From Aerosols to Cloud Microphysics Paolo Laj Laboratoire de Glaciologie et Géophysique de l’Environnement Grenoble - France

Clouds and the global Energy budget (SW radiation)

Apollo 11 image of Africa & Europe At any time, 30% of the Earth’s surface is covered by clouds Some interesting numbers Clouds increase the global reflection of solar radiation from 10 to 30%, reducing the amount of solar radiation absorbed by the Earth by about 44 W/m².

Clouds and the global Energy budget (LW radiation)

Apollo 11 image of Africa & Europe At any time, 30% of the Earth’s surface is covered by clouds Some interesting numbers Clouds increase the global reflection of solar radiation from 10 to 30%, reducing the amount of solar radiation absorbed by the Earth by about 44 W/m². This cooling is offset somewhat by the greenhouse effect of clouds which reduces the outgoing longwave radiation by about 31 W/m². Thus the net cloud forcing of the radiation budget is a loss of about 13 W/m²

Different kind of Clouds Question : which kind of hydrometeors ?

Low overcast clouds result in cooling (35 W m −2 ± 9 W m −2 ) Thin high clouds result in warming (20 W m −2 ± 8 W m −2 ) Clouds and the redistribution of radiant energy within the atmosphere

Clouds and the global Energy budget (LW radiation) Objective of the lecture : 1- discuss the mechanisms by which anthropogenic activities may modify the Earth radiative budget (Cloud Radiative Forcing) 2- Focus on the aerosol/cloud interaction

Definition  What is an aerosol ? Particles +Gases = Aerosols

<1m/s~1 m -3 2mm-20mmSnowflakes Up to 30m/s~1 m -3 1mm-50mm Graupel and hail particles <1m/s1-100 l  m- 3mm Ice crystals <15cm/s~1 m  m- 6mm Raindrops <30cm/s cm -3 1  m-100  m Cloud droplets Terminal velocity Number concentration Size (diameter) Shape Hydrometeor Different kind of hydrometeors

Size range of aerosols

Seoul, Korea, April 10, 2006

Dust in Seoul, Korea April 8, 2006 PM10 level reached 2,070 ug/m 3.

Black Carbon on snow

Enhancement of pixel-average cloud spherical albedo sph on April 5 relative to that on April 2, as a function of LWP

Summary 1.Aerosol can scatter and absorb short- wave solar radiation 2.Aerosol can modify cloud microphysics and, in turn, change cloud reflectivity 3.Question: are these processes relevant in the global energy budget ?

Anthropogenic Radiative Forcing from IPCC Question : what is behind the large uncertainty for the cloud albedo effect ?

More than one indirect effect….. Question : how do we quantify the indirect effect ?

Cloud Albedo and cloud microphysical properties Cloud albedo effect (Twomey effect)

Cloud Albedo and cloud microphysical properties Cloud Geometry Question : what does this equation tells us ?

LWP and Cloud Optical depth Adiabatic assumption Qext = extinction coefficient LWP= Liquid Water Path (g m -2 ) Reff= effective radius

Cloud Albedo and Cloud Optical depth Question : implications of the R/  dependency ? a= empirical coefficient g = assimetry parameter (0.85 for clouds)

Cloud Albedo and Cloud Microphysics Aerosol influence on cloud albedo requires comparison not of the albedo values themselves but of the enhancement in albedo relative to that expected for the same LWP Question : can we measure it ? Which kind of clouds would you use ?

Cloud Albedo and Cloud Microphysics Pixel-average cloud spherical albedo as a function of vertical cloud LWP, for three satellite overpasses

Cloud Albedo and Cloud Microphysics Enhancement of pixel-average cloud spherical albedo sph on April 5 relative to that on April 2, as a function of LWP Enhancement against LWP shows maximum enhancement at intermediate values of LWP, for which sensitivity to increased cloud-drop number concentration is the greatest

Is LWP independent of CN ? Question : what can you say about this picture ?

Aerosol activation to cloud droplets

CNs and CCNs Higher hygroscopic fraction Lower hygroscopic fraction smaller size

ERCA School Grenoble- January2002 Equilibrium between aqueous solution and humid air Curvature (Kelvin) Effect Curvature (Kelvin) Effect: the saturation vapour pressure increases with increasing curvature Solute (Raoult) Effect Solute (Raoult) Effect: the presence of solutes in the drop decreases the saturation vapour pressure Cloud droplet formation The Köhler theory

The smaller the droplet, the greater the supersaturation (with respect to a flat surface) is needed to keep the droplet from evaporating Cloud droplet formation II Kelvin Effect

The vapor pressure for a solution drop is less than that for a plane of pure water The vapor pressure required to maintain equilibrium decreases as the drop radius decreases. This is opposite of the effect for curvature. Cloud droplet formation III Raoult Effect

We can combine the effects of curvature and solution. This curve, represented by the thick line at the right, is the Köhler curve. Initially the solution effect dominates, but as the drop gets bigger, the curvature effect takes over. When the drop is very large, neither effect dominates and the surface of the drop, to the water molecules, appears as a flat surface. Cloud droplet formation III Raoult + Kelvin Effect Question : what can we measure in the köhler equation ?

Köhler curves calculated for three aerosol dry sizes and two different aerosol chemical compositions. -inorganic aerosol with surface tension equal to that of pure water (dotted lines). -inorganic + organic aerosol and variable surface tension (solid lines). Effect of a lower surface tension on critical supersaturation due to organic substances

Modified Kolher Equation to include the effects of slightly soluble organic compounds

Derived parameter Growth Factor GF = D p 0 Measurement of HGF: Principle of Tandem-DMA

Measurement of CCNs

Measurement of HGF: Principle of Tandem-DMA

A simplified view of the Atmospheric Aerosols

Hygroscopic growth of laboratory aerosol mixtures Classic growth theory (soluble fraction) – Neglecting hydrophilic organic material and surface tension effect Zdanoski-Stokes-Robinson (ZSR) approach GF = (A GFA3 + B GFB3 + …) 1/3 Neglecting non-linearity of organic/inorganic mixture on water activity and suface tension

Interstitial Phase (RJI) Condensed Phase CVI Microphysics Condensed Phase (cloud impactor) Interstitial + Condensed Phases (Whole air) In-situ Characterisation of scavenging Question : How to characterize the scavenged aerosol fraction ?

Cloud Sampler I The original Sampler

Cloud Sampler I Passive Sampler

Cloud Sampler III Active String collector

Cloud Droplet Dynamics Overal Losses 20µm 5 m s -1 Analyzer Settling velocity: 1-2 cm s -1 Stopping distance: 0.5 cm Relaxation time: s -1 Stokes number: 1-2 Evaporation time : 1-5 s 50-80% 5-15% 60-80%

Interstitial Phase (RJI) Condensed Phase CVI Microphysics Condensed Phase (cloud impactor) Interstitial + Condensed Phases (Whole air) In-situ Characterisation of scavenging Question : How to characterize the scavenged aerosol fraction ?

Sampling cloud droplets Principle of a Counter Flow Virtual Impactor

Hygroscopic properties of natural atmospheric aerosols Scavenging efficiency primarily related to size (Dusek et al., 2006) Size distribution alone explains 84 to 96% of the variation in CCN Variations of CCN activation with particle chemical composition observed but secondary role. Personal comment: I’am not fully convinced…. Sellegri et al., 2003

Estimating the Indirect Effect Cloud Properties Macroscopic properties (horizontal and vertical distributions) Microphysical properties Cloud base height Cloud fraction Cloud top height Radar Doppler Radar reflectivity Aerosol Microphysical and chemical properties Aerosol number concentration Aerosol particle size Black carbon concentration Cloud condensation nuclei Hygroscopic growth chemical composition Particle size distribution Optical and radiative properties Aerosol absorption Aerosol extinction Aerosol scattering Backscattered radiation Optical depth Radiometric measurements active (such as radar and lidar) and passive (such as broadband radiometers and spectral sensors) longwave broadband Radiative heating rate longwave narrowband … Surface and column meteorology Advective tendency Atmospheric moisture Atmospheric pressure Atmospheric temperature Atmospheric turbulence Horizontal wind Planetary boundary layer height Precipitable water Radiative heating rate Vertical velocity Virtual temperature Pristine Air Mass

Estimating the Indirect Effect Cloud Properties Macroscopic properties (horizontal and vertical distributions) Microphysical properties Cloud base height Cloud fraction Cloud top height Radar Doppler Radar reflectivity Aerosol Microphysical and chemical properties Aerosol number concentration Aerosol particle size Black carbon concentration Cloud condensation nuclei Hygroscopic growth chemical composition Particle size distribution Optical and radiative properties Aerosol absorption Aerosol extinction Aerosol scattering Backscattered radiation Optical depth Radiometric measurements active (such as radar and lidar) and passive (such as broadband radiometers and spectral sensors) longwave broadband Radiative heating rate longwave narrowband … Surface and column meteorology Advective tendency Atmospheric moisture Atmospheric pressure Atmospheric temperature Atmospheric turbulence Horizontal wind Planetary boundary layer height Precipitable water Radiative heating rate Vertical velocity Virtual temperature Polluted air Mass Question : where to find the ideal conditions ?

Complex instrumentation Long-term observations Global coverage Direct measurment Fine scale 1,2,3,4D measurments Long-term observations Limited spatial coverage Indirect observation 1D sampling Short-term observations Noise Indirect measurement + - A need for a multiscale approach

Modelling the cloud albedo effect Global decrease in cloud droplet effective radius caused by anthropogenic aerosols, Global mean RF =0.52 W m –2 Over land = –1.14 W m –2 Over Oceans = –0.28 W m –2

One more problem: the ice phase

Anthropogenic effect of cloud dynamics

Conclusions

Inside a Cloud…. THank you for your attention