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 September 2008, L'Aquila, Italy

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

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 N a causes an increase in drop concentration N d : If nothing else changes, drops are smaller and cloud is more reflective Drop concentration, N d Aerosol concentration, N a Ramanathan et al. 2001

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

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

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

Microphysical Pathways

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

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) Aerosol - cloud drop - rain drop r = radius in  m n = number/litre v = fall velocity in cm s -1 Typical number concentrations: aerosol: cm -3 droplets: 10 2 cm -3 raindrops: cm -3 CCN: drawn 25x larger

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:  m – cm –Cloud scales: ~ 10 2 m – 10 3 km Range of temporal scales –Activation process (aerosol  droplet): seconds –Time to generate precipitation ~ 30 min –Cloud systems: days Coupled System –Multiple feedbacks

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  m) –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 wet droplet radius supersaturation McFiggans et al. 2006

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 Cloud? Clouds have fuzzy edges Model results: Koren and Feingold 2008 y-direction x-direction Linear intensity scale Log intensity scale

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 What is a Drop? The Kohler Curve Relationship between the supersaturation over a droplet and its wet radius at equilibrium wet droplet radius Supersaturation critical radius critical supersaturation Kelvin term (surface tension) Solute (Raoult) term

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

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

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

Aerosol Size distribution Critical radius (derived from the Kohler equation) Activated particlesunactivated

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

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

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

Observations of Aerosol Effects on Cloud  physics Drop size decreases with increasing aerosol (at constant LWP) Drop size aerosol extinction Drop size decreases with increasing aerosol (all LWP) Slope = 0.04 – Slope = 0.10 – 0.15 LWP = liquid water path N d N a In-situ measurements remote measurements Drop size aerosol index Clean cloud polluted cloud Droplet radius,  m dN/dlogr, cm -3

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

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

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

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

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

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

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

Precipitation: Macrophysics vs Microphysics Measurements show that Rainrate ~ H 3 /N or LWP 1.5 /N Some models suggest Rainrate ~ LWP 1.6 /N 0.7 Rain production is 2.5 x more sensitive to changes in LWP than changes in N d r e is a much less effective determinant of rainrate Rainrate, mm d- 1 H 3 N -1, m 6 Van Zanten et al Rainrate, mm d -1 r e,  m

Drop breakup fragments Two mm-size drops collide Parent drops 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