1. J. Redemann 1, B. Schmid 1, J.A. Eilers 2, R. Kahn 3, R.C. Levy 4,5, P.B. Russell 2, J.M. Livingston 6, P.V. Hobbs 7, W.L. Smith Jr. 8, B.N. Holben 4 1 Bay Area Environmental Research Institute, Sonoma, CA 2 NASA Ames Research Center, Moffett Field, CA 3 Jet Propulsion Laboratory, Pasadena, CA 4 NASA/Goddard Space Flight Center, Greenbelt, MD 5 Also at Science Systems and Applications Inc., Lanham, MD 6 SRI International, Menlo Park, CA 7 University of Washington, Seattle, WA 8 NASA Langley Research Center, Hampton, VA Suborbital measurements of spectral aerosol optical depth and its variability at sub-satellite-grid scales in support of CLAMS, 2001(off the central US East coast)

2. Contents 1.Motivation for studying spatial variability in AOD 2.AATS-14 aboard the UW CV-580 3.Comparisons of airborne AATS-14 to ground- based AERONET Cimel sunphotometer at COVE: implications for satellite and model validation 4.Spatial statistics of AOD off the US East Coast 5.Summary

3. Motivation 1) How comparable is a measurement at a single point in space and time to 2) What is the best strategy for validating a spatially averaged model- or satellite- derived quantity? a satellite/model grid box around it? a satellite/model grid box Δx and Δt away from it? another point measurement Δx and Δt away from it?

4. AATS-14 aboard the UW CV-580 1.Measures direct solar beam transmission @: 353.5 380.0 449.0 499.4 525.0 605.7 675.1 778.4 864.5 939.7 1019.1 1059.4 1241.3 1557.8 2139.3 nm 2.Yields: aerosol optical depth + aerosol extinction when A/C flies profiles columnar water vapor (ozone)+ water vapor (ozone) concentration when A/C flies profiles 3.Size: Telescope dome 8" OD (hemisphere) atop 5" H pedestal. (Total H: 9" above A/C skin), Inside A/C: 12" D x 18" H cylinder. 4.Weight: 131.6 lbs

5. Suborbital measurements of AOD - spatial vs. temporal variability: Temporal: Long-term Temporal: Short-term Spatial resolution Ground-based (e.g., AERONET) yesyes (15 min.)no Ship-based sunphotometer yes no?? Aircraft-based TDDR nopossibleyes 45 sec. ~ 3- 4 km Aircraft-based AATSnopossibleyes 2 sec. ~ 100-200 m

6. Location of nine low-level flight legs of the Convair-580 research aircraft in CLAMS. Green points mark locations of successful AATS-14 retrievals of AOD at flight altitudes below 80 m. Chesapeake Lighthouse

7. Statistics of AERONET vs. AATS-14 at COVE, July 10 – Aug.2, 2001

8. Chesapeake Lighthouse Dependence of mean AOD on averaging area around COVE - 1 50km 17km 6km

9. Dependence of mean AOD on averaging area around COVE - 2

10. Location of nine low-level flight legs of the Convair-580 research aircraft in CLAMS. Green points mark locations of successful AATS-14 retrievals of AOD at flight altitudes below 80 m. Chesapeake Lighthouse

11. Spatial variability in AODs (0.354, 0.499, 1.019 and 1.558 µm) Absolute differenceRelative difference [%] Characteristic length scale (CLS) for AOD: CLS  15-20km

12. Convair-580 flight tracks relative to the location of MODIS level 2 (MOD02_L2) aerosol retrieval boxes

13. Wavelength [m] AOD Comparison of spectral AODs derived from AATS-14 and MODIS (MOD04_L2) 0.3 0.5 0.7 1.2 1.6 2.0

14. MISRMODIS Scatter plot of AATS-14 and regional mean MISR AOD (standard algorithm, left) and MODIS level 2 AOD (10x10km, nadir, right)

15. Summary 1.Comparisons between airborne AATS-14 in the vicinity of and AERONET Cimel derived AOD’s directly at the Chesapeake Lighthouse show good agreement. 2.Mean AODs in the vicinity of the Chesapeake Lighthouse (COVE) vary by as much as 28% when averaged over satellite-grid size areas (17 and 50km radius) in one third of the studied cases. 3.At UV and mid-visible wavelengths, the largest absolute gradients in AOD were 0.1-0.2 per 50 km horizontal distance. In the near IR, analogous gradients rarely measured 0.05. Relative AOD variability during low-level legs of 11 research flights was in average ~30% over distances of ~50 km. 4.MODIS-AATS-14 comparisons are weakly correlated (r 2 =0.53). However, 70% of MODIS AODs are within pre-launch uncertainty estimates. 5.MISR-AATS-14 comparisons are strongly correlated (r 2 =0.94). There remains a constant offset of ~0.06. Resolution of the offset problem promises to bring MISR and AATS-14 into very good agreement. 6.A good way to validate a model or satellite AOD product is to assess and stay within the characteristic aerosol length scale (CLS, 15-20km in CLAMS), which is likely to be a function of time and validation site itself. Acknowledgements NASA EOS validation program (M. King) NASA Radiation Science Program (D. Anderson) NASA New Investigator Program (M.-Y. Wei)