Dusty gust fronts and their contributions to long-lived convection in West Africa/

Abstract

To model and predict the behavior of West African storms and mesoscale convective systems (MCSs), we must understand the life cycle of gust fronts, which invariably accompany thunderstorms and often initiate them. In this thesis, I track 40 gust fronts observed during summer 2006 by the MIT radar in Niamey, Niger and characterized with ground station measurements. A novel technique is developed using satellite infrared observations to track these fronts' propagation over a much longer distance than the <80 km enabled by radar; gust fronts are shown to propagate over >1000 km (mean 750 km) and up to 24 hours, much further than has been previously demonstrated for large numbers of gust fronts. These gust fronts are often embedded in mesoscale convective systems (MCSs). It is shown how MCSs can be tracked in satellite imagery and lightning locations from a VLF intercontinental radio network are analyzed to yield valuable information about the long-range propagation of MCSs, including the most common kind of West African MCS - the westward-moving squall line. An automated method is developed to quantify lightning within an MCS (using a Lagrangian method to follow the storm). Continuous "stripes" of lightning activity, caused by squall lines, emerge in lightning Hovmollers over West Africa and are substantially longer than the typical wavelength of an an African Easterly Wave (AEW). These stripes are used to study the relationships among MCS development, extent, and propagation distance: MCSs with greater squall line lengths propagate greater distances on average, but no evidence is found to show that larger, deeper systems propagate any faster than smaller systems, contrary to results in the literature. Evidence is shown that in many cases continuity within a lightning stripe is mediated by gust fronts. MCSs were found to propagate distances greater than an AEW wavelength, but only in the absence of an AEW; it is shown that AEWs were absent for many key weeks in summer 2006.