Fabien Solmon Laboratoire d Aérologie Toulouse (LA)

Transcription

1 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

2 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 ) 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

3 F 0 absorption scattering Incoming solar flux Dust Short Wave radiative forcing Outgoing flux F 0.R 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 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 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)

4 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

5 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

6 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 p Q l Transport Primary Emissions Removal terms Physico chemical transformations Particles and chemical species considered SO 2 SO -- 4 BC(soot) OC(total organic carbon) DUST(4 bins) 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) µm µm µm 5-20 µm Presribed emission inventories On line emissions

7 dust aerosol module in the ICTP RegCM3 model Dust module in RegCM3 Marticorena and Bergametti, Alfaro et al., Saltation wind Soil granulometry 10 µm µm 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

8 dust animation (February 2000)

9 Validations MODIS Deep blue AOD MAR 2006 RegCM AOD

10 Seasonal scale 1.5 MISR AOD JJA ( ) RegCM AOD Vertical extinction profiles RegCM Lidar M Bour Senegal

11 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

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

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

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

15 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

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

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

18 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 % ) ( % ) (-3.0 % ) (-2 4% ) (-3.8 % ) (+ 2 0% ) (-7.9% ) (+ 1.4% ) (-6.1 % ) (+ 41% ) (+ 7.4 % ) (-92% ) ( % ) Im p rov m en t? n o n o yes yes yes yes

19 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

20 Simulated precipitation anomaly over the ocean? JJA 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

21 Simple SST experiment : SST* = SST 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

22 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

23 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

24 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.

25 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

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

27 Source SWAC, 1998

28 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.

29 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

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

31 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

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

33 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)

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

35 THANK YOU!