Limited-Area Ensemble Prediction: the ARPA-SMR LEPS system Stefano Tibaldi, Tiziana Paccagnella,

WORKSHOP ON SHORT-RANGE ENSEMBLE PREDICTION USING LIMITED-AREA MODELS Instituto National de Meteorologia, Madrid, 3-4 October 2002

Limited-Area Ensemble Prediction: the ARPA-SMR LEPS system

Stefano Tibaldi, Tiziana Paccagnella,

Chiara Marsigli, Fabrizio Nerozzi, Andrea Montani ARPA-SMR

With contributions from F.Molteni, R.Buizza and H.Hersbach, all at ECMWF, at some time

Limited-Area Ensemble Prediction: the ARPA-SMR LEPS system

Stefano Tibaldi, Tiziana Paccagnella,

Chiara Marsigli, Fabrizio Nerozzi, Andrea Montani ARPA-SMR

With contributions from F.Molteni, R.Buizza and H.Hersbach, all at ECMWF, at some time


• The need for the regionalisation of scenarios

• The LEPS approach– Methodology– Some case studies– Statistical evaluation

• COSMO-LEPS

• Concluding remarks

OUTLINE OUTLINE


THE NEED FOR REGIONALISATION OF SCENARIOS (1)


Despite the recent increase of computer power resources, which have allowed the development of more and more sophisticated NWP models, the accurate forecast of extreme weather conditions, especially when related to intense and localised precipitation structures, is still difficult.

This limitation is due, among other reasons, to the inherent low degree of deterministic predictability associated to this kind of phenomena.




Global-model ensemble systems have been shown to be important tools to tackle the predictability problem beyond day 3.

Operational ensemble systems are usually run at a coarser resolution with respect to single deterministic model integrations for obvious economy reasons.

The EPS skill in producing quantitative forecasts of intense and localised events in the short- and early-medium-range is still limited, although growing.


THE LEPS APPROACHTHE LEPS APPROACH

The main purpose of the LEPS project is to produce a system capable of providing the forecaster some probabilistic guidance to identify the possible occurrence of severe weather conditions in the time range:

“late-short-range (>48h) up to early-medium-range (120h)”.



LEPS is designed to combine in a single systemthe (supposed) ability of a global ensemble prediction system to generate an exhaustive set of large-scale evolution scenarios (through an adequate sampling of the phase-space in the neighbourhood of the best available initial conditions)

Withthe (supposed) capability of a LAM of detailing atmospheric phenomena on the local scales, particularly the precipitation field, in regions with complex orography


The obvious solution: one LAM integration

for each global EPS member

All the information from the global EPS is retained

BUT

it is hardly feasible on an operational basis (at least

at ARPA-SMR)

The B.F. (Brute Force) APPROACH

The B.F. (Brute Force) APPROACH


For each LAM run:

Computer time:

+120 hours of LAM integration

306 x 258 x 32 grid points

13 hours cpu on vpp5000

Data volume (IC and BCs):

0.9 GB for each member

B.F.: REQUIRED RESOURCES(evaluated on COSMO-LEPS

configuration)

B.F.: REQUIRED RESOURCES(evaluated on COSMO-LEPS

configuration)

51 runs

663 hours cpu!

46 GB!


The LAM is nested in only a limited number of

members selected from the global EPS, the

Representative Members

Some of the information from global EPS is lost

BUT

the operation becomes feasible on an operational basis



Most Representative Member

one per cluster choice is based on selected 3D fields: has to be: the closest to the mean of its own cluster AND the most distant to the other clusters’ means 5 runs instead of 51, 102 or 153!!


Dim 2

Initial conditions Dim 1

LEPS – Limited area Ensemble Prediction SystemLEPS – Limited area Ensemble Prediction System

Dim 2

Possible evolution scenarios

Dim 1 Initial conditions

EPS and ensemble size reduction

Cluster members chosen as representative members (RMs)

LAM integrations driven byRMs

LAM scenario

LAM scenario

LAM scenario


LEPS – Limited area Ensemble Prediction SystemLEPS – Limited area Ensemble Prediction System

Dim 2

Initial conditions

Dim 1

LAM scenario

LAM scenario

LAM scenario

PROBABILITY MAPS: WEIGHTING??????PROBABILITY MAPS: WEIGHTING??????

precipitation scenarios

precipitation scenarios


The basic idea was to leave the task of exploring the phase space to the global EPS, while the LAM has to zoom in the forecast,

producing adequately intense local phenomena (e.g. precipitation maxima),

but….


Precip outliers 91 ECMWF EPS DJF 1999/2000

TP ACC 24 H BELOW MIN

ABOVE MAX

OUTLIERS %

D+3 8.3 5.6 13.9 D+5 4.6 2.8 7.3

EPS

both 4.1 1.7 5.8

D+3 13.6 24.9 38.6 D+5 9.7 17.2 26.9

5 RM

(c.linkage) both 9.4 16.8 26.1


QUESTIONS:

Are we adequately sampling the space of possibilities (i. e. the phasespace around the initial conditions)?


The same 700hPa isoline plotted for the 51 Members at + 72

The same 700hPa isoline plotted for the 51 Members at + 120

Spread: EPS started at 14 May 1999, 00 UTC


Z 500 DJF 1999-2000AREA 28°N - 60°N; 18°W - 45°E

0.0

0.5

1.0

1.5

2.0

0 2 4 6 8 10 12 14

FC DAY

MSS

MSE EM

MSE CTRL

Spread vs errorMean square error/mean square spread


How can we improve our exploration

of the phase space, increasing the

available number of EPS forecasts?

By resurrecting the old concept of

time-lagged ensemble forecast!

The Super-Ensemble!


Precip outliers 91 ECMWF EPS DJF 1999/2000

TP ACC 24 H BELOW MIN

ABOVE MAX

OUTLIERS %

D+3 8.3 5.6 13.9 D+5 4.6 2.8 7.3

EPS

both 4.1 1.7 5.8

D+3 13.6 24.9 38.6 D+5 9.7 17.2 26.9

5 RM

(c.linkage) both 9.4 16.8 26.1


LEPS super-ensembleLEPS super-ensemble

start of integrations

102members Day n EPS

Day n

Day n-2 EPS

Day n-2

clustering time

Verification period


102 members (50 + 1)*2

5 representative members (RMs)

Summary of the LEPS Summary of the LEPS methodology methodology

5 LAMBO integrations nested on 5 RMs:

LEPS - Limited-area (High Resolution) Ensemble Prediction System

5 LAMBO integrations nested on 5 RMs:

LEPS - Limited-area (High Resolution) Ensemble Prediction System

5 clusters

Hierarchical Cluster Analysismethod: Complete Linkage

area: Southern Europefields: 4 variables at 4 levels (3D cluster)

number of clusters: fixed to 5

Hierarchical Cluster Analysismethod: Complete Linkage

area: Southern Europefields: 4 variables at 4 levels (3D cluster)

number of clusters: fixed to 5

Representative Member Selection one per cluster

base on the nearest (3D fields) to the mean of its own cluster AND the most distant to the other clusters’

means

Representative Member Selection one per cluster

base on the nearest (3D fields) to the mean of its own cluster AND the most distant to the other clusters’

means

Super ensemble:2 global ensembles starting 5/3 days before the

verification time

Super ensemble:2 global ensembles starting 5/3 days before the

verification time


LEPS:

SOME CASE STUDIES

LEPS:

SOME CASE STUDIES


Soverato: 8, 9, 10 - Sep - 2000








Soverato flood


Soverato (Calabria) flood

ECMWF proxy of 24h cumulated precipitation 10/09/2000 at 00Z

91


Between 9 and 10 September, rainfall peaks above 300 mm in 24 Between 9 and 10 September, rainfall peaks above 300 mm in 24 hours were recorded close to the village of Soverato this causing hours were recorded close to the village of Soverato this causing landslides, great disruption and losses oflandslides, great disruption and losses oflife.life.

8 9 10

8 9 10


ECMWF probability maps for 24h cumulated precipitation exceeding P

threshold at +60h

P > 20 mm/ 24 h

P > 50 mm/ 24 h

P > 100 mm/ 24 h

Ensemble T255


30

100


P > 20 mm/ 24 h

P > 50 mm/ 24 h

P > 100 mm/ 24 h

LEPS probability maps for 24h cumulated precipitation exceeding P threshold at

+60h

LEPS – 5 LAMBO runs driven by the 5 RMs selected from 153 members of the Super-Ensemble TL255


47

75 5

8

42


Probability maps for 24h cumulated precipitation exceeding threshold at +60h

Brute-force approach

51 LAMBO runs on ensemble EPS TL255L40

(51 members)P > 50 mm / 24 h

33

23

Soverato (Calabria) flood: BF vs LEPS

2554

LEPS:

5 weighted LAMBO runs on ensemble EPS TL255L40

P > 50 mm / 24 h


MAP IOP2b: 19, 20 - Sep - 1999






24 hours observed precipitation from 06 to 06 UTC

MAP IOP 2B, 20-21 September 1999


ECMWF probability maps for 24h cumulated precipitation exceeding threshold at +66

hours

ECMWF EPS forecasts 51 members

P > 20mm/24h

MAP IOP 2B

P > 50mm/24h

98%


LEPS probability maps for 24h cumulated precipitation exceeding threshold at +66

hours

LEPS:

5 LAMBO runs at 20 km on Super-Ensemble TEPS TL159

P > 20mm/24h

MAP IOP 2B

P > 50mm/24h


Probability maps for 24h cumulated precipitation exceeding threshold at +66

hours

P > 50mm/24h

P > 50mm/24h

Brute-force approach

51 LAMBO runs on TEPS TL159 (51 members)

MAP IOP 2b BF vs LEPS LEPS:

5 weighted LAMBO runs at 20 km on

ensemble EPS TL159

Documents

Limited-Area Ensemble Prediction: the ARPA-SMR LEPS system Stefano Tibaldi, Tiziana Paccagnella,