severe weather forecasting in africa – development of severe weather – ervin zsoter 1 / 55...

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

[email protected]

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

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16


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


Different NWP parameters showing different scale

30°N

40°N

50°N

60°N

70°N

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60°W

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40°W

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20°W

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0°

20°E

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30°N

40°N

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20°E 40°E

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270

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30°N

40°N

50°N

60°N

70°N

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20°E 40°E

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60°E

0.1

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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

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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


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)

…


What do we need for severe weather development?

Unstable atmosphere

Enough moisture

Rising motion trigger in the

atmosphere

By saying in a simplified way:


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


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)


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


Global: Convective cloud types (2)

proxy distribution of deep and shallow convective clouds as obtained from IFS


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°

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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


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°

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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


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


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


Example for (equivalent) potential temperature field


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


Example of relative vorticity field


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


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


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.


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



-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


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 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



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



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


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


Tropospheric systems

trough

ridgeridge

PVA NVARegion of upper-level divergence

Region of upper-level convergence

500 hPa geop. height

H L Surface


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


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


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


French Floods: 1/2 December 2003 (3)

Precipitation verification for deterministic forecast

old

Thin numbers=Obs

Thick numbers= max. Forecast values


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”


South Africa “tomorrow”


Weather situation on Thursday in South Africa

Rapid cyclognesis along the vorticity advection

severe weather forecasting in africa – development of severe weather – ervin zsoter 1 / 55...

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