Performance of the ARPA-SMR limited-area ensemble prediction system: two flood cases

13 Pages·2002·0.86 MB·English

NonlinearProcessesinGeophysics(2001)8:387–399
Nonlinear Processes
in Geophysics
(cid:13)cEuropeanGeophysicalSociety2001
Performance of the ARPA-SMR limited-area ensemble prediction
system: two flood cases
A.Montani1,C.Marsigli2,F.Nerozzi1,T.Paccagnella1,andR.Buizza3
1RegionalMeteorologicalServiceARPA-SMR,Bologna,Italy
2DepartmentofEarthSciences,UniversityofBologna,Bologna,Italy
3EuropeanCentreforMedium-RangeWeatherForecasts,Reading,UK
Received: 26September2000–Revised: 9March2001–Accepted: 10May2001
Abstract.TheperformanceoftheARPA-SMRLimited-area termine the reliability of the deterministic forecasts for dif-
EnsemblePredictionSystem(LEPS),generatedbynestinga ferenttime-ranges. Infact, ensemblesystemsenableoneto
limited-areamodelonselectedmembersoftheECMWFtar- find the probabilities associated with the different evolution
geted ensemble, is evaluated for two flood events that oc- scenarios of the atmosphere and their operational use pro-
curred during September 1992. The predictability of the vides an estimate of the day-to-day evolution of the proba-
eventsisstudiedforforecasttimesrangingfrom2to4days. bilitydistributionfunctionoftheatmosphericflow(Molteni
Theextenttowhichfloodslocalisedintimeandspacecanbe etal., 1996). AtECMWF,asintheothercentres, computer
forecast at high resolution in probabilistic terms was inves- powerresourceshavegreatlyincreasedinthelastfewyears,
tigated. Rainfall probability maps generated by both LEPS therebyenablingtheoperationaluseofensembleswithmore
andECMWFtargetedensemblesarecomparedfordifferent and more members and at higher and higher spatial resolu-
precipitation thresholds in order to assess the impact of en- tion(atthetimeofwriting,51membersrunatabout120km
hanced resolution. At all considered forecast ranges, LEPS ofhorizontalresolutionwith40verticalmodellevels;Buizza
performsbetter,providingamoreaccuratedescriptionofthe etal., 1998). Nevertheless, computeravailabilityisalsothe
eventwithrespecttothespatio-temporallocation,aswellas main limitation which prevents the running of ensembles at
itsintensity. Inbothfloodcases,LEPSprobabilitymapsturn an even higher resolution (Molteni et al., 2001; Marsigli et
out to be a very valuable tool to assist forecasters to issue al.,2001). Infact,whilethemedium-rangepredictabilityof
floodalertsatdifferentforecastranges. Itisalsoshownthat thelarge-scaleflowisusuallywellcapturedbytheECMWF
at the shortest forecast range, the deterministic prediction ensembleand,ingeneral,byallensemblesystems,thefore-
provided by the limited area model, when run in a higher- castoflocaleventsispoorerduetotherelativelylimitedhor-
resolution configuration, provides a very accurate rainfall izontalresolution. Thislimitationcanheavilyaffectthepre-
pattern and a good quantitative estimate of the total rainfall diction of local and intense rainfall events, especially over
deployedinthefloodedregions. Europeduringthewarmseason(Buizzaetal.,1998). Inthe
last few years, much effort has been devoted to the setup
of probabilistic forecasts for short time-ranges and over lo-
calised regions. Therefore, it has been necessary to further
1 Introduction
enhancethepresent-dayconfigurationofensemblesystems.
Among the different approaches used to tackle this prob-
In recent years, the main weather centres have greatly in-
lem,aglobal-modeltargetedensemblepredictionsystem(re-
creased the use of probability forecasts in order to predict
ferredtoasTEPS)isbeingdevelopedatECMWF(Hersbach
theglobalevolutionoftheatmosphericflowbeyondtheshort
etal.,2000). IntheTEPSexperiments,theperturbationsthat
range(namely,fromday2onward). TheNationalCenterfor
differentiate the initial conditions of the ensemble system
Environmental Prediction, NCEP (Toth and Kalnay, 1993),
are given by linear combinations of singular vectors (SVs)
the Recherche en Pre`vision Nume`rique, RPN (Houtemaker
targeted to maximize the 48-hour total energy perturbation
et al., 1996) and the European Centre for Medium-Range
over the European area (instead of the extra-tropical north-
Weather Forecasts, ECMWF (Buizza and Palmer, 1995;
ernandsouthernhemisphere, asintheEPS,theoperational
Molteni et al., 1996) have developed operational ensemble
Ensemble Prediction System). In the last two years, a joint
systems to estimate the predictability of the flow and to de-
research group, including the Regional Meteorological Ser-
Correspondenceto: A.Montani viceofEmilia-RomagnaARPA-SMR(inBologna,Italy),the
([email protected]) RoyalNetherlandsMeteorologicalInstituteandOsloUniver-
388 A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem
sity, is carrying out a verification-experimentation program verificationcarriedoutagainstobservedvaluesconfirmsthe
to test the validity of running a TEPS system in order to highaccuracyofLEPSpredictionsofrainfall.
provideinitialandboundaryconditionstolimited-areamod- The task attempted in this paper is the combination of
els, as well as to improve the probabilistic predictability of the benefits gained by using TEPS instead of EPS to drive
rareevents(e.g.rainfallexceeding20mm/day)withaglobal LEPShigh-resolutionmembers, aswellastheinvestigation
model. A study performed by Hersbach et al. (2000) over of shorter forecast ranges when the differences in either the
51 case studies suggests the better performance of TEPS boundaryortheinitialconditionsassociatedwiththelimited-
over EPS for the short-range (up to 72–96h) prediction of areamodelcancontributetotheenhancementofthespread
severeeventsovertheEuropeanarea. Nevertheless,alimita- obtained by the LEPS integrations. This work investigates
tionstillremainsinthehorizontalresolutionofTEPSmem- the extent to which the configuration TEPS-LEPS can pro-
bers (the same as EPS), which prevents the spatial-detailed videinformationaboutthepossibilityoffloodoccurrenceat
predictionofthosefields(e.g.rainfall,surfacetemperature), different time ranges, thereby assisting the forecaster in the
where mesoscale and orographic-related processes can play confirmation or dismissal of flood alerts. The 4-day, 3-day
acrucialrole. and 2-day predictability of two flood events that occurred
As an alternative approach, which makes use of global during September 1992 is studied. More precisely, this
models, Atger (1999) and Ziehmann (2000) propose the work investigates the assistance provided to a forecaster by
“poor man’s EPS”, generated by joining together forecasts probability products for the different forecast ranges. Once
fromdifferentweathercentres.Thisapproachimplicitlycon- highprobabilitiesoffloodoccurrencehavebeenpredicted,a
sidersmodeluncertaintiesandprovidesreliableprobabilistic “higher-resolution”runofthelimited-areamodelwillalsobe
forecastsintheearlyaswellasinthemediumrange. performedinordertoenhancethespatialdetailoftheregions
affectedbytheeventandtohaveamoreaccuratequantitative
Another approach, which takes into consideration uncer-
estimateoftheamountofexpectedrainfall.
taintiesinthemodelformulation,butusingregionalmodels,
The paper is organised as follows: Sect. 2 reviews the
ispresentedbyStensrudetal.(1999aand1999b). Limited-
methodologyfollowedintheTEPS-LEPSconfigurationand
area ensembles are generated by perturbing model physics,
Sect.3presentsabriefsynopticdescriptionofthecasestud-
as well as by generating random initial conditions with the
ies. TheexperimentalsetupisdescribedinSect.4,whilethe
samemodel. Resultsareparticularlyencouragingwhenen-
results from TEPS and LEPS integrations are presented in
semblesarecreatedbyperturbingmodelphysics.
Sects.5and6. Finally,theconclusionsaredrawninSect.7.
Alternatively, NCEP has recently developed the short-
range ensemble forecasting (SREF) system, based on high-
resolution versions of NCEP’s Eta and Regional Spectral 2 Methodology
Model. This system is optimised for prediction ranges up
to 72h and turns out to be very useful for quantitative pre- As described by Hersbach et al. (2000), TEPS consists of
cipitationpredictions(Tractonetal.,1998). 51integrationsoftheECMWFoperationalglobalmodel,run
In this study, the attention is focussed on the devel- attheresolutionTL159L40(correspondingtoabout120km
opment of the Limited-area Ensemble Prediction System in the horizontal with 40 vertical levels). One integration
(LEPS), presented in Molteni et al. (2001) and Marsigli (control) is initialised with the unperturbed analysis; in all
et al. (2001) and developed at ARPA-SMR. In LEPS, theothers,theanalysesareperturbedin50differentwaysby
a clustering-selection technique is first applied to group adding/subtractingproperlyscaledandtargetedinitialpertur-
ECMWF ensemble members (with a grid scale of approx- bations(SVsoptimizedtomaximizethegrowthoverthere-
imately 120km) into clusters and a representative member gionextendingbetween35◦N–75◦Nand40◦W–30◦E).Un-
(RM) from each cluster is chosen. Then, each RM is used like in Hersbach et al. (2000), where the optimization time
to provide initial and boundary conditions for the integra- intervalwassetto72h,heretheperturbationenergyismax-
tions with a high-resolution limited-area model (20km of imisedoverashortertimeinterval(48h),sinceitisthought
grid spacing). Hence, the perturbations for the LEPS are thatthelinearassumptionunderwhichSVsarecalculatedis
of the same scale as the global-model ensemble and there morecloselyrespectedinthefirsttwodaysofevolution.
is no account of high-resolution perturbations. In Marsigli ConcerningtheLEPSconfiguration,themethodologyfol-
etal.(2001),the5LEPSintegrationsaretreatedasasmall- lowed to perform the cluster analysis and then to select the
sizehigh-resolutionensembleoveralimiteddomainandthe representativemembersisoutlinedasfollows(formorede-
usual probability products, characteristic of EPS, are pre- tail,thereaderisreferredtoMoltenietal.,2001).
paredforforecastrangesof120h. Fromtheresultsobtained
– AllTEPSmembersareintegratedforth(inthiswork,
so far, it appears that the high-resolution limited-area inte-
t =48, 72and96h,dependingonthepredictionrange
grationshaveagreaterspatialdetailintheprecipitationpat-
ofinterest).
terns and they are capable of predicting, a few days ahead,
theoccurrenceofintenseprecipitationoverlocalisedregions, – At forecast time t, the 51 members are grouped into
therebyenablingtheissueofanearlyfloodalert.LEPSgives 5 clusters, using as a discriminating variable a combi-
higherprobabilitiesofheavyprecipitationthanEPS,andthe nationoffourvariablesatfourpressurelevels(thetwo
A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem 389
horizontal wind components, the geopotential height ofLAMBO,thereaderisreferredtoJanjic(1990)andMar-
andthespecifichumidityat500,700,850and925hPa). siglietal.(2001).
– Allclusteringvariablesarestandardised(foreachvari-
able,themeanovertheclusteringareaiscalculatedand
3 Casestudydescriptions
subtracted from any grid-point value. Then the result
is divided by the standard deviation, so as to obtain a
Thecasestudiesinvestigatedinthispaperdealwithtwoflood
non-dimensionalquantity).
events which took place over southern France and north-
– ThedistancesbetweenallTEPSmembersarecomputed western Italy towards the end of September 1992 (Vaison-
ina16-dimensionalspace(4variablesat4pressurelev- la-RomaineandGenovaflood,respectively). Duringthispe-
els). riod,characterisedbyhighlyunstableconditionsintheatmo-
– The complete-linkage algorithm (Wilks, 1995) is ap- sphericflow,mesoscaleconvectivesystemsdevelopedwhich
pliedtoconstructthe5clusters. Theclusteranalysisis affectedthesouthernsideoftheAlpsandcausedheavypre-
performedoversouthernEurope(approximately53◦N– cipitationeventsoverthatregion.
35◦N5◦W–25◦E).
3.1 Vaison-la-Romaineflood
– Theclustersarenumberedinsuchawaythatthecontrol
forecastisalwaysinthefirstcluster.
At 12:00UTC on 21 September 1992, the mean sea level
– Onerepresentativemember(RM)isselectedfromeach pressurepatternoverwesternEurope(notshown)waschar-
cluster using the same variables that are used to con- acterised by the Azores’ high migration southeastwards; at
structtheclustersinordertodiscriminateamongmem- thesametime,anothersurfacelowthatwascenteredoffthe
berswithinthesamecluster. TheRMofaclusteristhe western coast of France was moving eastwards. During the
member closest to the elements of its own cluster and afternoon of 21 September, a cold front reached the French
mostdistantfromthoseoftheotherclusters. Atlanticcoast,andmesoscaleconvectivesystemsdeveloped
aheadofthefront(notshown).Alowpressuresystem,deep-
Therefore, 5 TEPS members (one RM per cluster) are se- ening over the French Mediterranean coast, was associated
lected. Each member is representative of a possible evolu- withthesteadyflowofwarm,moistairfromthesouthwest.
tion scenario highlighted by the model, with the possibil- Thesurfaceflow,enhancedbythemountains’channellingef-
ityoftheoccurrencebeingproportionaltothepopulationof fect,wascharacterisedbyanintense,low-leveljetstreamin
the cluster. Once the RMs are determined, they are used as theGulfofLion(Senesiet. al.,1996). On22September,the
boundary and initial conditions for the integration with the axis of the trough tilted and mesoscale convective systems
limited-area model that is run for th every time. This pro- started to develop to the north of the French Mediterranean
duces a5-memberhigh-resolutionensembleoveralimited- coast. Intense precipitation was recorded over many loca-
area (hence, LEPS) with a small set of evolution scenarios tionsofthisregion,withrainfallratesofabout200mmina
atahighresolution. Hence, itispossibletoconstructthose fewhours. Intheearlyhoursof23September,thelow-level
probabilityproducts(e.g.probabilitymapsforrainfall,tem- circulationwasdominatedbya700hPageopotentialheight
perature and winds exceeding particular thresholds) on the troughatabout43◦N6◦E(topleftpanelofFig.1). Thevil-
basis of the information provided only by the limited-area lageofVaison-la-Romaine(atabout44◦N5◦E)sufferedthe
integrations. As explained in Molteni et al. (2001), the de- heaviestprecipitation,withmaximaofabout300mmin24h
cision to choose a limited-area ensemble with a 5-member observedduring23September(thiscasestudyisreferredto
population is driven by the need to keep a sufficient num- asthe“Vaisoncase”intheremainingpartofthiswork).Sec-
ber of different evolution scenarios, as well as by computer ondaryrainfallmaximawerealsorecordedabout100kmto
powerconstraints,sinceitwouldbeveryexpensivetooper- the northwest of the village, as well as over northwestern
ationallyrunamorepopulatedLEPSwiththepresentcom- Italy (bottom left panel of Fig. 1). Precipitation peaks of
puter facilities at ARPA-SMR. Nevertheless, the configura- about 250mm were observed close to the town of Savona,
tion presented in this paper for the TEPS-LEPS system is a atabout44.3◦N8.4◦E(notshown).
suitablecompromiseamongthedifferentneedsandcouldbe
wellimplementedonanoperationalbasis. 3.2 Genovaflood
The limited-area model used to construct the LEPS is
theLimited-AreaModelBOlogna(LAMBO),operationalat The synoptic charts (not shown) relative to 12:00UTC on
ARPA-SMR since 1993. LAMBO, based on a version of 26 September 1992 indicated, at both 500 and 700hPa, an
the NCEP Eta Model, has a horizontal resolution of 20km intense southwesterly flow blowing over the western Medi-
with32verticallevelsanditisoperationallyruntwiceaday terranean. In the following hours, the area affected by this
for 72 and 84h (nested on ECMWF high-resolution deter- warm and moist flow was located more and more to the
ministicforecastsof00:00and12:00UTC,respectively). Its east, while an upper-level minimum was placed over Iberia
domain covers the Italian peninsula and the Alpine region at 00:00UTC on 27 September. At this time, a surface low
(1◦E–25◦E, 36◦N–50◦N). For a more detailed description wasevidenttothenorthwestoftheBalearicIslands,atabout
390 A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem
Figures
Milano
Marseille
Milano
Marseille
Fig.1. Leftcolumnpanels: meansealevelpressure(solidcontours)and700hPageopotentialheight(dashedcontours)analysesvalidat
06:00UFTiCg.on1.23LSeefpt–tecmobleurm1n99p2a(ntoeplsl:eftmpeaannel–)saenad–cleumveullapterdesrsauinrfeall(s(oinlimdmc)onobtoseurrvse)daonvder7th0e02h4Phapegreioodpeontdeinntgiaalththeeigsahmtetime
(bottomleftpanel)fortheVaisoncase. Rightcolumnpanels: thesameasbefore, butvalidat12:00UTCon28September1992forthe
Genova(cdaasseh.eCdonctoonutroinutresr)vaalns:al1yhsPeasavnadli1ddaatm6.UOTbsCerovafti2o3nsSferopmtemthebedrat1a9s9et2so(tfotphe–lMefetsopsacnaelel)AalpnidnecuPrmojuelcatt(eVdairsaoinncfaaslle)(ianndfrom
theCentrodiricercaInMonitoraggioAmbientale(Genovacase). Inthebottomrowpanels,onlyafewstationdataareplottedforclarity
purposes.
mm) observed over the 24–hour period ending at the same time (bottom–left panel) for the Vaison case.
42◦N2R◦iEgh.tI–nctohleufmolnlopwainneglsh:outhrse,sbaomthethaesubpepfoerr-el,evbeultmvainli-dat1t2owUnT.CPorefc2ip8itSateipotnemrabteesra1b9o9v2ef2o0r0thmemG/deanyovwaecreasree.corded
imum and the surface cyclone traveled eastwards, a mean over several places along the coast (bottom right panel of
sea levCelonmtionuimriunmterovfal1s0:114hhPPaaawndas1edvaidme.nOt obvseerrvtahteioGnuslffromtFhiegd.a1t)a,stehtussocfatuhseinMgesseovsecrealdeaAmlapgiense,Parsowjeecllt(aVsatwisooncasual-
of Genova at 00:00UTC on 28 September. During the first tiesamongthelocalpopulation.
hours ocfasteh)isanddayf,rothmetshoeuCthe,nstoruotdhwireiscteerrclyaIflnoMwoimniptoarcatignggioAmbientale(Genovacase). Inthebottom–rowpanels,
theLiguriacoast(northwesternItaly)wasintense,asshown
inthetoonplyrigfehwtpsatnaetiloonfdFaitga.a1r.eDpuloettteodthfeorocrolagrriatyphriecafsoornc.- 4 Experimentconfiguration
ingoftheAppeninesandtheMaritimeAlps,intenseprecip-
itation was widespread in these areas, as well as along the
For both cases, the 4-day, 3-day and 2-day predictability is
coast. In fact, during 27 and 28 September, heavy rainfall
studied. TEPS and LEPS forecasts are always started and
wasobservedovertheLiguriaregion,withpeaksofintensity
close to Genova (44.25◦N, 8.51◦E) and to the west of the 22 verifiedat12:00UTC.Table1reportstheresultsoftheclus-
ter analysis performed after 96, 72 and 48h of integrations
A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem 391
Table1. Clusterpopulationforthetwoexperiments: Vaison(rows Table2. Vaisonflood: clusterpopulationversusZ700root-mean-
3–5,clustertime:12:00UTCon23September1992)andGenova squareforecasterror(inmandaveragedovertheclusteringarea)
(rows6–8,clustertime:12:00UTCon28September1992).There- fortheRMsofthe5clustersandforthedifferentforecastranges
sultsoftheclusteranalysisforthe4-dayforecastrangearereported
inrows3and6;forthe3-dayrange,inrows4and7;forthe2-day
experimentname clusterpopulation Z700rmserror
range,inrows5and8
17 19
7 47
experiment Clusterpopulation
VLR+4 16 18
name clst1 clst2 clst3 clst4 clst5
9 26
VLR+4 17 7 16 9 2 2 39
VLR+3 16 8 14 11 2
16 29
VLR+2 25 10 7 6 3 8 38
VLR+3 14 24
GE+4 28 5 5 5 8
11 30
GE+3 14 6 10 13 8 2 32
GE+2 29 9 10 2 1
25 22
10 21
VLR+2 7 19
6 15
3 31
forthetwocasestudies. TheexperimentsrelativetotheVai-
sonfloodstarton the19, 20and 21September (denoted by
VLR+4,VLR+3andVLR+2,respectively)andarever-
ifiedonthe23September,whiletheexperimentsrunforthe
5 Vaisoncase
Genovacasestartonthe24,25and26September(GE+4,
GE+3andGE+2)andareallverifiedon28September.
5.1 Four-dayforecast
TheinformationprovidedbyTEPSandLEPSproductsis
comparedforthedifferentpredictionranges,withparticular The top row panels of Fig. 2 report the TEPS probability
attentiontothequantitativepredictionofrainfall. Concern- maps (for rainfall exceeding 20mm/day) generated in the
ingtheECMWFproducts,theprobabilitymapsareobtained abovedescribedconfigurations. Bycomparingtheforecasts
intwodifferentways: withtheobservedrainfallvaluesofFig.1(bottomleftpanel),
one notices that the performance is quite poor for the “51-
memberTEPS”,aswellasforthe“5-memberTEPS”.Inboth
– by considering all 51 global-model integrations (as in cases, the 4-day forecasts indicate a chance of rain above
Molteni et al., 1996); hence, the maps are constructed 20mm/dayinPiedmont,ontheItaliansideoftheAlps,while
inthetraditionalway(“51-memberTEPS”); very little precipitation is expected in the region of Vaison.
The10%contourcoverstheregionofVaisononlyinthe“5-
member TEPS” probability map (top right panel), although
– byconsideringonlythe5RMs;themapsaregenerated the differences between the two configurations are almost
byweightingeachRMintegrationaccordingtotheclus- negligibleatotherthresholds(notshown).Atthe50mm/day
terpopulation(“5-memberTEPS”). threshold,theprobabilityisbelow10%inbothcases(andthe
panelisnotshown).Themisplacementoftheregionaffected
byheavyrainisdueto,amongotherfactors,thewrongposi-
Ontheotherhand,LEPSprobabilitymapscanbegenerated tioningofa700hPatrough(notshown)thatislocatedtoofar
byeitherweightingornotweightingeachofthe5LAMBO totheeastinalmostalltheTEPSforecastswhencomparedto
integrations according to the population of the cluster in theanalysis. AccordingtomanyECMWFforecasts,thesys-
whichtheRMwasselected. AlthoughMarsiglietal.(2001) temseemstomoveveryquicklyandtoalreadyhavecleared
found a relatively low sensitivity of the LEPS results to the theFrenchareaby23September.
weighting procedure, we chose to weight each LAMBO in- When LEPS integrations are considered, the situation is
tegration. Weighting should favour the transfer of informa- slightly better, but far from satisfactory; most of the errors
tionabouttheatmosphericflowpredictabilityfromtheglobal presentintheglobalforecastscanbefoundagaininLAMBO
TEPStotheLEPS.Twoprecipitationeventshavebeencom- integrations. The rainfall pattern is more detailed and the
pared, specifically “24-hour accumulated precipitation ex- probabilities are higher, but the highest peaks of precipita-
ceeding 20 and 50mm”. For reason of brevity, the Vaison tion are still located over northern Italy, rather than south-
caseisdescribedingreaterdetail,whiletheGenovafloodis ern France. In the LEPS forecasts, there is a 16% proba-
presentedinamorecursoryway. bility of the rainfall rate exceeding 20mm/day in the vicin-
392 A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem
Marseille Marseille
Fig. 2. Vaison flood. 4-day predic-
tions:51-memberand5-memberTEPS
probabilitymapsforrainfallexceeding
20mm/day (top left and top right pan-
els, respectively) and the LEPS prob-
ability maps for rainfall exceeding 20
and50mm/day(bottomleftandbottom
right panels, respectively). All maps
are valid at 6:00UTC on 23 Septem-
Marseille Marseille
ber1992(cumulatedbetweenforecasts
t+66handt+90h).Contoursarelo-
catedatevery10%.
Fig.2. Vaisonflood. 4–daypredictions: 51–memberand5–memberTEPSprobabilitymapsforrainfall
iteyxcoeefdtihnge2c0omrrme/cdtaylo(tcoapt–iloefnta(nbdottotpo–mriglhetfptanpealsn,erlesopefcFtivigel.y)2a)n,dpLrEe-PSprobraebcilaitsytmearprsofrosrroafinfeaallch TEPS RM in terms of the geopoten-
cipitationbeingalsoverylikelyalongthenorthwesternItal- tialheightat700hPa(hereafter,Z700)calculatedatthefinal
exceeding20and50mm/day(bottom–leftandbottom–rightpanels,respectively).Allmapsarevalidat6
ian coast. On the other hand, the bottom right panel shows time(andaveragedovertheclusteranalysisdomain).Forthe
thattheprobabilityofrainfallexceeding50mm/dayisabove 4-dayrange,onenoticesthattheforecasterrorsarehigherfor
UTCof23/9/1992(cumulatedbetweenforecastst+66handt+90h).Contoursevery10%.
10%onlyovernorthernItaly,afewhundredkmseastofthe theRMsofthe2ndand5thclusters, withthelowestvalues
flooded regions (out of the plotted area). It is worth point- achievedbythe1stand3rdclusters.
ingoutthat1outofthe5LAMBOruns(notshown)nested Therefore,the4-dayforecastsbyboththeTEPSandLEPS
ontheRMofthe5thcluster(populatedbyjust2members) systems are quite inaccurate, with the probability of flood
predictsabout40mmofrainfallinthecorrectlocation. Al- occurrence being very low. These results are worse than
thoughtheobservedamountismuchhigher(asinFig.1),itis those obtained by Marsigli et al. (2001), where the 5-day
encouragingtonoticethatalready4daysahead,aheavyrain- forecast was studied and the EPS instead of the TEPS was
fallscenariowashighlightedbyatleastonehigh-resolution used. ThisisconsistentwiththetrendobservedbyHersbach
forecast. Thisscenarioisdepictedashighlyunlikelybythe et al. (2000), who found that at this forecast range, the tar-
23
cluster analysis; hence, its weight on the final probability getedperturbationsstarttoleavetheareaofinterest,thereby
mapislow. Inthiscase,itwouldhavebeenmoreconvenient worseningtheTEPSperformance. Onealsonoticesthatthe
to generate LEPS maps by giving the same weight to each probabilitymapsgeneratedbythe“51-membersTEPS”and
LAMBOintegrationsinordertoincreasetherelativeimpor- the“5-memberTEPS”arealmostidentical. Sincethisresult
tanceofthemoreaccurateintegration.Infact,theprobability is also confirmed by the other experiments, the maps rela-
mapsobtainedinthisfurtherconfiguration(notshown)indi- tivetothelatterconfigurationwillnotbeshowninthenext
cate a 40% chance of rainfall exceeding 20mm/day in the sections and, for the global ensemble, the attention will be
Vaisonareaandthesignalforapossiblefloodisclearerthan focussed on the probability products generated by the “51-
before.Therefore,itlooksasiftheRMoftheleastpopulated membersTEPS”.
clusteristheclosesttotheverifyinganalysisintermsoftotal
precipitation. 5.2 Three-dayforecast
Theresultsarepartlydifferentiftheattentionisfocussed
onthecirculationfield. Table2reports,fortheexperiments When the 3-day forecast range is considered, the perfor-
relative to the Vaison flood, the root-mean-square (rms) fo- mance of the TEPS and especially the LEPS integrations
A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem 393
Marseille Marseille
Fig. 3. Vaison flood. 3-day predic-
tions: TEPS(toprow)andLEPS(bot-
tomrow)rainfallprobabilitymapsvalid
at6:00UTCon23September1992(cu-
Marseille Marseille mulatedbetweenforecastst+42hand
t +66h)forrainfallexceeding20(left
column)and50(rightcolumn)mm/day.
Contoursarelocatedatevery10%.
Fig.3.Vaisonflood.3–daypredictions:TEPS(toprow)andLEPS(bottomrow)rainfallprobabilitymaps
imvaplidroavt6esU,TaCsofs2h3o/9w/1n99i2n(cuFmigu.lat3ed.beTtwheeenlefofrteccaostlsut+m4n2hpaandnet+ls66ohf)forrainitfyal)l,extcheuesdinggiv20in(lgefta hypothetical forecaster the possibility to
thefigureshowaveryhighprobabilityofrainfallexceeding issueawarningthreedaysbeforetheweathereventhitsthe
column)and50(rightcolumn)mm/day.Contoursevery10%.
20mm/dayoverabroadregionbetweensouthernFranceand region.
Italy, including the area of the flood. Nevertheless, none of
the ECMWF ensemble members predicts rainfall rates ex- 5.3 Two-dayforecast
ceeding 50mm/day over the region affected by the flood;
therefore,thetoprightpanelofthefiguredoesnotshowany Astheforecastrangeshortensto2days,thepredictionaccu-
indicationofpossibleheavyrainintheVaisonarea. Onthe racyincreasesfurther. IntheTEPSruns,theclusteranalysis
otherhand,thebottomrightpanelshowsthatthe72hLEPS indicates that the most likely evolution scenario is provided
forecastsindicatea25%probabilityofheavyrainfall(above by the first cluster (see Table 1) and that the control fore-
50mm)overaregionwhichincludesVaison. Inadditionto cast is the RM of this cluster (the most populated one with
this,thebottomrowpanelsprovidemoredetailedprecipita- 25 members). In addition to this, the Z700 prediction er-
tionpatternsincomparisonwiththeTEP2S4probabilitymaps. rors of 4 out of 5RMs are lower than in the previous cases
TheZ700forecastsindicatethatthegeopotentialheightpat- (accordingtoTable2),suggestinganimprovedperformance
ternspredictedbysomeTEPSmembersareclosetotheveri- of the ECMWF TEPS for this range. In the top left panel
ficationpatterns(notshown),althoughseveralECMWFruns ofFig.4,therainfallprobabilitymapobtainedbytheTEPS
predictthelocationofthelow-leveltroughtoofartotheeast integrationsindicateahighprobabilityofrainfallexceeding
incomparisonwiththeanalysis. Table2showsthatthelow- 20mm/day over a broad region, with two peaks in proba-
est Z700 prediction errors are obtained by the RMs of the bility (about 100%) over southern France and northwestern
two most populated clusters (1st and 3rd ones), which are Italy. Asinthepreviouscase, theprobabilityoftherainfall
representativeofthemostlikelyevolutionscenarios. rateexceedingthe50mm/daythresholdisbelow10%inthe
Therefore,thoughtheTEPSprobabilitymapsareofgreat- region affected by the flood (top right panel of the figure).
er assistance than in the previous case, they still do not On the other hand, LEPS integrations predict heavy rainfall
indicate a clear probability of the rainfall rate exceeding scenarios in the correct region. The bottom row panels of
50mm/day. On the other hand, LEPS runs are more accu- Fig. 4 show that the probability maps at high-resolution in-
rateandtheprobabilitymapsprovideenoughinformationto dicate for the lower threshold a rainfall pattern with more
suggestheavyrainasapossiblescenario(25%ofprobabil- detailsascomparedtothatobtainedbytheECMWFintegra-
394 A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem
Marseille Marseille
Marseille Marseille
Fig.4.Vaisonflood.2-daypredictions:
the same as Fig. 3, but for rainfall cu-
mulatedbetweent+18handt+42h.
Fig.4.Vaisonflood.2–daypredictions:thesameasFig.3,butforrainfallcumulatedbetweent+18hand
tito+n4s2;h.this reflects, among other factors, a better description theregionsmostaffectedbytheweatherevent.
oforography. Thebottomrightpanelofthefigureconfirms
thelikelihoodofextremerainfall,alreadyhighlightedbythe 5.4 Higherresolutionrun
72hruns, withan84%maximumofprobabilityforprecip-
itation exceeding 50mm/day. In particular, the probability Onceafloodscenariooveraregionispredictedwithahigh
patternclearlyshowsthemainpeakatabout45◦N4◦E,very probabilityofoccurrence(asinthe48hLEPSruns),itisof
closetotheactuallocationoftheflood. Astheseveryhigh crucialimportancetohaveaverydetaileddescriptionofthe
valuessuggestaclearchanceofaflood,theycouldassistthe precipitationpatternwithintheregionitself.Hence,itcanbe
forecasterinissuingawarning,therebyalertingcivilprotec- necessary to further augment the resolution of the limited-
tion agencies two days before the event. These results con- area model in order to have an enhanced description of the
firmthelimitationoftheTEPS’abilitytopredictheavypre- orographyandtherelatedphysicalprocesses. Thismayhelp
cipitation.Nevertheless,itshouldbealsonotedthatthereso- to provide a more reliable quantitative estimate of the total
lutionoftheTEPSmembers(TL159corre2s5pondingtoabout amountofrainfallwhichisabouttofallovertheregion. In
120kmingrid-pointspace)maynotbesufficientenoughto ordertoaddressthispoint,onefurtherrunwasperformedat
properlydescribethoseorographiceffectsthatplayacrucial ahigherhorizontalresolution(5insteadof20km, stillwith
roleindeterminingtheconditionsforheavyprecipitationin 32verticallevels), nestedonthat48hLAMBOrunrelative
thiscasestudy. Thisisunderlinedbythedifferentprobabil- totheRMofthemostpopulatedcluster(thefirstonewith25
ity patterns obtained by the TEPS and LEPS in the 20mm members,accordingtoTable1). Duetothelimitationsinthe
threshold (left column panels of Fig. 4). One notices that computerpower,thedomainofintegrationwaskeptsmaller
theTEPSmapscannotresolvethenarrowstripofheavypre- thaninthepreviousintegrationsandcenteredapproximately
cipitation over the Liguria coast (northwestern Italy), high- overtheregionaffectedbytheflood1.
lighted,ontheotherhand,bytheLEPSrunsandconfirmed Figure5showstheprecipitationpatternpredictedbythis
bytheobservations. Therefore,whileintheTEPSprobabil- high-resolution run over its domain. This LAMBO integra-
ity maps the signal for heavy precipitation is still weak, the
informationprovidedbytheLEPSrunsismuchmoreaccu- 1Itisalsoworthnotingthatthedeterministichigherresolution
rateandrealistic, therebypredictingwithhighspatialdetail approach is prohibitive for ARPA-SMR computing facilities with
forecastrangesexceeding48h
A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem 395
Fig. 5. Vaison flood. Total precipi-
Marseille
tation cumulated between t + 18 and
t+42hpredictedbyLAMBO(“higher-
resolution” configuration), starting at
12:00UTC on 21 September 1992.
Contour intervals: 2, 10, 50, 75, 100,
150and200mm.
Fig.5.Vaisonflood.Totalprecipitationcumulatedbetweent+18andt+42hpredictedbyLAMBO(“higher–
resolution”configuration),startingat12UTC21September1992. Contourintervals: 2,10,50,75,100,
tionisveryaccurateintermsoftotalprecipitation:itpredicts 6.1 Four-dayforecast
a15ra0iannfda2ll00mmamx.imum of about 170mm in an area very close
tothatwhichwasactuallyflooded,wherethemaximumob-
At the longest forecast range, both the global and, to a
served value was 159mm. In addition to this, the amounts
lesser extent, the limited-area ensembles misplace the lo-
of rainfall predicted along the coast are quite close to those
cation of the region affected by heavy rain. The amount
observed and reported in Fig. 1. The predictions of other
of precipitation is underestimated over the Liguria region
fields(notshown),suchasZ700ormeansealevelpressure,
and over-estimated over southern France. The left panel of
are also quite accurate, indicating the benefits gained by an
Fig. 6 shows the TEPS probability map for rainfall exceed-
enhancementofhorizontalresolutionforthepredictionofa
ing20mm/day, representingabroadregionwithhighprob-
low-levelmeteorologicalfield. abilityofprecipitationrangesfrom4◦Eto9◦E,witha85%
maximumneartheFrench-Italianborder. Therightpanelof
Fig. 6 shows the more detailed rainfall pattern of the LEPS
integrations, which predict the high possibility of precipita-
tionexceeding20mm/dayallalongthecoastandnotonlyto
26 the west of 9◦E. A probability peak of 81% is well evident
6 Genovacase
just over the town of Genova, although rainfall exceeding
20mm/day is expected almost everywhere in northwestern
Italy. At the 50mm/day threshold, not a single run by ei-
TheresultsobtainedbytheTEPSandLEPSintegrationsfor
ther TEPS or LEPS predicts heavy rainfall in northwestern
theGenovafloodarepartlydifferentfromthosedescribedin
Italy,butratheroversouthernFrance(whereheavyrainwas
the previous section for the Vaison case. The TEPS perfor-
not actually observed) and the probability maps for rainfall
mance is much more accurate than in the previous case at
exceeding this threshold are not shown. In particular, the
anyrangeandthisreducestheamountofinformationgained
LEPSperformanceatthe50mm/daythresholdisquitepoor:
when the limited-area ensemble is considered. Hence, the
a70%probabilitymaximumislocatedatabout44◦N4◦E,
betterperformanceoftheLEPSrunsislessevidentthanbe-
completely misplaced in comparison to the actual flood lo-
fore. Inthefollowing,theattentionisfocussedonthe4-day
cation,andthereisnoindicationofthepossibilityofaflood
and2-dayforecasts(3-dayforecastsarenotshown,sincethe
overtheLiguriaregion.
results are almost identical to those obtained for the 2-day
range),aswellasonthehigherresolutionrun. Therefore, at the longest forecast range, both the TEPS
396 A.Montanietal.: TheARPA-SMRlimited-areaensemblepredictionsystem
Milano Milano
Fig.6.Genovaflood.4-daypredictions:TEPS(leftpanel)andLEPS(rightpanel)rainfallprobabilitymapsvalidat12:00UTCon28Septem-
ber199F2i(gc.u6m.uGlateendobveatwfleoeondt.+4–7d2ahyapndretd+ic9ti6ohn)sf:oTrEraPinSfa(llleefxtcpeaednienlg)2a0ndmLmE/dPayS.(Croignhtotuprsanareel)lorcaaintefdalaltpevroerbya1b0il%it.ymaps
valid at 12 UTC of 28/9/1992 (cumulated between t+ 72h and t+ 96h) for rainfallexceeding 20 mm/day.
and LEPS maps indicate the possibility of rainfall over the 6.3 Higherresolutionrun
region of Genova, although the intensity of the event is not
Contoursevery10%.
properlycapturedatthisstage. Asinthepreviouscase,afurther48hLAMBOrunwasper-
formedinthehigherresolutionconfiguration(5kmofhori-
zontalresolution)toobtainamoredetailedquantitativerain-
6.2 Two-dayforecast fallestimateoverthefloodedregion. Figure8showsthere-
sultsobtainedbythehigherresolutionintegrationnestedon
theRMofthefirstcluster, whichisthemostpopulatedone
Attheshorterpredictionranges,theaccuracyofbothglobal with 29 members, as in Table 1. The rainfall forecast in-
and limited-area ensemble forecasts increases. Figure 7 dicates the heaviest precipitation rates over the coast to the
shows the performance of TEPS and LEPS for the 2-day west of Genova, with peaks above 180mm/day, and a sec-
forecast range (top row and bottom row panels, respective- ondary maximum also evident along the western Alps, at
ly). Onenoticesthattheglobalmodelpredictsthehighpos- about45◦N7◦E.Adirectcomparisonwiththeobservations
sibilityofrainfalltoofartothewest,whencomparedtothe in the bottom right panel of Fig. 1 shows that although this
observationsinthebottomrightpanelofFig.1.Infact,atthe run properly forecasts heavier precipitation over the west-
lowerthreshold, thereisaverybroadprobabilitymaximum ernratherthantheeasterncoast,therainfallmaximaarestill
covering the northwestern coast for almost 500km (top left located too far to the west. In fact, the 300mm peak over
panel),whiletheprobabilityofrainfallexceeding50mm/day the town of Genova is missing and the Z700 forecast (not
showsamorelocalisedpeak,witha76%maximumatabout shown) indicates the small-scale trough slightly misplaced
43◦N6◦E(toprightpanel). TheLEPSperformanceissimi- to the west (in comparison with its actual position). Never-
lartothatobtainedbytheTEPS,atleastforthegeographical theless,boththeintensityandthetimingoftheweatherevent
locationoftheregionsaffectedbyheavyprecipitation,since arewellcaptured,therebyenablingaforecastertoalert,those
the westward displacement is evident but to a lesser extent organizationsinvolvedinpublicsecurity,aswellasthelocal
also in the LAMBO integrations. The bottom left panel of populationtwodaysbeforetheevent.
Fig. 7 shows a probability maxima over northwestern Italy
and southern France, while at the 50mm/day threshold, a
7 Conclusions
broadpeakintherainfallprobabilityisevidentallalongthe
coasttothewestofGenova(bottomrightpanel).
TheperformanceofLEPS,theARPA-SMRlimited-areaen-
Therefore, in the LEPS maps, the probability of heavy semble prediction system nested in the experimental TEPS,
rainfall is always higher than in the ECMWF runs, with was tested, thereby investigating the predictability of two
the horizontal details also more accurate than in the global floodweathereventsfortheforecasttimesrangingbetween
runs. Nevertheless,bothensemblespredictarainfallpattern 2 and 4 days. LEPS is composed of five runs of the high-
slightly misplaced tothewestwhencomparedtotheobser- resolution limited-area model LAMBO, nested on five se-
vations. 27 lected members of the ECMWF TEPS, the recently devel-