Organisation
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Organisation. 9 written exercises 1 presentation (30 November) 2 computer exercises. Cloud Physics - Content. Introduction water clouds - nucleation of cloud droplets - droplet growth - growth of droplet populations ice phase - nucleation - growth mechanism - habits

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Organisation

Organisation

9 written exercises

1 presentation (30 November)

2 computer exercises

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Cloud physics content

Cloud Physics - Content

  • Introduction

  • water clouds- nucleation of cloud droplets- droplet growth- growth of droplet populations

  • ice phase- nucleation- growth mechanism- habits

  • precipitation- warm and cold rain- radar rmeteorology- thunderstorms

  • measurements of cloud parameters

  • modeling of clouds- spectral models- cloud parametrizations in NWP and climate models

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Repetition ice clouds

Repetition Ice Clouds

Heterogeneous Freezing

  • How do ice crystals form?

  • And at which temperatures?

Homogeneous Freezing

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Repetition ice clouds1

Repetition Ice Clouds

  • What are ice nuclei and how frequent are they?

Example: Kaolinit (Mineral, Al4[(OH)8|Si4O10] )ice nuclei below -10° C → droplet freezing

ice nuclei below - 20° C (and no saturation to water) → deposition nucleation

Wikipedia

silver iodide

lead iodide

Kaolinit

South. Hem.(Expansion)

South. Hem. (Mixing)

North. Hem. (Expansion)

Antarctica

Fig. 6.30 Wallace & Hobbs

Fig. 6.31 Wallace & Hobbs

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Ice multiplication hallett mossop process

Ice multiplication – Hallett-Mossop Process

  • The process (splintering following riming) was first reproduced in the laboratory by Hallett and Mossop (1974)

  • Ice production only occurs at between -3°C and -8°C (with a pronounced peak in the middle of the range) and in the presence of both large (>24 µm) and small droplets.

http://bio-ice.forumotion.com/t19-what-really-is-the-hallett-mossop-process

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Examples of ice clouds

Examples of Ice Clouds

  • Lee clouds

  • frontal cirrus

  • contrails

  • tropical cirrus

  • "subvisible cirrus"

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Lee clouds

Lee Clouds

Orographic wave disturbances can lead to updrafts of a few meter per second

This can cause lifting of up to a kilometer (corresponding to 10 K) producing sufficient supersaturation.

In comparison frontal cirrus shields caused by lifting at cold or warm fronts are connected to velocities in the order of a few cm/s.

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Frontal cirrus

Frontal Cirrus

frequent in mid latitudes

Seifert and Crewell, 2008

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Contrails

Contrails

Boucher, 2011

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Aircraft emissions

Aircraft Emissions

Source: Stefan Borrmann

10-65 g/kg CO

3-6 g/kg NOx

CnHm

3160 g/kg CO2

0.01-0.03g/kg soot

O2

1230 g/kg H2O

N, S

1 g/kg SO2

Emission indices in gramm emission per kg kerosine

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Contrail formation

H

SO

2

4

Contrail Formation

Isobaric mixing of hot exhaust gases

with cold dry environment

H2SO4

H2O

n

H

O

.

2

Ruß

±

OH, H2O

X

Ion-cluster

ice crystals

S

SO

2

H2SO4

H2O

H2SO4

H2O

n

H

O

.

2

0.01 s

0.1 s

1s

Age of exhaust plume

Source: Stefan Borrmann

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Contrails1

Contrails

  • isobaric mixing of hot exhaust gases with cold dry environment

  • nucleation at supersaturation in respect to liquid water – soot particles, volatile exhaust particles and background aerosol serve as condensation nuclei

  • diffusional growth of activated particles

  • homogeneous or heterogeneous freezing of drops

  • fast growth of ice crystals

  • evaporation of crystals in non-saturated environment:- in stratosphere within seconds- HNO3 can delay evaporation

  • model studies indicate studies show that contrails can also be formed without soot as sufficient background aerosol is available leading to larger ice crystals

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Subvisible cirrus

Subvisible Cirrus

  • low optical thickness (VIS) from 0.03 to 0.05; low concentration (~25 l-1)

  • nearly transparent in VIS but not in the longwave reducing outgoing longwave radiation by a few Wm-2

  • clouds are so thin that they cannot be detected by conventional instruments but by satellites

  • are probably formed by outflow of cumulus nimbus, ceasing contrails or slow large scale lifting

  • moisture at upper troposphere/lower stratosphere (UTLS) is important criteria but not known sufficiently well

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Thin and subvisible cirrus

Thin and subvisible Cirrus

Kübbeler et al., 2011, ACP

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

IV Precipitation Formation

Cloud droplets are rather small (ca. 10 μm) and occur in large populations (up to 1000 per cm3).

Populations are rather stabile and show little interaction mostly general growth by diffusion

Precipitation is formed then populations become instable precipitation particles are so large that they don‘t completely evaporate when falling

  • direct collision and coalescence of drops

  • interaction of water drops and ice crystals ice crstals growth at the expense of water drops

warm rain

cold rain

growth processes lead to precipitation formation starting from condensation nuclei within 20 min

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Coagulation growth

Coagulation Growth

16

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Break up of drops

Break-up of Drops

Fig. 6.26 Wallace & Hobbs


Temporal development of dsd

Temporal Development of DSD

equilibrium is established after some time

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Formation of Cold Rain

Franklin 1789, Wagner 1911, Bergeron 1933, Findeisen 1938

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Formation of Cold Rain

Freezing + fast diffusional growth

Cirrus shields, Amboss

- 40 ° C

- 15 ° C

Collision +

coalescence

updraft zone

0 ° C

melting

layer

base of

cumulonimbus

clouds

rain out,

or graupel/hail

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Stratiform Precipitation at Mid-Latitudes

melting layer

fall streaks

Radar time-height section

Wallace & Hobbs Fig. 6.45

Deposition growth

1 mm ice plate falls through with LWC=0.5 gm-3

→ spherical graupel r ~ 0.5 mm within a few minutes corresponding to vertical velocity of about 1m/s

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Forms of Solid Precipitation

Wallace & Hobbs Tab. 6.2

hexagonal plates

stellar crystals

hollow/solid columns

needles

dendrites

capped columns

irregular crystals

graupel

ice pellets

hail

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Growth of Ice Crystals

  • Water vapor deposition

  • growth rate of ice crystal depends on temperature andhumidity: optimum growth at -15°C.

  • Accretion

  • growth of ice crystals by riming with supercooled droplets

  • contact freezing leads to rimed particles

  • optimum in saturated layers of 0 to -10°C (Staudenmaier, 1999).

  • extreme riming leads to Graupel or snow pellets.

  • Aggregation

  • collision and coalescence of ice crystals

  • agglomeration is maximum close to 0°C.

  • ice crystals with dendritic shape can mechanistically can get caught and generate large aggregates

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Diffusional Growth

Normalized growth rate of ice crystals at two different pressures as function of temperature (Rogers und Yau, 1989)

growth rate max ~ -15°

Example:

- 5 ° C → plate growthswithin 30 min to ~7μg (r=0,5mm)

drizzle drops of about r=0.13 mm

corresponding to falls speed of0.3 m/s

Diffusional growth produces only weak precipitation

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Growth by Riming

  • Mixed phase cloud

  • contact freezing of supercooled drops

  • Graupel

  • original form of crystal can‘t be seen anymore

  • e) sphericalf) conical

  • Hail

  • extreme form of riming

  • max D=13.8 cm and 0.7 kg

  • fast freezing causes liquid inclusions

lightly rimed needle

rimed column

rimed plate

rimed stellar

spherical gaupel

conicall gaupel

Wallace & Hobbs Fig. 6.41

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Formation of Hail

  • hailhas a minimumdiameterof 5 mm

  • forhailformationstrong updrafts(upto 50 m/s) and high amountsofsupercooledwaterareneeded

  • iceparticlescirculateoverthefullverticalrangeofthecumuluscloud multiple times - asthehail falls, itmaymelttovaryingdegreesandbepickedupagainandvcarried high intotheatmospheretore-freeze.

  • hailstonescontain a kernelwithsurroundingonion-likelayersand intransparent layers

  • alternatingglassyhailstonesaccumulatewater in thelower warm partofthecloudwhichfreezes in theuppercoldpart (wetand dry growthalternate) .

  • eachcycleadds an icelayerwetgrowth -> liquid waterspreadsacrosstumblinghailstonesandslowlyfreezesairbubblescanescaperesulting in a layerofclearicedry growth -> airbubblesare "frozen" in place, leavingcloudyice

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Hail Formation

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Growth by Aggregation

Collection efficiency = Collision efficiency x Coalescence efficiency

  • Collision efficiency probably larger than for water droplets because of the larger fall speeds

  • Collision of ice particle with supercooled droplets has a coalescence efficiency of about 1

  • Coalescence efficiency between ice particles is higher at higher temperatures and for dendrites (get stuck)

  • Observations: significant aggregation only at temperatures > -10°C

  • Diffusional growth for ice isfaster than for water light precipitation (formation without aggregation)

rimed needles

rimed columns

rimed frozen drops

dendrites

Wallace & Hobbs Fig. 6.44

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Growth by Aggregation

  • Agglomoration of ice crystals to form snow particles(get stuck, contact freezing)

  • collision rate depends on fall velocityGraupel (aggregates of frozen droplets) falls quickly depending strongly on diameter D (in cm) with D encircling the particle

  • snowflakes and rimed structures fall with about ~1 m/sFor D the diameter of melted particles

    Dendrites k~160n=0.3Columns and plates k~234n=0.3

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Terminal Velocity

Avramov, A., and J. Y. Harrington, 2010

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Mass-Size Relation

Depending on shape the relation between mass and size varies

D – maximum linear dimension of crystal in cmM – crystal mass in g

important for mass growth,remote sensing, microphysical modelling

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Relation Mass-Size

Avramov, A., and J. Y. Harrington, 2010

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Snow Flakes

Snow Rain

Snow

Snow ab

Roger&Yau20002

Sekhon&Sriwastava17802.21

D (mm)

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Artificial Precipitation

  • of high interest in dry regions

  • first tests in November 1946 (Langmuir)

  • selection of a cloud with high amount of supercooled water

  • cloud seeding with ice-freezing nuclei, e.g. dry ice (carbon acid) or silver iodide (AgJ)

  • non-satisfing results - no clear statistics possible

  • also used for hail protection (Rosenheim, Stuttgart), because clouds with early onset of precipitation can not build large number of supercooled drops

A Y-shaped path cut into a layer of super-cooled cloud by seeding with dry ice

Wallace & Hobbs Fig. 6.47

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Modification of Clouds and Precipitation

  • injection of large hydroscopic nuclei in warm clouds to stimulate collision induced growth to rain drops

  • injection of artificial ice nuclei into cold clouds (most likely not containing many nuclei) to stimulate precipitation formation via ice phase difficult

  • injection of high concentrations of artificial ice nuclei into cold clouds → drastical reduction of supercooled clouds → suppression of riming and aggregation

    precipitation suppression (in particular hail)

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Success or Pure Chance?

+ 10 min

+ 19 min

+ 48 min

Wallace & Hobbs Fig. 6.48

Icing causes latent heat release and supports buoyancy lifting above level of free convection

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Artifical Clouds

Paper plant

contrails

industries release heat, water vapor & cloud active aerosol (CCN & IN)

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Exercise Precipitation Processes

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Characteristic Scales of Precipitation

Ascent/lifting is caused by

O(1000 km)

O(100 km)

O(10 km)

Synoptic Processes

Orographic lifting

Buoyancy/ secondary circulations

Stratiforme precipitation (30 %)

Convective precipitation (70 %)

Humidity content

O(1000 km)

O(10 – 100 km)

Advection

Modification by evapotranspiration and secondary circulation

QNV Bonn 12/03 Karlsruhe

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Characteristic Scales of Precipitation

Free

Troposphere

CAPE

Boundary Layer

turbulent mixing

mountain flow

Orography

Vegetation

Surface

Heat andhumidity influence

Mesoscale circulation

Wolkenphysik, Susanne Crewell, SS 2007


Organisation

Secondary Circulations

S. Raasch and G. Harbusch, 2001:An Analysis of Secondary Circulations and their Effects Caused by

Small-Scale Surface Inhomogeneities Using LES. Boundary-Layer Meteorol., 101, 31-59.

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Precipitation Process

  • spatial coverage, intensity and life cycle of a precipitation event is mainly determined by - vertical velocity and- available moisture

  • precipitation formation is prefered when - large variation of drop sizes - large vertical extent of clouds- strong updrafts exist

Stratiform precipitation: extended, continuous precipitation connected with largescale ascent due to frontal or orographic lifting or horizontal convergence

Convective precipitation: local showers connected with cumulus scale convection in unstable air mass

In reality fluent transition between stratiform and convective

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Formation of Cold Rain

  • If parts of the water cloud are above the freezing level precipitation formation can be initiated

  • ice grows at the cost of supercooled water

  • ice crystals fall faster than cloud droplets

  • growth by riming (water droplets are caught by collisions)

  • growth by aggregation (ice paricles connect via contact freezing or entangling)

  • Iceparticlesfromthemiddleortheupperpartofthecloudcan

  • reachthegroundasiceparticles

  • meltandreachthegroundas rain

  • melt, reachthegroundas rain andrefreezethere

  • melt, refreezeandreachtheground in form ofgraupel

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Precipitation Types

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Freezing Rain

Supercooled drops reach surface which is colder than 0°C

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Precipitation efficiency

Precipitation Efficiency

  • Relation between water vapor flux into cloud andsurface precipitation

  • What causes high efficiency?

  • effectiveness of collision-coalescence process

  • high humidity convergence

  • reduced evaporation

  • warm surface layerhigh humidity uptake, effective development of warm rain

  • enhanced residence time in cloud large height range leads to lowervelocities and better efficiency of warm and cold precipitation process

  • low Lifting Condensation Level (LCL)low evaporation below cloud

  • vertical wind shear

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Thunderstorms

Single cell thunderstorms

Multi cell thunderstorms

Super cell thunderstorms

  • (conditionailly) unstableairmasses

  • substantial boundarylayermoisture

  • lowlevelconvergence (orliftingtoreleasetheinstability)

  • strong updrafts, heavy rain, lightning, hail

Life cycle and intensity increases from single to super cells. Single cells hardly produce tornados – super cells frequently

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Stratification

Convective available potential energy

Convective inhibition

Level of equilibrium

LFC

environmental

lapse rate

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Stratification

Level of equilibrium

LFC

environmental

lapse rate

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Classical Diurnal Cycle

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Classical Development

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Single Cell Storms

Ordinary or single cell storm

Phase transition releases energy and supports further growth

  • Classification into different phases

  • Cumulus-State: Development, updrafts in largest part of the cell

  • Mature-State: Simultaneous existence of up and downdrafts Falling water (precipitation) initiates downdrafts by viscous drag of wateron air and cooling connected with evaporation

  • Dissipating phase: Downdrafts prevent further growth, strong precipitationand "downbursts" (air cooled by precipitation falls down, is horizontally deflected and becomes turbulent)

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Single Cell

10- 15 min 15-30 min ca. 30 min

cold nucleus

change of surface wind

http://www.crh.noaa.gov/mkx/slide-show/tstm/

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Repetition storm development

Repetition: Storm development

  • What different types of thunderstorms exist? How are they characterized?

  • How does a atmospheric stability develop on a fair weather day?

  • What are the different stages within the life cylce of a single cell storm?

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Single Cell Storms

Ascent above zero degree line is important for producing lightning (electrification due to ice particle collisions). With high buoyancy (unstable stratification), but low shear, typical summer thunderstorms made ot of „single cells“ are formed:

Single cell storms are short lived (ca. 30 min to 1 h), are isolated and rarely bring hail or downbursts

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Multi cell storm

Multi Cell Storm

  • Large shear (change of environmental with height) causes horizontal shift of up- and downdraft.

  • Both can interact and build uo a longer lasting circulation with subsequently growing cells.

  • Cold surface air that develops in association with thunderstorm downdrafts due to evaporation of rain (cold pools).

  • have longer life time (ca. 1 h to 3 h)

  • bring hail and downdrafts more often

  • are not necessarily isolated

  • Tornados or downbursts can occur with this storm type.

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Organisation

Coldpool

formation

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Multi cell storms

Multi Cell Storms

rather complex development in time and space


Super cell storms

Super Cell Storms

  • In contrast to multi cell storms are characterized by strong rotation.

  • Updraft elements combine into one rotating updraft and explode vertically (no competition of cells).

  • Can cause severe damage by string winds and hail :

  • are long-lived (ca. 1 h to 6 h)

  • often bring hail or downbursts

  • are not necessarily isolated

  • propagate with about 60 km/h

  • Tornados or downbursts are most frequently connected to this storm type

http://www.hprcc.unl.edu/nebraska/spc_radar.html

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Super cells

Super Cells

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


Global distribution of thunderstorms

Global Distribution of Thunderstorms

METSWN, Susanne Crewell & Ulrich Löhnert, WS 2012/13


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