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

A series of mesoscale convective systems (MCSs) was observed in the UK during one of the Intensive Observation Periods (IOP 3) in the Convective Storm Initiation Project (CSIP). A detailed case-study of one of the MCSs, involving some novel interpretation of single Doppler radar data, reveals the evolution of stacked slantwise circulations associated with elevated upright convection. A feature of this occasion wasthe existencenear the surfaceof astrongundercurrent of cool air flowing against the direction of travel of the storm. The air feeding both the upright and slantwise ascent originated not from near the surface but, rather, from layers between 1 and 3 km. The upright convection was intense at first and was accompanied by a region of dry subsidence surrounding it, together with the alternating layers of slantwise ascent and descent within the storm's precipitation area. When the elevated upright convection weakened, vigorous slantwise motions continued for sometime, but their slope decreased from the initial 1 in 4 to 1 in 9. The lowermost layer of slantwise descent corresponded to a moderately intense rear-inflow jet. The rear-inflow jet did not penetrate the cool undercurrent and reach the surface; instead, beneath where it impacted the warm air capping the undercurrent, the undercurrent took on the structure of a gravity wave without stagnation. The raised head of the undercurrent lifted overlying air of high wet-bulb potential temperature sufficiently to overcome the convective inhibition.