Idea Transcript
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 increased upper level clouds reduced upper level clouds
induced subsidence suppresses convection
increased moisture transport, AEW
increased low level clouds
reduced SW cools land
Increased rainfall
dust source
reduced SW cooler ocean
Caribbean
Lau et al. 2009, Geophys. Annales
cooler land
W. Africa Gulf of Guinea
Saharan Dust-Atmospheric Water Cycle Interaction from Satellite Observations (Cloudsat-Calipso, MODIS, AIRS, TRMM) and MERRA-reanalysis
Westerh Atlantic/Caribbean
Off W. Africa coast
West Africa Land
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
ITR MDR
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
El Nino-SST effect
Dust-SST “Solar Dimming” effect
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)
SST covariance
2006 minus 2005, June AOD
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 (