**4. Results**

#### **4.1. Environmental analysis**

A major facet of this work was conducting in-depth synoptic analyses of 44 individual events that occurred between 2001 and 2015. The first part of this analysis focused on the evolution of the upper-level dynamics by looking at 300 mb heights, divergence, and winds. The main focus was assessing upper-level divergence with respect to the number of tornadoes that occurred during each event. The average maximum value for 300 mb divergence among the 44 events was 4.32 × 10−5 s−1, whereas the majority of tornadic TCs had divergence values between 4 and 6 × 10−5 s−1 (see **Table 1**). This suggests that upper-level divergence had at least some influence on tropospheric mass evacuation near convective cells moving ashore by enabling thunderstorm updrafts to reach higher altitudes. However, previous work has established that low- and mid-level dynamics play a larger role in TC-induced tornadogenesis [5, 7, 8, 17].

In examining the middle levels, the analyzed parameters included 500 mb heights/temperatures/winds and 500 mb height and absolute vorticity. Collectively, the 500 mb height, temperature, and wind values had respective average values of 580.35 dm, −4.33°C, and 35.47 kts. For the most tornadic TCs, the majority of 500 mb height values ranged between 5 dm above and below the average value. The 500 mb temperature values had a larger range of variability, with the most tornadic TCs having values range from −8 to 5°C. For the most tornadic TCs, the 500 mb wind values ranged from 25 to 60 kts. The second aspect evaluated at the 500 mb level was 500 mb height and absolute vorticity (for the 15 data-ready cases) for which the average value was 37.47 × 10−5 s−1. It is important to note that the 500 mb vorticity values ranged from 24 × 10−5 to 58 × 10−5 s−1, indicating that greater vorticity was not correlated with increasingly tornadic TCs. For a deeper look into these mid-level parameters, refer **Table 1** for average value comparisons between all of the events and the most tornadic events.

In stepping down to lower levels, 700 mb heights, temperatures, and winds were the focus. For the most tornadic TCs, the average 700 mb height value was calculated to be 306.16 decameters. Yet, among the various case studies, the 700 mb height values range from 292 to 316 dm. This indicated that the most tornadic TCs occurred with both below- and aboveaverage 700 mb heights, suggesting that the proximity of the TC center to the coastline may have played a key role. As a landfalling TC moves inland, the associated pressure gradient weakens and the lowest minimum central pressure rises. This process is a direct consequence of the transient balance between the rate of mass adjustment toward the center (i.e., via cyclonic inflow toward the TC center) and the rate of tropospheric mass evacuation [3, 7]. The most tornadic TCs, occurring with above-average heights, likely unfolded due to rising mid-level heights coupled with a conditionally unstable convective boundary layer during the postlandfall phase of a particular TC [3, 9, 11]. In examining the 700 mb temperatures, the average value was 9.34°C, and most tornadic TCs were close to this value when measured near the height of each event. By analyzing the 700 mb winds, the average value came out to be 39.09 kts. However, during the most tornadic TCs, the 700 mb wind values ranged from 25 to 90 kts, which is higher than the typical values. For these lower level parameters, refer **Table 1** for comparisons of the average values for all of the events as compared to the most tornadic events. During some landfalling TCs in which 30 or more tornadoes were reported, the mean effective bulk shear value was 40 kts; thus, it is plausible that the larger number of tornadoes was due to larger magnitudes of the low-level shear, coupled with the 850 mb wind data and other lowlevel data discussed below.


of the upper-level dynamics by looking at 300 mb heights, divergence, and winds. The main focus was assessing upper-level divergence with respect to the number of tornadoes that occurred during each event. The average maximum value for 300 mb divergence among the 44 events was 4.32 × 10−5 s−1, whereas the majority of tornadic TCs had divergence values between 4 and 6 × 10−5 s−1 (see **Table 1**). This suggests that upper-level divergence had at least some influence on tropospheric mass evacuation near convective cells moving ashore by enabling thunderstorm updrafts to reach higher altitudes. However, previous work has established that low- and mid-level dynamics play a larger role in TC-induced tornadogenesis

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

In examining the middle levels, the analyzed parameters included 500 mb heights/temperatures/winds and 500 mb height and absolute vorticity. Collectively, the 500 mb height, temperature, and wind values had respective average values of 580.35 dm, −4.33°C, and 35.47 kts. For the most tornadic TCs, the majority of 500 mb height values ranged between 5 dm above and below the average value. The 500 mb temperature values had a larger range of variability, with the most tornadic TCs having values range from −8 to 5°C. For the most tornadic TCs, the 500 mb wind values ranged from 25 to 60 kts. The second aspect evaluated at the 500 mb level was 500 mb height and absolute vorticity (for the 15 data-ready cases) for which the average value was 37.47 × 10−5 s−1. It is important to note that the 500 mb vorticity values ranged from 24 × 10−5 to 58 × 10−5 s−1, indicating that greater vorticity was not correlated with increasingly tornadic TCs. For a deeper look into these mid-level parameters, refer **Table 1** for average value

In stepping down to lower levels, 700 mb heights, temperatures, and winds were the focus. For the most tornadic TCs, the average 700 mb height value was calculated to be 306.16 decameters. Yet, among the various case studies, the 700 mb height values range from 292 to 316 dm. This indicated that the most tornadic TCs occurred with both below- and aboveaverage 700 mb heights, suggesting that the proximity of the TC center to the coastline may have played a key role. As a landfalling TC moves inland, the associated pressure gradient weakens and the lowest minimum central pressure rises. This process is a direct consequence of the transient balance between the rate of mass adjustment toward the center (i.e., via cyclonic inflow toward the TC center) and the rate of tropospheric mass evacuation [3, 7]. The most tornadic TCs, occurring with above-average heights, likely unfolded due to rising mid-level heights coupled with a conditionally unstable convective boundary layer during the postlandfall phase of a particular TC [3, 9, 11]. In examining the 700 mb temperatures, the average value was 9.34°C, and most tornadic TCs were close to this value when measured near the height of each event. By analyzing the 700 mb winds, the average value came out to be 39.09 kts. However, during the most tornadic TCs, the 700 mb wind values ranged from 25 to 90 kts, which is higher than the typical values. For these lower level parameters, refer **Table 1** for comparisons of the average values for all of the events as compared to the most tornadic events. During some landfalling TCs in which 30 or more tornadoes were reported, the mean effective bulk shear value was 40 kts; thus, it is plausible that the larger number of tornadoes was due to larger magnitudes of the low-level shear, coupled with the 850 mb wind data and other low-

comparisons between all of the events and the most tornadic events.

[5, 7, 8, 17].

level data discussed below.

**Table 1.** Total events average and most tornadic event average values for 300 mb divergence, 500 mb heights/ temperatures/winds and absolute vorticity, 700 mb heights/temperatures/winds, 850 mb heights/temperatures/winds, 0–1 and 0–3 km EHI, 100 mb mean parcel LCL height, BRN shear, effective bulk shear, and mean sea-level pressure.

Further, low-level analysis involved examining the 850 mb heights/temperatures/winds, 850 mb moisture transport vector, and 850 mb temperature advection (the last two parameters only having 15 data-ready cases). The average 850 mb height, temperature, and wind values were calculated to be 142.41 dm, 17.14°C, and 40.91 kts. A critical finding was that the 850 mb heights associated with the more tornadic TCs were near or below the average value, indicating that lower 850 mb heights favored more tornadoes, which agreed with the previous work [7]. For the most tornadic TCs, 850 mb temperatures stayed near or below average, illustrating that TCs which maintained lower mid-level temperatures tended to produce more tornadoes. This is a result of those TCs maintaining warmer near-surface temperatures and cooler mid-level temperatures, strengthening vertical wind shear, which is a critical aspect of TC-induced tornadogenesis. This concurs with the previous work finding that TCs that maintain a warmcore structure aloft, while developing a cold-core structure near the surface, develop stronger vertical wind shear [7]. Consequently, this stronger vertical wind shear fosters an environment more conducive for TC-induced tornadogenesis. Finally, 850 mb wind speeds for the larger tornado producing TCs were at or above the average value, indicating that stronger low-level winds are more favorable for tornadogenesis assuming the presence of weaker winds at the surface. For the 850 mb parameters, refer **Table 1** for comparisons of the average values for all of the events as compared to the most tornadic events.

Another important component was the 850 mb moisture transport (flux) vector, which is the product of the wind speed (m/s) and the mixing ratio (g/g) at 850 mb. This product (studied for 15 data-ready cases) showed that within 12 h of TC-tornadogenesis, there was a large quantity of warm/moist air advected into the tornadic regions. This provided abundant water vapor, which has been shown to be crucial for TC tornadogenesis as well as more common Great Plains tornadic supercells [9, 18]. The last 850 mb product being analyzed was 850 mb temperature advection that among the 15 data-ready events had an average value of −2.31 × 10-5°C s−1. It was found that 850 mb temperature advection among the most tornadic TCs ranged from 10 × 10−5 to −20 × 10−5°Cs−1 near the time of maximum tornadogenesis. More specifically, this meant that during the most tornadic TCs there was either weak warm-air advection or weak/moderate cold-air advection occurring on a 3 h timescale. Taking past research into consideration, it makes sense that the most tornadic TCs occurred with below-average 850 mb temperatures [7]. For cases with lower 850 mb temperatures, more research is needed to better understand why comparable numbers of tornadoes occur when the vertical temperature gradient is weaker.

Other critical components of TC-induced tornadogenesis include the presence of vertical wind shear and more near-surface variables. In regards to the near-surface domain, analyses were conducted for 0–1 km EHI, 0–3 km EHI, 100 mb mean parcel LCL height (measured in meters AGL), BRN shear, effective bulk shear, MSL pressure, and precipitable water depth (all of which had only 26 data-ready cases except for MSL for which data were available for all 44 events). Taking the EHI into consideration, it is worth noting that the representative equation is

$$\text{EHI} = \left(\frac{\text{CAPE}}{\text{storm} - \text{relativehelicity}}\right) \times 160,000;$$

which is the ratio between CAPE and low/mid-level wind shear represented by storm-relative helicity, which indicates the relative importance of each variable. The average values for 0–1 km EHI and 0–3 km EHI were 2.83 and 3.31, respectively, with the most tornadic TCs having greater than average values for both variables. This indicates the greater value of buoyancy relative to wind shear for tornado producing events. In addition, EHI values larger than one indicate a greater potential for tornadoes up to EF-3 strength, this is a very propitious result that needs to be further evaluated as also discussed by Eastin and Link [19]. The next parameter evaluated was the 100 mb mean parcel LCL height that was given in meters AGL. The average value for the 100 mb mean parcel LCL heights was calculated to be 640.38 m AGL for the 26 data-ready cases. Most tornadic TCs had 100 mb mean parcel LCL heights ranging from 140 m AGL below to 100 m AGL above average. This provided supporting evidence that lower LCL heights are more favorable for TC-induced tornadogenesis (i.e., 100 mb mean parcel LCL heights under 1000 m AGL) [9, 19]. For these near-surface parameters, refer to **Table 1** for comparisons of the average values for all of the events as compared to the most tornadic events.

The last part of the synoptic analysis considered the following variables: BRN shear, effective bulk shear, MSL pressure, and precipitable water depth. BRN shear is the square of the bulk vector difference between the 0 and 500 m AGL mean wind (both pressure weighted) and then multiplied by one half (http://www.spc.noaa.gov). It has been found that higher BRN values are linked to an increasing risk of supercells based on BRN values being defined by the ratio of buoyancy and shear. This supports the findings from the EHI analyses, which is contingent upon the ratio of convective available potential energy (CAPE) and wind shear in the form of storm-relative helicity [11]. The average BRN shear value was 106.4 m2 s2 , whereas the most tornadic TCs had values ranging between 80 and 220 m2 s2 . This is important because BRN shear values at or above 40 m2 s2 support environments more conducive for TC-induced tornadogenesis [20]. Looking at effective bulk shear, the average value was calculated to be 41.74 kts, whereas the most tornadic TCs had values ranging from 40 to 60 kts. This is important based on previous work finding that supercell development becomes more likely as effective bulk shear increases to 25–40 kts or greater (http:////spc.noaa.gov). Upon inspecting the evolution of precipitable water, it became clear that values in the vicinity of a landfalling TC were consistently over 2.00 in. This concurs with previous work finding that high water vapor content is essential for TC-induced tornadogenesis due to the necessity for deep moisture within the convective boundary layer [8, 9, 16, 19]. For these additional near-surface parameters, refer to **Table 1** for aforementioned average value comparisons.

The final aspect being assessed is mean sea-level pressure, whose average value among the 44 events was calculated to be 985.55 mb. It is important to mention that the lowest mean sealevel pressure values were all calculated when the storm was approaching landfall (i.e., the TC center was within 160 km of the coastline). For the most tornadic TCs, the mean sea-level pressure values ranged anywhere from 949 to 1008 mb. This indicated significant variability among the surface pressures of landfalling TCs, suggesting that a more intense TC at the time of landfall does not directly correlate with the production of more tornadoes. However, previous work has shown that the majority of tornadoes occur with rapidly weakening TCs upon landfall (i.e., TC centers rapidly filling in terms of the net mass adjustment) [7]. For a more detailed comparison of the average values for all of the events as compared to the more tornadic events, refer to **Table 1**.

#### **4.2. Forecast analysis**

more conducive for TC-induced tornadogenesis. Finally, 850 mb wind speeds for the larger tornado producing TCs were at or above the average value, indicating that stronger low-level winds are more favorable for tornadogenesis assuming the presence of weaker winds at the surface. For the 850 mb parameters, refer **Table 1** for comparisons of the average values for all

Another important component was the 850 mb moisture transport (flux) vector, which is the product of the wind speed (m/s) and the mixing ratio (g/g) at 850 mb. This product (studied for 15 data-ready cases) showed that within 12 h of TC-tornadogenesis, there was a large quantity of warm/moist air advected into the tornadic regions. This provided abundant water vapor, which has been shown to be crucial for TC tornadogenesis as well as more common Great Plains tornadic supercells [9, 18]. The last 850 mb product being analyzed was 850 mb temperature advection that among the 15 data-ready events had an average value of −2.31 × 10-5°C s−1. It was found that 850 mb temperature advection among the most tornadic TCs ranged from 10 × 10−5 to −20 × 10−5°Cs−1 near the time of maximum tornadogenesis. More specifically, this meant that during the most tornadic TCs there was either weak warm-air advection or weak/moderate cold-air advection occurring on a 3 h timescale. Taking past research into consideration, it makes sense that the most tornadic TCs occurred with below-average 850 mb temperatures [7]. For cases with lower 850 mb temperatures, more research is needed to better understand why comparable numbers of tornadoes occur when the vertical temperature

Other critical components of TC-induced tornadogenesis include the presence of vertical wind shear and more near-surface variables. In regards to the near-surface domain, analyses were conducted for 0–1 km EHI, 0–3 km EHI, 100 mb mean parcel LCL height (measured in meters AGL), BRN shear, effective bulk shear, MSL pressure, and precipitable water depth (all of which had only 26 data-ready cases except for MSL for which data were available for all 44 events). Taking the EHI into consideration, it is worth noting that the representative

> CAPE EHI 160,000; storm relativehelicity æ ö = ´ ç ÷ è ø -

which is the ratio between CAPE and low/mid-level wind shear represented by storm-relative helicity, which indicates the relative importance of each variable. The average values for 0–1 km EHI and 0–3 km EHI were 2.83 and 3.31, respectively, with the most tornadic TCs having greater than average values for both variables. This indicates the greater value of buoyancy relative to wind shear for tornado producing events. In addition, EHI values larger than one indicate a greater potential for tornadoes up to EF-3 strength, this is a very propitious result that needs to be further evaluated as also discussed by Eastin and Link [19]. The next parameter evaluated was the 100 mb mean parcel LCL height that was given in meters AGL. The average value for the 100 mb mean parcel LCL heights was calculated to be 640.38 m AGL for the 26 data-ready cases. Most tornadic TCs had 100 mb mean parcel LCL heights ranging from 140

of the events as compared to the most tornadic events.

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

gradient is weaker.

equation is

By examining this issue with a social impact perspective, it is clear that the public's information outlet is contingent upon the communication of operational meteorologists. This section is a detailed analysis of 124 archived mesoscale discussions from the SPC beginning in August 2004. The forecasters responsible for these mesoscale discussions include the following: C. Broyles, G. Carbin, Dr. A. Cohen, S. Corfidi, M. Darrow, S. Dial, R. Edwards, S. Goss, J. Grams, J. Hart, R. Jewell, B. Kerr, E. Leitman, Dr. P. Marsh, M. Mosier, J. Peters, J. Picca, J. Rogers, R. Thompson, B. Smith, and the late Jonathan Racy. The impetus for studying these archived mesoscale discussions was to look for a trend in forecaster improvement over the last 10–15 years. The primary focus was assessing the timeliness and accuracy of warnings conveyed by forecasters. Mesoscale discussions for events during and after 2004 were chosen since that was when the SPC began publishing graphics to visually illustrate their dialogue. The addition of graphics provided further insight into the thoughts of forecasters during a particular situation. The principal factors being studied were the number of times the following words appeared at least once in a given discussion: high temperature(s) or surface heating, instability (i.e., CAPE (the amount of energy available for convection), SBCAPE(surface-based CAPE → the value of CAPE relative to an air parcel rising from the lower planetary boundary layer), MLCAPE(mixed-layer CAPE → CAPE calculated with values of temperature and moisture from the lowest 100 mb above ground level), DCAPE(downdraft-CAPE → CAPE calculation which estimates the strength of rain and evaporatively cooled downdrafts), wind shear (i.e., bulk shear, low-level shear, sheared profile, etc.), storm relative helicity (SRH) in the 0–1 or 0– 3 km layer, and precipitable water (http//www.spc.noaa.gov). The intention was to assess which factors were most critical for TC-induced tornadogenesis from the standpoint of SPC mesoscale forecasters.

Of the 124 mesoscale discussions, 49 discussions (39.52%) had at least one mention of surface heating and/or high temperatures. Then, 47 of the 124 discussions (37.90%) contained at least one mention of instability terminology. Also, 100 of the 124 discussions (80.65%) contained at least one mention of wind shear terminology. In addition, 66 of the 124 discussions (53.23%) contained at least one mention of storm relative helicity terminology. Finally, 7 of the 124 discussions (5.65%) contained at least one mention of precipitable water. Collectively, wind shear was a prominent topic of many discussions, coinciding with the past research findings that strong low-level wind shear is essential for TC-induced tornadogenesis [10, 16].

In addition, the frequent presence of surface heating and/or high temperature wordings, instability terminology, and storm-relative helicity terminology indicated the issues' prevalence to forecasters involved in watch and/or warning coordination [14]. The main point ascertained from the relative frequency of the discussion parameters from 2004 to 2015 is that SPC forecasters clearly have improved during the latter dual-polarization era that began in late mid to late 2012 across much the contiguous United States (particularly along the Gulf and East Coast regions). This margin of improvement is distinguished by a greater presence of wind shear terminology that as previously stated is crucial to TC-induced tornadogenesis. This may suggest that advanced remote sensing capabilities, as in the implementation of dualpolarization radar technology, in concert with high-resolution rapid-scan satellite imagery, have bolstered forecaster analysis quality.

## **5. Conclusions**

Given the growth of the global population, especially those living near coastlines, and the consistent threats associated with landfalling TCs, there is a growing need for improvements to the modeling and forecasting of TC intensities and trajectories. As a nearly 50-year period of research has shown, tremendous progress has been made in understanding the dynamics that govern TC-induced tornadogenesis. Yet, considerable work still awaits the atmospheric science community.

The results of the environmental analysis showed that upper- and mid-level factors help facilitate TC-induced tornadogenesis. In some cases, upper-level divergence appeared to support tropospheric mass evacuation, bolstering upward vertical velocities within convective cells. It is imperative to note that upper-level divergence was not essential based on several cases with 10+ tornadoes occurring without significant upper-level divergence. Inspecting the middle levels, 500 mb heights/temperatures/winds experienced notable variability that was likely due to differences prior to and after landfall (i.e., pre-landfall TCs were associated with lower 500 mb heights, lower 500 mb temperatures, and stronger 500 mb wind speeds, whereas post-landfall tornadic TCs were the opposite coupled with a greater tendency for synoptic interactions). In regards to 500 mb absolute vorticity, larger values were not synonymous with greater tornado production.

Analysis of 700 and 850 mb data revealed that most tornadic TCs occurred with at or belowaverage heights, near-average temperatures, and above-average winds. The tendency for at or below-average heights coupled by near-average temperatures (i.e., average values being 9.34°C at 700 mb and 17.14°C at 850 mb) backs the presence of a notable vertical temperature gradient that is pivotal in facilitating TC-induced tornadogenesis. Approaching the surface, the variables, i.e., 100 mb mean parcel LCL heights, BRN shear, effective bulk shear, and mean sea-level pressure, showed convincing output (as shown in **Table 1**). Collectively, it is clear that the essential components most responsible for TC-induced tornadogenesis reside within the planetary boundary layer coupled with some relevant mid-level factors.

However, as with many aspects of meteorology, no individual numerical weather prediction product (i.e., a deterministic model forecast) can be the sole product with which we determine the likelihood of a particular TC producing or not producing tornadoes. Rather, it is with the integration of ensemble forecasts and appropriate temporal and/or spatial parameterizations that atmospheric research will fortify additional headway in generating more accurate TC forecasts. By the collaborative efforts of both atmospheric researchers from the United States and other countries around the world, this is the method by which atmospheric science will most productively push forward in this pursuit of knowledge.
