Mesovortex Circulations Seen by Airborne Doppler Radar within a Bow-Echo Mesoscale Convective System

Abstract

During the spring of 1991, scientists from the National Severe Storms Laboratory conducted a field observational program to obtain a better understanding of the processes responsible for organizing and maintaining the dynamical and electrical structure of mesoscale convective systems (MCSs), as well as mechanisms acting to organize and propagate the dryline. Extensive use was made of a relatively new observing tool, the airborne Doppler radar installed on one of the NOAA P-3 research aircraft, to map the precipitation and kinematic structure of large convective systems. The radar was operated in an innovative scanning mode in order to collect pseudo dual-Doppler wind data from a straightline flight path. This scanning method, termed the fore/aft scanning technique (FAST), effectively maps out the three-dimensional wind field over mesoscale domains (e.g., 80 km? 100 km) in ?15 min with horizontal data spacing of 1?2 km. Several MCSs were observed over central Oklahoma during May and June of 1991, and one such system exhibiting a ?bow-echo? structure is described. Many observed features of this MCS correspond to structures seen in nonhydrostatic numerical simulations. These features include a pronounced bulge or ?bow? in the convective line (convex toward the storm's direction of propagation), a strong descending rearinflow jet whose axis is aligned with the apex of the bow, and a cyclonic vortex (most pronounced at heights of 2?3 km) situated in the trailing stratiform region lateral to the axis of strongest rear inflow. Doppler-derived wind analyses reveal the likely role played by the mesoscale circulation in twisting environmental vertical shear and converging ambient vertical vorticity in maintaining and amplifying the vortex. The relatively detailed yet horizontally extensive airflow analyses also reveal the utility and advantages of airborne Doppler radar in the study of large convective systems.