The formation of a forward-tilting cold front with multiple cloud bands during superstorm 1993

Abstract

A mesoscale-model simulation is used to examine the evolution of the cold front and accompanying cloud bands in eastern Mexico associated with Superstorm 1993 (12-14 March). The simulated cold front differed in structure and evolution from a classical cold front, in agreement with evidence from observations and European Centre for Medium-Range Weather Forecasts analyses. The surface cold front, as defined by the leading edge of strong northerlics and cold advection, initially possessed a rearward tilt with height over southern Texas. Within 6 h, the leading edge of the front moved equatorward and developed a large-scale forward till of greater than 200 km in the horizontal from the surface to 700 hPa. This forward tilt occurred as a mid- to upper-tropospheric baroclinic zone arrived from over the Sierra Madre, descended into eastern Mexico, and interacted with the surface cold front. Embedded within this large-scale forward tilt was a locally enhanced horizontal potential temperature gradient that also tilted forward ∼ 100 km from the surface to 850 hPa. Tilting frontogenesis associated with ascent at the leading edge of the surface front was responsible for the smaller-scale forward-tilting structure. This surface-based ascent is believed to have caused the primary cloud band observed from satellite imagery that is coincident with the leading edge of the front, whereas a second region of ascent, elevated at the leading edge of the mid- to uppcr-tropospheric baroclinic zone, is believed to have caused the prefrontal cloud band revealed by satellite imagery. Subsidence behind the forward-tilting cold-frontal structure at and above 850 hPa (and concomitant divergence underneath) resulted in frontolysis of the surface front, and eventually the dissipation of the primary cloud band, leading to the dominance of the prefrontal cloud band. Finally, the Superstorrn 1993 cold front is compared and contrasted to nonclassical cold-frontal structures found in the literature and a general context for frontal interaction is discussed.