The Mesoscale Structure of a Nocturnal Dryline and of a Frontal–Dryline Merger

Abstract

A horizontal gradient in moisture, termed the dryline, is often detected at the surface over the southern Great Plains of the United States during the spring and early summer. The dryline exhibits distinct diurnal variations in both its movement and structure. Recent research has focused on dryline structure during the afternoon and evening, particularly showing how strong (?1?5 m s?1) ascent frequently creates an environment favorable to the initiation of convection, quite close (within ?10 km) to the dryline interface. To date, however, there have been very few detailed analyses of the dryline interface at night, so that the nocturnal behavior of the interface predicted by theory and numerical studies is relatively poorly evaluated. In this study, special observations taken by a Doppler lidar, serial rawinsonde ascents, and a dual-channel microwave radiometer are utilized to describe the behavior of a nocturnal dryline observed on 12?13 May 1985. The analysis presented here reveals that the mesoscale structure of the nocturnal dryline prior to the formation of deep convection is a gently sloping, slow-moving interface. The movement of the dryline at night was related to the evolution of the low-level jet within the moist air. Wavelike structures and evidence for vertical mixing were observed in the moist air as low Richardson numbers occurred below the height of the jet. The previously discussed strong ascent is largely lacking in the present nocturnal case so that the circulations inherent to an undisturbed dryline at night are far less favorable for the initiation of deep convection than in the afternoon and early evening. In the present case, severe convection developed as a weak cold front approached and merged with the nocturnal dryline and the environment rapidly destabilized. Between soundings taken 2.5 h apart, the convective available potential energy increased from 524 to 3417 J kg?1 and the absolute value of the convective inhibition decreased from 412 to 9 J kg?1. The vertical shear of the horizontal wind also dramatically increased with time, so that the bulk Richardson number was within values normally associated with supercell convection. The timescale of the changes in stability and in the moisture field (?1?2.5 h) has implications for the type of observing network needed to nowcast severe convection and for assessing the performance of research and operational numerical models.