OBSERVATIONAL STUDY OF A MIDWESTERN
SEVERE WIND MESOSCALE CONVECTIVE SYSTEM (MCS) ON 29 JUNE 1998:
A SINGLE DOPPLER ANALYSIS STUDY
Jason T. Martinelli
Saint Louis University, Saint Louis
, Missouri
Ron W. Przybylinski
National
Weather Service Office, Saint Charles, Missouri
Yeong-Jer Lin
Saint Louis University, Saint Louis, Missouri
INTRODUCTION
During the early afternoon of 29 June 1998, a severe squall line traversed across central Iowa, producing widespread straight-line wind damage from gusts exceeding 50 m s-1, and several weak to moderate tornadoes (F0-F2). During the early stages of the Mesoscale Convective System (MCS), the overall storm complex contained several hybrid High-Precipitation (HP) supercells. This study utilizes high-resolution reflectivity and and single Doppler velocity data from the Des Moines, Iowa (KDMX) WSR-88D radar site. The first part of this study will focus on the storm morphology and velocity structures as the system travels across central Iowa. Specifically, we will investigate the evolution of one of the embedded HP storms and its associated circulations. The second part of this study will examine a tornadic mesocyclone at close ranges to the radar. We will compare our results with observations documentated by Burgess and Magsig (1998) (hereafter BM98) who examined several tornadic vorticies at close ranges to a WSR-88D Doppler radar.PAST STUDIES
Bluestein and Jain (1985) (hereafter BJ85) categorized severe squall line development as typically occurring in one of four ways: broken line, broken areal, back building, and embedded areal. Rasmussen and Rutledge (1993) (hereafter RR93) used Doppler radar observations to identify common patterns in the evolution of the reflectivity and flow structure of squall lines. They found four distinct phases of reflectivity structure throughout the storms lifecycle: formative, intensifying, mature, and dissipating. Weisman (1992, 1993) numerically simulated linear convection in the midwest and found four identifiable stages of idealized bow echo development. These stages are characterized by the relationship between the cold pool and the low-level ambient shear. Most of the recent studies of the structure and evolution of bow echoes and circulation characteristics have focused on the mature stage of MCS evolution (e.g. Heinlein et al. 1998; Funk et al. 2000). Other papers have examined bow echoes during both the intensifying and mature stages (e.g. Schmocker et al. 1998). Our investigations of the 29 June 1998 MCS event examines the storm and circulation evolution during the intensifying and early part of the mature stage of MCS evolution (pre-bow echo and early stages of bowing). The number of studies covering only this stage of MCS evolution is limited.SYNOPTIC ENVIRONMENT
The 1200 UTC 29 June synoptic environment played an important role in the growth and sustenance of the squall line. The mid- and upper-level flow was nearly zonal with a 60 m s-1 jet streak located over South Dakota and Minnesota. Flow at 850 mb was southwesterly, with a 18 m s-1 low-level jet extending from the Oklahoma panhandle to central Illinois. The squall line formed along and just north of a surface warm front extending from east-central Nebraska through southwest Wisconsin. The large convective system moved east-southward along a nearly stationary surface boundary stretching from central Iowa through northwest Illinois. Unstable air (max theta-e CAPE 3325 J kg-1) was feeding into the boundary from the southwest, as indicated by the 1200 UTC sounding from Valley, Nebraska (not shown). The 1700 UTC wind data from the Slater, Iowa profiler, showed a broad, curved, vertical wind shear profile suggestive of the potential for supercell development. The magnitude of the 0-6 km shear was 30 m s-1.STORM SCALE STRUCTURE AND EVOLUTION
The reflectivity pattern at 1716 UTC showed a cluster of organized deep convective storms stretching from 120 km northeast of KDMX to 80 km north of Omaha (OAX) (Fig.1). This pattern resembled the initial stage of broken-areal formation as described by BJ85. Three HP storms containing persistent rotating updraft centers, were also detected west and northwest of KDMX (Storms A, B, and C). In each of these storms, the origins of these mesocyclones occurred at mid-levels, similar to observations recorded by Burgess et al. (1982). Of the three storms, we will focus on the evolution of Storm B and its associated circulations at an initial range of 100 km northwest of KDMX.
Figure 1: WSR-88D PPI reflectivity display at the 0.5o elevation angle for 1716 UTC 29 June 1998.
A reflectivity cross section (not shown), oriented perpendicular to the leading edge of
Storm B at 1716 UTC, showed a tall convective tower extending to a height of 15 km. This finding is consistent with observations of RR93 intensifying stage of MCSevolution and Weisman's Stage II (tall echo stage). RR93 noted that during this stage of a MCS, the cells at the leading edge grow to their greatest vertical extent and the convective updrafts attain their highest updraft speeds. Weisman (1993) found that during this stage in an MCS's lifecycle, the circulation due to the
cold pool increases in strength until it balances the circulation due to the low-level ambient vertical wind shear. Subsequently, strong and erect updrafts are produced.
Figure 2: Map designating the tracks of C1, C2, and C4a.
Tracks of the vortices studied are shown in Fig. 2. The first circulation (C1) identified with Storm B, was observed at 1716 UTC between 3.5 and 8 km, with a maximum rotational velocity (Vr) of 23 m s-1, and a diameter of 5 km (Fig. 3). At a range of 100 km, C1 immediately met the criteria of a strong mesocyclone (Andra 1997). The observation of a mesocyclone originating at mid-levels is consistent with findings of Burgess et al. (1982) where vorticies associated with supercells originated from mid-level beginnings. C1 formed in the vicinity of a weak echo region (WER) on the leading edge of the Storm B. The strongest cyclonic shears remained at mid-levels between 1716 and 1737 UTC with a consistent diameter of 5 km. Throughout the lifespan of C1, weaker shears were found below the strongest shears. After 1741 UTC the vortex became embedded near the storm's high reflectivity core, and became indistinguishable after 1751 UTC.
Figure 3:Plot of height (in km) versus time of Vr values for C1.
A second vortex (C2) was identified at 1731 UTC, south of the path of C1. This circulation also revealed mid-level beginnings with the strongest cyclonic shears (23 m s-1) located at a height of 5 km (Fig. 4). Overall depth of C2 was 3 km and its diameter was roughly 3.5 km. C2 maintained its strongest shears aloft until 1751 UTC. At 1746 UTC, C2 attained its greatest vertical depth of approximately 8 km and overall diameter of 6 km. After 1751 UTC, magnitudes of the cyclonic shears of C2 decreased in intensity as it traversed into the high reflectivity core of Storm B. C2 appeared to take a similar trajectory to C1's path where it gradually advected rearward, into the storm's high reflectivity core.
Figure 4: Plot of height (in km) versus time of Vr values for C2.
At 1811 UTC, the MCS had solidified into a line nearly 200 km long, orientated east-northeast-west-southwest across west-central and central Iowa (Fig. 5). Two embedded HP storms were detected, with Storm B located approximately 50-80 km northwest of KDMX.
Figure 5: WSR-88D PPI reflectivity display at the 0.5o elevation angle for 1811 UTC 29 June 1998.
The vertical reflectivity cross-section of Storm B at 1811 UTC, showed a WER along the HP storm's forward flank with a 50 dBZ core extending to a height of approximately 11 km (Fig. 6). The convective towers remained nearly vertically erect, similar to observations recorded by RR93 during the later part of the intensifying stage and early part of the mature stage of MCS evolution. Surface gusts associated with Storm B exceeded 35 m s-1 over southwest portions of Boone county Iowa (50 km northwest of KDMX).
Figure 6: WSR-88D RHI reflectivity display for 1811 UTC 29 June 1998.
The corresponding velocity cross-section taken at 1811 UTC showed a steep ascending branch of the front-to-rear flow (FTR) and a gradual descending rear inflow (Fig. 7). This type of flow structure appears to be common during the `early part' of the mature stage of MCS evolution. The steep ascending branch of the FTR shown is in contrast to the gradual ascending branch shown in the later part of the mature stage (RR93) and Weisman's Stage III.
Figure 7: WSR-88D RHI SRM display for 1811 UTC 29 June 1998. (Storm velocity of 11.0 m s-1, from 270.0o.
EXAMINATION OF A TORNADIC CIRCULATION (C4a) AT CLOSE RANGES
Recent studies examining tornadic vortices at close ranges from BM98 have shown a consistent picture of development of vortex evolution just before and during tornado formation. They include: 1) before tornado formation - strong rotation well above cloud base (at 0.7 km or 2500 ft) and strong convergence below cloud base, 2) just before tornado formation - strong rotation well above cloud base, strong rotation near cloud base, and strong convergence near and below cloud base, and 3) at tornado formation - strong rotation through a deep column, including below cloud base, with maximum rotation just above cloud base. KDMX allows us to view the vortex characteristics of circulation 4a (C4a) at 26 and 8 km away from the radar respectively. C4a formed at 1806 UTC along the leading edge of Storm B. At this time, C4a revealed an overall depth of 4 km with a core diameter of 2.5 km. After 1811 UTC, C4a deepened while the magnitude of (Vr)s exceeded 25 m s-1 within the lowest 6 km of the mesocyclone (1821 UTC). The first report of a tornado occurred at approximately 1820 UTC southeast of Berkley, Iowa in southwest Boone county (Storm-Data). Tornadogenesis occurred just prior to C4a reaching its greatest depth (8 km) and strongest Vr values (25-28 m s-1) (Fig. 8).
Figure 8: Plot of height (in km) versus time of Vr values for C4a.
At 1821 UTC, C4a was located approximately 26 km away from KDMX. The proximity of C4a to the radar allows us to examine the detailed structures of this vortex (Fig. 9). In comparing our observations to BM98, C4a revealed a cyclonic convergent velocity signature at the lowest slice 0.5o (0.4 km AGL) and a symmetrical vortex structure at the 1.5o slice (1.0 km AGL) and above. The first tornado was occurring at this time. A pure convergent velocity signature was absent. The following volume scan recorded from KDMX at 1833 UTC, showed a nearly symmetrical vortex at the lowest two slices (0.5o and 1.5o) (0.1 km / 0.2 km AGL) and above. These observations are in agreement with those recorded by BM98. It is however interesting to note that strong convergence (delta V 58 m/s) was detected along the northern periphery of C4a's core, or 8 km northwest of KDMX. Subsequent elevation slices above the convergent velocity signature exhibited a cyclonic convergent velocity couplet (above 1.0 km). The overall velocity pattern is complex at this time and the limited sampling resulted in bringing only a partial picture to the overall vortex structure of C4a. However, the velocity structure at 1833 UTC does suggest the possibility of a `double core' vortex structure.
Figure 9: WSR-88D PPI SRM display at the 0.5o elevation angle for 1821 UTC 29 June 1998.
SUMMARY
An MCS producing several swaths of damaging winds and tornadoes traveled across most sections of central Iowa during the early to mid afternoon of 29 June 1998. The storms evolved in a highly unstable -strong shear environment. The Slater, Iowa profiler at 1700 UTC revealed `deep-layer' shear (broad curved profile) suggesting the potential for supercell development. From 1700 to 1730 UTC, a cluster of severe storms across central and eastern Iowa gradually evolved into a nearly solid linear convective line similar to the intensifying stage shown by RR93. Storms B and C, 60-80 km northwest of KDMX, exhibited hybrid High Precipitation supercellular characteristics with rotating cyclonic centers having mid-level origins. Damaging winds were reported with the HP storms. One storm (Storm B) and its associated circulations was closely examined throughout much of its lifecycle from 1716 UTC through 1833 UTC. Characteristics of the first two mesocyclones (C1 and C2) showed that the strongest cyclonic shears generally remained within the 4 to 6 km layer of the storm's mid-level region. These vortices weakened at mid-levels as they traversed into the high-reflectivity core region of the HP storm. The fourth mesocyclone (C4a) formed along the forward flank of the HP storm, originating at low-levels, and deepened and intensified during the subsequent volume scans. This vortex was responsible for spawning several tornadoes rated (F1 - F2 intensity) over parts of southern Boone and Dallas counties in Iowa. The tornadoes occurred just prior to C4a reaching its greatest depth and strongest magnitudes of rotational velocities. Overall mesocyclone structure of C4a at ranges below 30 km was compared to observations by BM98. During the period of tornadic activity, C4a's core structure appeared to be similar to observations recorded by Burgess where strong rotation was observed through a deep column. Much study has been completed on the mature stage of MCS evolution. However much work is still needed to understand the MCS evolution from its early stages to mature stage.REFERENCES
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