1 / 30

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

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).

deva
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 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).

  2. 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.

  3. 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 The BAS cloud instruments Knowing these is important as they determine cloud reflectivity • 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?

  4. The Antarctic Peninsula region Wilkins Larsen B 1750 km Larsen C 1540 km Scale comparison • Consists of a long ridge of high mountains (up to ~2000 m high). • Have 14 flights worth of cloud data = Rothera BAS base Topography

  5. Case study – lenticular clouds in Marguerite bay Altitude (m) Approx wind direction • 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

  6. 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

  7. The overall picture Blue - indicates small particles, probably droplets Green – large particles, probably ice Grey - both. Approx wind direction

  8. Flight segment20:20-20:40 UTC Blue - indicates small particles, probably droplets Green – large particles, probably ice Grey - both. Approx wind direction

  9. 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

  10. Not many observed ice crystals to base the statistics on.

  11. 3500 m 3000 m Blue - indicates small particles, probably droplets Green – large particles, probably ice Grey - both. 2500 m 2000 m 1500 m

  12. Ice Nuclei concentration parameterisations Heterogeneous Ice Nuclei (IN) • 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.

  13. Ice Nuclei concentration parameterisations Immersion and contact IN • 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

  14. 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

  15. 3500 m 3000 m Blue - indicates small particles, probably droplets Green – large particles, probably ice Grey - both. 2500 m 2000 m 1500 m

  16. Summary of another flight • Cold temperatures of cloud over the mountain. • Fairly low concentrations and large ice particles:- Ice mass (mg m-3) Ice number

  17. Cold temperatures of cloud over the mountain. • Fairly low concentrations and large ice particles:- Ice mass (mg m-3) Ice number • Hallet Mossop splinter production zone • Plenty of liquid water available. • Lots of ice splinter columns observed:- Liquid water

  18. Ice Nuclei parameterisation comparison • 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. • Observed ice generally < 1 per litre • Suggests possible overestimation of parameterisations?

  19. 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.

More Related