Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 1 / 55
Synoptic-scale forcing mechanisms – development of severe weather
Ervin Zsoter
ECMWF, Meteorological Operations Section
With contributions from:
Peter Bechtold and Mark Rodwell
30°N
40°N
50°N
60°N
70°N
60°W
60°W
40°W
40°W
20°W
20°W
0°
0°
20°E
20°E 40°E
40°E
60°E
60°E
-0.5
-0.25
0
0.25
0.5
1
2
3
4
6
8
16
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 2 / 55
Forecasting time and space scale
Time scale
Spa
ce s
cale
Tornadoes
Supercell thunderstorm
Mesoscale convective complex
Tropical cyclones
Storm trackMonsoon
Extra-tropical cyclones
Climate change
min hours days weeks months years
~100 m
~ 10 km
~100 km
~1000 km
Global
Pacific SSTs
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 3 / 55
Different NWP parameters showing different scale
30°N
40°N
50°N
60°N
70°N
60°W
60°W
40°W
40°W
20°W
20°W
0°
0°
20°E
20°E 40°E
40°E
60°E
60°E
-0.5
-0.25
0
0.25
0.5
1
2
3
4
6
8
16
30°N
40°N
50°N
60°N
70°N
60°W
60°W
40°W
40°W
20°W
20°W
0°
0°
20°E
20°E 40°E
40°E
60°E
60°E
270
275
280
285
290
295
300
305
310
315
320
30°N
40°N
50°N
60°N
70°N
60°W
60°W
40°W
40°W
20°W
20°W
0°
0°
20°E
20°E 40°E
40°E
60°E
60°E
0.1
0.5
1
2
3
5
8
10
15
20
30
Z500
PV330K
Precipitation
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 4 / 551 2 3 4 5 6 7 8 9 10Lead-time (days)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
An
om
aly
Co
rrel
atio
n S
kill
ACC for European Z500
(top), 2 (middle), PRECIP (bottom)
Z500
& 2 on 2.5o grid, 24h-PRECIP at SYNOP locations and divided by climatology
M.J.Rodwell
90% Confidence Interval
20052004200320022001
Annual-mean of ACC for Europe
Z500
2
PrecipSYNOP
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 5 / 55
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)
…
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 6 / 55
What do we need for severe weather development?
Unstable atmosphere
Enough moisture
Rising motion trigger in the
atmosphere
By saying in a simplified way:
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 7 / 55
JJA
DJF
mm day-1
“MONSOON” FROM ARABIC WORD “MAUSIM”: SEASONAL REVERSAL OF WINDS
FARMERS MORE INTERESTED IN SEASONAL CYCLE OF RAINS
BOTH ASPECTS ARE LINKED
MONSOONS:ASIAN & AUSTRALIAN,NORTH & SOUTH AFRICAN,SOUTH AMERICAN & MEXICAN
WINDS IMPORTANT FOR ARAB MERCHANT SAILORS
Observed Precipitation, V925 and Z500
SUSTAIN HALF THE WORLD’S POPULATION
Monsoon
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 8 / 55
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)
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 9 / 55
Global: Precipitation and Convection (1)
It’s 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
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 10 / 55
Global: Convective cloud types (2)
proxy distribution of deep and shallow convective clouds as obtained from IFS
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 11 / 55
0° 0°
20°N20°N
40°N 40°N
40°W
40°W 30°W
30°W 20°W
20°W 10°W
10°W 0°
0° 10°E
10°E 20°E
20°E 30°E
30°E
METEOSAT 7 First Infrared Band Wednesday 13 April 2005 0600UTC
0° 0°
20°N20°N
40°N 40°N
40°W
40°W 30°W
30°W 20°W
20°W 10°W
10°W 0°
0° 10°E
10°E 20°E
20°E 30°E
30°E
RTTOV generated METEOSAT 7 First Infrared Band (10 bit)Tuesday 12 April 2005 12UTC ECMWF Forecast t+18 VT:Wednesday 13 April 2005 06UTC
How well we predict the tropical convection in the short range
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 12 / 55
0° 0°
20°N20°N
40°N 40°N
40°W
40°W 30°W
30°W 20°W
20°W 10°W
10°W 0°
0° 10°E
10°E 20°E
20°E 30°E
30°E
METEOSAT 7 First Infrared Band Friday 15 April 2005 1200UTC
0° 0°
20°N20°N
40°N 40°N
40°W
40°W 30°W
30°W 20°W
20°W 10°W
10°W 0°
0° 10°E
10°E 20°E
20°E 30°E
30°E
RTTOV generated METEOSAT 7 First Infrared Band (10 bit)Thursday 14 April 2005 12UTC ECMWF Forecast t+24 VT:Friday 15 April 2005 12UTC
How well we predict the tropical convection in the short range
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 13 / 55
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
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 14 / 55
TThe conservative (Lagrangian way – following the particle in move) quantities are the
best in monitoring, detecting structures, evolution of flow, etc.
Some basics of dynamic meteorology – termodynamic flow tracers
dt
dT Compression / expansion
dt
d
z
gN
2
TThe temperature is not conservative, the Lagrangian variation is driven by two factors
– EEffect of pressure change of the particle– EEffect of heat exchange with diabatic sources +
Diabatic sources
BBy combining the temperature change with the 1st term we
get the potential temperature (Θ)
– 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
Diabatic sources
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 15 / 55
Example for (equivalent) potential temperature field
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 16 / 55
DDivergent fieldDivergence (convergence) and vorticity - flow tracers of the “dynamics”
Vorticity due to wind shear
Vorticity due to
curvature
Parcel gain positive (cyclonic) vorticity
CConvergent field
TThe rotation is described by the vorticity (ξ)– 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
kfa
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 17 / 55
Example of relative vorticity field
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 18 / 55
SSimilarly to the idea of potential temperature the potential vorticity can be defined (PV)
Flow tracers of the “dynamics” – Potential vorticity
pfgPV
)(
hPa
K
p 100
10
222
142
10
1)
100
10)(10)(10(
msmkg
hPa
hPa
KssmPV
For hidrostatic atmosphere with potential temperature used as vertical coordinate FFor typical midlatitude, synoptic flow PV has an order of 10-6
PVUkgKsm 110 1126
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 Θ 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
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 19 / 55
Flow tracers of the “dynamics” – Potential vorticity
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-ө
Zonal average of 10 cold seasons (1986-1995)
From METEO France training material
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 20 / 55
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
Hydrostatic balance
– No explicit vertical accelerations are allowed
zgvkf pg
– 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.
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 21 / 55
Quasi-Geostrophic Height Tendency 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
Derived from the quasi-geostrophic termodynamic equation and the quasi-geostrophic vorticity equation
Which is the thickness of a layer defined by two pressure surfaces P1
ΔΦpp
t
p
vp
fvf
p
fggg
20
02
2202
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 22 / 55
Quasi-Geostrophic Height Tendency Equation
-12
-14
-16 -12Vg
Vg
Δη
-f0 |Vg| | Δη | cos 1800 > 0
< 0, height falls
-f0 |Vg| | Δη | cos 00 < 0
> 0, height rises
Interpretation of Term B advection of the Absolute Geostrophic Vorticity by the Geostrophic Wind
Δη
Φ-1ΦΦ+1 Φ+1Φ
Φ-1
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 23 / 55
warm air advection
cold air advection
500 mb
700 mb
850 mb
thickness increases
thickness decreases
Cold air advection decreasing with height height falls at 700 mb
Same result for warm air advection that increases with height
500 mb
700 mb
850 mb
old position of 700 mb
Interpretation of Term C Vertical Variation of the Geostrophic Thickness Advection
Quasi-Geostrophic Height Tendency Equation
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 24 / 55
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
p
vvp
f
p
fggg
202
2202 1
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 25 / 55
Quasi-Geostrophic Diagnostic Omega Equation
500 mb
300 mb
700 mb
negative vorticity advection
positive vorticity advection
V g g < 0
V g g > 0
Therefore, ω < 0 (i.e., upward vertical motion)
PVA NVA
Interpretation of Term C Vertical Variation of the Geostrophic Advection of the Absolute Geostrophic Vorticity
0)(0
ggvp
f
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 26 / 55
Quasi-Geostrophic Diagnostic Omega Equation
NoteP
VP
VP
g
g
:
0
0
0
Since Φ 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:
Δ2(CAA) will be < 0; therefore LHS of omega equation is < 0 and ω will be > 0 (downward vertical motion)
Δ2(WAA) will be > 0; therefore LHS of omega equation is >0 and ω will be < 0 (upward vertical motion)
Interpretation of Term C Vertical Variation of the Geostrophic Advection of the Absolute Geostrophic Vorticity
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 27 / 55
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
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 28 / 55
Tropospheric systems
trough
ridgeridge
PVA NVARegion of upper-level divergence
Region of upper-level convergence
500 hPa geop. height
H L Surface
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 29 / 55
Jet streams
gv
gv
gv
warm
Surface
gvgv
gv
0g
0g
avDt
vD gg
Dt
vD ggav
cold
Forcing of ageostrophic
circulations/convection in
the right entrance and left
exit side of upper-level Jet
Thermally indirect circulation
Thermally direct circulation
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 30 / 55
Ex. French Floods: 3 December 2003 (1)
upper/lower-level 48h Forecast
c
45N
10E10W 0
20E45N
10W 10E0
b
250 hPa Wind, 330 K PV, 850 Thetae 925 hPa Wind, 330 K PV, 850 Thetae
Upper-level divergence and lower level convergence
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 31 / 55
45N
0 10E
a
10W
20E
French Floods: 3 December 2003 (2)
Comparison 48h Forecast and Analysis
45N
10W 10E0
b
925 hPa Wind, 330 K PV, 850 ThetaeAnalysis 48h Forecast
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 32 / 55
French Floods: 1/2 December 2003 (3)
Precipitation verification for deterministic forecast
old
Thin numbers=Obs
Thick numbers= max. Forecast values
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 33 / 55
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”
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 34 / 55
South Africa “tomorrow”
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 35 / 55
South Africa “tomorrow”
Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 36 / 55
Weather situation on Thursday in South Africa
Rapid cyclognesis along the vorticity advection