Mesoscale Modification of Synoptic Situations from Thunderstorm Development as Revealed by ATS III and Aerological Data

Abstract

Synoptic and dynamic aspects of the development of the tornado-producing thunderstorms of 23 April 1968 were studied by combining conventional observation data with ATS III pictures, with special emphasis on the interaction between the thunderstorms and the large-scale field. Results of the analysis revealed a significant modification of the moisture, thermal and wind fields in the vicinity of the area where storms developed. The formations of the large-scale cloud band and moist tongue were due to the mesoscale convective disturbances that developed along the frontal zone. It was concluded that the formation of a mid-tropospheric warm core accompanied by a strong upper outflow was the result of a process of so-called convective warming. Results of the analyses suggested that both the jet stream and the core of positive vorticity to the left of the jet stream were formed by the increased thermal gradient to the left of the warm core. It was inferred that the modified thermal and vorticity fields caused downward motion and produced the dry area to the rear of the storm area. Another characteristic aspect brought out by the analyses was the intensification of a low-level jet stream. The downward convective transport of the horizontal momentum in the shear flow intensified the low-level jet. The low-level jet increased the convergence of the water vapor flux ahead of the jet, thus maintaining the activity of the thunderstorms.