# Presentation: 904040

## Dust Aerosol Radiative Effects from Terra and Aqua

- Thomas A. Jones*,
- Sundar A. Christopher,
-  Jianglong Zhang,
- Lorraine Remer
*October 31, 2006*

## Outline

- Why are dust aerosols important?
- Goals of this research
- Data
- Assumptions
- Separation of AOT components
- Dust Radiative Effect
- Terra vs. Aqua differences
- Continuing Research

## Importance of Dust Aerosols

- Naturally occurring dust aerosols are major contributors to the Earth-atmosphere system
- Annual dust aerosol emissions range from 1000-3000 Tg
- Dust aerosols generally originate over desert regions such as the Sahara
- Atmospheric transport allows dust to spread far away from its source regions
- Uncertainty exists as to the contribution of land use change and anthropogenic aerosols to overall dust loading.

## Effect of Dust Aerosols

- Over open oceans, dust aerosols increase reflectivity, reducing incoming TOA solar (shortwave) flux
- A cooling effect
- Dust aerosols also absorb and emit outgoing longwave flux, but emit at colder temperatures than the background ocean
- A warming effect
- Previous research indicates that SW cooling generally exceeds LW warming, but the magnitude of the LW effect is largely unknown

## Goals

- Use satellite observations of aerosol optical thickness (AOT) and fine mode fraction (FMF) to determine the proportion of AOT due to dust aerosols
- Use satellite-derived TOA incoming SW and outgoing LW fluxes to determine the effect of dust AOT on the energy budget
- Compare SW and LW effects to produce a net dust radiative effect
- Does LW warming significantly offset SW cooling?
- Since the greatest concentration of dust aerosols occurs over the Atlantic ocean, west of Africa, this research was initially restricted to that domain.

## Data

- CERES Single Satellite Footprint (SSF)
- Terra FM1, Edition 2B data files
- CERES reports SW and LW TOA radiance at a ~20 km resolution which are converted to fluxes using ADMs (Zhang et al. 2005a)
- Data collected for June, July, and August between 2000 and 2005
- Spatial domain limited to tropical Atlantic (10-60°W, 0-30°N)
- MODIS
- Reports aerosol optical thickness at 0.55 m
- Combined with CERES footprint data using a point spread function
- Raw MODIS AOT available at higher resolution

## Assumptions

- Only over-ocean data considered.
- Pixels over land or near coast are removed
- Only clear-sky pixels considered
- MODIS Cloud Fraction < 1.0 %
- CERES Clear sky percent > 99.0 %
- Removes ~95% of total data
- Dust Radiative Effect statistics calculated from only data where dust AOT is > 0
- Dust AOT only valid where 0.3 ≤ FMF ≤ 0.9

## Separation of AOT

- We use Kaufman et al. (2005) technique to separate observed AOT into maritime, anthropogenic, and dust components
- Simple mathematical function
- Separates AOT components using assumed FMF characteristics of each
- Fmari = 0.3, Fdust = 0.5, Fanth = 0.9
- Assumes maritime optical thickness is a function of surface wind speed
- ma = 0.007W + 0.02

## Separation of AOT

- Kaufman et al. Dust AOT Equation:
- Uncertainties and limitations:
- Observed FMF bounded between 0.5 and 0.9
- For low observed AOT, this equation can return a negative value for dust AOT
- Dust AOT set equal to 0 in this case
- Kaufman et al. estimates a 15% uncertainty in component AOT using this technique
- Has a downstream effect on component radiative effect uncertainty

## Dust AOT Map

- Highest dust concentration

## Calculating Radiative Effect of Dust Aerosols

- Dust Radiative Effect is calculated by subtracting SW and LW fluxes containing aerosols (Faero) from a clear-sky, aerosol-free background (Fclr)
- This difference is then scaled by the ratio of dust AOT to total AOT to derive the component of forcing from dust
- The clear-sky, aerosol free background is derived by relating pixels where AOT < 0.2 to SW flux and deriving what the AOT=0 flux value should be.
- No adjustment for LW (Use AOT < 0.1)

## AOT-Flux relationship

- Shortwave

- Longwave

- SWFclr

## Diurnal and Sample-Bias Adjustments

- Instantaneous radiative effect numbers do not tell the whole story
- Terra only observes AOT and flux at ~10:30 local time
- Diurnal variability not sampled
- Use “diurnal adjustment” functions developed by Remer and Kaufman (2005)
- Diurnal effect ≈ instantaneous effect ÷ 2.0
- The CERES footprint is much larger than the MODIS footprint
- Due to clear-sky assumption, DRE is biased toward clear-sky regions
- AOT from the MOD08 data set were used to derive MODIS-only dust AOT, which is higher than the CERES- footprint dust AOT
- This difference (0.045) is used to adjust DRE upward

## Statistics Table

- Radiative effect values include diurnal and sample bias adjustments.

- Spatial and temporal domains are not an exact match.

- Uncertainty is ~20%

## Adjusted SWRE:

**Adjusted SWRE:**
- Maximum cooling corresponds to location of highest dust aerosol concentration

**Adjusted LWRE:**
- Correlation with AOT concentration much less
- Cooling along ITCZ

## DRE as a Function of Dust AOT

- SWeff = -33.8 Wm-2 -1

- LWeff = 2.3 Wm-2 -1

## Conclusions

- Dust aerosols have a measurable impact on both SW and LW fluxes
- For this region, almost all NRE can be attributed to dust aerosols
- The LW warming offsets SW cooling by approximately 15%
- A significant number
- Provides framework for global analysis

## Terra vs. Aqua DRE

- CERES SSF data from Terra and Aqua satellites were compared to examine the effect of different overpass times on AOT and DRE measurements
- 2003-2005, June, July, and August
- FM1 and FM3 instruments used
- Same Atlantic ocean domain as before
- Aqua satellite overpass time is approximately 3 hours after Terra (13:30 vs. 10:30 local time)

## AOT histogram

- Aerosol Optical Thickness

- Adjusted NET
- Dust Radiative Effect

- Terra > Aqua

## SWRE vs. Dust AOT

- SWRE - AOT relationship for Terra and Aqua is the same

## Terra – Aqua Net Radiative Effect

## Terra-Aqua Conclusions

- Terra AOT are slightly higher than corresponding Aqua AOT throughout this domain
- Differences are small and randomly distributed
- Differences in adjusted net dust radiative effect are small.
- AOT-SWRE relationship is the same
- Sample bias adjustment is larger for Aqua

## Ongoing Research

- Global Dust
- Use 2000-2001 CERES SSF data for global determination of dust radiation effect
- Also study sensitivity of FMF thresholds on component radiative effect results
- Averaging
- Performing an analysis of the statistical properties and assumptions inherent reported DRE values.

## References

- T.A. Jones and _**S.A. Christopher**_, Is the top of atmosphere Dust Net Radiative Different Between Terra and Aqua?, Geophysical Research Letters, **submitted, September, 2006 **
_**Christopher, S.A.**_ and T. Jones, Satellite-based Assessment of Cloud-free Net Radiative Effect of Dust Aerosols over the Atlantic Ocean, Geophysical Research Letters,- **revised** September 11, 2006 - 2006GL027783R
_**Christopher, S. A**_., J. Zhang, Y. J. Kaufman, and L. A. Remer (2006), Satellite-based assessment of top of atmosphere anthropogenic aerosol radiative forcing over cloud-free oceans, Geophys. Res. Lett., 33, L15816, doi:10.1029/2005GL025535.
- H. Yu, Y. J. Kaufman, M. Chin, G. Feingold, L. A. Remer, T. L. Anderson, Y. Balkanski, N. Bellouin, O. Boucher, _**S. A. Christopher**_, P. DeCola, R. Kahn, D. Koch, N. Loeb, M. S. Reddy, M. Schulz, T. Takemura, M. Zhou, A review of measurement-based assessment of aerosol direct radiative effect and forcing,  Atmos. Chem. Phys. 6, 613-666, 2006.
- Zhang, J., _**S.A. Christopher**_, L.A. Remer and Y.J. Kaufman, Shortwave Aerosol Cloud-Free Radiative Forcing from Terra, I: Angular Models for Aerosols, Journal of Geophysical Research -Atmospheres, D10, S23, doi:10.1029/2004jd005008, 2005.
- Zhang, J., _**S.A. Christopher**_, L.A. Remer and Y.J. Kaufman, Shortwave Aerosol Cloud-Free Radiative Forcing from Terra, II: Global and Seasonal Distributions Journal of Geophysical Research -Atmospheres, D10, S24, doi:10.1029/2004jd005009, 2005.
- Anderson, T.L., R.J., Charlson, N. Bellouin, O. Boucher, M. Chin, _**S.A. Christopher**_, H.J. Haywood, Y.J. Kaufman, S. Kinne, J. Ogren, L.A. Remer, T. Takemura, D. Tanre, O. Torres, C.R.Trepte, B.A. Wielicki, D. Winker, H. Yu, A-Train strategy for quantifying direct aerosol forcing of climate: Step-wise development of an observational basis for aerosol optical depth, aerosol forcing efficiency, and aerosol anthropogenic fraction, Bulletin of the American Meteorological Society, 2005, 1795-1809.
_**Christopher, S. A**_., and J. Zhang (2004), Cloud-free shortwave aerosol radiative effect over oceans: Strategies for identifying anthropogenic forcing from Terra satellite measurements, Geophysical Research Letters, 31, L18101, doi:10.1029/2004GL020510.

## Questions

## The End

- Who am I, and why am I here?