Interactions between a Developing Mesoscale Convective System and Its Environment. Part II: Numerical Simulation

Abstract

The 19 July 1993 mesoscale convective system (MCS), discussed in Part I, was simulated using the Regional Atmospheric Modeling System (RAMS). The model was initialized with variable physiographic and atmospheric data with the goal of reproducing the convective system and its four-dimensional environment. Four telescopically nested, moving grids allowed for horizontal grid spacings down to 1.67 km on the cloud resolving grid. Comparisons with the analysis show that the propagation, evolution, and structure of this MCS were well simulated. The simulation is used to further investigate the interactions between this MCS and its surrounding environment. In Part I, the Doppler-derived winds indicated that upshear (westward) propagating gravity waves left upper-tropospheric front-to-rear and midtropospheric rear-to-front flow perturbations in their wake. A similar flow structure developed in the simulated MCS, and unlike the Doppler results, the low-frequency waves that produced it were resolved in the data. In the simulation, much of the convectively generated temperature and momentum perturbations propagated westward with the waves, leaving a warm wake in the clear air trailing the system. Although the gravity waves traveled rearward, the perturbation flow in their wake was not strong enough to reverse the upper-tropospheric storm-relative winds. Thus, most of the anvil condensate advected ahead of the convective line. As the MCS encountered the low-level jet, the midtropospheric upward mass flux increased, but gravity wave motions became less detectable. The upper-tropospheric anvil pushed westward into the strong flow as the system expanded into a characteristically oval shape. Temperature and momentum perturbations propagated rearward along with the anvil in a disturbance that resembled an advective outflow. Unlike the gravity waves, this disturbance became almost stationary with respect to the ground, and it retained its continuity through the rest of the simulation. Vertical cross sections indicate that a large slab of convectively processed air had detrained into the upper troposphere. Prior to this event, much of the warm temperature anomalies generated within the convective towers either remained in the updrafts, or propagated outward with the gravity waves. Early on, individual updrafts were relatively erect and although condensate did detrain eastward in the forward anvil, the temperature anomalies did not propagate with it. In contrast, convective updrafts associated with the expanding oval anvil disturbance were more continuous, and they tilted strongly westward with height.