1.27 SEVERE CONVECTION ENVIRONMENT IN SOUTHEASTERN ROMANIA

1. S band - WSR-98D Gematronik C-band: METEOR 500C EEC C-band: DWSR-2500C 1.27 SEVERE CONVECTION ENVIRONMENT IN SOUTHEASTERN ROMANIA Aurora Stan-Sion and Cornel Soci National Meteorological Administration, 97 sos. Bucuresti-Ploiesti 013686 Bucharest, Romania contact address: aurora.stan@meteo.inmh.ro Context and objectives The Weather Service in Romania recently acquired several Doppler radars. Immediately upon installation our forecasters faced new challenging issues in interpretation of Doppler radar data because storms in Romania have never before been observed with Doppler radar; hence it was not known how different the fields of Doppler velocity would be and if signatures of severe weather phenomena could be readily detected. Also, there was a general belief that tornadoes do not occur in Romania. To understand the effects of the specificity of Romanian storms on the Doppler observations required time and presence of adverse weather. After nearly 5 years of observations several of the severe storms attributes have been documented. Although this is a short time for statistical evaluations answers to questions such as what is the prevalent storm type that causes severe weather are beginning to emerge. It is confirmed that rotating storms (mesocyclones) do occur and some have produced tornadoes. Romanian National Radar Network The first observed tornado was at Facaeni (Aug 2002) after which tornado threat in Romania was recognized.Thus the supercell storms become "targets" for nowcasters. Several cases were challenges of recognizing severe weather patterns and environmental mesoscale factors that favored the onset of supercelular thunderstorms in southeastern Romania. It seems that there the environment is more conducive to severe convection than in other regions of the country. Based on ALADIN limited-area model, the mesoscale features which favored development of supercells are analyzed. In this study four major type of convergence lines are identified in the southern part of Romania; for each of them a case study is briefly analysed. • Individual site radar products are available every 6 minutes, depending on the radar and mode of operation. • Three types of national radar mosaics are produced every 10 minutes. • The available national radar mosaic products include first tilt base reflectivity, echo top and composite reflectivity. • All radar products may be made available in near real-time Convergence lines triggering severe convection in South of Romania C type:in a weak synoptic flow, a thermal low can develop in the Baragan Plain giving birth to an associated convergence line. Usually, this configuration is responsible for severe hailstorms (see the radar image below). D type:in a southern synoptic flow, the breeze front advances inland and the convergence line is of “dry line” type. The southern flow brings heat and moisture that produces high instability (see the CAPE field below). The south-eastern flow in low levels generates high values of vertical wind shear. These are the main ingredients for supercells in this region (see also the radar image below). Case 12 August 2002. A type: the flow around the S like-shape of Carpathians Mountains induces a convergence zone oriented on a North-South line. If the eastern flow prevails, a back-building squall line forms over the hilly region having mainly a southern oriented exposure (see the satellite image below). B type: the flow around the S like-shape of Carpathians Mountains induces a convergence zone oriented on a West-East line. If the western flow prevails, a convergence line forms in the Baragan Plain and severe convection can be triggered (see the radar image below). Reflectivity field at first tilt. The image shows a hook echo of a classic supercell. The Facaeni tornado (F3+) case, 12 August 2002. Radar data provided by the C-band radar operating in Bucharest (EEC radar type). In the background, the Danube river is represented with dark blue. NOAA-11 image illustrating the so-called A type convergence line. Other boundaries and convergence lines are present as well. 1312 UTC 21 August 1999 (courtesy Martin Setvak). Reflectivity field (left hand-side) at first tilt and vertical cross-section on A-B direction (right hand-side) as indicated in the left image. Case 19 May 2001. Radar data provided by the C-band radar operating in Bucharest (EEC radar type). Radial velocities (left hand-side) field and reflectivity field (right hand-side) at first tilt provided by the EEC C-band radar operating in Bucharest. Case 1320 UTC 30 June 2004. A supercell developed in the vicinity of the convergence line. MSLP (black solid line) and 10m wind field analysis valid at 12 UTC 21 August 1999. The red line illustrates the 800m orography height. The convergence line is plotted with brown. 50m wind field analysis valid at 12 UTC 30 June 2004 as provided by the ALADIN model. The convergence line is plotted with brown. MSLP and 10m wind field analysis valid at 12 UTC 19 May 2001 (ALADIN model). The isobares with values below 1015 hPa are plotted using red dashed lines. ALADIN analysis for the Facaeni tornado. Left hand-side: CAPE (black solid line) and MOCON (black dashed line). Right hand-side: 10 m wind field. The damaged villages are shown with red. The Romanian borders are marked with blue dashed line. C O N C L U S I O N S • The complex orography in South of Romania induces particular air flow which can initiate deep convection in specific mezoscale circulations. Among the specific features of the Romanian topography are: • the S like-shape of Carpathians that produces prevailing northern and eastern low level flow in Baragan Plain. • the Black Sea with an important breeze circulation enhanced in southern flows • a predominant southern oriented slopes of the high orography. Thus, the mountain breeze has an intense circulation, sustaining and intensifying the southern low level jet . • The previous features belong to PBL and can be detected using satellite and radar images and can be analyzed using NWP outputs such as low level wind fields. Other model outputs like CAPE and MOCON can alert the forecaster if a specific ingredient can be present. Thus, the nowcasting of severe weather depends on how fast the forecaster is able to recognize a specific pattern. • The discontinuity lines were known in Romania but their influence on the onset of deep convection was not understood. The four major convergence types described in this study represents the beginning of a deeper understanding of the triggering mechanisms in southeastern Romania.