Short Course Challenges in Understanding Cloud and Precipitation

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Short Course Challenges in Understanding Cloud and Precipitation Processes and Their Impact on Weather and Climate Darrel Baumgardner PhD. Droplet Measurement Technologies [email protected] 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

Condensational Growth rate  1/D2

Observations

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)

Secondary Ice Production Collisions between ice crystals producing secondary fragments

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 for producing ice crystals. ‘Hallett-Mossop’ and droplet splintering are the only processes that have been replicated in the laboratory

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 All Clouds a) Aerosol/Cloud Interactions b) Inadvertent weather modification – do anthropogenic emissions increase or decrease precipitation?

III.

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)

1006UTC ~ T+1hr10:06 Model

1040UTC ~ T+1.5hr 10:40

1130UTC ~ T+2.5hr 11:30

1202UTC ~ T+3hr12:02

Just touching coast near the Humber

1342UTC ~ T+4.5hr 13:42

1526UTC ~ T+6.5hr 15:26

1708UTC ~ T+8hr17:08

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

Before

During

After

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.

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

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.

Growing

Mature

Dissipating

What are the Physics Behind Cloud Formation And Evolution to Precipitation?

Water vapor

Aerosol Particle

Sublimation Deposition

Serves as nuclei for heterogeneous nucleation

Condensation Evaporation

Serves as nuclei for ice .

Freezing

Ice Xtal

Droplet Growth

Graupel growth

Evaporation

Serves as aggregate embryo

A microphysical model must take each of these pathways into account.

Riming

Drizzle Drop Coalescence

Serves as aggregate embryo

Evaporation Melting

Aggregates

Melting

Rain Drop Freezing

This diagram summarizes the possible pathways to the formation of precipitation.

Serves as aggregate embryo

Break-up

Each arrow belongs to a process requiring individual numerical treatment/subroutine for the model simulation

Serves as hail embryo

Snow Pellet Temperature

Hail

Figure courtesy of S. Borrmann, U. Mainz

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Short Course Challenges in Understanding Cloud and Precipitation

Short Course Challenges in Understanding Cloud and Precipitation Processes and Their Impact on Weather and Climate Darrel Baumgardner PhD. Droplet Mea...

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