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Numerical Weather Prediction Numerical Weather Prediction Parametrization of diabatic processesParametrization of diabatic processes
Cloud ParametrizationCloud Parametrization
Adrian Tompkins
OutlineOutline
• LECTURE 1: Introduction to Cloud Issues– Physical processes to represent– What are the potential problems in GCMs?– History of cloud schemes
• LECTURE 2: Cloud Cover in GCMs
• LECTURE 3: The ECMWF cloud scheme
• LECTURE 4: Validation of cloud schemes
GCMs: Issues and GCMs: Issues and approachesapproaches
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 evaporation/sublimation of cloud and precipitation size
particles
Microphysical processes and Microphysical processes and water substanceswater substances• Nucleation of particles• Diffusion growth• Collision-Coalescence • Collection• Breakup of drops• Sedimentation• Ice enhancement• Melting• Evaporation/sublimation
• Involving:• Water vapour• Cloud liquid water• Precipitation liquid water• Cloud ice• Precipitation ice
Processes Uncertain - Parameterization often highly simplified
Reference: “Microphysics of Clouds and Precipitation” by H.R.Pruppacher and J. D. Kett and “A short course in Cloud Physics” by Rogers and Yau
(all subsequent diagrams from latter text unless otherwise stated)
Microphysics: Microphysics: Complex Complex System!System!
Overview of(1) Warm Phase Microphysics T>273K(2) Mixed Phase Microphysics 235K<T<273K(3) Pure ice Microphysics T<235K
Droplet ClassificationDroplet Classification
Nucleation of Water:Nucleation of Water:Homogeneous NucleationHomogeneous Nucleation
• Drop of pure water forms from vapour
• Kelvin’s formula for critical radius for initial droplet to be “survive”
• strongly dependent on supersaturation
• requires several hundred percent supersaturation (not observed in the atmosphere),
sLv
vlc
eeTR
Rln
2
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)
• Particles are called Cloud Condensation Nuclei (CCN)• CCN always present in sufficient numbers in lower and
middle troposphere• Activation at supersaturations of <1%• Therefore we assume that condensation occurs if
RH>100% (n.b. deep convection)
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
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
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)
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 above these 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
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 supersturated 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
lP eqcG
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
Homogeneous/heterogeneous Homogeneous/heterogeneous nucleation nucleation
• However even in “polluted” NH air, homogeneous nucleation can dominate in strong updraughts
• However wind fluctuations can occur on mesoscale length-scales comparible or smaller to 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)
• Recent observations (e.g. Mozaic aircraft and Microwave sounders) have revealed such values are common.
3000 km supersaturated segment observed ahead of front
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
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 prognostica diagnostic
ql prognostica prognostic(directly orindirectly)
Radiationeffects
Prescribedzonal meanalbedo andemissivity
a diagnostic[usually f(RH)]ql prescribed
a = as cloudscheme
a = as cloudscheme
Convection No cloudinteraction
acu = f(CP)ql cu prescribed
acu = f(CP)ql cu prescribed
Direct link toa and ql
Microphysics none none Simple bulkmicrophysics
Complex bulkmicrophysics(ice)
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