From Cooling to Heating – Influence of Aerosol Sky Cover on the Critical Single Scatter Albedo

1. From Cooling to Heating – Influence of Aerosol Sky Cover on the Critical Single Scatter Albedo Erez Beit-Halachmi2 Carynelisa Erlick-Haspel1 Nir Shaviv2 • Racah Institute of Physics, Hebrew University of Jerusalem • Institute of Earth Sciences, Hebrew University of Jerusalem

2. Aerosol Single Scattering Parameters In global models, aerosol optical properties are generally characterized by three parameters: • optical depth (t), the e-folding parameter of intensity • single-scattering albedo (SSA; ), the ratio of the scattering coefficient to the total extinction coefficient • asymmetry factor (g), representing the strength of forward versus backward scattering

3. The Critical Single Scattering Albedo [Hansen et al., 1997] Aerosol impact on global mean surface temperature shifts from heating to cooling at  * ~ 0.86. • For  < ~0.86, aerosols absorb enough to warm the atmosphere and surface. • For  > ~0.86, aerosol scattering dominates. The critical SSA has been shown to be sensitive to: • the aerosol vertical profile • the aerosol distribution over land and ocean • feedback on clouds

4. Goal of This Study • Perform a sensitivity study with respect to aerosol sky cover.

5. Methods – Radiative Convective Model [Hu, 1994] • allows elaborate treatment of the transfer of radiation while accounting for vertical atmospheric motions in a globally averaged sense – well suited to sensitivity studies • radiative fluxes: discrete ordinates method • convection: convective adjustment, pseudo-adiabatic lapse rate is used as the "critical" lapse rate • cloud optical parameters calculated by drop equivalent radius • aerosol parameters are prescribed - direct aerosol effect only

6. Typical Temperature Profile

7. Methods – Obtaining the Global Average Temperature three scales of variation in aerosol sky cover: • Small scale - average the optical depths of the “polluted” and “clean” patches. • Large scale - average the equilibrium temperature resulting from areas with full aerosol cover and no aerosol cover • Local scale - average the radiative fluxes resulting from “polluted” and “clean” areas every model iteration

8. An illustration of the averaging methods An illustration of the averaging methods (1) (1) (2) (2) (3) (3) Aerosols Aerosols Aerosols Aerosols Aerosols Aerosols Clear sky Clear sky Clear sky Clear sky t t t 0 0 0 t t t 0 0 0 T( T( T( ) ) ) T( T( T( ) ) ) F( F( F( ) ) ) F( F( F( ) ) ) T T T T T T T T avg avg avg avg avg avg avg avg Methods – Obtaining the Global Average Temperature

9. Results – t Averaged, low aerosols

10. Results – All Three Averages, low aerosols differ by only 1%

11. Results – All Three Averages, high aerosols

12. Results • The results were found to be linear with optical depth, up to values exceeding the global mean. • The critical SSA was determined depending on the amount of sky covered by aerosols. Only with a sky cover of at least 50% did this transition from heating to cooling occur • All three averaging methods gave similar results. The differences in  * were around 1% • For the same sky cover, high altitude aerosols have a lower critical SSA

13. Conclusions • The critical SSA is highly sensitive to aerosol sky cover, varying from 0.88 to 1.00. This is important for legislating policies regarding particle emissions according to particle type. • The critical SSA obtained is within current measurements. A small change in the SSA of particles we emit can switch the climate direction between cooling and warming. • The global optical depth is low enough that the results are not sensitive to the assumptions made regarding obtaining the global average temperature.

14. Further Work • Couple parallel latitude runs with meridional energy fluxes to create a 2-D radiative convective model.