Book Description

Relatively little is known about how terrain affects convective storms. Many studies acknowledge the possible influence of terrain, but it is difficult to say how any particular storm is affected by terrain; this is largely because we cannot know how a particular storm would have evolved in the absence of terrain. In order to better understand how terrain affects storms and their ability to produce tornadoes, the effects of terrain on both storm environments and storm dynamics should be studied. The work presented here investigates both the influence of terrain on storm environments in northeastern Alabama (an area of considerable interest in some recent studies) and the effects of terrain on the tornadogenesis process in storms more generally. Storms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective storm environments remains limited. HRRR data are used in this study to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased. The influence of terrain on tornadogenesis is highly uncertain. Most observations of storms that produce tornadoes in complex terrain attribute the tornadogenesis to near-storm environment changes. However, these observations fail to give insight into changes to storm dynamics that may affect tornadogenesis. To assess how complex terrain may affect storms, 72 different simulations are generated in which a 250 m tall isolated hill is placed at different locations within the domain. These storms are objectively clustered according to when, where, and if tornadoes are generated. Over half of the storms fail to make tornadoes or make very brief tornadoes. Storms that do not make tornadoes have lower near-surface circulation and weaker vertical accelerations in low-levels of the storms than those that make tornadoes, which fits with current conceptual models of tornadogenesis. The storms that fail to make tornadoes are scattered throughout the domain, but the hills placed farthest south and west in the domain seem to be associated with lower rates of tornadogenesis. In a hill relative sense, tornadoes typically form to the south and east of the isolated hill. The north side of the hill contains lower storm-relative wind shear, which may lead to fewer storms producing tornadoes as they track north of the hill. Composite means are generated for storms that travel south and north of the hill, both for tornadic and nontornadic storms. Storms that travel south of the hill consistently have more buoyant air in the supercell's forward flank and have more circulation near the surface 5 minutes prior to tornadogenesis. Small changes in the track of the storm relative to the hill can have large outcomes in terms of the storm's ability to generate tornadoes. These changes are not easy to anticipate for any given hill position, but when viewing the ensemble as a whole, it is clear that the hill has some effect on where storms tend to produce tornadoes.