The Lubbock tornado: an elevated mixed layer case study

Abstract

The synoptic and sub-synoptic environment preceding the Lubbock tornado, 11 May 1970, is examined using surface, upper-air, radar and limited satellite imagery data. The standard analyses are complemented by five vertical cross-sectional analyses to highlight the natural partitioning of the lower atmosphere. This allows a more explicit identification of the airmass structure over west Texas and the surrounding geographical region. The combined analyses of the airmass structure are then applied to a conceptual model (Carlson and Ludlam, 1968) of a severe local storm environment. In this study, the conceptual model components expressed a relationship between the synoptic scale circulation pattern and topography of the southwest United States, making it a favorable location for the development of severe storm or tornadic activity. Within the model, the elevated mixed layer, or lid (Carlson and Ludlam, 1968), is especially important.

This study examines the components of the conceptual model as they existed on 11-12 May 1970. The synoptic and mesoscale atmospheric features are documented and applied to the conceptual model. Specifically, the diurnal dryline cycle, as described by Schaefer (1973), was isolated by meso-analysis techniques with additional documentation through radar analyses. Next, the cross-section analyses depicted the vertical partitioning of an elevated well-mixed layer or "lid" (Carlson and Ludlam, 1968) overlying low-level moist air advection. Finally, the combined contributions were organized to develop composite analyses to emphasize the significant contributions that allowed a sudden, focused release of latent instability from beneath the lid.

From the case study, the environmental stratification generally conformed to the conceptual model. The lid formed due to advection of a deep, mixed layer from Mexico over moist air from the Gulf of Mexico. The wet-bulb potential temperature within the moist air increased significantly during the daylight hours on 11 May 1970, reducing the negative buoyant area contained beneath the lid. Next, the outbreak of deep, focused convection was limited to the area over Lubbock where dryline forcing dynamics combined with destabilizing horizontal advection at approximately 700 mb. The environmental shear and advection tendencies were analyzed with hodograph and isentropic techniques. Finally, the composite analyses of the situation prior to the tornado event depicted significant convergence and thermodynamic alignment (Moller, 1979) northeast of Lubbock; however, the tornado occurred in the southwest quadrant of the dryline bulge. This is where maximum differential advection of the capping inversion occurred and underrunning resulted.