North African Dust Export and Impacts: An Integrated Satellite and Model Perspective.

D. A. Ridley1, C. L. Heald1, B. Ford 1

1 Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA

Seasonality Similar picture from MODIS and MISR

aerosol optical depth (AOD)

Both biomass and dust emissions in the Sahel during the winter season

Emissions from the Bodélé depression throughout the year, peaking during the spring season

Change in the latitude of the export with season

MISR AOD @ 550nm [2007]

GEOS-Chem Dust Modeling 2 x 2.5 resolution with 47 vertical levels

Dust emitted according to Zender et al. [2003] Dust emitted into four size bins:

0.1–1.0, 1.0–1.8, 1.8–3.0, and 3.0–6.0 µm radius

Optics improved using SHADE size distribution measurements

Mass Partitioning of Sub-Micron Dust

AOD decreased by 10-25%

Due to size dependence of scattering

NB no change in total dust mass

Equal vs realistic partitioning GEOS-Chem Annual AOD (2006)

Dust size distribution from the SHADE campaign (Highwood et al., 2003)

AOD contribution from dust size bins

Original uniform mass partitioningNew dust mass partitioning

Horizontal Structure The model captures the spatial

distribution, magnitude, and the seasonality of the AOD

Statistical comparison with both MISR and MODIS yields R>0.63 over the region shown

AOD at the Bodélé Depression underestimated by the model

Comparison with AERONET also made at eight sites over the three year period

GEOS-Chem AOD MISR AOD (@ 550nm) [2007]

Vertical StructureAerosol extinction across the Atlantic averaged over 15 deg latitudal bands [2007]

Seasonality in vertical distribution reproduced well Missing aerosol close to ocean surface – dust and marine?

Marine aerosol? Lapina et al. [2011]

Annual westward export 250±50Tg (240±80Tg from MODIS1) Deposition to Atlantic is slightly higher than other studies1,2,3,4

Deposition to the Amazon is lower than previously estimated1,5

Deposition of DustAnnual deposition of phosphorus (dust) in the Atlantic,

Caribbean, and Amazon

1 Kaufman et al., [2005]2 Ginoux et al., [2004]3 Prospero et al., [1996]4 Fan et al., [2004]5 Mahowald et al., 2005]

Tg

2006-

2008

Amazon Caribbea

n

Atlantic

DJF 3.6±2.5 0.4±0.1 56.3±24.2

MAM 7.5±1.4 3.1±1.3 51.2±7.3

JJA 5.1±0.9 19.5±2.4 67.0±3.0

SON 0.4±0.4 1.3±0.9 36.0±0.8

TOTAL

(Tg yr-1)

16.6±4.8 24.3±4.3 210.5±31.9

Annual average dust deposition totals [2006-2008]

Good correlation with Barbados deposition measurements (R=0.74) Similar monthly variability between model and observations Observations suggest more dust export during Fall season

Deposition of Dust to the Caribbean

TgBarbados dust deposition (in-situ data from J. Prospero)

Tg

Annual deposition of phosphorus (dust) in the Atlantic, Caribbean, and Amazon

    Dust Lifetime (days)

  Seaso

n

GEOS-

ChemMODIS Ratio

DJF 2006 4.4 4.3 1.0

  2007 1.8 3.3 0.5

  2008 1.8 3.4 0.5

MAM 2006 2.9 7.4 0.4

  2007 3.4 6.6 0.5

  2008 2.8 6.5 0.4

JJA 2006 3.8 5.2 0.7

  2007 5.1 7.5 0.7

  2008 4.1 14.0 0.3

SON 2006 2.1 2.7 0.8

  2007 2.2 3.4 0.6

  2008 2.2 2.0 1.1

Dust SinksEstimated dust lifetime from

seasonal AOD transects

Seasonally averaged AOD along Atlantic-outflow transects

Modeled lifetimes are typically 30-50% shorter than from MODIS

Consistently 50% shorter in Spring

MODIS (Deep Blue) AOD @ 550nm [2007]

Switching off convective removal increases deposition to Amazon by ~50%

AOD

Dust Sources

AOD

GEOS-Chem – MODIS AOD (550nm) Observed AOD (550nm) [2006]

Doubling emissions from Bodélé Depression improves agreement with MODIS and MISR

R increased by ~10% and RMS error decreased by ~35%

Dust deposited to Amazon increased by ~10%

GEOS-Chem predicts 17±5Tg/yr is deposited in the Amazon, less than half of the 50Tg/yr estimate from satellite

This contrasts with good model agreement with dust deposition measurements in Barbados (R=0.74) and westward dust export comparison with MODIS (250±50 and 240±80Tg/yr respectively)

Modeled dust loss due to wet deposition appears too rapid

A doubling of the dust from the Bodélé depression improves the agreement between the model and satellite observations

Even at the extreme the model and the satellite estimates of dust deposition to the Amazon cannot be reconciled

Conclusions

Time Series of AOD at AERONET sitesNew GEOSchemOld GEOSchemAERONETRange of obs