Use of data from Meteosat water vapour channel and surface observations for studying pre-convective environment of a tornado-producing storm

Abstract

An alternative to the upper air sounding approach is used for assessing potential instability in the environment of a tornado-producing storm on 15 May 1999. The storm developed over a mountain area of the most southern part of Bulgaria located close to the Mediterranean coast. Hourly High Resolution Image (HRI) data in water vapour (WV) channel of Meteosat are used to identify the continuously decreasing of mid- and upper level humidity over the upstream area of the tornado location within 9 h prior to the severe weather event. During the same period, three hourly data from six synoptic stations (altitude range: 140–1920 m) showed increasing of temperature and humidity of the low-level air mass around the area of subsequent development of the convective storm. A new quantity referred to as Potential Instability WV Index ( I WV) is proposed as a measure of potential for destabilisation of the air mass. The I WV uses a combination of two different data sources: thermodynamic parameters calculated from surface observations at synoptic stations; HRI Meteosat WV data (representative for water content in the middle and upper troposphere) averaged in an area of 7×7 pixels around the synoptic stations. Nine hours prior to the tornadic event, high and continuously increasing values of I WV are observed at the upstream area of the tornado release point where the pronounced ‘C’-shaped dark zone appeared in the imagery. The proposed WV Index is used in this study to reflect the potential instability in the pre-thunderstorm environment having moist surface air capped by a deep mid- to upper-tropospheric dry layer.