30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the...
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![Page 1: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/1.jpg)
30 years of African dust: From emission to depositionUsing GEOS-Chem and MERRA to determine the causes of variability and trendsDavid A. Ridley, Colette L. Heald
Acknowledgements: R. Lucchesi, M. Karki (NASA GMAO), J. Marsham and M. Hobby (U. of Leeds)This work is supported by internal funding at MIT
Toward resolution-independent dust emissions in global models: Impacts on the seasonal and spatial distribution of dustD. A. Ridley, C. L. Heald, J. R. Pierce and M. J. EvansGeophysical Research Letters, Vol 40, 1-5, doi:10.1002/grl.50409, 2013
![Page 2: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/2.jpg)
Motivation
• African dust accounts for half of global emissions (Huneeus et al., 2011)
• Wintertime dust emission and transport highly variable (Propero et al., 2013; Doherty et al., 2012; Ben-Ami et al., 2012)
• Land-use changes often cited as possible cause of change in dust emission over recent decades (Chiapello et al., 2005; Evan et al., 2011; Mulitza et al., 2010)
Annual Dust AOD (2012)
![Page 3: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/3.jpg)
Two Questions:
1. Can GEOS-Chem capture the emission and long-range transport of African dust over decadal timescales?
2. What drives the inter-annual variability in winter African dust emissions?
Are land-use changes important??
![Page 4: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/4.jpg)
Modelling Dust Emission
Wind Speed
Clay Fraction
Erodible Area
Soil Moisture
Vegetation
• DEAD dust scheme (Zender et al., 2003)
• TOMS AI dust source map (Ginoux et al., 2001, Prospero et al., 2002)
Ridley et al. (2013)
4 ° x 5° 2 ° x 2.5°
March 2012
• Weibull PDF used to represent sub-grid winds in GEOS-Chem
• Dust emission proportional to 10m-wind cubed• Wind threshold must be exceeded for emission
• Reduction in resolution-dependence of emissions
• Solving the resolution issue…
![Page 5: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/5.jpg)
Winter (DJFM) Dust AOD
GEOS-ChemMODIS
• Comparison with observations (AERONET, MODIS, MISR & surface concentration at Barbados) shows seasonality is captured.
(Ridley et al., 2012)
• Daily comparison (2000-2008) with AERONET and MODIS yields good spatial and temporal correlation in winter.
• Most events captured, however model underestimates peak AOD (R = 0.60 – 0.80 at key AERONET sites)
(MODIS dust AOD derived using Eq. A3 from Kaufman et al., 2005)
![Page 6: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/6.jpg)
Inter-annual variability downwind• Using 26 years of dust AOD (DAOD) derived
from AVHRR & MODIS satellite data (Evan & Mukhopadhyay, 2010)
• Significant decrease in dustiness of 11-20% per decade in both observations and model
![Page 7: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/7.jpg)
Is Land-Use Change Important?
• A greening of the Sahel over the past 30 years observed from AVHRR
• Varying vegetation alters emissions by only 1-5%• Accounting for dynamic vegetation may be more important in other
regions
• Using a relaxed source map modulated by surface bareness from AVHRR NDVI (Koven et al., 2006; Kim et al., 2013)
Sahel
![Page 8: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/8.jpg)
What causes the variability?
• Large scale changes in source wind account for a 30% of the variance in AOD downwind
Cause of variance in winter dust AOD
Region Source 10-m wind speed
Coastal Africa 31%
Barbados 32%
Cayenne 26%
• North Atlantic Oscillation (NAO) shown to correlate with Atlantic dust AOD and concentration (Moulin et al., 1997; Ginoux et al., 2004)
• GEOS-Chem coastal Africa dust AOD correlates with NAO (R=0.63)
![Page 9: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/9.jpg)
Two Answers:
1. GEOS-Chem captures the trend and variability in wintertime AOD downwind
2. Land use changes do not seem to drive changes in dust (dust variability is primarily controlled by meteorology)
![Page 10: 30 years of African dust: From emission to deposition Using GEOS-Chem and MERRA to determine the causes of variability and trends David A. Ridley, Colette.](https://reader035.fdocuments.in/reader035/viewer/2022081520/56649e0c5503460f94af503e/html5/thumbnails/10.jpg)
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What about summer?
Day
s
AERO
NET
AERO
NET
AERO
NET
Mod
el
Mod
el
Mod
el>0.750.25-0.75<0.25
Day
s
AERO
NET
AERO
NET
AERO
NET
Mod
el
Mod
el
Mod
el
>0.750.25-0.75<0.25
Win
ter
Sum
mer
AOD
AOD