The influence of atmospheric bores on nocturnal convection initiation in weakly forced synoptic environments

Abstract

This study creates a composite sounding for nocturnal convection initiation (CI) events under weakly forced conditions and utilizes idealized numerical simulations to assess the impact of atmospheric bores on these environments. Thirteen soundings were used to create this composite sounding. Common conditions associated with these weakly forced environments include a nocturnal low-level jet and a Brunt-Väisälä frequency of 0.011 s−1 above 900 hPa. The median lift needed for parcels to realize any convective instability is 490 m, the median convective-available potential energy of these convectively unstable parcels is 992 J kg−1, and the median initial pressure of these parcels is 800 hPa. An idealized numerical simulation was utilized to examine the potential influence of bores on CI in an environment based on the composite sounding. The characteristics of the simulated bore was representative of observed bores. The vertical velocities associated with this simulated bore was between 1 and 2 m s−1, and the net upward displacement of parcels was between 400 and 650 m. The vertical displacement of air parcels has two notable phases: lift by the bore itself and smaller-scale lift that occurs 100 – 150 km ahead of the bore passage. The pre-bore lift is between 50 and 200 m and appears to be related to low-frequency waves ahead of the bores. The lift with these waves was maximized in the low- to mid-troposphere between 1 km and 4 km AGL, and this lift may play a role in assisting CI in these otherwise weakly forced environments.