Simulations of an observed elevated mesoscale convective system over southern England during CSIP IOP 3

Simulations of an elevated mesoscale convective system (MCS) observed over southern England during the Convective Storm Initiation Project (CSIP) provide the first detailed modelling study of a case of elevated convection occurring in the UK. The study shows that many factors can influence the maintenance of elevated deep convection, from large-scale flow through to surface heating processes and diabatic cooling within the convective system. It is also shown that interactions and feedback mechanisms between a stable layer and the storm can act to maintain deep convection. The simulation successfully reproduced an elevated MCS above a low-level stable undercurrent, with a wave in the undercurrent linked to a rear-inflow jet (RIJ). Convection was fed from an elevated (840hPa) source layer with CAPE of about 350Jkg-1. The undercurrent in the simulation was approximately 1km deep, about half that observed. Unlike the observed MCS, a transition from elevated to surface-based convection occurred in the simulation due to the combined effects of a pre-existing large-scale θe gradient, advection and surface heating causing the system to encounter increasingly unstable low-level air and a shallower stable layer that was more susceptible to penetration by downdraughts. The transition to surface-based convection was accompanied by the development of cold-pool outflow and an increase in system velocity from about 6 to 10ms-1. Diabatic cooling from microphysical processes in the simulation enhanced the undercurrent and strengthened the RIJ. This strengthened the wave in the undercurrent and led to more extensive convection. The existence of a positive feedback process between the convection, RIJ and stable layer is discussed. Uncertainty in the synoptic scale generating errors in the undercurrent is shown to be a major source of error for convective-scale forecasts

Elevated convection and castellanus: Ambiguities, significance, and questions

The term elevated convection is used to describe convection where the constituent air parcels originate from a layer above the planetary boundary layer. Because elevated convection can produce severe hail, damaging surface wind, and excessive rainfall in places well removed from strong surface-based instability, situations with elevated storms can be challenging for forecasters. Furthermore, determining the source of air parcels in a given convective cloud using a proximity sounding to ascertain whether the cloud is elevated or surface based would appear to be trivial. In practice, however, this is often not the case. Compounding the challenges in understanding elevated convection is that some meteorologists refer to a cloud formation known as castellanus synonymously as a form of elevated convection. Two different definitions of castellanus exist in the literature-one is morphologically based (cloud formations that develop turreted or cumuliform shapes on their upper surfaces) and the other is physically based (inferring the turrets result from the release of conditional instability). The terms elevated convection and castellanus are not synonymous, because castellanus can arise from surface-based convection and elevated convection exists that does not feature castellanus cloud formations. Therefore, the purpose of this paper is to clarify the definitions of elevated convection and castellanus, fostering a better understanding of the relevant physical processes. Specifically, the present paper advocates the physically based definition of castellanus and recommends eliminating the synonymity between the terms castellanus and elevated convection