Meteorol. Atmos. Phys. 71, 139±155 (1999)
1
Department of Astronomy and Meteorology, University of Barcelona, Spain Department of Physics, University of Balearic Islands, Palma de Mallorca, Spain 3 Department of Environmental Engineering, University of Genoa, Italy 2
Storm Tracking and Monitoring Using Objective Synoptic Diagnosis and Cluster Identi®cation from Infrared Meteosat Imagery: A Case Study M. C. Llasat1 , C. Ramis2 , and L. Lanza3 With 14 Figures Received July 20, 1998 Revised June 21, 1999 Summary The present paper investigates the potential of combining image processing techniques based on cluster analysis of infrared (IR) Meteosat images with dynamic meteorological theory on synoptic systems. From this last point of view the highest probability of deep convective development is favoured where the overlapping of four mechanisms acting at synoptic scale is produced: upward quasi-geostrophic forcing, convergence of water vapour at low levels, convective instability in the lower troposphere and great convective available potential energy. Cloud tracking is performed over sequences of Meteosat IR images by using a shape parameterisation approach after appropriate ®ltering for non-signi®cant clouds and automated identi®cation of convective systems. The integrated methodology is applied to the case study of the heavy rainfall event which produced ¯oods in the South of France and the North of Italy on September 27±28th, 1992. The analysis focuses on the monitoring and explanation of the zones most affected by heavy rainfall with the aim of investigating possible improvements of the predictive potential of cloud tracking and allowing identi®cation of the areas which most lend themselves to ¯ash ¯oods for use in operational ¯ood forecasting applications.
1. Introduction During the month of September 1992 two catastrophic ¯ood events occurred over the South of France and the North of Italy (Fig. 1). The
®rst, which took place between the 22nd and 23rd, produced 42 casualties and 4 people missing in France. The second event started on the 26th and ended on September 28th and damages in France were more than FF 400 millions, 4 casualties and 1 missing person. This work deals with this second event. Meteorological analysis of heavy rainfall events which cause ¯ash ¯oods is being signi®cantly assisted, in highly developed countries, by the use of information received from satellite sensors. One of the most promising approaches to the satellite imagery relies on cluster analysis aimed at cloud tracking and storm identi®cation (Bolla et al., 1995; Boni et al., 1996). This kind of approach is particularly useful in those cases in which convective systems move quickly and heavy rainfalls affect different zones. Also, when a single convective system is easily identi®able in satellite images ± as it is the case with mesoscale convective systems (MCS) ± storm identi®cation and tracking techniques based on automated image processing algorithms seem to hold promising perspectives for ¯ood monitoring and forecasting applications. In Spain it is quite common to ®nd moving systems which in the course of their travel affect
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Fig. 1. Map of West Mediterranean Area. It includes the locations referred to in the text
Andalucia, then Levante (Eastern Spain) and ®nally Catalonia (Northeast Spain) (Fig. 1). This was the case in the ¯ooding of 19 October 1982 which claimed 39 victims in the Levante region of Spain, with over 500 mm of rainfall in 24 hours. This same system had previously given rainfalls exceeding 100 mm in 24 hours in Andalucia and later again exceeded 100 mm in Catalonia. By no means infrequent, either, are those situations connected with heavy rainfalls which affect Catalonia and the South of France (Llasat and Puigcerver, 1994). An example of such situations was the serious ¯oods that occurred between 6 and 8 November 1982. In addition to affecting the region of the Eastern Pyrenees±with total rainfall for the three days up to 610 mm±150 mm (more than double the monthly average) was recorded in South Andalucia on 6 November, while in the Massif Central (France) 548 mm was recorded over the three days. A similar situation had arisen in October 1940, with more than 800 mm in 24 hours in the Eastern Pyrenees (Llasat, 1993). Similarly, the heavy rainfall events that occurred in October 1986 and 1987, and again in November 1988, with more than 200 mm in 24 hours, caused ¯oods in both the French and Spanish zones (Llasat and Rodriguez, 1992; Ramis et al., 1994, 1995). In relation to ¯ood events, studies generally end up concentrating on the most severely
affected zone, occasionally thereby forsaking an overall view. This is particularly important in those situations which affect different countries, where studies unfortunately tend to restrict themselves to their own country frontiers. The case of 26±28 September 1992 which is analysed here is an example of shifting and reinforcement of a convective system which affected zones situated in three different countries: Spain, France and Italy (Fig. 1). Over the course of the paper an attempt is made to explain the location of the zones most severely affected by heavy rainfall day after day, using Meteosat images and objective synoptic diagnosis. 2. The Event The ®rst heavy rainfalls were recorded in the South of Catalonia and in Levante (Spain) on September 26th, with total quantities exceeding 100 mm in many places (Fig. 2). In France, the ®rst heavy rainfalls were recorded at midday on the 26th, mainly affecting the basins of the Eastern Pyrenees. In this region, the maximum recorded was 324 mm, 93 mm of which were recorded between 20 and 21 hours, local time. On the 27th the rains mainly affected the Liguria region (Italy), where a total of 459 mm was recorded in 24 hours, with a maximum mean hourly intensity of 75 mm, between 15 and 16 hours, local time. On the 28th the rains mainly
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Fig. 2. Map showing the zones affected by the high rainfalls (dotted) over the days 25 to 28 September 1992 and the maximum total precipitation (mm) in different places (only cumulated precipitation above 100 mm are showed)
affected the Toscana region, with a maximum of 148 mm. On the 29th and 30th some weak precipitation was still being recorded. In France, this rainfall led to swelling of some rivers in the Eastern Pyrenees. Fortunately, the VincËa reservoir reduced the ¯ow of the T^et from 2045 m3/s (the estimated ¯ow which would have occurred without the reservoir) to 1115 m3/s, avoiding a catastrophic ¯ooding in the urban areas near the coast. Even so, four people died and another person disappeared in France. Total losses were put at 400 million francs and a state of natural disaster decreed in the greater part of the Eastern Pyrenees regions. The main hydrometeorological feature of this phenomenon was not the maximum rainfall recorded, but the large area affected by the high rainfalls: 150 mm were recorded in 4 hours over 1600 km2. In Italy, there were a few casualties in the urban area of the town of Genova ± where a few streams ¯ooded out ± and more than US $12.000.000 was needed for urgent salvage work. Extensive information on other hydrometeorological aspects and on damage caused by the 26±28 September 1992 event can be found in Mission Interservice de l'Eau (1993), Blanchet and Deblaere (1993), Leroux (1993) and in the report published by Gruppo Nazionale per la Difesa dalle Catastro® Idrogeologiche (1994). It is important not to confuse the event under study in this paper with the one recorded between 22 and 23 September 1992 in the South of
France, with 42 people killed and 4 missing (Comby, 1993). This ®rst episode also affected the North of Italy, and speci®cally Liguria. The studies by Erpicum (1993), Mottet et al. (1993), Comby (1993), Blanchet and Deblare (1993), Leroux (1993), Chastan et al. (1993), Berenguer (1993), and the monographic study produced by Gruppo Nazionale per la Difesa dalle Catastro® Idrogeologiche (1994) look at this episode in detail. 3. Methodology 3.1 Meteorological Aspects The development of convection with potential for ¯ash ¯ooding needs instability (latent or potential), high values of relative humidity in all the troposphere resulting in signi®cant values of precipitable water in the troposphere, moisture and low-level convergence and lifting mechanism (McGinley, 1986). The instability and humidity are part of a favourable environment, which combined with the upward vertical motion is covered by the synoptic scale. On the other hand, the focusing mechanism that triggers the development of convective cells usually goes into the mesoscale. Due to the lack of data appropriate for a mesoscale study, only the synoptic analysis has been made in this work. In this paper, the synoptic diagnosis was made through determination of derived ®elds from the
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numerical analysis products obtained by the Instituto Nacional de MeteorologõÂa (1992) (INM) of Spain as initial values for its Limited Area Model (LAM), which was running in 1992. The data generated were geopotential height, temperature, relative humidity and wind components (DõÂaz-Pabon, 1988). Values were available on a 0.91 (latitude/longitude) grid in the horizontal and standard pressure levels in the vertical. Following the results obtained in previous works (Ramis et al., 1994, 1995) the diagnosis consisted in the identi®cation of those zones where there was quasi-geostrophic upward forcing at 850 hPa, moisture convergence at 1000 hPa and convective instability between 1000 and 500 hPa, every 12 hours. The upward quasi-geostrophic forcing on isobaric surfaces (hereafter FQ) has been calculated using the Q-vector formulation of the !equation (see Hoskins and Pedder, 1980). For this purpose, the non-initialized geopotential height and temperature ®elds were ®ltered to eliminate short wavelengths by using the procedure described by Gomis and Alonso (1988). The horizontal distribution of convective instability in the lower troposphere was determined by the difference between the equivalent potential temperatures at 500 and 1000 hPa. Equivalent potential temperatures have been calculated by using Bolton's (1980) expression. Bearing in mind the strong relationship between rainfall intensity and water mass ¯ux, a fourth factor has been introduced into the synoptic analysis: the Convective Available Potential Energy (CAPE) (Weisman and Klemp, 1986). Indeed, as Doswell (1993) shows, a convective storm with a strong updraft has higher water vapour mass ¯ux than a convective storm with a weak updraft. Simple parcel theory indicates that the maximum updraft speed is directly related to the CAPE square root. Although CAPE is usually calculated for radiosounding ascents at one point, in this paper its spatial distribution has been obtained considering each grid point as a sounding. 3.2 Meteosat Analysis The use of satellite imagery is very well suited for accompanying diagnosis methodologies developed on the basis of conventional datasets.
In particular, the information about the radiance temperatures of the tops of the clouds±as provided by the images from the Meteosat geostationary platform in the thermal infrared band (IR)±is very useful to detect deep convective systems. The IR images obtained are formed by arrays of 25002500 discrete elements (pixels) respectively that correspond to resolutions of 75 km2 in middle latitudes. Procedures for the automated acquisition, preprocessing, georeferencing and analysis of halfhourly images in real time have been developed in order to identify convective cloud systems and eventually track their evolution both in space and time (Bolla et al., 1995; Lanza and Conti, 1995). The concept of cloud tracking relies on the assumption that the areas presenting the highest probability of heavy rainfall within the observed cloud system are actually clustered, so as to re¯ect the structure of typical MCS. This means that the pixels showing the lowest values of radiance temperatures in the IR images are aggregated in clearly distinguishable clusters which are easily and automatically identi®ed in each on the images from a sequence, and then tracked, with reference to some suitable cluster characteristics. The image processing methodology applied in this work is described in Lanza and Conti (1995). In general terms it is as follows: initially low and middle elevation clouds, associated with low probability of rainfall, are ®ltered out from each image in order to speed up the procedure, using a ®xed threshold (253 K) previously de®ned on the basis of the meteorological and climatic characteristics of the area. The main cloud clusters are identi®ed starting from a single numerical seed which initiates the process and a series of iterations performed on the same images while testing the compactness of the cluster and the opportunity to subdivide it into two independent ones. The cluster is then represented through an equivalent elliptical shape and the main geometrical characteristics are evaluated, such as the position of the centroid, the cluster area and the principal inertial moments and radii. The tracking is then carried out in this parameter space in order to derive the cluster evolution in time and space. Forecasting is possible over a lead time not exceeding one or two images in the future (half-an-hour and one hour, respectively), using a
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linear autoregressive procedure on the last available images. 4. Meteorological Analyses of the Event 4.1 Synoptic-Scale Analysis In general terms, the situation was characterised at 300 and 500 hPa by the evolution, between 26 and 28 September, from a meridian circulation to a Western zonal circulation, as a result of the shift of a long-wave trough from the Gulf of CaÂdiz towards Sardinia. The winds at 500 hPa exceeded 30 kn over the affected zones, veering from S-SW on the 26th to SW on the 27th and WSW on the 28th. The meteorological situation on 25 and 26 September was characterized at 1000 hPa/surface
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(Fig. 3a) by a low located over the Northwest of the Iberian peninsula, which advected warm and humid Atlantic air over Spain, and an anticyclone located over the centre of Europe and Scandinavia, which extended its in¯uence to the Western Mediterranean where it produced moderate winds and warm advection from the Southeast. On 25 September this advection affected mainly the Alboran Sea and the East coast of Spain. During the afternoon of 26 September the Atlantic low and the anticyclone started to move to the Northeast. At 0000 UTC 28 September the low was centred over the Mancha Channel. Due to this movement a strong warm advection over the Gulf of Lyon and the Southeast of France occurred during the afternoon and the night of 26 September. This tongue of warm air continued its displacement towards
Fig. 3. Synoptic situation at 0000 UTC 26 September 1992; a) 1000 hPa; b) 850 hPa. Contour interval for height 30 gpm (full line), for temperature 4 C (dashed line); c) 500 hPa (height -full line- and relative vorticity -dashed line). Contour interval for height 60 gpm, for relative vorticity 4 10ÿ 5 sÿ 1; d) Quasi-geostrophic vertical forcing at 850 hPa. Solid and dashed lines indicate forcing of upward and downward vertical motion, respectively. Isoline interval 8 10ÿ 18 m kgÿ 1 sÿ 1
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Fig. 4. As for Fig. 2 but for 0000 UTC 27 September 1992
the East, affecting the Tyrrhenian Sea and the Gulf of Genoa on 27 September (Fig. 4a), and the entire Italian peninsula on 28 September (Fig. 5a). Between 1200 UTC 25 September and the 1200 UTC 26 September, a secondary warm low arose over the North of the Algerian coast and the Alboran Sea. This shallow secondary low was a consequence of an Algerian cyclogenesis as shown on the mesoscale maps published by the INM (INM, 1992). Due to this secondary low the pressure gradient near the Gulf of Lyon was increased during the 26 September, and, consequently, the winds over Catalonia and the Eastern Pyrenees were increases, too. On September 27 the new position of this Algerian low veered the wind towards the South of France. At 850 hPa the pattern was very similar to that at the surface. Besides the European anticyclone, a second anticyclonic zone had developed over the Northeast of Algeria (Fig. 3b) giving way to a
single anticyclone at 0000 UTC 27 September. As a consequence of this low-high dipole, the Southern winds and the warm advection formed at 1000 hPa over the Western Mediterranean increased. This warm advection started in the afternoon of 25 September, increased during the afternoon of 26 September due to the warm air ¯ux from the South (Fig. 4b) and, ®nally, at 1200 UTC 27 September it affected all Italy due to the veering of the wind to the SW (Fig. 5b). The places most affected by the high rainfalls were those most affected by the warm and humid advection. On 25 September a closed cold low was located at 500 hPa to the Northwest of the surface low with a trough to the West of the Iberian Peninsula. Apart from a major cyclonic vorticity advection (CVA) over the West of Spain due to this trough, a secondary nucleus of cyclonic vorticity, related to a short wave trough situated
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Fig. 5. As for Fig. 2 but for 0000 UTC 28 September 1992
over the Alboran Sea, was centred over the island of Majorca. Since the afternoon this secondary nucleus had moved to the North and at 0000 UTC 26 September it was situated over the South of France (Fig. 3c). Meanwhile, the Atlantic cold low moved to the East and at 0000 UTC 27 September it affected the Northwest of the Iberian peninsula (Fig. 4c). At that moment the ridge over the central Mediterranean was completely developed and a thermal ridge occupied the Western Mediterranean and the South of France. As a consequence, a warm air ¯ux from the South impinged over the Gulf of Lyon and the Gulf of Genoa, although only the Gulf of Lyon was affected by the CVA. Over the course of that day the cold low continued its movement to the East and arrived to Catalonia in the afternoon, it being possible to see a weak CVA over the Gulf of Genoa which increased during the night (Fig. 5c). This situation gave strong winds at 500 hPa and, at 300 hPa, it was possible
to see jet-stream bifurcation as a consequence of the long wave trough. Diagnosis of FQ on 25 September at 1200 UTC showed that at low levels there was upward forcing over Levante (Spain), the Pyrenean region and the Southwest of the Iberian Peninsula, but downward forcing over the Catalan coast. FQ at 500 hPa showed a strong upward forcing near the Northwest of Spain, over the Atlantic low, and a secondary maximum between the Catalan coast and the centre of France. On 26 September at 0000 UTC the Mediterranean circulation at low levels appeared as two strong dipoles of FQ (Fig. 3d), the ®rst one situated between Morocco and the Balearic Islands and the second between the Balearic Islands and the Pyrenees, while over the Gulf of Genoa there was a downward forcing. Twelve hours later the downward forcing between the Balearic Islands and the Catalan coast had been absorbed by the most extensive upward forcing, which at 1200
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UTC spread from the North of Africa up to the Northwest of Spain, with a maximum over the Catalan coast. At 500 hPa a weak downward forcing was found over the region of upward forcing at low levels. At 0000 UTC 27 September (Fig. 4d) the greater upward forcing at low levels affected the entire Gulf of Lyon having extended to 500 hPa. This maximum at 850 hPa moved to the East and 12 hours later affected the Southeast of France, while a weak downward forcing was still present over the Gulf of Genoa. Finally, on 28 September, the upward forcement region situated over the Southeast of France affected the North of Italy and a new positive region was situated between the islands of Sardinia and Sicily, affecting the centre of Italy (Fig. 5d). 4.2 Instability Diagnosis and Moisture Flux Divergence Fields The space distribution of instability has been evaluated using the difference between the equivalent potential energy at 500 hPa and at 1000 hPa as an index which allows identi®cation of the areas with potential instability (Ramis et al., 1994, 1995; Llasat et al., 1996). On 26 September at 0000 UTC a zone with strong instability was spread along all the South of the Western Mediterranean (Fig. 6a), having a maximum over the Alboran Sea. At noon this instability zone affected all the East of the Iberian Peninsula, with a maximum centred over the Levante (Spain) region and a secondary maximum over the Gulf of Lyon. The next day the potential instability covered all the West Mediterranean Sea and its boundary coast from Catalonia to Italy (Fig. 6b). In the course of that day the region with instability moved to the East, and 24 hours later, on 28 September at 0000 UTC (Fig. 6c), it was bounded by the Tyrrhenian Sea and the Adriatic Sea. The beginning of this strong instability and its subsequent movement were related with the warm advection detected in the synoptic overview. The CAPE presented a space distribution similar to that of the potential instability, with values above 2000 J/kg every day and with maxima near the zones affected by the high rainfalls (Fig. 7a, 7b and 7c). Taking into account the wind direction and the satellite images (see next section), those maxima or their
Fig. 6. Equivalent potential temperature difference between 500 hPa and 1000 hPa at 0000 UTC a) 26 September 1992, b) 27 September 1992, c) 28 September 1992. Solid and dashed lines represent positive and negative values, respectively. Contour intervals of 4 C
possible localisation some hours after or before the 0000 UTC or the 1200 UTC, coincided with the ¯ood zones.
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Ramis et al. (1994). By 0000 UTC 25 and 26 September (Fig. 8a) the moisture convergence over the Mediterranean was centred over the Tunisian coast and the Alboran Sea. This second
Fig. 7. Convective Available Potential Energy at 0000 UTC a) 26 September 1992, b) 27 September 1992, c) 28 September 1992. Contour interval is 400 J/kg
The low level divergence/convergence of moisture was calculated for the level of 1000 hPa. This layer has been selected as the most representative one based on the results shown in
Fig. 8. Moisture divergence (full line) and convergence (dashed line) at 1000 hPa for 0000 UTC a) 26 September 1992, b) 27 September 1992, c) 28 September 1992. Contour interval is 2 mg mÿ 2 sÿ 1 hPaÿ 1
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convergence zone extended along the Spanish Mediterranean coast. Over the next 48 hours this zone of convergence moved Eastward to the Central Mediterranean following the same trajectory as the instability zones and the upward vertical forcing (Fig. 8b and 8c). 5. Satellite Image Processing On 26 September, the sequence of Meteosat IR images shows a cloud band with a minimum radiance temperature of the cloud top over the North of Spain and the Gulf of Lyon. The advection of vorticity and the movement towards the East deformed the cloud band. Ten hours later the cluster was situated over the Italian Peninsula. Figure 9 (a±d) show the Meteosat images in the IR band for a) 27 September 0130 UTC, b) 27 September 1130 UTC, c) 27 September 2300
UTC and d) 28 September 1130 UTC. It can be observed that in the early morning of the 27th the nucleus of the convective system was situated over the South of France. By investigation of the whole sequence of half-hourly Meteosat images it can be observed that the system became deeper as it moved towards the East and at 0530 UTC on the 27th a very cold nucleus is observed over the North of Italy, where it remained motionless, but extending its radius of action until the morning of the 28th. It should be noted that by 0900 UTC a second nucleus was born to the South of the ®rst, which then grew and ended up merging with the ®rst on the 28th. To show the stationary nature of the convective system, Fig. 10 presents the evolution of the MCS identi®ed by means of cluster identi®cation performed on the half-hourly Meteosat images in the IR band between 1900 and 2300 UTC.
Fig. 9. Meteosat images in the IR band for a) 27 September 0130 UTC, b) 27 September 1130 UTC, c) 27 September 2300 UTC and d) 28 September 1130 UTC
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Fig. 10. Cluster identi®cation for the sequence of half-hourly Meteosat images in the IR band from 1900 UTC to 2230 UTC of 27 September 1992
The Meteosat primary images on 27 September showed a larger area presenting radiance temperature values below 253 K, allowing the identi®cation of a typical Mesoscale Convective System (MCS) (it did not acomplished all the conditions needed for being a Mesoscale Convective Complex, proposed by Maddox (1980). This MCS moved quite rapidly from Southern France in a Southeasterly direction, driven by the outline of the orography that runs almost parallel
to the Tyrrhenian coastline of Italy, while the entire cloud system was moving slowly Eastwards. Figure 11 (after Boni et al., 1996) shows the total areal coverage of the MCS from 0130 UTC of 27th September to 2230 UTC of 28th September 1992. Results of the cloud tracking exercise in the case of the event of September 26±28th, 1992, are synthetically reported in the diagram of Fig. 12.
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Fig. 11. Total areal coverage of the MCS from 0130 UTC of 27 September to 2230 UTC of 28 September 1992. Colour levels represent classes of persistence of the cloud coverage over each pixel. (After Boni et al., 1996)
6. Composite Charts and Meteosat Images The main aim of this paper is to show the advantage of the use of the proposed composite charts to identify the zones which most favour the development of heavy rainfall. As it was proposed in previous papers (Ramis et al., 1994, 1995; Llasat et al., 1996), those zones were identi®ed by overlapping of the zones with positive vertical quasi-geostrophic forcing at 850 hPa, negative potential temperature difference between 500 hPa and 1000 hPa and moisture ¯ux convergence at 1000 hPa. In this paper, CAPE values above 1800 J/kg have also been considered. On the other hand, it is also necessary to take in account the wind at low levels over the area affected by the high rainfalls. Figure 13(a-g) show the zones of superimposition of the four ®elds from 25 September 1992 at 1200 UTC to 28 September at 1200 UTC, with a
time interval of 12 hours between each panel. The ®gures also indicate the direction of the dominant wind at steering level over the zone of superimposition. In the ®rst place, it is possible to observe throughout the entire episode three dominant zones: a very small one, related with the Atlantic low, over the West of the Iberian Peninsula which could be observed only on the 26th (1); a secondary zone over the North of Africa, mainly Tunisia and Libya (2); and ®nally, the main zone situated over the Mediterranean (3). These last two zones were ®rst detectable on 25 September at 12 UTC, the one situated over the Eastern Spanish coast being possibly related with the rainfall recorded in that zone. Twelve hours later, the small centres situated over Levante and France had disappeared, while the zone detected over the North of Morocco had moved towards the Balearic Sea. There also appeared at that time a zone of overlapping to the West of Portugal which moved towards the
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Fig. 12. Cloud tracking monitoring for the high rainfall event of September 27±28
interior of the Peninsula, and by 24 hours later had disappeared. At 1200 UTC of 26 September, both the Balearic nucleus and the Tunisian nucleus had moved Northwards, enlarging the affected area. If the direction of the predominant winds in the low and middle troposphere was taken into account and the overlapping area was conserved, it is possible that zone 3 could by the evening and night have affected the Northeast of the Iberian Peninsula and Southeast France,
where the main rainfalls were recorded during the evening and night of 26 September. Unfortunately, the lack of data between 26th September 1200 UTC and 27th September 0000 UTC made the corroboration of this hypothesis impossible. Monitoring of the 27th and 28th through the above ®gures allows a coincidence to be detected between the zones most favourable to unstabilization and the nucleus or nuclei of the convective system. There are some apparently favourable
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Fig. 13. Temporal evolution of the composite charts made by overlapping of the zones with positive vertical quasi-geostrophic forcing at 850 hPa, negative potential temperature difference between 500 hPa and 1000 hPa, moisture ¯ux convergence at 1000 hPa and CAPE values above 1800 J/kg (dotted zones); a) 1200 UTC 25 September 1992, b) 0000 UTC 26 September 1992, c) 1200 UTC 26 September 1992, d) 0000 UTC 27 September 1992, e) 1200 UTC 27 September 1992, f ) 0000 UTC 28 September 1992, g) 1200 UTC 28 September 1992. Shaded zones denote the zone of greatest vertical upward forcing (above 16 10ÿ 18 m kgÿ 1 sÿ 1)
zones in which hardly any convective cloudiness was detected (North of Libya, centre of the Iberian Peninsula, etc.), which was probably due to lack of the mesoscale lifting mechanisms needed to produce triggering of the latent instability, such as incidence perpendicular of low level winds to the coast and to the coastal mountain ranges. Figure 14a shows the superimposition and merging of the zones favourable to convection between the 25th at 1200 UTC and the 26th at the same time. The common zone permits diagnosis of the long duration of the conditions favourable to instability and the generation of
heavy rainfalls over a single region, which would enhance the degree of organisation of the convection necessary for generation of the MCS observed in the satellite images. Figure 14b provides the same type of information relating to the time interval between 0000 UTC of 27 September and 1200 UTC of the 28th. A shifting of the zone placed over the Balearic Islands towards the West can be observed and its merger with the zone placed near Sardinia. Another nucleus appeared in which the instability conditions persisted for more than 24 hours, coinciding with the nucleus of the area affected by the MCS of Fig. 11.
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Fig. 14. Integration of the overlapping zones between a) 25 September 1200 UTC and 26 September 1200 UTC, b) 27 September 0000 UTC and 28 September 1200 UTC
Fig. 13 (continued)
7. Conclusions Cloud tracking techniques are not ``physically based'' in the sense that no understanding of the meteorological evolution of the observed event is involved in the procedure. This resolves into quite good predictions of the short-term (1/2±2
hours) evolution of cloud systems, as the temporal sampling of the satellite sensor is far shorter than the scale of convection development and decay within the MCS. However, low predictive capabilities are observed in the long term due to the missing of any prognostic meteorological indicator by the cloud tracking techniques which essentially involve image processing algorithms. Meteorological analysis suffers from the lack of data, but provides a physical understanding of the process. This resolves into the perspective of constraining on the basis of some forcing parameter capable of improving the accuracy of medium-term prediction. In this paper, remotely sensed information and objective synoptic diagnosis have been used to identify areas where the main localisation and triggering processes took place during a well-
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monitored case study from both the traditional and remote sensing perspective. The overlapping of meteorological maps and satellite images has shown that the occurrence of an MCS in the Western Mediterranean region could be related to the presence of the four key factors selected (water vapour convergence at 1000 hPa, quasigeostrophic vertical forcing at 850 hPa, instability between 1000 and 500 hPa and CAPE values above 1800 J/kg) and to an intense ¯ow which impinges perpendicularly to the coast and the nearby mountain regions, triggering the potential convection. Synoptic analysis was possible only in the diagnosis stage within the case study analysed. However high resolution LAM outputs may be used to forecast meteorological scenarios that are prone to produce MCS formations over the target regions. The development of cloud tracking techniques that include constraining of the prediction of the MCS displacement and evolution to the synoptic conditions as determined by objective analysis is a promising perspective in the direction of providing more physically based methodologies for use in ¯ood forecasting applications. Acknowledgements This research was supported by the Commission of the European Communities, within the framework of the STORM project (Contract No. EV5V-CT92-0167), and Project FLOODAWARE (Contract No. ENV4-CT96-0293) and by the CICYT AMB95-0671. Partial support comes from DGICYT under PB94-1169-C02-02 grant. References Berenguer, J., 1993: Les dommages causes a l'agriculture en Vaucluse et dans le sud dromois le 22 septembre 1992. Rev. de Geographie de Lyon, 68(2±3), 171±174. Blanchet, G., Deblaere, J. C., 1993: L'episode pluvio-orageux catastrophique de septembre 1992 dans le sud-est de la France: analyse pluviometrique et meteorologique. Rev. de Geographie de Lyon, 68(2±3), 129±138. Bolla, R., Boni, G., La Barbera, P., Lanza, L., Marchese, M., Zappatore, S., 1995: The tracking and prediction of high intensity rainstorms. Remote Sensing Reviews, 14, 151± 184. Bolton, D., 1980: The computation of equivalent potential temperature. Mon. Wea. Rev., 108, 1046±1053. Boni, G., Conti, M., Dietrich, S., Lanza, L., Marzano, F. S., Mugnai, A., Panegrossi, G., Siccardi, F., 1996: Multisensor observations during the ¯ood event of 4±6 Novem-
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Ramis, C., Alonso, S., Llasat, M. C., 1995: A comparative study between two cases of extreme rainfall events in Catalonia. Surveys in Geophysics, 16, 141±161. Weisman, M. L., Klemp, J. B., 1986: Characteristics of isolated convective storms. In: Ray, P. S. (ed.) Mesoscale Meteorology and Forecasting, Amer. Meteor. Soc., pp. 331±358. Authors' addresses: Maria-Carmen Llasat, Department of Astronomy and Meteorology, University of Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain (e-mail:
[email protected]); Climent Ramis, Department of Physics, University of Balearic Islands, Ca. de Valldemossa, km 7.5, E-07071 Palma de Mallorca, Spain; Luca Lanza, Environmental Engineering Department, University of Genoa, Via Montallegro, 1, I-16145 Genoa, Italy.
