Deutsches Zentrum fuÈr Luft- und Raumfahrt (DLR), Institut f�ur Physik der Atmosph�are, Oberpfaffenhofen, Germany

Heavy Precipitation in the Alpine Region (HERA): Areal RainfallDetermination for Flood Warnings Through in-situ Measurements,Remote Sensing and Atmospheric Modelling

H. Volkert

With 8 Figures

Received April 14, 1999Revised August 3, 1999

Summary

Background and key results of the EU-funded projectHERA are presented. The identi®cation of the growingpossibilities to use forecast precipitation ®elds as input forhydrological models is followed by a broad assessment ofthe state of the art regarding the determination of theatmospheric part of the hydrological cycle, with thegeographical focus on the mountainous region of the Alpsin the heart of Europe. This includes the construction of(north)Alpine radar composites for nine episodes; a ®rstsystematic cross-validation on a daily basis of fouroperational forecasting models with a trans-nationalgridded observational dataset; several detailed case studiesinvolving research versions of operational forecast modelsand the latest advances in model resolution and micro-physical parameterizations; new algorithms for Dopplerradar retrievals over complex terrain; and syntheticmodelling studies with governing parameters derived fromthe selected cases to investigate some basic processes inisolation. Finally, implications for proto-type applicationsof forecast areal precipitation ®elds in the hydrologicalmodelling of mountainous catchments are given.

1. Introduction

The picture of a hydrological cycle as anabstract, but persuasive concept for the move-ment and distribution of water in all its threephases within the layers above, at and below theEarth's surface is of such a basic nature that itfeatures prominently already in primary school

curricula. But its extremely complex and variablespatio-temporal nature renders its quantitativedescription an as yet unreached but importantgoal of basic and applied research.

Extreme cases of long lasting precipitationevents and resulting ¯oods are recorded inancient, prescienti®c reports (e.g., about theDeluge, O.T., 1st book Mose 7, 17±24) or haverepeatedly spurred institutional efforts regardinga better management during hazardous situationsor even reliable warnings before such events. InBavaria, for instance, meteorological and hydro-technical institutes had been separately inaugu-rated after 1878, when the devastating ¯oods of1899 were followed by an increase of staff in allprovinces of the kingdom and eventually by acooperative agreement concerning ¯ood warn-ings in 1910 (Volkert, 1983, pp. 65, 70, 203).

The European Commission included ` ¯oodhazard'' in its portfolio of research programmesin 1986 (Casale et al., 1998), embedded into themore general theme of ` natural risk and civilprotection'' (Horlick-Jones, 1995). The severewinter ¯oods of Rhine and Meuse in 1993/94and 1995 were taken as a timely reminder foreven closer coordinated efforts and led to theinception of the Concerted Action ` river basinmodelling, management and ¯ood mitigation

Meteorol. Atmos. Phys. 72, 73±85 (2000)

(RIBAMOD)'' (Moore, 1998). In parallel some 20research projects obtained European fundingunder the heading ` technologies to forecast,prevent and reduce hydrological and hydrogeo-logical risks'' within the ` environment and cli-mate'' part of the Fourth Framework Programme.

The present state of affairs may be sketched asfollows:

a) ¯ood hazard is an old, clearly recognizedproblem area calling for a truely interdisci-plinary approach (Cluckie, 1998);

b) the one outstanding feature of ¯ood assess-ment which is readily accepted is that it isvery dif®cult (Cluckie and Han, 1995);

c) clear-cut splits in subdomains are necessary tomake progress step by step and to aidcooperations across well de®ned interfaces;natural subdomains are speci®c scienti®cdisciplines, various spatial domains or differ-ent temporal horizons.

One such subdivision is given in Fig. 1 andserves to put the studies of this volume intoperspective. The very transient, i.e. time depen-dent, hand-over of water from the atmosphere tothe various catchment areas of the topographi-cally structured ground and its subsequent

collection in rivers (of lower dimensionality) isquite straight forward and somewhat in parallelto the established scienti®c disciplines meteorol-ogy, geography, and hydrology. Less evident is aconsistent allocation of data, tools, and above allthe desired results within such a framework.

A concentration on long term issues, such asrisk assessment, calls for climatological timeseries of rain and river gauge networks, possiblyaugmented by a database of past ¯oods (Garrote,1995; Marcuello, 1998). This data in turn can beused to determine return periods, for prede®nedthreshold values, to ®nally arrive at suitabledesigns of, e.g., urban canalisations or dams.Reliable ¯ood warnings necessitate actual riverlevel information from upstream gauges, possi-bly augmented by real-time estimates of arealprecipitation from a radar network, depending onthe scale and steepness of the catchment underconsideration (Cluckie, 1999).

Considering the increasing skill and spatio-temporal resolution of numerical weather pre-diction (NWP) models, the vision of integratingforecast precipitation data into ¯ood warningsystems is both natural and demanding, espe-cially for extended mountainous region such asthe Alps in the heart of Europe. Ad-hoc feasibiltystudies have dealt with such precipitation fore-casts regarding ¯ash ¯oods in mountainousterrain (Quiby and Schubiger, 1998) and thesimulation of the longer lasting extended pre-cipitation system which caused the Oder ¯ood ofJuly 1997 (Keil et al., 1999).

The Alps are generally regarded as Europe's` water tower'', as they serve to enhance the

Fig. 1. Generic subdivision of the geophysical domain ofthe ¯ood hazard issue into scienti®c disciplines, physicalcompartments (boxes) including the hand-over of water(ellipses), and the complex ®eld spanned by data, tools,results (elongated diamonds) which inherently depends onthe envisaged time horizon. The dots at the bottom point toother facets, e.g., socio-economic impacts. All aspects inbold face are dealt with in project HERA

Photo: Coffrini; Sonntags-Zeitung of 26 Sept. 1993

Fig. 2. Cars buried in mud and uprooted trees after thedevastating ¯ash ¯ood of the river Saltina in the town ofBrig, Switzerland on 24 September 1993

74 H. Volkert

ambient rainfall (as every major mountain range)and feed it into the large river systems of Rhone,Rhine, Danube, Po and their respective tribu-taries. Frei and Sch�ar (1998) recently presenteda ®rst consistent, quality controlled 20 yearprecipitation climatology which made use ofmore than 6600 stations from various networkswithin the different Alpine countries. Amongthe outstanding results were the quanti®cationsof maxima in both the mean precipitation(> 1800 mm/a) and its standard deviation atthe northern and southern ¯anks of the massifand their distribution in a spotty pattern ratherthan in a uniform zone.

These climatological precipitation maxima areto some extent made up of singular episodes ofextremely high rainfall amounts over compara-tively short time periods. A frequent conse-quence are (¯ash) ¯ood situations, which canlead to quite unexpected and severe conse-quences as in Brig, when on 24 September1993 masses of gravel and mud suddenly ®lledthe dammed bed of river Saltina and then spilledover into the town centre (Fig. 2). Aftercontinuing strong precipitation the water levelof the neighboring Lago Maggiore on theregion's south side had risen by 3.7 m sometwo weeks later (Fig. 3). The location and size ofthe lake (212 km2) embedded within the complextopography of southern Switzerland and adjacentItaly is visualized in Fig. 4.

The database about natural disasters ofMunich Re, a large international reinsurancecompany, lists the following Alpine ¯oodepisodes as particularly severe disaster situationswith profound socioeconomic impact: 20±22September 1992 in the Alpine foothills ofsouth-eastern France (38 deaths; main damagesin the town Vaison-La-Romaine; economic lossesexceeding 4 � 108 US$); 20±28 September 1993in the south-western Alps (16 deaths; largedamages in the Swiss town Brig [see Fig. 2];economic losses about 15 � 108 US$); and 4±6November 1994 in the Italian Piedmont region(64 deaths; economic losses exceeding 90 � 108

US$).During the 1990's the meteorological services,

university departements and research laboratoriesin all Alpine countries became increasinglyaware that the adequate observation of the richrange of mesoscale atmospheric phenomenainstigated by the Alps, their basic understanding,and successful prediction constitute both dif®cultand desirable enterprises. Description and simu-lation of precipitation processes are one coretopic of the ungoing ` Mesoscale Alpine Pro-gramme (MAP)'' (Binder and Sch�ar, 1995),which will culminate in a Special ObservingPeriod (MAP±SOP) from September to Novem-ber 1999.

All the above aspects served to motivate theestablishment and de®nition of a research projectunder the title ` Heavy precipitation in theAlpine region (HERA)'', which was submittedto the ` hydrological risks'' topic within the EU

Photo: Rita Tognini

Fig. 3. 9 October 1993: Inundation of the lake-sidepromenade of Locarno at the northern shore of LagoMaggiore. The lake level was 3.7 m above normal due toabout 800 � 106 m3of excess water

Picture: David Bresch with ARGOS software

Fig. 4. Elevated view from the south onto the region aroundLago Maggiore (193 m MSL). Notable are the deep valleyswith ¯oor heights below 300 m between mountain ridgeswith snow covered summits in the range from 2000 to3000 m. Locarno (Lo; Fig. 3) and Monte Lema radar site(1620 m; L; Fig. 8) are indicated in red

Heavy Precipitation in the Alpine Region: Areal Rainfall Determination 75

research programme ` environment and climate''.Project HERA of®cially started on 1 December1996 and continued till 31 March 1999. Thereminder of the present overview article outlinesthe project's aims and structure of six work-packages (section 2), highlights major projectresults (section 3) and attempts a balancedconclusion and outlook (section 4). Detailedaccounts on the various results obtained duringthe conduct of HERA are given in the otherarticles of this volume.

2. Project Aims and Structure

The Alpine region consisting of the numerousmassifs divided by deep valleys and the adjacentforeland were selected as the geographical focusof a truely European dimension. Furthermore itwas decided to concentrate on a number ofrecent, but past precipitation episodes and tomake use of a variety of available data typeswhich had been collected independent of HERA.

Nine institutions from ®ve Alpine countriesmade up the partnership: from Germany (DE)DLR Oberpfaffenhofen, Institut f�ur Physik derAtmosph�are and Meteorologisches Institut derUniversit�at M�unchen (MIM); from Austria (AT)Institut f�ur Meteorologie und Geophysik derUniversit�at Wien (IMGW) and Institut f�urMeteorologie und Geophysik der Universit�atInnsbruck (IMGI); from Italy (IT) Istituto diScienze dell'Atmosfera e dell'Oceano (till 1998:FISBAT), and Servizio Meteorologico Regionale(SMR), both in Bologna; from France (FR)Centre National de Recherches M�et�eorologiques(CNRM) of M�et�eo-France and Laboratoired'A�erologie (LA) of Universit�e Paul Sabatier,both in Toulouse; and from Switzerland (CH)Institut f�ur Atmosph�arische Wissenschaft andGeographisches Institut of ETH, Z�urich.

The overall objectives of the project werede®ned as follows:

± to establish a multi-year systematic collectionof heavy precipitation episodes by processingcombined data sets from ground based radar,surface and upper air observations;

± to analyse major precipitation events byapplying operational and research-type numer-ical weather forecast models and to quantifythe underlying physical processes;

± to adapt airborne radar algorithms to measure-ments over mountainous terrain; and

± to develop and to implement new highresolu-tion numerical techniques (nonhydrostatic gridnesting).

More speci®cally, answers to questions such asthe following ones were sought:

± What is the spatial extent and temporalduration of severe precipitation events in theAlpine region?

± How predictable are such events using opera-tional mesoscale NWP models?

± Which are the essential dynamical processesfor engendering copious precipitation inmountainous terrain? Does the relative impor-tance of forcing processes differ between thenorthern and southern ¯ank of the Alps? Howimportant are ampli®cation effects throughsynchronous processes acting in differentheight levels?

± What can be gained from observational andmodelling techniques under development?Which spatial and temporal data resolutioncan be realistically expected by hydrologists,geologists and warning services?

The entire work share was split in a projectstructure consisting of six work packages to

Table 1. Distribution of the Workshare Between the Six Work Packages and the Nine Partner Institutions Introduced inSection 2; ??: Group Responsible for Work Package; ?: Participating Group

Country DE AT CH IT FRInstitution DLR MIM IMGW IMGI ETH FISBAT SMR CNRM LA

A1 ?? ? ?A2 ? ? ?A3 ? ? ?? ? ?B1 ??B2 ? ? ?B3 ? ?? ?

76 H. Volkert

which the partners were to contribute as given inTable 1:

A1: systematic monitoring of precipitation sys-tems from recent years;

A2: coherent evaluation of operational precipi-tation forecasts;

A3: detailed investigation of major cases;B1: feasibility of airborne Doppler radar techni-

ques over complex orography;B2: implementation of advanced modelling

techniques; andB3: process studies of basic mechanisms.

The work packages A1, A2, A3 placed theemphasis on observed cases and the applicationof established techniques, while packages B1,B2, B3 concentrated on process studies of keyphysical mechanisms and advanced techniqueswhich are under development.

Regarding the applied methods the projectaimed at a balanced blend ofobservations from

± advanced ground based radars (some Dop-plerized/polarmmetric),

± routine surface networks (partly auto-matted),

± routine upper air soundings; and

simulations from

± operational weather forecasting models,± research models concentrating on key

mechanisms,± tests of the coming generation of opera-

tional models

in order to obtain an as complete as possiblepicture of heavy precipitation events in the Alpineregion and the short range predictability for theatmospheric part of the hydrological cycle.

3. Key Results

This section outlines what is considered to be theprinpical achievements made in the course ofHERA. The sequence of presentation follows thestructure of work packages as described above.More details can be found in the bulk of all thereferenced papers, which are also contained inthis volume of Meteorology and AtmosphericPhysics.

3.1 Systematic Monitoring

For a long time rain gauge networks served as thestandard tool for precipitation monitoring.Although they usually have the highest spatialdensity of all meteorological surface networks, areliable quanti®cation of areal precipitation ishampered by their modest temporal resolution(merely daily sums at the vast majority ofstations) and by the inadequacy of pointmeasurements for the determination of a ®eldas highly structured as rainfall.

During the past 40 years radar techniques havebeen more and more re®ned and are nowoperationally used to determine quasi-instanta-neous rain rates from the echoes that precipitat-ing particles generate when hit by micro-waveelectro-magnetic radiation which had beenemitted from a radar installation (Austin, 1999).Most meteorological services now cover theirdomains with radar networks. But routine trans-national links were hardly available in the Alpineregion before the start of HERA.

Hagen et al. (this volume) outline the com-bined use of radar data from France, Switzerland,Germany, and Austria and of a 25 year dailyprecipitation climatology (Frei and Sch�ar, 1998;augmented by the years 1991±95) to monitordevelopment, progression and spatial extent ofmesoscale convective systems (MCS) along thenorthern side of the Alps. Nine precipitationepisodes were selected from the years 1992, 94,95, 96 in order to construct HERA radar compo-sites combining data of up to twenty radarscovering an area of about 1,250� 650 km2. Agrand total of 1104 half-hourly pictures havebeen produced spanning a period of 23 days. Allpictures (GIF-format) and data®les are collectedin the user section of the MAP Data Centre atETH Z�urich (http:/ /map.ethz.ch/mm-doc/HERA.htm).

The four panels of Fig. 5 span 4.5 h and clearlydepict complexity, size and propagation speed ofa semi-coherent precipitation pattern with muchvariability in the appearance (and disappearance)of convective cells. Hagen et al. (this volume)give statistical details about, inter alia, size,duration and total rain volume for all 15 MCSwhich occured during the selected HERA epi-sodes. On 22 July 1995, for instance, more than5 � 109 m3 rainfall were determined from the

Heavy Precipitation in the Alpine Region: Areal Rainfall Determination 77

radar measurements during little more than 8 h.From the combination of several years of radardata and longer term rain gauge measurements itwas estimated that a large MCS (more than30 mm/d precipitation over an area of more than40,000 km2) has to be expected in the northernAlpine region every 6 years.

An automatted monitoring of convective cellswithin MCS is possible through a combination ofradar measurements with data from lightningdetection systems operated by electricity provi-ders (Steinacker et al., this volume). Preferredtracks were found along large Alpine valleys andtheir ¯anks.

3.2 Coherent Evaluation of OperationalForecasts

Although numerical weather prediction withelectronic computers has a nearly 50 year history,operational forecasts including mesoscale pre-cipition ®elds only became generally available

during the past decade. The explorative study ofNickerson et al. (1986) already stressed thenecessity for a combined view of air¯ow overmountains and precipitation, but also demon-strated the considerable dif®culties of a consis-tent model evaluation. If forecast precipitation®elds are eventually to be used for hydrologicalwarning schemes, a strigent short term qualityassessment of operational precipitation forecastsis mandatory.

A, perhaps unexpected, dif®culty consists inthe provision of a reliable calibration standard,i.e. a consistent, quality controlled, daily pre-cipitation climatology for horizontal areas com-parable with the computational gridsize as onlyrecently put forward by Frei and Sch�ar (1998).Mladek et al. (this volume) use this informationtogether with daily data provided by theforecasting centres of Deutscher Wetterdienst(DWD; Offenbach), M�et�eo-France (Toulouse),Servicio Meteorologico Regionale (Bologna),and Schweizerische Meteorologische Anstalt

Courtesy: Martin Hagen

Fig. 5. Time evolution (with 1.5 h intervals) of the convective cells (red and purple) imbedded in a multiline frontal system on23 July 1996. The varying number of available radars (8 to 11) should be noted

78 H. Volkert

(Z�urich) for parts of the MAP seasons in autumn1995 and 1996. The complete analysis domainand its division in subareas is shown in Fig. 6together with the respective mean forecastquality for the test period in 1995. In generalthe models are able to reproduce the observedprecipitation with some tendency of overpredic-tion. The latter is most striking for subarea 60 ofelevations exceeding 2000 m. This ®nding initself stresses the importance of a specialobserving period of measurements with a spatialand temporal resolution much enhanced overroutine practise, as it will be undertaken inautumn 1999 within MAP. But it also points topossible uncertainties in the observational data-base for high Alpine elevations.

Mladek et al. (this volume) present a wealth ofstatistical information in charts, scatter plots,global skill scores for various thresholds andsample time-series of these. They also attempt aclassi®cation of precipitation events dependingon the predominant wind direction. Thus workpackage A2 was successful in providing a ®rstlarge step towards a systematic quality controland model intercomparison procedure for pre-cipitation forecasts, especially over mountainousterrain. However remaining technical barriersalso became evident, as resolution and durationof the observational database has to be increasedtogether with longer archived series of consistentmodel results, i.e. those not in¯uenced by the

ongoing modi®cations in the forecasting systems.The European meteorological services areencouraged to seriously consider the conduct ofa multiyear re-analysis project for central Europefollowing the global scale example of theEuropean Centre for Medium-range WeatherForecasts (ECMWF; Gibson et al., 1997). As aside-effect this would certainly ease the access tohitherto neither easily nor inexpensively avail-able high resolution European climate data(Hulme, 1994).

3.3 Detailed Case Studies

Precipitation episodes with ¯ooding potential arerare and mostly single events. Therefore a closerinspection of capabilities and current limitingfactors of some European operational NWPmodels is approriate for such episodes.

Buzzi and Foschini (this volume) apply theresearch NWP model BOLAM3 to ®ve recent¯ooding episodes on the southern-western tosouth-eastern ¯anks of the Alps. They investi-gated the impact of variable horizontal gridresolution from 40 down to 4 km and found anincrease of simulated rainfall amounts for ®nerresolutions but no statistical signi®cance (due tothe small sample size).

Cacciamani et al. (this volume) use theoperational model LAMBO of the regionalmeteorological service (SMR) of the Italian

Courtesy: Philippe Bougeault

Fig. 6. Summary of intercomparison of operational precipitation forecasts. Left: Total area (00) and subareas used for theintercomparison over 164 days in autumns 1995 and 1996: Massif Centrale (10), western Alps (20), northern Alps (30),Dinaric Alps (40), southern Alps (50), elevations above 2000 m (60; dark grey), and elevations above 700 m (70; light anddark grey). Right: Observed versus forecast mean daily precipitation (mm/d) by (sub)areas and agency: M�et�eo-France (FR),Servizio Meteorologico Regionale (IT), and Schweizerische Meteorologische Anstalt (SW)

Heavy Precipitation in the Alpine Region: Areal Rainfall Determination 79

region Emilia-Romagna for numerical experi-ments of three south-Alpine ¯ooding events.Beside the effect of horizontal resolution theyfocus on the sensitivity of the simulated rainfallto the speci®ed initial conditions. A newlydeveloped nudging scheme, which allows toinput detailed surface observations into thesimulation run, helps to provide better results,especially regarding the ®ne scale structure of theprecipitation patterns.

Keil and Volkert (this volume) apply theresearch version of DWD's Deutschland Modell(DM) to four of the north-Alpine cases whichwere selected for the HERA radar composites(see subsection 3.1). The model independentvalidation data comprise the regular raingaugenetwork of DWD, the HERA composites, Dop-pler winds from the DLR research radar, andmeteorological time series at a ground station.

Three consecutive hourly precipitation amountsand the resulting three-hourly sum (Fig. 7)

reveal: the quite transient nature of a precipita-tion ®eld linked to a cold front passage; thenevertheless spotty nature of the calculated rainamounts made up of grid-mesh pixels sized14� 14 km2 (eight pixels equal 1� difference inlatitude); and the local three-hourly maximaexeceeding 10 mm across Switzerland as wellas in the region of rivers Neckar and southern Isar.A four-day budget evaluation from hourly simula-tion data over the Isar-Amper catchment (made upof 45 grid-meshes) involves the atmosphericmoisture terms, the resulting convective andgrid-scale precipitation components and arealweighted upward motion. It demonstrates boththe potential and the present (spatial) limitationsof this current atmospheric forecasting system.

Ho®nger et al. (this volume) report about a ®ne-scale investigation of 14 summer-time precipita-tion events from 1997 in the intra-Alpine regionaround Innsbruck. The study included the set up ofa dedicated network with 5 to 10 min recording

Courtesy: Christian Keil

Fig. 7. Transient nature of precipitation over the north Alpine foreland as simulated for three consecutive hourly intervals on5 July 1996 and the resulting three-hourly precipitation (RR)

80 H. Volkert

interval. Rainfall of up to 30 mm/6 h and distinctgradients over short distances were documented.This material may prove to provide valuableveri®cation material for future simulations whenlarge inner-Alpine valleys can be resolved.

3.4 Feasibility of Doppler Radar Techniques

The preceeding subsections indicated that moredetailed and accurate observations of strongprecipitation systems are still necessary in orderto improve the simulation capabilities. Dopplerradars have the unique capacity to providesimultaneous information about the ¯ow andprecipitation ®elds. In the past, algorithms for¯ow determination were not well suited forapplications near highly variable orography, asunaccounted ground clutter and orographicallyinduced small scale circulations deterioratedthe retrieved wind information. The additionaladvantage of an airborne system, i.e. movable

around the area of interest, could up to now onlybe applied over sea or ¯at terrain.

Georgis et al. (this volume) present extendedretrivial algorithms including variational meth-ods for ground-based and airborne dual Dopplerradar measurements over complex terrain, suchas the Alps or the Rocky Mountains. A hierarchyof tests is preformed, which comprise radarobservations for analytically speci®ed ¯ows overidealized terrain, numerically simulated windand re¯ectivity ®elds for the Brig ¯ood event (22September 1993; see also Stein et al., thisvolume), and actual measurements during ashower episode over the Colorado Rockies.

During the MAP±SOP in autumn 1999 severalAmerican and French teams will operate ground-based and airborne Doppler-radars particularly inthe region around Lago Maggiore (Fig. 4). Thenthese new methods are indispensible for adetailed documentation of the life-cycle oforographically and convectively induced storms.

Courtesy: Evelyne Richard

Fig. 8. Six-hourly accumulated rainfall pattern during the Brig ¯ash¯ood event (22 September 1993, 12 to 18 UT; in mm) asdetermined from 36 radar measurements from Monte Lema (at 8.8� E, 46.1� N, 1620 m; a) and by a high resolution simulationincluding ice-phase parameterizations (b). The numbers (mm) denote rain gauge measurements from Italian and Swissstations. See text for the discrepancy in colour scale. The 1� � 1�grid (centre at 9� E, 46� N) and an approximated Italo-Swissborderline aid the orientation

Heavy Precipitation in the Alpine Region: Areal Rainfall Determination 81

3.5 Advanced Modelling Techniques

Already during the de®nition phase of HERA itwas felt that the new generation of meso-scaleNWP model, which then started to be developed,will offer good prospects for a distinct progressin quantitative rainfall forecasting. The followingthree features are considered important for arealistic and well resolved simulation of preci-pitation over complex terrain: (i) the applicationof a prognostic equation for the vertical velocity(non-hydrostatic model; in contrast to currenthydrostatic models which diagnose upwardmotion from a more approximated version ofmass continuity); (ii) the use of mesh sizesclearly below 10 km (by interactive grid nesting);and (iii) the inclusion of ice phase processes inthe microphysical parameterizations. All thesepoints are being combined in the Meso-NHmodel.

Stein et al. (this volume) give a brief technicaldescription of these novel features and presentsystematic sensitivity studies comprising about10 numerical experiments for each of the ¯ood-ing events of Vaison-La-Romaine (22 September1992) and Brig (22±24 September 1993). Specialefforts were undertaken to obtain improvedinitial conditions, e.g. by indirectly inferringthe moisture structure over the Golfe du Lionfrom infrared satellite imagery, to obtain six-hourly rainfall amounts from Italian and Swissrain gauges (with the kind assistance of the EU-fellow project RAPHAEL), and to determineradar rainfall estimates from the early test phaseof the Monte Lema radar in southern Switzer-land.

Six-hourly accumulated rainfall during theBrig episode from three independent data sourcesare jux-taposed in Fig. 8. The raingauge data(black numbers) are seen as reliable pointinformation revealing the distinct local maxima(42 mm/6 h in the more central part; 28 mm/6 hat the western edge) and the border to areaswhich remained dry. The summed-up radarinformation (Fig. 8a) gives the two-dimensionalbanded structure of the precipitation ®eld with adouble core wide band west of the 9th meridianand three slim ones east of it. The northern edgeof the strong maximum is apparently under-estimated, probably due to beam blocking byintermediate summits. Altogether the radar

derived rain rates are too high by about a factorof 4 (due to calibration problems during the earlyoperational phase of the radar in 1993; MartinHagen: personal communication) Therefore thecolour scale was adjusted accordingly.

The Meso-NH simulation result with 2.5 kmhorizontal resolution and including ice-phasemicrophysics (Fig. 8b) contains the followingimportant features: the model mesh-size isapproaching the size of the radar pixels; thebanded structures are well captured including thenarrow dry zones; the quantitative agreementwith the surface observations is remarkable; thenorthward extent of the wide band is differentfrom the radar information, but coincides wellwith the surface data. Having in mind thecomplex orography of the area (Fig. 4), it isimportant to note the decisive interplay betweenmountain induced vertical circulations, the gen-eration of precipitation and the possible spilloveracross mountain chains into different catchments(as it was the case in Brig situated well north ofthe observed and simulated precipitation max-imum). It appears that in this respect theadvanced modelling techniques will attain greatadditional value in the future.

3.6 Basic Mechanisms

A project dealing with the current state ofresearch regarding the determination and simula-tion of areal precipitation would be incomplete,if it did not include studies dealing with the basicmechanisms involved and intrinsic dif®cultieswhich impede large gains in model skill.

Fehlmann and Quadri (this volume) indicatethat the vast majority of severe rainstorms, inparticular on the Alpine southside, was to a goodmeasure determined by anomalies in atmosphericlevels high above the mountains. Consistentvariation of upper level structures within theaccuracy of routine measurements in this regionreveals a remarkable impact on amount andlocation of simulated precipitation maxima. Thisleads to the hypothesis that misforecasts are tosome extent due to the insuf®ciently knownatmospheric state well above and a day or twoprior to a severe precipitation event. Realistichindcast simulations are carried out for the entire®ve October months of the years 1987±90 and1992. All strong south-Alpine precipitation

82 H. Volkert

events contained in the period exhibit a verysimilar synoptic scale structure at upper levels.This aids the de®nition of more re®ned observa-tional strategies which will be tested during theMAP±SOP in autumn 1999.

Inspired by the severe precipitation event,which induced the Piedmont ¯ood of November1994, Schneidereit and Sch�ar (this volume) carryout a hierarchy of idealized numerical experi-ments with dry and moist atmospheres anddifferent approximations of the arc-shaped Alpineorography. The importance of a high velocity( jet) ¯ow at lower levels and the Earth's rotation(Coriolis effect) for suf®ciently strong liftingbecomes evident as well as the role of the Alpinearc for the concentration of precipitation in acomparatively small area.

Peristeri et al. (this volume) investigate thegenesis conditions for thunderstorms and squalllines at the Alpine north-side by taking basicparameters from the 1994-HERA-cases. Espe-cially in summer large-scale south-westerly ¯owtransports low-level moisture from the Mediter-ranean around the western Alps to southernGermany where mountain-induced wave effectstend to trigger convective instability. Frequentlyoccurring pre-frontal convergence-lines are shownto have a signi®cant impact on the strength ofthunderstorm forming convective cells.

Fantini and Davolio (this volume) report on anattempt towards a theoretical treatment of thecombined study of atmospheric frontal zones,which generate uplift and precipitation, andisolated orography.

4. Conclusions

On the term of HERA numerous innovative andvaluable conclusions can be drawn and these arerecorded below.

Several important technical achievementswere made concerning the areal rainfall determi-nation over complex terrain, as the setup oftransnational radar composites (meanwhile alsoavailable as real time application), a ®rstobjective quality assessment of operationalmesoscale precipitation forecasts, the develop-ment of extended Doppler radar retrieval algo-rithms, or the successful demonstration of highresolution forecasts including ice-phase pro-cesses. Computerized deliverables are available

to interested parties via the MAP Data Centre atETH, Z�urich.

Tangible answers were found to the questionswhich had been put forward at the outset of theproject. The size of strong precipitation eventsregularly exceeds 10,000 km2. At the northernside of the Alps a west-to-east propagation ismost frequent, while the Alpine shape has thepotential to lock precipitation systems on itssouthern side over a relatively small area forseveral days. Current NWP systems exhibitgrowing skill in simulating precipitation, but areliable ground truth is not generally available,especially for higher elevations. Nearly all strongrainfall events are a complicated interplay ofseveral atmospheric mechanisms. South-Alpineevents are regularly triggered by upper levelanomalies. The coming generation of NWPsystems will provide routine forecasts withmeshsizes of a few square-kilometres for, say,three-hourly periods.

Project HERA much pro®ted from being fullyintegrated in the ungoing Mesoscale AlpineProgramme (MAP) with its pre-established con-tacts between the partners of the consortium. Thesigni®cant, but still only partial (about 55%)funding by the CEC proved to be extremlyimportant to further strengthen these research linksand to enable notably younger scientists to becomeinvolved into a truely European co-operation.

Concerning interdisciplinarity good integra-tion was achieved between the diverse disciplinesin the atmospheric sciences, as radar engineering,high-end numerical modelling, micro-physics, orconventional data collection. But any follow-onproject in the ¯ood-forecasting domain has toinstall close links with the hydrological model-ling community. Furthermore, the emphasis is tobe shifted closer towards integrated applicationsfor speci®c cases (for a recent example in Canadasee Benoit et al., 2000).

As a natural outlook we envisage the collec-tion of atmospheric and hydrological data inunprecedented spatio-temporal resolution duringthe MAP±SOP in autumn 1999, partly by makinguse of techniques which were developed withinHERA. These then await their use in a convincingdemonstration of what integrated atmosphericand hydrological forecasts should be able toroutinely achieve in the not too distant future tothe bene®t of many of the well over 10 millions

Heavy Precipitation in the Alpine Region: Areal Rainfall Determination 83

inhabitants of the Alpine region; all the more, asintense precipitation events are likely to becomeeven more frequent (Frei et al., 1998; Karl andH�aeberli, 1998).

Acknowledgements

As the coordinator of project HERA it is the author'spleasure to explicitly acknowledge the dedicated efforts ofall partners of the consortium to achieve the commonresearch aims as they had been envisaged during theplanning process and to make available their results.Gerhard Berz, Angelika Wirtz (Munich Re, M�unchen)and Remo Cavalli (SMA, Locarno) kindly granted back-ground information. Ian Cluckie (Univ. of Bristol) gener-ously provided a preprint of the proceedings from a recentAdvanced Study Course on real time flood forecasting.Huw Davies (ETH Z�urich) patiently ameliorated a foreign-er's use of English. Winfried Beer, Martin Leutbecher, andAnja Trudel (DLR, Oberpfaffenhofen) gave valuabletechnical assistance for the layout of text and figures. Thisstudy was in parts supported by the European Commission,Programme ` Environment and Climate'' under contractENV4-CT96-0332.

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Author's address: Hans Volkert, DLR Oberpfaffenhofen,Institut f�ur Physik der Atmosph�are, D-82234 Wessling,Germany (e-mail: Hans.Volkert@dlr.de).

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