6.1 HISTORY OF TORNADO RESEARCH Howard B. Bluestein School of Meteorology, University of Oklahoma, Norman 1. INTRODUCTION 1953) by radars at the Illinois State Water Survey and M. I. T., respectively. Other The focus of this presentation is on early similar observations were made at Texas measurements and observations of A&M by Stuart Bigler. These early tornadoes, laboratory-vortex models, observations showed that hook echoes are severe-storm intercept field programs related to tornadoes. (“storm chasing”), in-situ measurements From the late 1950s onward, and made both intentionally and by chance, especially in the 1960s and 1970s, Ted Doppler radar observations of tornadoes Fujita at the University of Chicago made and their parent storms, and theory. painstaking analyses of tornado damage, Numerical modeling will be discussed very tornado photographs, and integrated these briefly, in deference to the later presentation analyses with “mesoanalysis” of surface by Morris Weisman in 9.1. data. Based on photographs of the Fargo, All of the aforementioned topics will be ND tornado of June 1957, he introduced discussed in some sense of, but not exactly, terminology such as “wall cloud” and “tail in chronological order. I will, however, cloud,” which persists to this day. He attempt to convey a sense of historical introduced what became known as the context. I do not have enough time here to “Fujita Scale” in an attempt to estimate wind detail or critique each contribution in this speeds in tornadoes on the basis of the short overview. It is recognized at the outset characteristics of damage inflicted. While not that there is overlap among the sub- calibrated, his technique remains today, disciplines of tornado research and that though in a modified form. After viewing there has been much cross-fertilization of tornado damage from aircraft, he ideas. Branches of research topics are documented cycloidal ground marks and highly intertwined and as such one cannot associated them with multiple-vortex describe progress in a linear fashion. tornadoes. On the basis of aircraft flights For the sake of brevity, I will not spend over a tornadic storm in 1977 he suggested much time, if any, on tornado forecasting, that downdrafts in thunderstorms may play a aspects of convective storms not related to role in tornadogenesis. tornadoes, acoustic-wave research, In the late 1950s, Chester and Harriett electrical research, recent contributions from Newton suggested in a seminal paper based newer instruments such as UAVs, portable upon analysis of a squall line in 1949, that surface probes, etc., data assimilation, non- vertical shear in the environment of a supercell waterspouts and other funnel convective storm leads to propagation of its clouds/small vortices, damage studies, and updraft as a result of interactions between exotica such as rocket probes . The period the storm updraft and the momentum in its of time covered in my review is from the environment at different heights. Their idea early 1950s until just before VORTEX2. later became the physical basis for our later understanding of supercell behavior, which 2. EARLY MEASUREMENTS AND is intimately related to tornadogenesis. OBSERVATIONS On 4 May 1961, Neil Ward, who was participating in the National Severe Storms After the Thunderstorm Project in the late Project (NSSP), made the first scientific 1940s, serendipitous radar observations of tornado chase. He traveled with the tornadic storms were first made in 1953 in Oklahoma Highway Patrol and, receiving Illinois (9 April) and Massachusetts (9 June information by phone about echoes seen on a WSR-57 radar, observed a tornado in one knows all the variables in space as a western Oklahoma. He postulated, on the function of time without having to measure basis of his visual observations, that cold them, and one can more easily conduct outflow in the parent storm may have played controlled experiments since one can easily a role in tornadogenesis. Keith Browining vary parameters and boundary conditions. and Ralph Donaldson, in an important 1963 Most recently, Dave and Steve Lewellen at paper, noted a similarity of a radar vault to West Virginia University have been doing that observed in the famous Wokingham, LES model experiments and are able to England hailstorm of 9 July 1959. They reproduce vortex behavior with startling showed an echo hole in the storm, a feature resemblance to reality. we now recognize as being associated with Both laboratory models and numerical tornadoes and/or strong updrafts. simulations have demonstrated the Keith Browning continued his work with importance of the swirl ratio to vortex an analysis of a tornadic supercell in behavior and the phenomenon of vortex Oklahoma on 26 May 1963. Observed by breakdown. Bob Davies-Jones at NSSL has the Weather Radar Lab in Norman and also analyzed lab model behavior and documented visually by Ralph Donaldson, it produced a long-standing and widely was suggested in 1965 that there is an reproduced figure illustrating the effect of ordered sequence of events leading up to swirl ratio on tornado structure and behavior. tornadogenesis. Lewellen and Lewellen have recently proposed that “corner flow collapse” may be 3. LABORATORY MODELS responsible for triggering tornadoes when the rear-flank downdraft (RFD) cuts off flow In order to understand how intense around a developing vortex. vortices such as tornadoes interact with the ground, laboratory models of vortices, 4. THEORY “vortex chambers,” have been built. They have been used to elucidate tornado Theoretical work has progressed in a behavior, but do not address the question of number of areas. Bob Gall at the Univ. of how tornadoes form in storms, because the Arizona in the late 1970s and early to mid parent storms are not represented, save for 1980s did linear stability analyses of vortices an exhaust fan that simulates the updraft. and explained the multiple-vortex Neil Ward at NSSL in 1972 reported on phenomenon in terms of a dynamic his early laboratory studies conducted in the instability. Bob Walko in the early 1990s at mid 1960s. His seminal work was followed OU pioneered the use of a simple, idealized by experiments at the University of model to elucidate physical mechanisms of Oklahoma (OU) with the “Viney tornado vortex formation. Jeff Trapp and Brian simulator” by Martin Jishke and Gene Fiedler at OU in the 1990s continued studies Wilkins and students. John Snow, Chris in the same vein, while more recently Paul Church and graduate students conducted Markowski at Penn State has conducted similar experiments in Purdue beginning in more studies. These “toy” models make use the late 1970s. In the mid 1990s, Snow and of positively and/or negatively buoyant his group pioneered the use of a laser bubbles released in environments of vertical Doppler velocimeter to map the wind field shear. without disturbing the flow and without Lance Leslie and Roger Smith in 1978 having to resort to the use of sometimes introduced the “dynamic pipe effect” (DPE) smelly tracers. (Smoke was used in the as an explanation of how vortices produced simulator at OU.) aloft can propagate downward. Jeff Trapp Numerical simulations of laboratory and Bob Davies-Jones, twenty years later, vortices were first done by Rich Rotunno, at did idealized numerical simulations to NCAR, in a series of studies conducted in identify conditions amenable to the DPE and the late 1970s and early 1980s. An those not amenable to the DPE. advantage of numerical simulations is that Observations of the DPE were made using WSR-88D Doppler radar data by Jeff Trapp tornadogenesis. A statistical study of radar and collaborators. case studies by Aaron Kennedy at OU and Over the years there has been a flurry of collaborators was inconclusive. Bob Davies- studies trying to explain tornado Jones picked up on suggestions by Ted windspeeds. Doug Lilly in an influential Fujita in the early 1970s and by Das in the NCAR publication (an unrefereed report) in early 1980s that rain-filled downdrafts might 1969 explained the hydrostatic initiate tornadogenesis. Davies-Jonesʼ consequences of a vortex in cyclostrophic theory is that descending rain curtains balance. Dergagarbedian and Fendell in transport momentum downward, which can 1970 came up with a theory for the lead to vortex intensification. Most recently, maximum windspeeds in tornadoes based Byko and collaborators used mobile Doppler on hydrostatic balance. Brian Fiedler, as a radar data and idealized modeling to post doc at NCAR in the mid 1980s, worked investigate the causes and consequences of with Rich Rotunno to show how the the DRCs. “thermodynamic speed limit” may be exceeded when there is a supercritical end- 5. EARLY SEVERE-STORM INTERCEPT wall vortex. Bob Walko in 1988 investigated ACTIVITIES the causes of subsidence inside tornadoes. It had been postulated that subsidence The reader is referred to Bluestein (1999) inside tornadoes could be responsible for for a summary of severe-storm intercept warming and a hydrostatic warm core. field programs prior to the late 1990s. In Doug Lilly at OU in the mid 1980s 1963 Frank Ludlam, a prominent British introduced the idea that helicity might act to convection researcher at Imperial College in stabilize storms and promote longevity. He London, stated in an AMS Monograph that emphasized the importance of Beltrami flow. “…the tornado, which energetically is only a Bob Davies-Jones at NSSL, on the other detail in the severe storm. However, its hand, emphasized the importance of the importance as a hazard and the interest of related streamwise vorticity in forming the problems which it poses makes it mesocyclones. He subsequently published a desirable to indicate its probable place in the series of papers investigating the baroclinic cumulonimbus model.” Storm chasers, generation of vortices and the propagation beginning in the early 1970s set out to do of convective storms.