The evolution of an MCS over southern England. Part 2: Model simulations and sensitivity to microphysics

Abstract

AbstractSimulations using the Met Office Unified Model at 1 km horizontal grid spacing of a Mesoscale Convective System (MCS) with a cold pool which propagated across southern England on 25 August 2005 are validated using detailed observations from the Convective Storm Initiation Project (CSIP). Early organisation of the system is not especially well treated, but the model goes on to form a system which developed qualitative and quantitative features remarkably similar to the observations.A sensitivity study suggests that the initial linear system is driven by the position of a low‐level ‘lid’ and upper‐level instability, the linear organisation being promoted by a weak rear‐inflow jet forced by the upper‐level warm anomaly in the cloud anvil. A weak cold pool develops in the absence of ice‐phase processes, but this does not promote system propagation. Strengthening and descent of the rear‐inflow jet, and acceleration of the system, is promoted by the additional heating through glaciation and cooling through snow evaporation. The surface cold pool and gust front are further strengthened by snow melting and rainfall evaporation. With ice‐phase processes present, the cold pool strengthens as a result of the system development and its strength is broadly correlated with system propagation speed during the middle phase of the system's lifetime. Propagation enables the convective band to ‘sweep up’ any convective cells which trigger ahead of the system.The differing scales of microphysical processes means that it is difficult to form a steady‐state system. The observed transition phase corresponds to an increase in the slantwise nature of the flow in the storm and in the low‐level cooling by rainfall evaporation near the gust front, combined with a change in ambient conditions (advection over the sea) which eventually enable the cold pool to propagate ahead of the system.