Rear-flank Downdraft Research Articles

Direct Surface Thermodynamic Observations within the Rear-Flank Downdrafts of Nontornadic and Tornadic Supercells

Despite the long-surmised importance of the hook echo and rear-flank downdraft (RFD) in tornadogenesis, only a paucity of direct observations have been obtained at the surface within hook echoes and RFDs. In this paper, in situ surface observations within hook echoes and RFDs are analyzed. These “mobile mesonet” data have unprecedented horizontal spatial resolution and were obtained from the Verifications of the Origins of Rotation in Tornadoes Experiment (VORTEX) and additional field experiments conducted since the conclusion of VORTEX. The surface thermodynamic characteristics of hook echoes and RFDs associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and RFDs are favorable (or unfavorable) for tornadogenesis. Tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy, potential buoyancy (as measured by the convective available potential energy), and equivalent potential temperature in the RFD increase, and as the convective inhibition associated with RFD parcels at the surface decreases. It is hypothesized that evaporative cooling and entrainment of midlevel potentially cold air may play smaller roles in the development of RFDs associated with tornadic supercells compared to nontornadic supercells. Furthermore, baroclinity at the surface within the hook echo is not a necessary condition for tornadogenesis. It also will be shown that environments characterized by high boundary layer relative humidity (and low cloud base) may be more conducive to RFDs associated with relatively high buoyancy than environments characterized by low boundary layer relative humidity (and high cloud base).

Mobile Mesonet Observations on 3 May 1999

Abstract Two long-lived tornadic supercells were sampled by an automobile-borne observing system on 3 May 1999. The “mobile mesonet” observed relatively warm and moist air, weak baroclinity, and small pressure excess at the surface within the rear-flank downdrafts of the storms. Furthermore, the downdraft air parcels, which have been shown to enter the tornado in past observational and modeling studies, were associated with substantial convective available potential energy and small convective inhibition. The detection of only small equivalent potential temperature deficits (1–4 K) within the downdrafts may imply that the downdrafts were driven primarily by nonhydrostatic pressure gradients and/or precipitation drag, rather than by the entrainment of potentially cold environmental air at midlevels.

Hook Echoes and Rear-Flank Downdrafts: A Review

Abstract Nearly 50 years of observations of hook echoes and their associated rear-flank downdrafts (RFDs) are reviewed. Relevant theoretical and numerical simulation results also are discussed. For over 20 years, the hook echo and RFD have been hypothesized to be critical in the tornadogenesis process. Yet direct observations within hook echoes and RFDs have been relatively scarce. Furthermore, the role of the hook echo and RFD in tornadogenesis remains poorly understood. Despite many strong similarities between simulated and observed storms, some possibly important observations within hook echoes and RFDs have not been reproduced in three-dimensional numerical models.

Observations of Tornadoes and Other Convective Phenomena with a Mobile, 3–mm Wavelength, Doppler Radar: The Spring 1999 Field Experiment

In the spring of 1999 a field experiment was conducted in the Southern Plains of the United States, during which a mobile, millimeter-wavelength pulsed Doppler radar from the University of Massachusetts, Amherst, was used by a storm-intercept team from the University of Oklahoma to collect data in tornadoes and developing tornadoes. With a 0.18° beam antenna, resolution as high as 5–10 m in the azimuthal direction was attained in a tornado on 3 May. Data collected in three supercell tornadoes are described. Features such as eyes, spiral bands, and multiple vortices/wavelike asymmetries along the edge of the eyewall are discussed. Winds approaching 80 m s−1 were resolved without folding using the polarization diversity pulse pair technique. Two tornadoes formed at an inflection point in reflectivity where the hook echo and apparent rear-flank downdraft intersected. Finescale transverse bands of reflectivity were evident in one hook echo. Data in a dust devil are also described. Numerous other datasets collected in mesocyclones are also noted. A plan for future data analysis is suggested and a plan for future experiments and upgrades to the radar are proposed.

A Numerical Simulation of Cyclic Mesocyclogenesis

A three-dimensional nonhydrostatic numerical model, the Advanced Regional Prediction System, is used to study the process of cyclic mesocyclogenesis in a classic supercell thunderstorm. During the 4-h simulation, the storm’s mesocyclone undergoes two distinct occlusions, with the beginning of a third indicated at the end of the simulation. The occlusion process exhibits a period of approximately 60 min and is qualitatively similar in each case. Initial midlevel (3–7 km) mesocyclogenesis proceeds according to the “classic” picture, that is, via tilting of streamwise environmental vorticity. The development of an evaporatively driven rear-flank downdraft (RFD) signals the beginning of the occlusion process. The developing RFD wraps cyclonically around the mesocyclone, causing the gust front to surge outward. Simultaneously, the occluding mesocyclone rapidly intensifies near the surface. Trajectory analyses demonstrate that this intensification follows from the tilting and stretching of near-ground (<500 m) streamwise vorticity produced by baroclinic generation, crosswise exchange, and streamwise stretching along descending parcel trajectories in the RFD. The surging gust front also initiates updraft development on the downshear flank at midlevels, resulting in a two-celled updraft structure. As the near-ground mesocyclone becomes detached from the gust front due to the developing occlusion downdraft, the upshear updraft flank weakens as its conditionally unstable inflow is cut off at low levels; at the same time, the downshear updraft flank continues to develop eastward. The end of the occlusion process is signaled as the old near-ground mesocyclone becomes completely embedded near the surface in divergent outflow beneath the decaying updraft and is advected away by the mean flow. Near-ground mesocyclogenesis is initiated in the new updraft in a process nearly identical to that of the initial mesocyclone. However, after the first occlusion, near-ground equivalent potential temperature and buoyancy contours are fortuitously oriented such that streamwise baroclinic generation can proceed without delay. Thus, although the initial occlusion requires two hours to become fully organized, the second occurs only one hour later. In effect, the occlusion appears to set the stage for more rapid development of subsequent mesocyclones.

The Occurrence of Tornadoes in Supercells Interacting with Boundaries during VORTEX-95

Abstract During the Verifications of the Origins of Rotation in Tornadoes Experiment, nearly 70% of the significant tornadoes occurred near low-level boundaries not associated with the forward or rear flank downdrafts of supercells. In general, these were preexisting boundaries readily identified using conventional data sources. Most of the tornadoes occurred on the cool side of these low-level boundaries and generally within 30 km of the boundaries. It is likely that the low-level boundaries augmented the “ambient” horizontal vorticity, which, upon further generation in the forward-flank region, became sufficient to be associated with tornadic low-level mesocyclones. Some implications for forecasting and further research are discussed.

近接した雷雲内の中層渦によって強化された中層のrear inflowに伴うマルチセル型雷雲の発達過程

A meso-β-scale convective system (MCS) passed over the Tokai District of Japan on 8 Sep., 1994. Only one multicell thunderstorm (Storm C) exceeded the tropopause level in the MCS. This paper presents the evolution of the kinematic and radar echo structures of Storm C observed by the dual-Doppler radar system of Nagoya University. Storm C lasted more than 2 hours and its elongated axis was normal to the direction of mean cell motion. The early developing stage of Storm C was characterized by a downdraft on the left flank (left flank downdraft or LFD) and the development of cells on the right flank above the outflow of the LFD. A mid-level rear inflow descending within the storm and a new downdraft on the rear flank (rear flank downdraft or RFD) were observed in the explosively developing stage. The mid-level rear inflow was accelerated and localized in the vicinity of an anticyclonic vortex developed in a nearby decaying storm. Cells developed strongly above the merged outflows of the LFD, the RFD and the descending mid-level rear inflow. In the decaying stage, the intensified RFD and mid-level rear flow divided the main updraft region on the right flank into two and their outflows prevented the outflow of the LFD from spreading to the right flank. On the basis of the observational results, the effect of the vortex on the behavior of the rear inflow as well as the influence of the rear inflow on the evolution of Storm C were discussed.

Some Noteworthy Aspects of the Hesston, Kansas, Tornado Family of 13 March 1990

Abstract This paper considers a tornadic storm that struck south-central and eastern Kansas on 13 March 1 990. Most of the devastation was associated with the first tornado from the storm as it passed through Hession, Kansas. From the synoptic-scale and mesoscale view- points, the event was part of an outbreak of tornadoes on a day when the tornado threat was synoptically evident, Satellite imagery, combined with conventional data, suggest that the Hesston storm was affected by a preexisting, mesoscale outflow boundary laid down by morning storms. Radar and satellite data give clear indication of the supercellular character of the storm, despite limited radar data coverage. Because of the considerable photographic coverage, several interesting features of the storm were recorded and are analyzed here. These include the following: 1)the movement and dissipation of a cloud band associated with an apparent rear-flank downdraft; 2) a transition from a rather large funnel through an apparent dissipation to the...

Observations Related to the Rotational Dynamics of the 20 May 1977 Tornadic Storms

Abstract Observations of the 20 May 1977 tornadic storms are used to evaluate recent theories on the initiation of rotation at mid-and low levels and to verify recent thermodynamic retrieval results. Using the lengthy data record from a variety of sensors available for this day, it appears that the mechanism that initiates low-level rotation is different from that at midlevels. Attempts to identify the source of the low-level rotation as vertical tilting baroclinically generated horizontal vorticity were inconclusive. The recent thermodynamic retrieval results of Hane and Ray and of Brandes for these storms are in good agreement with independent measurements where available. However, verification is hindered by the sparseness of these measurements. Noticeable differences in the region of the rear-flank downdraft suggest that there is room for improvement in the retrieval methods. Investigation of the cyclic generation of rotation along gust fronts indicates that the source of low-level rotation is not der...

A Study of the Tornadic Region within a Supercell Thunderstorm

The transition of a supercell thunderstorm into its tornadic phase is investigated through high-resolution numerical cloud model simulations initiated within the interior portion of a previously simulated mature supercell storm. With the enhanced grid resolution, the low-level cyclonic vorticity increases dramatically, and the gust front rapidly occludes as small-scale downdrafts develop in the vicinity of the low-level center of circulation. As the occlusion progresses, a ring of high-vorticity air surrounds the circulation center and could be conducive to multiple vortex tornado formation. Numerous features of the simulated transition bear resemblance to those observed in tornadic storms. In the model simulation, the large low-level vorticity is generated through the tilting and intense stretching of air from the inflow side of the storm. This vertical vorticity is derived from the horizontal vorticity of the environmental shear and also from horizontal vorticity generated solenoidally as low-level air approaches the storm along the forward flank cold outflow boundary. Intensification of the rear flank downdraft during the occluding phase is dynamically driven by the strong low-level circulation.

On certain points in the construction of marine boilers

A laboratory tornado simulator was designed, constructed and tested to generate tornado-like vortices for the purpose of quantifying tornado-induced aerodynamic loads on civil engineering structures. This simulator generates a vortex that can translate along a ground plane to interact with models of structures on the ground. The simulator uses a “rotating forced downdraft” technique that loosely matches the rear flank downdraft (RFD) phenomenon often emphasized as important to tornadogenesis. The rotating forced downdraft is generated using a 1.83 m diameter fan and an annular duct suspended from an overhead crane that can translate along a 10.4 m ground plane. Measurements were conducted to quantify the simulator’s flow structures. An 18-hole pressure probe was used to traverse and map the vortex flow field. The flow structures of the vortices were validated by comparing with mobile Doppler radar observations of two major tornados. Results show that tornado vortex radii from 0.23 to 0.56 m can be generated. A wide range of maximum tangential velocities are also possible (from 6.9 to 14.5 m/s). The simulator generates a range of vortex structures from single-celled vortices to two-celled vortices with corresponding swirl ratios ranging from 0.08 to 1.14. Comparisons of general flow structure and normalized profiles of tangential velocity showed excellent match between simulator and radar data. The range of tornado sizes together with model structures of 1/100 to 1/500 scale will allow extensive examination of tornado-induced wind loads on terrestrial structures. The effects of a tornado’s size, translation speed and detailed flow structure on these loads can now be studied.