Short Course Challenges in Understanding Cloud and Precipitation Processes and Their Impact on Weather and Climate

Short Course Challenges in Understanding Cloud and Precipitation Processes and Their Impact on Weather and Climate Darrel Baumgardner PhD. Droplet Measurement Technologies February 18-22 3:30-4:30 pm break 4:45-5:30 pm

Class II

Your Instructor. UNAM (2006) NCAR (1986)

I am the daughter of Earth and Water And the nursling of the Sky; I pass through the pores of the ocean and shores; I change, but I cannot die. For after the rain when with never a stain The pavilion of Heaven is bare, And the winds and sunbeams with their convex gleams Build up the blue dome of air, I silently laugh at my own cenotaph, And out of the caverns of rain, Like a child from the womb, like a ghost from the tomb, I arise and unbuild it again. The Cloud Percy Bysshe Shelley In memory of Peter V. Hobbs (1936-2005) Wallace, J.M. and P.V. Hobbs, 2006: Atmospheric Science: An Introductory Survey, Second Edition, Elsevier, Oxford, England 484 pp.

1.0 Overview of Clouds and Precipitation 1.1 Clouds and Climate 1.2 Clouds and the Hydrological Cycle 1.3 Some Issues Related to Cloud and Precipitation Physics 2.0 Microphysical Properties of Clouds and Precipitation 2.1 Bulk properties 2.1.1 Number, area and mass concentrations 2.1.2 Scattering, absorption and extinction coefficients, optical depth, effective diameter wavelength dependent 2.1.3 Chemical composition (inorganic and organic ions, elemental carbon, bioaerosols 2.1.4 Electrical fields 2.2 Size dependent properties (Size distributions) 2.2.1 Mass, density and morphology 2.2.2 Optical cross section and phase function as function of wavelength. 2.2.3 Area surface and projected 2.2.4 Fall velocity 2.2.5 Electric charge

3.0 Review of Fundamental Processes 3.1. Thermodynamic structure of the atmosphere and its relationship to cloud formation and evolution. 3.1.1 Vertical profile of temperature and humidity 3.1.2 Adiabatic temperature and water content, supersaturation. 3.1.3 Maximum vertical velocities and relationship to droplet activation 3.2. Particle Formation 3.2.1 Droplet activation Koehler theory and droplet nucleation 3.2.2 Crystal nucleation homogeneous freezing, deposition, immersion freezing, contact nucleation 3.3 Particle Growth 3.3.1 Diffusional growth-condensation and deposition, Werner- Bergeron 3.3.2 Collision growth -coalescence, aggregation,riming and multiplication 3.4 Particle lifetime 3.4.1 Entrainment and mixing 3.4.2 Radiation cooling and evaporation 3.4.3 Raindrop and ice crystal break-up

4.0 In Situ Measurement Techniques 4.1 Nuclei properties 4.1.1 CCN Parallel plate and continuous flow counters 4.1.2 IN CFDC, PINC, ZINC, SPIN 4.2 Impaction and Replication 4.2.1 Historical 4.2.2 Measurement Principles and Implementation 4.2.3 Measurement Issues 4.3 Single Particle Size and Morphology Measurements 4.3.1 Single Particle Light Scattering 4.3.2 Single Particle Imaging 4.3.3 Imaging of Particle Ensembles Holography 4.3.4 Measurement Issues

4.0 In Situ Measurement Techniques (continued) 4.4 Integral Properties of an Ensemble of Particles 4.4.1 Thermal Techniques for Cloud LWC and IWC: Measurement principles and implementation 4.4.2 Optical Techniques for the Measurement of Cloud Water 4.4.3 Measurement Issues 4.5 Emerging Technologies 4.5.1 Backscatter Cloud Probe with Polarization (BCPOL) 4.5.2 The Cloud Particle Spectrometer with Polarization Detection(CPSPD) 4.6 Data Analysis 4.6.1 Quality assurance detecting spurious particles, sample size, coincident problems, measurements of temperature and humidity in cloud, etc. 4.6.2 Image analysis from optical array probe measurements 4.6.3 Interpretation of measurements Correlation versus causation, size distributions, etc.

Interaction between radiation and particles (aerosol and cloud) Two processes can occur, depending on the incident wavelength (solar or terrestrial) : Scattering: the incident radiation is re-radiated by the particles but with a different intensity and direction. Absorption: the incident radiation is transformed into radiation at different wavelength (like heat).

x y z partícula ),, ( ),, ( r ó z y x Incident plane electromagnetic wave Interaction between radiation and particles (aerosol and cloud) The energy that is produced from the interaction between a particle and incident light is described by: I 0 d d r I sin 2 2 0 ) / (2 ),, ( r F I I ),, ( F = scattering function

Interaction between radiation and particles (aerosol and cloud) The total scattered energy by the particle in all directions is expressed as a function of the scattering cross section. C scat 1 1 I o (2r / ) 2 0 0 2 I 2 0 0 r 2 sin d d F(,, ) sin d d And the scattering efficiency Q scat C scat Area

Interaction between radiation and particles (aerosol and cloud) Similarly the absorption cross section, C a, is defined as the fraction of incident energy that is absorbed per unit area of the particle and the absorption efficiency, Q abs, is the efficiency divided by particle area. The extinction efficiency is a measure of how much of the incident radiation is removed by the particle through scattering and absorption, expressed as: Q ext Q scat Q abs

Interaction between radiation and particles (aerosol and cloud) Particles small with respect to the wavelength of the incident radiation (Rayleigh scattering): Qdisp 8 3 x 4 m Re m 2 2 1 2 2 Q abs m Im m 2 4x 2 1 2 x d p / Size parameter << 1 m=n - in : Refractive index Examples: air molecules and visible radiation cloud particles and radar wavelengths

Interaction between radiation and particles (aerosol and cloud) Particles that are large with respect to the wavelength, i.e. x d p / 1 Q disp 2 The scattering efficiency is TWICE the actual cross sectional area (remember Q disp = C disp /area). This is the geometrical scattering region where diffraction is an important component of scattering (more on this later).

Interaction between radiation and particles (aerosol and cloud) Particles with sizes close to the wavelength (x ~ 1) Mie region Mie (1908), solved the Maxwell equations for the special case of spheres when wavelength of the incident light and referective index is known.

Interaction between radiation and particles (aerosol and cloud)

Interaction between radiation and particles (aerosol and cloud) For multiple particles, the extinction coefficient is defined as : 0 b d 4) Q ( x, m) n ) d( d 2 ( p / ext d ( d p p where: n d (d p ) is the concentration of particles with diameter d p b( ) b scat ( ) b ( ) abs )

Climate forcing: General concepts Climate is a result of radiative processes in the atmosphere, oceans, surface and biosphere. Changes in the incoming solar radiation or outgoing terrestrial radiation creates a new energy equillibrium.

Climate forcing: General concepts Atmospheric aerosols can have a direct forcing and an indirect forcing on climate. I partícula nube I partículas

Natural and human forcing of climate change

Direct Forcing Direct refers to the interaction between solar and terrestrial infrared radiation with aerosol particles before they become cloud particles. This magnitude of the interaction depends on the particle concentration, size, shape, and composition of the particles. This produces a net cooling effect due to the solar radiation that is scattered back to space. However, aerosol particles like black carbon can produce net warming

Direct effect example: Eruption of Mt. Pinatubo Optical depth measurements with SAGE II

The Indirect Effect of Aerosol Particles The concentration of water droplets depends directly on the concentration of aerosol particles that can form these droplet, cloud condensation nuclei (CCN) and the vapor pressure of water with respect to the equilibrium saturation vapor pressure. Natural and anthropogenic aerosol particles can serve as CCN, depending on their size and composition (hygroscopicity).

The Indirect Effect of Aerosol Particles An increase in anthropogenic sources of CCN can increase the reflection (albedo) of clouds, by increasing the droplet concentration while decreasing the average diameter. This effect was named the indirect effect of aerosols by Twomey (1974)

Evidence for the indirect Twomy effect in this satellite image of clouds off the coast of California. The ship tracks are a result of high reflectivity regions in the marine stratus clouds formed by increased concentrations of small droplets formed on the sulfate particles from emissions by ships.

The Indirect Effect of Aerosol Particles A larger concentration of droplets with smaller size implies a reduction in the precipitation efficiency with an associated increase in cloud lifetime, affecting the hydrological cycle. It is the impact on the albedo that we usually associate with the indirect effect; however, the impact on the lifetime is referred as the second indirect effect.

Why adding more CCN decreases average droplet size and increases cloud lifetime Low concentration of CCN Form cloud droplets in supersaturated environment That grow until environment is no longer supersaturated Some grow to raindrops that fall out and cloud dissipates

Why adding more CCN decreases average droplet size and increases cloud lifetime High concentration of CCN Form cloud droplets in supersaturated environment That grow much slower as they compete for available vapor No rain forms, cloud lasts longer

Direct effect vs indirect effect I 0 I ref = AI 1, A=albedo I 1 = e - 1 Aerosol layer = Optical depth = B ext dz B ext ~ ND 2 I 2 = e - 2 N a = 10 4 cm -3 D a = 0.05 m

I 0 A cloud >> A aerosol Cloud layer N n = 100 cm -3 D n = 10 m n / a =(N n /N a )(D n /D a ) 2 = (.01)(200) 2 = 400 Aerosol layer B ext ~ ND 2 N = 10 4 cm -3 D = 0.05 m

We can t understand clouds without understanding aerosols! Homogeneous nucleation (droplet formation from only water molecules). Droplets form by the simultaneous collisions of water molecules. Cluster The nucleation rate (J) vs supersaturation (S v,w ) S v,w (%) 200% 600% J 1.9 x 10-112 6.0 x 10 3 (cm - 3 s -1 ) S v,w de las nubes nunca excede 5% - las nubes no se pueden formar a través de este mecanismo!! Embriones

Definitions of Saturated, Subsaturated and Supersaturated Subsaturated is when more water molecules are escaping the droplet (evaporation) than are diffusing to it (condensing) Saturated is when the rate of water molecules diffusing to the water droplet is equal to the rate of molecules leaving. This is also call a state of equilibrium/ Supersaturated is when more water molecules are diffusing to the droplet (condensation) than are escaping (evaporation)

Heterogenous nuclation The activation (formation of a water droplet) from an aerosol particle as a CCN depends on the size and chemistry (hygroscopicity) of the particle and the water vapor pressure with respect to the saturation vapor pressure. Φ σ n a, n b n x Φ = contact angle = f(diameter) σ = surface tension = f(chemistry) n a, n b n x = chemical potential

Some Outstanding Problems in Cloud Microphysics I. Warm Clouds a) Stratiform i) Drizzle formation ii) Geoengineering b) Cumulus i) Spectra broadening ii) Rain formation II. III. Cold Clouds a) Ice formation processes i) Homogeneous and heterogeneous nucleation ii) Ice multiplication b) Cirrus and Contrails i) Impact on climate ii) Cirrus evolving from contrails iii) Lightning generation All Clouds a) Aerosol/Cloud Interactions b) Inadvertent weather modification do anthropogenic emissions increase or decrease precipitation?

How do marine stratus clouds form drizzle and precipitation? Marine stratus are shallow with low concentrations of small droplets Observations show presence of drizzle droplets 50-100m diameter Questions: How does drizzle form without coalescence (low concentrations and small droplets inhibit)? Sometimes preferentially at cloud tops. What role does mixing and entrainment play? Can radiative cooling at the cloud tops enhance condensational growth?

Do we understand marine stratus cloud formation processes well enough to generate them artificially?

Official Statement from the International Commission on Clouds and Precipitation * That further research is pursued to better understand the fundamental science and possible efficacy of radiation management climate engineering schemes. That climate engineering research be conducted in an open and independent manner that engages public participation, and is used to properly assess the potential risks involved. That research activities must include studies of the human impacts, ethics, legal and political impacts of climate engineering Given the poor state of the current knowledge on clouds, aerosols, precipitation and their interactions, the ICCP does not support the implementation of climate engineering and does not expect that climate engineering can solve the global warming problem. * Thara and I are members of the ICCP commission.