Airflow patterns in two mesoscale convective systems

Abstract

Single Doppler observations from an operational 5-cm radar of two mesoscale convective systems show mesoscale kinematic structures of the order of tens of kilometers. Two cases observed during routine operational scanning illustrate the presence of various low-level jets and their relationship with precipitation. One case illustrates dual rear jets associated with a mesoscale convective system (MCS) observed on 17 July 1986. The MCS developed on a wave along a warm front and was in steady state for much of the observation period. This wave was not resolvable in the synoptic data. Primary features were a leading narrow convective rain band 20 km in width, a low reflectivity transition zone 30 km in width, with a trailing region of rain which included a bright band at a height of 4 km. From the operational PPI radial velocity images, the patterns showed three-dimensional kinematic structure which included a mid-tropospheric rearward flow and two “dry” rear inflow jets. These jets were deduced to flow toward the rear of the convective band and then diverge with one descending anti-cyclonically and the other ascending cyclonically. These jets were below the bright band found in the trailing rain region. A “warm” cross-flow was also observed ahead of the convective line. A second case illustrates a MCS that evolved much more rapidly. The presence of the rear jet preceded the full development of the trailing rain region. These two MCS cases illustrate not only the complex three-dimensional structure of these systems which are very difficult to interpret for the operational nowcaster but also the mesoscale analogy to the conveyor-belt model of synoptic precipitation systems.