4 th European Conference on Severe Storms 10 - 14 September 2007 - Trieste - ITALY NOWCASTING SEVERE STORMS IN THE CENTRAL AREA OF THE STATE OF SÃO PAULO WITH THE AID OF TITAN Ana Maria Gomes and Gerhard Held Instituto de Pesquisas Meteorológicas, Universidade Estadual Paulista, Bauru, S.P., Brazil ana@ipmet.unesp.br; gerhard@ipmet.unesp.br (Dated: September 12, 2007) I. INTRODUCTION Until recently very little was available at IPMet to be deployed as a nowcasting tool, once there was no automatic procedure for detecting and tracking potentially severe cells and to forecast their evolution and displacement. Through a collaborative effort with scientists from the National Center for Atmospheric Research (NCAR), the software system TITAN (Thunderstorm Identification, Tracking, Analysis and Nowcasting; Dixon and Wiener, 1993) was made available to analyze radar data information from both radars operated by IPMet. The implementation in IPMet´s computer system was done with the assistance of NCAR staff that helped with all necessary computer routines to be adapted for direct access to the data format generated by both Doppler radars (Kokitsu, 2005). The results that will be presented here are part of the project validation of the severe events in the central area of the State of São Paulo in compliance with some objectives of the SIHESP (Sistema Hidrometeorológico do Estado de São Paulo) project. The main objective of the study is to verify the potential of the new tools available with the TITAN system and then to transfer the results to the operational sector of IPMet. Results related to a severe event that occurred on 17 October 1999, causing extensive damage by hail, will be presented as an example. II. DATA AND METHODOLOGY The Doppler radars are located at Bauru (Lat: 22.36°S, Lon: 49.02°W, 624 m amsl) and 240 km further west at Presidente Prudente (Figure 1). The main charac- teristics of the radar are: 2° beam width and ranges of 450 km for surveillance and 240 km in volume scan mode (11 elevations: 0.3° to 34.9°), 1 km radial and 1° azimuthal resolution, and temporal resolution of 15 min or less, recording reflectivity, radial velocity and spectral width. TITAN was used in the ARCHIVE mode and the tracking properties form the base for the analysis here. A TITAN cell was defined by the 40 dBZ threshold for the reflectivity with a minimum volume of 50 km 3 , observed at least in two volume scans (15 minutes). For all storms complying with or exceeding the adopted criteria for storm properties, the hail metrics, as well as the forecasting of its evolution, were determined. Severe Storm Parameter Indicator Besides tracking and nowcasting the movement of storm cells, TITAN has incorporated algorithms that allow identification of potentially severe storm “signatures”, such as hail metrics, to produce probability of hail (POH), based on Waldvogel et al. (1979). This implies that hail occurs always when the 45 dBZ reflectivity is present at 1.4 km or more above the freezing level. Another parameter, FOKR (Foote-Kraus) index, also related to hail-producing storms, was developed by Foote et al. (2005) and first applied to hail-producing storms in Argentina. Also used for the analysis here is the SSS (Storm Structure Severity) index, developed by Visser (2001) for hailstorms on the South African Plateau. FIG. 1: Doppler radar network of IPMet (BRU = Bauru; PPR = Presidente Prudente), showing range rings for 240 and 450 km. III. DISCUSSION AND RESULTS Storm Overview The severe event on 17 October 1999 developed and evolved in an environment under the influence of a baroclinic system reaching the State of São Paulo. A squall line with multicellular storm characteristics, showing several intense cells forming ahead of the frontal disturbance, with an extended trailing stratiform area, can be seen in Figure 2. FIG 2: Composite reflectivity field for the Bauru and Presidente Prudente radars, 240 km range, on 17/10/1999, 11:22 LT. The ellipses show the areas in excess of 40 dBZ identified and tracked by TITAN (yellow: past history of the storm; blue: present; red: nowcasting for the next 60 minutes). Hail Event of 17 October 1999 The most intense cell, labeled A, located south of Marilia (M) at 11:22 LT, is shown in Figure 2 with its trajectory and forecast for the next 60 minutes, highlighting the areas with reflectivities in excess of 40 dBZ. The cell A, that produced the hailstorm had an average speed of 60 to 65 km.h -1 , with reflectivities in excess of 60 dBZ, reaching a