VERSILIA FLOOD: A PREFRONTAL SUPERCELL SYSTEM EMBEDDED IN WEAK FLOW IMPINGING ON NEAR COASTAL MOUNTAIN RANGE

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Emery Hawkins
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1 VERSILIA FLOOD: A PREFRONTAL SUPERCELL SYSTEM EMBEDDED IN WEAK FLOW IMPINGING ON NEAR COASTAL MOUNTAIN RANGE (1)P.Frontero, (2)P.Binder, (1)S.Pugnaghi, (1)L.Lombroso, (1)R.Santangelo, (3)A.Buzzi, (4)T.Paccagnella, (4)A.Selvini (1) Dip.Sci.Ing. Sez. Osservatorio Geofisico Univ. Modena, I (2) Swiss Meteorological Institute, Zurich, CH (3) FISBAT CNR Bologna, I (4) Serv.Met.Reg. Bologna, I Abstract A deep and stationary convective event occured over Versilia (a part of the Tuscany region, Italy) on 19 June 1996 producing heavy precipitation during twelve hours (maximum accumulated value 478 mm). It initially formed at the coastline and its growth was supported by lifting of the weak flow towards the Apuane Alps. Cases of this type highlight the need for accurate and spatially highly resolved observations for analysis and prediction. We collected non standard data and performed special numerical simulations with different LAMs to improve investigation of a case which produced many injuries to the local people. Introduction The Versilia event (Italy) on 19 June 1996 was a severe convective case leading to heavy precipitation. The major synoptic feature was a cold front approaching from the north, but which was still on the northern side of the Alps during the critical time period. The far influence of this cold front in combination with the prevailing W SW warm advection over the Mediterranean in the lower part of the troposphere created the prerequisite conditions for the supercell thunderstorm to develop. The synoptic forcing in this case was very weak, more important was the meso scale flow and the convection enhanced by local orography. Only few observations from the GTS network are available in the area of the meso scale phenomenon. The Versilia event represents a convergence divergence field case that needs a better comprehension by means of models and this is one of the main scientific objective of MAP. The heavy precipitation system exhibited characteristics similar to other events [Frontero et al., 1995], [Frontero et al., 1996] seen in the same area. These cases highlight the need for more high resolution observations in this area, also to have better initial conditions for LAMs. Synoptic situation The synoptic situation at 500 hpa on 19 of June shows a ridge with axis from Algeria up to Brittany. A geopotential minimum is found over southern Sweden. This setting produced a northwestern flow regime over Italy with a wind speed of about 10 m/s. The curvature of the geopotential field was mainly anticyclonic. Temperature over Northern Italy at this altitude was about 15 C due to cold advection. At 850 hpa warm advection from W SW over the sea led to instability [Berliner Wetterkarte, 1996]. The surface pressure field was well levelled around 1015 hpa with a light cyclonic curvature. The lower part of a cold front over central Europe reached the northern Italian regions. As a whole, the synoptic situation, does not exhibit spectacular features. Precipitation From the measurements of the meteorological stations in the Versilia area it can be seen that precipitation began at 3:00 UTC on June 19 and lasted until 18:00 UTC producing a maximum of accumulated precipitation of 478 mm at the Pomezzana rain gauge station. Around Pomezzana St, in an area of about 10x20 km 2 (see the rectangular area in Fig. 1a) the accumulated precipitation exceeded 100 mm. Outside this area the accumulated precipitation decreased rapidly and at the nearby synoptic stations of Pisa, Firenze and Perugia precipitation amounts of: 0.0, 0.6 and 0.0 mm were observed respectively. The event had two intensity maxima, the first between 5 and 6 UTC and the second around 13:00 UTC of minor intensity. file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 1/10

2 It is interesting to know that in this region the annual precipitation has a local maximum of about 4000 mm, due to its particularly sensitive position linked to the local orography. This is the value measured at Mt. Pania (1800 m), inside the rectangular area of Fig. 1a. The gradient of precipitation towards the coast is very sharp. On the coast, only 14 km away from Mt. Pania, the annual accumulated precipitation is mm. Case study The cell was triggered over the sea, close to the coast of La Spezia, at about 1:00 UTC. It was observed by the NOAA and METEOSAT satellites. The satellite images show an initial eastward displacement toward the Apuane Alps; the cell had a circular shape in this phase. Then it was oriented SE along the Apennines' ridge and a clockwise rotation became apparent. The hodograph had the same rotation while the supercell grew instigating new cells at its right flank [Bluestein, 1993]; this can be seen on both satellite images and by the precipitation distribution. In spite of the elipsoidal of the cloud system in the second phase, the precipitation continued above the same area. NOAA AVHRR images taken at about 13:30 UTC in channel 1 (visible) and channel 4 (thermal infrared) show that a great part of the cloud system was a sort of plume [Levizzani et al., 1996], since the high level cloud were blown off in the upper wind direction. Under this plume there was no precipitation. By means of the TeraScan software package, using TIR channels, we determined the SST. A difference of about two degrees resulted between the Ligurian sea (21.5 C) and the Thyrrhenian sea (23.5 C). With the same package TOVS data were analyzed to obtain geopotential, wind, temperature and dew point temperature fields at standard pressure levels. The minimum of geopotential at 850 hpa southeast of the Apuane Alps (Fig. 1b) corresponds to a well defined downstream lee depression [Carlson, 1991] and the dew point temperature shows a dry air nucleous in the same area. The background of both Fig. 1a and 1b is the brightness temperature obtained by AVHRR channel 4; bright and dark colours indicate warm and cold surface respectively. We collected all the radiosoundings performed around the region of interest: Ajaccio, Cagliari, Nimes, San Pietro Capofiume (Bologna) and Milano, all of them show no significant instability while the considered LAMs: BOLAM (FISBAT CNR, Italy), LAMBO (SMR, Italy) and SM (SMI, Switzerland) show different instability characteristics for this area. The SM profiles over the Ligurian sea show in the first two or three model layers a very stable (i.e. temperature inversion) saturated boundary layer. This inversion is maintained by the warm W SW advection, previously quoted, of potentially very unstable but dry air and by the relatively cold sea surface. This gives the characteristic shape of the profile over the sea (see Fig. 2a). The profiles over the land have the same main structure (see Fig. 2b), but here no capping inversion exists and some lifting at the coast and the mountain behind is exerted on the flow. Quite an impressive picture is given by the 10 m wind (see Fig. 3a). The low level flow exhibits significant convergence towards the Versilia coast. This feature is connected to a distinct vorticity pattern. Flow splitting in the stable marine boundary layer is seen around Corsica. The northern branch of the split flow converges with the flow coming down the Rhone vally and turning to the east. However, this relevant flow characteristic can not be observed by the SYNOP stations on the coast. BOLAM identified the same low level flow pattern as the SM. At 850 hpa westerly flow can be seen in Fig. 3b. The model wind speed at this altitude is in fair agreement with the mountain station observations. An other relevant aspect is derived by the analysis of the cross sections of the horizontal wind speed (see Fig. 4a) and of omega, connected to the vertical motion [Kurz, 1990] (see Fig. 4b). In the W E cross section of Fig. 4a a core of wind speed of 20 m/s at about 950 hpa is clearly visible over the Ligurian sea. It is a kind of low level boundary layer jet. Fig. 4b shows a reduction of stability between 900 and 800 hpa close to the Versilia coast and to the right of the low level jet; this reduction is of course associated with an ascending vertical motion. Behind Apennines, positive values of omega indicate descending air motion (foehn effect). This in part explains the geopotential pattern of Fig. 1b. Finally a synthetic description of the event is given by the windvector and moisture convergence analysis shown in Fig. 5 (map obtained at about 250 m a.g.l.). A significant moisture convergence reaches its maximum value in the area where there was the maximum of precipitation. In the Tiedke parameterization scheme for convection, as used in the SM, this quantity is crucial for the onset and maintenance of convection; the 24h maximum precipitation amount forecasted by the SM for the period in question was 155 mm (most precipitation forecasted of the three considered LAMs). Conclusion The supercell was triggered over the sea, near the coast and grew above the Versilia area, producing a maximum of 478 mm during twelve hours. The key factors leading to the event were: 1) the vertical structure of the atmosphere with its blocked boundary layer over the sea and significant instability potential aloft (large CAPE values); file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 2/10

3 2) the low level jet formed by the convergence of the flow deflected around Corsica and the Alps leading to moisture convergence and lifting onshore. The geometry of the flow is mainly driven by the local topography. Synoptic forcing in this case was very weak and therefore the local and mesoscale conditions were responsible for the outbreak of the severe convection. Finally we remark a lack of vertical observations in this sensitive area (Gulf of Genoa); this was a factor reducing the reliability of the LAM forecasts. Aknowledgement Observed precipitation data, from the station network of the italian Servizio Idrografico e Mareografico Nazionale, have been made available for research purposes by the Italian Department for Civil protection. We thank Matthias Jaeneke (Deutscher Wetterdienst) for the information and F. Parmiggiani (IMGA CNR, Italy) for the NOAA images. Reference Berliner Wetterkarte, Institute fur Meteorologie der Freie Universitat Berlin, Germany, 1996 Bluestein H.B., 1993 Synoptic dynamic meteorology in midlatitudes, Volume I II Oxford University Press, Carlson T.N., 1991 Mid Latitude Weather Systems HarperCollins Academic 1991 Frontero P., L. Lombroso, S. Pugnaghi, R. Santangelo, 1995 November 1994 Piedmont flood nowcasting contribution MAP Newsletter n 3, 1995 Frontero P., L. Lombroso, S. Pugnaghi, R. Santangelo,A. Buzzi, T. Paccagnella, P. Binder, 1996 Heavy precipitation over the Ligurian area: The case of 4 6 October 1995 MAP Newsletteter n 5, 1996 Kurz M.M., 1990 Methods of synoptic diagnosis, DWD Zentralamt, 1990 Levizzani V., M.Setvak, R.A.Rabin, C.A.Doswell III, P.K.Wang, 1996 Storm top structure as seen from NOAA AVHRR imagery: A need for interpretation MAP newsletter n 5, 1996 file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 3/10

Figures Fig. 1a: The dark shade over the sea is the cloud system during its early stage at about 2 UTC on 19 June 1996.

4 Figures Fig. 1a: The dark shade over the sea is the cloud system during its early stage at about 2 UTC on 19 June The rectangle shows the area with accumulated precipitation greater then 100 mm. The spot inside the rectangle indicates the station where there was the maximum precipitation of 478 mm. file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 4/10

Fig. 1b: TOVS geopotential contour lines at 850 hpa. The minimum to the southeast of the Apuane Alps corresponds to a downstream lee depression. Fig.

5 Fig. 1b: TOVS geopotential contour lines at 850 hpa. The minimum to the southeast of the Apuane Alps corresponds to a downstream lee depression. Fig. 2a: Characteristic thermodynamic profile as seen by the Swiss Model (SM) over the sea west southwest of Versilia: 19 June UTC (right) and 06 UTC (left). file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 5/10

3a: SM predicted (+6h) wind field at 10 m for 19 June 1996

6 Fig. 2b: Same as Fig. 2a but for a Versilia land point. Fig. 3a: SM predicted (+6h) wind field at 10 m for 19 June UTC. Note the convergence offshore Versilia. file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 6/10

7 Fig. 3b: Same as Fig. 3a but for the 850 hpa wind field. Flow splitting around Corsica is almost absent. file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 7/10

8 Fig. 4a: Vertical cross section at about 44 N. Equivalent potential temperature (solid lines, every 2 degrees) and wind speed (shading and bold dashed isolines for 10, 13 and 16 m/s) as predicted by the SM (19 June UTC + 6h). The mountain range to the left (west) are the Maritime Alps, the ridge to the right (east) are the Apuane Alps (Apennines). file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 8/10

9 Fig. 4b: Vertical cross section at about 44 N cutting the Apuane Alps. Equivalent potential temperature and vertical velocity (omega). Upward motion is indicated by dashed lines. file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 9/10

Fig. 5: Wind field and moisture convergence at about 250 m a.g.l. (SM 19 June 1996 00 UTC + 6h).

10 Fig. 5: Wind field and moisture convergence at about 250 m a.g.l. (SM 19 June UTC + 6h). Maximum value in the Versilia region is s 1. file:///e:/luca%20archivio/ufficio/siti%20web/htdocs/papers/00463.html 10/10

A Cyclogenesis south of the Alps. Manfred Kurz Neustadt/Weinstraße

A Cyclogenesis south of the Alps Manfred Kurz Neustadt/Weinstraße A cyclogenesis south of the Alps Surface maps 06-11-99, 00 UTC The occluded frontal system of a quasi-stationary low above the North Sea