Nowcasting thunderstorms in complex cases using radar data

Alessandro Hering*Stéphane Sénési#

Paolo Ambrosetti*Isabelle Bernard-Bouissières#

*MeteoSwissML/RASA, Locarno-Monti

#Météo-FranceDPREVI/PI, Toulouse

TRT application output (Thunderstorms Radar Tracking)

13dBZ 25dBZ 40dBZ 55dBZ

330 km 100 km 17:00 21:00

TRT input/output

Swiss radar network:• 3 volumetric C-band Doppler radars• 20 elevation scan (-0.3° / 40°) every 5 min

Input:

1. Radar• Cartesian composite (3 radars); 5 min • vertical maximum projection (from 12 CAPPI between 1 and 12 km)• resolution: 2 km on 16 reflectivity classes between <13 and >55 dBZ

2. Lightning data (Météorage): CG

Output: TRT-objects (attributes: geographical location, area, motion vector, velocity, trajectories, lightning...)

Visualisation: - 2003: Browser - 2006: NinJo workstation (P.Joe, 7.13)

Monte Lema, 1625 m46.04N, 8.83E

TRT cells detection principle

• TRT developed by MeteoSwiss and Météo-France (RDT)

• TRT algorithm: modification of RDT satellite-algorithms for Alpine radar images

• ADAPTIVE REFLECTIVITY THRESHOLDING: cells at individual thresholds

50 150100

dBmin = 36

45

54

Range [km]

Re

flect

ivity

[dB

Z]

ΔdBT

ΔdBT ΔdBT = 6dB

ΔdBT

cell 1cell 2 cell 3

dBth

dBth

TRT tracking principle

• Method: GEOGRAPHICAL OVERLAPPING of cells - Advection (estimated displacement velocity or cross-

correlation)

• Complex cases / splits / merges considered

• Good tracking also for small objects

t0+Δt

t0

t0+adv

v(t0)+

+

A L P

S

ITALY

FRANCE

GERMANY5

50 k

m

600 km

MAX > 55 dBZ

>55 [dBZ]36 [dBZ]

TRT drill down product

TRT cell velocity estimates

Previous algorithm Improved actual algorithm

• Unstability of centroid velocity caused by splits, merges, significant area changes• New: cross-correlation at object scale (∆area > 30%) and centroid displacement• Temporal smoothing filter• Better performance in complex cases

splits/merges

08.05.2003 15:00-23:05 UTC

1

10

100

1000

10000

0 60 120 180 240 300 360 420 480 540Time [min]

Are

a [k

m2]

0

20

40

60

80

100

120

45 dBZ

48 dBZ

51 dBZ

54 dBZ

57 dBZ

CG -

CG +

CG tot

08.05.2003 15:00-23:05 UTC

1

10

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10000

0 60 120 180 240 300 360 420 480 540Time [min]

Are

a [k

m2]

0

20

40

60

80

100

120

45 dBZ

48 dBZ

51 dBZ

54 dBZ

57 dBZ

CG -

CG +

CG tot

08.05.2003 15:00-23:05 UTC

1

10

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1000

10000

0 60 120 180 240 300 360 420 480 540Time [min]

Are

a [k

m2]

0

20

40

60

80

100

120

45 dBZ

48 dBZ

51 dBZ

54 dBZ

57 dBZ

CG -

CG +

CG tot

TRT cell area and CG lightning evolution

• Cell area evolution at different reflectivities (>45 dBZ) in 3 dB steps; real-time• Complex case: splits/merges, significant area changes• Total, negative/positive CG lightning

Broadcasting “Thunderstorm Flash-news”

• During this summer MeteoSwiss started the diffusion of heavy thunderstorms warnings based on TRT and other sources

• In whole Switzerland for the general public and the authorities

• Use simple flash-news diffused by local and national radio stations

• Lead time: 30-120 min.

• June-September 2005: 70 Flash-news on 18 warning days

• Forecasters: substantial TRT contibution to flash-news

Actual Radar image available

TRT algorithm (+ other data sources)

Analysis by forecaster

Edit Flash message

Transmission of Flash message SMS, fax...

Message Broadcast by radio (local, national)

Actions by users (authorities, general public)

“Thunderstorm Flash-news”: tipical timing

A severe thunderstorm is presently located over Geneva and will probably move in the next 60 minutes to the region of Lausanne. It can produce wind gust over 75 km/h or hail.

Time [min]0

2

4

12

16

24-50

19

?

“Thunderstorm Flash”: experience summer 2005

Problems Solutions

Long cycle (analysis + dissemination) Further process automation

Priority setting of TS cells Objective severity classification (NinJo)

Phase estimation / early detection Cells phase classification (NinJo)

Wind Gust and Hail forecast Local adapted algorithms

Flash Flood forecast (stationary cells) Rain field extrapolation / accumulation

Coarse localization in forecast Higher resolution / shorter forecast time

TRT: Summary and outlook

• TRT: automatic identification, tracking and monitoring of convective systems using radar and lightning data- Adaptive reflectivity thresholding- Splits / merges, complex cases- Time histories of cell attributes

• 2003: TRT operational at MeteoSwiss for nowcasting

• 2005: thunderstorms Flash-news warnings (positive preliminary assessment)

• 2006: visualisation in the NinJo workstation

• Outlook: more extensive use of 3D reflectivity data(echo top, VIL, probability of hail,... )