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Retrieving aerosol optical depth above clouds using OMI near UV observations

Retrieving aerosol optical depth above clouds using OMI near UV observations. Omar Torres NASA Goddard Space Flight Center, 613.3 Greenbelt, MD Hiren Jethva Department of Atmospheric and Planetary Sciences Hampton University, Hampton, Va. Pawan K. Bhartia

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Retrieving aerosol optical depth above clouds using OMI near UV observations

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  1. Retrieving aerosol optical depth above clouds using OMI near UV observations Omar Torres NASA Goddard Space Flight Center, 613.3 Greenbelt, MD Hiren Jethva Department of Atmospheric and Planetary Sciences Hampton University, Hampton, Va. Pawan K. Bhartia NASA Goddard Space Flight Center, 613.3 Greenbelt, MD 2011 Aerosol Update NASA-GSFC March 29, 2011

  2. -Carbonaceous aerosols (OC/BC) are frequently observed above clouds. -The TOA radiative forcing of absorbing aerosols changes from cooling for cloud free conditions to warming when located above clouds. -It is estimated that BC above low clouds accounts for 20% of the global burden but 50% of the forcing [Zarzycki and Bond, GRL, 2010] -The Aerosol Index (AI) is a qualitative proxy of aerosol absorption. It unambiguously detects the presence of absorbing aerosols (carbonaceous and desert dust aerosols) above clouds -A retrieval scheme to quantify the absorbing aerosol load above clouds is discussed.

  3. Sensitivity Analysis (1) Effect of Absorbing Aerosols above clouds on TOA reflectance (388 nm) Cloud only Cloud and aerosol (ssa: 0.94) Cloud and aerosol (ssa: 0.85)

  4. Sensitivity Analysis (2) Effect of Absorbing Aerosols above clouds on measured Aerosol Index (388 nm) AAE=2.7 (OC) AAE=1.9 (OC) AAE=1.0 (BC) No aerosols -OC-containing aerosols produce a larger signal -OC-containing aerosols are a more realistic representation of carbonaceous aerosols(Jethva and Torres, ACP 2010, submitted)

  5. Retrieval of AOD Above Clouds from OMI Observations -OMI detects the unambiguous AI signal of absorbing aerosols above clouds. Observed AI above clouds depends mainly on: -Aerosol Optical Depth -λ-dependent aerosol SSA (angstrom absorption exponent, AAE) -Cloud Optical Depth -Aerosol-Cloud separation (second order effect) Retrieval Approach: -Cloud effects must be accounted for since the AI of aerosols above clouds is enhanced by cloud brightness. • -Assume Geometric Cloud Fraction: 1.0 • (R388 > 30%, AI > 1.0) • -Use retrieved SSA from cloud free case. • -Assume AAE • -Obtain Aerosol Optical Depth (AOD) and • Cloud Optical Depth (COD) Absorbing Aerosol Index 388 nm reflectance

  6. Benchmark case: OMI COD retrieval when no aerosols are present No aerosols above cloud OMI COD No aerosol-case, Oct 13, 2006 MODIS COD

  7. A-train Observations of aerosols above clouds OMI August 4, 2007 CALIOP MODIS CALIOP MODIS OMI

  8. Case Study 1: August 4, 2007 Low COD SSA (388 nm) = 0.88 (12ºS,10E) AAE = -2.2 (Kirchstetter et al, 2004) High AOD High COD Low AOD OMI retrieved COD-AOD are not generally correlated

  9. Comparison of OMI and MODIS Cloud Optical Depth Retrievals August 4-2007

  10. Correlation between Aerosol Index and Aerosol Optical Depth August 4, 2007

  11. Case study 2: September 7, 2008 SSA (388 nm) = 0.90 (11ºS,12E) AAE = -2.2 (Kirchstetter et al, 2004) AI AOD COD

  12. September 7, 2008: Correlation Analysis MODIS COD VS OMI COD

  13. Concluding Remarks • -For overcast pixels, OMI Aerosol Index above clouds depends on aerosol optical • depth and single scattering albedo, and cloud optical depth. • -Weak dependence on aerosol-cloud separation is observed. • -A retrieval approach to obtain COD and AOD above clouds from OMI near-UV • observations has been developed. • Preliminary results are encouraging: • No AOD-COD correlations are observed. • OMI retrieved COD is correlated to MODIS COD • OMI COD up to 20% larger than MODIS product.

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