**2. Literature review**

operational forecasters. As the United States has become more industrialized, a larger percentage of the population has settled in coastal regions. As a consequence, the inherent dangers linked to landfalling TCs have become a more prevalent issue as more people are living in areas which are geographically and economically vulnerable to TCs [1, 2]. TC-induced tornadoes can occur during the evening or overnight hours, complicating visual confirmation due to the lack of daylight and/or being rain-wrapped. One of the goals associated with TC forecasting is the accurate anticipation of the timing and location of tornadogenesis with respect to a TC landfall. The tornadoes that do occur in association with TCs are rated based upon the Enhanced Fujita (EF) damage rating scale. A large portion of TC-induced tornadoes are of EF-2 intensity or weaker (maximum winds up to 117 kts), with a small percentage reaching EF-3 (maximum winds between 118 and 143 kts) and even more rarely EF-4 intensity (maximum winds between 144 and 174 kts) [3, 4]. This is reflected in **Figure 1** that illustrates the TC-tornado intensity breakdown based on the analysis in Tropical Cyclone Tornado

**Figure 1.** The breakdown of the percentage and number of the 1139 tornadoes that occurred between 1995 and 2009 TC

The motivation for presenting the content in this chapter is twofold. The first part involves revisiting previous research and assessing how it has benefited the atmospheric research and forecasting communities over the past few decades. The second part is assessing where atmospheric research needs to head moving forward based on the forthcoming results being presented. This chapter is organized as follows. Section 2 consists of an inclusive literature review covering 47 years of TC-induced tornadogenesis research, focusing on relevant statistics and dynamics. Section 3 presents an overview of the data sources that were utilized and the methods by which different contents were evaluated. Section 4 provides a detailed explanation of the results that are divided into two parts: a comprehensive synoptic analysis of 44 United States TC landfall events and detailed analyses of operational forecaster perspectives from the Storm Prediction Center (SPC). The objective of the SPC is to "deliver timely

Records for the Modernized NWS Era [4].

52 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

based on their damage rating [4].

Past research involving TC-induced tornadogenesis dates back to the middle of the twentieth century. The onset of the satellite era stimulated the atmospheric research community to study mesoscale features embedded within synoptic-scale systems more comprehensively. A paramount issue is the forecasting of tornadoes induced by landfalling TCs along the United States mainland. Over the past six decades considerable progress has been made in understanding the atmospheric dynamics associated with TC-induced tornadogenesis. Early work, which studied TC-induced tornadogenesis, primarily focused on the collection of tornado reports and generating initial theories to explain the variable occurrence of tornadoes during TC landfall events [5, 6].

The work of Smith [6] was composed of compiling a TC-induced tornado climatology using tornado reports from 1955 to 1962. This work proposed a climatological TC tornadogenesis model based on the forward speed of TCs as well as influences from the dynamically driven frictional convergence and strong low-level vertical shear in the northeast quadrant of landfalling TCs. It was noted that some tornado reports are marginal due to the issues such as insufficiently educated spotter reports and/or limited remote sensing capabilities. Nonetheless, this earlier work provided the basis for further progress, starting with Pearson and Sadowski's work [5]. Their work expanded upon [6] by conducting a more detailed assessment of location and timing of tornadogenesis relative to the TC center position. The work of Pearson and Sadowski [5] also provided evidence for tornadogenesis occurring 12 h or more ahead of the arrival of hurricane-force winds. This discovery was monumental as it showed that TCinduced tornadoes occur farther from the center than previously believed.

Novlan and Gray [7] analyzed TC-induced tornadogenesis reports between 1948 and 1972 for the United States and between 1950 and 1971 for typhoons that impacted Japan. The compiled analysis of tornado reports revealed that the number of tornadoes initiated by a given TC was regulated by the rate of cooling aloft, the change in wind speed from the surface to 850 mb, and rising surface pressure values near the TC center upon landfall. It was found that tornadic TCs had weaker winds at the surface and stronger winds aloft, or stronger vertical wind shear [7–9]. In addition, the ambient surface temperatures of tornadic TC environments were lower than those which did not produce tornadoes. The temperatures at 850 mb also remained high in both scenarios, indicating the presence of a low-level cold core that maintained a stronger vertical wind shear profile [7]. This process enhanced cumulus downdraft potential, supporting stronger low-level horizontal wind shear based on a stronger vertical temperature gradient within the tornadic TC environments. Consequently, this induced intense small-scale regions of convergence and rotation, which were objectively associated with the TC-induced tornadogenesis [7].

Moving into the 1980s, Gentry [8] studied TC-induced tornado reports between 1973 and 1980 coupled with considerations based on past research. A core part of his work was appending data from [7] to develop a more comprehensive analysis of tornado reports based on their position relative to the coastline and their distance from TC center positions. A major finding was the majority of tornadoes occurred within 100 km of a given TC center and/or between azimuths of 20° and 120°. The results from [8] reaffirmed that strong vertical wind shear and strong vertical temperature gradients (i.e., cold-core to warm-core changes with height in tornadic TCs) are most responsible for generating environments conducive for tornadogenesis.

In the 1990s, McCaul and Weisman [10] studied composite profiles of temperature, moisture, and wind fields coincident with tornadic TC environments between 1948 and 1986. The premier finding was that helicity (i.e., helical flow) and vertical wind shear parameters were best correlated with TC-induced tornadogenesis. A second major finding was that the number and intensity of TC-induced tornadoes increased in accordance with the increasing TC size and intensity. A final important result from this work was that TCs landfalling along the East Coast produced fewer tornadoes than those that made landfall along the Gulf Coast. Spratt et al. [11] analyzed tornadic mesocyclones associated with two mature TCs that were not close to landfall near the time of tornadogenesis (i.e., Tropical Storm Gordon (1994) and Hurricane Allison (1995)). The primary result was the confirmation of several similarities between TCinduced tornadic cells and Great Plains supercells. In spite of the TC-induced convection being much lower-topped, the ratio of the depth of rotation to storm top was comparable between TC-induced tornadic cells and more common Great Plains supercells. The shallower depth of these rotating cells presented the issue of nondetection based on weaker capabilities of the Weather Surveillance Radar-1988 Doppler (WSR-88D) radar technology. Another important similarity was the persistent nature observed within both Great Plains supercells and TCinduced tornadic supercells.

Heading into the twenty-first century, many breakthroughs advanced the comprehension of dynamics relevant to TC-induced tornadogenesis. The focus of McCaul et al. [12] was studying the remnants of Tropical Storm Beryl (1994) with data from the WSR-88D radar at Columbia, South Carolina, and the National Lightning Detection Network. A major finding was the persistence of offshore low-topped supercells, one of which lasted 11 h and generated multiple tornadoes based on radar data over the course of 6.5 h. In addition, many cases showed a decrease in the frequency of cloud-to-ground (CG) lightning strikes or no lightning activity at all within 30 min of tornadogenesis [12]. Schultz and Cecil [3] conducted a detailed analysis of 1800 TC-induced tornadoes that occurred between 1950 and 2007 to develop an even more extensive TC-tornado climatology as shown in **Figure 2**. Their results supported hypotheses from previous work regarding the differences between inner- and outer-region tornadoes within TC circulations [6, 7]. Another major finding was that the outer-region tornadoes (greater than 200 km from the TC center) exhibited a stronger diurnal signal, with many tornadoes occurring in the afternoon. On the other hand, inner-region tornadoes typically occurred within about 12 h of landfall (without preference for time of day). As stated in [8], the majority of TC tornadoes (60%) occurred within 100 km of the coast. These events include all of the tornadoes in close proximity to the TC core near the time of landfall as well as Evaluating the Progress of Atmospheric Research in Understanding the Mechanics Behind Tropical... http://dx.doi.org/10.5772/64142 55

Moving into the 1980s, Gentry [8] studied TC-induced tornado reports between 1973 and 1980 coupled with considerations based on past research. A core part of his work was appending data from [7] to develop a more comprehensive analysis of tornado reports based on their position relative to the coastline and their distance from TC center positions. A major finding was the majority of tornadoes occurred within 100 km of a given TC center and/or between azimuths of 20° and 120°. The results from [8] reaffirmed that strong vertical wind shear and strong vertical temperature gradients (i.e., cold-core to warm-core changes with height in tornadic TCs) are most responsible for generating environments conducive for tornadogenesis.

54 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

In the 1990s, McCaul and Weisman [10] studied composite profiles of temperature, moisture, and wind fields coincident with tornadic TC environments between 1948 and 1986. The premier finding was that helicity (i.e., helical flow) and vertical wind shear parameters were best correlated with TC-induced tornadogenesis. A second major finding was that the number and intensity of TC-induced tornadoes increased in accordance with the increasing TC size and intensity. A final important result from this work was that TCs landfalling along the East Coast produced fewer tornadoes than those that made landfall along the Gulf Coast. Spratt et al. [11] analyzed tornadic mesocyclones associated with two mature TCs that were not close to landfall near the time of tornadogenesis (i.e., Tropical Storm Gordon (1994) and Hurricane Allison (1995)). The primary result was the confirmation of several similarities between TCinduced tornadic cells and Great Plains supercells. In spite of the TC-induced convection being much lower-topped, the ratio of the depth of rotation to storm top was comparable between TC-induced tornadic cells and more common Great Plains supercells. The shallower depth of these rotating cells presented the issue of nondetection based on weaker capabilities of the Weather Surveillance Radar-1988 Doppler (WSR-88D) radar technology. Another important similarity was the persistent nature observed within both Great Plains supercells and TC-

Heading into the twenty-first century, many breakthroughs advanced the comprehension of dynamics relevant to TC-induced tornadogenesis. The focus of McCaul et al. [12] was studying the remnants of Tropical Storm Beryl (1994) with data from the WSR-88D radar at Columbia, South Carolina, and the National Lightning Detection Network. A major finding was the persistence of offshore low-topped supercells, one of which lasted 11 h and generated multiple tornadoes based on radar data over the course of 6.5 h. In addition, many cases showed a decrease in the frequency of cloud-to-ground (CG) lightning strikes or no lightning activity at all within 30 min of tornadogenesis [12]. Schultz and Cecil [3] conducted a detailed analysis of 1800 TC-induced tornadoes that occurred between 1950 and 2007 to develop an even more extensive TC-tornado climatology as shown in **Figure 2**. Their results supported hypotheses from previous work regarding the differences between inner- and outer-region tornadoes within TC circulations [6, 7]. Another major finding was that the outer-region tornadoes (greater than 200 km from the TC center) exhibited a stronger diurnal signal, with many tornadoes occurring in the afternoon. On the other hand, inner-region tornadoes typically occurred within about 12 h of landfall (without preference for time of day). As stated in [8], the majority of TC tornadoes (60%) occurred within 100 km of the coast. These events include all of the tornadoes in close proximity to the TC core near the time of landfall as well as

induced tornadic supercells.

**Figure 2.** The 1800 tornadoes plotted within the ranges of 200, 400, and 600 km from the respective United States coastlines [3].

tornadoes embedded in rain bands far from the TC center. The last major finding was that the tornadic threat can be found to persist for 2–3 days after landfall and as far as 400–500 km inland from the TC center [3, 13].

**Figure 3.** A map of initiation points of 1163 United States TC-induced tornadoes which occurred between 1995 and 2010, damage rating 48 bins as labeled [4].

Edwards [14] studied a TC-tornado dataset dating from 1995 to 2009 from which various graphical and statistical analyses were generated. He conducted a detailed analysis of the position and intensity of 1139 tornadoes that occurred in association with TCs during this 15 year period as shown in **Figure 3**. One revealing result was that the 1139 tornadoes broke down such that there were 722 F0-tornadoes (63.39%), 339 F1-tornadoes (29.76%), 75 F2-torndaoes (6.58%), and 3 F3-tornadoes (0.26%). This statistical breakdown revealed that an overwhelming percentage of the TC-induced tornadoes were characterized by a weaker intensity (i.e., winds of 63 kts or less). The aforementioned TC-induced tornado distribution is comparable to tornado statistics from 1970 to 2002, which were compiled by McCarthy and Schaefer [15]. Within the aforementioned 32-year tornado climatology, the following intensity percentage distribution was found: 39, 36, 19, 5, and 1% for F0, F1, F2, F3, and F4 tornadoes, respectively [15]. The second finding was the apparent peak hours of TC-induced tornado occurrence between 18:00 UTC and 00:00 UTC as reflected in **Figure 4** [14]. This 6-h window in which TCinduced tornadoes occurred most frequently provided evidence for the strong influence of the diurnal cycle on these times of tornadogenesis. This highlighted the importance of diabatic heating and its influence on convective available potential energy (CAPE) values.

**Figure 4.** TC-tornado events by UTC time, in 3-hourly groupings. Yellow bars correspond to the local diurnal cycle 19 along the Gulf and Atlantic Coasts, while dark blue bars correspond to the nocturnal cycle. Purple bars correspond to the period of transition between the maximum diurnal cycle impacts and the overnight hours. Periods end in the minute 20 before the labeled times, e.g., "21–00" covers 2100–2359 UTC [4].

Edwards et al. [16] studied a 2003–2011 subset of the Storm Prediction Center (SPC) TC tornado records dataset in conjunction with environmental convective parameters derived from the SPC's hourly mesoscale analysis archive. A key difference observed between TC and non-TC tornado environments was that TC-tornado environments exhibited deeper tropospheric moisture coupled with the reduced lapse rates (near moist adiabatic) and lower CAPE. There was also a proposed objective, which is to study TC-tornado environments more consistently in order to provide higher-quality data in real time, which may improve the overall effectiveness of operational forecasts in TC-tornado prone locations.
