Sensitivity of cloud droplet nucleation to kinetic effects and varying updraft velocity

1. Sensitivity of cloud droplet nucleation to kinetic effects and varyingupdraft velocity Ulrike Lohmann, Lisa Phinney and Yiran Peng Department of Physics and Atmospheric Science Halifax, Nova Scotia, Canada Richard Leaitch Meteorological Service of Canada

2. Adiabatic parcel model (Leaitch et al. 1986) RH Clouds 100% Air parcel Air parcel • 98 size bins • Single mode ammonium sulfate aerosol based on measurements from Nova Scotia, Canada • Parcel model starts at RH=99%, T=10°C, p=800 hPa

3. ARG parameterization • Cloud droplet activation parameterization from Abdul-Razzak and Ghan (ARG) [2000] based on Köhler theory sulfate sea salt

4. Cloud droplet number versus aerosol concentration from RACE data (North Atlantic) and ARG parameterization

5. Effect of varying updraft velocity

6. Wet radius R from parcel model PM, critical radius Rcrit, and liquid water content LWC from PM and calculated from droplets at their equili-brium size LWCequi as a function of dry radius aminPM aminARG

7. Cloud droplet number conc. versus aerosol conc. Rm = 0. 035 mm s = 1.35 V = 10 cm s-1 amin amin V = 50 cm s-1 V = 200 cm s-1

8. Cloud droplet number conc. versus aerosol conc. Rm = 0. 065 mm s = 1.54 V = 10 cm s-1 V = 50 cm s-1 V = 200 cm s-1

9. Cloud droplet number vs. sulfate aerosol number 800 SO only (rm=0.08 m) N 600 d 4 400 mode (low windspeed: Nss1= 6 cm-3, Nss2= 1 cm-3) SO N 200 d 0 4 100 1000 10000 SO mode (high windspeed:Nss1= 72 cm-3, Nss2= 4 cm-3) N i) 10 cm s-1 d 4 2500 NaCl mode 1&2 (high windspeed, rm,1=0.1 m rm,2=1 m) N 2000 d 1500 1000 500 0 100 1000 10000 ii) 50 cm s -1 N SO only d 4 SO mode (low windspeed) N d 4 N SO mode (high windspeed) d 4 N d Parcel Model ARG parameterization Parcel Model b) a) ARG Parameterization 800 Nd [cm-3] 600 400 200 0 Na [cm-3] 100 1000 10000 10 cm s-1 10 cm s-1 i) 10 cm s -1 2500 2000 1500 1000 ` 500 0 100 1000 10000 50 cm s-1 50 cm s-1 ii) 50 cm s -1 8000 8000 6000 6000 4000 4000 2000 2000 0 0 100 1000 10000 100 1000 10000 iii) 200 cm s -1 200 cm s-1 200 cm s-1 iii) 200 cm s -1

10. Relative contribution of sulfate particles to the total cloud droplet number concentration (Ndso4/Nd) for the two sea salt scenarios.

11. Reduction in radiative forcing (DF) due to sulfate aerosols that results from using the ARG parameterization as compared to the parcel model (PM) : F = 1/3 Ac(1- Ac)FoTa2 N/N Ac=0.5 (cloud albedo) Fo=343 Wm-2 (incident SW radiation) Ta=0.76 (SW atmospheric transmittance above cloud)

12. Reduction in radiative forcing (DF) due to sulfate that results from using the ARG parameterization to calculate Nd as compared to PM for sulfate and 2 sea salt modes that arise from 5 m/s winds

13. Reduction in radiative forcing (DF) due to sulfate that results from using the ARG parameterization to calculate Nd as compared to PM for sulfate and 2 sea salt modes that arise from 17 m/s winds

14. Part II: Effect of varying updraft velocity on CDNC Using ACE2 data of Tenerife, June/July 1997

15. Vertical velocity spectra during ACE2

16. Aerosol size distributions during ACE2

17. Effect of varying updraft velocity on CDNC

18. Conclusions • Using a representative updraft velocity instead of its probability density function overestimates CDNC • Abdul-Razzak&Ghan (ARG) parameterizationunderpredicts cloud droplet number concentration (CDNC) because of an overestimation of the condensation rate onto larger particles, especially for V < 50 cm s-1 and Na > 500 cm-3. • Relative contribution of sulfate aerosols to CDNC is lower for ARG parameterization than for parcel model in low to moderate updrafts. • This leads to an underestimation of 6-15 W m-2 for the first indirect aerosol effect for typical marine clouds. •  one solution: there is a new cloud droplet activation parameterization by Nenes and Seinfeld (poster # 439), that takes kinetic effects properly into account