Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Airborne measurements of spectral direct aerosol radiative forcing - a new aerosol gradient method applied to data collected in INTEX/ITCT/ICARTT, 2004 J. Redemann (1), P. Pilewskie (2), P. Russell (4), J. Livingston (3), S. Howard (1), J. Pommier (1), B. Schmid (1), W. Gore (4), 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

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Contents  Description of the airborne instrumentation used in this study  Methodology for an observationally-based estimate of aerosol radiative effects  Comparison of results to ground-based methods  Summary

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Jetstream-31 (J31) in ITCT-INTEX Intercontinental Transport and Chemical Transformation-Intercontinental Chemical Transport Experiment

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Methodology for radiative effect studies from AOD gradients

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 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) →  F net /  AOD = 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

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005

J31 Flight 12, 21 July 2004 Slope yields measure of forcing efficiency, i.e. forcing=f(AOD)

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005

Full spectrum of absolute and relative forcing efficiency

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Summary of 14 gradient cases

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 R 2 -statistic – univariate vz. multivariate regressor model

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005

nm net flux change for 10 ICARTT cases, assuming ssa=0.9-1, g= Conversion from instantaneous to 24h-average forcing efficiency

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Bush and Valero, 2003 ACE-Asia Previous methodologies for aerosol radiative forcing studies Flux nm [Wm] Flux nm [Wm -2 ] GMT on Day 106 AOD (500nm) Flux nm [Wm] Flux nm [Wm -2 ]

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Comparison of 24h-avg. forcing efficiencies at the surface to previous work surface [Wm -2 ] Spectral rangeregioncomments This work 45.8± ± – 0.7  m 0.35 – 1.67  m Gulf of Maineairborne Kim et al., (non-dust) (dust) 0.3 – 4.0  m Korean peninsula + East China Sea surface Kim et al., – 4.0  m East Asiasurface Meywerk and Ramanathan, * (84 at noon) 0.4 – 0.7  m East Asia, INDOEX-1 ship Bush and Valero, ± ± – 0.7  m 0.3 – 3.81  m Kaashidhoo, INDOEX surface Bush and Valero, ± ± – 0.7  m 0.3 – 3.81  m Gosan, ACE-Asia surface * Converted from reported value at solar noon

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Summary 1.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. 2.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. 3.We found  cos(  )/  «1 &&   nm >0.05 to be necessary criteria for the gradient forcing method. 4.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 ( nm), with a mean of -79.6Wm -2 and a standard deviation of 21.8Wm -2 (27%). 5.The mean instantaneous forcing efficiency for the visible plus near-IR wavelength range ( nm) was derived to be ‑ 135.3Wm -2 with a standard deviation of 36.0Wm -2 (27%). 6.An analytical conversion of the instantaneous forcing efficiencies to 24h- average values based on a 2-stream approximation yielded -45.8±13.1Wm -2 (mean±std) for the visible and -82.9±23.1Wm -2 (mean±std) for the visible plus near-IR wavelength range, respectively.

Redemann, ICARTT workshop, Durham, NH, Aug.9-12, 2005 Acknowledgements Funding for the J-31 aircraft modifications through NASA RSP, TCP + SSP, and for ICARTT measurements and data analysis through NOAA ACCP are gratefully acknowledged.