The relation between nocturnal MCS evolution and its outflow boundaries in the stable boundary layer: An observational study of the 15 July 2015 MCS in PECAN

The vertical structures of a leading outflow boundary ahead of a continental nocturnal MCS and of the upstream environment are examined in order to answer the question of whether this vertical structure affects new cell formation and thus MCS maintenance. The MCS in question, observed on 15 July 2015 as part of the Plains Elevated Convection at Night (PECAN) experiment, formed near sunset as a surface-based, density current-driven system. As the night progressed and a stable boundary layer developed, convection became elevated, multiple fine lines became apparent (indicative of an undular bore), and convection increasingly lagged the outflow boundary. Bore-like boundaries became most apparent where the outflow boundary was oriented more perpendicular to the low-level jet, and the lower troposphere was more susceptible to wave trapping. This case study uses a rich array of radiosonde data, as well as airborne Raman lidar and ground-based interferometer data, to profile the temperature and humidity in the lower troposphere. In all soundings, the lifting of air in the residual mixed layer over a depth corresponding to the Raman lidar observed vertical displacement reduced CIN to near zero and enabled deep convection, even though most unstable CAPE steadily decreased during the evolution of this MCS. Both types of outflow boundaries (density currents and bores) initiated convection that helped maintain the MCS. In the case of density currents, cold pool depth and wind shear determined new cell formation and thus MCS maintenance. For bore-like boundaries, bore transformation and propagation were additional factors that determined whether convection initiated and whether it contributed to the MCS or remained separated.