Mixed-phase layer clouds

Mixed-phase layer clouds

Chris Westbrook and Andrew Barrett

Thanks to Anthony Illingworth, Robin Hogan, Andrew

Heymsfield and all at the Chilbolton Observatory

What is a mixed-phase cloud?

• Cloud below 0°C where liquid water droplets and ice crystals

co-exist

• Interesting because Wegener, Bergeron & Findeison showed that

this situation is unstable

• Difference in vapour pressure between liquid water and ice

surfaces means droplets evaporate, and ice crystals grow at their

expense

• Apparent implication is that mixed-phase clouds should be rare

• Reality: mixed-phase clouds can persist for hours, even days

Observations

• Millimetre radar and near-infrared lidar together

• Powerful technique:

– liquid droplets very reflective to lidar

– ice particles dominate radar

ice crystal virga

ice crystals

cloud top ≈ -12°C

red stripe = base of liquid layer

Mechanisms for nucleating ice

deposition

immersion

condensation

contact

• Know that pure water droplets freeze spontaneously at ≈-37°C

• In warmer clouds, need an aerosol particle to form ice

cooling, time cooling, time

diffusion

Mechanisms for nucleating ice

JUST NEED RH

TO BE HIGH ENOUGH

THESE ONES NEED A LIQUID WATER CLOUD

• Know that pure water droplets freeze spontaneously at ≈-37°C

• In warmer clouds, need an aerosol particle to form ice

So how important is liquid water for

forming ice in the atmosphere?

Use radar and lidar observations over 4 years to identify ice cloud layers – how many have liquid water at the top?

Find that in clouds > -22C almost all ice clouds have a liquid water top

Tells you droplets are needed to nucleate ice in these clouds – deposition nucleation is not important *because don’t see any ice clouds

without liquid in them]

Fraction of liquid-topped ice clouds drops off at colder temperatures:

50% at -27C

- increased deposition nucleation activity, or Bergeron-Findeison process?

0% at -37C [homogeneous freezing]

Implications:

• Cloud scheme should not form ice without liquid

water content for temperatures > -22°C

• Means simulation of supercooled liquid is important

if we want to successfully model the ice phase in

mid-level clouds

• Very frequent occurrence of liquid water at top of

mid-level clouds is important for radiation: optical

depth of liquid cloud is ≈10x that of ice cloud for

given water content

Focus of rest of talk

• Will concentrate on persistent thin mixed-phase

layer clouds such as Ac

• Typical structure: thin supercooled liquid layer at

top, ice crystals nucleated in layer, growing in it,

and falling below for ≈ 1km

• Relatively idealised setting to investigate basic

physics of how ice is formed

• Important for radiation

[see also Hogan et al 2003, QJRMS]

Key questions

• How much ice is nucleated in the liquid

water layers?

• How does that ice evolve and fall out?

• And how does the supercooled liquid

Microphysical structure

• Example from 18 May 2008: persistent Ac with virga

Liquid water dominates optical properties.

What did the model predict?

Colours are ice Contours are liquid

Cloud fraction much too low Cloud structure is wrong – no liquid at top

Microphysical structure

• Enormous reflection from ice crystals if lidar beam exactly vertical • Caused by mirror reflections from oriented plate-like ice crystals

Microphysical structure

Confirmed by polarisation radar pointing at 45°

- Horizontally polarised return is much stronger - Oriented pristine crystals dominate

General picture:

<ZDR> profiles from 6

persistent Ac clouds during May 2008 – same signature Images from 2DS cloud probe in flight through an Ac cloud, top -13C

Conclusion:

• Vapour growth dominates in these

clouds which are vapour-rich, but

which have low liquid water path

and are geometrically thin

• Typical habits are planar types

(plates, dendrites, stellars etc) –

reflects the typical temperature

range for persistent supercooled

layers ≈-10 to -20°C

Dynamics

Shallow mixed layer in top 500m

Radar spectral width – measure of turbulence

Blue values = still air

Dynamics

• PDF of vertical velocity of liquid

droplets at top (tracers for air

motion) from Doppler lidar

• Mean close to zero, negatively

skewed

• Narrow intense downdrafts

surrounded by broad weaker

updrafts

• Indicates overturning is driven

from top-down by radiative

cooling

Fluxes of ice, vapour and liquid

• Flight over Chilbolton 18 Feb 2009

• Very persistent layer of Ac, top -13C, lasted over

site for > 1 day

• Flight took place over 4 hour period in the

afternoon

Liquid water and θ

e

profiles

• Liquid water profile ~ adiabatic

• Droplet concentration constant 50/cc

• 25g/m

_{liquid water path}

equiv. potential temperature profile:

500m deep well mixed layer,

turbulence: well-mixed layer 500m deep at cloud top stable below

ice crystals falling 0.5-1m/s

Supercooled-top cloud, ice virga1km deep

Flux of ice crystals

Conclusion:

Flux ≈ 50/m

_/second

= 1.810

_/m

_/hour

… and it goes on for hours …

• Measure size spectra over 100km legs using CIP probe

• Calculate ∫n(D) v(D) dD

• Some uncertainty on v(D) so try different relationships and use spread as error bar

Ice nuclei budget

Flux of ice out layer = 50/m

_/second

According to De Mott et al (2010) concentration of ice nuclei at this temperature is ≈ 0.5/litre

Total depth of well-mixed layer is 500m, so at this rate available ice nuclei are completely depleted in about 1 hr.

But we sampled in situ for 4 hours, and flux at end was similar to that at the start… In fact, the radar observations show production of ice continuing for at least 24 hrs! Entrainment of fresh IN?

• to keep a steady supply of IN at this rate, would need entrainment velocity of 5cm/s (large) – no sign of cloud top rise

• RH is only 7% above cloud top. Mix in this air at 5cm/s and cloud quickly evaporates!

These long-lived supercooled clouds seem to be able to maintain a

steady production of ice over many hours: why aren’t IN depleted?

• Suggest a time-dependent freezing mechanism

•Most studies assume ice nuclei are sparse and efficient – once cooled to critical

temperature, freezing is immediate

• Suggest also have droplets which contain more inefficient ice nuclei, which freeze

randomly and slowly over time

• MODEL IMPLICATION: Could parameterise a steady flux of ice crystals as a

function of temperature?

•Met Office cloud scheme already sort-of simulates nucleation like this – automatic

replenishment of ice nuclei if all ice has fallen out

• CRMs with coupled cloud & aerosol– may be artificially simulating depletion of ice

nuclei in long lived mixed-phase clouds

Vapour / liquid water budget

Can estimate growth of ice at expense of liquid water:

dm/dt  capacitance  supersaturation

• know water saturated environment, temperature

• capacitance for planar habits is weak function of shape

≈ 0.3  maximum dimension, to within 20% or so (Westbrook et al 2008, JAS) • Now integrate over CIP size spectrum, and over supercooled layer depth:

• dLWP/dt ≈ 2g/m

_/hr

_{(vs 25g/m}

_{measured liquid water path)}

• Complete glaciation takes 12 hours

• Bergeron-Findeison process in these clouds is slow

- could be

offset by a weak net radiative cooling of cloud layer (≈1K/day)

LW cooling

- destabilises top 500m of cloud - small net cooling to liquid layer

overturning

- well mixed layer

mixed-phase layer

500m

500m

dry air: evaporation of ice

Schematic diagram of mixed-phase Ac

stable layer

stable, potentially warm, often very dry air

ice virga

How can we successfully

simulated these clouds in a GCM?

• Investigate case studies using 1D model forced

by ERA interim

• Edwards & Slingo radiation

• Non-local mixing scheme

• MetO cloud microphysics, but can be altered

• Idea is to test sensitivities – what do we need to

do to get these clouds to come out right?

Effect of resolution on persistence of liquid water

Black contours are liquid water, colours are ice water content

• High res simulation: less ice near cloud top, persistent liquid layer

• Low res simulation: more ice water at top, cloud glaciates in ≈ 1.5 hrs

• Reason: gradient of IWC near top due to sedimentation is not resolved =

too much ice in grid box = big vapour flux from liquid to ice.

• Note +ve feedback: more liquid → more LW cooling → more liquid → ...

Need liquid to have a chance to get going for radiation to do its bit

How high does the vertical

resolution need to be?

• Only get close to convergence once Δz<100m

• Typical grid spacing in mid-troposphere: 400m ECMWF; 250m MetO UK4

Parameterise for low res GCMs

Parameterise ice, temperature & humidity profiles in 500m grid box Calculate mean grid box process rates from that parameterised profile Leads to more liquid, less ice growth

Effect of the size spectrum

• Growth rate integrated over size spectrum to get dIWC/dt (and hence - dLWC/dt) • Usually represented as an exponential in models

Concentration = N0 exp (-λD)

N0 diagnosed from temp

Con ce n tra tio n (lo g sca le) Particle size

Slope λ determined from IWC

Warmer temperatures N0 lower, meaning fewer, bigger particles for given IWC

In MetO model N0(T) is fixed function.

Have calculated N0 from large in-situ data set – see what happens when use these

Effect on ice growth and fall out

For low IWC clouds (eg Ac)

growth of ice is 2x bigger

than it ought to be

Effect on ice growth and fall out

For low IWC fall out rate is

half as fast as it should be

Net effect – ice sucks out

vapour too fast and sticks

Fix: fit N0 = f(T,IWC)

Long term evaluation of model: CloudNet

• Major failing is inability to fill the grid box – cloud

fraction is too low in mid-level clouds

• All model resolutions equally poor

• Problem with sub-grid humidity PDF?

• Not unique to Met Office!

34 34 Extending European Profiling Network 4 to 7 stations;

Launch of FP7 ‘Actris’ May 11: + 3 more sites in D.

Will soon be able to test model clouds over wide range of conditions

eg. supercooled Sc in Finland, very clean air at Mace Head in W. Ireland, etc.

Summary

• Liquid water is key to nucleation of ice in mid-level clouds –

no ice formed below water saturation

• Presence of liquid at top of these clouds makes them much

more optically thick than they would otherwise be

• Well-mixed through top 500m = 1-2 GCM grid boxes

• Flux of ice is ≈ steady, but entrainment is weak – implies

time-dependent freezing

• Vapour growth dominates – flux of vapour is relatively

weak however – offset by net LW cooling

• To model properly, particularly need to sort out

– Sub-grid structure of ice water content and humidity

– Ice size spectrum

What next?

• Need more basic observations of Ac

• A lot to be learned from aircraft measurements

colocated with radar/lidar (eg over Chilbolton)

• Need to build up an idea of crystal fluxes as

function of temperature to parameterise

• Quantify radiation/liquid water budget,

understand how modulated by temperature, SW

heating etc.