Intellectual merit: The Verification of Origins of Rotation in Tornadoes Experiment #2 (VORTEX 2) is a multi-scale investigation of factors that lead to tornadogenesis. VORTEX 2 is a follow on to VORTEX 1 whose field phase was conducted during the Spring of 1994 and 1995. The VORTEX 1 advanced knowledge of the kinematic structures of tornadic and nontornadic storms and provided some hints as to the sensitivity of the evolution of supercell storms and tornadogenesis to very fine spatial scale heterogeneity. The VORTEX 2 community intends to extend and build upon the results of VORTEX 1. The research supported under this proposal addresses the stormscale circulations, and thermodynamic and precipitation variability that affect the development and distribution of vorticity in tornadic storms. The primary data set common to these research objectives are Doppler and polarimetric radar observations over the entire breadth and depth of the parent tornadic storm and its immediate environment. C-band radars, with their ability to both penetrate heavy precipitation and sense the clear-air inflow associated with supercell storms, are required. Although the Principal Investigators will cover the entire depth of the storm with two C-bands, frequent coordinated sampling of the lowest 3 kilometers will be made with a X-band dual-polarimetric radar to examine the microphysics and dynamics of the rear-flank downdraft and angular momentum beneath the mesocyclone. These three radars will be tightly coordinated to provide the storm-scale circulation and precipitation structure needed to place finer-scale observations from other instruments into the context of larger-scale storm and environmental flow.
The scientific objectives of this research are to study: (i) vorticity dynamics associated with low-level rotation in supercells, (ii) the role of angular momentum transfer on tornadogenesis (iii) precipitation and thermodynamics of the rear-flank downdraft, (iv) the impact of environmental heterogeneity (including boundaries) on storm evolution, (v) the role of cell mergers on tornadogenesis, (vi) the evolution of polarimetric signatures within tornadic and non-tornadic supercells, and (vii) improvements in the analysis and forecasting of tornadic storms through the use of ensemble Kalman-filtering data assimilation.
Broader impacts: According to the 1997 Workshop on Societal and Economic Impacts of Weather, tornadic storms account for about 73 fatalities each year and have an average economic impact of about $700M. While lead times associated with tornado warnings have increased with the National Weather Service (NWS) Doppler modernization and forecaster training program, false alarm rates have essentially remained flat. The lack of improvement in false alarms reflects lack of understanding of the physical processes that can cause or limit tornadogenesis. The research conducted here will improve fundamental knowledge, enabling better conceptual models and improved forecasting ability. The improved data assimilation and numerical weather prediction techniques will aid forecasters in their efforts to better warn the public. The independent dual-polarization and multiple-Doppler derived wind fields will aid in storm process and model validation studies. The dual-polarization data will provide an early indication of polarimetric signatures associated with tornadogenesis, if they exist, which will maximize the benefit of the dual-polarimetric upgrade of the NWS operational radar network.
The datasets will also be used in university classes from freshman orientation to graduate level term projects. Computer-based teaching modules will be developed and made available to the community.
According to the 1997 Workshop on Societal and Economic Impacts of Weather, tornadic storms account for ~73 fatalities each year and have an average economic impact of ~$700M. While lead times associated with tornado warnings have increased with the National Weather Service (NWS) Doppler modernization and forecaster training program, the false alarm rates have remained flat. The lack of improvement in false alarms reflects a lack of understanding of the physical processes that cause or limit tornadogenesis. The research conducted here was aimed at improving fundamental knowledge, enabling better conceptual models and improved forecasting ability to better warn the public. The first part of the project involved a large data collection effort called the second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2) sponsored by the National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA). We fielded two C-band Shared Mobile Atmospheric Research and Teaching (SMART) Radars and the newly developed National Severe Storms Laboratory’s (NSSL) X-band mobile radar (Figure 1). More than two dozen severe storms, many of which were tornadic, were observed by the mobile radars. Data from the mobile radars were used to examine the internal structure and evolution of tornadic and non-tornadic storms. Figure 2 shows the radar structure and airflow relative to the moving storm (left column) and vertical air motion with contours of "vertical vorticity" overlaid (right column) at three times during the life of a tornadic storm observed over central Oklahoma. Vertical vorticity is related to the spinning motion of air and indicates the strength of the circulation. The fastest spin is located in the center of the contours and denoted by an "x" in panels (d-f). At the earliest time (top row Fig. 2), the vertical vorticity field was elongated and relatively weak even though the storm had a pronounced hook-echo radar structure (Fig. 2a). A rear-flank downdraft (RFD, panel d) occurred along the southern end of the elongated vortex. This RFD acted to organize and strengthen the circulation, as evidenced by the analysis at 001929 Z (second row, Fig.2). An occlusion downdraft (denoted by arrows in panels e, f) formed in response to the strengthening vorticity. At the same time, an updraft formed along the northern edge of the vorticity center, as shown by the orange area just above the x in panel f. With time, the updraft grew and intensified as it rotated around the center of circulation (Fig. 3). By 003939 Z (second row of Fig. 3), the circulation was embedded in updraft, which resulted in rapid intensification (panel 3f) and aided the development of a tornado. To provide greater insight into smaller-scale circulations within the hook-echo region, a novel trajectory mapping method was developed. Times series of radar analyses were used to compute trajectories of air motion backwards in time to determine their starting position. Trajectories were initiated at every grid point in the analysis domain. By mapping the prior altitude of air at a particular time, it was possible to estimate which part of the circulation contained buoyant low-level air and which part contained cooler, less buoyant air from the rear-flank and occlusion downdrafts. An example of the trajectory mapping analysis is shown in Figure 4. The colors refer to the source altitude of air about eight minutes prior to the analysis time. The contours show observed vertical vorticity at the analysis time. Since the figure is for the 5 km level at 004500 Z, warm colors indicate regions in which air came up from lower altitudes while cool colors indicate air from the same or higher altitude. The center of circulation is embedded in likely buoyant air that came up from lower levels. But, as indicated by the thin region of blue wrapping around the circulation from the northwest, the circulation is surrounded by a narrow ribbon of air that came down from a higher altitude. A similar evolution occurred in a severe supercell thunderstorms observed near Greensburg, KS (Fig. 5). However, unlike the previous case, this storm did not later produce a tornado. As in the earlier case, a rear-flank downdraft aided in the organization of the low-level vertical vorticity and the circulation became embedded in upward motion (Fig. 5a, b). The circulation intensified (Fig. 5c, d). But a sudden outflow surge emanated from the hook-echo region (Fig. 5e, f) and undercut the circulation, leading to its demise (Fig. 5g, h). The outflow was observed to be strongly negatively buoyant, indicating that the thermodynamic nature of the storm’s environment is another factor that must be considered in determining the likelihood of tornadogenesis. The research conducted here also led to educational and public outreach activities (Fig. 6) to foster interest in science and engineering needed to ensure development of the next generation of meteorologists.
