1.29 LIFE CYCLE OF CONVECTIVE CELLS WITH RAPID SCAN SATELLITE AND RADAR DATA IN THE EASTERN ALPINE REGION

Transcription

1 1.29 LIFE CYCLE OF CONVECTIVE CELLS WITH RAPID SCAN SATELLITE AND RADAR DATA IN THE EASTERN ALPINE REGION Friedrich Wölfelmaier 1, Veronika Zwatz-Meise 2 1 ZAMG, Regional center Styria, Graz, Austria, 2 ZAMG, Department for Synoptic Meteorology, Vienna, Austria 1. INTRODUCTION The understanding of the life cycle of convective cells is important for Nowcasting of these cells. In the following work 25 life-cycles of convective cells were analysed with a focus on the growing and mature stage. The full life-cycle was studied in the radar reflectivity. The case studies, all from the year 3, were selected mainly within the Austrian state of Styria, which lies in the southeast of the country and has a high frequency of thunderstorms. Some cases were chosen from other places in Austria. analysis was performed manually with the MAVIS visualisation system. Investigated parameters for the analysed convective cell: 2-D radar reflectivity (Maxcappi), 1-min resolution IR- Cloud top temperature (CTT) from rapid scan, 1-min resolution number of lightning flashes, 1 minutes resolution VIS brightness, 3 minutes resolution 2. METHOD Only cases with intense thunderstorm activity were selected for the analysis. As a selection criteria the radar reflectivity in the MAXCAPPI (maximum projection to the ground) had to exceed 54 dbz, which was the highest of seven intensity classes of the radar reflectivity colour scale in MAVIS (Met Austria Visualisation System). Lightning activity was also a necessary condition for the choice of the cases. Heavy thunderstorms are often organised in multi-cell storms. The selected cases contain mostly multi-cell storms and some single cell and supercell cases. For a multicell storm the life cycle of one individual cell was analysed. For about half of the cases local hail warnings were issued via SMS. The 3. Results 3.1 Convective cell life cycle phases in the radar reflectivity For the duration of the different life cycle phases, criteria were selected to define the growing phase, the maximum phase and the dissipating phase in the radar reflectivity. In the IR-temperature mainly the growing stage was studied. a) growing phase in radar reflectivity rise time: dbz - 54 dbz min 3- min 9- min rise time / min Figure 1: Frequency of radar reflectivity rise time from to 54 dbz during the growing phase * Corresponding author address: Friedrich Wölfelmaier, ZAMG, Flughafenstrasse 4, 73 Feldkirchen, Austria; f.woelfelmaier@zamg.ac.at

2 rise time: 3 dbz - 54 dbz min 4-6 min > 6 min rise time / min Figure 2: Frequency of rise time of radar reflectivity from 3 to 54 dbz during the growing phase b) maximum phase in the radar reflectivity duration of the maximum phase with 54 dbz (red) min -9 min > 9 min time / min Figure 3: Frequency of the maximum phase length in the with radar reflectivity c) dissipating phase in the radar reflectivity dissipating phase, 54 dbz - dbz % 7% 6% 5% 4% 3% % 1% % 1-3 min 4-7 min decay time Figure 4: Frequency of decay time of radar reflectivity from 3 to 54 dbz

3 3.2 CTT during the radar life cycle CTT at the beginning of the 54 dbz radar reflectivity Cloud top temperature at start of 54 dbz radar signal 6 4 Mean: 5,4 C < -52 C -52 to -37 C -37 to -27 C cloud top temperature Figure 5: Cloud top temperatures at the beginning of the 54 dbz radar signal When the cloud top temperature in the IR Satellite image goes below -52 C, the radar signal often exceeds an intensity of 54 dbz. This happens in 64 % of the analysed cases. In 28 % of the cases the radar signal exceeds 54 dbz, if the cloud top temperature falls below a value between 37 C and 52 C. In 8 % of the cases 54 dbz in the radar is reached, when the cloud top temperature lies between 27 C and 37 C Minimum CTT during the whole life cycle Minimum cloud top temperature during full life cycle 6 4 < -52 C -4 C cloud top temperature Mean: 6, C Fig. 6: Minimum cloud top temperature during the whole life cycle The minimum CTT during the whole life cycle falls below -52 C in 96% of the cases, only in one case (4 % of the cases) it is 4 C.

4 3.2.2 Temporal correlation between IR-temperature and radar life cycle Figure 7: Median of the convective cell lifecycle phases for radar with corresponding run of the IR temperature curve In Figure 7 the median of the radar life cycle phases is plotted together with run of the CTT curve. In the radar life cycle the median of the growing phase lasts 5 minutes, the phase with maximum reflectivity 4 minutes and the dissipating phase 25 minutes. The drop of the CTT from C to 53 C lasts 6 minutes in the median. In the developing stage of a cell the cloud top temperature falls rapidly. The early mature stage is marked, where the temperature curve becomes flat. In this transition of the cloud top temperature curve, the strongest radar signals begin to occur. The strongest radar reflectivities start 1 minutes after the time, when the IR-temperature curve of the cell becomes flat. During the time with very low ( -53 C) and rather constant CTT, repeated phases with strong radar signals can occur until the time when the CTT begins to fall. Sometimes the convective cloud associated with a radar echo is hidden by the higher cloud tops of a close by and further developed cell. In these cases it is only possible to analyse the CTT of a whole multicell system. 3.3 VIS-brightness maximum Radar reflectivity maximum The brightness maximum occurs mainly to 3 minutes after the radar reflectivity maximum time difference of occurence VIS maximum - Radar maximum 6 4 VIS before radar VIS after radar -3 - min -3 min 4-6 min time / min Figure 8: Time shift in occurrence of radar maximum and VIS-brightness maximum

5 3.4 Lightning frequency radar reflectivity The maximum lightning density is reached during the phase of the strongest radar signal. Within this phase it varies. 5. Acknowledgements This work has been supported by the INTERREG IIIB project METEORISK and ZAMG. Thanks are also due to the support of the remote sensing department of ZAMG. 4. Conclusions and Outlook From the analysed cases characteristic phases of the 2-D radar reflectivity lifecycle could be identified. The correlation of radar with CTT, VIS brightness and lightning frequency shows typical common patterns of the cell development. 6. References Auer, M., 2: Betrachtung konvektiver Systeme unter Zuhilfenahme von Meteosat-6 rapd scan während MAP-SOP, Master thesis, University of Vienna