1. Introduction
On 19 August 2007, the remnants of Tropical Storm (TS) Erin underwent a rapid overland reintensification that produced an eyelike structure in radar imagery similar to that commonly seen over open water. This reintensification was unusual in that it occurred approximately 800 km from the coastline over central Oklahoma and resulted in stronger maximum sustained surface winds (25 m s−1 with gusts up to 36.7 m s−1) and a lower minimum sea level pressure (1001.3 hPa) than was observed at any time the system was over water (18 m s−1 and 1003 hPa, respectively; Knabb 2008). Erin’s progression over central Oklahoma resulted in record rainfall amounts, with widespread 24-h storm-total rainfall amounts exceeding 100 mm and a maximum storm-total amount of 325 mm near Eakly, Oklahoma (Monteverdi and Edwards 2010).
Emanuel et al. (2008) documented observations of TC reintensification over the hot, sandy soils of northern Australia. They suggested that favorable surface conditions such as sufficiently hot, sandy, and moist soils can contribute to overland reintensification via increase of surface enthalpy fluxes. According to Emanuel (2008), a similar mechanism may have been responsible for Erin’s postlandfall redevelopment over the moist and relatively sandy soils of western Oklahoma. Furthermore, Evans et al. (2011) examined the simulated impacts of the abnormally moist soils over Texas and Oklahoma on the reinvigoration of TC Erin. Their results showed that latent heat fluxes associated with increased soil moisture content led to weaker mixing, increased boundary layer moisture and instability, and deep moisture availability that may have limited cold pool development and propagation. These conditions ultimately favored warm-core redevelopment and, subsequently, the reintensification of the primary circulation near the surface. Montgomery et al. (2006, 2009) documented that idealized TCs could intensify even in environments with weak latent heat fluxes through vortex stretching induced by rotating deep convection.
Despite the system’s overland reintensification and tropical characteristics, the National Hurricane Center did not reclassify Erin as a tropical entity while over Oklahoma because of the short duration of the organized convection and speculation that upper-level baroclinic forcing largely contributed to the strengthening of the system (Knabb 2008). In contrast to tropical cyclones (TCs) over water, the reintensification of TS Erin occurred over dense data networks that included surface, radar, and total lightning observations, thereby providing a unique opportunity to study the electrical and polarimetric-radar-derived microphysical characteristics of the reintensification process in detail that is currently not feasible well offshore over open water.
For years, TCs were believed to be weakly electrified with minimal lightning activity, particularly in the inner-core and eyewall regions (e.g., Black and Hallett 1999). In recent years, however, several studies have documented this is not always the case (e.g., Molinari et al. 1994, 1999; Shao et al. 2005; Price et al. 2009), particularly as a storm undergoes intensity fluctuation. The greatest lightning densities are typically detected in the outer rainbands, while densities in the eyewall and inner-core convection (i.e., within the 0–100-km radius) tend to be smaller and more sporadic (e.g., Lyons and Keen 1994; Simpson et al. 1998; Cecil et al. 2002; Squires and Businger 2008). Consistent with the relationships of a storm’s electrical characteristics and its microphysical and dynamic structures, several observational studies have revealed that rapidly intensifying hurricanes often produce abundant inner-core and eyewall lightning activity during intensification phases (Molinari et al. 1994, 1999; Shao et al. 2005; Squires and Businger 2008; Price et al. 2009; Fierro et al. 2011). This relationship with intensification has been successfully simulated by Fierro et al. (2007, 2013) and Fierro and Reisner (2011).
Although several studies have suggested that lightning bursts in the eyewall and inner core are associated with TC intensification (e.g., Leary and Ritchie 2009), it still remains unclear how well lightning bursts (either in the outer bands or inner core) can reliably indicate any changes in intensity. Some studies have suggested eyewall lightning bursts may be a precursor to intensification (e.g., Lyons and Keen 1994; Molinari et al. 1994; Price et al. 2009; Thomas et al. 2010), while others have documented bursts occurring either at the beginning of or during intensification (Molinari et al. 1999; Squires and Businger 2008; Thomas et al. 2010; Fierro et al. 2011; DeMaria et al. 2012). DeMaria et al. (2012) suggested that lightning bursts in the outer bands may be better correlated with TC intensification than eyewall lightning bursts. Furthermore, their study suggested that inner-core lightning bursts may rather indicate the end of an intensification phase.
Since the advent of dual-polarization weather radar, several studies have documented the utility of polarimetric data in identifying microphysical properties within convective storms, as well as providing detailed observations of storm structure and development (e.g., Herzegh and Jameson 1992; Zrnić and Ryzhkov 1999; Ryzhkov et al. 2005b; Kumjian et al. 2012). Polarimetric precipitation measurements are dependent on the size, shape, orientation, and phase of hydrometeors; thus, measurements from dual-polarization radars can provide valuable insight into the hydrometeor characteristics within a storm (e.g., Herzegh and Jameson 1992; Doviak and Zrnić 1993; Straka et al. 2000). In addition to being used to infer microphysical properties of storms, polarimetric radar data are also useful for identifying regions of intense convection. For example, vertical columns of enhanced ZDR extending from the surface to above the freezing level indicate lofting of large mixed-phase hydrometeors by an updraft (e.g., Herzegh and Jameson 1992; Loney et al. 2002; Ryzhkov et al. 2005c; Kumjian and Ryzhkov 2008; Picca and Ryzhkov 2010; Kumjian et al. 2012). Kumjian et al. (2012) demonstrated that the height of a “ZDR column” is strongly correlated with updraft strength. Furthermore, these ZDR columns are often topped by reduced ρhv values, due to the freezing and wet growth of lofted precipitation particles, leading to the formation of rimed ice (e.g., Picca and Ryzhkov 2010; Kumjian et al. 2012). This is important since rebounding collisions between riming graupel and cloud ice particles (via the noninductive process) is believed to be the primary mechanism by which convective clouds become electrified (e.g., Takahashi 1978; MacGorman and Rust 1998; Saunders and Peck 1998; Takahashi and Miyawaki 2002; Saunders 2008; Emersic and Saunders 2010).
This study examines the electrical and polarimetric-radar-derived microphysical characteristics of the eyewall and outer bands convection during Erin’s reintensification period over central Oklahoma. The analysis primarily focuses on the eyewall and inner-core convection to identify possible relationships between inner-core convective bursts and TC intensification. To achieve this goal, the evolution of Erin’s total lightning flash rates and locations is examined relative to polarimetric radar and high-resolution surface observations from the Oklahoma Mesonet. To the authors’ knowledge, these unprecedented high-quality datasets provide a first opportunity to examine in detail high-temporal resolution three-dimensional total lightning fields and associated microphysical evolution at the cell scale within an intensifying tropical system. This study also provides potential insight into the promising utility of dual-polarization and total lightning observations toward diagnosing and predicting TC intensity fluctuations.
2. Overview of TS Erin
TS Erin formed over the Gulf of Mexico and reached tropical depression (TD) status at approximately 0000 UTC 15 August 2007 (Knabb 2008; Fig. 1). As the system progressed toward the Texas coastline, it strengthened over the warm waters of the Gulf of Mexico and reached TS status by 1800 UTC 15 August (18 m s−1 maximum sustained winds; Knabb 2008). Erin maintained TS status for less than a day before weakening to a TD and making landfall at San Jose Island, Texas, at approximately 1030 UTC 16 August. After landfall, it continued on a northwestward track into west Texas as a TD before weakening further into a remnant low and tracking northeastward toward Oklahoma along the northwestern periphery of a strong midlevel ridge. As the remnants of Erin moved over Oklahoma early on 19 August and impinged upon an eastward-moving upper-level shortwave trough and greater boundary layer moisture, the thunderstorm activity abruptly increased on its eastern semicircle and, later, near its center of circulation. This sudden increase in convection was associated with abundant lightning activity, which was detected by the Oklahoma Lightning Mapping Array (OK-LMA; MacGorman et al. 2008) and the National Lightning Detection Network (NLDN; Biagi et al. 2007; Cummins and Murphy 2009).
National Hurricane Center’s best track of TC Erin, 15–19 Aug 2007. Shaded circles indicate the system’s location at 0000 UTC and open circles indicate its location at 1200 UTC, for the given day. Minimum pressure (hPa) over water and land are indicated by the purple arrows. [Image is from Monteverdi and Edwards (2010).]
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
While passing over west-central Oklahoma, the convection associated with Erin’s remnants continued increasing and, shortly after, was coincident with the unexpected formation of an eyelike feature that was clearly discernable in Weather Surveillance Radar-1988 Doppler (WSR-88D) imagery (Fig. 2; Knabb 2008). During this increase in convective activity (strength and areal coverage), the system produced abundant lightning activity. The eyelike feature became distinguishable between approximately 0800 and 1300 UTC 19 August, and fluctuated in size while convective cells pulsed within the eyewall (Arndt et al. 2009). Additionally, several spiral rainbands, reminiscent of those of tropical systems’ outer rainbands, formed east, southeast, and south of the newly formed eye. After 1300 UTC, the storm rapidly weakened and the eye dissipated as the eastward-moving shortwave trough progressed ahead of the surface low (Knabb 2008).
Oklahoma Mesonet surface observations of maximum wind speed and minimum pressure (reduced to sea level), with overlaid KTLX radar reflectivity at (a) 0400, (b) 0630, (c) 0725, (d) 0950, and (e) 1115 UTC 19 Aug 2007. (f) Color scales for radar reflectivity (dBZ) and surface MSLP (mb).
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
3. Instrumentation and data processing
This study utilizes datasets from the following instruments/networks: (i) polarimetric radar data from the National Oceanic and Atmospheric Administration (NOAA)/National Severe Storms Laboratory KOUN WSR-88D in Norman, Oklahoma; (ii) three-dimensional (3D) total lightning data from the OK-LMA; (iii) ground-flash lightning data from the NLDN; and (iv) surface observations from the Oklahoma Mesonet (Fig. 3). These instrumentation and datasets are discussed in more detail below.
Map of Erin’s observational domain. KOUN (red dot) is located approximately 42 km east-southeast of the center of the OK-LMA network (orange star). OK-LMA VHF receivers (9 stations during observation period) are indicated by green dots. Oklahoma Mesonet surface-observing stations are indicated by black squares, with characteristic spacing of 30 km. The red-shaded circle (150-km radius) represents the domain of KOUN. The purple-shaded circle (100-km radius) delimits the area in which 3D source locations are most accurately mapped. The yellow-shaded region (200-km radius) delimits the area in which two-dimensional (2D) data are mapped well. Also displayed is an overlaid track of Erin’s circulation center from 0300 to 1300 UTC 19 Aug 2007. The orange, green, and blue segments represent the cluster phase, eye phase, and dissipation phase of the system, respectively. Note: An OK-LMA station is collocated with the center of the OK-LMA network and a Mesonet station is collocated with KOUN.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
a. KOUN radar
The S-band KOUN simultaneously transmits and receives horizontally and vertically polarized waves (e.g., Doviak et al. 2000; Ryzhkov et al. 2005b). In addition to collecting conventional radar variables such as radar reflectivity factor at horizontal polarization ZH, Doppler velocity Vr, and spectral width συ, KOUN collects differential reflectivity ZDR, differential phase shift ΦDP, and the copolar correlation coefficient ρhv, which can be used to discern information on the microphysical characteristics of a storm. Further information on the theory and applications of weather radar polarimetry can be found in Herzegh and Jameson (1992), Zrnić and Ryzhkov (1999), Straka et al. (2000), Bringi and Chandrasekar (2001), and Ryzhkov et al. (2005b).
To ensure optimal quality, the radar data were processed according to the quality control procedures detailed by Schuur et al. (2003) and Ryzhkov et al. (2005a). After processing, the 5-min volume data were interpolated to a 3D-Cartesian grid with horizontal and vertical spatial resolutions of 0.5 km. The 3D grids were then used to generate constant altitude plan position indicator (CAPPI) and range–height indicator (RHI) images of ZH, ZDR, ρhv, and Vr.
b. Oklahoma Lightning Mapping Array
During the summer of 2007, the OK-LMA consisted of a network of nine ground-based measurement stations located in central Oklahoma (Fig. 3). These sensors measure the electromagnetic radiation emitted by lightning as it propagates through the atmosphere. The signals are detected in an available very high frequency (VHF) television band (60–66 MHz, channel 3; Thomas et al. 2004). The time and 3D location of radiation sources are determined by performing a least squares fit of measured differences in the time-of-arrival of signals received at six or more receiving stations, and are synchronized with global positioning system time (Rison et al. 1999). For this study, we limited the solutions we used to goodness-of-fit values of the reduced chi-square statistic (χ2) to ≤2 and signal detection by at least seven stations to increase degrees of redundancy. These requirements reduced the number of computed source locations from spurious local sources unrelated to lightning discharge. The 3D and 2D OK-LMA data are well mapped within 100 and 200 km of the center of the network, respectively. Outside of the 2D region, horizontal and vertical location uncertainties increase significantly with increasing distance from the center of the network (Krehbiel et al. 2000). This investigation focuses on data within the 3D observational domain of the OK-LMA network (Fig. 3). Additionally, according to Rison et al. (1999), LMA timing uncertainties are 40–50 ns, which lead to horizontal and vertical location errors over the network of approximately 50 and 100 m, respectively. For further information on the specifications and accuracy of LMAs, see Thomas et al. (2004).
Using a lightning flash sorting algorithm, the mapped VHF source points were grouped into flashes based on the time and distance between the sources, according to MacGorman et al. (2008). Locations of flash initiation points were determined only for flashes composed of at least 10 VHF source points; a detailed description of this procedure can be found in Lund et al. (2009). To analyze the evolution of lightning activity relative to the polarimetric data, flash and source rates were calculated in the same 5-min increments as the radar data.
c. National Lightning Detection Network
The NLDN (Cummins et al. 1998; Cummins and Murphy 2009) operates at frequencies between 0.5 and 400 kHz to detect and locate cloud-to-ground (CG) lightning flashes, which emit frequencies in the very low frequency (VLF; 3–30 kHz) band with peak frequencies near 10 kHz (Leary and Ritchie 2009). Throughout the continental United States, the NLDN provides CG flash detection efficiency in excess of 90% (Biagi et al. 2007; MacGorman et al. 2011). However, the network occasionally misclassifies intracloud (IC) flashes as low-amplitude positive CG flashes (Cummins et al. 1998; Biagi et al. 2007). Although some ground flashes do exhibit low peak currents (i.e., approximately less than 10 kA), smaller peak currents are typically associated with cloud flashes (Biagi et al. 2007; Emersic et al. 2011). Therefore, in this study, positive CG strokes with peak current less than 10 kA were neglected since they were likely IC flashes falsely identified by the NLDN system. Ground flash data are provided in flashes per 5 min, to align with the 5-min time resolution of the radar and total lightning data.
d. Oklahoma Mesonet
Surface observations of wind speed and pressure (interpolated to sea level) were obtained from the Oklahoma Mesonet (McPherson et al. 2007; Arndt et al. 2009), which comprises over 100 surface-observing stations throughout Oklahoma, with characteristic spacing of 30 km (Brock et al. 1995). Oklahoma Mesonet data are collected and transmitted to the Oklahoma Climatological Survey every 5 min.
4. Data observations and analysis
For this analysis, radar and surface observations were used to subjectively define three distinct structural phases of the system on 19 August 2007: (i) a convective cluster phase (0300–0753 UTC), when storm cells formed, intensified, and merged in and around the remnant low’s center of circulation; (ii) an eye phase (0753–1013 UTC), when the eyelike signature appeared and was most distinct; and (iii) a dissipation phase (1013–1300 UTC). The convective cluster phase, coincident with the reintensification of Erin’s remnants, consisted of a widespread group of strong convective cells primarily located on its southeastern quadrant that progressively wrapped around the center of circulation, forming TC-like precipitation bands (Figs. 2a–c). This axisymmetrization of convection around the center of circulation was coincident with a relatively rapid deepening of the system and an increase of the maximum surface wind speeds (Fig. 4). The cluster phase was also associated with the greatest rainfall accumulations. This analysis primarily focuses on the eye phase, which began at the time weak ZH echoes of the eyelike signature were first observed (0753 UTC). After the eye appeared, it filled in and then reappeared (0753–0833 UTC; not shown), eventually becoming distinct with well-defined eyewall convection (Fig. 2d). During the dissipation phase, the system retained an eye and eyewall convection, but the signatures rapidly deteriorated as the cells within the eyewall convection began to collapse (e.g., Fig. 2e).
5-min total flash rates (red) vs 5-min maximum observed wind speeds (green; m s−1) for (a) total storm and (b) 50-km data (0300–1300 UTC 19 Aug 2007). The vertical dashed lines indicate the onset times of the eye phase (0753 UTC) and dissipation phase (1018 UTC). Winds speeds in (a) and (b) are data for the entire domain.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
a. Analysis of Erin’s electrical structure
This section examines flash rates, ground flash data, and time–height density plots of VHF source locations in conjunction with wind and pressure observations from the Oklahoma Mesonet, to analyze simultaneously the overall trends of the electrical evolution/structure of Erin and its changes in intensity. Since inner-core dynamics are believed to play a major role in TC intensification (e.g., Emanuel 1997; Kelley et al. 2004; Rogers 2010), these data are also analyzed within a more confined area of ≤50-km radius from the observed center of circulation (which includes the eyewall convection as well as a small portion of the main precipitation band) as inferred from observations from the Oklahoma Mesonet. The two datasets provide insight into possible relationships between the evolution of lightning activity within Erin’s inner core (i.e., within the 100-km radius) and structural and intensity changes of the system. Note that observations prior to approximately 0500 UTC were outside of the OK-LMA’s 3D range, resulting in larger location and detection errors of the VHF sources.
1) Flash rates and surface observations
Figure 4 displays the total lightning (IC plus CG) flash rates (number of flashes per 5 min) and maximum surface wind speeds for the entire storm (Fig. 4a) and 50-km disk (Fig. 4b), over the observational period 0300–1300 UTC. An overall increase in flash rates was observed after 0430 UTC, with a peak and subsequent decrease immediately before the onset of the eye phase (0730–0745 UTC; Figs. 4a,b). The maximum peak [1380 flashes (5 min)−1 for the whole system and 300 flashes (5 min)−1 for the innermost 50-km range] at 0730 UTC was preceded by a rapid increase in flash rates starting at 0640 UTC. This peak in lightning indicated a transition in the system’s convective structure and intensity. Two secondary maxima in flash rates occurred at 0800 and 0900 UTC during the eye phase, with two distinct storm-total (inner 50 km) peaks of about 1000 (250) flashes (5 min)−1, respectively. After 0900 UTC, flash rates steadily declined. Overall, flash rates were largest during the hour just prior to the onset of the eye phase and during approximately the first half of the eye phase.
Almost all maximum surface wind speeds ranged between 15 and 35 m s−1 during the analysis period, with a notable increase beginning at ~0430 UTC (Figs. 4a,b). The greatest speeds occurred from about 0500 to 0820 UTC (until shortly after the onset of the eye phase), as the convection within the main precipitation band in the southeast quadrant wrapped around the center of circulation to form an eyelike feature. This overall wind speed increase was coincident with a well-defined increase in flash rates during the cluster phase. A maximum surface wind speed of 33.5 m s−1 was recorded at 0525 UTC and a secondary maximum surface wind speed of 32.6 m s−1 was recorded at about 0725 UTC, each occurring prior to the peak in lightning activity and before the formation of the eyelike feature (Fig. 4). Soon after these peaks, the maximum wind speed (and storm-total lightning rate) began to gradually decline, with the overall tendency being a continual decline in both fields throughout the remainder of the eye phase and dissipation phase (Fig. 4). In general, the greatest maximum wind speeds occurred before and during the period of greatest flash rates, during the reintensification (convective cluster) period and the initial 20 min of the eye phase.
Figures 5a and 5b display the total flash rates, CG flash rates, and IC flash rates for the entire storm and inner 50 km, respectively, overlaid with the minimum mean sea level pressure (MSLP). Ground flash rates exhibited trends similar to those of total flash rates in both the total storm and the inner 50 km. After 0430 UTC, CG flash rates increased until they peaked at nearly the same time as the total flash rates (0730 UTC; Figs. 5a,b). The storm-total peak in Erin’s CG flash rates [roughly 250 flashes (5 min)−1] is comparable to average CG flash rates documented by Zhang et al. (2012) for northwest Pacific TCs. Thereafter, CG flash rates steadily declined and eventually leveled off at approximately 10 flashes (5 min)−1 during the dissipation phase. Trends in IC flash rates were similar to those in total flash rates, for both analysis regions. Throughout the observational period, IC flashes composed an average of about 79% of all flashes in the whole storm and about 83% of all flashes within the inner 50 km, consistent with the ratios observed within continental storms over a wide variety of climatological regions (MacGorman et al. 1989; MacGorman and Rust 1998, p. 190). Lightning activity in the inner 50 km ceased after about 1115 UTC.
5-min total flash rates (red), IC flash rates (light blue), and CG flash rates (green) for (a) total storm and (b) 50-km data (0300–1300 UTC 19 Aug 2007). The 5-min minimum observed pressure (dark blue) data have been reduced to sea level. The vertical dashed lines indicate the onset times of the eye phase (0753 UTC) and dissipation phase (1018 UTC).
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
Minimum MSLP rapidly decreased from 0410 to 0520 UTC before reaching a steady state at roughly 1004 hPa. This steady-state period was followed by a secondary sudden 3-hPa decrease from 0715 to 0725 UTC, when the lowest observed MSLP of 1001.3 hPa was measured (Arndt et al. 2009; Figs. 5a,b). This minimum occurred 28 min before the onset of the eye phase, and just 5 min before the peak in total flash rates. Shortly after the total lightning peak, the minimum MSLP increased to approximately 1004 hPa at the beginning of the eye phase, decreased again to 1003 hPa shortly after 0800 UTC, and then rose to 1005 hPa at 0830 UTC, after which it fluctuated between 1004 and 1005.5 hPa until 1200 UTC before rising to 1006.6 hPa shortly before 1300 UTC. Overall, the evolution of flash rates and minimum observed MSLP exhibited a well-defined inverse relationship (negatively correlated) throughout the observation period; the rapid decrease in MSLP occurred at nearly the same time as the rapid increase in flash rates, similar to TCs over water (e.g., Molinari et al. 1994, 1999; Fierro et al. 2011). Furthermore, comparing Figs. 4 and 5, the maximum observed wind speeds occurred during the rapid increase in flash rates and rapid decrease in MSLP.
2) VHF source densities
(i) Total storm densities
A time–height density plot of VHF sources (Fig. 6) illustrates the vertical distribution of sources during the evolution of Erin’s remnants over Oklahoma. Densities were calculated by counting the number of source points within 5-min-by-250-m bins. Figure 7 provides plan view and east–west vertical displays of the VHF source densities to illustrate the evolution of the convection and determine which areas contributed to the maximum source densities. Note that the density scales are different from one time to another, as described in the caption (Fig. 7).
Time (UTC)–height (km) plot of VHF source density (0300–1300 UTC 19 Aug 2007). The vertical dashed lines indicate the onset times of the eye phase (0753 UTC) and dissipation phase (1018 UTC). The vertical solid red lines indicate the times of maximum observed wind speed (33.53 m s−1 at 0525 UTC and 32.63 m s−1 at 0725 UTC) and minimum observed MSLP (1001.3 hPa at about 0725 UTC). Units of densities are number of source points within 5-min-by-250-m bins.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
Density of VHF sources detected by the OK-LMA, during 10-min periods (on 19 Aug 2007) at (a),(b) 0550–0600 UTC; (c),(d) 0730–0740 UTC; (e),(f) 0930–0940 UTC; and (g),(h) 1150–1200 UTC. Cooler colors represent the least densities, while warmer colors represent the greatest densities. Note: densities are on different scales than those of the time–height plots and other density plots in this manuscript. Also, because each time-period plots’ scale depends on the number of source points in the plot, the scales for the various time periods are different. In (b),(d),(f),(h) horizontal (plan) views of the source densities are shown, and in (a),(c),(e),(g) the x–z views from the south (as if looking from south toward north) are shown. The green squares indicate locations of the OK-LMA network stations.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
During the convective cluster phase (0300–0753 UTC), OK-LMA lightning source densities increased within the 5.5–11-km layer between 0445 through 0630 UTC, with the greatest densities observed between 6 and 7 km (Fig. 6). As noted previously, the initial increase in total lightning flash rate occurred as MSLP was decreasing and the maximum surface winds were increasing. Also during this period, low-to-moderate source densities formed transient secondary maxima within an upper layer at 12–17 km MSL. The maximum altitude of source densities >3 × 103 occurred near the time of a relative minimum in MSLP and the peak in maximum observed wind speeds (Figs. 4, 5, and 6), denoted by the first red-colored vertical bar in Fig. 6. As shown in Figs. 7a,b, the large densities at the upper levels were primarily associated with a well-defined region of maximum densities within the main precipitation band in the southeast quadrant of the storm, located within 100 km of the center of circulation (i.e., within the inner core). The vertical extent of VHF source densities in this maximum extended to 18 km (Fig. 7).
Beginning at approximately 0700 UTC, source densities increased throughout most of the depth of the storm, with densities >1.1 × 104 reaching a maximum altitude of 12 km (Fig. 6) just 5 min after a secondary maximum in wind speed and the minimum MSLP were observed (Figs. 4, 5, and 6). Densities pulsed vertically to approximately 16 km, with larger densities between 5 and 12 km (Fig. 6). The greatest densities observed throughout the analysis period occurred between 6 and 7.5 km, from approximately 0730 through 0753 UTC.
During this period of rapidly increasing VHF source densities, cyclonic structure became more defined about the center of rotation, with pronounced densities in the eyewall and maximum densities within the large region of deep convection forming the main precipitation band on the southeast side of the storm (Figs. 7c,d). In the northern portion of the eyewall, secondary maxima in VHF sources were found to rotate around the storm center toward a more persistent maximum (centered at roughly −100 km west, 55 km north) that pulsed northwest of the circulation center. This pulse is hereafter referred to as an eyewall convective event (CE). Increases in lightning activity indicate increasing electrification, typically caused by an increased volume of graupel and/or small hail aloft, strengthening updrafts, and a greater likelihood of heavy rainfall (e.g., MacGorman et al. 1989; Wiens et al. 2005; Kuhlman et al. 2006; Deierling and Petersen 2008). Overall, the convective cluster phase was the most electrically active period throughout the reintensification over Oklahoma, exhibiting bursts of lightning both northwest and southeast of the center of circulation (within a 100-km radius).
During the eye phase (0753–1013 UTC), densities tended to gradually decrease with height until approximately halfway through the period (i.e., about 0900 UTC; Fig. 6). A secondary region of moderate-to-high densities of 1.1 × 104 to 1.3 × 104 occurred at altitudes between 8 and 11 km from roughly 0830 to 0915 UTC, just prior to the time at which the eyewall structure was best defined according to both low-level reflectivity (Fig. 2d) and VHF source density (Figs. 7e,f). This bilevel structure in VHF source densities continued throughout the remainder of the eye phase. Source densities during the second half of the phase were much smaller than during the first half. This reduction in densities may seem peculiar because the eyewall convection had the clearest eye structure and greatest base-scan reflectivities during this time. However, the increased lightning densities within the eyewall appeared to be more than offset by reduced VHF source densities within the previously widespread cluster of maximum densities in the main precipitating band.
The dissipation phase (1013–1300 UTC) was characterized by further decreases in VHF source densities, as Erin’s convection rapidly weakened (Fig. 6). After about 1050 UTC, the greatest decreases in densities occurred below 7 km, while the top contours of density tended to decrease slightly in height toward the end of the phase, which was likely caused by heavier particles falling within the collapsing convective cells while the lighter charged particles (ice crystals/small aggregates) in the upper levels of the cells and stratiform anvils descended more slowly. As during the period shown in Figs. 7g,h, appreciable VHF source densities around the eyewall tended to disappear during the dissipation phase, although there were occasional small bursts of VHF sources in the eyewall (not shown). The overwhelming majority of sources during the dissipating phase were associated with the weakened rainband convection.
In summary, source densities peaked just after the lowest observed MSLP, and just prior to the onset of the eye phase, followed by a gradual decrease throughout the remainder of the observation period. Most importantly, after the eye formed, the evolution of the convection within the inner core (i.e., within the 0–100-km radius) northwest and southeast of the circulation center dominated the evolution in the vertical distribution of VHF sources.
(ii) Inner-50-km source densities
The vertical distribution of VHF sources within 50 km of the center of circulation is analyzed here to distinguish and contrast its evolution relative to the whole system. During the convective cluster phase (0300–0753 UTC), densities increased after approximately 0500 UTC and exhibited a distinct bilevel distribution, with maxima within the 4.5–7- and 8–11.5-km layers (Fig. 8). Until about 0730 UTC, the greatest densities occurred within the lowest level. Note that the bilevel signature began at about the same time as the peak in maximum wind speeds was observed, as well as during the period of rapid pressure falls (Figs. 4 and 5). The larger densities and most pronounced bilevel signature (5- and 10.5-km levels; Figs. 9a,b) were associated with the strengthening convection north of the center of circulation (i.e., with the CE).
As in Fig. 6, but the data are restricted to within 50 km of the center of circulation (0300–1300 UTC 19 Aug 2007).
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
As in Fig. 7, but the data are restricted to within 50 km of the center of circulation during (a),(b) 0700–0710 UTC; (c),(d) 0800–0810 UTC; (e),(f) 0840–0850 UTC; and (g),(h) 1000–1010 UTC.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
The greatest source densities within 50 km of the center of circulation occurred during the eye phase (0753–1013 UTC; Fig. 8). Throughout the first half of the eye phase (between 0753 and 0900 UTC), densities pulsed vertically up to about 15 km. Two brief maxima occurred between 9 and 11.5 km, each lasting roughly 10 min. One even briefer maximum occurred between 4.5 and 5.5 km. These dramatic signatures were associated with large densities within both the eyewall convection northwest of the center of circulation and the primary outer precipitating band east and southeast of the center, both having bilevel maxima in the vertical projections (Figs. 9c,d and 9e,f). During the second half of the eye phase, the maximum height of sources tended to decrease (Fig. 8). Except for the lower 4.5–6-km layer, the vertical source densities decreased at most heights. These maxima in densities at lower altitudes were associated with the CE in the eyewall west of the center of circulation (Figs. 9g,h). By 1000 UTC, the eyewall had become more clearly structured, and the deep, highly electrified convective cluster within the main precipitating band contained the vast majority of the sources at upper levels. The dissipation phase (1013–1300 UTC) was characterized by rapidly decreasing lightning activity within the inner 50 km, which ceased after 1130 UTC.
The storm-total and inner-50-km source density data helped diagnose the overall evolution of the inner-core convection. Furthermore, the deep and highly electrified convective cluster within the main precipitating band had a considerable impact on the evolution of the vertical distribution of sources throughout the analysis period. After the dissipation of this convection, the overall densities declined while densities within the eyewall CE remained distinct.
b. Analysis of Erin’s polarimetric structure
As Erin’s remnants organized throughout the cluster and eye phases, several precipitation features were observed by the KOUN polarimetric radar, some with characteristics similar to that of tropical systems over water. This section first presents observations of the eyewall CE mentioned earlier, followed by a description of Erin’s eyewall convection. Second, this section describes an example of a distinct ZDR column within a strong convective cell, followed by an example of a depolarization signature that indicated significant electrification at the top of a convective column.
1) Eyewall convective event
At 0803 UTC, Erin had yet to exhibit an eye or organized eyewall, but the center of circulation (approximately 94 km from KOUN) was strengthening as it progressed east-northeastward over central Oklahoma. A region of intense convection began within the northwest quadrant of the eyewall (ZH reaching 56–60 dBZ and ZDR reaching 2.25 dB; Fig. 10), persisted in nearly the same location for approximately one hour, and then rotated cyclonically (Fig. 11) toward the southern region of the eyewall, where it dissipated shortly after 1003 UTC (Fig. 12). This finding is similar to behavior documented within TCs, in which convection develops in the downshear to downshear-left quadrants and then rotates upshear (e.g., Frank and Ritchie 2001; Corbosiero and Molinari 2002; Reasor et al. 2009; Fierro and Reisner 2011). The cyclonic rotation of the CE was evident in a succession of images throughout the 0800–1000 UTC period (not shown). The early stages of this region of intense eyewall convection were first observed during the cluster phase, northwest of the center of circulation.
CAPPIs of (a) ZH, (b) ZDR, and (c) ρhv at 3 km, and (d) OK-LMA 5-min total lightning source density (from the 0803 UTC 19 Aug 2007 volume scan). In each CAPPI, ZH is contoured at 30, 40, and 50 dBZ. The greatest source densities are represented by red, and the smallest source densities are represented by blue. The sounding-indicated 0°C level at this time was approximately 4.3 km.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
As in Fig. 10, but at 0938 UTC 19 Aug 2007.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
As in Fig. 10, but at 1003 UTC 19 Aug 2007.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
At 0938 UTC, the eyewall convection had become better organized, and ZH and ZDR within the CE reached 60 dBZ and 3–3.5 dB, respectively (among the greatest values observed in the dataset; Fig. 11). By 1003 UTC, the eye was clearly discernable and had ZH as high as 56 dBZ and ZDR as high as 3–3.5 dB (Fig. 12). As in Figs. 11 and 12 and throughout the evolution of the CE, an “arc” of reduced ρhv values (as low as 0.85) was evident within the eyewall convection, collocated with high ZH and ZDR values as they rotated cyclonically toward the southern eyewall region. The arc was observable in a layer from 3 to 4.5 km (not shown), indicating extreme differential sedimentation of precipitation particles (Kumjian and Ryzhkov 2012) due to heavier particles falling from the melting layer. Size sorting was further suggested by areas of enhanced ZDR offset from areas of high ZH (e.g., at roughly −72 km west, 41 km north in Fig. 12; Kumjian and Ryzhkov 2012). Shortly after 1003 UTC, the CE dissipated south of the center of circulation, immediately followed by the sudden weakening of the eyewall convection. This timing suggests the CE may have helped sustain the eyewall.
While the CE rotated counterclockwise around the eye, large ZDR values extended up to (and sometimes above) the freezing level, while ZH > 36 dBZ often reached 11 km, suggesting the presence of deep convective updrafts. This CE is comparable to hot towers observed within the eyewalls of Hurricanes Dennis (2005; Guimond et al. 2010), Rita (2005; Fierro et al. 2011), and Karl (2010; Reinhart et al. 2014). In the remainder of the eyewall convection, 36-dBZ echo tops rarely exceeded the 6-km level throughout the analysis period, consistent with observations of tropical oceanic profiles of ZH (e.g., Zipser and Lutz 1994; Trier et al. 1996, 1997; May and Rajopadhyaya 1999; Fierro et al. 2009, 2012); therefore this deep CE was an easily discernable feature.
OK-LMA data reveal the CE was collocated with a local maximum in total lightning activity. Compared to other regions of the eyewall, this CE was the most electrically active and exhibited the greatest VHF source densities (e.g., Figs. 11d and 12d). These CE observations are revealing and consistent with Kelley et al. (2004), who suggested that a TC is 70% more likely to intensify when deep convection erupts close to the center of circulation. Furthermore, the deep, highly electrified CE in the present case appears similar to hot towers and convective bursts that have been related to TC intensification by other investigators (e.g., Molinari et al. 1994, 1999; Black and Hallett 1999; Guimond et al. 2010; Fierro et al. 2011).
2) Sloping eyewall
Polarimetric imagery of Erin’s distinct eyewall feature, having similar traits as that of eyewall convection found within mature TCs over water, is presented. At 0858 UTC 19 August, a well-defined asymmetric eyewall structure along the 313° azimuth can be seen (Fig. 13). A distinct melting layer was evident in the ρhv signature (ρhv < 0.95) at approximately 4.5 km, and appeared elevated at the locations of convective towers, particularly within the eyewall convection. The ZH and Vr signatures both revealed organized eyewall convection that sloped outward with height from the center of the eye, with a 45° slope from the vertical axis, concordant with in situ observations within TCs (e.g., Jorgensen 1984; Marks 1985; Marks and Houze 1987).
(a) CAPPI of ZH at 3 km at 0858 UTC 19 Aug 2007. The black line indicates the location of the RHIs; ZH is contoured at 30, 40, and 50 dBZ. RHIs of (b) ZH, (c) ZDR, (d) ρhv, and (e) Vr at 313° azimuth at 0858 UTC 19 Aug 2007. In each RHI, ZH is contoured at 20, 30, 40, and 50 dBZ. Lightning initiation points are indicated by the white dots. Initiation points were plotted if they occurred within 5 min of the KOUN volumetric scan and were located within 2.5 km of either side of the azimuth. The sounding-indicated 0°C level at this time was approximately 4.3 km.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
Within the northwest eyewall convection, a column of enhanced ZDR values up to 2.75 dB (and associated ρhv values as low as 0.96) extended up to the 5.5-km level, indicating the presence of large, wet hydrometeors within a strong updraft (Figs. 13c,d). Although the ZH values within the center of the eye were 20–36 dBZ (Fig. 13b), values near 0 dBZ were observed at different times and azimuths (not shown). Lightning initiation points were located at heights ranging from 2 to 9 km (between 110 and 115 km, from the KOUN radar), along the outer horizontal ZH gradient within the northwest quadrant. These points were also located along a deep low-level convergence zone (Fig. 13e). This supports the conclusions of Rust et al. (1982), Proctor (1991), and Lund et al. (2009) that lightning initiations tend to occur at the edges of high ZH cores, within regions of large horizontal gradients of ZH and vertical velocity. The sloping eyewall, strong rising motion within the eyewall (inferred from the observed substantial change in Doppler velocity across the eyewall, on the order of ~24 m s−1 over a distance of ~20 km), and weak reflectivities within the eye all indicate that Erin exhibited TC-like characteristics during its eye phase.
3) ZDR columns and depolarization streaks
As Erin reintensified during the convective cluster stage, deep convection extended throughout the region of the main outer precipitating band southeast of the center of circulation (in contrast to the shallower convection nearest the center of circulation), with echo tops >36 dBZ commonly exceeding 11 km (up to around 14 km; Fig. 14b). These strong convective cells were occasionally associated with vertical columns of enhanced ZDR (extending to as high as 7 km), surmounted by pockets of reduced ρhv values (as low as 0.85) and lightning initiation points (e.g., Figs. 14c,d). However, there were much fewer ZDR columns extending above the freezing layer than one would typically see in continental systems (e.g., Brandes et al. 1995; Kumjian and Ryzhkov 2008). These columns were found to occur more frequently within Erin’s outer precipitating bands than within the eyewall (e.g., Fig. 13). Some of the deep convective cells were topped by dense clusters of lightning initiation points (e.g., Fig. 14), indicating that strong updrafts were likely lofting supercooled precipitation particles above the freezing layer.
(a) CAPPI of ZH at 3 km at 0743 UTC 19 Aug 2007. The black line indicates the location of the RHIs; ZH is contoured at 30, 40, and 50 dBZ. RHIs of (b) ZH, (c) ZDR, and (d) ρhv at 246° azimuth at 0743 UTC 19 Aug 2007. In each RHI, ZH is contoured at 20, 30, 40, and 50 dBZ. Lightning initiation points are indicated by the white dots. Initiation points were plotted if they occurred within 5 min of the KOUN volumetric scan and were located within 2.5 km of either side of the azimuth.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
According to Ryzhkov and Zrnić (2007), strong electrostatic fields within thunderstorms can orient ice crystals, which can depolarize the transmitted radar signal. Figure 15 (0653 UTC) illustrates the most distinct depolarization streak signature identified during the observational period. This remarkable signature was located along the 212° azimuth, which cut through the southern portion of the primary outer precipitating band (Fig. 15a). A radial streak of enhanced ZDR values extended outward from a range of approximately 65 km and a height of approximately 7.5 km, with a large region exceeding 5 dB (Fig. 15c). The streak was collocated with a region of enhanced ZH at a range of 100–105 km from KOUN and a height of about 7–14 km. Additionally, a dense cluster of lightning initiation points was present at upper levels (at a range of 100–105 km from KOUN and height of 10–14 km), further illustrating the electrified environment. The ZDR signature in the corresponding plan position indicator (PPI) image at an elevation angle of 6.42° (Fig. 16b) displayed radial streaks of enhanced positive and negative ZDR, exceeding 5 and −2 dB, respectively. These streaks were ubiquitous within the domain (Fig. 16b), suggesting strong electrification throughout the layer. Though depolarization streaks occurred most frequently during the convective cluster phase (within the eyewall, main precipitating band, and leading stratiform regions), the signatures were also observed during the eye and dissipation phases.
As in Fig. 14, but at 212° azimuth and 0653 UTC 19 Aug 2007.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
PPIs of (a) ZH and (b) ZDR at an elevation of 6.42° at 0653 UTC 19 Aug 2007.
Citation: Monthly Weather Review 142, 6; 10.1175/MWR-D-13-00360.1
5. Discussion and summary
The analysis revealed that Erin bore many key similarities to those of TCs over open water. In particular, the eyewall convection tilted outward with height with an average slope of 45° from the vertical axis (consistent with e.g., Jorgensen 1984; Marks 1985). Relatively strong rising motion existed within the eyewall (inferred from the observed substantial change in Doppler velocity across the eyewall) and reflectivities near 0 dBZ occupied the eye. Additionally, relatively few ZDR columns extended above the freezing level, in stark contrast to continental convection. The strongest convection and greatest lightning activity were concentrated within the primary precipitation band southeast of the center of circulation, and within the eyewall CE (each within Erin’s inner core, radius ≤100 km), consistent with studies focusing on TCs (e.g., Simpson et al. 1998; Fierro et al. 2011; Reinhart et al. 2014). Furthermore, this work identified a localized region of intense convection that formed within the eyewall during reintensification, which appeared to bear some similarities to the “convective bursts” reported within intensifying TCs over water (e.g., Molinari et al. 1994, 1999; Black and Hallett 1999; Hendricks et al. 2004; Montgomery et al. 2006; Guimond et al. 2010; Fierro et al. 2011). For example, this CE was characterized by a deep convective updraft (as indicated by columns of high ZDR occasionally extending above the freezing level) and enhanced electrification, and it also pulsed as it rotated cyclonically around the eye, then persisted in nearly the same location for approximately one hour before dissipating in the southwest quadrant. Since the eyewall convection rapidly weakened immediately after the dissipation of the eyewall CE, the CE may have contributed to temporarily sustaining the eyewall. Although Erin bore many similarities to those of oceanic TCs, one distinct difference was that, rather than exhibiting a circular eyewall surrounded by rainbands, Erin’s eyewall convection was connected to its primary precipitation band, a radar reflectivity characteristic reminiscent of midlatitude cyclones.
The results of this study are a first step toward understanding how total lightning and dual-polarization observations might be used to diagnose intensification of land-falling tropical systems. With the launch of the Geostationary Operational Environmental Satellite R series (GOES-R) in 2015 [with its Geostationary Lightning Mapper (GLM)], total (CG plus IC) lightning data will be available for continuous systematic monitoring of tropical systems located over the Americas and adjacent oceanic regions (Gurka et al. 2006; Fierro et al. 2013; Goodman et al. 2013). Also, with recent polarimetric upgrades to the U.S. WSR-88D network, dual-polarization data will be available for multiple coastal locations, allowing for observations of the microphysical properties of TC clouds and precipitation, and their relation to intensity changes.
The 3D lightning morphology within Erin was well correlated with the intensity of the deeper convection during its reintensification. Differences in the distribution of lightning activity were attributable to variations in the depth of convection (e.g., MacGorman et al. 1989), with convection being deeper and producing more lightning in the primary outer precipitating band than within the eyewall. Within the main precipitating band, 36-dBZ echo tops commonly exceeded 11 km and occasionally reached 14 km. Within the inner-50-km radius, flash rates were generally smaller and 36-dBZ echo tops were shallower, typically ≤6 km, although CE towers reached 11 km during peak intensity. Depolarization streaks were ubiquitous within the system, indicative of the strongly electrified environment aloft.
It is clear that deep, electrified convection near the center of circulation preceded the formation of Erin’s distinct eyelike signature. More than two hours before the remnants of Erin formed an eye, the minimum MSLP dropped significantly and the wind speeds increased. These changes corresponded to an increase in flash rates and in VHF source densities, especially within 50 km of the circulation center at altitudes near 9–10 km. As Erin deepened to its minimum MSLP and the eye began to form, total flash rates, IC flash rates, CG flash rates, and the height and magnitude of VHF source density all exhibited a sharp increase. Storm-total peak values of these lightning parameters all occurred a few minutes after Erin reached its maximum intensity (in terms of minimum MSLP and maximum wind speeds), which was approximately 25 min before the beginning of the eye phase. The height and magnitude of VHF source densities within 50 km of the circulation center also increased at this time, but reached their maximum values a few minutes after the beginning of the eye phase. Lightning remained very active during the first half of the eye phase, but declined rapidly during the second half (when the eyewall convection appeared most clearly in ZH).
The decline in lightning activity was particularly rapid within 50 km of the circulation center. The dramatic reduction in electrification in this region during the last half of the eye phase was a result of the deep convective cluster in the main precipitating band, southeast of the center of circulation, shifting outside the 50-km radius. Although the lightning activity of this cluster clearly dominated the overall electrical evolution of the system, the relatively smaller contribution of the highly electrified eyewall CE remained important.
Since convective growth and various measures of updraft strength are positively correlated with IC and total flash rates (e.g., MacGorman et al. 1989; Reap and MacGorman 1989; Baker et al. 1995; Wiens et al. 2005; Calhoun et al. 2013), the data for Erin suggest updrafts became more vigorous shortly before the formation of Erin’s eye and remained vigorous during the first half of the eye phase, consistent with the taller, more distinct ZDR columns observed during this period. In the past years, several studies documented that enhanced eyewall lightning activity occurs in some TCs during intensification (e.g., Molinari et al. 1994, 1999; Kelley et al. 2004; Price et al. 2009; Abarca et al. 2011; Fierro et al. 2011), which appears consistent with the findings herein.
Time series of electrical activity and surface observations demonstrate that the maximum lightning activity occurred prior to and during Erin’s reintensification. The increase in lightning activity as Erin’s MSLP decreased suggests deep convection may have played a role in Erin’s intensification. Furthermore, the dissipation of the eyewall following the disappearance of the CE in the eyewall suggests the deep CE may have played a role in eyewall maintenance. Observations of Erin’s polarimetric and electrical structure and evolution support that electrically active inner-core convective bursts can precede or accompany TC intensification and suggests that inner-core lightning outbreaks may be a useful tool in predicting potential TC intensity changes.
Acknowledgments
This work evolved from a master’s thesis (by the first author) at the University of Oklahoma. The authors thank the NSSL/CIMMS employees who maintain and operate the OK-LMA and KOUN polarimetric radar for research-grade operations. Funding was provided by NOAA/Office of Oceanic and Atmospheric Research under NOAA–University of Oklahoma Cooperative Agreements NA17RJ1227 and NA11OAR4320072, U.S. Department of Commerce, and by the U.S. National Weather Service, Federal Aviation Administration, and Department of Defense program for modernization of NEXRAD radars. Partial support for the research was provided by the National Science Foundation Grant ATM-0233268 and ATM-0924621. The authors thank the Oklahoma Climatological Survey for supplying the Oklahoma Mesonet data used in this study. We would also like to thank Dr. David Jorgensen of NSSL for providing a thorough review of this manuscript, and two anonymous reviewers for useful comments that helped improve the manuscript.
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