Parameterization in models Introduction to cloud issues

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Parameterization in modelsIntroduction to cloud
issues
tompkins_at_ictp.it
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Clouds in GCMs - What are the problems ?
Many of the observed clouds and especially the
processes within them are of subgrid-scale size
(both horizontally and vertically)
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Macroscale Issues of Parameterization
VERTICAL COVERAGE
Most models assume that this is 1
This can be a poor assumption with coarse
vertical grids. Many climate models still use
fewer than 30 vertical levels currently, some
recent examples still use only 9 levels
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Macroscale Issues of Parameterization
HORIZONTAL COVERAGE, a
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Macroscale Issues of Parameterization
Vertical Overlap of cloud
Important for Radiation and Microphysics
Interaction
500m
100km
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Macroscale Issues of Parameterization
In cloud inhomogeneity in terms of cloud particle
size and number
500m
100km
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Macroscale Issues of Parameterization
Just these issues can become very complex!!!
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Clouds in GCMs - What are the problems ?
Clouds are the result of complex interactions
between a large number of processes
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Clouds in GCMs - What are the problems ?
Many of these processes are only poorly
understood - For example, the interaction with
radiation
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What do we want to represent?
Complexity
Cloud Mass
Single Moment Schemes
Double Moment Schemes
Spectral/Bin Microphysics
Most GCMs only have simple single-moment schemes
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Clouds in GCMs - How ?
Main variables Cloud fraction, a - refers to
horizontal cover since cloud fills vertical
Cloud condensate mass (cloud water and/or ice),
ql.
Diagnostic approach
Prognostic approach
NOT DISTINCT - CAN HAVE MIXTURE OF APPROACHES
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Cloud microphysical processes

• We would like to include into our models
• Formation of clouds
• Release of precipitation
• Evaporation of both clouds and precipitation
• Therefore we need to describe
• the change of phase from water vapour to water
  droplets and ice crystals
• the transformation of small cloud droplets/ice
  crystals to larger rain drops/ice particles
• The advection and sedimentation/falling of these
  species
• the evaporation/sublimation of cloud and
  precipitation size particles

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Microphysics Complex System!

• Overview of
• Warm Phase Microphysics Tgt273K
• Mixed Phase Microphysics 250KltTlt273K
• Pure ice Microphysics Tlt250K

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Droplet Classification
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Important effects for particle activation
Planar surface Equilibrium when ees and number
of molecules impinging on surface equals rate of
evaporation
Surface molecule has fewer neighbours
Curved surface saturation vapour pressure
increases with smaller drop size since surface
molecules have fewer binding neighbours.
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Nucleation of WaterHomogeneous Nucleation

• Drop of pure water forms from vapour
• Small drops require much higher super saturations
  
• Kelvins formula for critical radius for initial
  droplet to be survive
• strongly dependent on supersaturation
• Would require several hundred percent
  supersaturation (not observed in the atmosphere).

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Nucleation of WaterHeterogeneous Nucleation

• Collection of water molecules on a foreign
  substance, RH gt 80 (Haze particles) (Note, not
  same when drying)
• These (hydrophilic) solluble particles are called
  Cloud Condensation Nuclei (CCN)
• CCN always present in sufficient numbers in lower
  and middle troposphere

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Important effects for particle activation
Planar surface Equilibrium when ees and number
of molecules impinging on surface equals rate of
evaporation
Surface molecule has fewer neighbours
Curved surface saturation vapour pressure
increases with smaller drop size since surface
molecules have fewer binding neighbours. Effect
proportional to r-1
Dissolved substance reduces vapour pressure
Presence of dissolved substance saturation
vapour pressure reduces with smaller drop size
due to sollute molecules replacing solvent on
drop surface (assuming esolluteltev) Effect
proportional to r-3
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Heterogeneous Nucleation
Haze particle in equilibrium
e/es equilibrium
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Parameterizing Nucleation and droplet growth

• Nucleation Since Activation occurs at
  supersaturations less than 1 most schemes
  assumes all supersaturation is immediately
  removed as liquid water
• Note that this assumption means that models can
  just use one prognostic equation for the total
  water mass, the sum of vapour and liquid
• Usually, the growth equation is not explicitly
  solved, and in single-moment schemes simple
  (diagnostic) assumptions are made concerning the
  droplet number concentration when needed (e.g.
  radiation). These often assume more CCN in
  polluted air over land.

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Diffusion growth (water)

• Nucleation small droplets
• once droplet is activated, water vapour diffuses
  towards it condensation
• reverse process evaporation
• droplets that are formed by diffusion growth
  attain a typical size of 0.1 to 10 mm
• rain drops are much larger than that
• drizzle 50 to 100 mm
• rain gt100 mm
• other processes must also act in precipitating
  clouds

For r gt 1 mm and neglecting diffusion of
heat DDiffusion coefficient, SSupersaturation No
te inverse radius dependency
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Collision-Coalescence

• Drops of different size move with different fall
  speeds - collision and fusion
• large drops grow at the expense of small droplets
• Collection efficiency low for small drops
• process depends on width of droplet spectrum and
  is more efficient for broader spectra - Paradox
• large drops can only be produced in clouds of
  large vertical extent Aided by turbulence and
  entrainment
• important process for low latitudes where deep
  clouds of high water content are present

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Parameterizating Autoconversion of cloud drops
to raindrops
Autoconversion (Kessler, AMS monogram 1969)
Sundqvist, QJRMS, 1978
what are the issues for data assimilation?
Non-local collection
PPrecipitation Flux
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Schematic of Warm Rain Processes
Heterogeneous Nucleation RHgt78 (Haze)
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Ice Nucleation

• Ice processes complex and poorly understood
• Droplets do not freeze at 0oC!
• Can also be split into Homogeneous and
  Heterogeneous processes
• Processes depend on temperature and history of
  cloud
• Homogeneous freezing of water droplet occurs
  between 35 and 40oC (often used assumption in
  microphysical schemes).
• Frequent observation of ice at warmer
  temperatures indicates role for heterogeneous
  processes

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Ice Nucleation

• Spontaneous freezing of liquid droplets smaller
  than 5 mm requires temperature less than -40oC.
• Observations of liquid in cloud are common at
  -20oC.
• Ice crystals start to appear in appreciable
  numbers below around -15oC.
• Heterogenous Nucleation responsible Process less
  clear
• Ice nuclei Become active at various temperatures
  less than 0oC, many fewer
• Observations
• lt -20oC Ice free clouds are rare
• gt -5oC ice is unlikely
• ice supersaturation ( gt 10 ) observations are
  common

Fletcher 1962
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Heterogeneous Nucleation
I will not discuss heterogeneous ice nucleation
in great detail in this course due to lack of
time and the fact that these processes are only
just starting to be tackled in Large-scale
models. See recent work of Ulrike Lohmann for
more details
aerosol
Supercooled drop
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Ice Habits
Ice habits can be complex, depends on
temperature influences fall speeds and radiative
properties
http//www.its.caltech.edu/atomic/snowcrystals/
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Mixed Phase clouds Bergeron Process (I)

• The saturation water vapour pressure with respect
  to ice is smaller than with respect to water

• A cloud, which is saturated with respect to water
  is supersaturated with respect to ice !

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Bergeron process (II)
Ice particle enters water cloud
Ice particles grow at the expense of water
droplets
Cloud is supersaturated with respect to ice
Diffusion of water vapour onto ice particle
Cloud will become sub-saturated with respect to
water
Water droplets evaporate to increase water vapour
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Modification of Sundqvist to take Bergeron
Process into account
Sundqvist, QJRMS, 1978
Collection
Bergeron Process
Otherwise, most schemes have neglected ice
processes, removing ice super-saturation al la
Warm rain See Lohman and Karcher JGR 2002(a,b)
for first attempts to include ice microphysics in
GCM
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Aggregation

• Ice crystals can aggregate together to form snow
• Temperature dependent, process increases in
  efficiency as temperature exceeds 5C, when ice
  surface becomes sticky
• Also a secondary peak between 10 and 16C when
  dendrite arms get entangled

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Riming

• If vapour exceeds the water saturation mixing
  ratio, water can condense on ice crystal, and
  then subsequently freeze to form graupel Round
  ice crystals with higher densities and fall
  speeds than snow dendrites
• Graupel and Hail are also formed by aggregating
  liquid water drops in mixed phased clouds
  (riming)
• If the Latent heat of condensation and fusion
  keeps temperature close to 273K, then high
  density hail particle forms, since the liquid
  water spreads out before freezing. Generally
  referred to as Hail The higher fall speed (up
  to 40 m/s) imply hail only forms in convection
  with strong updraughts able to support the
  particle long enough for growth

http//www.its.caltech.edu/atomic/snowcrystals/
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Aggregation and Riming Simple stratified
picture
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Ice Habits
Ice habits can be complex influences fall speeds
and radiative properties
Note shape/diameter distribution not monotonic
with height, Turbulence!
From Fleishauer et al 2002, JAS
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Falling Precipitation

• Need to know size distribution
• For ice also affected by ice habit
• Poses problem for numerics

Courtesy R Hogan, U. Reading
From R Hogan www.met.rdg.ac.uk/radar
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Pure ice Phase Homogeneous Ice Nucleation

• At cold temperature (e.g. upper troposphere)
  difference between liquid and ice saturation
  vapour pressures is large.
• If air mass is lifted, and does not contain
  significant liquid particles or ice nuclei, high
  supersaturations with respect to ice can occur,
  reaching 160 to 170.
• Long lasting contrails are a signature of
  supersaturation

Institute of Geography, University of Copenhagen
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Heteorogeneous Nucleation

• On the other hand, in air polluted by organic and
  mineral dust, supersaturation achieve perhaps
  130.
• Research is ongoing to determine the nature of
  ice nuclei at these colder temperatures
• This process probably more prevalent in NH where
  air is less clean

22nd April 2003 Modis Image from
http//modis.gsfc.nasa.gov
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Schematic of parcel evolution, Tlt-38oC
RHice
160
RHlt160
Lifting/cooling
130
100
aerosol
Few IN
Many IN
No IN
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Simple Parcel model calculationsMaximum humidity
important for determining number of ice crystals
nucleated
From Ren and Mackenzie QJRMS 2005
Lift (cool) parcel
Parcel containing aerosols
Single aerosol type
4 major issues!
e.g. Jensen 94, Spice et al. 97, Gierens 2003,
Ren and Mackenzie 2005
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Cooling rates-Vertical velocity

• Even in polluted NH air, homogeneous nucleation
  can dominate in strong updraughts
• Knowledge of local velocities is crucial
• However important wind fluctuations occur on
  length-scales comparable to/smaller than grid
  length
• Upscale cascade from turbulence
• Gravity waves
• Subgrid instabilities (cloud top instabilities)
• It is clear that models are deficient in
  representing these
• E.g Lohmann shows inadequacy of ECMWF model, and
  how enhancement of turbulence activity can
  produce improved spectra

From haag and Kaerchner
aircraft
ECMWF resolved motions only
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Limb Sounder and Mozaic Data(Pictures courtesy
of Klaus Gierens and Peter Spichtinger, DLR)

• Observations (e.g. Mozaic aircraft and Microwave
  sounders) have revealed supersaturated states are
  common, also seen in radiosonde datasets

3000 km supersaturated segment observed ahead of
front
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Summary Warm Cloud
E.g Stratocumulus
Evaporation
Condensation
(Rain formation - Fall Speeds - Evaporation of
rain)
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Summary Deep Convective Cloud

• Heteorogeneous Nucleation of ice
• Splintering/Bergeron Process

• Melting of Snow and Graupel

• Precipitation Falls Speeds
• Evaporation in Sub-Cloud Layer

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Summary Cirrus cloud
Homogeneous Nucleation (representation of
supersaturation)
Heterogeneous Nucleation (representation of
nuclei type and concentration)
Sedimentation of Ice crystals? Size distribution
and formation of snow?
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Cloud Schemes - A Brief History
C
C
C
C
C
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Simple Bulk Microphysics
VAPOUR (prognostic)
Evaporation Condensation
CLOUD (prognostic)
Evaporation
Autoconversion
RAIN (diagnostic)
WHY?
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Microphysics - a complex GCM scheme
Fowler et al., JCL, 1996
Similar complexity to many schemes in use in CRMs
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Cloud Cover Why Important?
In addition to the influence on radiation, the
cloud cover is important for the representation
of microphysics
Imagine a cloud with a liquid condensate mass
ql The incloud mass mixing ratio is ql/a
Complex microphysics perhaps a wasted effort if
assessment of a is poor