Downdraft-Generated Gravity Waves and Their Role in the Maintenance of Nocturnal Elevated MCSs

The genesis and propagation of elevated nocturnal mesoscale convective systems (MCSs) are not well understood or simulated in numerical models. The Weather Research and Forecasting (WRF) model is used here to investigate whether downdrafts within an MCS can impinge on the stable nocturnal boundary layer (SNBL), creating gravity waves within the MCS that can couple with the convection, maintaining the MCS. A real-data simulation of a nocturnal elevated MCS that occurred on 9-10 August 2014 was performed. NEXRAD imagery showed that this storm produced a prominent bore ahead of the system that was maintained for several hours. The bore generated a series of gravity waves, which appeared to initiate short-lived convection in their wake. The simulation was initialized using NAM and uses a 200 m moving nest to resolve the small-scale convective processes within the system. About 15 km behind the convective line, the stable nocturnal boundary layer (SNBL) is compressed by a downdraft immediately above it; just ahead of this, a wave is present which results in a decrease in potential temperature of about 1 K and upward vertical velocities of about 2 m s-1. The exact depth of the impression on the SNBL varies from about 200-400 m, velocity of downdraft varies from about 0.5 to 2 m s-1, and the amplitude of the wave is about 300 m. These features are consistently present over the period of approximately one hour. Sensitivity tests removing the effects of latent heating and precipitation drag are used to identify their role on the wave generation and maintenance. The role of the wave in maintaining convection will also be discussed.