1. Sensitivity of cloud droplet nucleation to kinetic effects and varying updraft 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) Cloud s Air parcel RH 100% 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. Cloud droplet activation parameterization from Abdul-Razzak and Ghan (ARG) [2000] based on Köhler theory ARG parameterization 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 R crit, and liquid water content LWC from PM and calculated from droplets at their equili- brium size LWC equi as a function of dry radius a min PM a min ARG

7. V = 50 cm s -1 V = 10 cm s -1 V = 200 cm s -1 R m = 0. 035  m  = 1.35 Cloud droplet number conc. versus aerosol conc. a min

8. R m = 0. 065  m  = 1.54 V = 50 cm s -1 V = 10 cm s -1 V = 200 cm s -1 Cloud droplet number conc. versus aerosol conc.

9. Cloud droplet number vs. sulfate aerosol number 0 2000 4000 6000 8000 100100010000 iii) 200 cm s Parcel Model a) 0 200 400 600 800 100100010000 i) 10 cm s b) ARG Parameterization 0 500 1000 1500 2000 2500 100100010000 ii) 50 cm s ` 0 2000 4000 6000 8000 100100010000 iii) 200 cm s 200 cm s -1 10 cm s -1 50 cm s -1 200 cm s -1 N d SO 4 only (r m =0.08  m) N d SO 4 mode (low windspeed: N ss 1 = 6 cm -3, N ss 2 = 1 cm -3 ) N d SO 4 mode (high windspeed:N ss 1 = 72 cm -3, N ss 2 = 4 cm -3 ) N d NaCl mode 1&2 (high windspeed, r m,1 =0.1  m r m,2 =1  m) Parcel ModelARG parameterization N d [cm -3 ] N a [cm -3 ]

10. Relative contribution of sulfate particles to the total cloud droplet number concentration (N dso4 /N d ) for the two sea salt scenarios.

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

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

13. Reduction in radiative forcing (  F) due to sulfate that results from using the ARG parameterization to calculate N d 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. Using a representative updraft velocity instead of its probability density function overestimates CDNC Abdul-Razzak&Ghan (ARG) parameterization underpredicts cloud droplet number concentration (CDNC) because of an overestimation of the condensation rate onto larger particles, especially for V 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 Conclusions