A Climatology of supercells in Romania

Transcription

1 A Climatology of supercells in Romania Bogdan Antonescu, Daniel Carbunaru, Monica Sasu, Sorin Burcea, and Aurora Bell National Meteorological Administration, Sos. Bucuresti-Ploiesti 97, Bucharest , Romania 1. Introduction Bogdan Antonescu Supercells in Romania can be severe and cause significant damage and loss of life. Forecasters have difficulty forecasting their initiation, severity and associated weather. Conceptual models, based on radar, satellite and synoptic observations have been developed for convective storm initiation (Stan-Sion and Antonescu, 2006). These have provided invaluable guidance for forecasters. But Romanian forecasters are still challenged to recognize environmental mesoscale factors that favor the onset of supercellular thunderstorms, mainly in southeastern Romania, where the environment tends to be more conducive to severe convection than in other parts of the country. In this paper, for the first time in Romania radar climatology of supercells for south-eastern Romania (Dobrogea) was developed, in order to understand the spatial and temporal distribution and the frequency of prolific severe report of supercells in this region. 2. Data and methods A five years storm supercells climatology was developed for) for south-eastern Romania. This climatology includes the initiation point, the track of the storm, the termination point and also the duration of the supercells. The cases were selected from the convective season (May September) between 2003 and 2006, using data collected by WSR-98D radar installed at Medgidia (Ioana et al., 2004) (Fig. 1). Using geographic information systems (GIS) the radar-based supercell climatology was developed into a geographic database that enable a powerful spatial analysis and future use of the dataset (Hocker and Basara, 2008). For the case selection a similar procedure whit that developed by Hocker and Basara (2008) was used: 1. the storm initiation was determined when the mesocyclon, with difference in azimuthal velocity > 7 m s -1. was observed at the first radar tilt; 2. the centroid coordinated were collected every 6 min; 3. the duration of storm was greater than 30 min; 4. initiation of the supercell was determined when the first reflectivities values greater 40-dBZ were observed; 5. the last radar scan with an observed mesocyclon was considered as the termination of supercell. Following this procedure approximately 282 supercells were identified during 2003 and Results 3.1 Monthly distribution of supercells The monthly percentage of supercells is shown in Fig. 1. About 60 % of the supercells were observed in July August. The minimum values for flash counts are recorded in September (10 %) and May (14 %). Hocker and Basara (2008) found that for 943 supercells observed between 1994 and 2003 in Oklahoma (USA), the supercells totals increased significantly from March (~ 6%) to April (~ 20%), followed by a strong peak in May (~ 35%), for August and September supercells were infrequent (~ 3%). 3.2 Number of days with supercells The monthly distribution of the number of days with supercells (Fig. 2) shows a similar distribution with the monthly variation. This indicates that supercells are in general isolated events, rather than outbreaks.

2 FIG. 1. Monthly variation of the supercells between 2003 and The monthly percentage of the annual number of supercells for each year is represented on the ordinate. FIG. 2. Monthly supercells days variation between 2003 and The monthly percentage of the annual number of supercells for each year is represented on the ordinate. 3.3 Diurnal variation supercells Figure 3 portrays the diurnal variation of the supercells with a resolution of 60 min, as a percentage of the total number supercells recorded during the study. The maximum of the supercells activity occurs at 1400 UTC and a secondary maximum at 1800 UTC. The supercells cycle to be related to the diurnal cycle, with thunderstorms developing in the afternoon when solar heating is at maximum.

3 FIG. 3. Diurnal distribution of supercells from 2003 to 2006, with a time resolution of 60 min. The hourly percentage of the total number of supercellsis represented on the ordinate. 3.4 Spatial distribution of supercells initiation point The spatial distribution of the initiation point of supercells is presented in Fig. 4. The density map is based on dividing the study area into a grid with 20-km cells. The results show that the main activity region is situated in the eastern and western part of Bucharest city and also in the vicinity of the Black Sea (Fig. 4). FIG. 4. Spatial distribution of the initiation point of all supercells observed during the study period ( ), overlaid with the mean track (grey vector) and mean initiation location (organge circle).

4 3.5 Duration of supercells The time between the initiation and the termination point of the supercells is represented in Fig. 5. In general supercells have a life time between min (~ 65%). Fig. 5 Duration of supercells with a resolution of 30 min. 3.6 Supercells track direction In Fig. 6 is represented the number of supercells observed during the study period as a function of direction. In general, supercells (~ 25%) are moving from south-west toward north-east. Fig. 6 Diagram depicting the number of supercells observed between 2003 and 2006 as a function of direction.

5 3.6 Mean monthly variation of the initiation point of supercells The mean monthly initiation point of supercells in moving toward inland north-west between May and July and then toward Black Sea from August to September. Fig. 6 Mean monthly variation of the initiation point of supercells observed between 2003 and Conclusions In this paper the spatial and temporal characteristic of supercells in south-eastern Romania were analysed, the results include de following: About 60 % of the supercells were observed in July August. The minimum values for flash counts are recorded in September (10 %) and May (14 %). The maximum of the supercells activity occurs at 1400 UTC and a secondary maximum at 1800 UTC. The initiation point of supercells is situated in the eastern and western part of Bucharest city and also in the vicinity of the Black Sea. In general, supercells (~ 25%) are moving from south-west toward north-east. References Ioana, M.,V. Ivanovici, E. Cordoneanu, D. Banciu, A. Apostu, and B. Ford, 2004: SIMIN The integrated system for meteorological surveillance, forecast and alert in Romania. Preprints, 20th Int. Conf. on IIPS for Meteorology, Oceanography, and Hydrology, Seattle, WA, Amer. Meteor. Soc., [Available online at paper_70954.htm.] Hocker, James E., Jeffrey B. Basara, 2008: A Geographic Information Systems Based Analysis of Supercells across Oklahoma from 1994 to J. Appl. Meteor. Climatol., 47, Stan-Sion,A., and B.Antonescu, 2006: Mesocyclones inromania Characteristics and environments. Preprints, 23rd Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc., 9.4. [Available online at pdf.]