VOL. 11, NO.4 WEATHER AND FORECASTING DECEMBER 1996

The Vaison-La-Romaine Flash Flood: Mesoscale Analysis and Predictability Issues

STEÂ PHANE SEÂ NEÂ SI,PHILIPPE BOUGEAULT,JEAN-LUC CHEÁ ZE,PHILIPPE COSENTINO, AND ROSE-MAY THEPENIER Groupe de MeÂteÂorologie de Moyenne Echelle, MeÂteÂo-France, Centre National de Recherches MeÂteÂorologiques, Toulouse, France (Manuscript received 8 May 1995, in ®nal form 23 July 1996)

ABSTRACT On the morning and early afternoon of 22 September 1992, a ¯ash ¯ood (220 mm of rain in 3 h) occurred in the city of Vaison-La-Romaine, located in southeastern France, causing numerous casualties and considerable property damage. It was generated by a combination of several mesoscale convective systems ahead of a slow- moving cold front associated with a cutoff low. The large-scale setting and a mesoscale analysis of the case, together with estimates of radar-derived rain accumulations, are presented. The mesoscale analysis demonstrates the complexity of the case, which involved ®ve precipitation systems. Orographic in¯uences generated a cold pool and focused convective cell development in a con®ned area, enabling two precipitation systems to be quasi- stationary. Precipitation forecasts by different versions of two models (a ®ne mesh version of an operational limited area model, and an operational stretched global model) are summarized. They demonstrate the extent to which realistic rainfall forecasts may be produced for extreme meteorological events such as this. One model exhibits a rather precise and realistic distribution and evolution of the precipitation patterns, while all show signi®cant accumulations. Finally, some of the objective pieces of information useful for nowcasting rainfall location and duration for such an event are discussed.

1. Introduction a variety of data sources including pro®ler, aircraft, and Doppler radar data (e.g., McGinley et al. 1991; Brew- Forecasting severe convection and ¯ash ¯ooding ster et al. 1994; Beckman and Polston 1995). remains a dif®cult challenge to national weather ser- The availability of dense and frequent surface data vices and meteorological researchers. Maddox et al. in real time may also be insuf®cient. Even when com- (1979) and Chappell (1986) have studied quasi-sta- prehensive data are available, much effort and much tionary convective events that have led to severe and catastrophic ¯ooding. In analyzing 150 ¯ash ¯ood processing is required in order to use them effectively events over the United States, Maddox et al. (1979) (e.g., Moller et al. 1994). Numerical models are em- were able to de®ne three consistent and typical sce- ployed in only a limited number of experiments ori- narios involving lower- and upper-level patterns as- ented toward explicit real-time simulation of convec- sociated with excessive heavy rainfall. Chappell tion or convective organization types (e.g., Janish et al. (1986) explained the mechanisms enabling rain gen- 1995). In France, severe convection is rare (at least eration to remain stationary. before the recent succession of events), and conse- Forecasters at operational ®eld of®ces rarely bene®t quently few speci®c experiments or case studies have from the most advanced research concerning convec- addressed this issue. To contribute to the understanding tion. Moreover, in most weather services the amount of such extreme events in this region, researchers and of time forecasters can usually spend on a case is lim- forecasters must therefore rely on case studies that have ited, and the observations and numerical analysis and been documented using all the data available off-line. forecast tools available to forecasters today may be The present study evaluates a particular extreme largely insuf®cient for effectively employing recent re- event in a quasi-operational observing and modeling search results in forecast operations. Another consid- framework, and addresses some issues of predictability erable problem in many countries is the lack of high- in this context. For the investigation we employ surface resolution upper-air data for detailed situation analysis observations provided by automated weather stations, and model initialization. To solve this problem implies 5-min and 1-km resolution PPI radar data, and numer- operating assimilation systems capable of coping with ical modeling tools that are slightly more advanced than current operational tools (i.e., the same model but with increased resolution). During the late morning of 22 September 1992, sev- Corresponding author address: SteÂphane SeÂneÂsi, MeÂteÂo-France, eral mesoscale convective systems generated a ¯ash CNRM/GMME/PI, 42. Av. G. Coriolis, F-31057 Toulouse Cedex, France. ¯ood in the city of Vaison-La-Romaine, France, on the E-mail: [email protected] southern foothills of the Alps and not far from the Med- 417 ᭧ 1996 American Meteorological Society

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Unauthenticated | Downloaded 10/02/21 07:15 AM UTC 418 WEATHER AND FORECASTING VOLUME 11 iterranean coast (Fig. 1). These systems produced 300 mm (11.8 in.) of rain in 24 h (Fig. 2), with 220 mm (8.7 in.) occurring in 3 h near the city. Surrounding areas also recorded intense precipitation (224 mm in 3.5 h in Barnas, ArdeÁche, some 90 km away) and re- ported damage, but not at the magnitude that struck in and around Vaison-La-Romaine where 35 fatalities oc- curred, hundreds of houses were destroyed, and prop- erty damage was estimated at nearly one billion dollars. Benech et al. (1993) state that over a 30-yr period in France a rainfall accumulation of over 300 mm has occurred on only 15 occasions during the later half of the year, and the return period for the value measured in Vaison-La-Romaine (179 mm in 2 h) is estimated between 40 and 150 years, depending on various as- sumptions. As reported by Jacq (1994), 119 cases of rainfall exceeding 190 mm in 24 h have been recorded during the last 37 years in southeastern France. The Mediterranean climate is known for its high precipita- tion rates during autumn (Jacq 1994; Martyn 1992) when unstable synoptic conditions are reinforced by warm and moist advection due to southerly ¯ows driv- FIG. 2. Rain accumulations for 22 September 1992. Thick lines ing Mediterranean air inland. Nevertheless, good doc- represent the 24-h rain accumulation analysis (in mm) beginning at umentation of such dramatic events is rare. MeÂteÂo- 0600 UTC 22 September. The stars indicate the rain gauge network. France internal studies have documented few severe Thin lines are elevation contours at 300 and 900 m. convection cases (four events that caused considerable destruction during the 5-yr period ending in 1991). However, Pircher and Saix (1991) and Barret et al. (1994) do describe similar cases in the same vicinity as the ¯ood we analyze here. The 22 September 1992 event was well forecast by MeÂteÂo-France. (The hydrological watch, however, which is not performed by MeÂteÂo-France, is not dis- cussed here.) According to Benech et al. (1993), 18 h before the ¯ood a warning was issued at a severity level rarely used (no more than 10 times a year for the whole of France), and shortly before the onset of the heavy rainfall, another bulletin was issued forecasting rainfall amounts of 200 mm in 24 h, with rates up to 100 mm in 3 h in the lower RhoÃne Valley and Alps foothills. These authors point out that rainfalls of this magnitude are not by themselves hazardous in all locations. They cite that 450 mm of rainfall caused only minor damage some 100 km away on the day prior to this event. But this case has demonstrated, at least to meteorologists, that the hydrological con®guration of the small catch- ment and subcatchments of the OuveÁze River, which passes through Vaison-La-Romaine, can make this re- gion prone to ¯ash ¯ooding. One can also state that even if the meteorological forecast had been successful, it may not have been accurate enough for hydrological forecasting for this catchment. Ultimately, the quality of the meteorological forecast relied on the forecasters' expertise in interpreting model output (the operational limited area model produced a largely underestimated FIG. 1. Geography and topography of the event area. The contours correspond to 300-, 900-, and 2100-m levels. GL stands for Gulf of but somewhat consistent picture of the situation; see Lion. The sounding locations and main mountain ranges are indi- section 4) and on their knowledge of the synoptic en- cated. The box shows the domain of the mesoscale analysis. vironment associated with such events.

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FIG. 3.(a) MeÂteÂo-France's of®cial surface analysis for 1200 UTC 21 September. Frontal drawings and observation plots are conventional. Cold front symbols with open triangles signify a cold front aloft. (b) The analysis of mean sea level pressure (solid line at 2-hPa intervals), pseudo-wet-bulb potential temperatures (dashed line at 2ЊC intervals), and wind barbs for 0600 UTC 22 September.

In this paper we present the synoptic and mesoscale wind shifts became clearer. Figure 3b illustrates, at aspects of the 22 September 1992 event in sections 2 0600 UTC 22 September 1992, the sharp gradient in and 3, respectively. In section 4 we provide an over- the pseudo-wet-bulb potential temperature ®eld asso- view of the NWP forecast. Section 5 concludes with a ciated with the cold front along the eastern Spanish discussion of some nowcasting issues, namely, the rain- coast and extending into southern France. This gradient fall location and duration nowcast. zone was also clearly delimiting different wind orien- tations. The cold sector exhibited dewpoint depressions up to 15Њ around the Spanish Mediterranean coast. The 2. Synoptic setting cold front zone later showed a slow eastward move- a. Surface analysis ment, and it reached Italy late the following night, still with active convection ahead of it. MeÂteÂo-France's of®cial analysis for 1200 UTC 21 The other noticeable feature in Fig. 3b is the meso- September 1992 is represented in Fig. 3a, illustrating scale low centered over the French Mediterranean the mean sea level pressure ®eld. The Azores 1028-hPa coast. It was situated between a weak ridge associated high was moving east-southeastward, while a 1000-hPa with the cold front, in southwestern France, and a low, centered off the western coast of France, was mov- marked ridge in the eastern part of the area established ing eastward and becoming less organized and ®lling. by the Alps blocking the easterly ¯ow. This low drove Owing to the low number of ship observations, the a nearly steady southerly to southeasterly ¯ow into the frontal analysis was based largely on satellite image Mediterranean coast, with speeds reaching 20 m s 01 . interpretation for the eastern Atlantic. It shows a small- The ¯ow contained warm, nearly saturated air with a scale classic front drawn with a limited warm sector dewpoint depression less than 2Њ and often less than 1Њ and a secondary cold front related to a comma-cloud (with temperatures around 22ЊC). The long-range mar- pattern. Convective activity was already occurring near itime trajectory of this advection can be traced back to the Mediterranean coast of France, as represented in North Africa. The channeling effects of the Spanish Fig. 3a by a cold front aloft (i.e., open triangles), which Plateau, the Pyrenean and Alpine Ranges, and the Mas- caused soil saturation and played a role in the subse- sif Central helped generate a low-level jet in the Gulf quent ¯ooding. of Lion. We see here that one of the necessary ingre- Convection developed during the afternoon of 21 dients for deep convection, namely, strong moist ad- September ahead of the Atlantic cold front, which vection, was clearly present, with orographic factors reached the French southwestern coast around 1800 leading it to remain in the same location and to generate UTC. Convection became linearly organized by 0000 upward motion. (Figures 1 and 2 illustrate the instru- UTC, and surface temperature gradients and surface mental local features and topography at two scales.)

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1) EVOLUTION OF MASS AND TEMPERATURE FIELDS The 500-hPa analysis, as derived by the operational Peridot ®ne mesh analysis suite (Imbard et al. 1986; Durand and Bougeault 1987), is shown 9 h before the event began (Fig. 4a) and during the event (Fig. 4b). At 0000 UTC (Fig. 4a) a large-sized trough was reach- ing the French Atlantic coast, west of a well-marked meridional ridge over central Europe. It contained a cutoff low at 5490-gpm height. Small-scale cold ad- vection was showing in the southern part of the low at the 500-hPa (see label CA) and 300-hPa levels. A cloud pattern was associated with the frontal limit, with enhanced convective activity in its southern part. The 1200 UTC situation (Fig. 4b) showed a tilting of the trough (labeled TA) and a new thermal short wave (labeled CA) along with a well-de®ned meso- scale convective system developing just north of the French Mediterranean coast. This general upper-air pattern resembles the one described by Browning and Hill (1984) for the case of a mesoscale convective sys- tem, except for a more southeastern position of the low. It also matches one of the types de®ned by Maddox et al. (1979) and then further explained by Chappell (1986) for ¯ash ¯ood producing synoptic situations, namely, the synoptic type. At lower levels, strong warm and moist advection from North Africa and Spain was showing as a tongue shape following the 850-hPa equivalent potential tem- perature contours and matching the one depicted at the surface in Fig. 3b (and a south-southwest ¯ow not shown here).

2) JET STREAKS AND LIFT Figure 5 illustrates jet streaks and the vertical veloc- ity ®eld evolution after successive Peridot analyses. The wind speed maxima analysis was done indepen- dently of height. It shows, for 0000 UTC, 22 September (Fig. 5b), a jet down to 350 hPa on its southern part, with two jet streaks reaching 45 m s 01 and strong cur- vature. Keyser and Shapiro (1986) and Cammas and Ramond (1989) have discussed the in¯uence of cur- vature on the ageostrophic circulation around jet streaks, which modi®es the double convergence/diver- gence dipole at entrance and exit regions, depending on the location of the jet streak relative to the trough axis. The transverse circulation, due to acceleration/ deceleration, may have a varying effect on the longi- tudinal circulation (due to curvature). The case at hand shows at 0000 UTC some similarity to case 1 of Cam- mas and Ramond (1989), where the downstream jet FIG. 4. Upper-level ®elds at 0000 and 1200 UTC 22 September. streak lies between a trough and a ridge and is followed Thick lines represent 500-hPa surface geopotential contours at 40- by an upstream jet streak (albeit, the scale involved m intervals. Dashed lines are 500-hPa surface temperature con- tours at 1ЊC intervals. Areas with Meteosat infrared equivalent here is somewhat smaller). temperatures below 040ЊC are shaded. TA is the trough axis. CA The mountain ranges also played a large role in driv- represents cold advections. [Note: cross-section line for Fig. 7 is ing the vertical velocities. The sequence of panels in shown in (a)].

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FIG. 5. Jet streaks and vertical velocity ®elds. Thick lines represent troposphere-wide maximum winds (m s 01 ) thres- holded above 35 m s 01 at5ms01 intervals. Thin lines rep- resent vertical velocities at 400 hPa (cm s 01 )at5cms01 in- tervals (dashed is upward). The ¯ood location is marked with a cross (1). (a) 1200 UTC 21 September. (b) 0000 UTC 22 September. (c) 1200 UTC 22 September. Value at label S is 50.8 cm s 01 , and at label U 023.5 cm s 01 .

Fig. 5 shows the combined effect of these factors: at at 1200 UTC (Fig. 5c), the upward southern core ex- 0000 UTC, a dipole of vertical velocities appeared tended to the up¯ow slopes of the Massif Central and around the PyreÂneÂes, with strong subsidence on the Alps (above the ¯ood location), while the trough and northern side (see labels S and U), and upward velocity jet streak moved eastward. We see here that another showed along the Spanish Mediterranean coast. Later, necessary ingredient for deep convection is present,

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3) STABILITY The 0000 UTC sounding (Fig. 6a) at Palma de Ma- jorca, located upstream of the ¯ood and ahead of the frontal zone and associated convection, was represen- tative of the advected ¯ow. It indicated a ``loaded gun'' type sounding (Fawbush and Miller 1953), with a moist, shallow marine layer showing weak static sta- bility, capped by a strong temperature inversion and a dry neutral warm layer (extending from 900 to 630 hPa). This dry layer originated from the high terrain over North Africa and traveled through only subsiding or slightly ascending regions (cf. Fig. 5). The upper layer, from 630 hPa upward, was more moist and, from 480 hPa upward, stable for moist ascent. The overall convective potential for this sounding can be scaled by its low Showalter index of 04Њ and by the large amount of convective available potential energy (CAPE), which reached 2662 J kg 01 for a parcel rising from the surface. Vertical wind shear was low (7 m s 01 ), giving a bulk Richardson number of 148 (Weisman and Klemp 1986), favorable for multicellular convection. The precipitable water in the lowest 150-hPa layer amounted to 16.7 mm (0.66 in.) and to 30.2 mm (1.2 in.) from the surface to 500 hPa. By 1200 UTC, the CAPE value had decreased to 1626 J kg 01 (still for a parcel rising from the surface). Figure 6b shows the 0000 UTC sounding at Nimes where an elevated capping layer (from 750 to 600 hPa) was present. A virtual potential temperature analysis shows that the layer extending from 700 to 573 hPa was dry adiabatic. A very dry layer extended from 570 hPa upward. The CAPE reached 383 J kg 01 . In order to put this value in perspective, because it is low with respect to values encountered in the midwest of the United States during heavy convective activity, a cli- matology of CAPE values was computed for a 28-yr period for September morning soundings in Nimes. For 706 cases, the mean value is 96 J kg 01 , and this case ranks in the upper 8% of the distribution. We computed the CAPE using the Nimes midnight sounding, but us- ing for the rising parcel surface characteristics from a FIG. 6. Relevant environmental soundings at two locations: Palma de Majorca (a) and Nimes (b) for 0000 UTC 22 September. (See ground station located near the coast and at a time when Fig. 1 for locations.) Inserts are hodographs with axes labeled in diurnal heating has already begun but before convec- ms01 and heights in thousands of m. tive systems have swept the coast. We therefore chose the Montpellier station at 0700 UTC. This gave the much more signi®cant CAPE value of 2452 J kg 01 ,as well as a large bulk Richardson number. The low-level The vertical structure at 0000 UTC along a cross (150 hPa) precipitable water content of 12.4 mm ranks section through the frontal limit, as derived from the in the upper third of a 12-yr September climatology, Peridot analysis (Fig. 7, see cross section line on Fig. while the content for the surface to 500-hPa layer is 4a) demonstrated the following: signi®cantly high at 30.6 mm (i.e., in the upper 12%). The wind hodograph showed a clockwise turning of i) the cold air mass was partly hanging over the the wind with height, which is favorable for organized moist and warm advection along the Spanish coast (la- convection. beled SpC) and slightly west of it, at the 600-hPa level;

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line, panels 0830±1100 UTC) that weakened rapidly while moving northeastward. At 0930 UTC, a warm spot was appearing (labeled C). At 1030 UTC, a new very cold cell formed in the vicinity of the ¯ood loca- tion, with a ring shape (labeled D), and almost instan- taneously expanded into a V-shaped large cell pointing toward the sea (see the 1100 UTC panel, label E1, and 1230 UTC panel, label F). This seems to match both the ``large-scale wedge'' type of Sco®eld's (1985) classi®cation for heavy rain producing systems and the enhanced-V pattern, with more than one V in the cloud shield, as also described by McCann (1983). This V-shaped cell remained nearly stationary for 3 h (from 1100 to 1400 UTC), and then it slowly shifted eastward in the same orientation while deforming slightly (see panels 1500 and 1730 UTC), marking the end of the ¯ooding period. At 1730 UTC, the main cell, while not matching the enhanced-V pattern, showed an interesting couplet of cold and warm pockets (labeled G1). At 2130 UTC, another cell development can be seen northwest of Palma de Majorca (labeled G2), staying off the coast while maturing, and progressively FIG. 7. Cross section along the line in Fig. 4a across the cold front. capturing the warm, moist in¯ow of the main cell. This Thick lines represent potential temperatures (in K at 4 K intervals). Thin lines are equivalent potential temperatures (in K at 2.5 K inter- new cell also exhibited some features of the enhanced- vals). Dashed lines indicate humidity above 60% (at 15% intervals). V pattern (labeled H2), features that are clearer in the SpC signi®es Spanish Mediterranean coast. old cell (labeled H1). Finally, both cells merged around 2300 UTC. One can incidentally note that none of the cloud pat- ii) conditional instability was occurring over the en- terns matched the set of criteria that de®nes Mesoscale tire Mediterranean portion of the cross section; and, Convective Complexes (Maddox 1980). iii) this instability was mitigated around the Spanish coast by the already developed convection. 3. Mesoscale c. Cloud patterns At the mesoscale, the interesting topographical fea- tures (Fig. 9) are the three mountain ranges, with the At the smaller end of the synoptic scale, enhanced Alps and the Massif Central forming a funnel toward Meteosat infrared imagery shows a remarkable devel- the Mediterranean Sea. The ¯ood location (labeled opment (Fig. 8), despite a lack of some images at cru- Vaison) stands on the eastern side of this funnel, in the cial times (1130, 1200, 1300 UTC). Beginning as a Alps foothills. A small range of hills (labeled Alpilles weakly organized and meridionally oriented zone of on the insert), which lies in the southern part of the convection located on the eastern part of the PyreÂneÂes funnel, will be referred to later in the discussion. at 0130 UTC, two mesoscale convective systems (MCS) formed over the Gulf of Lion and the southern side of the Massif Central (see panel 0530 UTC). The a. Sketch of the precipitating systems northern system already exhibited the enhanced-V pat- Figure 10 summarizes the ®ve different precipitation tern (labeled A) described by McCann (1983), which systems involved in the ¯ood, which we are going to is signi®cantly correlated with severe events. McCann describe, as derived from radar data: explained that this pattern incorporates a relatively warm pocket for which three possible generation mech- i) a system with an overall linear shape (labeled QL anisms can be invoked. This enhanced-V pattern ap- in panel 0730 UTC) that remained almost stationary peared at different times during the day, in a more or along the steep foothills of the Massif Central; less clear way, as it sometimes only formed into a small ii) a fast moving line of intense cells, which and somewhat circular shaped warm spot. At 0700 will be shown to be a squall line (labeled SL, panel UTC, the V pattern was not very sharp and somewhat 0730 UTC); asymmetrical (labeled B). iii) a very fast moving cell, showing some features Image loops show that the two MCS cloud shields of a supercell (labeled SC, panel 1015 UTC); merged at 0830 UTC and then formed a northwest± iv) a regenerative stationary convective line (la- southeast-oriented line of cold tops (dotted±dashed beled SCL, panel 1115 UTC); and

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FIG. 8. Sequence of Meteosat infrared images for 22 September. The area is slightly larger than the thick frame in Fig. 1. Observation times are indicated in upper-left corners. See text for labels.

v) a very active N±S-oriented frontal convective Fig. 8, while system FL generated the narrow V-pat- line (labeled FL, panel 1215 UTC). tern labeled F. The remainder of this section explains how the var- Relating these systems to satellite imagery, ious precipitation systems interacted and led to the through loops of superimposed satellite and radar ¯ooding. One can summarize it this way: the heavy data, shows that i) systems QL and SL are the two precipitation at the ¯ood location (see label in Fig. 10) whose cloud shields merged at 0830 UTC, ii) system was due basically to the stationary character of the re- SC generated the ring-shaped cold zone depicted in generative convective line (SCL) and the slow move- Fig. 8 (labeled D), and iii) system SCL generated ment of the frontal line (FL) near Vaison-La-Romaine. the complex V-shaped pattern labeled E1 and E2 in The ¯ood location was determined by the out¯ows of

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Unauthenticated | Downloaded 10/02/21 07:15 AM UTC DECEMBER 1996 S EÂ NEÂ SI ET AL. 425 the earlier systems QL and SL, in conjunction with oro- graphic factors. Our basic data are i) conventional ground station observations (the network is sketched in Fig. 9); ii) radar data from a conventional, 10.7-cm wavelength, pseudo-CAPPI radar with a 5-min time resolution [it is located in NimesÐFig. 9Ðand belongs to the French Aramis network (Gilet 1984; CheÁze 1990)]; and iii) cloud-to-ground lightning data from a LLP-like (lightning location and protection; Krider et al. 1976, MacGorman and Taylor 1989) network covering con- tinental France (Tourte et al. 1988). Objective analyses of surface parameters were performed using a cubic spline analysis package that provides an objective eval- uation of the standard observation error (Craven and Wahba 1979). b. Mesoscale analysis

1) EVOLUTION OF SYSTEM QL System QL (Fig. 11, panels 0600 and 0800 UTC) was roughly linear and southwest±northeast-ori- ented along the Massif Central foothills. It was active from 0100 UTC, on the southwest end of the Massif Central foothills, up to 0900 UTC (see some rem- nants on the corresponding panel in Fig. 11) near the RhoÃne Valley, where it touched with the northern part of system SL (described below). It was rather poorly documented in the morning by the then failing Nimes radar, and the composite images from the two closest radars showed a discrete progression by cell regeneration to the northwest. Some of these cells were strong (as far as it can be deduced from this long-range radar data) and stationary at midslope. The cold out¯ow of this system, located on the north- ern side of the northernmost cell, ®rst exhibited a signature in the potential temperature ®eld at 0400 UTC, then very clear at 0600 UTC (see label CP on corresponding panel), and up until merging with sys- tem SL. It also displayed re¯ectivities reaching 52 dBZ (65 mm h01)1 and at some points 56 dBZ (115 mm h 01 )1 at 0730 UTC. A few surface stations provided in situ observations of this system, con®rming the high peak precipitation rates: 35 mm in 30 min at 0600 UTC in La Grande Combe (see locations in Fig. 9), more than 30 mm during four 30-min periods in Barnas (totaling 224 mm FIG. 9. Mesoscale topography, toponymy, and observation station net- from 0530 to 0900 UTC), and more than 50 mm in 30 work locations. Contours represent elevation (in m at 300-m intervals). min (ending at 0800 UTC) in L'Estables. Wind gusts Small crosses are automated station locations. Squares are stations with 01 pressure data (with varying temporal resolution). Large crosses indicate around the main cells reached 25 m s . The maximum the geographical locations referred to in the text and are represented in rain accumulation generated by system QL was 273 subsequent ®gures, together with the hatched rectangle surrounding Vai- son-La-Romaine and two of the elevation contours (300 and 900 m).

1 All the rainfall intensities in this paragraph are computed with a raw Marshall±Palmer conversion: Z Å 200 R 1.6 after calibrating the mm in Montpezat-sous-Bauzon. This system presents re¯ectivities. The details of the calibration are beyond the scope of some similarities with the Big Thompson ¯ash ¯ood this paper. (Caracena et al. 1979) in that it was generated by a

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180 mm h 01 ), and its longitudinal extension was 200 km, while its transverse extension was about 60 km. Its dissipating stage occurred around 1015 UTC, after which the system reached the 1600-m level of the Alps foothills (see some remnants in Fig. 11, panel 1000 UTC, northeast corner). This was a region where the potential for convective feeding was obviously much reduced, both at low levels (the marine advection did not reach this region) and at upper levels (the system being too far east of the cold advection). The stratiform part of the rain then widened and weakened. The pres- sure rise associated with this squall line can be seen clearly in Fig. 11 (panel 0900 UTC, labeled MH) and as a reinforcement of the surrounding ridges at 1000 UTC (one labeled Ridge in the north, the other unla- beled in the east). The surface signature for this squall line was remarkable for this part of western Europe (Fig. 12). The pressure rise at Carpentras (labeled ``Carp.'' in Fig. 9, south of Vaison) was 5.5 hPa in 18 min, with a maximum rate of 2.4 hPa in 6 min. The potential temperature decreased by 4.6ЊC (from 21.4Њ to 16.8ЊC) in 6 min during the heaviest precipitation (16.4 mm during this 6-min interval) and just follow- ing a dewpoint rise of 1.5Њ. The most striking feature is certainly the cold pool associated with the line. It is clearly visible near the coast in Fig. 11 at 0800 UTC (labeled CP). It was subsequently reinforced at 0900 UTC (also labeled), probably through a merging with system QL's out¯ow. It then extended some 80 km to the rear of the line. FIG. 10. Nimes radar images showing the main precipitating systems. See text for system label names and Fig. 9 for geography. This led to the building of a cold wedge in the RhoÃne Valley funnel (panel 1000 UTC, labeled CW). The cold wedge was also shaped by the radiational heating that occurred on the western part of the coast, as soon strong and moist low-level ¯ow impinging on the Mas- as the stratiform shield had left the zone (panel 1000 sif Central foothills, in the presence of instability and UTC, labeled RH). This warming somewhat hid the capping, and with cell redevelopment in the opposite advance of the cold front in the temperature ®eld by direction of the cell movement, leading to stationary creating intricate potential temperature contours; wind rain generation along the ridge. These are also the main shifts depicted the passage of the cold front better, but characteristics of the Rapid City ¯ash ¯ood, as reported only in the ¯at terrain near the coast. by Maddox et al. (1978), except for the location of the Because the cold pool was so instrumental in the cold front and the in¯uence of afternoon heating. generation and persistence of the ensuing systems, it is important to describe its characteristics in more 2) EVOLUTION OF THE SQUALL LINE (SL) detail. The largest temperature drop was 9.2ЊC en- countered in Vinsorbes, but 4ЊC temperature drops System SL started as a circular system (according to were commonly measured. The thickness of the cold satellite images and some long-range radar data) pool can be estimated by using surface-measured around 0400 UTC in the extreme northeast of Spain, characteristics during the line passage and under the and it acquired a linear shape around 0700 UTC. In its hydrostatic assumption by DP Å g 1 H 1 Dr, where later stage, system SL showed most characteristics of DP is the pressure rise, H is the thickness, and Dr is a squall line, the most noticeable being its mode of the density rise. This formula also assumes a very propagation and its cold rear sector. Propagation was simple conceptual model of the squall line with two almost continuous, at the time and space resolution of sectors showing uniform densities. When applied to the available radar data, except for a few discrete new station measurements collected at low elevations and cell developments along the gust front. The system ¯at areas in the squall line path, the formula estimates reached a speed of 20 m s 01 while heading toward the varying thickness values during its mature stage: northeast and remained in an active state until 0930 from 1600 m in the center of the line (e.g., in Car- UTC. Its re¯ectivity reached 59 dBZ (approximately pentras and Nimes) to approximately 450 m at both

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FIG. 11. Mesoscale ®eld analysis. Thick lines are contours of the analyzed sea level pressure ®eld (in hPa at 1-hPa intervals). Dashed lines represent surface potential temperature contours (in ЊCat2Њintervals). Barbs are conventional. Backgrounds are composite images from three radars (but only two remote ones for panels 06 and 08). Upper-left corner values for each panel are in h (UTC). See text for system labels and Fig. 9 for geographical information.

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FIG. 11. (Continued)

ends (e.g., Salon and MonteÂlimar). An alternate es- 3) EVOLUTION OF SYSTEM SC timate of the cold pool depth can be derived from Around 0930 UTC, two convective cells appeared in elevations of close observation stations: the temper- the vicinity of the Alpilles hills, some 40 km inland ature drop varied from 0.5ЊC in Sault at 670 m to (Fig. 13, ®rst panel, labeled SC and SCL). The north- 3.1ЊC in Savoillan at 520 m (see Fig. 9 for locations), ern cell rapidly strengthened in re¯ectivity, reaching 56 so the cold pool showed in Savoillan and not in Sault. dBZ by 0950 UTC. At 1005 UTC, it split into two cells These stations are separated by 12 km and are on (Fig. 13, panel 1010, labeled SC and SCS); the north- opposite sides of Mont Ventoux; consequently, their ern one (labeled SC) traveled at a sustained speed of respective elevations could frame the actual cold 29.5 m s 01 (covering 212 km between 0930 and 1130 pool height, even if the orography largely in¯uenced UTC) on the east ¯ank of the RhoÃne River. It reached the pool's shape. a re¯ectivity of 62 dBZ near Visan (panel 1020 UTC), The surface precipitation associated with the squall where a rain gauge registered more than 50 mm of rain line varied in both time and space. Its lowest peak rate 2 01 01 in 6 min. Along its path wind gusts reached 25 m s was 36 mm h (measured in 6 min) with an accu- and at one point it destroyed a stadium roof (panel 0950 mulation of 7.6 mm in 24 min near Nimes around 0830 UTC, labeled St). UTC when the southern end of the line left the coast. The peak rate increased to 164 mm h 01 with an accu- mulation of 41 mm in 24 min in Carpentras around 0915 UTC when the line reached the foothills of Mont 2 The archive was not designed to handle accumulations larger than this 50 mm in 6-min thresholds, and this threshold value is therefore Ventoux. There was also a noticeable decrease in the also used to represent missing data and other measurement problems. rates and accumulations when going from the middle The time series nevertheless leads us to trust the effectiveness of this of the line toward both of its ends. value.

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to further prove this classi®cation. The most striking feature of system SC is its movement, both in speed and direction. The system's anomalous direction (with respect to systems SCS and SCL, described below) can be explained by its left split character. We can only explain its anomalously high speed by citing the remark of Weisman and Klemp (1986), namely, that once formed, a supercell propagates somewhat indepen- dently of the out¯ows, with its own dynamics. The reason why system SC, the left split, was fa- vored with respect to its right-mover split SCS is not clear. Klemp and Whilemson (1978) numerically dem- onstrated that left movers are favored by counterclock- wise curvature of the hodograph in the lower levels, and Houze et al. (1993) also found a slight but signif- icant counterclockwise turning layer in the mean ho- dograph associated with their left-mover cases. In the present case, the only available observation is the FIG. 12. Surface parameters at Carpentras. Theta stands for the Nimes sounding at 0000 UTC (Fig. 6), whose hodo- potential temperature, and theta'w for the pseudo-wet-bulb potential graph was clearly turning clockwise. Nevertheless, the temperature. local modi®cation due to the squall line passage and the ®nescale orography may have had a large impact on modifying this observed hodograph. System SC showed aspects of a supercell (e.g., Weisman and Klemp 1986), at least those accessible 4) EVOLUTION OF THE STATIONARY CONVECTIVE with conventional radar data, namely i) its signi®cant LINE (SCL) duration (see above); ii) its single updraft (see Fig. 13, Systems SCL and FL (described below) were the this was also con®rmed through visual inspection of two heaviest rain- and ¯ood-generating systems of color-coded images); iii) its continuous propagation, this case. The evolution of system SCL is depicted con®rmed by image loops; iv) its steady character, at in Fig. 13. It generated as an initial cell a bit south least between 1020 and 1110 UTC; and v) some hints of SC (panel 0940 UTC) but then followed a slightly of the mesolow associated with the vortex: a pressure more eastward track. It was then fed by a series of drop on the order of 2 hPa occurred in Carpentras, Or- powerful cells that were generating roughly over the ange, and MonteÂlimar (labeled in appropriate panels of Alpilles hill (see label on panel 1035 UTC), which Fig. 13) just before the passage of system SC; the max- faced the maritime moist advection (reaching then imum rate was 1.4 hPa in 6 min at Carpentras at 1006 17 m s 01 with a 16 g kg 01 mixing ratio). There were UTC, and a cooling on the order of 1.3ЊC was observed two or three main cells at any given time, all follow- afterward. ing the north-northeast orientation of the line. With Some features of system SC, however, are inconsis- the cell generation location being ®xed, this was a tent with the supercell archetype. Chisholm and Renick favorable con®guration for establishing this station- (1972) described a typical re¯ectivity pattern (repro- ary multicellular system and generating heavy local duced by Houze and Hobbs 1982) showing high values rain with continued cell passages. and high gradients in the rear of a supercell. In the case The northern end of the line was located around Vai- at hand, the re¯ectivity pattern at 1030 UTC (see Fig. son, in the foothills of Mont Ventoux, where the cells 13), which overall displays the best match with the remained for a signi®cant duration, intensifying up to typical pattern, does not show (including using color 59 dBZ (177 mm h 01 ). The surface precipitation as- images) the highest re¯ectivities in the rear, but rather sociated with system SCL was actually heavy in these in the front, and the actual pattern is mirrored with re- surroundings: more than 50 mm in 6 min (please note spect to the direction of movement. the remark made earlier for Visan when interpreting Two other facts lead us to believe that system SC this value) and 100 mm in1hinVaison, and 66 mm belonged instead to the ``intermediary left-moving in 30 min in Carpentras. The line organization lasted storm'' type described by Houze et al. (1993) as oc- from 1010 to 1140 UTC; the last stage of system SCL curring under some given conditions. These facts are was actually de®ned by its merger with system FL (see i) the marked advance of the pressure drop ahead of panels 1140 and 1200 UTC in Fig. 13). Finally, one system SC and ii) the respective directions of move- can note that system SCL did reinforce the cold wedge ment of systems SC and SCS after the split, which al- in the RhoÃne Valley, creating again a cold pool with a lows to qualify SC as left moving. We obviously lack potential temperature below 18ЊC (see Fig. 11, panel 3D observations and do not possess enough radar scans 1100 UTC, label CP).

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FIG. 13. Small-scale radar images. Same geographical information as in Fig. 9. See text for system labels.

5) EVOLUTION OF THE FRONTAL LINE (FL) of the foothills and the progression of the western cold sector with its overhanging low equivalent po- The frontal convective line produced the largest tential temperatures. It began to organize into a mul- rain accumulations. It appeared as a broken line at ticellular line around 1000 UTC when the wind shift two principal locations (labeled FL in Fig. 13, panel line associated with the western cold sector passed 1120 UTC). The northern segment was as an inten- the ridge line of the Massif Central during its east- si®cation of a set of scattered and moderately in- ward progression. tense cells that developed following the passage of Figure 13 also depicts the generation of the south- the squall line around 0930 UTC, on the eastern ern segment of system FL (panel 1120 UTC, labeled foothills of the Massif Central. The formation mech- FL, down arrow). A shallow convective zone was anism of these initial cells seems quite straightfor- also showing across the Gulf of Lion, clearly mark- ward: an interaction of the southeasterly maritime ing the limit of the cold air (refer back to Fig. 10, advection with the barrier formed by the topography panels 1115 and 1215 UTC). The line of intense con-

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Unauthenticated | Downloaded 10/02/21 07:15 AM UTC DECEMBER 1996 S EÂ NEÂ SI ET AL. 431 vection continued to organize and strengthen while c. Rainfall slowly moving eastward and then ®nally reaching the stationary convective line (SCL) around 1140 UTC 1) RAIN ACTIVITY EVOLUTION OF SYSTEMS SL, SCL, (Fig. 13, panel 1140 UTC). This merger intensi®ed AND FL the precipitation. The peak re¯ectivity then reached In order to describe quantitatively the evolution of 59 dBZ. Some pixels were noted at 64 dBZ but they systems SC, SCL, and FL, we computed the temporal were collocated with (normally much less intense) sequences of integration of i) the electrical activity ground clutter, making these values somewhat ques- measured as a rate of ¯ashes (estimated at 5-min in- tionable. From 1200 to 1400 UTC, the eastward pro- tervals) and ii) the rain production estimated from ra- gression of the line was slow (5 m s 01 ), allowing for dar data. Each integration was computed over the area the large rain accumulation described below, espe- of each system (the areas were hand delineated using cially around Vaison, which was located under the radar data and frequently enough to adequately trace even more stationary northern part of the line (see the movement of the systems). panels 1140±1230 UTC). We used radar data (despite the known problems The evolution of the line was a kind of discrete arising from using such data to estimate rainfall quan- upwind regenerative mechanism, with individual titatively) because of its good spatial and temporal cov- cells propagating northward with a speed of 16 erage, which largely outperformed the rain gauge net- ms01. By 1300 UTC, the frontal line tilted clock- work for this case (the network sketched in Fig. 2 pro- wise, this being apparently linked with the cold sec- vides only daily accumulations). We performed only a tor progression on the coast. A mesoscale high raw adjustment to the rain observations at the surface, (1015 hPa) formed on its northern end (Fig. 11, and we extrapolated from the rain cell movement to panel 1300 UTC, labeled CP / MH) through a com- estimate the rain rates above the ground clutter around bination of the large-scale cold air advection with Mont Ventoux (more sophisticated estimates have also an accumulation of cold downdrafts that reinforced been computed and are described below). the existing cold pool. This mesohigh closely re- Figure 14 summarizes the change in the activity of sembles the type of ``mesohigh event'' for ¯ash the systems for these two quantities. The thick solid ¯oods as described by Maddox et al. (1979). The line shows total rain production, integrated over the potential temperature dropped down to 15ЊC in the radar image domain, while the two thick-dashed lines cold pocket on the west side of this mesohigh. The represent integrations on a system by system basis for marine advection veered from southeast to south systems SL and SCL. The interval or difference be- just ahead of the line while weakening, and the tween the upper and lower curves represents the early southern portion of the line slowly lost its activity rain production activity for systems QL and FL; how- while accelerating its eastward movement. By 1530 ever, it includes a slight underestimation up until 1030 UTC, the southern part of the line disappeared, as UTC due to rain over discarded ground clutter zones the northeastern part was joined by a new develop- on the Massif Central foothills. After 1030 UTC it es- ment originating from the coast, very similar to the timates the activity of system FL correctly. The curves former one. This new system, although not de- in Fig. 14 show the following: scribed here, caused extensive damage around the Gulf of Genoa after being reinforced by a new i) the increased activity of the squall line (SL) when southern cell (refer to section 2). it reached the very ®rst foothills of the Alps (at 0830 The surface signature of this frontal convection UTC) and its quick decay thereafter; where maxi- line was marked by a very large temperature con- mum rain production rate sampled over its area (in- trast: the largest drop occurred in Sault, from 17.5Њ cluding the stratiform part) reached 32 1 10 6 kg s 01 to 9.3ЊC within 30 min, and 6ЊC drops were com- at 0730 UTC; monly encountered. Rainfall accumulations and rain ii) a quick startup of system SCL, marked at 1000 rates between 1150 and 1500 UTC were also note- UTC; where its total rain production amounted to 1 worthy. The highest rainfall rate was more than 50 1 10 11 kg between 0930 and 1130 UTC; mm in 6 min in Saint-Saturnin (same remark as for iii) an unchanged trend in the total rain activity at Visan and Vaison), and nine stations in a 50 km 1130 UTC when system SCL was joined by system FL 1 50 km square recorded peak rates higher than 100 and then disappeared; and 01 mm h . In this area, the accumulations were larger iv) an overall maximum rainout of 54 1 10 6 kg s 01 than 50 mm with a maximum of 123 mm in Saint- at 1300 UTC [which represents, for instance, six times Saturnin. the Big Thompson storm maximum rate; see Caracena The signature of the cold front is also clearly seen at et al. (1979)], and a total rain production by system 1300 UTC (Fig. 11) in the wind shifts that occurred FL of 6.3 1 10 11 kg from 0940 to 1430 UTC. all along the western part of the coast. The warm pocket due to radiational heating still remained in this region The strong correlation between the rain and electrical around Nimes. activity of convective systems has been documented

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FIG. 14. Evolution of rain and electrical activity for the systems (spatial integrations). Solid lines are volumetric rain rates. Lines with symbols are ¯ash rates. Upper curves are integrations over the area in Fig. 13. Lower curves are integrations over evolving zones surrounding the re- spective systems (SL and SCL). The difference between the curves represents the activity of systems QL and FL (before and after 0930 UTC, respectively;). over many years (e.g., Battan 1965; Kinzer 1974; Piep- instances: when catching up to system SCL, and around grass et al. 1982). For the case here, this correlation is 1200 and 1230 UTC. demonstrated clearly in Fig. 14 for all the systems. For The ratio of electrical activity to rain production systems SCL and FL, the curves superimpose closely seems larger for the mature stage of system SL, whose except for small-scale variations for system FL and main physical mechanisms clearly differ from those of some enhanced electrical activity for system FL at three the stationary systems FL and SCL, and matches the value reported by Battan (1965), namely, 3 1 10 4 m3 per cloud-to-ground ¯ash (please note that no radar rain data show between 0730 and 0815 UTC, and that the correlation cannot be estimated over this period). Higher values of volumetric rain per ¯ash occur for the entire lifetime of system SCL and for the growing phase of system FL. These values are actually higher than those reported by the authors cited above, but the reason for this is unclear.

2) HIGH-RESOLUTION RADAR ESTIMATES We found it useful to employ radar data to estimate the temporal evolution of rainfall in the catchment of the OuveÁze River upstream of Vaison-La-Romaine be- cause the results could be used to simulate the hydrol- ogy for this case and also be compared with other data sources. We performed a careful calibration of the radar data (whose detailed description is beyond the scope of this paper). The calibration, nonetheless, incorpo- rates a convective rain rate to re¯ectivity law [Z Å 300 R 1.35 , after Sekhon and Srivastava (1971)] and, follow- ing Delrieu's proposal (Delrieu et al. 1991), an atten- FIG. 15. Rain rate and accumulated rainfall amounts averaged over the OuveÁze River catchment upstream of Vaison-La-Romaine as de- uation for the radar signal by rain (as detailed in CheÁze duced from radar data. Accumulation starts at 0900 UTC. (Note: 1994), which proved useful in this case, even for a 10- there is a factor of 2 scale difference between the two curves.) cm wavelength radar.

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A spatial integration of the high-frequency calibrated the jet intensity was slightly underestimated. The fore- data was computed for the 591 km2 catchment (Fig. cast 10-m-height wind shift line associated with the 15). This ®gure shows the two distinct contributions cold front in the south of France was located within from systems SCL and FL around 1130 and 1300 UTC, two grid points of the analyzed location. respectively. One also notices the squall line contri- The 24-h range forecast (valid for 0000 UTC 23 bution in the ®rst half-hour of the period (with half the September) exhibited larger errors. Although it posi- peak rate of the main systems), which de®nitely helped tioned the slightly weak 500-hPa low correctly, it over- saturate the soil and amplify the ¯ood. For systems estimated the 500-hPa temperature ®eld meridional SCL and FL, the catchment mean reached about 45 temperature gradient values and hence limited the east- mm h 01 . Employing this value in an overly simpli®ed ward movement of cold air in northern France. The jet hydrological model, with no catchment delay and a streak was largely underestimated (by 10 m s 01 in 100% precipitation ef®ciency, would generate a ¯ow southeast France) and its easterly shift was slightly un- exceeding 4000 m3 s 01 in Vaison. A hydrological and derestimated. The surface low was also shifted 120 km hydraulic study (Chastan et al. 1993), which examined to the southwest with respect to the analysis. the ¯ow with entirely different methodologies and data- sets, estimated the actual peak ¯ow at around 1500 (ii) Rainfall location m3 s 01 . This value is consistent with the one (4000 m3 s 01 ) computed without taking hydrological and hy- The 3-h rainfall accumulation forecast ®elds (not draulic aspects into account. Our computations also shown) displayed convective activity that differed from show that the accumulation on the catchment over the the actual ®elds in two ways. entire period reached the equivalent of a 76-mm ho- i) Up to 1200 UTC, the simulation showed rather mogeneous rainfall (Fig. 15). strong convective activity in the southern part of the Mediterranean, south and southeast of Majorca, at 4. Simulations places where Meteosat data showed only scattered and warm clouds. Meanwhile, a moderate convective core a. The operational forecast was simulated developing in the Gulf of Lion and 1) MODEL CHARACTERISTICS shifted to the north, progressively covering the lower RhoÃne Valley. This latter system was, considering the In September 1992 the operational suite at MeÂteÂo- model's 35-km grid size, quite a good simulation of the France was based on a global analysis and forecast combined QL and FL systems, with a maximum ac- model and the Peridot limited area model (Imbard et cumulation of 16 mm from 0900 to 1200 UTC. al. 1986; Ducrocq and Bougeault 1995). The main ii) After 1200 UTC, the model failed to simulate the characteristics of the operational model at the time were eastward movement of system FL. The precipitation i) the use of primitive equations on 15 sigma levels; pattern remained stationary on the west ¯ank of the ii) a Kuo precipitation scheme (Geleyn 1985); RhoÃne River, with a strengthening at ®rst (a 23-mm iii) a dedicated mesoscale analysis scheme, allow- accumulation from 1200 to 1500 UTC) and then a slow ing Advanced Very High Resolution Radiometer and unrealistic shift toward the north. (AVHRR) 3 radiance analysis; and iv) a 35-km grid size. (iii) Rainfall accumulations Despite the relatively poor quality of the operational 2) THE FORECAST forecast (as far as detailed behavior of the frontal zone (i) Mesoscale context forecast is concerned), the model did capture a signi®cant event, and therefore one may wish to assess the quality The 12-h operational forecast (not shown), when of the rainfall accumulations from various perspectives. compared to the 1200 UTC 22 September operational The 24-h accumulation ®eld is compared to observa- analysis (Fig. 4b), positioned the center of the 500-hPa tions in section 4c below, but here we consider the op- low some 350 km to the north-northwest (the low be- erations perspective in which the forecasters have a ing, however, correctly predicted). Consequently, more or less thorough knowledge of the model's draw- there was an underestimation of up to 8 m s 01 in the backs and biases. It is of interest to compare the max- 500-hPa winds around the lower RhoÃne Valley. The imum rainfall accumulations, not to observations, but overall shape of the temperature low was well forecast, to the model climatology. but the forecast showed an overly warm cold air zone We computed a climatology of the model's 24-h (upto1.9ЊC too warm) located west of 0Њ longitude rainfall accumulation over a 450 km 1 450 km area and in Spain. On the other hand, the warming due to centered on Vaison. The climatology was based on 350 convection over southern France was underestimated morning runs of the model from June to October for 2 by up to 1ЊC. The jet streak location was correctly fore- years and part of a third year. The maximum 24-h rain cast, albeit slightly too far west in its southern part, and accumulation forecast for this case (92.2 mm) is in the

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Unauthenticated | Downloaded 10/02/21 07:15 AM UTC 434 WEATHER AND FORECASTING VOLUME 11 very tail of the distribution. Only three cases out of ues of this coef®cient, within the accepted limits, with- these 350 exhibited larger accumulations. Repeating out much change in the results. The simulation reported the same analysis on the part of this total rainfall that below had this coef®cient set to 0.3. had been generated only by the convection parameter- ization scheme shows a less exceptional case (8 situ- 3) RAINFALL FORECASTS ations, out of 350, did encounter larger accumulations). These ®gures show that both the convection parame- This simulation shows a rather realistic representa- terization scheme and the resolved precipitation were tion of rainfall concerning the morphology and chro- particularly active in this case. nology (Fig. 16). It was able to catch the behavior early on of system QL (albeit with a slight southerly shift; b. Fine-mesh simulation Fig. 16, panel 0530 UTC) and of system FL up to 1400 UTC (panels 1130 and 1430 UTC). It failed, however, 1) REANALYSIS to correctly simulate system SL. This may be traced to the nonprediction of the southern convective core that In order to produce the best possible initial state, the generated it (see panel 0530 UTC), but some features case was reanalyzed, starting at 0000 UTC 21 Septem- of this squall line's dynamic may have been captured. ber and going until 1200 UTC 22 September with 12- One notices a core of convective activity matching its h time steps. The observations used included all the southeastern end (panel 0730 UTC, and the correct routine observations from the operational suite, which movement of this core during 1 h; not shown). were enhanced by specially acquired TOVS [TIROS The simulation of the other systems (SC and SCL) (Television Infrared Observation System) Operational is of course beyond the capabilities of this type of Vertical Sounder] and MSU (microwave sounder unit) model. This may partly explain the model's failure to radiance data. We should recall that the Peridot analysis catch the clockwise tilting of the southern end of sys- system is based on optimal interpolation. It was among tem FL (by underestimating the cold pool in the RhoÃne the few operational systems making direct use of ob- Valley; see the description earlier). Despite these var- served satellite radiances as predictors of temperature ious discrepancies, one can see that the 12-h forecast and moisture increments between the ®rst guess and derived from the 10-km simulation would have been of the analysis. The use of these radiances effectively led great interest in an operational context. It also serves to a better analysis of the upper-air moisture over the to document the adequacy of such a model con®gura- Mediterranean for this case, improving the subsequent tion as a forecasting tool for heavy convective events. forecast. An assessment of the quantitative forecast is performed in section 4c. 2) DESCRIPTION OF THE SIMULATION On this case we also ran the research version of the 4) MODEL DESCRIPTION OF THE CONVECTIVE Peridot model, which has been developed at Centre Na- ENVIRONMENT tional de Recherches MeÂteÂorologiques over many years The quality of the ®ne-mesh simulation allows us to (e.g., Ducrocq and Bougeault 1995). Based on the describe some aspects of the environment of the con- code of the operational model, this version features an vective systems. The three factors necessary for intense improved resolution of about 10 km, as well as various convection are instability, moisture, and upward ver- advanced physical parameterizations: turbulence, sur- tical velocity, and we analyze them using cross sections face processes, and convection. For the experiment re- normal and parallel to the simulated frontal line, prior ported here, the main ingredient was the Bougeault to the development of systems SCL and FL, at 0700 (1985) convection scheme, including the newly devel- UTC (Fig. 17a). We ®nd the following. oped downdraft representation (Ducrocq and Bou- geault 1995). i) Instability: there is a decrease in equivalent po- The 10-km model was run on a domain of about tential temperature with height above 850 hPa over 1000 km 1 1000 km centered over Vaison. The initial the Mediterranean Sea and near the coast (Fig. 17b), data were provided by an interpolation of the above- in the region between longitudes 2.5Њ and 6ЊE (Fig. mentioned 0000 UTC 22 September reanalysis, and all 17c). At lower levels, instability only occurs over the boundary values were supplied by a 35-km simu- the sea south of 43ЊN (Fig. 17b), and a shallow cold lation starting from the same reanalysis (labeled BOU layer is found in the RhoÃne Valley below a homo- hereafter). Thus, the 10-km simulation was, in effect, geneous air mass around 320 K. In the east±west one-way nested into the 35-km simulation. The inclu- direction (Fig. 17c), stable conditions also appear at sion of the downdraft convection scheme certainly was very low levels except on the central portion of the one of the major modi®cations to the model. The down- domain (from 3Њ to 5ЊE). The already developed con- draft intensity is governed by an empirical coef®cient: vection is therefore occurring as a chimney around the percentage of the convective rainfall evaporated to longitude 4.3ЊE (Fig. 17c), ahead of the simulated create the downdraft. We tried several reasonable val- frontal zone (labeled SFL).

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FIG. 16. One-hour precipitation accumulation as forecast by the ®ne-mesh simulation (solid lines in mm at 10-mm intervals). Radar images are represented as grayscale backgrounds (with the same scale as in Fig. 11). Upper-left corner values are radar observation times. The model precipitation ®elds are hourly accumulations centered on the radar observation times. For 0530 UTC, the only available radar data are from two remote radars.

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FIG. 17. Fine-mesh simulation, valid at 0700 UTC. (a) 800-hPa level winds (larger arrows corresponding to 25 m s 01 ), 1-h precipitation accumulations (contour intervals are 5 mm), and cross-section lines used as in other panels. See Fig. 9 for geography (b) Meridional cross section of equivalent potential temperatures (measured in K at 2-K intervals) and circulation along the RhoÃne Valley. (c) As in (b) but for a quasi-zonal cross section. (d) Mixing ratios (solid lines measured in 10 01 gkg01 at2gkg01 intervals) and the wind component normal to the quasi-zonal section (dashed lines measured in m s 01 at 2.5 m s 01 intervals). All cross sections have pressure as the vertical coordinate and geography as the horizontal coordinate [i.e., latitude for (b), longitude for (c) and (d)].

ii) Moisture: the distribution of high moisture is iii) Upward motion: Fig. 17c shows the main fea- tightly linked to the Mediterranean in¯uence with high tures of the circulation across the simulated frontal zone mixing ratios (peak values above 14 g kg 01 ) in the low (labeled SFL). Above the PyreÂneÂes (on the left, around levels and in the southern half of the domain between 2ЊE), an orographic wave generates upward motion in 2.5Њ and 5.5ЊE (Fig. 17d), and values around 9 g kg 01 the stable cold air mass (while northward, the Massif in the foothills of the Alps. Central forces an upward motion west of the main pre-

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Unauthenticated | Downloaded 10/02/21 07:15 AM UTC DECEMBER 1996 S EÂ NEÂ SI ET AL. 437 cipitation core; not shown). East of the frontal zone fore noon. The 3-h accumulation ending at 0900 UTC (where upward velocity reaches 1 m s 01 ), a zone of exhibits a pattern roughly corresponding to system QL mesoscale subsidence is revealed that generates a light (not shown). This simulation depicts both precipitation low-level easterly return ¯ow (labeled RF), thus in- cores with rather realistic shapes. While slightly over- creasing convergence and thermal contrast in the estimating the rain production (3.9 vs 3.0 1 10 9 m3 ), RhoÃne Valley. Figure 17b shows a subsidence zone it underestimates accumulations observed above 70 (labeled Sub), ahead of the frontal zone, from the coast mm (lower-right panel). line to 100 km inland, which clearly inhibits convective Label 10K is the 10-km resolution run of the Peridot development in this region where the powerful SCL model described above (section 4b). It of course shows system occurred 2.5 h later. One can explain this dis- the more detailed patterns due to its ®ner resolution. It crepancy by a marked overestimation of the southward simulates the Vaison-La-Romaine precipitation core extension of the shallow RhoÃne Valley cold layer at with a location error on the order of 30 km, a realistic low levels, due possibly to either inadequate behavior shape, and a relatively precise maximum (above 200 of the convection parameterization for this case or to mm, for an observed value of 300 mm). The north- bad initial conditions. western core is not so well simulated (90 km too far south), and the total rain production is overestimated c. Comparison of several rainfall forecasts by 50% (see lower-right panel). ARP is a forecast based on the Arpege/IFS fore- Several models and model con®gurations have been casting suite (Courtier and Geleyn 1988; Courtier et al. tested. Their quality is assessed here on the basis of 1991). Arpege replaced Peridot as the operational rainfall accumulations starting at 0600 UTC and cov- model at MeÂteÂo-France in 1993. Its main feature is a ering the entire rainy period over the domain shown in stretched global spectral grid focusing the model res- Fig. 18. olution around a given pole of interest, while keeping The ground truth is given by the data described in it a global model. The present simulation was per- section 3c (also shown in Fig. 18, panel OBS). The formed with T127 truncation and a 3.5 stretching fac- simulation labels used hereafter are also the panel tor, which gave a physical space resolution equivalent names in Fig. 18. The lower-right panel of Fig. 18 il- to the OPE run (35 km) in the domain of interest. The lustrates a dif®cult test of the rainfall forecasts; that is, convection scheme is after Bougeault (1985) as in the for each of the four forecast ®elds of Fig. 18, we thres- BOU run above. The rainfall patterns show a frontal holded the ®eld at given threshold values and computed line moving too slowly with a ®rst core drifting slowly a spatial integration of these thresholded rainfalls, over northward and a second one generating just east of Vai- the area of the panels. The thresholds stand on the x son, when the line crosses the RhoÃne Valley (this event axis, and the integrated values on the y axis. When being simulated 6±8 h too late). This run simulates an compared to the similar curve derived from the ob- underestimated northwestern core and an eastern one served rainfall ®eld, each forecast curve shows to what above 50 mm that catches some aspects of the real evo- extent the corresponding run was able to simulate the lution in the afternoon, but it neglects the Vaison-La- combined effects of the stationary character of the sys- Romaine core. Its integrated rain production (lower- tems and the rainfall rates at each accumulation level. right panel) closely matches that of the BOU run, and In Fig. 18, the forecast panel labels are as follows. one should remember that both simulations use the OPE is the operational forecast with the Peridot same convection scheme (with a resolution-dependent model, using a Kuo-type convection scheme, with a 35- tuning for the stretched Arpege run), but different anal- km grid mesh. This simulation has been described yses (ARP does not take into account the satellite data above (section 4a). It underestimates by a factor of 2 included in the BOU analysis). the total rain production (lower-right panel). It catches In summary, one can state the following. the northwestern precipitation core (but largely under- estimates the peak value: 65 vs 300 mm) and misses i) All models show signi®cant accumulations completely the Vaison-La-Romaine core. The under- (above 75 mm over large areas) and exhibit the basic estimation is more pronounced for values above 20 mm stationary character of the rainfall, albeit with some of accumulation (lower-right panel). lack of realism. BOU is a forecast performed with the Peridot model ii) Only two simulations (BOU and 10K) were able using the same grid mesh as OPE but based on the to simulate the unusual location of the rainfall core east above-mentioned reanalysis and with the Bougeault of the RhoÃne River (only the early QL system occurred (1985) mass ¯ux convection scheme (refer to section in a rather typical location). 4b). The rainfall patterns in this simulation ®rst show iii) The Kuo-type convection scheme clearly pro- a powerful frontal line that is more than 4 h late in its duced too weak rain accumulations. easterly movement across the RhoÃne Valley, then is too iv) The mass ¯ux convection scheme is moderately slow (remaining around Vaison-La-Romaine until well adapted to this case for the 35-km resolution sim- 2300 UTC), and ®nally extends too far southward be- ulations.

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FIG. 18. Comparison of observed and model-simulated rainfall accumulations (in mm) for the 24-h period beginning 0600 UTC 22 September. Contour line spacing is in 50-mm intervals. Panel OBS and the heavy contours (on each panel) represent observed rainfall accumulation ®elds. Panels 10K, BOU, OPE, and ARP depict the rainfall accumulation forecasts for various models. The lower-right panel grayscale is for the forecast rainfall accumulation; this panel also displays the curves of spatially integrated rain (over the domain of the panels) vs a lower integration threshold. Other features are as in Fig. 9.

v) The 10-km resolution run gives the more re- rainfall was particularly poor after 1200 UTC in all alistic chronology and spatial distribution of rainfalls simulations (albeit with varying intensities). and was able to simulate accumulations above 150 mm. It largely overestimated all accumulations be- 5. Discussion of the nowcasting of the event low 75 mm. In addition to the NWP forecasts described above, An examination of shorter period accumulations for there is a strong need for tools and techniques for now- all simulations indicates that most of them were un- casting such events. Given the data available here, two derestimated and that large accumulations were gen- points may be examined further: i) forecasting the lo- erated by an over simulation of the stationary character cation of heavy rainfall and ii) forecasting the end of of the rainfall. It also indicates that the chronology of the heavy rainfall.

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Unauthenticated | Downloaded 10/02/21 07:15 AM UTC DECEMBER 1996 S EÂ NEÂ SI ET AL. 439 a. Forecasting the heavy rainfall location by its cold pool and the enhanced convergence that its associated mesohigh generated around Vaison-La-Ro- At short ranges, one could try to ®nd objective pieces maine (with a 10-min-mean wind reaching 25 m s 01 of information to help forecast accurately the location on the coast east of the Alpilles). Also noticeable is the of heavy rain. In this context, with the potential insta- good match between the saturated conditions and the bility capped by an inversion (according to the Nimes track of system SC and cells associated with system sounding), it is well known that the occurrence of the SCL (in Fig. 19 the axis of saturation is labeled ``T± ®rst updraft drives almost all following convective cells T''). Of course, these explanations for determining and that one should therefore focus on system SC, the heavy rain locations of systems SC and SCL cannot be initial supercell. After a thorough examination of half- taken as a predictive method based on this single case. hourly surface analyses, the most objective pieces of As far as the frontal convective line is concerned, information that could explain why system SC initiated one could only argue that its reinforcement on the Alps south of Vaison-La-Romaine are (Fig. 19) foothills was foreseeable, as was not the case for the i) the lift source due to (a) the low-level ¯ow observed additive effect with system SCL; its propa- crossing small hills (the Alpilles), (b) the general con- gation was unusual, with a jump across the southern vergence in the funnel-shaped RhoÃne Valley (refer end of the range, close to the coast, between 1400 and back to Fig. 11, panel 0900 UTC where the wind shift 1500 UTC (refer back to Fig. 8, panels for both times, line appears clearly), and (c) the cold pool created by or to Fig. 16, panel 1430 UTC, eastern part, which the squall line in the upper RhoÃne delta, and shows both steps of the jump). ii) the marine moisture source, which saturated the ¯ow in the lower RhoÃne delta (with less than 5-hPa b. Forecasting the end of heavy rain lifting needed to reach a lifted condensation level). Watching the moist in¯ow of a convective system There might also be upper-level conditions favorable using surface observations may be effective for antic- to this location, but the observations are insuf®cient to ipating its activity when there is no decoupling between assess this correctly. The squall line occurrence seems the system and the surface conditions. Given the lo- to have played a key role in this combination of factors, cation for which the forecast is to be made and the

FIG. 19. Surface conditions before the onset of supercell system SC at 0900 UTC. Thick and thin lines are contours of the potential and pseudo-wet-bulb potential temperature ®elds, respec- tively, at 1Њ intervals. Wind barbs are conventional (in kn). The background image is a digital elevation model. The T±T axis shows the maximum saturation axis.

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FIG. 20. South±north moisture ¯ux computed from measurements in Arles (with a 1-h time shift) and global rain production. general movement of individual cells to the north- noted by Chappell (1986), ``some of the larger and northeast, one can, in this case, ®nd in the upstream longer-lived quasi-stationary MCS develop under direction an intriguing characteristic at Arles, near the strong baroclinic conditions,'' even if the high wind coast (see the location in Fig. 10 or Fig. 19). As shown speeds encountered in such contexts are able to gen- in Fig. 20, the south to north moisture ¯ux at this point erate high-velocity storms. This ¯ood situation matches exhibited a strong correlation with the overall activity the ``synoptic'' type of Maddox's classi®cation (Mad- of the convective systems (as computed in section 3c), dox et al. 1979) and incorporates the following con- and especially with both SCL and FL systems, while ditions: slow-moving cold front, proximity of a for- showing a predictive potential on the order of 1.5 h. merly stationary ridge, strong low-level jet and low- This ®nding has already been stated by other authors level moisture advection, combination of upper-level (e.g., Caracena et al. 1979 or Doswell et al. 1995 in a jet streaks and orographic lifting, large convective po- more general way). The most dif®cult point in using tential, and conditional instability associated with such a ®nding as a forecast method is of course to ®nd marked capping over a large region. the relevant observing stations for any given combi- Mesoscale processes, through orography and nation of convective system and forecast location. For boundaries, were central in organizing a very large this case, we tried to use other ground stations to com- variety of precipitating systems in this event: an up- pute the in¯ow; 28 relevant stations were scanned, but slope-type stationary system, a squall line, a left- few of them were available along the shore. When us- moving supercellular system, a multicellular station- ing a more western location, the match between rainfall ary convective line, and another, slow moving, mul- and electrical activity is not so good (the initial squall ticellular line associated with the cold front. The line shows a signature, but the later systems do not). squall line and the orography played key roles in fo- When using a more eastern location, the match stays cusing further convective development, through a good, but the anticipating character is reduced consid- cold pool and a mesohighÐthe type of ``mesohigh erably (in agreement with the slow eastward movement event'' that has been documented by Maddox et al. of system FL). (1979) and Chappell (1986). This case also matches characteristics of two of the 6. Conclusions classes de®ned by Sco®eld (1985) for heavy precipi- tation, namely the ``large-scale wedge'' class, for sat- The Vaison-La-Romaine ¯ash ¯ood occurred in a ellite and conventional data, and the ``regenerative'' synoptic environment favorable for such events. As class when going to mesoscale analysis. It showed two

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Unauthenticated | Downloaded 10/02/21 07:15 AM UTC DECEMBER 1996 S EÂ NEÂ SI ET AL. 441 scales of regenerative mechanisms: the cell scale, as in for providing the spline package, M. Derrien for the the frontal line or the stationary line, and the system TOVS, AVHRR, and MSU data used in the reanalysis, scale, as shown in satellite imagery. the local of®ces (especially the Vaucluse and Gard of- The variety of convective organization types en- ®ces) for the high temporal resolution data, the three countered in the same place during6hisnoteworthy regional of®ces (DIRSE, DIRSO, DIRCE) for provid- and represents a serious challenge to all aspects of fore- ing the mesonetwork data, J.-L. Piriou for providing casting, and even when only forecasting convective or- missing Meteosat slots and high-resolution visible im- ganization type. Given the paucity of observations, es- ages, J. M. Piriou and E. Bazile for providing the ARP pecially wind pro®les, one could hardly get help from simulation results, J.-M. Audouin for performing an in- the proposed rules that use hodograph and bulk Rich- termediate simulation, the Rhea Company for having ardson number (Weisman and Klemp 1986) to explain initiated the operation of 1-km resolution radar data, R. such a spectrum of organization types. Darros for digitizing some data, P. Zwack and V. Duc- The operational model forecast did capture a severe rocq for improving the initial manuscript, B. Bloom- event, even if it showed some lack of realism in the ®eld for editing it, and the three reviewers for numerous chronology of the rainfall and frontal system move- suggestions. ment. A forecast, based on a hydrostatic model with a 10-km mesh and on a reanalysis, showed fairly realistic REFERENCES rainfall ®elds, both spatially and chronologically, over 10 h. Twenty-four-hour rainfall simulations by four Barret, I., V. Jacq, and J.-C. Rivrain, 1994: A situation generating torrential rains in the Mediterranean area (the 22±23/09/93 models were evaluated; they displayed varying realism, stormy case in south-eastern France) (in French). La MeÂteÂo- good positioning of rainfall core locations, and a gen- rologie, 7, 38±60. eral overestimation of the spatially integrated accu- Battan, L. J., 1965: Some factors governing precipitation and light- mulations. ning from convective clouds. J. Atmos. Sci., 22, 79±84. We discussed two aspects of nowcasting such a case: Beckman, S. K., and K. L. Polston, 1995: An evaluation of the RUC predicted winds and 404 MHz pro®ler observed winds at 0300 in particular, the anticipating (or informing) value of and 0600 UTC. Preprints, 14th Conf. on Weather Analysis and monitoring the system in¯ow, and that explaining Forecasting, Dallas, TX, Amer. Meteor. Soc., 188±193. heavy rainfall locations can only be done in an a pos- Benech, B., H. Brunet, V. Jacq, M. Payen, J.-Ch. Rivrain, and P. teriori way, through a careful analysis of all mesoscale Santurette, 1993: The Vaison-la-Romaine catastrophe and the heavy rains of September, 1992: Meteorological aspects (in data. French). La MeÂteÂorologie, 1, 72±90. The prospects of nowcasting such cases lies in bridg- Bougeault, P., 1985: A simple parameterization of the large-scale ing the gap between the very short range and the inter- effects of cumulus convection. Mon. Wea. Rev., 113, 2108± mediate-range forecasts and providing effective tools 2121. for forecasters. Very short range forecasts, around 1 h, Brewster, K., F. Carr, N. Lin, J. Straka, and J. Krause, 1994: A local analysis system for initializing real-time convective-scale mod- need the analysis tools used in the present study and els. Preprints, 10th Conf. on Numerical Weather Prediction, more systematic studies in order to tune these tools. Portland, OR, Amer. Meteor. Soc., 596±598. They can also take advantage, albeit with care, of ex- Browning, K. A., and F. F. Hill, 1984: Structure and evolution of a trapolation methods. mesoscale convective system near the British Isles. Quart. J. Roy. Meteor. Soc., 110, 897±913. There is also a need for a somewhat intelligent sys- Cammas, J.-P., and D. Ramond, 1989: Analysis and diagnosis of the tem capable of supporting forecasters with automatic composition of ageostrophic circulations in jet-front systems. tracking and characterization of convective systems Mon. Wea. Rev., 117, 2447±2462. with some understanding of the main mechanisms in- Caracena, F., R. A. Maddox, L. R. Hoxit, and C. F. Chappell, 1979: volved, and with some knowledge of past events. But Mesoanalysis of the Big Thompson storm. Mon. Wea. Rev., 107, 1±17. there is apparently no way to bridge the gap by enlarg- Chappell, C. F., 1986: Quasi-stationary convective events. Mesoscale ing the useful time range of such methods without the Meteorology and Forecasting, P. S. Ray, Ed., Amer. Meteor. support of a well-forecast convective environment. We Soc., 289±310. think that mesoscale models, run frequently enough Chastan, B., O. Gilard, and J. Lavabre, 1993: The dif®culties in rapid estimation of ¯ood ¯owsÐThe example of the OuveÁze river and fed with all available information, are necessary to ¯ood on the 22nd September 1992 (in French). Revue de GeÂo- provide a consistent picture of this convective environ- graphie de Lyon, 68, 2±3, 153±158. ment, if not of the convection itself. The main problems , 1994: On off-line correction of radar signal attenuation by rain. then lie in the assimilation of nonstandard data and ini- Proc. COST 75 Weather Radar Systems Int. Seminar, Brussels, tialization with convection present. Belgium, European Commission, 135±143. Chisholm, A. J., and J. H. Renick, 1972: The kinematics of multicell and supercell Alberta hailstorms. Research Council of Alberta Acknowledgments. This study was supported in part Hail Studies, Rep. 72-2, 53 pp. by the French Centre National de la Recherche Scien- Courtier, P., and J.-F. Geleyn, 1988: A global numerical weather ti®que (Grant INSU/PNRN/94/ATP/630/RH05) and prediction model with variable resolutionÐApplication to the shallow water equations. Quart. J. Roy. Meteor. Soc., 114, by the French MinisteÁre de l'Environnement (Grant 1321±1346. 93326). The authors would like to thank J.-L. Boichard , C. Freydier, J.-F. Geleyn, F. Rabier, and M. Rochas, 1991: The for the display tools used in this study, G. Desroziers Arpege project at MeÂteÂo-France. ECMWF 1991 Seminar Pro-

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ceedings: Numerical Methods in Atmospheric Models, Reading, Kinzer, G. D., 1974: Cloud-to-ground lightning versus radar re- Berkshire, United Kingdom, ECMWF, 193±231. ¯ectivity in Oklahoma thunderstorms. J. Atmos. Sci., 31, Craven, P., and G. Wahba, 1979: Smoothing noisy data with spline 787±799. functions. Numer. Math., 31, 377±403. Klemp, J. B., and R. B. Wilehmson, 1978: Simulations of right- and Delrieu, G., J.-D. Creutin, and I. Saint-AndreÂ, 1991: Mean K± R re- left-moving thunderstorms produced through storm splitting. J. lationships: Practical results for typical radar wavelengths. J. Atmos. Sci., 35, 1097±1110. Atmos. Oceanic Technol., 8, 467±476. Krider, E. P., R. C. Noyle, and M. A. Uman, 1976: A gated, wide- Doswell, C. A., III, H. E. Brooks, and R. A. Maddox, 1995: Flash board magnetic direction ®nder for lightning return strokes. J. ¯ood forecasting: An ingredients-based methodology. Preprints, Appl. Meteor., 15, 301±306. Fifth Workshop on Operational Meteorology, Edmonton, AB, MacGorman, D. R., and W. L. Taylor, 1989: Positive cloud-to- Canada, Atmospheric Environment Services/Canadian Meteo- ground lightning detection by a direction ®nder, J. Geophys. rological and Oceanographic Society, 149±156. Res., 94(D11), 13 313±13 318. Ducrocq, V., and P. Bougeault, 1995: Simulations of an observed Maddox, R. A., 1980: Mesoscale convective complexes. Bull. Amer. squall line with a meso-beta scale hydrostatic model. Wea. Fore- Meteor. Soc., 61, 1374±1387. casting, 10, 380±399. , L. R. Hoxit, C. F. Chappell, and F. Caracena, 1978: Compar- Durand, Y., and P. Bougeault, 1987: Peridot objective analysis (in ison of meteorological aspects of the Big Thompson and Rapid French). EERM Internal Note 193, 71 pp. [Available from City ¯ash ¯oods. Mon. Wea. Rev., 106, 375±389. CNRM/DOC, 42, Avenue G. Coriolis, F-31057 Toulouse , C. F. Chappell, and L. R. Hoxit, 1979: Synoptic and meso- Cedex, France.] alpha scale aspects of ¯ash ¯ood events. Bull. Amer. Meteor. Fawbush, E. J., and R. C. Miller, 1953: A method for forecasting Soc., 60, 115±123. hailstone size at the earth's surface. Bull. Amer. Meteor. Soc., Martyn, D., 1992: Developments in Atmospheric Sciences. Vol. 18, 34, 235±244. Climates of the World, Elsevier, 435 pp. Geleyn, J.-F., 1985: On a simple, parameter-free partition between McCann, D. W., 1983: The enhanced-V, a satellite observable severe moistening and precipitation in the Kuo scheme. Mon. Wea. storms signature. Mon. Wea. Rev., 111, 887±894. Rev., 113, 405±407. McGinley, J. A., S. C. Albers, and P. A. Stamus, 1991: Validation Gilet, M., 1984: The French weather radar networkÐA status report. of a composite convective index as de®ned by a real-time local analysis system. Wea. Forecasting, 6, 337±356. Proc. Nowcasting II Symp., Norrkopping, Sweden, European Moller, A. R., C. A. Doswell III, M. P. Foster, and G. R. Woodall, Space Agency, 417±422. 1994: The operational recognition of supercell thunderstorm Houze, R. A., Jr., and P. V. Hobbs, 1982: Organization and structure environments and storm structures. Wea. Forecasting, 9, of precipitating cloud systems. Advances in Geophysics, Vol. 327±347. 24, Academic Press, 221±315. Piepgrass, M. V., E. P. Krider, and C. B. Moore, 1982: Lightning , W. Schmid, R. G. Fovell, and H.-H. Schiesser, 1993: Hail- and surface rainfall during Florida thunderstorm. J. Geophys. storms in Switzerland: Left movers, right movers, and false Res., 87(C13), 11 193±11 201. hooks. Mon. Wea. Rev., 121, 3345±3370. Pircher, V., and F. Saix, 1991: Multiscale case study of the sustained Imbard, M., A. Joly, and R. Juvanon du Vachat, 1986: The Peridot thunderstorm activity associated with the Nimes ¯ood event on model for NWP: Formulation and manual (in French). EERM 3 October 1988. Preprints, 25th Radar Conf., Paris, France, Internal Note 161, 70 pp. [Available from CNRM/DOC, 42, Amer. Meteor. Soc., 428±431. Avenue G. Coriolis, F-31057, Toulouse Cedex, France.] Sco®eld, R. A., 1985: Satellite convective categories associated with Jacq, V., 1994: Inventory of torrential rain situations in south-east heavy precipitation. Preprints, Sixth Conf. on Hydrometeorol- France: Period 1985±1994. MeÂteÂo-France Remarkable Phe- ogy, Indianapolis, IN, Amer. Meteor. Soc., 42±51. nomena Series 3, 190 pp. [Available from MeÂteÂo-France, Sekhon, R. S., and R. C. Srivastava, 1971: Doppler radar observations SCEM/DOC, 42 Av. Coriolis, 31057 Toulouse Cedex, of drop-size distribution in a thunderstorm. J. Atmos. Sci., 28, France.] 983±994. Janish, P. R., K. K. Droegemeier, X. Ming, K. Brewster, and J. Levit, Tourte, J. L., F. Helloco, M. Le Boulch, and J. Hamelin, 1988: First 1995: Evaluation of the advanced regional prediction system results obtained with the MeÂteÂorage thunderstorm monitoring (ARPS) for storm-scale operational forecasting. Preprints, 14th system. Preprints, Eighth Conf. on Atmospheric Electricity, Conf. on Weather Analysis and Forecasting, Dallas, TX, Amer. Uppsala, Sweden, Amer. Meteor. Soc., 341±346. Meteor. Soc., 224±229. Weisman, M. L., and J. B. Klemp, 1986: Characteristics of isolated Keyser, D., and M. A. Shapiro, 1986: A review of the structure and dy- convective storms. Mesoscale Meteorology and Forecasting, S. namics of upper-level frontal zones. Mon. Wea. Rev., 114, 452±499. Ray, Ed., Amer. Meteor. Soc., 351±358.

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