
•The balance between these effects is very sensitive to the dust SSA which affects the intensity of
precipitation decrease vs. increase as well as the latitudinal limit between these two responses.
•When SST are prescribed to the model, only diabatic warming is effective and more convection and
precipitation are obtained in the dust outflow region. When SST are allowed to feedback (cooling due to
decrease of incoming SW radiation) the response could be of opposite sign due a decrease of latent heat
and moisture availability for deep convection.

Interaction between atmospheric chemistry, climate, and biogeochemical cylces over
West Africa : scientific context …
Intercontinental transport
Global chemistry
O 3
OH
etc
Heterogeneous chem.
gas/ particles /clouds
Climate change
Regional dynamics
Radiative forcings
Deposition
SO 4
RCOOH
O3
H2O
HNO3
OC
DUST
VOC
NH3
etc …
NOx
BC
OCEAN
Regional climatic and
Environmental impacts
TROPICAL FOREST
SAVANE HUMID SEMI-ARIDE
SAVANNAH
Primary Emissions
Anthro. pressure
DRY SAVANE SAVANNAH ARIDE
DESERT

e.g observed aerosol mixture ( MODIS obsrevation, june 4 2004)

Source SWAC, 1998

Large and increasing anthropogenic emissions in Africa
Diversity of emissions and complexity of tropical atmospheric chemistry
(Natural and anthropogenic compounds)
Sensitivity of societies and ecosystems to environmental change
Scientific challenges ….
To better understand complex climate / chemistry interactions over Africa.
To perform regional impact studies using emission, land use change and climate
scenarii.

Aerosol feedback on regional climate.
Role of anthropogenic and natural aersosol (Dust, biomass burning, energy …)
aerosols via direct and indirect effects …
Haywood et al., 2008, JGR
Dry season

Observation and simulation of aerosol hygroscopicity within convective systems ( from
Crumeyrolle et al., 2009, ACP)

Aerosol sectional model qqs-
40µm
Gaseous chemistry
in each bin:
Absorption/adsorption
+ internal equilibrium
SO 2
NOx
COV
Inorganic
Condensable
H 2
SO 4
NH 3,
HNO 3
Organic
Condensable
SOAA, SOAB
Water
sulphate
SOAA
SOAB
OCp
BC
sea-salt
dust
nitrate
pH
ammonium
Optical
properties
Hygroscopic
properties

Biogeochemical feedbacks…
e.g. 1 Deposition of nitrogen to savannah ecosystem ..
Nitrate associated to dust large particles ( IDAF / Lamto / ivory coast )

e.g. 2 : Dust iron solubility and bioavailability to the Atlantic ocean ecosystems
Influence of mineralogy ( Journet et al., 2009)
Influence of atmospheric processing ( Desboeufs et al;, 2008)

Impact of regional processes on global chemistry and regional air quality
Ozone, particles …
e.g. : monsoon dynamic and inter-hemispheric transport of ozone ..

THANK YOU !