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Parameterization in modelsParameterization in modelsIntroduction to cloud issuesIntroduction to cloud issues
Clouds in GCMs - What are the Clouds in GCMs - What are the problems ?problems ?Many of the observed clouds and especially the processes within them are of subgrid-scale size (both horizontally and vertically)
GCM Grid cell 40-400km
~50
0m
~100km
Macroscale Issues of Macroscale Issues of ParameterizationParameterization
VERTICAL COVERAGEMost 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
~50
0m
~100km
Macroscale Issues of Macroscale Issues of ParameterizationParameterization
HORIZONTAL COVERAGE, a
~50
0m
~100km
Macroscale Issues of Macroscale Issues of ParameterizationParameterization
Vertical Overlap of cloudImportant for Radiation and Microphysics Interaction
~50
0m
~100km
Macroscale Issues of Macroscale Issues of ParameterizationParameterization
In cloud inhomogeneity in terms of cloud particle size and number
~50
0m
~100km
Macroscale Issues of Macroscale Issues of ParameterizationParameterization
Just these issues can become very complex!!!
Clouds in GCMs - What are the Clouds in GCMs - What are the problems ?problems ?
convection
Clouds are the result of complex interactions between a large number of processes
radiation
turbulence
dynamics
microphysics
Clouds in GCMs - What are the Clouds in GCMs - What are the problems ?problems ?
Many of these processes are only poorly understood - For example, the interaction with radiation
Cloud-radiation interaction
Cloud macrophysics Cloud microphysics “External” influence
Cloud fraction and overlap
Cloud top and base height Amount of
condensate
In-cloud conden-sate distribution
Phase of condensate Cloud particle
size
Cloud particle shape
Cloud environment
What do we want to What do we want to represent?represent?
Ice mass
Ice number
small ice
Medium ice
Large ice
Most GCMs only have simple single-moment schemes
Ice Mass
Liquid Mass
CloudMass
Complexity
“Single Moment”Schemes
“Double Moment”Schemes
“Spectral/Bin”Microphysics
Clouds in GCMs - How ?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
,, 111 tt
fa nn
,, 112 tt
fq nnl
Prognostic approach
)()()( aDaSaAt
a
)()()( llll qDqSqA
t
q
NOT DISTINCT - CAN HAVE MIXTURE OF APPROACHES
Cloud microphysical Cloud microphysical processesprocesses• 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
Microphysics: Microphysics: Complex Complex System!System!
Overview of(1) Warm Phase Microphysics T>273K(2) Mixed Phase Microphysics 250K<T<273K(3) Pure ice Microphysics T<250K
Droplet ClassificationDroplet Classification
Important effects for particle Important effects for particle activationactivation
TrRe
re
lvs
s
2
exp)(
)(
Planar surface: Equilibrium when e=es and number of molecules impinging on surface equals rate of evaporation
Curved surface: saturation vapour pressure increases with smaller drop size since surface molecules have fewer binding neighbours.
Surface molecule has fewer neighbours
radius drop
droplet of tension Surface
r
Nucleation of Water:Nucleation of Water:Homogeneous NucleationHomogeneous Nucleation
• Drop of pure water forms from vapour
• Small drops require much higher super saturations
• Kelvin’s 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).
sLv
vlc
eeTR
Rln
2
droplet of tension Surface
Radius Critical
cR
Nucleation of Water:Nucleation of Water:Heterogeneous NucleationHeterogeneous Nucleation
• Collection of water molecules on a foreign substance, RH > ~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
Important effects for particle Important effects for particle activationactivation
Planar surface: Equilibrium when e=es and number of molecules impinging on surface equals rate of evaporation
Curved surface: saturation vapour pressure increases with smaller drop size since surface molecules have fewer binding neighbours.Effect proportional to r-1
Presence of dissolved substance: saturation vapour pressure reduces with smaller drop size due to sollute molecules replacing solvent on drop surface (assuming esollute<ev)Effect proportional to r-3
Surface molecule has fewer neighbours
Dissolved substance reduces vapour pressure
Heterogeneous NucleationHeterogeneous Nucleation
“Curvature term”Small drop – high radius of curvature
easier for molecule to escape
“Solution term”Reduction in
vapour pressure due to dissolved
substance
activated"" 12.0
,01.1/
mr
ese
e/e s
equi
libriu
m
Haze particle in equilibrium
Parameterizing Nucleation Parameterizing Nucleation and droplet growthand 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.
Diffusion growth (water)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 m
• rain drops are much larger than that
– drizzle: 50 to 100 m– rain: >100 m
• other processes must also act in precipitating clouds
)1(1
STR
De
rdt
dr
vL
s
For r > 1 m and neglecting diffusion of heat
D=Diffusion coefficient, S=SupersaturationNote inverse radius dependency
Collision-CoalescenceCollision-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
Parameterizating Parameterizating “Autoconversion” of cloud drops “Autoconversion” of cloud drops to raindropsto raindrops
Autoconversion (Kessler, AMS monogram 1969)
otherwise0
if0critcrit
lllll qqqqc
t
q
qlqlcrit
Gp
Sundqvist, QJRMS, 1978
2
0 1 crit
l
l
q
q
ll eqc
t
q
qlqlcrit
Gp“Non-local” collectionPcF 11 1
1F1F
P=Precipitation Flux
what are the issues for data assim
ilation?
Heterogeneous NucleationHeterogeneous NucleationRH>78% (Haze)RH>78% (Haze)
Schematic of Warm Rain Schematic of Warm Rain ProcessesProcesses
CCN
~10 microns~10 microns
RH>100.6%RH>100.6%““Activation”Activation”DiffusionalDiffusional
GrowthGrowth
Different fall speedsDifferent fall speeds
Coalescence
Ice NucleationIce 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
Ice NucleationIce Nucleation
• Spontaneous freezing of liquid droplets smaller than 5 m 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: – < -20oC Ice free clouds are rare– > -5oC ice is unlikely– ice supersaturation ( > 10% ) observations are
common
Fletcher 1962
Heterogeneous NucleationHeterogeneous Nucleation
Supercooled drop
aerosol
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
Ice HabitsIce HabitsIce habits can be complex, depends on temperature: influences fall speeds and radiative properties
http://www.its.caltech.edu/~atomic/snowcrystals/
Mixed Phase clouds: Bergeron Mixed Phase clouds: Bergeron Process (I)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 !
Bergeron process (II)Bergeron process (II)
Ice particle enters water cloud
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
Ice particles grow at
the expense of water droplets
Modification of Sundqvist to take Modification of Sundqvist to take Bergeron Process into accountBergeron Process into account
Sundqvist, QJRMS, 1978
2
0 1 crit
l
l
q
q
ll eqc
t
q
qlqlcrit
Gp
Collection
Bergeron Process
PcF 11 1
TcF 2681 22
21FF21FF
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
AggregationAggregation
• 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
RimingRiming
• 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/
Aggregation and Riming: Aggregation and Riming: Simple stratified picture Simple stratified picture
Ice HabitsIce Habits Ice habits can be complex: influences fall speeds and radiative properties
From Fleishauer et al 2002, JAS
Note shape/diameter distribution not monotonic with height, Turbulence!
Falling PrecipitationFalling Precipitation
• Need to know size distribution• For ice also affected by ice habit• Poses problem for numerics
From R Hogan www.met.rdg.ac.uk/radar
Cou
rtes
y: R
Hog
an,
U.
Re
adin
g
Pure ice Phase: Homogeneous Pure ice Phase: Homogeneous Ice NucleationIce 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
Heteorogeneous NucleationHeteorogeneous 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
Schematic of parcel evolution, T<-Schematic of parcel evolution, T<-3838ooCC
No IN Few IN Many IN
160%
130%
100%
RHice
Lifting/cooling
aerosol
RH
<1
60%
Simple Parcel model calculationsSimple Parcel model calculationsMaximum humidity important for determining number of Maximum humidity important for determining number of ice crystals nucleatedice crystals nucleated
From Ren and Mackenzie QJRMS 2005
e.g. Jensen 94, Spice et al. 97, Gierens 2003, Ren and Mackenzie 2005
Parcelcontainingaerosols
Lift (cool)parcel
Single aerosol type
4 major issues!
Cooling rates-Vertical velocity 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
ECMWF resolved motions only
aircraft
Limb Sounder and Mozaic DataLimb Sounder and Mozaic Data(Pictures courtesy of Klaus Gierens and Peter Spichtinger, DLR)(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
Summary: Warm CloudSummary: Warm Cloud
E.g: Stratocumulus
Condensation
(Rain formation - Fall Speeds - Evaporation of rain)
Evaporation
Summary: Deep Convective Summary: Deep Convective CloudCloud
• Precipitation Falls Speeds• Evaporation in Sub-Cloud Layer
• Heteorogeneous Nucleation of ice• Splintering/Bergeron Process• Melting of Snow and Graupel
Summary: Cirrus cloudSummary: 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?
Cloud Schemes - A Brief Cloud Schemes - A Brief HistoryHistory
60s
Condensation(non-convective)
qv > qs
Radiationeffects
Prescribedzonal meanalbedo andemissivity
Convection No cloudinteraction
Microphysics none
60s 70s
Condensation(non-convective)
qv > qs qv > qs
Radiationeffects
Prescribedzonal meanalbedo andemissivity
a diagnostic[usually f(RH)]ql prescribed
Convection No cloudinteraction
acu = f(CP)ql cu prescribed
Microphysics none none
60s 70s 80s
Condensation(non-convective)
qv > qs qv > qs ql prognostica diagnostic
Radiationeffects
Prescribedzonal meanalbedo andemissivity
a diagnostic[usually f(RH)]ql prescribed
a = as cloudscheme
Convection No cloudinteraction
acu = f(CP)ql cu prescribed
acu = f(CP)ql cu prescribed
Microphysics none none Simple bulkmicrophysics
60s 70s 80s 90s-
Condensation (non-convective)
qv > qs qv > qs ql prognostic a diagnostic
ql prognostic a prognostic (directly or indirectly)
Radiation effects
Prescribed zonal mean albedo and emissivity
a diagnostic [usually f(RH)] ql prescribed
a = as cloud scheme
a = as cloud scheme
Convection No cloud interaction
acu = f(CP) ql cu prescribed
acu = f(CP) ql cu prescribed
Direct link to a and ql
Microphysics none none Simple bulk microphysics
Complex bulk microphysics (ice)
C
C
CC
C
Simple Bulk MicrophysicsSimple Bulk Microphysics
VAPOUR (prognostic)
CLOUD (prognostic)
RAIN (diagnostic)
WHY?
Evaporation
Autoconversion
Evaporation
Condensation
Microphysics - a “complex” Microphysics - a “complex” GCM schemeGCM scheme
Fowler et al., JCL, 1996
Similar complexity to many schemes in use in CRMs
Mostly treated as instant“No supersaturation
assumption”
“Threshold” linear or exponential terms
with efficiency adjustments
)( vsat qqKR
Cloud Cover: Why Important?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
a largea small
GC
M g
rid b
ox
precipitation not equal in each case sinceautoconversion is nonlinear
Reminder: Autoconversion (Kessler, 1969)
otherwise0
if0critcrit
llllP
qqqqcG
Complex microphysics perhaps a wasted effort if assessment of a is poor