Saharan Dust Induced Radiation-Cloud-Precipitation-Dynamics Interactions William K. M. Lau NASA/GSFC Co-authors: K. M. Kim, M. Chin, P. Colarco, A. DaSilva
Atmospheric loading of Saharan dust Annual emission of Saharan dust is estimated to be 3-4 billion tons with large uncertainties Increased Saharan dust outbreak since the 1970 s, associated with the prolonged drought in Sahel ; decreasing loading over Atlantic, since 1990 s associated with Sahel rainfall recovery Possible weather and climate impacts SAL dry air may suppress tropical cyclogenesis, and/or hurricane formation Microphysics effects: suppress warm rain; increase ice-nucleation and deep convection Solar Dimming Effect Elevated Heat Pump (EHP) Effect (Lau et al 2006, 2009 and others) MODIS Images
Our Research on Saharan Dust Atmospheric Water Cycle Interaction Event-based dust outbreak composite analysis from satellite observations (Wilcox et al. 2010.) GCM modeling studies of dust radiative-dynamic interactions (Lau and Kim 2009.) Impacts of Saharan dust on hurricanes (Lau and Kim 2007 a, b.) Long-term variability (> decadal time scales) see also workshop poster Impacts on forecasts of AEWs and tropical cyclogensis (Oreste et al. 2009, 2011..) Dust induced diurnal variability in W. African monsoon (Kim et al., 2009) Transport and interactions with African Easterly Waves (AEW) talk following this, by K. M. Kim Aerosol indirect (microphysics) effects - a wild card
Atlantic ITCZ Region
Aug 28, 2006 West Africa East Atlantic Caribbean N 10N Eq S
Wilcox et al. 2010
Model: NASA fvgcm with McRAS - 2.5x2.0 horizontal resolution and 55 vertical sigma level - prognostic cloud water schemes, and liquid- and ice-phase cloud microphysics (Sud and Walker 1999, 2003) - radiative transfer scheme of Chou and Suarez (1994, 1996). (SW+LW) - Prescribed global aerosol forcing: seasonally varying 3-D distribution of five aerosol species (dust, black carbon, organic carbon, sulfate and sea salt) derived from the Goddard Chemistry Aerosol Radiation Transport (GOCART) model (Chin et al. 2002, 2004). Model Experiment - Seven-month integration with coupled MLO model were performed with NASA fvgcm/mcras for Apr-Oct of the years 2000-2007 without aerosol radiative forcing (Control Run or NA) - AA: Same as NA, but with aerosol radiative forcing - AA : Other experiments with different dust absorption properties - Ensemble mean difference (AA-NA) will be discussed. More Recent experiments are carried out with interactive (radiatively) aerosols
AA-minus-NA surface fluxes, and surface temperature anomalies
East-West Cross-section (5N-15N) North-South Cross-section (10W-10E)
GEOS GCM simulation of rainfall enhancement at the northern edge of the Atlantic ITCZ (southern edge of the Saharan dust layer)
EHP effect is minimized for reflecting dust SSA > 0.95
Anomalous atmospheric water cycle and surface temperature induced by the Elevated-Heat-Pump (EHP) effect of Saharan Dust over the West Africa/Atlantic region reduced upper level clouds increased upper level clouds induced subsidence suppresses convection increased low level clouds increased moisture transport, AEW Increased rainfall cooler ocean reduced SW cooler land reduced SW cools land W. Africa dust source Caribbean Gulf of Guinea Lau et al. 2009, Geophys. Annales
Saharan Dust-Atmospheric Water Cycle Interaction from Satellite Observations (Cloudsat-Calipso, MODIS, AIRS, TRMM) and MERRA-reanalysis
Westerh Atlantic/Caribbean West Africa Land Off W. Africa coast
E W
- Impacts of Saharan Dust on Tropical Cyclogenses and Hurricanes
Contrasting the 2005 and the 2006 hurricane seasons 2005 15H, 12TS, 2TD 2006 5H, 4TS How Nature foiled the 2006 hurricane forecasts Lau and Kim (2007, Eos)
JAS (2006 minus 2005) wind and circulation anomaly
ITR MDR El Nino-SST effect ITR MDR Dust-SST Solar Dimming effect Dust appears to be the primary cause for SST cooling in the ITR; El Nino is associated with reduced SST (and increased vertical shear) in the eastern equatorial Atlantic, suppresses cyclogensis
Increased pre-season (JJA) dust activity is associated with subsequent (JASON) cooler Atlantic SST and less hurricanes Covariance (sign reversed) of MODIS AOD (JJA) to hurricane activity (JASON) 2006 minus 2005, June AOD SST covariance SST difference Lau and Kim (2008, GRL): Cooling of the Atlantic by Saharan dust
Long-term (> decadal scale) Variations of Saharan Dust and Possible Climatic Impacts
Lower Saharan dust loading over N. Atlantic and reduced hurricane geneses along the MDR in recent decades Distribution of August-September (AS) mean dust optical depth simulated by GOCART model forced with GEOS 4 assimilation data for (a) 1998-2007 and (b) 1980-1989. Green dots indicate the tropical storm genesis locations during the same periods.
August-September mean dust distribution simulated by GOCART driven by GEOS-4 assimilation
Simulated August-September mean difference in dust AOD between periods 1998-2007 and 1980-1989.
August-September difference between periods 1998-2007 and 1980-1989 of (a) sea surface temperature (C) and (b) outgoing longwave radiation (W/m2)
GEOS5 5-day forecast of dust
Conclusions Radiative forcing by Saharan dust at the surface and in the atmosphere (~ ± 20-40 W m -2 ) have strong impacts on multi-scale variability of the Atlantic ITCZ, West Africa monsoon rainfall, AEW, AEJ, with possibly impacts on long-term Atlantic hurricanes trends. Enhanced Saharan outbreak tends to: - Change the surface and atmospheric energy balance (SW, LW, LH, SH) of the underlying surface, cooling the upper ocean (<0.5C) and land surfaces ( 1-2C), and warm ( 0.2-0.5 C /day) the upper dust layer and atmosphere above depending on SSA of dust - Enhance rainfall in the northern edge of the WAM and Atlantic ITCZ abutting the southern edge of the Saharan dust layer - Generate anomalous Walker and local Hadley cells, with increased rainfall over WAM land, and eastern equatorial Atlantic. (Lau et al. 2009, Wilcox et al. 2010) - Reduce the amplitude of diurnal cycle of rainfall over WAM land region draws Atlantic ITCZ, and WAM rainfall (Kim et al. 2009) - Enhance the amplitude of the AEWs, and shift the AEJ poleward; alter transport pathways of Saharan dust (Kim et al., 2009).