Short Course Challenges in Understanding Cloud

and Precipitation Processes and Their Impact on Weather and Climate

Darrel Baumgardner PhD.

Droplet Measurement Technologies darrel.baumgardner@gmail.com

February 18-22 3:30-4:30 pm

break 4:45-5:30 pm

Second Class

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

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. 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

III. All Clouds a) Aerosol/Cloud Interactions

b) Inadvertent weather modification – do anthropogenic emissions increase or decrease precipitation?

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. 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

III. All Clouds a) Aerosol/Cloud Interactions

b) Inadvertent weather modification – do anthropogenic emissions increase or decrease precipitation?

Simple Condensational Growth Model Observations

Condensational Growth rate 1/D2

Simple Condensational Growth Model Observations

Broader observed spectra lead to much faster coalescence because of the difference in terminal velocities. Models predict rain in clouds with warm tops in > 60 minutes. Rain is observed in warm clouds in < 30 minutes.

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. 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

III. All Clouds a) Aerosol/Cloud Interactions

b) Inadvertent weather modification – do anthropogenic emissions increase or decrease precipitation?

Ice Multiplication processes were hypothesized when many more ice crystals were measured than predicted from simple ice nucleation versus temperature relations. Gulteppe et al., “ICE CRYSTAL NUMBER CONCENTRATION VERSUS TEMPERATURE FOR CLIMATE STUDIES, INTERNATIONAL JOURNAL OF CLIMATOLOGY, Int. J. Climatol. 21: 1281–1302 (2001)

Collisions between ice crystals producing secondary fragments

Secondary Ice Production

Q1: Hallet Mossop process more explanation is requested, in general and also if IN play a role ? Q2:Any measurements made for aerosol processing by clouds ? how ? Q3: How about the shape of ice crystals in clouds and how it impacts the radiative forcing, how it is differentiated only of contrail for eg. ? Q4: Contrail forcing at night was maximum as it is only LW forcing (SW forcing is zero ?) or because more ice crystals form ? Q5: temperature range from ice crystals to droplets ? and vice versa ?

Hallett-Mossop Secondary Ice Production

Initially, coalescence produces small supercooled raindrops (300-500 µm) which freeze then collide with droplets, forming coating of rime (supercooled droplets freezing on ice surface). When this piece of graupel, up to 1-2 mm wide, hits larger droplets they may eject ice shards. These ice shards grow into needles or columns by vapour deposition to form precipitation, and possibly also more ice-particle-generating graupel. Ice production only occurs at between -3°C and -8°C and in the presence of both large (>24 µm) and small droplets.

Very little known about this mechanism forproducing ice crystals.

‘Hallett-Mossop’ and droplet splintering are theonly processes that have been replicated in thelaboratory

Hallett-Mossop Secondary Ice Production

Secondary Ice Production Artifact?

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. 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. All Clouds a) Aerosol/Cloud Interactions

b) Inadvertent weather modification – do anthropogenic emissions increase or decrease precipitation?

Issues

• Aviation impacts on climate: radiative forcing due to

• CO2 emissions ~ 0.03 Wm-2

• Contrail-induced cirrus ~ 0.03 Wm-2 IPCC(2007)

• Indirect radiative impacts of aviation • contrail formation, persistence and growth

• modification of natural cirrus properties via impacts of emitted aerosols

Heymsfield et al (2010), BAMS: Contrail Microphysics

• Well-established theory of contrail formation • Range of existing microphysical obs. but with sub-optimal

instrumentation (prone to shattering artefacts) • Lack of recent studies of aerosol emission characteristics for

current- and future-generation engines • Difficulty of measurement in key regions to fully-characterize

the evolution of a contrail (plume-mixing region, vortex region)

• Need for lab and field obs. of soot IN activity – fresh and aged • Need for large-scale “closure” experiments to link contrails

sources, vapour availability, microphysical characteristics and radiative impact

Yang et al. (2010) BAMS: Contrails and Induced Cirrus: Optics and Radiation

• Ice habit of cirrus and contrails: when are they similar or different?

• Single-scattering properties of contrail ice.

• Possible need for separate parametrization in GCMs if optical properties are significantly different

• Ambiguity of identifying contrail cirrus when evolved beyond the linear stage

• Need for satellite climatologies of contrail cirrus

• Need for supporting field campaigns

Haywood et al. (2009): A case study of the radiative forcing of persistent contrails evolving into

contrail-induced cirrus, J.Geophys.Res.

• AWACS aircraft flying large circles off the east coast of England

• Contrail drift simulated using the Met Office NAME atmospheric dispersion model: Lagrangian particles transported by dynamical fields from operational Unified Model forecast.

• IR satellite images from sequence of polar-orbiters (NOAA 15/17/18, Metop-A, TERRA)

10:06 1006UTC ~ T+1hr

Model

10:40 1040UTC ~ T+1.5hr

11:30 1130UTC ~ T+2.5hr

12:02

Just touching coast near the Humber

1202UTC ~ T+3hr

13:42 1342UTC ~ T+4.5hr

15:26 1526UTC ~ T+6.5hr

17:08

Contribution from other contrails

1708UTC ~ T+8hr

How much of this cloud cover would have been present if the airmass hadn’t been seeded by contrails?

Spangenberg, D. A., P. Minnis, S. T. Bedka, R. Palikonda, D. P. Duda and F. G. Rose (2013), Contrail radiative forcing over the Northern Hemisphere from 2006 Aqua MODIS data, Geophys. Res. Lett., 40, doi:10.1002/grl.50168. Largest forcing is at night

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. 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. All Clouds a) Aerosol/Cloud Interactions

b) Inadvertent weather modification – do anthropogenic emissions increase or decrease precipitation?

Aerosol processing by clouds

No change in the aerosol properties

Before During After

Aerosol particles are removed or transformed when a CCN forms a droplet that then collects a non-activated particle (inertial scavenging). When the droplet evaporates, the new aerosol particle has larger mass and diameter, and possibly a new composition

Before During After

Aerosol particles are removed when two CCN form droplets that collide and coalesce and form a larger droplet. When this droplet evaporates, the new, residual aerosol particle has larger mass and diameter, and possibly a new composition

Before During After

Aerosol particles are removed when droplets collide and coalesce, form a rain drop that precipitates. This raindrop can remove other aerosol particles by inertia scavenging.

Before During After

The adsorption by droplets of some types of gases, like SO2, will also change the mass and composition of the aerosol particles.

Before During After

Cloud processing can change the morphology (shape) of the particles by adding a layer of water.

Before During After

Aerosols processed by clouds may form clouds and precipitation faster and easier!

No rain Rain

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. 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. All Clouds a) Aerosol/Cloud Interactions

b) Inadvertent weather modification – do anthropogenic emissions increase or decrease precipitation?

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 ….for clouds with low updraft and small vertical development.

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

Formation of precipitation: natural cloud condensation and ice nuclei

Growing Mature Dissipating

Growing Mature Dissipating

The number of cloud droplets activated under polluted conditions is not less necessarily than pristine clouds – they just take longer to activate and hence form higher in clouds and change the dynamics and rate of precipitation formation.

What are the Physics Behind Cloud Formation

And Evolution to Precipitation?

This diagram summarizes the possible pathways to the formation of precipitation. A microphysical model must take each of these pathways into account. Each arrow belongs to a process requiring individual numerical treatment/subroutine for the model simulation Figure courtesy of S.

Borrmann, U. Mainz

Hail

Aggregates

Snow Pellet

Rain Drop

Drizzle Drop

Droplet

Growth Evaporation

Evaporation Coalescence

Serves as nuclei for heterogeneous nucleation

Aerosol Particle

Serves as nuclei for ice .

Water vapor Sublimation Deposition

Condensation Evaporation

Break-up

Freezing

Melting

Freezing

Ice Xtal

Serves as aggregate embryo

Serves as aggregate embryo

Serves as aggregate embryo

Serves as hail embryo

Riming

Graupel growth

Melting

Temperature