Transcript In situ aircraft observations of Antarctic Peninsula clouds

In-cloud aircraft observations over the Antarctic Peninsula
Daniel Grosvenor, T. Choularton, M. Gallagher, K. Bower, J. Crosier (University of
Manchester, UK);
Thomas Lachlan Cope and Russell Ladkin (British Antarctic Survey).
Daniel Grosvenor, Thomas Choularton, Martin Gallagher (University of Manchester, UK);
Thomas Lachlan Cope and John King (British Antarctic Survey).
•Contents
•Antarctic clouds
•Flight 102 – In situ observations of
lenticular clouds
•Ice number observations – can
they give an estimate of Ice Nuclei
concentrations?
•How do they compare to current
ice parameterisations (based on
non-Antarctic clouds)?
•Are Antarctic clouds different?
•Flight 104
•Ice observations at colder
temperatures
•Ice formed by the Hallet Mossop
process.
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 probe
Measures 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
Scale
comparison
Wilkins
•Consists of a long ridge of high
mountains (up to ~2000 m high).
Larsen C
•Have 14 flights worth of cloud data
= Rothera BAS base
1540 km
Topography
1750 km
Larsen B
Case study – lenticular clouds in Marguerite bay
Altitude (m)
Altitude (m) from 19.6861 to 22.0281 UTC
550
-6 6
.5
-6 7
-6 6
3000
-6 7
2500
-6 6
.5
-6 8
-6 7
.5
-6 8
.5
-6 8
Approx
wind
direction
1000
-6
4
300
1500
-6
6
350
2000
-6 8
400
-6 7
.5
-7 0
Y (km)
450
-7 2
-6
6
-7 0
500
250
200
200
-6 8
-6 9
.5
250
-6 9
300
500
-6 8
.5
-6
8
350
400
X (km)
450
500
550
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.
= Rothera BAS base
Clouds most likely formed on the
crests of lee waves.
Lee wave clouds
Droplet
formation
Flow over mountain sets off
vertical motions
Droplet
evaporation
Aerosol (CCN/IN) processing
through successive clouds?
Stable air on
downwind side allows
vertical oscillations
•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.
Approx
wind
direction
Flight segment
20:20-20:40 UTC
Blue - indicates
small particles,
probably droplets
Green – large
particles,
probably ice
Approx
wind
direction
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.
3000 m
Blue - indicates
small particles,
probably droplets
2500 m
Green – large
particles,
probably ice
Grey - both.
2000 m
1500 m
-3
Ice mass (mg m ) for flight 102 on
1
Total ice mass
0.9
0.8
Ice mass (mg m-3)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
160
170
180
190
200
210
220
Distance along flight track (km)
230
Ice Nuclei concentration parameterisations
Heterogeneous Ice Nuclei (IN)
-20
Ice nuclei concentations
These include deposition IN
(direct nucleation from vapour
phase) and condensation IN
(liquid droplet nucleated first,
which then freezes)
Temperature (oC)
-15
Numbers of up to ~0.150.35 per litre predicted for the
temperature range of the
lenticulars for the WRF
scheme
-10
WRF (Coooper)
Fletcher
-5
0
0.5
1
1.5
2
Heterogeneous IN number cocentration (L -1)
2.5
Actual ice concentrations of 0.1-0.45 per litre observed. Thus IN parameterisation
seem to be of the right magnitude.
Ice Nuclei concentration parameterisations
Immersion and contact IN
-15
Ice number formation rates for P=650 mb, QC=0.1 g m -3
•Bigg’s (immersion IN) - IN already
contained within droplets
•Contact IN – when droplets collide with
airborne IN and freeze
-14
-13
Gives a rate of freezing – need to
estimate a period of nucleation to get a
concentration
Temperature (oC)
-12
-11
-10
-9
-8
-7
-6
-5
0
WRF Biggs
Contact freezing
Biggs + contact freezing
1
2
Ice number production rate (L -1 s-1)
3
x 10
-4
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.
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
3000 m
2500 m
Blue - indicates
small particles,
probably droplets
Green – large
particles,
probably ice
Grey - both.
2000 m
1500 m
Summary of another flight
Altitude (m) from 18.6612 to 21.9909 UTC
-6 8
-7 0
-6 6
-6
4
-6 7
4000
-6 5
.5
-6 6
.5
500
-6 5
-6
6
550
3000
450
-6
6
-6
6
-6 6
.5
400
-6 8
-6 7
2500
2000
-6 7
.5
-6 8
300
-6 8
.5
-6
2
350
-6
4
Y (km)
3500
1500
-6
7
1000
250
-6 9
-6
6
200
300
350
500
-6
8
400
450
500
X (km)
-6
7.5
550
600
650
Ice number
Ice mass (colours; mg m -3) and number for flight 104 for 18:39:41 to 21:59:27
-25
-20
3.5
Ice mass
(mg m-3)
-15
Temperature (oC)
4
3
2.5
-10
2
-5
1.5
0
1
5
10
0.5
0
1
2
3
-1
Ice number (L )
4
5
0
Cold temperatures of cloud over the mountain.
Fairly low concentrations and large ice
particles:-
Ice mass (colours; mg m -3) and number for flight 104 for 18:39:41 to 21:59:27
-25
-20
3.5
Ice mass
(mg m-3)
Temperature (oC)
-15
Ice number
4
3
2.5
-10
2
-5
1.5
0
1
5
10
0.5
0
1
2
3
4
5
0
Ice number (L-1)
LWC&altitude (colours; m) for flight 104 for 18:40:12 to 21:58:25
-25
Temperature (oC)
4000
Liquid water
Cold temperatures of cloud
over the mountain.
Fairly low concentrations
and large ice particles:-
-20
3500
-15
3000
2500
-10
2000
-5
1500
0
5
1000
500
0
0.1
0.2
0.3
0.4
0.5
0.6
Hotwire LWC (g m-3)
0.7
0.8
Hallet Mossop splinter
production zone
Plenty of liquid water
available.
Lots of ice splinter columns
observed:-
Ice Nuclei parameterisation comparison
-20
Ice nuclei concentations
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.
Temperature (oC)
-15
-10
WRF (Coooper)
Fletcher
-5
0
0.5
1
1.5
2
Heterogeneous IN number cocentration (L -1)
2.5
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.
Observed ice generally < 1 per litre
Suggests possible overestimation of parameterisations?
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 (where was 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.