Brief history of tornado research

The History of Tornado Research and Instrumentation

1. Introduction

Tornadoes have been a weather phenomenon that has plagued scientists with more questions than answers throughout the years and still do to this day. While much advancement has been made since the late 1900’s, many unknowns still remain. The first documented case study of both storm spotter observation of a tornado and correlated Doppler radar signature dates back to 1973 (C. Doswell, 1974). This case study became the basis for radar-indicated warnings in 1988 when the Doppler radar network was implemented in the United States (Crum, 1993). The first known storm chasers, whom ventured out to chase storms and tornadoes, date back to the 1950’s. Roger Jensen in 1953 and David Hoadley in 1956 began chasing storms in an attempt to see the ever elusive tornado (Bluestein, 1999). Several government and privately funded research projects have occurred throughout the last century and all have tried to unveil the mystery of tornadoes. However, the prevailing question remains: why do some supercell thunderstorms produce tornadoes and others with the same conditions don’t? Several theories have emerged throughout the years of tornadogenesis research. This paper will attempt to provide the history of tornado research and several instruments and methods scientists utilized to achieve such hypothesis.

2. Results

2.1. Past research projects

2.1.1. Tornado Vortex Signature (TVS)

According to the National Severe Storms Laboratory (NSSL), an NSSL team intercepted a storm producing a tornado in 1973 that was being scanned by Doppler radar. The team recorded the life cycle of the tornado and compared it with the resulting Doppler radar imagery. The first tornado vortex signature (TVS) was then identified. This discovery led to the nationwide deployment of Doppler radars by the National Oceanic and Atmospheric Administration (NOAA).

2.1.2. The Tornado Intercept Project (TIP)

In 1975, the NSSL launched their first official field project on severe storms and tornadoes. At the time, crews intercepting storms were fed live radar information via the NSSL office and the term “nowcaster” was born. The presence of anticyclonic tornadoes was confirmed during this project (Burgess, 1976).

2.1.3. Joint Doppler Operational Project (JDOP)

In 1976, the NSSL was tasked with proving Doppler radar capabilities and that it could improve warning for severe thunderstorms and tornadoes. In 1979, this led to the implementation of Doppler radar by the National Weather Service (NWS), U.S. Air Force’s Air Weather Service and the Federal Aviation Administration (FAA). In 1978, the project was successful in issuing severe thunderstorm and tornado warnings based on Doppler radar signatures. Even with its success, it took ten years for the national operational Doppler radar system to be implemented.

The TIP and JDOP projects provided insightful knowledge of radar and visual observations to provide the first discovery of supercell tornadogenesis and their relation with the rear flank downdraft (RFD) (Doswell, 1979). The RFD became the leading hypothesis for tornadogenesis. Another key finding during this project was the categorisation of landspout tornadoes. A term coined by H. Bluestein, landspout tornadoes were observed to occur without the presence of a parent mesocyclone and were not easily identified by Doppler radar (Bluestein H. , 1985).

2.1.4. Totable Tornado Observatory (TOTO)

TOTO was the first project by the NSSL to attempt to deploy instruments in the path of tornadoes (Figure 1). Between 1981 and 1984, NSSL attempted to intercept tornadoes and deploy a “pod” called TOTO. This pod had weather instruments to measure wind speeds near the ground, barometric pressure, moisture and temperature. However, the team was unsuccessful at deploying instruments within a tornado. The TOTO project was abandoned since it was concluded that the chances of placing an instrument probe inside a tornado were very slim.

2.1.5. Various research instruments used

During this time, several other projects were ongoing as well. In 1982, small rockets were used as instrument probes to be launched in tornadoes. These were successfully launched at severe tornado-producing storms, however the rockets did not survive the extreme conditions around tornadoes (Colgate, 1982). From 1976 to 1981, sound recording instruments were deployed to record the sound made by tornadoes (Arnold, 1983).

Figure 1. TOTO in a wind tunnel at Texas A&M University on 21 Mar 1983 (copyright H. Bluestein).

This project was initiated by the eyewitness reports depicting tornadoes sounding like a train or waterfall. This project was unsuccessful. After learning the difficulties of TOTO, another instrumental probe was developed named the “turtle” due to its appearance. The advantages of this instrument probe were its lightweight and easy to deploy capabilities (Figure 2). The turtles were deployed in a grid to enhance chances of intercepting tornadoes. The results of turtle deployments were the pressure drops around tornadoes and in mesocyclones of around 5mb (Brock, 1987).

2.1.6. The Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX)

In 1995, the first VORTEX project was born. This project provided the research team with mobile Doppler radar called Doppler On Wheels (DOW). This provided the NSSL team with mobility to scan developing supercells and tornadoes. The project provided revolutionary data on tornadic storms.

Figure 2. Oklahoma University student Mike Magsig displaying a “turtle” in Sep 1993 (copyright H. Bluestein).

The VORTEX project resulted in qualitative changes in the understanding of storms that produce tornadoes. The research project suggested that tornadogenesis

is perhaps a fragile process that may depend on unobserved differences within supercells and their environments (Wurman, 2012). Resulting observations confirmed the role of the rear-flank downdraft, the presence of strong low-level mesocyclones and that violent tornadoes often were associated with pre-existing mesoscale boundaries (Markowski, 1998).

2.1.7. VORTEX-2

The second Verification of the Origins of Rotation in Tornadoes Experiment ran in the months of May and June in the United States during 2009 and 2010. VORTEX-2 was the largest and most ambitious study focused on improving the understanding of tornadoes, with the goal to study tornadogenesis, tornado structure and improving forecasts (Wurman, 2012). The project employed 13 mobile mesonet vehicles equipped with instruments to measure temperature, relative humidity, wind speed and direction and pressure. Comparisons between tornadic and non-tornadic storms were expected to increase the understanding between them and provide more accurate watches and warnings.

Figure 3. Doppler velocity and reflectivity images of the 5 Jun 2009 Goshen County, Wyoming, tornadic supercell observed by seven VORTEX2 radars at approximately 2216 UTC. Viewing angles, native radar resolution, wavelength, and range to the tornado all affect the appearance of the supercell, hook echo, and tornadic region. RDOW: Rapid-Scan DOW, SR1: SMART-Radar 1, UMXP: UMASS XPol (Wurman, 2012).

2.2. The first measurement inside a tornado

2.2.1. The Hardened In-Situ Tornado Pressure Recorder (HITPR)

In 2002, Tim Samaras and his team successfully deployed and measured data inside a tornadoes core. On May 7th 2002, several tornadoes touchdown near the city of Pratt, Kansas. The strongest tornado of the day, an F3 tornado, was sampled by the HITPR probe. The probe was designed to measure static pressure in winds of 40 meters/second or higher and to survive the harsh environment within the tornado’s core (T. M. Samaras, 2004).

Equation 1. Relation for the pressure coefficient (Cp) equation (T. M. Samaras, 2004).

The probe measures the free-static pressure by taking advantage of its aerodynamically profiled body as seen in Figure 3. The pressure coefficient is calculated using Equation 1. The pressure on the probe was measured for different radii and azimuthal angle. When Cp is zero, the pressure at the surface is equal to the static pressure (Ps) (T. M. Samaras, 2004). For wind speeds of 45m/s and higher, Cp was found to be independent of the wind speed. According to Samaras et al. (2004), at 24m/s, the pressure coefficient (Cp) dependence on the length of the body was shifted slightly due to Reynolds number effects (laminar and turbulent flows). The pressure coefficient becomes important when analysing wind speed and direction.

Figure 3. (a) HITPR 1 (and 2) probe (diameter = 0.51 m), (b) video probe (diameter = 0.76 m) and (c) mobile mesonet (three cars on left) (Karstens, 2010).

2.2.2. Estimation of wind speeds and direction

The angular dependence of the pressure coefficient (Cp) serves as a basis for estimating the wind speeds and direction. According to Samaras et al. (2004), it was possible to establish a method to determine the wind direction as shown in Figure 4. Figure 4 shows the relation between the negative pressure coefficient (Cp) and the angular variation at a radius of 10cm. Since the pressure is highest facing the wind, the highest-pressure value will indicate where the wind is coming from. According to Samaras et al. (2004), the measurement of the highest pressure is equal to the free-static pressure (Ps).

Figure 4. Angular dependence of negative pressure coefficient (Cp) at a radius of 10cm and wind speed of 90m/s (T. M. Samaras, 2004).

Once we know the direction of the wind, we can determine the velocity using Equation 2.

Equation 2. Equation used to infer the wind speed from pressure measurements.

Equation 2 is possible since we know our measured pressure (Pm), the measurement of our highest pressure is equal to the static pressure (Ps) and the pressure coefficient (Cp) is calculated using Equation 1. Since pressure is measured at all times at several angles by the HITPR, the pressure at two different angles can be used to calculate the velocity of the wind.

2.2.3. Measurements inside a tornado

Image 1. Image of large F-3 tornado after it passed over the HITPR probe on May 7th 2002 (T. M. Samaras, 2004).

Image 1 depicts the first tornado ever measured from the inside. According to Samaras et al. (2004), the tornado only hit the side of the HITPR instrument, but it measured a 24 millibar pressure drop. Figure 5 and Figure 6 depict the measured pressure before, during and after the tornado passed the instrument and the wind speed measurements of the tornado respectively. As shown in Figure 5, a significant pressure drop is recorded from initial 920mb pressure to below 900mb.

Figure 5. Recorded pressure in millibars (mb) for the Pratt, Kansas tornado on May 7th 2002 (T. M. Samaras, 2004).