Mid-level jet in intense convective environment as seen in the 7.3 μm satellite imagery
Résumé
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.