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North African Dust Export: A Global 3-D Model Analysis Using MODIS, MISR, CALIPSO, and AERONET Observations

Abstract

David A. Ridley1, Colette L. Heald1

1Department of Atmospheric Science, Colorado State University

Dust Size Distribution Deposition To Amazon & Caribbean

Dust Climatology

Uncertainties In Dust Processes

Conclusions

 Mineral dust is arguably the most important aerosol in terms of the impact upon the radiation budget in equatorial regions. It is also a source of nutrients for regions such as the Amazon via long range transport. Understanding the life cycle of airborne dust particles is essential for accurate prediction of potential changes in radiative forcing and nutrient availability as the climate changes.

A synergy of ground and space-based platforms are used to understand the climatology of trans-Atlantic dust on daily to annual timescales. This is used to assess our understanding of African dust transport in the GEOS-Chem model, with focus on the sensitivity to microphysical emission and removal processes. Long-range transport of African dust is found to be highly sensitive to certain wet deposition and size distribution assumptions. A significant improvement in agreement between the model and observations of aerosol optical depth is achieved using more realistic parameterizations. Using the improved model we estimate the mass of mineral dust that is available as a nutrient source for the Amazon. This is compared with calculations based on satellite observations.

2006 Amazon Caribbean Total

DJF 1.9 0.2 28.7

MAM 6.2 2.0 43.6

JJA 4.0 16.4 66.5

SON 0.5 1.0 35.1

Total (Tg/yr) 12.6 19.6 173.9

Reducing the efficiency of wet deposition (wash out, rain out, and convective uplift removal) by 50% increases the mass of dust deposited in the Amazon by 10-30%. Therefore it is unlikely that over zealous wet deposition is entirely responsible for the discrepancy between modeled and MODIS-derived dust deposition to the Amazon Basin.

• The way sub-micron dust is treated in the GEOS-Chem model has been improved. Comparison with AERONET and MODIS observations of AOD show that better agreement is achieved over the majority of North African source regions.

• Comparison between MODIS and GEOS-Chem AOD suggests that the model is removing ~50% too much aerosol as it is transported across the Atlantic. • In GEOS-Chem, ~13Tg of dust is deposited in the Amazon Basin annually. This is similar to estimates made from the AVHRR satellite instrument (13Tg/yr), but significantly less than derived from MODIS more recently (~50Tg/yr)

• Reducing wet removal efficiency by 50% in the model increases the dust deposited in the Amazon by 10-30%. Therefore this alone cannot explain the discrepancy between dust deposition to the Amazon derived from model and satellite observations.

• Further research into the importance of the vertical distribution and the frequency of rainfall (rather than total) in the model is currently underway.

Tg/monthThe change in dust deposition when wet removal efficiency is reduced by 50% is shown for the March – May period

The model captures this seasonality with a peak in deposition in the Amazon during Spring. Over 95% of the deposition is via wet removal processes. The annual total of ~13Tg that is deposited to the Amazon in the model is in agreement with estimates made using the AVHRR satellite (Swap et al., 1992). However, this is considerably lower than the 50Tg estimated from MODIS observations more recently (Kaufman et al., 2005). The total dust exported to the Atlantic Ocean is towards the lower limit of that calculated from MODIS in the same study (240±80 Tg/yr).

African dust emissions are observed in MISR and MODIS AOD retrievals throughout the year, with clearly active key source regions, such as the Bodélé Depression. A peak in AOD is seen during the summer months (JJA) due to Saharan emissions. During these months transport pathways carry dust west towards the Caribbean, and south west towards South America during the winter.

Seasonal maps of AOD for MISR (left column) and MODIS (right column) for 2006. The columns correspond to winter, spring, summer, and fall seasons (top to bottom). AOD observed at AERONET stations is plotted as filled circles. All available AERONET data is used for each season,

and only stations with over ten days of available data in the season are displayed. The black circles highlight the Bodélé Depression.

Below, the monthly mean AOD from MISR and MODIS averaged over six regions influenced by African dust are shown for each of the three year periods. The inter-annual variability is relatively small compared to the seasonal variability for the three years studied.

In both MISR and MODIS observations the seasonality in AOD can be seen to follow a distinctly different pattern above and below a latitude of 15N (top and bottom row, respectively). This is primarily due to there being a mix of biomass burning and dust aerosol during the winter and spring months in the Sahel region (south of15N latitude).

200620072008

MISRMODIS

Sub-micron dust preferentially scatters more radiation than an equal amount of coarse dust in the visible. Thus an accurate representation of the size distribution of simulated sub-micron dust is critical for reproducing observed AOD. Partitioning of the sub-micron dust into four sizes has been implemented based on observations during the SAFARI and DABEX campaigns (Highwood et al., 2003; Osborne et al., 2008). This improved partitioning has significant effects on the optical properties of the dust.

The annual difference in AOD (at 550nm) resulting from altering the partitioning of dust mass in the sub-micron sizes is shown between 30N and 30S (bottom panel). The AOD contribution from each of the seven size bins is shown for both new (red) and original (blue) dust partitioning for four locations across the African dust outflow (top panel). The locations of the size distributions are numbered on the map of AOD difference.

Key uncertainties in modeling the long range transport of dust include the soil composition, vertical distribution, and wet removal processes.

The model has been compared with MODIS AOD in dust outflow regions to determine if dust is being removed too rapidly.

For three out of four seasons the AOD decreases ~50% faster in GEOS-Chem than in MODIS along the export pathway. This indicates that aerosol is being removed at a faster rate in the model than observations suggest.

Latitude

AO

D

Longitude

MODIS

GEOS-Chem

Mgeos /Mmod = 0.9 Mgeos /Mmod = 1.6

Mgeos /Mmod = 1.5

Mgeos /Mmod = 1.6

Latitude Latitude Latitude

-0.5 -0.25 0 0.25 0.5The seasonally averaged AOD along transects are shown for MODIS (solid) and GEOS-Chem (dashed) in

the top panels. The bottom panel shows the difference in AOD between GEOS-Chem and MODIS for the summer season. The difference between GEOS-Chem and AERONET are shown as circles on the map.

The arrow denotes a typical transect direction, although this varies with time of year.

Several studies have shown that a significant amount of African dust is transported to the Amazon, primarily between March and May. It has been estimated that 50Tg of mineral dust would be required, annually, to maintain the nutrient levels measured in the Amazon rainforests (Swap et al., 1992).

The total amount of dust mass deposited (Tg/gridbox) during the March – May period through both wet and dry processes. The boxes show the regions enclosed when claculating the deposition to the Caribbean and Amazon.

The AOD is reduced by up to 0.15 annually with the new sub-micron dust partitioning. This generally gives better agreement with both AERONET and MODIS close to source (characterized by a reduction in bias and slight increase in correlation).

Time series of monthly AOD are shown for the 2006-2008 period at six AERONET sites heavily influenced by African dust aerosol. Observations are shown for AERONET (black circles), MODIS (red crosses), and

MISR (blue diamonds) all sampled to the same days. The AOD for the original (dashed line) and modified (solid line) version of GEOS-Chem is shown. The

GEOS-Chem AOD solely from dust is also shown for the original (orange) and modified (yellow) versions.

daridley@engr.colostate.edu

The monthly average aerosol plume height is shown for March (top) and June (bottom) of 2006. Only plumes over 1km high are included. Extinction profiles (left panel) are shown at the source (black) and downwind (blue) for both months.

The long range transport of dust is dependent on plume height. The model is able to show seasonality in the vertical distribution of the dust and descending of the plume during transport over the Atlantic Ocean.

We are currently using CALIPSO satellite lidar observations to understand, quantitatively, how well the vertical distribution is characterized by the model.

March 2006

June 2006