Centrifuging of Hydrometeors and Debris in Tornadoes: Radar-Reflectivity Patterns and Wind-Measurement Errors

Abstract

High-resolution Doppler radar observations of tornadoes reveal a distinctive tornado-scale signature with the following properties: a reflectivity minimum aloft inside the tornado core (described previously as an "eye"), a high-reflectivity tube aloft that is slightly wider than the tornado core, and a tapering of this high-reflectivity tube near the ground. The results of simple one-dimensional and two-dimensional models demonstrate how these characteristics develop. Important processes in the models include centrifugal ejection of hydrometeors and/or debris by the rotating flow and recycling of some objects by the nearsurface inflow and updraft.Doppler radars sample the motion of objects within the tornado rather than the actual airflow. Since objects move at different speeds and along different trajectories than the air, error is introduced into kinematic analyses of tornadoes based on radar observations. In a steady, axisymmetric tornado, objects move outward relative to the air and move more slowly than the air in the tangential direction; in addition, the vertical air-relative speed of an object is less than it is in still air. The differences between air motion and object motion are greater for objects with greater characteristic fall speeds (i.e., larger, denser objects) and can have magnitudes of tens of meters per second. Estimates of these differences for specified object and tornado characteristics can be obtained from an approximation of the one-dimensional model.Doppler On Wheels observations of the 30 May 1998 Spencer, South Dakota, tornado demonstrate how the apparent tornado structure can change when the radar-scatterer type changes. When the Spencer tornado entered the town and started lofting debris, changes occurred in the Doppler velocity and reflectivity fields that are consistent with an increase in mean scatterer size.

Spatial and Temporal Characteristics of Polarimetric Tornadic Debris Signatures

“…Characteristics of fields of tornadic debris were examined by Dowell et al (2005) in a modeling study. They concluded that the concentration of debris in a strong vortex is quickly reduced by centrifuging and that the centrifuging effect is more significant for larger debris elements.…”

“…A tornado can occur in a region of relatively few scatterers, leading to low SNR values that complicate r hv interpretation (Schultz et al 2012b). TDSs can also be wider than the associated tornado because of debris centrifuging (e.g., Dowell et al 2005). Last, a TDS often persists for a significant time after tornado demise, which makes sense given observations of long-range debris transport (e.g., Magsig and Snow 1998).…”

Low-Level Winds in Tornadoes and Potential Catastrophic Tornado Impacts in Urban Areas

“…In addition, radars measure scatterer motion, not air motion, resulting in possible underestimation of the latter in intense tornadoes (Dowell et al 2005). The stan- dard height for meteorological wind measurements (World Meteorological Organization 2004), and that now used to correlate wind speeds and damage to buildings (Marshall 2004), is 10 m AGL and is a 3-s average.…”

“…The tornadoes might have been even more intense during the periods when they were not observed by DOWs. Dowell et al (2005) suggest that the effects of debris centrifuging result in the underestimation of tornado winds by radars such as the DOWs, particularly in small, intense tornadoes such as BC. A hypothetical "hybrid" tornado's (HB) parameters represent the large size of the MH tornado and the peak intensity of the BC tornado.…”

Tornado funnel-shaped cloud as convection in a cloudy layer

“…In paper Souza et al (2000) the heat engine frame Correspondence to: P. Rutkevich (pbrutkevich@gmail.com) work is used to formulate a theory for circulations driven by surface heterogeneities in sloping terrains. Dowell et al (2005) considers how the objects in a tornado respond to a specific vortex flow. Wicker and Wilhelmson (1995) consider a three-dimensional numerical simulation of tornado genesis within a supercell.…”