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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ABSTRACT: Using a case study of a severe convective event as an example, a framework for interpreting 6.2 µm channel satellite imagery that enables to indicate upper-level conditioning of the convective environment is presented and discussed. In order to illustrate the approach, all convective cells during the summer of 2007 that produced precipitations over Bulgaria are considered. They are classified regarding the observed moisture pattern in mid-upper levels as well as the low-level conditions of air humidity and convergence of the flow. Water vapour (WV) images are used to study the evolution of the upper-level moist and dry structures. The proposed interpretation is that the role of the upper-level dry boundaries identified in the WV imagery as favoured areas for the initiation of deep moist convection cannot be understood (and hence cannot be forecasted accurately) by considering them in isolation from the dynamic rate at which they are maintained.The paper examines the 23 June 2006 flash flood in Sofia city as a case, in which the operational forecast of the National Institute of Meteorology and Hydrology of Bulgaria based on the mesoscale NWP model ALADIN underestimated the severity of the convective process. A comparison between the satellite water vapour imagery and the corresponding geopotential field of the dynamical tropopause, expressed in terms of potential vorticity (PV), shows an error in the performance of the ARPEGE operational numerical model. There is an obvious mismatch between the PV anomaly structure and the dry zone of the imagery. The forecast field shows underestimation of the tropopause height gradient and displacement of the PV anomaly to the southwest of the real position seen in the satellite image. It is concluded that the observed poor forecast is a result of the ARPEGE failure to treat correctly the interaction between the PV anomaly and the low-level warm anomaly.Atmospheric Research 07/2009; 93(1):295-303. · 2.20 Impact Factor
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ABSTRACT: The paper presents results that shed some more light on the mid- to upper-level dynamics, responsible for development of intense convection, as seen by satellite images in the water vapour channels. The study has also the ambition to help forecasters to improve their abilities in nowcasting strong convective events.In addition to the well-known upper-level dynamic structures visible in the 6.2 µm imagery, typical moisture boundaries related to mid-level jet streams can be distinguished in 7.3 µm images. About 20 cases of severe convection developing over southern Europe between 2004 and 2007 were studied. In 80% of the cases, a mid-level jet (MLJ) is present at about 600 or 700 hPa in a south-westerly flow. In these cases, the distinct MLJ boundary in 7.3 µm image grey shades is a signature for the 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.Images in the 7.3 µm and 6.2 µm channels are used to detect coupling between low- and mid-level conditions dynamics associated with intense convective developments. As a tool for water vapour imagery analysis in diagnosing this context, “dynamic wind shift”−defined as difference in the position of mid- and upper-level jets over a short distance, seen in the WV images−is considered. Two types of intense convective developments over the Mediterranean are distinguished, associated with “smooth” and “sharp” dynamic wind shift conditions, upstream of the zone of intense convection.Atmospheric Research - ATMOS RES. 01/2009; 93(1):277-285.