Daniel Grosvenor, Thomas Choularton, Martin Gallagher (University of Manchester, UK); Thomas Lachlan...

Daniel Grosvenor, Thomas Choularton, Martin Gallagher (University of Manchester, UK);

Thomas Lachlan Cope and John King (British Antarctic Survey).

Daniel Grosvenor, T. Choularton, M. Gallagher, K. Bower, J. Crosier (University of Manchester, UK);

Thomas Lachlan Cope and Russell Ladkin (British Antarctic Survey).

In-cloud aircraft observations over the Antarctic Peninsula

•ContentsContents•Antarctic cloudsAntarctic clouds•Flight 102 – In situ observations of Flight 102 – In situ observations of lenticular cloudslenticular clouds

•Ice number observations – can Ice number observations – can they give an estimate of Ice Nuclei they give an estimate of Ice Nuclei concentrations?concentrations?•How do they compare to current How do they compare to current ice parameterisations (based on ice parameterisations (based on non-Antarctic clouds)?non-Antarctic clouds)?•Are Antarctic clouds different?Are Antarctic clouds different?

•Flight 104Flight 104•Ice observations at colder Ice observations at colder temperaturestemperatures•Ice formed by the Hallet Mossop Ice formed by the Hallet Mossop process.process.

Antarctic Clouds – why the interest?

• Antarctica covers a very large area and cloud cover is extensive.

• Therefore they are important in determining the radiation balance of the Earth.

• Yet, little is known about Antarctic clouds due to a lack of in-situ observations – representing them in climate models is therefore difficult.

• Satellite observations have the potential to give us widespread cloud information, but retrievals require assumptions about cloud properties often based on mid-latitude clouds.

The BAS cloud instruments

•The CAPS instrument•Consists of 3 instruments

CAS (Cloud Aerosol Spectrometer)Size distributions of particles 0.61-50 μm in diameter

CIP (Cloud Imaging Probe)Takes images of particles 25-1550 μm in diameter (mainly ice)Can calculate size distributions from these

Hotwire probeMeasures the liquid water content

•How do Antarctic clouds differ from mid-latitude ones?Cloud Condensation Nuclei (CCN) concentrations?Ice Nuclei (IN) concentrations?Different CCN/IN sources – e.g. Bio IN?

Knowing these is important as they determine cloud reflectivity

The Antarctic Peninsula region

Larsen B

Larsen C

Wilkins

1540 km

1750

km

Topography

Scale comparisonScale comparison

•Consists of a long ridge of high mountains (up to ~2000 m high).

= Rothera BAS base

Case study – lenticular clouds in Marguerite bay

200 250 300 350 400 450 500 550200

250

300

350

400

450

500

550

X (km)

Y (

km)

Altitude (m) from 19.6861 to 22.0281 UTC

-7

2

-70

-70

-68

-68

-66

-66

-64

-69.5

-69

-68.5

-68.5

-68

-68

-68

-67.5

-67.5

-67

-67

-66.5

-66.5

-66

500

1000

1500

2000

2500

3000

Approx wind direction


Altitude (m)Altitude (m)

Deep low in the North Weddell Sea.

Led to a strong cross Peninsula flow (east to west).

Large stacks of lenticulars were developing over the mountains.

But flew through bands of lenticulars developing out into Marguerite Bay.

Clouds most likely formed on the crests of lee waves.

= Rothera BAS base

Flow over mountain sets off vertical motions

Stable air on downwind side allows vertical oscillations

Lee wave clouds

•Since such clouds are likely to have been recently formed and are not likely to be deep they are quite simple•May therefore be useful to look at ice nucleation

The overall picture



Blue - indicates small particles, probably droplets

Green – large particles, probably ice

Grey - both.



Grey - both.

Flight segment20:20-20:40 UTC





Grey - both.



Grey - both.

Examining the lee waves

• Gravity waves of temperature amplitude 2-5 oC.

• Horizontal wavelengths of 9-10 km.

• Predominately liquid formed at the crests of the gravity waves.

• But some ice too

• Ice present on the downward part of the waves – likely sedimentation from above

• RHi > RH at these temperatures – so would be supersaturated w.r.t. ice if are forming liquid

Not many observed ice crystals to base the statistics on.



Grey - both.



Grey - both.

3500 m

3000 m

2500 m

2000 m

1500 m



Grey - both.



Grey - both.

3500 m

3000 m

2500 m

2000 m

1500 m



Grey - both.



Grey - both.

3500 m

3000 m

2500 m

2000 m

1500 m

Distance along flight track (km)

Ice

ma

ss (

mg

m-3

)

Ice mass (mg m-3) for flight 102 on

160 170 180 190 200 210 220 2300

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Total ice mass

Ice Nuclei concentration parameterisations

Heterogeneous Ice Nuclei (IN)Heterogeneous Ice Nuclei (IN)

Heterogeneous IN number cocentration (L-1)

Te

mp

era

ture

(oC

)

Ice nuclei concentations

0 0.5 1 1.5 2 2.5

-20

-15

-10

-5

WRF (Coooper)Fletcher

These include deposition IN (direct nucleation from vapour phase) and condensation IN (liquid droplet nucleated first, which then freezes)

Numbers of up to ~0.15-0.35 per litre predicted for the temperature range of the lenticulars for the WRF scheme

These include deposition IN (direct nucleation from vapour phase) and condensation IN (liquid droplet nucleated first, which then freezes)

Numbers of up to ~0.15-0.35 per litre predicted for the temperature range of the lenticulars for the WRF scheme

Actual ice concentrations of 0.1-0.45 per litre observed. Thus IN parameterisation seem to be of the right magnitude.

Actual ice concentrations of 0.1-0.45 per litre observed. Thus IN parameterisation seem to be of the right magnitude.

Ice number production rate (L-1 s-1)

Te

mp

era

ture

(oC

)

Ice number formation rates for P=650 mb, QC=0.1 g m-3

0 1 2 3

x 10-4

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

WRF BiggsContact freezingBiggs + contact freezing

Immersion and contact INImmersion and contact IN

Ice Nuclei concentration parameterisations

•Bigg’s (immersion IN) - IN already contained within droplets•Contact IN – when droplets collide with airborne IN and freeze

Gives a rate of freezing – need to estimate a period of nucleation to get a concentration

•Bigg’s (immersion IN) - IN already contained within droplets•Contact IN – when droplets collide with airborne IN and freeze

Gives a rate of freezing – need to estimate a period of nucleation to get a concentration

Estimation of ice duration of formation

•Icy regions at gravity wave crests are ~5 km wide.•Wind speeds of ~20 m/s. Gives an ice forming time of ~250 s.

Gives an ice concentration of ~0.015-0.040 per litre for -11 to -14 oC.For higher LWC of 0.2 g m-3 get 0.015-0.080 per litre.

•Icy regions at gravity wave crests are ~5 km wide.•Wind speeds of ~20 m/s. Gives an ice forming time of ~250 s.

Gives an ice concentration of ~0.015-0.040 per litre for -11 to -14 oC.For higher LWC of 0.2 g m-3 get 0.015-0.080 per litre.

Actual ice concentrations of 0.1-0.45 per litre observed. Thus immersion ice parameterisations are on the low side of the observed ice concentrations.Heterogeneous IN likely slightly dominant over Biggs and contact freezing, according to parameterisations.

Overall, observed ice concentrations similar to predicted IN concentrations

Actual ice concentrations of 0.1-0.45 per litre observed. Thus immersion ice parameterisations are on the low side of the observed ice concentrations.Heterogeneous IN likely slightly dominant over Biggs and contact freezing, according to parameterisations.

Overall, observed ice concentrations similar to predicted IN concentrations



Grey - both.



Grey - both.

3500 m

3000 m

2500 m

2000 m

1500 m



Grey - both.



Grey - both.

Summary for this flightGravity waves allowed liquid and ice supersaturation and thus droplet and ice nucleation.

Ice numbers that approximately agree with typical IN parameterisations

Deposition/condensation IN predicted to be dominant over immersion nucleation.

Except in the likely aircraft seeding region

How representative of IN concentrations are the observed ice crystal concentrations?

No ice observed in some of the liquid regions – why?

Summary of another flight

300 350 400 450 500 550 600 650200

250

300

350

400

450

500

550

X (km)

Y (

km)

Altitude (m) from 18.6612 to 21.9909 UTC

-70

-68

-68

-66

-66

-66

-64

-64

-62

-69

-68.5

-68

-68

-67.5

-67.5

-67

-67

-67

-66.5

-66.5

-66

-66

-65.5

-65

500

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3500

4000

0 1 2 3 4 5

-25

-20

-15

-10

-5

0

5

10

Ice number (L-1)

Tem

pera

ture

(o C)

Ice mass (colours; mg m-3) and number for flight 104 for 18:39:41 to 21:59:27

0

0.5

1

1.5

2

2.5

3

3.5

4

Cold temperatures of cloud over the mountain.Fairly low concentrations and large ice particles:-


Ice mass (mg m-3)Ice mass (mg m-3)

Ice numberIce number

0 1 2 3 4 5

-25

-20

-15

-10

-5

0

5

10

Ice number (L-1)

Tem

pera

ture

(o C)

Ice mass (colours; mg m-3) and number for flight 104 for 18:39:41 to 21:59:27

0

0.5

1

1.5

2

2.5

3

3.5

4

Hallet Mossop splinter production zonePlenty of liquid water available.Lots of ice splinter columns observed:-

Hallet Mossop splinter production zonePlenty of liquid water available.Lots of ice splinter columns observed:-

Ice numberIce number

Liquid waterLiquid water

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-25

-20

-15

-10

-5

0

5

Hotwire LWC (g m-3)

Tem

pera

ture

(o C)

LWC&altitude (colours; m) for flight 104 for 18:40:12 to 21:58:25

500

1000

1500

2000

2500

3000

3500

4000

Ice mass (mg m-3)Ice mass (mg m-3)



Ice Nuclei parameterisation comparison

Observed ice generally < 1 per litreSuggests possible overestimation of parameterisations?

Observed ice generally < 1 per litreSuggests possible overestimation of parameterisations?

Heterogeneous IN parameterisations estimate ~2.25 L-1 at T= -20 oC Biggs freezing (immersion IN) of up to ~0.02 L-1 s-1

predicted for T= -20 oC and LWC=0.3 g m-3.

Wind speeds of 18 m/s at T=-20 oC. Ice present over distances of ~25 km.Gives estimate of ~>20 per litre at coldest temperatures.

Heterogeneous IN parameterisations estimate ~2.25 L-1 at T= -20 oC Biggs freezing (immersion IN) of up to ~0.02 L-1 s-1

predicted for T= -20 oC and LWC=0.3 g m-3.

Wind speeds of 18 m/s at T=-20 oC. Ice present over distances of ~25 km.Gives estimate of ~>20 per litre at coldest temperatures.

Heterogeneous IN number cocentration (L-1)

Te

mp

era

ture

(oC

)

Ice nuclei concentations

0 0.5 1 1.5 2 2.5

-20

-15

-10

-5

WRF (Coooper)Fletcher

Conclusions

• Lee wave (lenticular clouds) likely provide a good “natural laboratory” to look at Ice Nuclei numbers.

• Mostly liquid formed in the gravity wave crests, but some ice was observed in the crests and in the troughs (likely precipitated from above).

• Ice numbers were consistent with IN parameterisations for the -11 to -14 oC temperature range.

• Parameterisations suggested that deposition/condensation IN likely the biggest source of ice.

• Likely seeding of ice from aircraft exhaust – caution required in data interpretation and flight track planning.

• For the colder clouds at -20 oC the parameterisation numbers were considerably higher than those observed (factor of 20 or so).

• In the -3 to -8 oC temperature range the Hallet Mossop process was observed producing more ice particles – concentrations up to 3.5 per litre.

• This process is likely to be important for glaciating Antarctic clouds given the likely low IN concentrations.

Daniel Grosvenor, Thomas Choularton, Martin Gallagher (University of Manchester, UK); Thomas Lachlan...

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