Mixed-phase layer clouds

Transcription

1 Mixed-phase layer clouds Chris Westbrook and Andrew Barrett Thanks to Anthony Illingworth, Robin Hogan, Andrew Heymsfield and all at the Chilbolton Observatory

2 What is a mixed-phase cloud? Cloud below 0 C where liquid water droplets and ice crystals coexist 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

3 Observations Millimetre radar and near-infrared lidar together Powerful technique: liquid droplets very reflective to lidar ice particles dominate radar red stripe = base of liquid layer 1 Dec 2010 ice crystal virga cloud top -12 C ice crystals

4 Mechanisms for nucleating ice Know that pure water droplets freeze spontaneously at -37 C In warmer clouds, need an aerosol particle to form ice deposition cooling, time immersion cooling, time condensation immediate freezing contact diffusion

5 Mechanisms for nucleating ice Know that pure water droplets freeze spontaneously at -37 C In warmer clouds, need an aerosol particle to form ice JUST NEED RHice TO BE HIGH ENOUGH deposition cooling, time immersion cooling, time condensation immediate freezing THESE ONES NEED A LIQUID WATER CLOUD contact diffusion

6 So how important is liquid water for forming ice in the atmosphere? Westbrook and Illingworth (2011) Geophys. Res. Lett. in press. 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]

7 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

8 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] Poorly simulated in GCMs

9 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 persist in spite of the loss of water to ice?

10 Microphysical structure Example from 18 May 2008: persistent Ac with virga cloud top = -15C Liquid water dominates optical properties.

11 What did the model predict? Colours are ice Contours are liquid Cloud structure is wrong no liquid at top Cloud fraction much too low

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

13 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 - pristine planar crystals

14 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 Magono & Lee (1966)

15 Dynamics Radar spectral width measure of turbulence Blue values = still air Red = turbulence Shallow mixed layer in top 500m

16 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

17 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

18 Liquid water and θe profiles Liquid water profile ~ adiabatic Droplet concentration constant 50/cc 25g/m 2 liquid water path equiv. potential temperature profile: 500m deep well mixed layer, bounded by stable air above & below

19 Supercooled-top cloud, ice virga1km deep drizzle at surface ice crystals falling 0.5-1m/s turbulence: well-mixed layer 500m deep at cloud top stable below

20 Flux of ice crystals 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 15:30 UTC 12:20 UTC these ones are in liquid layer 11:20 UTC 15:20 UTC 12:40 UTC Conclusion: Flux 50/m 2 /second = /m 2 /hour and it goes on for hours

21 Ice nuclei budget Flux of ice out layer = 50/m 2 /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!

22 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

23 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 2 /hr (vs 25g/m 2 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)

24 Schematic diagram of mixed-phase Ac stable, potentially warm, often very dry air LW cooling - destabilises top 500m of cloud - small net cooling to liquid layer overturning - well mixed layer mixed-phase layer quasi-steady flux of ice 500m ice virga stable layer 500m dry air: evaporation of ice

25 How can we successfully simulated these clouds in a GCM? Andrew Barrett 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?

26 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

27 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 LWP out by a factor of 10 at this resolution, even when everything else is right!

28 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

29 Concentration (log scale) 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 Slope λ determined from IWC Particle size 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 real size spectra...

30 Effect on ice growth and fall out For low IWC clouds (eg Ac) growth of ice is 2x bigger than it ought to be

31 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 around too long in thin clouds

32 Fix: fit N0 = f(t,iwc) Bias is eliminated

33 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 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. - different characteristics to Chilbolton? 34

35 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

36 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. c.d.westbrook@reading.ac.uk