Radar, profiler, and radiometer analysis of the effects of multiple bores/solitary waves on the stability of the NBL and associated CI

Atmospheric bores may form when a density current, often produced by other convection, impinges on the stable nocturnal boundary layer (NBL) (e.g., Crook 1988; Rottman and Simpson 1989). It has been shown that atmospheric bores may destabilize the boundary layer through mixing or permanent upward displacements (e.g., Koch et al. 1991), and the lifting may also aide in convective initiation (e.g., Locatelli et al. 2002). Solitary waves or solitons may also destabilize the NBL (e.g., Rottman and Einaudi 1983). In this study, multiple bores and solitary waves, initiated by density currents from various convective systems and arriving from different directions, were observed over northern Alabama during the evening hours of 26 June 2008. The wave features passed within 50 km of the UAH ARMOR dual-polarimetric radar, and were also sampled by the UAH Mobile Integrated Profiling System (MIPS), including a 915 MHz wind profiler and a 12-channel microwave profiling radiometer (MPR). The main purpose of this study is to quantify the vertical displacements of air parcels by the wave features, and the thermodynamic changes resulting from the destabilizing effects of these features as they pertain to convective initiation. One of the wave features being examined was associated with prolific convective initiation (CI).

Doppler radar reflectivity and velocity data show the bores and waves propagating in the NBL. The 915 MHz wind profiler detected the vertical and horizontal motion perturbations associated with the passing wave features. A time-to-space conversion was performed on the most vigorous wave feature, assuming that the feature was two-dimensional. This allowed for flow trajectories through the wave feature to be calculated, that showed significant vertical displacements (some over 1 km) and vertical spreading of the parcel trajectories within the lowest 2 km AGL. MPR potential temperature calculations showed the vertical spreading of isentropes in two wave features, indicating a decrease in static stability (in some cases, N decreased by more than 50%). Convective parameters including CAPE, CIN, LCL, and LFC were also calculated using MPR temperature and humidity measurements. These measurements clearly showed a decrease in CIN and a lowering of the LFC behind the main wave feature. This decrease in stability and convective inhibition, perhaps aided by the vertical motion associated with the wave, resulted in CI behind the main wave feature.