Synoptic-scale forcing mechanisms

1
Synoptic-scale forcing mechanisms development
of severe weather

• Ervin Zsoter
• ECMWF, Meteorological Operations Section
• ervin.zsoter_at_ecmwf.int

With contributions from Peter Bechtold and Mark
Rodwell
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Forecasting time and space scale
Climate change
Global
Pacific SSTs
Monsoon
Storm track
Space scale
Extra-tropical cyclones
1000 km
Tropical cyclones
100 km
Mesoscale convective complex
10 km
Supercell thunderstorm
100 m
Tornadoes
years
min
hours
days
weeks
months
Time scale
3
Different NWP parameters showing different scale
Z500
PV330K
Precipitation
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Annual-mean of ACC for Europe
Z500
?2
PrecipSYNOP
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What sort of events are we interested in?

• Heavy rainfalls floods, land slides, etc.
• Thunder strikes
• Tornadoes
• Devastating wind storms
• Tropical cyclones related events (wind and rain
  again)

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What do we need for severe weather development?
By saying in a simplified way

• Unstable atmosphere
• Enough moisture
• Rising motion trigger in the atmosphere

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Observed Precipitation, V925 and Z500
mm day-1
Monsoon
JJA
MONSOON FROM ARABIC WORD MAUSIM SEASONAL
REVERSAL OF WINDS
WINDS IMPORTANT FOR ARAB MERCHANT SAILORS
FARMERS MORE INTERESTED IN SEASONAL CYCLE OF RAINS
DJF
BOTH ASPECTS ARE LINKED
MONSOONS ASIAN AUSTRALIAN, NORTH SOUTH
AFRICAN, SOUTH AMERICAN MEXICAN
SUSTAIN HALF THE WORLDS POPULATION
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Monsoons forecast a problem on different scales

• Short range Single rain events within an active
  phase of the monsoon (influenced by Mesoscale
  convective systems, Easterly waves)
• Medium-range/extended-range Alternation of
  active and quiescent monsoon phases (influenced
  by MJO, Kelvin waves)
• Interannual variability Annual variation of
  precipitation intensity and position (influenced
  by Astronomical factors, SST distribution,
  surface conditions, EL NINO)

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Global Precipitation and Convection (1)

• Its raining again 2000/2001 rainfall rate as
  simulated by IFS CY30R2 and compared to GPCP obs
• About 3 mm/day is falling globally, but most i.e.
  5-7 mm/day in the Tropics

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Global Convective cloud types (2)proxy
distribution of deep and shallow convective
clouds as obtained from IFS
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How well we predict the tropical convection in
the short range
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How well we predict the tropical convection in
the short range
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Basic mechanisms for the development of eddies

• WWave instabilities important for synoptic-scale
  meteorology are zonally asymmetric perturbations
  (eddies) to zonally symmetric flow field
• BBaroclinic instability
• BBaroclinic instability is a wave instability
  associated with vertical shear of the mean flow.
• BBaroclinic instabilities grow by converting
  potential energy associated with the mean
  horizontal temperature gradient.
• TThe temperature gradient must exist to provide
  thermal wind balance for vertical shear
• BBarotropic instabilities
• BBaroclinic instability is a wave instability
  associated with a horizontal shear in a jet like
  current
• BBarotropic instabilities grow by extracting
  kinetic energy from the mean flow field

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Some basics of dynamic meteorology termodynamic
flow tracers

• TThe conservative (Lagrangian way following the
  particle in move) quantities are the best in
  monitoring, detecting structures, evolution of
  flow, etc.

• TThe temperature is not conservative, the
  Lagrangian variation is driven by two factors

Compression / expansion
Diabatic sources

• EEffect of pressure change of the particle
• EEffect of heat exchange with diabatic sources

• BBy combining the temperature change with the 1st
  term we get the potential temperature (T)

Diabatic sources

• The pot. temperature is a tracer for the particle
  if the evolution is adiabatic
• Important role - static stability (N - buoyancy
  frequency or Brunt-Vaïsala frequency), on a
  synoptic-scale it is always positive, vertical
  motions are forced

• If an air particle is raised or lowered under the
  effect of the vertical motion associated with a
  convergence or divergence area, the static
  stability compels it to return towards its
  initial level

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Example for (equivalent) potential temperature
field
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Divergence (convergence) and vorticity - flow
tracers of the dynamics

• DDivergent field

• CConvergent field

• TThe rotation is described by the vorticity (?)
• TThe rotation is linked both to the motion of the
  Earth, and to the rotation component of the wind
• VVorticity is the measure of spin around the
  vertical axis of an object

• Vorticity due to wind shear

Parcel gain positive (cyclonic) vorticity

• Vorticity due to curvature

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Example of relative vorticity field
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Flow tracers of the dynamics Potential
vorticity

• SSimilarly to the idea of potential temperature
  the potential vorticity can be defined (PV)

For hidrostatic atmosphere with potential
temperature used as vertical coordinate

• FFor typical midlatitude, synoptic flow PV has an
  order of 10-6

Values less than 1.5 PVU (or 2.0) are
associated with the troposphere

• IIt is conserved in frictionless and adiabatic
  motion and on constant T surface it is advected
  like a passive tracer
• Its field shows more structure than the more
  traditional but equivalent approach of
  considering the geopotential height on constant
  pressure surface

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Flow tracers of the dynamics Potential
vorticity
From METEO France training material

• There is a strong transition between the high
  potential vorticity values (in the stratosphere)
  and the low values (in the troposphere)
• It is particularly rapid when following the iso-?
• Zonal average of 10 cold seasons (1986-1995)

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How to diagnose synoptic-scale motions

• Quasi-Geostrophic Assumption
• The assumption of the balance, in the atmosphere,
  between the horizontal Coriolis force and the
  horizontal pressure force selectively used in the
  momentum and thermodynamic equations

• Specifically, horizontal winds are replaced by
  their geostrophic values in the horizontal
  acceleration terms of the momentum equations, and
  horizontal advection in the thermodynamic
  equation is approximated by geostrophic
  advection.
• This approximation is not accurate in situations
  in which the ageostrophic wind plays an important
  advective role, for example, around fronts and
  jets.
• It helps us to understand how the mass and
  momentum fields interact on the synoptic scale to
  create vertical circulations which result in
  sensible weather.

• Hydrostatic balance
• No explicit vertical accelerations are allowed

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Quasi-Geostrophic Height Tendency Equation

• Derived from the quasi-geostrophic termodynamic
  equation and the quasi-geostrophic vorticity
  equation

A B
C Term A
three-dimensional Laplacian of the height
tendency Term B advection of the absolute
geostrophic vorticity by the geostrophic
wind Term C vertical variation of the
geostrophic thickness advection
P2
Which is the thickness of a layer defined by two
pressure surfaces
?F
P1
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Quasi-Geostrophic Height Tendency Equation

• Interpretation of Term B advection of the
  Absolute Geostrophic Vorticity by the Geostrophic
  Wind

??
??
-12
-16
Vg
-12
Vg
-14
F1
F-1
F-1
F
F1
F
-f0 Vg ?? cos 1800 gt 0 lt 0, height
falls
-f0 Vg ?? cos 00 lt 0 gt 0, height
rises
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Quasi-Geostrophic Height Tendency Equation

• Interpretation of Term C Vertical Variation of
  the Geostrophic Thickness Advection

500 mb
warm air advection
thickness increases
700 mb
cold air advection
thickness decreases
850 mb
Cold air advection decreasing with height height
falls at 700 mb Same result for warm air
advection that increases with height
500 mb
old position of 700 mb
700 mb
850 mb
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Quasi-Geostrophic Diagnostic Omega Equation
A
B C Term
A three-dimensional Laplacian of omega Term B
vertical variation of the geostrophic advection
of the absolute geostrophic vorticity Term C
Laplacian of the geostrophic advection of
thickness
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Quasi-Geostrophic Diagnostic Omega Equation

• Interpretation of Term C Vertical Variation of
  the Geostrophic Advection of the Absolute
  Geostrophic Vorticity

300 mb
lt 0
negative vorticity advection
500 mb
positive vorticity advection
gt 0
700 mb
PVA
NVA
Therefore, ? lt 0 (i.e., upward vertical motion)
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Quasi-Geostrophic Diagnostic Omega Equation

• Interpretation of Term C Vertical Variation of
  the Geostrophic Advection of the Absolute
  Geostrophic Vorticity

Since F increases as pressure decreases
For cold air advection
For warm air advection
Since the three-dimensional Laplacian operator
changes the sign of the function on which it
operates, we can see that ?2(CAA) will be lt 0
therefore LHS of omega equation is lt 0 and ?
will be gt 0 (downward vertical motion) ?2(WAA)
will be gt 0 therefore LHS of omega equation is
gt0 and ? will be lt 0 (upward vertical motion)
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Quasi-Geostrophic Theory

• As consequence of the Q-G vorticity equation
• The only way that the relative vorticity can
  change locally in a quasi-geostrophic atmosphere
  is through
• geostrophic advection
• divergence/convergence (i.e., shrinking or
  stretching the column)
• As a consequence of the Q-G height tendency
  equation
• The only way that the thickness can change
  locally in a quasi-geostrophic atmosphere is
  through
• geostrophic advection
• adiabatic heating/cooling of the layer through
  vertical motion

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Tropospheric systems
500 hPa geop. height
ridge
ridge
NVA
PVA
Region of upper-level divergence
trough
Region of upper-level convergence
H
L
Surface
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Jet streams
warm
Forcing of ageostrophic circulations/convection
in the right entrance and left exit side of
upper-level Jet
Surface
cold
Thermally indirect circulation
Thermally direct circulation
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Ex. French Floods 3 December 2003
(1)upper/lower-level 48h Forecast
250 hPa Wind, 330 K PV, 850 Thetae
925 hPa Wind, 330 K PV, 850 Thetae
Upper-level divergence and lower level convergence
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French Floods 3 December 2003 (2)Comparison 48h
Forecast and Analysis
925 hPa Wind, 330 K PV, 850 Thetae
Analysis
48h Forecast
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French Floods 1/2 December 2003 (3)
Precipitation verification for deterministic
forecast
old
Thin numbersObs Thick numbers max. Forecast
values
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French Floods 1-4 December 2003 (4)
The role of the EPS provide probabilities. Here
lagged EPS forecasts verifying at the same time
the closer to the event the better
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South Africa tomorrow
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South Africa tomorrow
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Weather situation on Thursday in South Africa
Rapid cyclognesis along the vorticity advection