The Fate of Saharan Dust Across the Atlantic and Implications for a Central American Dust Barrier

Transcription

1 The Fate of Saharan Dust Across the Atlantic and Implications for a Central American Dust Barrier Ed Nowottnick 1, Peter Colarco 1, Arlindo da Silva 2, Dennis Hlavka 3, and Matt McGill 3 1 Climate and Dynamics Branch, NASA GSFC, Greenbelt, MD 2 Global Modeling and Assimilation Office, NASA GSFC, Greenbelt, MD 3 Mesoscale Atmospheric Processes Branch, NASA GSFC, Greenbelt, MD

2 Motivation: CPL Total Attenuated Backscatter (532 nm) during NASA TC 4 field campaign on 7/19/2007 PACIFIC CLOUDS DUST CARIBBEAN Is this barrier a real, persistent feature? If so, what is the cause of this barrier?

3 MODIS Aqua and Terra Seasonal Climatology ( ) The barrier is most persistent during the summer GOAL: Use a global aerosol transport model to understand the cause of the barrier

4 The NASA GEOS 5 Modeling System An atmospheric general circulation model and assimilation system RESOLUTION: 4 5, 2 2.5, , , horizontal 72 hybrid sigma levels in the vertical MODES: climate, assimilation, and replay AEROSOLS: Goddard Chemistry, Aerosol, Radiation, and Transport (GOCART) model. Dust, Sea salt, Sulfate, Black and Organic Carbon. DUST: 5 radius transport bins: 0.1 1, 1 1.8, 1.8 3, 3 6, 6 10 μm. Parameterizations for emission, sedimentation, turbulent deposition, large scale and convective scavenging (Chin et al., 2002). Optics from Global Aerosol Dataset (Kopke, 1997). SIMULATION SETUP: replay for the TC 4 time period (July 2007)

5 July 2007 GEOS 5 Comparison to MODIS GEOS 5 captures the barrier, but not as well defined when compared to MODIS!

6 Controls on Saharan Dust During Transport Is the barrier due to transport, loss, or a combination? Define vertically integrated dust mass continuity equation: q (P L) Q t (1) (2) (3) where qis the column dust load and Q is the vertically integrated dust mass flux: q z top z 0 air dz Q z top z 0 z top z 0 air u dz i air v dz j where is dust mixing ratio, uand vare the east west and north south wind speeds at each model level. The dust continuity equation has 3 terms: the storage term (1), the production loss term (2), and the transport term (3). Accumulation within a column results from net P L and dust import/export from transport.

7 Controls on Saharan Dust During Transport q (P L) Q t P L and transport terms align

8 Significance of Loss Processes Convective scavenging dominates over the barrier region but our representation of the dust barrier was not as sharp when compared to observations.

9 Sensitivity Analysis Four sensitivity simulations considered: 1. Doubled dust convective scavenging efficiency 2. Doubled dust convective scavenging efficiency, increased large scale dust scavenging efficiency (treated as a hydrophilic aerosol) 3. No large scale scavenging of dust 4. No wet removal of dust Goal: Compare the roles of transport and loss in establishing the Central American dust barrier for each simulation

10 Results: Loss Processes vs. Transport Best match achieved when dust treated as a hydrophilic aerosol. Barrier not present when convective scavenging was not simulated.

11 Results: Loss Processes vs. Transport

12 Results: Loss Processes vs. Transport Experimen t/satellite Net North ward Mass Transport (Tg) and Barrier Contrib ution (%) Net Mass Loss from Remov al (Tg) and Barrier Contrib ution (%) 80 W, 90 W, and Net Change in Westward Transport (Tg) Mass Barrier Efficienc y Total AOT Coarse Mode AOT 1. Baseline % % Doubled Convec tive Scaveng ing % % Wet Remov al Treated As Other Aerosols % % No Large Scale Scaveng ing % % No Wet Remov al % % MO DIS-Terra Loss processes are twice as significant in establishing the barrier as transport.

13 Conclusions Using GEOS 5, controls on dust transport were explored during the TC 4 field campaign: The baseline simulation produced a weaker representation of the Central American dust barrier Subsequent sensitivity studies suggested that the wet removal of dust aerosols be treated the same as hydrophilic aerosols as it led to a better representation of the barrier Component analysis showed that both transport and loss processes have a role in defining the barrier Loss processes had about twice the impact as transport processes for best case scenario Of the wet removal processes, convective scavenging had the greatest contribution towards the barrier The barrier would not exist without removal by convective scavenging Future Work: Continue analysis will on longer time scales to determine variability in the roles of scavenging/transport.

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15 Central American Dust Barrier Case Study: 7/19 Is the barrier due to transport, loss, or a combination?

16 Central American Dust Barrier Case Study: 7/19 GEOS 5 captures the vertical and horizontal extent of the dust event.