1. Support from NSF and Clean Air Task Force
The role of radiative-dynamic feedbacks in amplifying surface warming by Arctic stratus in polluted conditions
Tim Garrett
University of Utah
Collaborations with Chuanfeng Zhao, Melissa Maestas, Mike Zulauf, and Steve Krueger at UU
Support from NSF and Clean Air Task Force

2. Arctic Stratus at full resolution (30 m x 300 m)

4. Cloud Radiative Forcing
Most Polluted

5. F(LW) = T4 Winter – Enhanced Cloud Longwave Emissivity (+ΔT)
Thin, polluted cloud.
Better insulator. Heat is
trapped and re-emitted.
[Garrett and Zhao, Nature, 2006]
Thin, clean cloud
Poor insulator
Heat escapes
F(LW) = T4

6. Cloud emissivity depends foremost on cloud thickness
Garrett et al. (2002) JAS

7. Cloud emissivity also depends on re and potentially also Arctic pollution
Garrett et al. (2002) JAS

8. Measurements ARM remote sensing NOAA aerosol Barrow Site
ERS-Gome satellite Ozone profile
Temperature and
water vapor profiles

9. Upper haze
quartile
Clean
Polluted
Lower
haze quartile
All
Low
Cloud

10. Liquid Cloud

11. LES Modeling

12. Polluted, 44 hrs
Clean, 44 hrs
Polluted, 48 hrs
Clean, 48 hrs
Cloud forms
Polluted cloud is thicker

15. FLW
FLW
FLW

17. Summary
Seasonal pollution is associated with changes in low-level Arctic cloud properties
Smaller radii
Larger LWP too!
Extra higher longwave cloud emissivity
When polluted and cloudy, extra warming is between 10 and 20 W/m2 more than clean and cloudy

18. Summary
Increases in downwelling longwave fluxes occur in late winter and early spring, at the beginning of the melting ‘push’

19. Summary
The CRF increases are larger than would be expected from effective radius decreases alone
There may be interesting interactions between pollution and cloud dynamics, associated with enhanced cloud top radiative cooling
Cloud cover
Cloud circulations
Cloud top entrainment
Cloud forcing?