The evolution of an MCS over southern England. Part 1: Observations

Observations are described of a Mesoscale Convective System (MCS) with a cold pool which propagated across southern England on 25 August 2005. The observations were made as part of the Convective Storm Initiation Project (CSIP). The observed MCS structure broadly followed well-established theories, including the presence of a weak rear-inflow jet. In detail, however, unsteady transitions occurred involving the formation of two distinct lines of showers ahead of the initial linear system. In each case the cold pool merged with cold downdraughts from the new showers leading to a discontinuous propagation of the system. One of these lines formed independently of the MCS, very probably on a sea-breeze convergence line. The mechanism for formation of the other is unknown, but it is possible that it was triggered by ascent associated with gravity waves generated by the MCS. The merged cold pool was deeper and colder and propagated faster than the original system, eventually forming a bow echo and arc cloud as it propagated across the English Channel. Until completion of the merger, the propagation velocity of the overall system had been controlled by a combination of the above mechanisms rather than simply by cold pool dynamics.