1Institute of Atmospheric Physics ASCR - Czech republic
2Czech Hydrometeorological Institute - Czech republic
Instability indices as well as other quantities used as convective event precursors are useful though not sufficient tools for meteorologists when forecasting deep convection. They can be easily calculated from diagnostic and prognostic data sets and they can be well physically interpreted. There are number of studies assessing the predictive ability of these precursors (see e.g. Peppler, Lamb, 1989 or Huntrieser et al., 1997). In the Czech territory several instability indices were evaluated (see e.g. Strachota, 1966; Táborský, 1966, Řezáčová, Motl, 1990) with thunderstorm occurrence identified by SYNOP reports. At present it is possible to use radar data as area information about precipitation occurrence including flood rainfall and hail. Moreover there is a 30-year data series of aerological data from Praha - Libuš station that is long enough to evaluate statistical properties of precursors.
The aim of the work presented in this contribution was to statistically assess the set of thermodynamic and dynamic precursors of severe convection, above all the occurrence of severe convective precipitation. Predictive ability of precursors was studied in connection with predictands based on radar measurements.
The set of precursors was assembled according to the selection of suitable quantities from the work Huntrieser et al., 1997. It was extended by some specific indices used in the Czech weather service. The set of 25 precursors was selected. This set consisted of 12 thermodynamic indices, 5 quantities connected with convective available potential energy (CAPE started from the surface and various convective levels and CIN value) and 2 quantities characterizing the wind shear. Several basic convective levels (LCL, CCL, LFC, EL) were added to the set. Several quantities are experimentally calculated from actual aerological data by the program, created within the framework of this work.
The set of predictands based on radar measurements from the years 1998 - 2001 was created in the next step. Maximum reflectivity values were divided into three categories and for each of them additional variables characterizing the area extent and duration of the event were evaluated. Based on the comparison of this set with the values of precursors the probabilities of convective events occurrence for individual classes of precursors were evaluated.
Besides the connection with radar data the precursors were also tested on significant cases of downbursts, tornadoes and strong hails during the years 1998 - 2001. The work was concentrated mainly on CAPE - vertical wind shear relationship.
The contribution summarizes the results of three domains:
• Statistical distribution of the values of precursors depending on the year and day period and their correlation.
• Conditional probabilities of categories and their combinations for individual intervals of the values of precursors.
• The application of precursors calculation and derived probabilities on situations when severe convective events occurred.
The work was supported by the project GAČR 205/00/1451 that is focused on severe convective events on the Czech territory.
Huntrieser, Schiesser, Schmid, Waldvogel, 1997:Wea. Forecasting, 12, 108-123
Peppler, Lamb, 1989:MWR, Vol. 117, No. 6, 1156-1180
Strachota, 1966 (in Czech):Met. Zpr., 19, 105
Táborský, 1966 (in Czech):Met. Zpr., 19, 164
Řezáčová, Motl, 1990:Studia geoph. Et geod., 34, 147 - 166