Aerosol-cloud-climate interactions: modeling and observations at the cloud scale Graham Feingold

1. Aerosol-cloud-climate interactions: modeling and observations at the cloud scale Graham Feingold NOAA Earth System Research Laboratory Boulder, Colorado ISSAOS 2008 AEROSOLS and CLIMATE CHANGE 22 - 26 September 2008, L'Aquila, Italy

2. Why do we care about aerosol-cloud interactions? • Planetary albedo is strongly affected by clouds • Large uncertainty in aerosol effects on albedo and radiative forcing • Larger uncertainty in aerosol effects on cloud albedo and radiative forcing Radiative Forcing

3. Drop concentration, Nd Aerosol concentration, Na How does aerosol affect clouds ? • Aerosol particles, or that component that act as cloud condensation nuclei (CCN) are a necessary ingredient of clouds • An increase in aerosol concentrations Nacauses an increase in drop concentration Nd: If nothing else changes, drops are smaller and cloud is more reflective Ramanathan et al. 2001

4. dynamics dynamics  cloud: updraft, LWP, drop activation, growth cloud  dynamics: drizzle, evaporation dynamics  aerosol: transport aerosol  dynamics: radiation cloud mphysics aerosol radiation  cloud: Surface heating drives convection; Reduced shortwave at the surface reduces clouds radiation aerosol: Aerosol absorbs, scatters Modifies heating profiles and surface fluxes radiation A Complex, Coupled System with Myriad Feedbacks • Aerosol-Cloud-Dynamics-Radiation-Chemistry-Land-surface aerosol  cloud: Activation, growth, precipitation cloud  aerosol: aq. chemistry, collection, washout, nucleation

5. Which clouds matter most ? • Low clouds provide strong shortwave forcing • Strong contrast with underlying dark ocean • Radiate at ~ same To as ocean therefore no longwave effect • High clouds (cirrus) provide longwave forcing by trapping outgoing longwave radiation • Persistent and frequently occurring clouds

6. Which clouds matter most ? • Low clouds provide strong shortwave forcing • Strong contrast with underlying dark ocean • Radiate at ~ same To as ocean therefore no longwave effect • High clouds (cirrus) provide longwave forcing by trapping outgoing longwave radiation • Persistent and frequently occurring clouds

7. Microphysical Pathways

8. Ingredients for Cloud Formation: Dynamics vs. Microphysics • Clouds are formed by dynamics • Updrafts due to convection, orographic uplift • Expansion, cooling, generation of supersaturation • Aerosol particles do not form clouds! • Aerosol particles (particularly CCN) are an essential ingredient for droplet formation • Aerosol particles modify cloud microphysical and optical properties

9. Aerosol - cloud drop - rain drop r = radius in mm n = number/litre v = fall velocity in cm s-1 Typical number concentrations: aerosol: 103 - 104 cm-3 droplets: 102 cm-3 raindrops: 10-5 cm-3 CCN: drawn 25x larger McDonald (1958) • 6 order of magnitude change in mass from typical CCN to cloud droplet! • 10 orders of magnitude mass change from cloud droplet to large raindrop! • factor of 650 increase in fall velocity (droplet to raindrop)

10. The Scope of the Aerosol  Cloud Problem • Involves complexity in both aerosol and clouds • Range of spatial scales • Aerosol particles 10s – 1000s nanometres • Cloud drops/ice particles: mm – cm • Cloud scales: ~ 102 m– 103 km • Range of temporal scales • Activation process (aerosol droplet): seconds • Time to generate precipitation ~ 30 min • Cloud systems: days • Coupled System • Multiple feedbacks

11. supersaturation wet droplet radius What is a Drop? • Drops are an aerosol (suspended particles in the air) • Drops can be distinguished from dry or humidified particles using some (somewhat) arbitrary criteria: • Size (e.g diameter > 2 mm) • Volume of water vs. particle • Optical detection (e.g., can a light-scattering device see them?) • Activation (have they passed the critical radius on the Kohler curve) • Continuum from dry particle  humidified haze particle  droplet McFiggans et al. 2006

12. y-direction x-direction What is a Cloud? Just as the distinction between aerosol and cloud is somewhat arbitrary, so is the distinction between the cloudy atmosphere and the “clear-sky” atmosphere Clouds have fuzzy edges Cloud? Linear intensity scale Log intensity scale Model results: Koren and Feingold 2008

13. critical supersaturation Kelvin term (surface tension) Supersaturation Solute (Raoult) term wet droplet radius critical radius What is a Drop? The Kohler Curve Relationship between the supersaturation over a droplet and its wet radius at equilibrium Kohler curve describing the equilibrium growth of a particle at a given supersaturation for one particle size and composition. It does NOT predict the size of a cloud droplet size distribution

14. critical supersaturation Kelvin term (surface tension) Supersaturation Solute (Raoult) term wet droplet radius critical radius The Kohler Curve:

15. droplet radius surface tension mass of solute Molecular weight of water Molecular weight of solute “Van’t Hoff factor” ~ number of dissociated ions The Kohler Curves: Equilibrium particle size • Easier to activate droplets at: • lower ss • lower Ms • higher msolute • higher nF smaller particle larger particle 5 vs 2: effect of composition 2 vs 3 vs 4 effect of mass Wallace and Hobbs 2006 Rasool 1973

16. CCN Relationship between particle diameter and critical supersaturation Adipic acid (not very soluble) Insoluble but wettable particle Ammonium sulfate (very soluble) Adapted from Hings et al. (2008) ACPD

17. Aerosol Size distribution unactivated Activated particles Critical radius (derived from the Kohler equation)

18. Azores Florida Arctic Wallace and Hobbs 2006 (after Hudson and Yum 2002) Cloud condensation Nuclei (CCN) Typical Measured CCN “activation spectra” Sometimes approximated by N=CSk Note logarithmic scales

19. Clean cloud Pressure, hPa dN/dlogr, cm-3 effective radius drop conc polluted cloud Droplet radius, mm Garrett and Hobbs (1996) Drop-size distributions Polluted cloud Polluted cloud Clean cloud • Warm clouds: • Drop size increases with height • Drop conc ~ constant with height • Polluted clouds: more numerous, • smaller drops

20. How do we measure cloud and rain drops? In-Situ (typically airborne) • Size distribution, Liquid Water Content, Extinction Remote Sensing (Radar, radiometer) • Drop sizes, liquid water path

21. Clean cloud Drop size dN/dlogr, cm-3 aerosol extinction polluted cloud Droplet radius, mm Nd Na Observations of Aerosol Effects on Cloud mphysics Drop size decreases with increasing aerosol (at constant LWP) Drop size decreases with increasing aerosol (all LWP) Slope = 0.10 – 0.15 remote measurements Drop size Slope = 0.04 – 0.085 LWP = liquid water path aerosol index In-situ measurements

22. Cloud optical depth Drop size Drop conc CCN conc Cloud depth Year Remote sensing Boers et al. 2006

23. Collector drop (larger fall velocity Vx) condensation coalescence Collected droplets (small fall velocity Vy) Growth Processes • Condensation growth does not produce precipitation in warm clouds • Collision-Coalescence produces precipitation “Gravitational Kernel” E(x,y) = collection efficiency Wallace and Hobbs 2006

24. Collector drop radius Collected drop radius, mm Droplet coalescence • Coalescence does not become important until collector droplets reach sizes of 20 mm diameter • drop mass  raindrop mass 10 orders of magnitude! Small droplets don’t collide easily with large droplets E(x,y) = collection efficiency Wallace and Hobbs 2006

25. Effect of Aerosol on Precipitation Formation Aerosol significantly reduces the ability of a cloud to generate precipitation (all else equal) (Gunn and Phillips 1957; Warner 1967) Clean Polluted Na = 300 cm-3 Na = 50 cm-3 t=0 t=0 r=20 mm r=20 mm t=10 min t=10 min Drop radius, mm

26. Effect of Giant Aerosol on Precipitation Formation Giant CCN ~ few mm in (dry) size produce collector droplets r ~ 20 mm Clean clouds: active coalescence process anyhow; Giant nuclei have no effect Polluted clouds: more significantly affected by giant nuclei Clean Polluted Na = 50 cm-3 Na = 300 cm-3 t=0 t=0 r=20 mm t=10 min t=10 min Drop radius, mm Including 1/l GCCN

27. Liquid water path Precipitation Influence of Giant CCN on precipitation in Stratocumulus (1/litre) • Significant increase in precip due to ~1/litre Giant CCN • More particles does not always mean smaller drops Feingold et al. 1999

28. Influence of Giant Nuclei on mphysics and Cloud Albedo No giant CCN Drop size Drop Number No giant CCN With Giant CCN Significant reduction in cloud albedo due to ~1/litre Giant CCN Cloud Albedo Feingold et al. 1999

29. Rainrate, mm d-1 Rainrate, mm d-1 re, mm H3 N-1, m6 Precipitation: Macrophysics vs Microphysics • Measurements show that Rainrate ~ H3/N or LWP1.5/N • Some models suggest Rainrate ~ LWP1.6/N0.7 • Rain production is 2.5 x more sensitive to changes in LWP than changes in Nd • re is a much less effective determinant of rainrate Van Zanten et al. 2005

30. Two mm-size drops collide Parent drops fragments Drop breakup • May enhance cloud’s ability to precipitate • by “seeding” the cloud with more precip • embryos • Affects the raindrop size distribution and the • ability to measure rainrate from radar • (radar reflectivity-rainrate relationships) • Affects subcloud evaporation (fragments • evaporate more efficiently) • Drives stronger downdrafts Coalescence vs Breakup: Depends on the energy of the collision