Structure and evolution of a mesoscale convective system near the British Isles

Abstract

SUMMARYFrequent digital cloud imagery from Meteosat and precipitation patterns from radar have been analysed together with surface observations and conventional and satellite soundings, to develop a conceptual model of a small mesoscale convective system. The system, observed to the south-west of England, had many of the features previously found in the United States, where the term mesoscale convective system or complex is applied to a cluster of thunderstorms that generates a common cirrus shield of mesoscale dimensions. A mesoscale region of fairly uniform, essentially stratiform, rain developed beneath the cirrus shield downwind of the active convection. Thunder was widespread throughout the precipitation area which was also characterized by a surface mesohigh. The mesoscale convective system, and another similar one before it, lasted for about half a day each, during which time they produced large falls of rain over an extensive area. The mesoscale convective systems developed in a region of locally strong baroclinicity on the eastern flank of a cut-off low. They happened to form over the sea and were decoupled from the surface by a cool easterly boundary layer flow. Their development could be traced to the onset of advection from France of warm moist air with a wet-bulb potential temperature in excess of 18°C at 850mb. The arrival of this air coincided with the intensification of the SE-ly flow around the approaching cut-off low.

Observations of dual slantwise circulations above a cool undercurrent in a mesoscale convective system

“…Analyses of such events have revealed that the moist downdraughts sometimes do not penetrate to the surface through the large-scale low-level layer of cool air, e.g. Browning and Hill (1984) for a MCS in the UK, and Fortune et al (1992), Smull and Augustine (1993) and Trier and Parsons (1993) for large MCSs (Mesoscale Convective Complexes) in the USA. This was true also in the present case.…”

Cloud systems leading to flood events in Europe: an overview and classification

“…The phenomenology of extratropical cyclones and frontal systems is well depicted and understood in terms of conceptual models for higher European latitudes, and mesoscale convective systems have also been studied in the north Atlantic. Browning & Hill (1984) studied the development and evolution of a mesoscale convective system near the British Isles, in terms of Meteosat imagery, radar data and conventional meteorological soundings, and developed a conceptual model that recognises four stages of evolution. Local scale thunderstorms were studied in north-west Europe by Collier & Lilley (1994) addressing the initiation forecasting problem.…”

Analysis of a Small, Vigorous Mesoscale Convective System in a Low-Shear Environment. Part I: Formation, Radar Echo Structure, and Lightning Behavior

“…Mesoscale convective systems (MCSs), that is, systems of cloud and precipitation that include at least one thunderstorm cell during most of their lifetime (Zipser 1982), occur in a continuous spectrum from small, shortlived systems (e.g., Browning and Hill 1984;Knupp and Cotton 1987;Yuter and Houze 1995a) to long-lived mesoscale convective complexes (MCCs) exhibiting an average maximum area 1 of approximately 160 ϫ 10 3 km 2 and a lifetime of about 13 h (e.g., Maddox et al 1982). Figure 1 presents area and lifetime values for 161 MCCs observed over the United States for the 1985-87 period.…”