Airborne measurements of spectral direct aerosol radiative forcing a new aerosol gradient method app

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Airborne measurements of spectral direct aerosol
radiative forcing - a new aerosol gradient method
applied to data collected in INTEX/ITCT, 2004
J. Redemann (1), P. Pilewskie (2), J. Livingston
(3), S. Howard (1), P. Russell (4),
B. Schmid (1), J. Eilers (4), M. Wendisch
(5)(1) Bay Area Environmental Research
Institute, Sonoma, USA, (2) University of
Colorado, Boulder, USA, (3) SRI International,
Menlo Park, USA, (4) NASA Ames Research Center,
Moffett Field, USA, (5) Institute for
Tropospheric Research, Leipzig, Germany
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Contents

• A quick overview of ICARTT/INTEX/ITCT
• Description of the airborne instrumentation used
  in this study
• Methodology for an observationally-based estimate
  of aerosol radiative effects
• Comparison to other methods
• Summary

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ICARTT International Consortium for Atmospheric
Research on Transport Transformation
J31
ITCT NOAA
INTEX NASA
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Jetstream-31 (J31) in ITCT-INTEXIntercontinental
Transport and Chemical Transformation-Intercontine
ntal Chemical Transport Experiment
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Methodology for radiative effect studies from AOD
gradients
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Methodology for studying aerosol radiative
forcing from horizontal AOD gradients

• Measure simultaneous change in spectral aerosol
  optical depth (AATS-14) and spectral net
  irradiance (SSFR) ? DFnet/DAOD aerosol
  radiative forcing efficiency
• Observationally-based estimate of aerosol
  radiative effect
• Advantage over ground-based methods
  quasi-instantaneous because of short horizontal
  distances
• Need to check (and correct for) effects of
  changing solar zenith angle and changing column
  water vapor contents

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J31 Flight 12, 21 July 2004
Slope yields measure of forcing efficiency, i.e.
forcingf(AOD)
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Full spectrum of absolute and relative forcing
efficiency
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Summary of 14 gradient cases
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R2-statistic univariate vz. multivariate
regressor model
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Conversion from instantaneous to 24h-average
forcing efficiency
350-700nm net flux change for 10 ICARTT cases,
assuming ssa0.9-1, g0.65-0.7
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Previous methodologies for aerosol radiative
forcing studies
Flux 400-700nm Wm-2
GMT on Day 106
Flux 400-700nm Wm-2
AOD (500nm)
Bush and Valero, 2003 ACE-Asia
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Comparison of 24h-avg. forcing efficiencies at
the surface to previous work
Converted from reported value at solar noon
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Summary

• In INTEX/ICARTT/ITCT, we observed a total of 14
  horizontal AOD gradients, with 10 gradients well
  suited for an analysis using the gradient forcing
  method.
• More than half of the AOD gradients (at a
  wavelength of 499 nm) were greater than 0.1 and
  extended over distances less than 40 km.
• We found Dcos(q)/Dt1 Dt500nmgt0.05 to be
  necessary criteria for the gradient forcing
  method.
• Within the 10 case studies we found a high
  variability in the derived instantaneous aerosol
  forcing efficiencies (forcing per unit optical
  depth) for the visible wavelength range
  (350-700nm), with a mean of -79.6Wm-2 and a
  standard deviation of 21.8Wm-2(27).
• The mean instantaneous forcing efficiency for the
  visible plus near-IR wavelength range
  (350-1670nm) was derived to be -135.3Wm-2 with a
  standard deviation of 36.0Wm-2(27).
• An analytical conversion of the instantaneous
  forcing efficiencies to 24h-average values based
  on a 2-stream approximation yielded
  -45.813.1Wm-2 (meanstd) for the visible and
  -82.923.1Wm-2 (meanstd) for the visible plus
  near-IR wavelength range, respectively.