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 (IWV) is proposed as a measure of potential for destabilisation of the air mass. The IWV 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 IWV 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.
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- "Synoptic, dynamic and thermodynamic characteristics that support severe convective conditions and describe the development processes of SCW have been examined in numerous studies even though there are many difficulties and uncertainties in monitoring and forecasting SCW (Moncrieff and Miller, 1976; Weisman and Klemp, 1982; Adler et al., 1985; Porcu et al., 1999; Li et al., 2004; Cohen et al., 2007). Some parameters are presented to depict the temperature and moist conditions of convective air parcel and the atmospheric instability indices are proposed to indicate the favourable conditions for convective development (Showalter, 1953; Colby, 1984; Woodall, 1990; Doswell and Rasmussen, 1994; Williams and Renno, 1993; Georgiev, 2003). "
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ABSTRACT: The paper presents results, which may add to the understanding of mid- to upper-level dynamics responsible for development of intense convection as seen by satellite images in water vapour channels. In addition to the well-known upper-level dynamic structures visible in the 6.2 μm imagery, typical moisture features associated with low- to mid-level thermodynamic conditions can be distinguished in 7.3 μm images. For example In 73 % of the most severe convective cases over southern part of Europe in 2004 and 2005, a mid-level jet (MLJ) is present at about 600 hPa in a south- westerly flow and appears as a specific boundary on the 7.3µm image grey shades. In these cases, the distinct MLJ boundary in 7.3 μm image grey shades is also a signature for presence of a low-level baroclinic zone -related to the MLJ origin -that plays a critical role in destabilisation of the atmosphere for intense convection. The use of 7.3 μm and 6.2 μm images in diagnosis of convection over Europe and the Mediterranean area is stressed. This approach is based on imagery analysis for identifying dynamic features responsible for initiation and maintenance of intense convection. The WV images are used to distinguish types of coupling between low-/mid-level conditions and upper-level dynamics associated with intense convective developments.
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ABSTRACT: Meteosat water vapour (WV) imagery and data from synoptic surface observations are used to infer the potential instability in a case study of a severe storm close to the Mediterranean coast. The use of the Potential Instability WV Index (I WV) introduced in a previous study is discussed. The I WV is calculated using a combination of two different kinds of data: synoptic station observations of surface moisture/temperature and HRI Meteosat WV data averaged in an area of 7×7 pixels around the synoptic stations. It is shown that the WV index might serve for assessing potential instability and providing early warnings in cases, in which the convection is initiated by lifting, which starts at the surface. It is not useful if the lift comes from a synoptic-scale disturbance as mid-and upper-level fronts or jet maxima and the low-level moisture is not a significant ingredient of the storm environment.