The San Angelo, Texas, Supercell of 31 May 1995: Visual Observations and Tornadogenesis

Roger M. Wakimoto Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California

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Hanne V. Murphy Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California

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Huaqing Cai Advanced Study Program, National Center for Atmospheric Research,* Boulder, Colorado

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Abstract

Airborne radar analysis combined with cloud photogrammetry of a supercell that formed on 31 May 1995 near San Angelo, Texas, is presented. The superposition of the Doppler wind syntheses with the cloud pictures provides a unique view of the relationship between the low-level mesocyclone and the developing lowered cloud base. The mesocyclone accompanying the storm went through two periods of intensification. The end of the first period of intensification did not lead to the formation of a tornado. An occlusion downdraft formed but was primarily driven by negative buoyancy rather than a downward-directed perturbation pressure gradient. Recent studies have suggested that cold downdrafts inhibit the formation of tornadoes. The present study supports this hypothesis and provides the first quantitative view of the vertical structure of this downdraft. The mesocyclone evolved through a second period of intensification that produced a weak tornado that was visually apparent from the aircraft. No occlusion downdraft was resolved during this latter period. The unusual aspect of this tornado is that it appeared to develop outside of the low-level mesocyclone. This type of tornadogenesis has not been previously shown.

The National Center for Atmospheric Research is sponsored by the National Science Foundation

Corresponding author address: Dr. Roger M. Wakimoto, Department of Atmospheric Sciences, UCLA, 405 Hilgard Ave., Los Angeles, CA 90095-1565. Email: roger@atmos.ucla.edu

Abstract

Airborne radar analysis combined with cloud photogrammetry of a supercell that formed on 31 May 1995 near San Angelo, Texas, is presented. The superposition of the Doppler wind syntheses with the cloud pictures provides a unique view of the relationship between the low-level mesocyclone and the developing lowered cloud base. The mesocyclone accompanying the storm went through two periods of intensification. The end of the first period of intensification did not lead to the formation of a tornado. An occlusion downdraft formed but was primarily driven by negative buoyancy rather than a downward-directed perturbation pressure gradient. Recent studies have suggested that cold downdrafts inhibit the formation of tornadoes. The present study supports this hypothesis and provides the first quantitative view of the vertical structure of this downdraft. The mesocyclone evolved through a second period of intensification that produced a weak tornado that was visually apparent from the aircraft. No occlusion downdraft was resolved during this latter period. The unusual aspect of this tornado is that it appeared to develop outside of the low-level mesocyclone. This type of tornadogenesis has not been previously shown.

The National Center for Atmospheric Research is sponsored by the National Science Foundation

Corresponding author address: Dr. Roger M. Wakimoto, Department of Atmospheric Sciences, UCLA, 405 Hilgard Ave., Los Angeles, CA 90095-1565. Email: roger@atmos.ucla.edu

1. Introduction

The introduction of airborne Doppler radars has provided a unique opportunity to place these mobile platforms in close juxtaposition with severe storms (e.g., Wakimoto et al. 1996; Bluestein et al. 1997a,b; Dowell et al. 1997; Wakimoto et al. 1998; Wakimoto and Liu 1998; Wakimoto and Cai 2000; Ziegler et al. 2001; Dowell and Bluestein 2002a,b). Moreover, improvements in radar designs combined with increased skill in flying near these destructive storms has produced datasets that are able to resolve supercell structure in greater detail than was previously possible and also has raised a number of new questions concerning the processes that lead to tornadogenesis (e.g., Wakimoto et al. 1998; Trapp 1999; Wakimoto and Cai 2000; Dowell and Bluestein 2002a,b). Wakimoto and Liu (1998) and Trapp (1999) examined the Garden City, Kansas, mesocyclone during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX; Rasmussen et al. 1994) using two different airborne platforms. Both studies arrived at the same conclusion: intense rotation within the low-level mesocyclone prior to tornadogenesis results in a downward-directed perturbation pressure gradient that induces a downdraft near the central axis of the mesocyclone. This downflow terminates in low- level radial outflow that turns vertical in an annular updraft at an outer radius. This downdraft has been referred to as the occlusion downdraft by Klemp and Rotunno (1983) and results in a two-celled vortex (Rotunno 1984). Rotunno (1984) has shown that this vortex can be unstable to three-dimensional perturbations; accordingly, subsidiary vortices can form in an annular ring that surrounds the downdraft. Wakimoto and Liu (1998) and Trapp (1999) suggest that this mechanism could have triggered tornadogenesis in the larger-scale mesocyclone. Unfortunately, the examination of the Garden City supercell is only one case study, and its evolution may not be representative of other tornadic supercells.

On 31 May 1995, several supercells developed over central Texas near San Angelo during VORTEX. A total of 15 passes were made by one supercell using the National Center for Atmospheric Research (NCAR) Electra Doppler Radar (ELDORA), covering a period of over 2 h. The evolution of the low-level mesocyclone was documented, and the storm produced a weak tornado during the data collection period. The analysis of the San Angelo supercell presents an opportunity to document the similarities and differences with the Garden City tornadic storm using wind syntheses of comparable temporal and spatial resolution. This case is also enhanced by the excellent visibility of the San Angelo storm from the aircraft on this day. A number of photographs of the low-level structure of the storm were taken from the Electra aircraft. These pictures were photogrammetrically merged with the Doppler radar data in order to provide insight into the structural relationship between the mesocyclone and the visual characteristics of the storm beneath cloud base.

Section 2 provides a brief overview of VORTEX and the ELDORA platform. The origins of the San Angelo storm and the environmental conditions are presented in section 3. An overview of the supercell storm and the ELDORA flight tracks are shown in section 4. Section 5 presents dual-Doppler wind syntheses combined with cloud photogrammetry to document the evolution of the supercell, and section 6 provides a summary and concluding remarks.

2. VORTEX and ELDORA

a. VORTEX

The field phase of VORTEX occurred during the spring months of 1994 and 1995. The primary focus of the experiment was to study issues concerning tornadoes and tornadic storms. The main data platforms were a dozen instrumented vehicles, the National Oceanic and Atmospheric Administration (NOAA) P-3 (Jorgensen et al. 1983) and NCAR Electra aircraft (Hildebrand et al. 1994, 1996). The two aircraft were equipped with tail Doppler radars that were capable of collecting Doppler velocities that could be synthesized into three-dimensional wind fields. The reader is referred to Rasmussen et al. (1994) for more information on the experimental design and other observing platforms deployed during VORTEX.

b. ELDORA

Although there were a number of platforms deployed during VORTEX, the present case study was based solely on data collected by ELDORA. The P-3 and the ground teams were unavailable on this day. The capabilities of ELDORA are discussed in detail by Hildebrand et al. (1994, 1996) and Wakimoto et al. (1996), and the list of scanning parameters implemented during data collection on this day is shown in Table 1. The along-track (∼300 m) and sweep-angle (1.44°) resolutions and the range of the maximum unambiguous velocities (±78.8 m s−1) are noteworthy. The interested reader is referred to Hildebrand et al. (1994, 1996) for a comprehensive discussion of the radar design of ELDORA.

ELDORA uses a multiple-beam technique referred to as the Fore/Aft Scanning Technique (FAST). This technique requires the antennae onboard the aircraft to be scanning fore and aft relative to the normal to the fuselage by ±18.5°. Accordingly, the angle of intersection of the radar scans is nominally 37°. This angle was chosen to minimize the time between points where the fore and aft scans intersect, while ensuring that the geometric angle of intersection between the beams is large enough to lower the standard deviations of the estimates of the three-dimensional wind field to an acceptable level. A complete description of this scanning methodology can be found in Jorgensen et al. (1996).

3. Storm initiation and the environmental conditions

The upper-level flow (not shown) strongly supported the development of supercell storms over Texas. Southerly flow at low levels was advecting warm, moist air from the Gulf of Mexico. The morning sounding closest to the area where the storms formed was launched from Midland, Texas (Fig. 1). The black line in Fig. 1 represents the parcel path if the convective temperature was reached [∼3000 J kg−1 of convective available potential energy (CAPE)].

Deep convection initiated at several locations along a well-defined outflow boundary, but the first supercell appeared to develop at the intersection of the outflow and the dryline. The surface analysis of equivalent potential temperature at 2000 UTC (hereafter all times are in UTC) reveals the location of the dryline that had moved into western Texas by midafternoon (Fig. 2a). The elliptically shaped region that defines the main outflow boundary is shown by the low values of equivalent potential temperature in Fig. 2. Note the well-defined arc cloud in the image. The anvil associated with the supercell extends over a larger area at 2200 (Fig. 2b), and the location of the San Angelo tornado is indicated by the black star (the tornado was observed from the aircraft at 2239). The San Angelo storm propagated from 317° at 13 m s−1 based on the analysis of reflectivity data collected by ELDORA.

4. The ELDORA flight tracks, overview of the supercell echo, and the mesocyclone

The flight track of the Electra is shown in Fig. 3. The primary flight pattern during VORTEX was one in which the aircraft flew close to the storm and at low altitudes (300 m above ground level (AGL); hereafter all heights are AGL) in an approximate racetrack pattern. The distances were variable but were typically 10– 15 km away from the mesocyclone. The radar data collected while executing this pattern is able to resolve the hook echo and mesocyclone in detail. However, a wind synthesis can only be performed on the lower half of the storm since Doppler velocities collected at high elevation angles are highly influenced by the vertical velocities and terminal fall speeds of the hydrometeors. The tracks shown are consolidated into two primary regions (Fig. 3). One is north of the boxed-in area and represents a series of flybys by a supercell storm that developed near Sweetwater, Texas. Intervening convection (not shown) limited the sampling time of this storm. Accordingly, the Electra terminated data collection and flew farther to the south and initiated a series of flight legs on the newly formed San Angelo supercell.

An enlargement of the flight track flown by the San Angelo storm is shown in Fig. 4. Also plotted on the figure is the time series of the supercell echo based on radar reflectivity data recorded by ELDORA. The flight tracks by the storm were nominally 35–40 km long and oriented in an approximate east–west direction. The time to execute each leg varied between 5 and 10 min. Dual- Doppler syntheses were constructed for each of the legs past the storm. A description of the data methodology used to create the kinematic wind fields from ELDORA is presented in the appendix.

The focus of this study is the time period between 2151 and 2254 when a mesocyclone was sampled with high temporal and spatial resolution until it produced a weak tornado. There were nine passes by the storm during this period. The evolution of the vertical vorticity field at low levels is shown in Fig. 5. The mesocyclone undergoes two periods of intensification, one culminating in an intense circulation at 2204–2207 and the other at 2227–2232. The latter period is followed by the formation of a weak tornado. The location of the tornado is shown by the black dot.

The life cycle of the San Angelo low-level mesocyclone is different from the process of cyclic mesocyclogenesis documented in the literature (e.g., Burgess et al 1982; Alderman et al. 1999). Cyclic mesocyclogenesis is nominally associated with the decay of a mesocyclone and the development of a new mesocyclone at a disparate location. The San Angelo mesocyclone remained as a distinct feature for the entire time shown in Fig. 5.

There is a horseshoe-shaped pattern that develops in the vertical vorticity field at 2245–2254 after tornadogenesis, a common characteristic of a mesocyclone entering the tornadic stage (e.g., Lemon and Doswell 1979; Klemp and Rotunno 1983; Brandes 1993; Wicker and Wilhelmson 1995). Note that the tornado is not positioned at the maximum of vertical vorticity but, instead, is located to the south at its periphery. This will be examined in more detail in the next section.

5. Dual-Doppler wind syntheses and photogrammetric analysis

The San Angelo supercell was well sampled by ELDORA. Fortunately, the visibility on this day was among the best observed during VORTEX. Accordingly, a series of pictures were taken from the Electra of features at and below cloud base. To facilitate the interpretation of the ELDORA wind syntheses, the photographs of the visual characteristics of the storm were merged with the radar data using photogrammetric techniques. Photogrammetry requires knowledge of the focal length of the lens as well as the exact camera location/height and the azimuth angles to landmarks visible in the photographs. It is also critical to know the exact time of the photograph taken from an airborne platform. The location was determined from the global positioning system (GPS) data from the aircraft. Vanishing points from several roads that could be identified in the photograph were used to determine the azimuths (Abrams 1952). The azimuth angles are also used to determine the aircraft's nadir at the time of the photograph. The calculated nadir can then be compared with the GPS location to check the accuracy of the photogrammetric calculations. The next step is the construction of an azimuth- and elevation-angle grid that can be superimposed onto the photograph. A general discussion of cloud photogrammetry is given by Holle (1986).

The photographs used in this study were taken from slightly different ranges from the mesocyclone depending on the aircraft location at the time that the picture was taken. This effect was negated by enlarging or reducing each individual photograph to a normalized range of 15 km based on the known photogrammetric parameters. The resizing ensured that the relative dimensions of the Doppler wind syntheses were the same in all figures, hence facilitating comparisons between analysis times. Additional discussion of the superposition of the Doppler wind synthesis and the photographs taken from the aircraft is presented in the appendix.

a. 2151:02–2153:32 UTC

The analysis of the first pass by the storm is shown in Fig. 6. The San Angelo storm was in its formative stages; however, it evolved quickly. Vertical vorticity values in excess of 10 × 10−3 s−1 outline the location of the developing low-level mesocyclone (Fig. 6c). No closed circulation could be identified in the storm-relative wind field. In addition, a rotational couplet was not apparent in the single-Doppler velocities (Fig. 6b). The incipient stages of a rear-flank gust front, echo appendage, and weak echo region are also apparent (Figs. 6a and 6b). A quasi-circular feature below cloud base is evident in the photograph taken from the Electra (Fig. 7a).1 This is believed to be the developing wall cloud. Overall cloud base is approximately 2 km; however, the lowered cloud features extend down to near 1 km. The axis of the mesocyclone tilts through the middle of the lowered cloud base, as shown in the vertical vorticity profile (Fig. 7c). Note that the diameter of the lowered cloud base is larger than the area associated with vorticity values greater than 10 × 10−3 s−1.

Strong echoes, greater than 50 dBZ, within the developing appendage are located at the western periphery (left) of the lowered cloud base. The low-level inflow and main updraft into the storm can be seen to the east of the mesocyclone (Fig. 7d). Another updraft is located to the west of the lowered cloud-base feature, and weak negative vertical velocities are positioned near the center of the developing wall cloud. A retrieval of the perturbation pressure and thermal fields was performed at this time in order to examine the forcing mechanisms of the updrafts and downdraft (see the appendix for an explanation of the pressure retrieval). The majority of the air within the main updraft was positively buoyant (Fig. 7e).

The downdraft appears to be forced by a combination of precipitation loading and a weak downward-directed pressure gradient located above cloud base (Fig. 7f). Klemp and Rotunno (1983) have referred to downflow near the surface position of the mesocyclone as the occlusion downdraft. The downdraft is dynamically driven by a downward-directed pressure gradient that develops in response to strong low-level rotation within the mesocyclone that exceeds values aloft. The intense, low- level circulation produces a mesolow owing to the fluid shear terms in the diagnostic perturbation pressure equation (e.g., Klemp and Rotunno 1983; Brandes 1984). It should be noted that pressure retrievals are based on a diagnostic relationship with the wind field, so inferring causality between the downdraft and the perturbation pressure field should be viewed with some caution.

The presence of the downdraft near the mesocyclone shown in Fig. 7 is different from the cases documented by Wakimoto et al. (1998) and Wakimoto and Cai (2000) since the low-level mesocyclone is in its formative stages and the maximum vertical vorticity is not located near the surface. Indeed, a well-defined circulation is not evident in the synthesized wind field (see Fig. 6a). Moreover, the pressure gradient force is not the dominant mechanism forcing this downdraft, as documented by Klemp and Rotunno (1983).

b. 2204:34–2207:34 UTC

A hook echo formed by the next synthesis time at 2157:00–2202:01 (not shown). The rear-flank downdraft produced outflow that surged to the south producing an arc-shaped updraft region along the gust front. Maximum updrafts along the front were >10 m s−1 during the next pass by the storm at 2204:34–2207:34 and the vertical vorticity associated with the mesocyclone is greater than 15 × 10−3 s−1 (Fig. 8c). A circulation has developed in the storm-relative wind field (Fig. 8a), and a rotational couplet can be identified in the single-Doppler velocity field at low levels in Fig. 8b.

The major change in the visual characteristics of the storm is the appearance of an intense rainshaft (Fig. 9a) below cloud base. The vorticity within the mesocyclone aloft is >30 × 10−3 s−1, and the total horizontal wind field reveals a discontinuity across the axis of vorticity maximum (Fig. 9c). Also apparent in the figure is the anticyclonic vorticity to the west (left) of the mesocyclone (also see Fig. 8c), a frequently observed feature at low levels (e.g., Klemp and Rotunno 1983). A strong downdraft (speeds greater than 30 m s−1) has created a bimodal updraft structure (Fig. 9d) with the low-level mesocyclone positioned near the eastern updraft/downdraft interface (Fig. 9c). This evolution is similar to tornadic mesocyclones documented in the literature, although no tornado was noted.

The appearance and strength of the downdraft at this time is different from past studies since the largest vorticity values are aloft rather than at the surface (e.g., Klemp and Rotunno 1983; Wicker and Wilhelmson 1995; Wakimoto et al. 1998). The forcing mechanisms shown in Figs. 9e and 9f suggest that the vertical perturbation pressure gradient played a relatively minor role in creating this downdraft. The pressure gradient force is upward-directed at high levels. The downward- directed component is confined to the lowest levels and is weak. The major forcing for the observed downdraft is the shaft of negatively buoyant air (Fig. 9e) associated with evaporative cooling and a secondary contribution from precipitation loading (Fig. 9f). This cool air extends to the surface and spreads out behind the rear- flank gust front. The net forcing (buoyancy plus perturbation pressure gradient force plus precipitation loading) is negative at the location of the downdraft (not shown), supporting downward acceleration at this location.

c. 2210:21–2216:50 UTC

The weak echo region signifying the primary storm inflow and the hook echo are still evident at 2210–2216 (Fig. 10a). However, the mesocyclone has weakened (Fig. 10c) and a circulation can no longer be identified within the storm-relative wind field. Note the lack of a distinct rotation couplet in the single-Doppler velocities in Fig. 10b. An elongated band of updrafts still defines the position of the rear-flank gust front (Fig. 10c).

Low clouds developing along the gust front and an intense rainshaft obscured the western (left) part of the lowered cloud base (Fig. 11a). The profile of vertical vorticity (Fig. 11c) reveals that the weakening of the mesocyclone occurred at all levels. Indeed, peak values of vertical vorticity greater than 30 × 10−3 s−1 in Fig. 9c have fallen to values near 10 × 10−3 s−1 in Fig. 11c. The anticyclonic vorticity noted at the earlier time has also diminished. The downdraft has dissipated, but its remnants can be identified by the bimodal updraft structure (Fig. 11d).

The appearance of a strong occlusion downdraft within the mesocyclone leading to an updraft/downdraft structure wrapping around the low-level circulation is a common observation for mesocyclones entering the tornadic phase (e.g., Lemon and Doswell 1979). The occlusion downdraft, observed during the previous analysis time, was largely driven by evaporative cooling rather than a vertical gradient of the perturbation pressure, as has been observed in previous studies (e.g., Klemp and Rotunno 1983). Markowski et al. (2002) have suggested, based on direct surface measurements, that evaporative cooling could play a more important role in the formation of rear-flank downdrafts in nontornadic supercells. They surmised that the likelihood of tornadogenesis increased when surface buoyancy, among other variables, in the rear-flank downdraft increased. The results presented in Fig. 9 support this hypothesis and provide the first kinematic and thermodynamic depiction of this cold downdraft above the ground. This scenario has been referred to as “tornadogenesis failure” (Trapp 1999) and is now recognized as a common feature in supercells (e.g., Wakimoto and Cai 2000; Markowski et al. 2002).

d. 2227:01–2232:30 UTC

The pass by the storm at 2218–2222 is marked by a reintensification of the mesocyclone (not shown). This strengthening continues into the next analysis time (Fig. 12). The maximum vertical vorticity is >30 × 10−3 s−1 (Fig. 12c), and there is a well-defined circulation in the storm-relative wind field (Fig. 12a). The rotational couplet in Fig. 12b exhibits a clockwise rotation, suggesting that convergence exists.

The largest vorticity values within the mesocyclone are confined to low levels (Fig. 13c), unlike the previous intensification period when the largest values were located aloft. The mesocyclone circulation, rotational couplet, and the vertical velocity and vorticity are better defined in the enlargement shown in Fig. 14. The leading edge of the large gradients of Doppler velocity is consistent with the location of the gust fronts (Fig. 14b). An axis of vertical vorticity maximum extends to the northeast (Figs. 12c and 14c) along the forward-flank gust front. There is no obvious extension along the rear- flank gust front. Wakimoto et al. (1998) also noted a vorticity extension along the forward-flank near the Garden City mesocyclone prior to tornadogenesis, and Wicker and Wilhelmson (1995) documented a similar ridge in their simulation of tornado development.

An occlusion downdraft is not apparent, although the updraft still exhibits a bimodal structure (Fig. 13d). The bimodal pattern appears to be the result of the shallow downward-directed pressure gradient created in response to the developing mesolow (Fig. 13f). The negative pressure gradient retards the updrafts in the vertical column above the mesocyclone. Large values of negative buoyancy are located at low levels as the cold air begins to spiral around the southern periphery of the mesocyclone (Figs. 13e and 14c). The cold air behind the gust front produces a mesohigh (not shown), resulting in an upward-directed perturbation pressure gradient at low levels to the west of the mesocyclone (Fig. 13f).

e. 2236:40–2244:32 UTC

A tornado was observed from the Electra during the next synthesis time. The low-level mesocyclone is evident in the vorticity analysis and the storm-relative wind field (Fig. 15c). An enlarged view of this region is shown in Fig. 16. Axes of positive vertical vorticity extend from the mesocyclone along the forward- and rear-flank gust fronts. The wind synthesis is not able to resolve the tornado owing to the imposed filtering routine discussed in the appendix. Fortunately, the tornado was apparent in the single-Doppler velocity plot as a small rotational couplet (Figs. 15b and 16b) and is near the tip of the hook-shaped appendage (Figs. 15a and 16a). The black dot in Figs. 15c and 16c denotes the position of the tornado.

Davies-Jones et al. (2001) define the mesocyclone as the region characterized by vertical vorticity values greater than 0.01 s−1 based on numerous past studies. Accordingly, the tornado in Figs. 15c and 16c is located outside of the low-level mesocyclone. Brandes (1978), Dowell and Bluestein (2002a), and Wakimoto et al. (2003) have documented cases where the tornado was displaced from the geometric center of the mesocyclone; however, they were still located well within the mesocyclonic vertical vorticity. The San Angelo tornado appears to be an example of tornadogenesis occurring outside of a strong mesocyclone. Indeed, the tornado is positioned between the anticyclonic and cyclonic vorticity maxima in Figs. 15c and 16c.

This is not believed to be an example of a “gustnado” forming along the flanking line of a supercell (Davies- Jones 1986). The flanking line, as delineated in the updraft and vorticity analyses, is several kilometers to the east of the tornado (Fig. 16c). Hence, it appears that this tornado belongs to the general class of mesocyclonic tornadoes as discussed by Davies-Jones et al. (2001). The mechanism that produced the San Angelo tornado is not readily apparent in the present case.

The funnel cloud and a wall cloud can be seen in the photograph taken from the aircraft (Fig. 17a). The funnel is ∼1 km wide, and a surface dust cloud below the funnel was noted in the original pictures although it cannot be seen in the black and white reproduction shown in Fig. 17. The tornado was weak, but the F- scale rating is not known. An extensive aerial survey performed several days after the event could not identify a damage track, suggesting that the tornado was most likely in the F0–F1 range.

The position of the rotational couplet agrees with the location of the funnel shown in Fig. 17b. The tornado is embedded within convergent flow and an axis of strong updrafts >40 m s−1 (Figs. 17c and 17e). In addition, it is in a region characterized by a cyclonic wind shift at low levels (Fig. 17d) and located along the western edge of the hook echo (Fig. 17c; also apparent in Fig. 16a). The unusual aspect of this study was the absence of an occlusion downdraft during the sequence of events leading up to tornadogenesis. No downdraft near or within the mesocyclone was resolved during this time. Recall that an occlusion downdraft was apparent during the first period of intensification of the low-level mesocyclone (section 5b). It is possible that the downdraft developed and, subsequently, weakened between analysis times. However, other case studies with comparable temporal resolution based on airborne Doppler radar data have been able to resolve this downdraft (e.g., Wakimoto et al. 1998; Trapp 1999; Wakimoto and Cai 2000).

6. Summary and concluding remarks

A comprehensive dataset using airborne Doppler radar was collected on a supercell storm that produced a weak tornado near San Angelo, Texas. The interpretation of the radar analysis was facilitated by a series of photographs taken from the aircraft of features below cloud base. The superposition of the Doppler wind syntheses on top of the cloud pictures provides a unique view of the relationship between the low-level mesocyclone and the developing lowered cloud base. The only other study that illustrates this relationship was presented by Wakimoto and Liu (1998); however, their analysis was only for a single time.

The mesocyclone at low levels evolved through two periods of intensification. The first period was accompanied by a strong occlusion downdraft within the mesocyclone that resulted in a bimodal updraft structure and a horeshoe-shaped updraft at low levels. These observations often precede the formation of a tornado, although none was observed during this period.

The forcing mechanism of the occlusion downdraft was different from previous cases that have been reported in the literature. Past studies suggest that strong rotation within the low-level mesocyclone produces a mesolow owing to the fluid shear terms in the diagnostic perturbation pressure equation. The mesolow creates a downward-directed pressure gradient that is the primary forcing mechanism for the occlusion downdraft. In the present case, however, the strongest vorticity values were aloft, and the retrieved thermodynamic fields suggest that the downdraft was driven by negative buoyancy. Recently, Markowski et al. (2002) proposed, based on surface observations, that negative buoyancy within the downdraft increases the likelihood of “tornadogenesis failure.” Accordingly, the cold downdraft in the present case may have inhibited the formation of the tornado. The current study reveals the vertical kinematic and thermodynamic structure of this downdraft.

The San Angelo mesocyclone underwent a second period of intensification. Interestingly, no occlusion downdraft was resolved, although it is possible that the downdraft was present but formed between syntheses times (∼7 min). A brief and weak tornado was visually observed and also detected in the single-Doppler velocity data. The tornado was associated with a funnel cloud pendant from cloud base that was large (∼1 km in diameter). The tornado developed outside of the low- level mesocyclone as defined by the vertical vorticity pattern. Previous studies have shown tornadogenesis displaced from the geometric center but still within the mesocyclone circulation. Future research should endeavor to combine airborne observations with high temporal data collected by ground-based mobile radars. To date, this has yet to be accomplished.

Acknowledgments

Comments from two anonymous reviewers substantially improved an earlier version of this manuscript. Research results presented in this paper were supported by the National Science Foundation under Grants ATM-9801720 and ATM-0121048.

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  • Gal-Chen, T., 1978: A method for the initialization of the anelastic equations: Implications for matching models with observations. Mon. Wea. Rev, 106 , 587606.

    • Search Google Scholar
    • Export Citation
  • Gal-Chen, T., and R. A. Kropfli, 1984: Buoyant and pressure perturbations derived from dual-Doppler radar observations of the planetary boundary layer: Applications for matching models with observations. J. Atmos. Sci, 41 , 30073020.

    • Search Google Scholar
    • Export Citation
  • Hildebrand, P. H., C. A. Walther, C. L. Frush, J. Testud, and F. Baudin, 1994: The ELDORA/ASTRAIA airborne Doppler weather radar: Goals, design, and first field tests. Proc. IEEE, 82 , 18731890.

    • Search Google Scholar
    • Export Citation
  • Hildebrand, P. H., and Coauthors, 1996: The ELDORA/ASTRAIA airborne Doppler weather radar: High resolution observations from TOGA COARE. Bull. Amer. Meteor. Soc, 77 , 213232.

    • Search Google Scholar
    • Export Citation
  • Holle, R. L., 1986: Photogrammetry of thunderstorms. Thunderstorms: A Social and Technological Documentary, E. Kessler, Ed., Vol. 3, University of Oklahoma Press, 77–98.

    • Search Google Scholar
    • Export Citation
  • Jorgensen, D. P., P. H. Hildebrand, and C. L. Frush, 1983: Feasibility test of an airborne pulse-Doppler meteorological radar. J. Climate Appl. Meteor, 22 , 744757.

    • Search Google Scholar
    • Export Citation
  • Jorgensen, D. P., T. Matejka, and J. D. DuGranrut, 1996: Multi-beam techniques for deriving wind fields from airborne Doppler radars. Meteor. Atmos. Phys, 59 , 83104.

    • Search Google Scholar
    • Export Citation
  • Joss, J., and D. Waldvogel, 1970: Raindrop size distribution and Doppler velocities. Preprints, 14th Conf. on Radar Meteorology, Tucson, AZ, Amer. Meteor. Soc., 153–156.

    • Search Google Scholar
    • Export Citation
  • Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci, 40 , 359377.

  • Leise, J. A., 1982: A multidimensional scale-telescoped filter and data extension package. NOAA Tech. Memo. ERL WPL-82, 19 pp. [Available from NOAA ERL, 325 Broadway, Boulder, CO 80303.].

    • Search Google Scholar
    • Export Citation
  • Lemon, L. R., and C. A. Doswell, 1979: Severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis. Mon. Wea. Rev, 107 , 11841197.

    • Search Google Scholar
    • Export Citation
  • Markowski, P. M., J. M. Straka, and E. N. Rasmussen, 2002: Direct surface thermodynamic observations within the rear-flank downdrafts of nontornadic and tornadic supercells. Mon. Wea. Rev, 130 , 16921721.

    • Search Google Scholar
    • Export Citation
  • Mohr, C. G., L. J. Miller, R. L. Vaughn, and H. W. Frank, 1986: The merger of mesoscale datasets into a common Cartesian format for efficient and systematic analyses. J. Atmos. Oceanic Technol, 3 , 143161.

    • Search Google Scholar
    • Export Citation
  • Oye, R., C. Mueller, and S. Smith, 1995: Software for radar translation, visualization, editing, and interpolation. Preprints, 27th Conf. on Radar Meteorology, Vail, CO, Amer. Meteor. Soc., 359–361.

    • Search Google Scholar
    • Export Citation
  • Rasmussen, E. N., J. M. Straka, R. Davies-Jones, C. A. Doswell III, F. H. Carr, M. D. Eilts, and D. R. MacGorman, 1994: Verification of the origins of rotation in tornadoes experiment: VORTEX. Bull. Amer. Meteor. Soc, 75 , 9951006.

    • Search Google Scholar
    • Export Citation
  • Rotunno, R., 1984: An investigation of a three-dimensional asymmetric vortex. J. Atmos. Sci, 41 , 283298.

  • Roux, F., 1985: Retrieval of thermodynamic fields from multiple- Doppler radar data using the equations of motion and the thermodynamic equation. Mon. Wea. Rev, 113 , 21422157.

    • Search Google Scholar
    • Export Citation
  • Roux, F., 1988: The west African squall line observed on 23 June 1981 during COPT 81: Kinematics and thermodynamics of the convective region. J. Atmos. Sci, 45 , 406426.

    • Search Google Scholar
    • Export Citation
  • Roux, F., and J. Sun, 1990: Single-Doppler observations of a west African squall line on 27–28 May 1981 during COPT 81: Kinematics, thermodynamics and water budget. Mon. Wea. Rev, 118 , 18261854.

    • Search Google Scholar
    • Export Citation
  • Roux, F., V. Marécal, and D. Hauser, 1993: The 12/13 January 1988 narrow cold-frontal rainband observed during MFDP/FRONTS 87. Part I: Kinematics and thermodynamics. J. Atmos. Sci, 50 , 951974.

    • Search Google Scholar
    • Export Citation
  • Trapp, R. J., 1999: Observations of nontornadic low-level mesocyclones and attendant tornadogenesis failure during VORTEX. Mon. Wea. Rev, 127 , 16931705.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., and C. Liu, 1998: The Garden City, Kansas, storm during VORTEX 95. Part II: The wall cloud and tornado. Mon. Wea. Rev, 126 , 393408.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., and H. Cai, 2000: Analysis of a nontornadic storm during VORTEX 95. Mon. Wea. Rev, 128 , 565592.

  • Wakimoto, R. M., W-C. Lee, H. B. Bluestein, C-H. Liu, and P. H. Hildebrand, 1996: ELDORA observations during VORTEX 95. Bull. Amer. Meteor. Soc, 77 , 14651481.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., C. Liu, and H. Cai, 1998: The Garden City, Kansas, storm during VORTEX 95: Part I: Overview of the storm's life cycle and mesocyclogenesis. Mon. Wea. Rev, 126 , 372392.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., H. V. Murphey, D. C. Dowell, and H. B. Bluestein, 2003: The Kellerville tornado during VORTEX: Damage survey and Doppler radar analyses. Mon. Wea. Rev, 131 , 21972221.

    • Search Google Scholar
    • Export Citation
  • Wicker, L. J., and R. B. Wilhelmson, 1995: Simulation and analysis of tornado development and decay within a three-dimensional supercell thunderstorm. J. Atmos. Sci, 52 , 26752703.

    • Search Google Scholar
    • Export Citation
  • Ziegler, C. L., E. N. Rasmussen, T. R. Shepherd, A. I. Watson, and J. M. Straka, 2001: The evolution of low-level rotation in the 29 May 1994 Newcastle–Graham, Texas, storm complex during VORTEX. Mon. Wea. Rev, 129 , 13391368.

    • Search Google Scholar
    • Export Citation

APPENDIX

Radar Meteorology

The radar methodology is similar to the one used by Wakimoto et al. (1998). The along-track and sweep- angle resolutions were ∼300 m and 1.44°, respectively, during VORTEX. This sweep-angle resolution resulted in an effective sampling in the vertical of ∼400 m in the analysis domain. As a result, the reflectivity and Doppler velocity data were interpolated onto a Cartesian grid with horizontal and vertical grid spacing of 300 and 400 m, respectively. The lowest grid level was located at 300 m AGL.

The individual radar scans for each pass were time– space adjusted using a velocity of 317° at 13 m s−1 corresponding to the propagation speed of the storm. The aircraft motion was removed from the velocity data by using SOLO software (Oye et al. 1995). A Cressman (1959) filter was applied in the interpolation process with a radius of influence of 300 and 400 m in the horizontal and vertical directions, respectively. The synthesis of the radar data used Custom Editing and Display of Reduced Information in Cartesian Space (CEDRIC; Mohr et al. 1986). The hydrometeor fall speeds were estimated from the reflectivity–terminal fall speed relationship established by Joss and Waldvogel (1970) with a correction for the effects of air density (Foote and du Toit 1969). A three-step Leise (1982) filter, which considerably damps wavelengths up to 3.6 km and removes wavelengths of less than 2.4 km, was applied to the Doppler wind syntheses. Accordingly, the analyzed wind fields and derived quantities (e.g., vertical velocity and vorticity) are not able to resolve tornadic circulations. Instead, only the larger-scale features such as the mesocyclone and the attendant gust fronts are resolvable.

The vertical velocities were obtained from the anelastic continuity equation by upward integration of the horizontal convergence field. The integration terminated at the 4.8-km level owing to accrued errors. The emphasis in the present study is on the low-level wind fields. Therefore, the neglect of the kinematics in the upper regions of the storm is not considered serious. Additional details of the expected standard deviations of the vertical velocity fields can be found in Wakimoto et al. (1998) and Wakimoto and Cai (2000).

Gal-Chen (1978) first proposed the use of three-dimensional winds synthesized from multi-Doppler radar analyses to retrieve the total perturbation pressure and density patterns using a least squares method in a horizontal plane. The retrieval treats the pressure and temperature as unknown variables in the anelastic momentum equations and solves the derived Poisson equation using the Doppler wind field. While this method retrieves individual horizontal cross sections of perturbation pressure, it does not reveal its vertical structure. Roux (1985, 1988), Roux and Sun (1990), and Roux et al. (1993) were able to modify Gal-Chen's method in order to retrieve the full three-dimensional perturbation pressure. This was accomplished by deriving a thermodynamic equation that relates the advection of temperature to the latent heat released through condensation or absorbed through melting and evaporation while neglecting other diabatic heat sources and sinks. It is assumed that saturated and unsaturated conditions occur during the production and removal of precipitation, respectively. The time tendency term was also included since the temporal resolution of the Doppler syntheses was good in the present case.

A momentum check was performed to assess the quality of all of the retrievals. This check also provides a dynamic constraint since a value of 0 when calculating the total perturbation pressure implies that the anelastic vertical vorticity equation has been satisfied. The momentum checks for the present case ranged between 0.21 and 0.29, which is well within the acceptable values defined by Gal-Chen and Kropfli (1984).

Photogrammetric analyses were performed on a number of pictures of the storm as discussed in section 5. These pictures were combined with the dual-Doppler syntheses in order to facilitate the interpretation of the storm evolution. Figure A1 depicts the superposition of the grid points from the synthesis onto a photograph taken from the Electra at 2239:07 UTC. The grid points depicted are for the cross section that slices through the low-level mesocyclone. The figure illustrates the high spatial resolution of the wind syntheses for this and other analysis times shown in the paper.

Fig. 1.
Fig. 1.

Sounding launched from Midland, TX, on 31 May 1995 at 1100 UTC. The solid and dashed black lines represent temperature and dewpoint temperatures, respectively. Vertical profile of the wind is shown with the half barb, full barb, and flag representing 2.5, 5.0, and 25.0 m s−1, respectively. The thick, black line is the parcel path if the convective temperature was reached

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 2.
Fig. 2.

Surface analysis of equivalent potential temperature superimposed onto high-resolution visual satellite images at (a) 2000 and (b) 2200 UTC. The launch sites of the Midland (MAF) and Fort Worth (FTW), TX, soundings are shown in (b). The location of the San Angelo tornado is indicated by the black star in (b)

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 3.
Fig. 3.

The flight track of the Electra on 31 May 1995. The highlighted region is enlarged in Fig. 4. Times along the track are in UTC.

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 4.
Fig. 4.

Enlargement of the box shown in Fig. 3. Flight track of the Electra is shown by the thin black line. Times are in UTC. Evolution of the San Angelo echo at 700 m AGL based on radar reflectivity data collected by ELDORA. The time interval (UTC) for each echo composite is labeled on the figure. Radar reflectivity is drawn as gray lines, with values greater than 40 dBZ shaded gray

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 5.
Fig. 5.

Time evolution of the vertical vorticity field associated with the low-level mesocyclone at 300 m AGL. The black line delineates the path of the center of the mesocyclone. The flight track of the Electra is shown by the gray line. The location of a tornado is shown by the black dot. Contours of vertical vorticity are drawn every 5 × 10−3 s−1, as shown on the figure. The alternating dashed and solid contours are used to isolate the analysis at successive times

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 6.
Fig. 6.

Horizontal cross section of the ELDORA syntheses at 2151:02–2153:32 UTC at 700 m AGL. (a) Radar reflectivity and storm- relative winds. (b) Radar reflectivity and single-Doppler velocities. (c) Vertical vorticity and vertical velocity. Radar reflectivities are drawn as gray lines, with values greater than 40 dBZ shaded gray. Positive and negative values of single-Doppler velocities and vorticity are drawn as black and dashed black lines, respectively. Positive and negative values of vertical velocity are drawn as gray and dashed gray lines, respectively. The black dot represents the position of the maximum vertical vorticity. The flight track of the Electra is drawn on the figure. The black line represents the location of a vertical cross section shown in Fig. 7

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 7.
Fig. 7.

Vertical cross section of the ELDORA wind synthesis at 2151:02–2153:32 UTC superimposed onto a photograph of the San Angelo storm taken at 2152:00 UTC. (a) Azimuth- and elevation-angle grid and height scale valid at the distance of the mesocyclone. (b) Radar reflectivities. (c) Vertical vorticity and horizontal winds. Half barb, full barb, and flag represent 2.5, 5.0, and 25.0 m s−1, respectively. (d) Vertical velocity and winds in the plane of the cross section. (e) Thermal buoyancy. (f) Precipitation loading (dashed lines) and vertical perturbation pressure gradient force

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 7.
Fig. 8.
Fig. 8.

Same as Fig. 6, except for 2204:34–2207:34 UTC

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 9.
Fig. 9.

Same as Fig. 7, except for 2204:34–2207:34 UTC superimposed on a photograph of the storm at 2205:32 UTC

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 9.
Fig. 10.
Fig. 10.

Same as Fig. 6, except for 2210:21–2216:50 UTC

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 11.
Fig. 11.

Vertical cross section of the ELDORA wind synthesis at 2210:21–2216:50 UTC superimposed onto a photograph of the San Angelo storm taken at 2213:08 UTC. (a) Azimuth- and elevation-angle grid and height scale valid at the distance of the mesocyclone. (b) Radar reflectivities. (c) Vertical vorticity and horizontal winds. Half barb, full barb, and flag represent 2.5, 5.0, and 25.0 m s−1, respectively. (d) Vertical velocity and winds in the plane of the cross section

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 11.
Fig. 12.
Fig. 12.

Same as Fig. 6, except for 2227:01–2232:30 UTC. Black box in (a) is enlarged in Fig. 14

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 13.
Fig. 13.

Same as Fig. 7, except for 2227:01–2232:30 UTC superimposed on a photograph of the storm at 2230:03 UTC

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 13.
Fig. 14.
Fig. 14.

Enlargement of the horizontal cross section at 2227:01–2232:59 UTC at 700 m AGL. (a) Radar reflectivity and storm-relative winds. (b) Radar reflectivity and single-Doppler velocities. (c) Vertical vorticity and vertical velocity. Radar reflectivities are drawn as gray lines, with values greater than 40 dBZ shaded gray. Positive and negative values of single-Doppler velocities and vorticity are drawn as black and dashed black lines, respectively. Positive and negative values of vertical velocity are drawn as gray and dashed gray lines, respectively. The black dot represents the position of the maximum vertical vorticity.

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 15.
Fig. 15.

Same as Fig. 6, except for 2236:40–2244:32 UTC. Black box in (a) is enlarged in Fig. 16. Black dot represents the location of the tornado.

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 16.
Fig. 16.

Enlargement of the horizontal cross section at 2236:40–2244:32 UTC at 700 m AGL. (a) Radar reflectivity and storm-relative winds. (b) Radar reflectivity and single-Doppler velocities. (c) Vertical vorticity and vertical velocity. Radar reflectivities are drawn as gray lines, with values greater than 40 dBZ shaded gray. Positive and negative values of single-Doppler velocities and vorticity are drawn as black and dashed black lines, respectively. Positive and negative values of vertical velocity are drawn as gray and dashed gray lines, respectively. The black dot represents the position of the tornado.

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 17.
Fig. 17.

Vertical cross section of the ELDORA wind synthesis at 2236:40–2244:32 UTC superimposed onto a photograph of the San Angelo storm taken at 2239:07 UTC. (a) Azimuth- and elevation-angle grid and height scale valid at the distance of the mesocyclone. (b) Single-Doppler velocities. (c) Radar reflectivities. (d) Vertical vorticity and horizontal winds. Half barb, full barb, and flag represent 2.5, 5.0, and 25.0 m s−1, respectively. (e) Vertical velocity and winds in the plane of the cross section

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Fig. 17.
i1520-0493-132-5-1269-fa01

Fig. A1. Photogrammetric grid for the picture taken at 2239:07 UTC. The azimuth- and elevation-angle and dual-Doppler wind syntheses grids are shown. The black dots represent the position of the individual grid points. The cross section is the one that slices through the center of the low-level mesocyclone

Citation: Monthly Weather Review 132, 5; 10.1175/1520-0493(2004)132<1269:TSATSO>2.0.CO;2

Table 1.

ELDORA scanning mode

Table 1.

1

The quasi-circular nature of the lowered cloud base was evident based on the continuous observations from the Electra as it flew by the storm.

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    • Export Citation
  • Gal-Chen, T., and R. A. Kropfli, 1984: Buoyant and pressure perturbations derived from dual-Doppler radar observations of the planetary boundary layer: Applications for matching models with observations. J. Atmos. Sci, 41 , 30073020.

    • Search Google Scholar
    • Export Citation
  • Hildebrand, P. H., C. A. Walther, C. L. Frush, J. Testud, and F. Baudin, 1994: The ELDORA/ASTRAIA airborne Doppler weather radar: Goals, design, and first field tests. Proc. IEEE, 82 , 18731890.

    • Search Google Scholar
    • Export Citation
  • Hildebrand, P. H., and Coauthors, 1996: The ELDORA/ASTRAIA airborne Doppler weather radar: High resolution observations from TOGA COARE. Bull. Amer. Meteor. Soc, 77 , 213232.

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    • Export Citation
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    • Export Citation
  • Jorgensen, D. P., P. H. Hildebrand, and C. L. Frush, 1983: Feasibility test of an airborne pulse-Doppler meteorological radar. J. Climate Appl. Meteor, 22 , 744757.

    • Search Google Scholar
    • Export Citation
  • Jorgensen, D. P., T. Matejka, and J. D. DuGranrut, 1996: Multi-beam techniques for deriving wind fields from airborne Doppler radars. Meteor. Atmos. Phys, 59 , 83104.

    • Search Google Scholar
    • Export Citation
  • Joss, J., and D. Waldvogel, 1970: Raindrop size distribution and Doppler velocities. Preprints, 14th Conf. on Radar Meteorology, Tucson, AZ, Amer. Meteor. Soc., 153–156.

    • Search Google Scholar
    • Export Citation
  • Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci, 40 , 359377.

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    • Search Google Scholar
    • Export Citation
  • Lemon, L. R., and C. A. Doswell, 1979: Severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis. Mon. Wea. Rev, 107 , 11841197.

    • Search Google Scholar
    • Export Citation
  • Markowski, P. M., J. M. Straka, and E. N. Rasmussen, 2002: Direct surface thermodynamic observations within the rear-flank downdrafts of nontornadic and tornadic supercells. Mon. Wea. Rev, 130 , 16921721.

    • Search Google Scholar
    • Export Citation
  • Mohr, C. G., L. J. Miller, R. L. Vaughn, and H. W. Frank, 1986: The merger of mesoscale datasets into a common Cartesian format for efficient and systematic analyses. J. Atmos. Oceanic Technol, 3 , 143161.

    • Search Google Scholar
    • Export Citation
  • Oye, R., C. Mueller, and S. Smith, 1995: Software for radar translation, visualization, editing, and interpolation. Preprints, 27th Conf. on Radar Meteorology, Vail, CO, Amer. Meteor. Soc., 359–361.

    • Search Google Scholar
    • Export Citation
  • Rasmussen, E. N., J. M. Straka, R. Davies-Jones, C. A. Doswell III, F. H. Carr, M. D. Eilts, and D. R. MacGorman, 1994: Verification of the origins of rotation in tornadoes experiment: VORTEX. Bull. Amer. Meteor. Soc, 75 , 9951006.

    • Search Google Scholar
    • Export Citation
  • Rotunno, R., 1984: An investigation of a three-dimensional asymmetric vortex. J. Atmos. Sci, 41 , 283298.

  • Roux, F., 1985: Retrieval of thermodynamic fields from multiple- Doppler radar data using the equations of motion and the thermodynamic equation. Mon. Wea. Rev, 113 , 21422157.

    • Search Google Scholar
    • Export Citation
  • Roux, F., 1988: The west African squall line observed on 23 June 1981 during COPT 81: Kinematics and thermodynamics of the convective region. J. Atmos. Sci, 45 , 406426.

    • Search Google Scholar
    • Export Citation
  • Roux, F., and J. Sun, 1990: Single-Doppler observations of a west African squall line on 27–28 May 1981 during COPT 81: Kinematics, thermodynamics and water budget. Mon. Wea. Rev, 118 , 18261854.

    • Search Google Scholar
    • Export Citation
  • Roux, F., V. Marécal, and D. Hauser, 1993: The 12/13 January 1988 narrow cold-frontal rainband observed during MFDP/FRONTS 87. Part I: Kinematics and thermodynamics. J. Atmos. Sci, 50 , 951974.

    • Search Google Scholar
    • Export Citation
  • Trapp, R. J., 1999: Observations of nontornadic low-level mesocyclones and attendant tornadogenesis failure during VORTEX. Mon. Wea. Rev, 127 , 16931705.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., and C. Liu, 1998: The Garden City, Kansas, storm during VORTEX 95. Part II: The wall cloud and tornado. Mon. Wea. Rev, 126 , 393408.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., and H. Cai, 2000: Analysis of a nontornadic storm during VORTEX 95. Mon. Wea. Rev, 128 , 565592.

  • Wakimoto, R. M., W-C. Lee, H. B. Bluestein, C-H. Liu, and P. H. Hildebrand, 1996: ELDORA observations during VORTEX 95. Bull. Amer. Meteor. Soc, 77 , 14651481.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., C. Liu, and H. Cai, 1998: The Garden City, Kansas, storm during VORTEX 95: Part I: Overview of the storm's life cycle and mesocyclogenesis. Mon. Wea. Rev, 126 , 372392.

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  • Fig. 1.

    Sounding launched from Midland, TX, on 31 May 1995 at 1100 UTC. The solid and dashed black lines represent temperature and dewpoint temperatures, respectively. Vertical profile of the wind is shown with the half barb, full barb, and flag representing 2.5, 5.0, and 25.0 m s−1, respectively. The thick, black line is the parcel path if the convective temperature was reached

  • Fig. 2.

    Surface analysis of equivalent potential temperature superimposed onto high-resolution visual satellite images at (a) 2000 and (b) 2200 UTC. The launch sites of the Midland (MAF) and Fort Worth (FTW), TX, soundings are shown in (b). The location of the San Angelo tornado is indicated by the black star in (b)

  • Fig. 3.

    The flight track of the Electra on 31 May 1995. The highlighted region is enlarged in Fig. 4. Times along the track are in UTC.

  • Fig. 4.

    Enlargement of the box shown in Fig. 3. Flight track of the Electra is shown by the thin black line. Times are in UTC. Evolution of the San Angelo echo at 700 m AGL based on radar reflectivity data collected by ELDORA. The time interval (UTC) for each echo composite is labeled on the figure. Radar reflectivity is drawn as gray lines, with values greater than 40 dBZ shaded gray

  • Fig. 5.

    Time evolution of the vertical vorticity field associated with the low-level mesocyclone at 300 m AGL. The black line delineates the path of the center of the mesocyclone. The flight track of the Electra is shown by the gray line. The location of a tornado is shown by the black dot. Contours of vertical vorticity are drawn every 5 × 10−3 s−1, as shown on the figure. The alternating dashed and solid contours are used to isolate the analysis at successive times

  • Fig. 6.

    Horizontal cross section of the ELDORA syntheses at 2151:02–2153:32 UTC at 700 m AGL. (a) Radar reflectivity and storm- relative winds. (b) Radar reflectivity and single-Doppler velocities. (c) Vertical vorticity and vertical velocity. Radar reflectivities are drawn as gray lines, with values greater than 40 dBZ shaded gray. Positive and negative values of single-Doppler velocities and vorticity are drawn as black and dashed black lines, respectively. Positive and negative values of vertical velocity are drawn as gray and dashed gray lines, respectively. The black dot represents the position of the maximum vertical vorticity. The flight track of the Electra is drawn on the figure. The black line represents the location of a vertical cross section shown in Fig. 7

  • Fig. 7.

    Vertical cross section of the ELDORA wind synthesis at 2151:02–2153:32 UTC superimposed onto a photograph of the San Angelo storm taken at 2152:00 UTC. (a) Azimuth- and elevation-angle grid and height scale valid at the distance of the mesocyclone. (b) Radar reflectivities. (c) Vertical vorticity and horizontal winds. Half barb, full barb, and flag represent 2.5, 5.0, and 25.0 m s−1, respectively. (d) Vertical velocity and winds in the plane of the cross section. (e) Thermal buoyancy. (f) Precipitation loading (dashed lines) and vertical perturbation pressure gradient force

  • Fig. 7.

    (Continued)

  • Fig. 8.

    Same as Fig. 6, except for 2204:34–2207:34 UTC

  • Fig. 9.

    Same as Fig. 7, except for 2204:34–2207:34 UTC superimposed on a photograph of the storm at 2205:32 UTC

  • Fig. 9.

    (Continued)

  • Fig. 10.

    Same as Fig. 6, except for 2210:21–2216:50 UTC

  • Fig. 11.

    Vertical cross section of the ELDORA wind synthesis at 2210:21–2216:50 UTC superimposed onto a photograph of the San Angelo storm taken at 2213:08 UTC. (a) Azimuth- and elevation-angle grid and height scale valid at the distance of the mesocyclone. (b) Radar reflectivities. (c) Vertical vorticity and horizontal winds. Half barb, full barb, and flag represent 2.5, 5.0, and 25.0 m s−1, respectively. (d) Vertical velocity and winds in the plane of the cross section

  • Fig. 11.

    (Continued)

  • Fig. 12.

    Same as Fig. 6, except for 2227:01–2232:30 UTC. Black box in (a) is enlarged in Fig. 14

  • Fig. 13.

    Same as Fig. 7, except for 2227:01–2232:30 UTC superimposed on a photograph of the storm at 2230:03 UTC

  • Fig. 13.

    (Continued)

  • Fig. 14.

    Enlargement of the horizontal cross section at 2227:01–2232:59 UTC at 700 m AGL. (a) Radar reflectivity and storm-relative winds. (b) Radar reflectivity and single-Doppler velocities. (c) Vertical vorticity and vertical velocity. Radar reflectivities are drawn as gray lines, with values greater than 40 dBZ shaded gray. Positive and negative values of single-Doppler velocities and vorticity are drawn as black and dashed black lines, respectively. Positive and negative values of vertical velocity are drawn as gray and dashed gray lines, respectively. The black dot represents the position of the maximum vertical vorticity.

  • Fig. 15.

    Same as Fig. 6, except for 2236:40–2244:32 UTC. Black box in (a) is enlarged in Fig. 16. Black dot represents the location of the tornado.

  • Fig. 16.

    Enlargement of the horizontal cross section at 2236:40–2244:32 UTC at 700 m AGL. (a) Radar reflectivity and storm-relative winds. (b) Radar reflectivity and single-Doppler velocities. (c) Vertical vorticity and vertical velocity. Radar reflectivities are drawn as gray lines, with values greater than 40 dBZ shaded gray. Positive and negative values of single-Doppler velocities and vorticity are drawn as black and dashed black lines, respectively. Positive and negative values of vertical velocity are drawn as gray and dashed gray lines, respectively. The black dot represents the position of the tornado.

  • Fig. 17.

    Vertical cross section of the ELDORA wind synthesis at 2236:40–2244:32 UTC superimposed onto a photograph of the San Angelo storm taken at 2239:07 UTC. (a) Azimuth- and elevation-angle grid and height scale valid at the distance of the mesocyclone. (b) Single-Doppler velocities. (c) Radar reflectivities. (d) Vertical vorticity and horizontal winds. Half barb, full barb, and flag represent 2.5, 5.0, and 25.0 m s−1, respectively. (e) Vertical velocity and winds in the plane of the cross section

  • Fig. 17.

    (Continued)

  • Fig. A1. Photogrammetric grid for the picture taken at 2239:07 UTC. The azimuth- and elevation-angle and dual-Doppler wind syntheses grids are shown. The black dots represent the position of the individual grid points. The cross section is the one that slices through the center of the low-level mesocyclone

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