# Presentation: 659151

## 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

## 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

## Jetstream-31 (J31) in ITCT-INTEXIntercontinental Transport and Chemical Transformation-Intercontinental Chemical Transport Experiment

## Methodology for radiative effect studies from AOD gradients

## Methodology for studying direct aerosol radiative forcing from horizontal AOD gradients 

- Measure simultaneous change in spectral aerosol optical depth (AATS-14) and spectral net (down minus up) irradiance (SSFR) →                  		*E =* ***F**net **/ ****AOD * = aerosol radiative forcing efficiency
- E = E (wavelength, solar zenith angle, surface albedo, altitude of the aerosol layer, atmospheric composition and aerosol optical properties) ;
- Units: spectral - [W m-2 nm-1]   broadband - [W m-2]
- Here E = Einst./24h, surface, dark ocean (, 350-700nnm, 350-1670nm)
- Also  *e = E / F**down*
- Observationally-based estimate of direct aerosol radiative forcing
- 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

## Slide 6

## J31 Flight 12, 21 July 2004

- Slope yields measure of forcing efficiency, i.e. forcing=f(AOD)

## Slide 8

## Full spectrum of absolute and relative forcing efficiency

## Summary of 14 gradient cases

## R2-statistic – univariate vz. multivariate regressor model

- R2-statistic – univariate vz. multivariate regressor model

## Slide 12

## Slide 13

## Best-fit aerosol single scattering albedo  (avg. 350-700nm) derived using irradiance and AOD gradients

- Best-fit aerosol single scattering albedo  (avg. 350-700nm) derived using irradiance and AOD gradients

- July 21-23, 2004:
- Alaskan forest fires

## Spectral relative forcing, e=E/Fdown for all 10 ICARTT cases

## Conversion from instantaneous to 24h-average forcing (efficiency)

- 350-700nm net flux change for 10 ICARTT cases, assuming ssa=0.9-1, g=0.65-0.7

## Comparison of 24h-avg. forcing efficiencies at the surface to previous work

| Author | 24h-ARFE @ surface
[Wm-2] | Spectral range | region | comments |
| --- | --- | --- | --- | --- |
| This work
 | 45.8±13.1
82.9±23.1 | 0.35 – 0.7m
0.35 – 1.67m | Gulf of Maine | airborne |
| Kim et al., 2005 | 69 - 90 (non-dust)
91 - 104 (dust) | 0.3 – 4.0m | Korean peninsula + East China Sea | surface |
| Kim et al., 2005 | 55 - 76  | 0.3 – 4.0m | East Asia | surface |
| Meywerk and Ramanathan, 1999 | 48* (84 at noon)  | 0.4 – 0.7m | East Asia, INDOEX-1 | ship |
| Bush and Valero, 2002 | 38.5±4.0
72.2±5.5 | 0.4 – 0.7m
0.3 – 3.81m | Kaashidhoo, INDOEX | surface |
| Bush and Valero, 2003 | 42.2±4.8
73.0±9.6 | 0.4 – 0.7m
0.3 – 3.81m | Gosan,
ACE-Asia | surface |

- * Converted from reported value at solar noon

## 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 cos()/«1 && nm>0.05 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 -45.8±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.
- An estimation of the mid-visible aerosol single scattering albedo based on the irradiance and AOD gradients yielded generally lower values (~0.9) during the early parts of the experiment, broadly consistent with trajectory analysis and satellite imagery of Alaskan forest fire smoke.

## 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.