Numerical Weather Prediction Parametrization of Diabatic Processes Clouds (1) Cloud Microphysics Richard Forbes and Adrian Tompkins forbes@ecmwf.int.
Post on 27-Mar-2015
216 Views
Preview:
Transcript
Numerical Weather Prediction Numerical Weather Prediction Parametrization of Diabatic ProcessesParametrization of Diabatic Processes
Clouds (1)Clouds (1)Cloud MicrophysicsCloud Microphysics
Richard Forbes
and Adrian Tompkins
forbes@ecmwf.int
2
3
OutlineOutline
• LECTURE 1: Cloud Microphysics1. Overview of GCM cloud parametrization issues
2. Microphysical processes
2.1 Warm phase
2.2 Cold phase
3. Summary
• LECTURE 2: Cloud Cover in GCMs
• LECTURE 3: The ECMWF Cloud Scheme
• LECTURE 4: Validation of Cloud Schemes
4
1. Overview of GCM Cloud 1. Overview of GCM Cloud Parametrization IssuesParametrization Issues
5
1. Water Cycle
2. Radiative Impacts
3. Dynamical Impacts
The Importance of Clouds
6
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
7
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
8
Cloud Parametrization Issues:Cloud Parametrization Issues:
• Microphysical processes
• Macro-physical – subgrid heterogeneity
• Numerical issues )()()( llll qDqSqAt
q
Cloud Parametrization Issues:Cloud Parametrization Issues:Which quantities to represent ?Which quantities to represent ?
• Cloud water droplets• Rain drops• Pristine ice crystals• Aggregate snow flakes• Graupel pellets• Hailstones
9
• Note for ice phase particles: Additional latent heat. Terminal fall speed of ice hydrometeors significantly less (lower density). Optical properties are different (important for radiation).
10
Cloud Parametrization Issues:Cloud Parametrization Issues:Complexity ?Complexity ?
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
11
Cloud Parametrization Issues:Cloud Parametrization Issues:Diagnostic or prognostic Diagnostic or prognostic variables ?variables ?
Cloud condensate mass (cloud water and/or ice), ql.
Diagnostic approach
,, 11 tt
fq nnl
Prognostic approach
)()()( llll qDqSqAt
q
CAN HAVE MIXTURE OF APPROACHES
Advection + sedimentation
Sources
Sinks
12
Simple Bulk MicrophysicsSimple Bulk Microphysics
VAPOUR (prognostic)
CLOUD (prognostic)
RAIN (diagnostic)
Evaporation
Autoconversion
Evaporation
Condensation
Why ?
13
Microphysics - a “complex” Microphysics - a “complex” GCM schemeGCM scheme
Fowler et al., JCL, 1996
Similar complexity to many schemes in use
in CRMs
14
2. Microphysical Processes2. Microphysical Processes
15
Cloud microphysical processesCloud 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– Nucleation of water droplets and ice crystals from water vapour– Diffusional growth of cloud droplets (condensation) and ice crystals
(deposition)– Collection processes for cloud drops (collision-coalescence), ice crystals
(aggregation) and ice and liquid (riming) leading precipitation sized particles
– The advection and sedimentation (falling) of particles– the evaporation/sublimation/melting of cloud and precipitation size
particles
16
Microphysics: Microphysics: A complex A complex system!system!
Overview of(1) Warm Phase Microphysics T > 273K(2) Mixed Phase Microphysics ~250K < T < 273K(3) Pure ice Microphysics T < ~250K
17
2. Microphysical Processes 2. Microphysical Processes 2.1 Warm Phase 2.1 Warm Phase
18
Droplet ClassificationDroplet Classification
19
Nucleation of cloud droplets:Nucleation of cloud droplets: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
20
Nucleation of cloud droplets:Nucleation of cloud droplets: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 “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
21
Nucleation of cloud droplets:Nucleation of cloud droplets:Heterogeneous NucleationHeterogeneous Nucleation
• Collection of water molecules on a foreign substance, RH > ~80% (Haze particles)
• These (hydrophilic) soluble particles are called Cloud Condensation Nuclei (CCN)
• CCN always present in sufficient numbers in lower and middle troposphere
• Nucleation of droplets (i.e. from stable haze particle to unstable regime of diffusive growth) at very small supersaturations.
22
Nucleation of cloud droplets:Nucleation of cloud droplets: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 1/r (curvature effect)
Presence of dissolved substance: saturation vapour pressure reduces with smaller drop size due to solute molecules replacing solvent on drop surface (assuming esollute<ev)Effect proportional to 1/r3 (solution effect)
Surface molecule has fewer neighbours
Dissolved substance reduces vapour pressure
23
Nucleation of cloud droplets:Nucleation of cloud droplets: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
24
Diffusional growthDiffusional growthof cloud water dropletsof cloud water droplets
)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
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
-drizzle: 50 to 100 m
-rain: >100 m
Other processes must also act in precipitating clouds
25
CollectionCollectionCollision-coalescence of water dropsCollision-coalescence of water drops
• Drops of different size move with different fall speeds - collision and coalescence
• 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 (differential evaporation), giant CCNs ?
Rain drop shape Chuang and Beard (1990)
26
Parameterizing nucleation and Parameterizing nucleation and droplet growthdroplet 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.
27Microphysics - ECMWF Seminar on Parametrization 1-4 Sep 2008 27
Microphysical Parametrization Microphysical Parametrization “Autoconversion” of cloud drops to “Autoconversion” of cloud drops to raindropsraindrops
qlqlcrit
Gp
qlqlcrit
Gp
• Linear function of ql (Kessler, 1969)
• Function of ql with additional term to avoid singular threshold and non-local precipitation term (Sundqvist 1978)
• Or, for example, for a more complex double-moment parametrization (Seifert and Beheng,2001), derived directly from the stochastic collection equation.
Kessler
Sundqvist
otherwise0
if0critcrit
lllll qqqqc
t
q
2
0 1 critl
l
q
q
ll eqct
q1F
1F
28
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
29
2. Microphysical Processes2. Microphysical Processes2.2 Cold Phase 2.2 Cold Phase
30
Kepler (1611) “On the Six-Kepler (1611) “On the Six-Cornered Snowflake”Cornered Snowflake”
31
““The Six-Cornered Snowflake”The Six-Cornered Snowflake”
www.snowcrystals.comKen Libbricht
32
• Ice nucleation
• Depositional Growth (and sublimation)
• Collection (aggregation/riming)
• Splintering
• Melting
Ice Microphysical ProcessesIce Microphysical Processes
33
Ice NucleationIce Nucleation
• Droplets do not freeze at 0oC !
• Ice nucleation processes can also be split into Homogeneous and Heterogeneous processes, but complex and not well understood.
• Homogeneous freezing of water droplets occurs below about -38oC,
• Homogeneous nucleation of ice crystals dependent on a critical relative humidity (fn of T, Koop et al. 2000).
• Frequent observation of ice between 0oC and colder temperatures indicates heterogeneous processes active.
• Number of activated ice nuclei increases with decreasing temperature.
Fletcher 1962
• Observations: – < -20oC Ice free clouds are rare– > -5oC ice is unlikely (unless ice falling from above!)– ice supersaturation ( > 10% ) observations are
common
34
Ice Nucleation:Ice Nucleation: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 starting to be tackled in Large-
scale models. See recent work of Ulrike Lohmann for more
details
35
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
36
4( )
1s s
a si
CsFS D
L L RTRT k T Xe
Equation for the rate of change of mass for an ice particle of diameter D due to deposition (diffusional growth), or evaporation
Diffusional growth of ice crystalsDiffusional growth of ice crystalsDepositionDeposition
• Deposition rate depends primarily on• the supersaturation, s • the particle shape (habit), C • the ventilation factor, F
• The particular mode of growth (edge growth vs corner growth) is sensitive to the temperature and supersaturation
37
Diffusional growth of ice crystalsDiffusional growth of ice crystalsIce HabitsIce Habits
Ice habits can be complex, depend on temperature: influences fall speeds and radiative properties
http://www.its.caltech.edu/~atomic/snowcrystals/
38
Diffusional growth of ice crystalsDiffusional growth of ice crystalsAnimationAnimation
39
Ice Saturation
Homogeneous Freezing Temperature
Water S
aturationDiffusional growth of ice crystalsDiffusional growth of ice crystalsMixed Phase Clouds: Bergeron Process Mixed Phase Clouds: Bergeron Process (I)(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 !
40
Diffusional growth of ice crystalsDiffusional growth of ice crystalsMixed phase cloud Bergeron process Mixed phase cloud Bergeron process (II)(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
41
• Ice crystals can aggregate together to form “snow”
• “Sticking” efficiency increases as temperature exceeds –5C
• Irregular crystals are most commonly observed in the atmosphere (e.g. Korolev et al. 1999, Heymsfield 2003)
Collection processes:Collection processes:Ice Crystal AggregationIce Crystal Aggregation
Lawson, JAS’99
Field & Heymsfield ‘03
500 m
CPI Model
T=-46oC
Westbrook et al. (2008)
• Some schemes represent ice processes very simply, removing ice super-saturation (as for warm rain process).
• Others, have a slightly more complex representation allowing ice supersaturation (e.g. current ECMWF scheme).
• Increasingly common are schemes which represent ice, supersaturation and the diffusional growth equation, and aggregation represented as an autoconversion to snow or parametrization of an evolving particle size distribution (e.g. Wilson and Ballard, 1999). See Lohmann and Karcher JGR 2002(a,b) for another example of including ice microphysics in a GCM.
Parametrization of ice crystal Parametrization of ice crystal diffusion growth and aggregationdiffusion growth and aggregation
42
43
Collection processes:Collection processes:Riming Riming –– capture of water drops by capture of water drops by ice ice
• Graupel formed by collecting liquid water drops in mixed phased clouds (“riming”), particulaly when at water saturation in strong updraughts (convection). Round ice crystals with higher densities and fall speeds than snow dendrites
• Hail forms if particle temperature close to 273K, 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
44
Rimed IRimed Icce Crystalse Crystals
http://www.its.caltech.edu/~atomic/snowcrystals
45
Rimed Ice CrystalRimed Ice Crystal
emu.arsusda.gov
46
Heavily Rimed Ice CrystalHeavily Rimed Ice Crystal
emu.arsusda.gov
• Most GCMs with parametrized convection don’t explicitly represent graupel or hail (too small scale)
• In cloud resolving models, traditional split between ice, snow and graupel and hail but this split is rather artificial.
• Degree of riming can be light or heavy.
• Alternative approach:– Morrison and Grabowski (2008) have three ice prognostics, ice
number concentration, mixing ratio from deposition, mixing ratio from riming.
– Avoids artificial thresholds between different categories.
Parametrization of rimed ice Parametrization of rimed ice particlesparticles
47
48
Other microphysical processesOther microphysical processesSplintering, Shedding Evaporation, MeltingSplintering, Shedding Evaporation, Melting
• Other processes include evaporation (reverse of condensation), ice sublimation (reverse of deposition) and melting.
• Shedding: Large rain drops break up – shedding to form smaller drops, places a limit on rain drop size.
• Splintering of ice crystals, Hallet-Mossop splintering through riming around -5°C. Leads to increased numbers of smaller crystals.
49
Particle Size DistributionsParticle Size Distributions
Mass DistributionSize Distribution
From Fleishauer et al (2002, JAS) Field (2000), Field and Heymsfield (2003)
50
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
51
Image from Robin Hogan. Data from RCRU RAL.
Typical time-height cross section of a front from the vertically pointing 94GHz radar at Chilbolton, UK
Microphysics at the Cloud ScaleMicrophysics at the Cloud Scale
Melting
Ice Nucleation and diffusion growth Aggregation
Warm phase
54
3. Summary3. Summary
55
Summary: Warm CloudSummary: Warm Cloud
E.g: Stratocumulus
Condensation
(Rain formation - Fall Speeds - Evaporation of rain)
Evaporation
56
Summary: Deep Summary: Deep Convective CloudConvective Cloud
• Precipitation Falls Speeds• Evaporation in Sub-Cloud Layer
• Heteorogeneous Nucleation of ice• Splintering/Bergeron Process
• Melting of Snow and Graupel
57
Summary: Cirrus cloudSummary: Cirrus cloud
Homogeneous Nucleation(representation of supersaturation)
Heterogeneous Nucleation(representation of nuclei type and concentration)
Diffusional growth, Aggregation Sedimentation of ice crystals
58
Simple Bulk MicrophysicsSimple Bulk Microphysics
VAPOUR (prognostic)
CLOUD (prognostic)
RAIN (diagnostic)
Evaporation
Autoconversion
Evaporation
Condensation
59
Microphysics - a “complex” Microphysics - a “complex” GCM schemeGCM scheme
Fowler et al., JCL, 1996
Similar complexity to many schemes in use in CRMs
60
Summary 1: A complex system!Summary 1: A complex system!
• Molecular Scale– Nucleation/activation, Diffusion/condensation/evaporation
• Particle Scale– Collection/collision-coalescence/aggegation, Shedding/splintering
• Parcel Scale– Particle Size Distributions
• Cloud Scale– Heterogeneity– Interaction with the dynamics (latent heating)
61
Summary 2: Simplifying Summary 2: Simplifying assumptionsassumptions• Parametrization of cloud and precipitation microphysical
processes:– Need to simplify a complex system– Accuracy vs. complexity vs. computational efficiency trade off– Appropriate for the application and no more complexity than can be
constrained and understood– Dynamical interactions (latent heating), radiative interactions– Still many uncertainties (particularly ice phase)– Particular active area of research is aerosol-microphysics interactions.– Microphysics often driven by small scale dynamics…..
• Next lecture: Cloud Cover– Sub-grid scale heterogeneity– Linking the micro-scale to the macro-scale
62
ReferencesReferences
Reference books for cloud and precipitation microphysics:
Pruppacher. H. R. and J. D. Klett (1998). Microphysics of Clouds and Precipitation (2nd Ed). Kluwer Academic Publishers.
Rogers, R. R. and M. K. Yau, (1989). A Short Course in Cloud Physics (3rd Ed.) Butterworth-Heinemann Publications.
Mason, B. J., (1971). The Physics of Clouds. Oxford University Press.
Hobbs, P. V., (1993). Aerosol-Cloud-Climate Interactions. Academic Press.
Houze, Jr., R. A., (1994). Cloud Dynamics. Academic Press.
Straka, J., (2009). Cloud and Precipitation Microphysics: Principles and Parameterizations. Cambridge University Press. Not yet released!
top related