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AN INVESTIGATION OF MORPHOLOGY AND MESOVORTEX EVOLUTION WITHIN A MIDWESTERN QUASI-LINEAR CONVECTIVE SYSTEM USING CONVENTIONAL AND SINGLE-DOPPLER DATA

Jason T. Martinelli

Department of Atmospheric Sciences Creighton University Robert PaskenOmaha, and Nebraska Yeong-Jer Lin

Department of Earth and Atmospheric Sciences Saint Louis University RonSt. Louis,W. Przybylinski Missouri

NOAA/National Service Weather Forecast Office Abstract St. Charles, Missouri

In the morning hours of 29 June 1998, a line of severe convective traversed central Iowa, producing a wide swath of straight-line damage (several gusts exceeding 50 m s-1) and isolated weak-to-moderate intensity tornadoes (F0-F2). This quasi-linear convective system evolved into a severe line with three high- (HP) and multiple

was reported across many sections of the entire line, this paper focuses on the evolution of one particularlywell-defined active mid-level portion associated embeddedwith a nearly within continuous it. Although swath scattered of severe wind straight-line damage

the metropolitan area. and tornadic activity beginning 40 km northwest of Des Moines and extending through

One of the embedded HP supercells contained several mid-level rotating centers that exhibited descending vortex characteristics between 1716 and 1806 UTC; their strongest furthercyclonic matured shears persisted and several at mid non-descending levels of the circulation.tornadic and As non the tornadicstorm approached the occurredDes Moines metropolitan area between 1806 and 1833 UTC, the outflow-dominated HP The evolution of the near-storm vertical likely played a role in the system’s along a very progressive .

intensification and transition from a supercellular to a linear structure. Weather Surveillance Radar–1988 Doppler (WSR 88D) data from the NOAA/ (NWS)/Weather Forecast Office (WFO) Des Moines are used to document the storm reflectivity and velocity structures as the storm approached the Des Moines area. Time height rotational velocity (Vr) traces are used to show the characteristics of the circulations and illustrate the differences

Corresponding Author: Dr. Jason Martinelli, Department of Atmospheric Sciences, Creighton University, 2500 California Plaza, HLS 504, Omaha, NE 68178 or via e-mail: [email protected] Martinelli et al.

between the tornadic and non tornadic vortices. Specifically, we found that several deep vortices near the northern flank of the HP supercell appeared to be responsible for enhancing the mesoscale rear inflow and convective scale outflow. As the outflow accelerated, several non supercell tornadic and non tornadic vortices, which initially developed from low levels, rapidly deepened and intensified. One of these non-descending vortices was responsible for the tornadic activity northwest of Des Moines. 1. Introduction

June 1998, a severe quasi-linear During the early afternoon of 29 traversed central Iowa producing widespreadmesoscale convective straight-line system wind damage (QLCS) (gusts exceeding 50 m s-1 and several weak to moderate intensity tornadoes (F0-F2) (Fig. 1). overall storm complex contained Early in the system’s life-time, the (HP) supercells. The proximity to several embedded high-precipitation allowedthe WFO for Desa detailed Moines, examination Iowa WSR- of the88D kinematic provided features a wealth and of storm-scale data that cyclonic circulations associated with Fig. 1. early mature stages of its life-cycle. Severe weather reported between 1200 UTC 29 June 1998 and 0000 the QLCS during the intensifying and reference.UTC 30 June 1998 (NCDC 1998). The WSR-88D sites at Des Moines (KDMX) and mesovortex structure during and Davenport, IA (KDVN), as well as Omaha, NE (KOAX), are indicated for This paper examines the reflectivity the intensifying and early mature stage of QLCS evolution close range to a WSR-88D radar. Summary and conclusions (pre- and early bowing stage). Thus far, a limited 2.are Review presented of in Convective section 6. Storm Morphology number of studies have focused on this portion of a QLCS’s coincidentlifecycle. Section upper-air 2 provides and surface a literature synoptic review scale of conditionsconvective asstorm they morphology. played an important Section 3 describesrole in the the development antecedent and convective storm, which shows a persistent correlation The current operational definition of a supercell is a complexsustenance storm of the morphology QLCS. In sectionand radial 4, Dopplervelocity radarstructures data between vertical vorticity and vertical velocity (Weisman from WFO Des Moines, Iowa (KDMX) are used to describe the andconceptual Klemp 1982), models that on is, variations a storm that of thecontains supercell a persistent theme, rotating updraft. Moller et al. (1994) presented a set of portionas the systemof the convective approached line thethat Des contained Moines an metropolitan HP supercell area. Specifically, the evolution of one particularly intense advancing the basic model set forth by Browning (1964). and its associated storm-scale cyclonic circulations were however,In the United occurrences States, were the documented classic supercell across parts (Browning of the is examined in detail. This embedded hybrid HP supercell 1964) typically occurs in the southern Great Plains; line wind damage and a moderate intensity (F2). In sectionresponsible 5, the for structure a nearly continuous of a tornadic swath of severe straight- in close northern High Plains, Midwest, southern Gulf Coast, and Northeast. Research has shown that the HP supercell is the low-level velocity structure of the tornadic mesocyclone more common over the Midwest often due to a deeper low- proximity to the KDMX radar site is discussed. In particular, thelevel trailing moisture side regime of the mesocyclonecompared to withareas HP across storms the where Great Plains. Heavy precipitation is frequently observed along is examined in detail and the observations are compared 154with National those documented Weather Digest by Burgess and Magsig (1998; other supercell structures are often rain free (Moller et al. hereafter BM98) who examined several tornadic vortices at 1990; hereafter MDP90). Although these storms may not Storm Morphology and Mesovortex Evolution occur clearly isolated from surrounding convection, they systems, typically contain waves along the leading edge. Squall lines, which develop into damaging convective structuresremain distinctive suggestive in character. of rotation HP within supercell a region reflectivity of high characteristics often include spiral, banded-like reflectivity The term “line-echo-wave-pattern” (LEWP) coined by The accompanying mesocyclone often occurs along mesoscaleNolen (1959) wave showed pattern how within one part the of larger the line line. accelerates, Hamilton reflectivity. while an adjacent part decelerates, resulting in a sinusoidal HP storm and then migrates rearward into the heavy the storm’s forward flank within the inflow region of the (1970) noted the correlation of the accelerated part of orthe acceleratedLEWP to damaging segments straight-line within the winds larger and line. tornadoes. He also precipitation core. These types of storms can become Fujita (1978) used the term “bow echo” to name the bulges efficient , wind, and tornado producers. This is in itcontrast is quite to common the rear-flank for an mesocycloneHP supercell totypically transition associated from a deduced that the bulges in the reflectivity pattern were with the classic supercell (MDP90). MDP90 also noted that associated with a strong meso-β scale high-pressure center heads. and the crest with a meso-β scale low-pressure center. classic supercell, or into a bow echo with rotating comma Fujita (1978) put forth the first detailed morphological description of the bow echo. His well known conceptual MDP90 noted that QLCSs and bow echoes may contain model shows how an initial tall echo evolves into a bow- HP storms. These storms are frequently observed near and shaped line of convective cells as strong structuresouth of the with apex strongof large low-levelbowing convective systems. Such descend to the surface. At the time of strongest surface storms often exhibit a large kidney-bean shaped reflectivity winds, a spearhead echo may form. As the overall system thereflectivity storm’s gradients weak echo observed region along the eastern flank signifying may form in the vicinity of the (WER). Weak short-lived tornadoes

WER and along the storm’s rear HPflank stormsdowndraft. and These comma-shaped storms will typically appear as meso-β scale echoes within a larger, meso-α scale southernbowing segment. part of convectiveIsolated as lineswell as HP storms embedded over the and bow echoes tend to travel along al.pre-existing 1998). HP thermal storm mesovortices boundaries (Maddox et al. 1980; Markowski et of the thermal contrasts across a benefit from the solenoidal effects surface boundary and the amplified vertical wind shear (VWS) in the vicinity of the boundary. In the mid 1980s, a number 1989) of observationalfocused primarily studies on squall (e.g., the lines PRE- in STORMthe mature Project; stage Houze of their et al.lifecycle. studies led to the well known Fig. 2. conceptualA synthesis modelof these of observational a (1989) for Station plots and sea-level pressure in millibars (thin solid) valid at 1200 details]. darkenedUTC 29 June circles. 1998. The Temperatures location of the and stationary dew points front are isin indicated. degrees Fahrenheit and system [see Houze et al. wind barbs are in knots. The degree of cloud cover at each station is indicated by the Volume 31 Number 2 ~ December 2007 155 Martinelli et al. declines, the convective system evolves into a comma- 3. Synoptic Environment shaped echo pattern with a cyclonically rotating head at the northern end.

was The consistent pre-convective with those environment associated observed with warm on 29 season June 1998 over the midwestern portion of the United States

weak“progressive” synoptic-scale type forcing, (Johns northwesterly and Hirt flow 1987). at Specifically, this environment is characterized by relatively in the vicinity of the initiation area, and a quasi- 500-mb, advection of relatively warm, moist air at 850-mb in the west-east direction. Furthermore, there is likely to stationary boundary at the surface that is typically oriented shear present. In this type of environment, the derecho- be strong instability and moderate low-level unidirectional

componentproducing system of motion develops directed along into and the subsequently warm sector. moves nearly parallel to the existing thermal boundary with some

was The extending objectively from analyzed an area surface of low analysispressure at over 1200 southern UTC on 29 June 1998 is shown in Fig. 2. At this time, a stationary front This frontal feature then connects to a weak low pressure Ontario westward across Michigan and into western Iowa.

center situated over southwestern Kansas. Warm, moist southerly flow is evident south of the front. Analogous to the “progressive” type derechos described by Johns and Fig. 3. Hirt (1987), the QLCS developed in close proximity to the

Sounding valid at 1200 UTC 29 June 1998 from Valley, Nebraska (KOAX).

Fig. 4. Fig. 5.

Wind profiler data from Slater, Iowa. Altitudes on Temperatures850-mb and station dew plots, point heights depressions in meters are in (solid) degrees 1998.the y-axis are plotted in kilometers and wind barbs are and isotachs (dashed) valid at 1200 UTC 29 June 1998. in156 knots. National Data Weatherare plotted Digest from 1000 – 2100 UTC 29 June Celsius. Isotach magnitudes are in knots. Storm Morphology and Mesovortex Evolution

the warm sector as it propagated through Iowa. stationary front and subsequently moved along it and into

present The 1200 through UTC the Valley, lowest NE kilometer, (KOAX) Skew-T with sharp analysis drying on 29 June 1998 (Fig. 3) indicated that deep moisture is

above. Surface-based convective available potential energy-1. (CAPE) calculated using a forecast maximum temperature (dew point) of 38°C (19°C) yielded a value of 3110 J kg ofA more the warm realistic front CAPE yielded based approximately on an anticipated 5120 maximum J kg-1 and temperature (dew point) of 29°C (24°C) along and south

e a Lifted Index of -12°C. Surface to midlevel equivalent potential temperature (θ ) differences exceeded 30 K, and in combination with large wetbulb temperature differences, this could result in strong evaporative cooling in organized downdrafts that develop. Additionally, there inis a downdrafts nearly-adiabatic since layer the atmosphere located between is not 850 resistant and 500 to mb; this is also highly favorable to momentum transport

vertical displacements. The vertical shear profile at KOAX Fig. 6. ssuggests-1, respectively. supercells are possible. At 1200 UTC, the 0-3 and 0-6-km bulk shear magnitudes at KOAX were 10 and 20 m Temperatures 850-mb and temperature dew points (solid) are plotted and dew in degreespoint (dashed) analyses valid at 1200 UTC 29 June 1998. NOAA’s Slater, IA profiler (Fig. 4) shows the change in wind shear structure that occurred between 12 and 18 UTC Celsius. on 29 June 1998. During this time, the winds increased in-1 magnitude in the-1 3000 to 5000 m layer increasing both the 0-3 km and 0-6-km shear magnitudes from 5 and 20 m s to 20 and 35 m s , respectively. By 18 UTC, the wind profile

Fig. 8.

0.5 degree base reflectivity data from the KDMX radar at 1716 UTC 29 June 1998. Storms “A”, “B” and “C” Fig. 7. are identified. At this time, Storms A, B and C all contained persistent rotating cores, consistent observations of other Temperatures500-mb and station dew plots, point heights depressions in meters are in (solid), degrees supercells. For reference, KDMX is located in the lower and isotachs (dashed) valid at 12 UTC 29 June 1998. softwareright-hand package corner fromin northeast www.grlevelx.com. Polk County. The dBZ scale is on the left. All Volumeradar data 31 Number are plotted 2 ~ Decemberwith the GRLevel2 2007 157 Celsius. Wind barbs are in knots. Martinelli et al.

linear convective modes. is one that is generally supportive of both supercellular and 5) revealed a trough extending from Ontario into northern The 1200 UTC 850-mb analysis on 29 June 1998-1 (Fig.that extended from the northern Texas Panhandle into central New Mexico as well as a low-level jet (LLJ) of 17 m s intoKansas. central The Iowa 850-mb and temperature/dewample quantities of point moisture field present(Fig. 6) showed a thermal ridge extending from the Desert Southwest as evidenced by dew points exceedingo -1) 12°Cwas occurring from Texas in into Nebraska and Iowa. At this time, strong warm air advection (WAA) at 850-mb (> 2.5 C h the region, extending from western Kansas to the Iowa/ Fig. 9. Illinois border. Additionally,-1 -1 850-mb moisture convergence exceeding 0.5 g kg h e was was strongest over Nebraska and eastern Iowa with values Storm-relative velocity (SRV) radar data at 0.5 degrees from the-1 KDMX radar at 1716 UTC 29 June 1998. . The strongest advection-1. The locations of θ of also identified over eastern Nebraska and western Iowa at Mesovortexindicate the 1direction (C1) is circled. of motion A storm toward motion or away of 295°from atthe e advection 1200 UTC with values greater than 1.4 K h 35 kt (18.0 m s ) was used to compute the SRV. Arrows maximum values of moisture convergence and θ radar. At this time, C1 is located approximately 100 km convection.are consistent with those shown in Johns (1993) to be initial indicators of areas most favorable for initiation of northwest of KDMX. The SRV scale is on the left.

At 500-mb (Fig. 7), moderate west-northwest flow and broad diffluence is observed over Iowa. Consistent with Johns (1993), the QLCS developed south of the 500-mb jet 4.axis Storm that is locatedScale Structureover northern and South Evolution Dakota.

deep Examination convective of storms the system’s stretching overall from reflectivity 120 km northeaststructure at 1716 UTC on 29 June 1998, revealed a cluster of organized of KDMX to 80 km north of KOAX (Fig. 8). At this time, three discrete cells located to the west and northwest of KDMX were found to contain persistent rotating centers (Storms A, B, and C in Fig. 8). At this time, the convective system was associated with several severe hail and wind reports (NCDC Fig. 10. r -1 1998). In each of these storms, the mesocyclones (1982). (Andra The Rotational velocities are shown in m s 1997)remainder originated of this at paper midlevels, will investigate similar to one observations particularly of V trace of C1 from 1716 - 1751 UTC 29 June 1998. other supercells as recorded by Burgess et al. . Prior to 1716 and forafter much 1751 of UTC, the circulation’sC1 was indistinguishable. lifetime. Note how the initialintense range portion of 100 of km. the convective system (Storm B) and maximum values of rotation are maintained, > 4 km AGL its associated circulations as it approached KDMX from an stage the cells at the leading edge grow to their greatest vertical extent and the convective updrafts attain their revealed At 1716 a nearly UTC, a vertical reflectivity convective cross section,tower extending which was to oriented normal to the leading edge of Storm B (not shown), highest updraft speeds. Weisman (1993) found that during an altitude of approximately 15 km. This observation is Stage II, strong, erect updrafts are produced because of consistent with those made by Rasmussen and Rutledge the balance attained between the vorticity generated on (1993; hereafter RR93) during the QLCS’s intensifying stage, the downshear side of the cold outflow and the vorticity as well as Weisman’s Stage II of bow echo development (tall generated by the ambient VWS. echo158 Nationalstage; Weisman Weather 1993). Digest RR93 noted that during this The first storm-scale cyclonic circulation associated with Storm Morphology and Mesovortex Evolution

Fig. 11 Fig. 13.

. 1.5 degree SRV radar data from the KDMX radar at 0.5 degree base reflectivity data from the KDMX 1736 UTC 29 June 1998.-1 C1 is denoted by the small circle radar at 1811 UTC 29 June 1998. The black line represents and C2 is marked by the larger circle. A storm motion of the location of the cross-sections through storm ‘B” 295° at 35 kt (18.0 m s ) was used to compute the SRV. left.presented in Figs. 14 and 15. For reference, KDMX is Arrows indicate the direction of motion toward and away located in northeast Polk County. The dBZ scale is on the from the radar. The SRV scale is on the left.

Fig. 14.

Base reflectivity cross-section oriented normal to Fig. 12. r the leading edge of Storm “B” along the line shown in Fig. June 1998. Rotational velocities are shown in m s -1. Prior 13. Each horizontal line represents an increment of 10,000 V trace of C2 between 1731 and 1811 UTC 29 positionsft (3.048 km). of the The front-to-rear horizontal (FTR) dimension and rear-to-front of the image is 35 nm (64.82 km). The arrows represent the approximate to 1731 and after 1811 UTC, C2 was indistinguishable. Similar to C1, C2 generally maintained its strongest scale(RTF) is flows on the within right. the system. The location of a weak circulation > 4 km AGL. echo region (WER) is also indicated in the figure. The dBZ

Storm B (C1) was detected at 1716 UTC between 3.5 and 8 km r) above ground-1 level (AGL) in southwestern Calhoun County with supercells originated at the storm’s midlevels. C1 and(Fig. it 9). maintained At this time, a consistent C1’s maximum diameter rotational of 5 km velocity throughout (V was initially detected near a weak echo region (WER) on of 23 m s was located approximately 5 km AGL (Fig. 10) the leading edge of Storm B. Between 1716 and 1737 UTC, immediately met the criteria for a strong mesocyclone C1 maintained a diameter of approximately 5 km as well its depth (not shown). At an initial range of 100 km, C1 as consistently stronger shear magnitudes between 5 and (1982) who showed that vortices associated 7 km AGL. After 1741 UTC, the vortex became embedded (Andra 1997). These observations are consistent with near Storm B’s high reflectivity core and weakened (see Fig Burgess et al. 10). C1 became Volumeindistinguishable 31 Number after 2 ~ December 1751 UTC. 2007 159 Martinelli et al.

The second cyclonic vortex detected

within Storm B (C2) was initially identified at 1731 UTC, 3 km south of the path of C1, on the border of Calhoun and Webster Counties level(Fig. origin11; shown with itsat strongest1736). Consistent cyclonic shears with observations-1 of C1, C2 also revealed a mid-

(23 m s ) located near 5 km AGL. At this time, C2’s depth was 3 km and its diameter verticalwas approximately depth of approximately 3.5 km. Between 8 km 1731 and and 1746 UTC, C2 expanded to its greatest

largest diameter of 6 km. C2 maintained trace its in Fig 15. r strongestFig. 12). Then, shear the magnitudes cyclonic shear between associated 3.5 and 6 km AGL until 1756 UTC (see V SRV cross-section oriented normal to the leading edge of Storm “B” along the line shown in Fig. 13. Each horizontal line represents an uniform throughout its decreasing depth. with C2 weakened considerably and became increment of 10,000 ft (3.048 km). The horizontal dimension of the image is 35 nm (64.82 km). The arrows represent the approximate positions of C2 appeared to take a similar trajectory to the front-to-rear (FTR) and rear-to-front (RTF) flows within the system. C1, gradually advecting rearward into Storm The SRV scale is on the right.

Fig. 16(a).

0.5 degree base reflectivity data from the KDMX radar at 1806 UTC 29 June 1998. Circle marks location of C5 associated with (b). storm B while arrows indicate locations of the maximum inbound and outbound radial velocities, motionrelative ofto 295the KDMX radar. At this1 time, data suggests that Storm B was developing an appendage in response to the 160existence National of C5. WeatherC5 Digest is also shown in the 0.5 degree SRV data from KDMX radar valid at the same time. A storm o at 35 kt (18.0 m s- ) was used to compute the SRV. Storm Morphology and Mesovortex Evolution

B’s high reflectivity core and weakening. Between 1716 and 1806, numerous severe straight-line winds were reported (NCDC 1998). orientated At 1811 east-northeast-west-southwestUTC, radar data indicated that theacross MCS west- had solidified into a line that was nearly 200 km long and central and central Iowa (Fig. 13). At this time, Storm B was located approximately 50-80 km northwest of KDMX. The reflectivity cross-section (orientated normal to Storm B’s leading edge) taken at 1811 UTC revealed a WER along the HP storm’s southeast flank associated with new cell development between 4 and 7 km AGL (Fig. 14). At this time, the 50 dBZ core extended to a height of approximately-1 11 km (see Fig. 14). During this time, surface gusts associated with Storm B exceeded 35corresponding m s over velocity southwest cross-section portions (Fig. of Boone 15) showed County a Iowa, 50 km northwest of KDMX (NCDC 1998). The steep ascending branch of the front-to-rear flow (FTR), al.as well as the gradual descending rear inflow. These Fig. 17. observations are similar to those shown in Atkins et r -1 (2005) and indicate that the QLCS has entered the 29 June 1998. Rotational velocities are shown in m s . V trace of C5 between 1806 and 1821 UTC on beginning of its mature stage. However, the existence of a steep ascending branch of the FTR at this time is in The ”T” on the abscissa represents the time of the first contrast to the gradual ascending branch shown in the reported tornado touchdown. Prior to 1806 UTC, C5 was latter part of the mature stage (RR93) and Weisman’s indistinguishable. Unlike C1 and C2, C5 developed at low- Stage III (Weisman 1993). throughout this period. 5. Examination of a Tornadic levels and maintained very strong rotation < 4 km AGL Mesocyclone at Close Ranges with classic supercells that occurred in close proximity km and a core diameter of 2.5 km. In contrast to observations BM98’s examination of tornadic vortices associated of non-tornadic vortices C1 and C2, C5 originated in the lower- r development prior to and during tornadogenesis. They levels of Storm B and was completely contained within 4 km to a WSR-88D revealed a consistent pattern of vortex of the surface (see V trace in Fig 17). Between 1811 and 1821 r UTC,s-1 C5 rapidly deepened and intensified. At 1821 UTC, the noted that before tornado formation, Doppler radar magnitudeassociated with of the this V mesocyclone associated occurred with C5 at exceeded approximately 23 m indicated the existence of strong . rotation Just before well tornado above between 200 m and 7 km AGL. The first report of a tornado the cloud base (assumed to be 750 m) and strong convergence below the cloud base 1820 UTC southeast of Berkley, Iowa in southwest Boone formation, there was strong rotation well above the County (NCDC 1998). Tornadogenesis-1 occurred just prior r values (25-28 m s .) This is consistent with Finally,cloud base at the to time near of cloud tornado base formation, as well as strong the presence rotation to when C5 attained its greatest vertical depth (8 km) and of strong convergence near and below the cloud base. strongest V 2000). findings in other cases examined in the Mid-Mississippi Valley was present through a deep column, including below the region (Przybylinski et al. cloud base, with maximum rotation located at a height At 1821 UTC, one minute after tornado touchdown, C5 was just above the cloud base. located at a range of approximately 26 km from KDMX (Fig. At less than 30 km from the KDMX WSR-88D, the close 18a and 18c). At this time, C5 revealed a cyclonic-convergent proximity of mesovortex 5 (C5) allowed for a detailed velocity signature at the 0.5° elevation angle (0.4 km AGL; Fig. examination of its vortex characteristics. C5 was initially 18d) and a symmetrical vortex structure at the 1.5° elevation detected along the leading edge of Storm B at 1806 UTC angle (1.0 km AGL) and above (Fig 18b). This observation (Fig 16). At this time, C5 revealed an overall depth of 3 differs slightly fromVolume those 31 Number made 2 ~ byDecember BM98, 2007 who 161observed a Martinelli et al.

Fig. 18(a).

1.4 degree base reflectivity data from the KDMX radar at 1821 UTC on 29 June 1998. C5 is indicated by a circle while arrows mark the location of the maximum inbound and outbound radial velocities relative to the KDMX (b).radar. The dBZ scale is to the left. Note the existence of the as a result of the presence of C5. motion of 295o -1 1.4 degree SRV data from the KDMX radar valid at the same time. Circle and arrows are as in Fig. 18a. A storm (c). at 35 kt (18.0 m s ) was used to compute the SRV. The SRV scale is to the left.

(d.) As in Fig. 18a except 0.5 degree base reflectivity is shown.

As in Fig. 18b except 0.5 degree SRV data is shown. purely rotational signature at the lowest levels at the time -1) was detected in this case is likely associated with the rapid advancement with those recorded by BM98. However, it is interesting to of tornado formation. The observed convergent signature note that strong convergence (∆V of 58 m s along the northern periphery of C5’s core, 8 km northwest of the precipitation-loaded rear-flank-downdraft. Due to a convergentof KDMX. Subsequent velocity couplet. elevation The slices overall above velocity the convergent pattern at structurepower outage, showed the anext distinct volume hook scan echo recorded as a result from KDMX of the thisvelocity time signature is very complex (i.e., above and 1 km the AGL) loss exhibitedof data and a cyclonic- limited did not occur until 1833 UTC, at which time the reflectivity redistribution of precipitation due to the presence of C5 sampling resulted in obtaining only a partial understanding (Fig.19a and 19c). Additionally, the velocity structure at of the overall vortex structure of C5. However, the velocity 1833 UTC showed a nearly symmetrical vortex at the lowest structure at 1833 UTC (Fig. 19b and 19d) does suggest the two elevation slices (0.5° and 1.5°; 0.1 km and 0.2 km AGL) possibility of a “double core” vortex structure. 162and aboveNational (Fig. Weather 19b and Digest 19d). These observations agree Storm Morphology and Mesovortex Evolution

Fig. 19(a).

1.4 degree base reflectivity data from the KDMX radar at 1833 UTC on 29 June 1998. C5 is indicated by a circle while arrows mark the location of the maximum inbound and outbound radial velocities relative to the KDMX (b).radar. The dBZ scale is to the left. -1 1.4 degree SRV data from the KDMX radar valid at the same time. Circle and arrows are as in Fig. 19a. A storm (c).motion of 295° at 35 kt (18.0 m s ) was used to compute the SRV. The SRV scale is to the left.

(d). As in Fig. 19a except 0.5 degree base reflectivity is shown.

As in Fig. 19b except 0.5 degree SRV data is shown. 6. Summary and Conclusions e air northward into the region of severe storm Panhandle into south-central Nebraska, transported seasonhigh θ “progressive” derecho environment, the severe 1998, a cluster of strong to severe formed development over northeast Nebraska. Similar to a warm During the late morning and early afternoon of 29 June into northwest Iowa. The cluster matured into a line of storms formed along a west-east frontal boundary which over northeast Nebraska and moved southeastward discrete storms from north-central to west-central Iowa, extended from east-central Nebraska through southeast Iowa and into central Illinois. Surface dew points of 23°C, pooled along and south of the surface boundary through exhibiting high-precipitation supercell characteristics. The southeast Nebraska and into southwest Iowa, resulted in a discrete storms gradually evolved into a QLCS across east region of highly unstable air. Through mid day (1800 UTC), central through central Iowa. Many of these storms were tornadoes. the surface boundary moved little across south-central and responsible for the production of damaging winds, hail, and southeast Iowa. The southerly flow south of the boundary continued to transportVolume warm, 31 Number moist, 2 ~unstable December air 2007north and163 The 850-mb low-level jet, orientated from the Texas Martinelli et al. northeastward, resulting in continued pooling of surface storm-relative velocity data were taken normal to the line.

The initial cluster of severe storms likely formed in a progressed from the latter part of the intensifying stage to dew points along and to the south of this boundary. They showed that the internal flow structure of the MCS highly unstable, strongly sheared environment.-1 Although the mature stage of MCS development, as shown by RR93 the surface-based CAPE computed from the 1200 UTC Valley, and Smull and Houze (1987). Observations of a deep, NE (KOAX) sounding was 2493-1 over J kg central, a modified Iowa at sounding the time ascending branch of FTR flow and a gradual descending based on actual temperature and dew point values, resulted mesoscale rear inflow were noted during the mature stage. in CAPE exceeding 3500 J kg The reflectivity structure and development of C5, of convective initiation. A strong, deep-layer shear profile-1. developmenttracking within associated 30 km of with KDMX, classic was examined supercell in storms detail was evident by late morning from the Slater, Iowa profiler and compared to near-radar observations of tornadic weakwhen with magnitudes a magnitude of the of 0-6-km 12 m s shear-1. This reached is in contrast 30 m sto In contrast, the 0-3-km shear from the Slater (2000) profiler where was by BM98. C5 can be characterized as a non-descending overallmesocyclone height whereof 8 km. the The vortex strongest originated cyclonic below shears 4 were km, observations-1. The recorded pre-convective by Przybylinski thermodynamic et al. and shear deepened, and intensified within the next 20 minutes to an 0-3 km shear values for bow echo events exceeded primarily17 m s supercellular. detected within the lowest 3 km during the first 15 minutes profiles suggested that the convective mode would be etand al. between 3 and 6.5 km 20 minutes after initial detection. These observations are similar to those described by Trapp cells During into thea solid early line part of of convective the storm development, storms from thenortheast overall (1999). Tornadogenesis occurred just prior to C5 system reflectivity structure evolved from an area of broken reaching its greatest depth, similar to observations noted by aPrzybylinski radial convergent et al. (2000). pattern, The while 1806 a and nearly 1811 symmetrical UTC radial mid-levelto southwest rotating Iowa. centers Several (mesocyclones) elements of this located convective along velocity data at the 0.5° elevation (0.5 km AGL) showed system exhibited HP supercell reflectivity structures with low-level velocity structure was noted at the 1.4° (1.3 km AGL) and the downshear flank of each of these storms. The overall stronghigher radial elevation convergent angles. velocity These pattern observations was present are similar near storm reflectivity characteristics included strong to those made within classic supercells by BM98, where a reflectivity gradients along the storm’s downwind flank sectionscapped by taken high normal reflectivities to the leading aloft, suggesting convective the cells location within or below cloud base and strong rotation was noted above of the storm’s updraft region or WER. Reflectivity cross- ofcloud radial base. convergence During the at 1816 the lowest UTC volume elevation scan angle and andjust deepafter rotation tornado throughout occurrence, much Doppler of the velocity circulation's observations depth the line confirmed the existence of a WER. Several storm-scale mesovortices associated with Storm at the lowest elevation angle likely originating as a result B were identified during this period between 1700 and ofdiffer the slightly advancement from those of of the BM98; precipitation-laden the radial convergence rear- 1833 UTC, suggesting one variation of the HP supercell theme. Many of these rotating centers originated from mid- levels (4-6 km AGL) and met the criteria for a mesocyclone. andflank-downdraft. upward growth Independent of rotation that of convectiveinitiates at mode,low-levels the Mid-level vortex origination is related to the growth of a isobservation a useful indicator of rapid that increases will allow in warning low-level forecasters convergence to perturbation low-pressure area aloft which induces an upward directed perturbation pressure-gradient force on the right flank of the storm. This process results from the gain confidence in anticipating tornadic development and shear profile represented by a clockwise turning hodograph issuance of subsequent warnings. study(Bluestein expanded 1993). upward Similar andto observations downward by from Burgess mid-levels et al. (1982), storm-scale vortices (C1 and C2) sampled in this Acknowledgments r traces showed that the strongest cyclonic shears associated with these circulations during intensification. However, V The authors would like to express gratitude to the anonymous reviewers for their comments and withremained many at of mid-levels these storms (4-6 during km AGL) this andperiod. did not intensify role in greatly improving a previous version of this to low-levels. Damaging winds and hail were associated manuscript. convective line as it approached south-central Iowa and The overall MCS gradually changed into a nearly solid

164the DesNational Moines Weather area. Digest Cross-sections of reflectivity and Storm Morphology and Mesovortex Evolution

Authors Dr. Y. J. Lin

Dr. Jason Martinelli received his Master degree in from University of Wisconsin in 1964, and his Ph.D. in received his B.S. degree from the meteorology from the Department of Meteorology and State University of New York at Albany in 1998 and his Oceanography, New York University, in 1969. He joined the M.S. and Ph.D. degrees from Saint Louis University in Department of Earth and Atmospheric Sciences, Saint Louis 2001 and 2004, respectively. He joined the Department of University, in September 1969 as an Assistant Professor currentlyAtmospheric teaches Sciences several at Creighton upper-level University and graduate in January courses of of Meteorology. Later, he was promoted to Associate 2004 as an Assistant Professor of Atmospheric Sciences. He Professor in 1972 and Professor of Meteorology in 1976. After 36 years of service at Saint Louis University, he retired mesoscalein Radar and and Mesoscale radar meteorology. Meteorology. Recently, his research from the Department in July 2005. Currently, he has been Dr. Martinelli’s research interests are in the areas of inworking radar on meteorology a graduate-level during textbook the past in radar two meteorology. decades. It of case studies involving tornadic and non-tornadic quasi- The material included is largely based on his lecture notes has focused on observational and numerical simulations case studies in severe local storms using single- and dual- also includes recent findings extracted from numerous deeplylinear convective involved in systems student (QLCSs). advising and outreach as well. In addition to teaching and research, Dr. Martinelli is Doppler radar data. Dr. Lin has supervised many Ph. D. and M. S. graduate Thus far, he has supervised several M.S. students and many students. Additionally, he also advised undergraduate undergraduate students majoring in Atmospheric Sciences. students majoring in meteorology. He received the meteorology.He also enjoys visiting many of the local elementary Burlington Northern Foundation’s Faculty Achievement and middle schools throughout the year to talk about Award as the 1986-87 outstanding teacher of the year in Dr. Robert Pasken 1987. Dr. Lin’s research interests are mesometeorology and received his B.S. degree (1973) from radar meteorology. He published his research findings in the State University of New York at Brockport. He then several international and domestic professional journals. completed his M.S. degree (1974) at the University of Chicago theHe was case elected studies Fellowof tornadic of the and American non-tornadic Meteorological mesoscale under T.T Fujita. From 1974 to 1976, he was employed as a Society in 1998. More recently, his research focused on consultant for Ecology and Environment Inc. in Buffalo N.Y. Dr. Pasken received his Ph.D. in Meteorology in 1981 from convective systems (MCS) using WSR-88D data. He and Saint Louis University. After a one-year appointment as a his co-investigators have studied the observed strong mid- visiting professor at the University of Oklahoma, he returned altitude radial convergence (MARC) along the forward to Saint Louis University as an Associate Professor in the structuralflank of organized characteristics bowing and MCSs dynamic using processes, single-Doppler which data. lead Science and Mathematics Department in Parks College of Emphasis is placed on the characteristics, including both Saint Louis University. Dr. Pasken was transferred to the Earth and Atmospheric Sciences Department of Saint Louis theto strong initial convergence onset of damaging in the mid-levelssurface winds of multicell associated bowing with University when Parks College was reorganized in late squallconvective lines. lines, and the utility of the MARC signature to 1997. Dr. Pasken’s teaching interests include Computer Ron W. Przybylinski ).Techniques His in research Meteorology interests as includewell as the Instrumentation dynamic and and Remote Sensing Techniques (particularly Doppler has been the Science and Operations recovery of thermodynamic parameters via dual and single PriorOfficer to at1991, the Ron NOAA/National served as forecaster Weather and Service station Weatherscientist thermodynamic structure of sub-tropical squall lines, the Forecast Office (WFO) in St. Louis, Missouri since 1991.

Doppler radars, as well as the use of High Performance at the NWSFO in Indianapolis, Indiana. Much of his early Computing tools to solve complex problems. Recent work on the study of bow echo evolution evolved during field work included participation in the NASA Tropical his tenure at Indianapolis. He received his B.S and M.S. Rainfall Measurement Mission (TRMM), ground-truth in meteorology from Saint Louis University in 1977 and field experiment in Brazil (TRMM-LBA), and the Kwajalein radar.1981. HeIn lateis currently 1980’s, ahe principle served asinvestigator a project leaderon the onsevere the Islands (KWAEX). Operational Test and Evaluation of the WSR-88D Doppler

straight-line winds component of the COMET Cooperative Project with SaintVolume Louis 31University. Number 2 ~ December 2007 165 Martinelli et al.

References of tornadic interactions with thermal Maddox, R. A., L. Mon R. Hoxit, Wea. Revand C. F. Chappell, 1980: A study 28th Andra,Conference D. L., 1997: on The Radar origin Meteorology and evolution of the WSR-88D boundaries. ., 108, 322-336. mesocyclone recognition Algorithm. Preprints, The occurrence of tornadoes in supercells interacting , Austin, TX, Amer. Markowski, P. M., E. N. Rasmussen, and J.Wea. M. Straka, Forecasting 1998:, Meteor. Soc., 364-365. with boundaries during VORTEX-95. Atkins, N. L., C. S. Bouchard, R. W. Przybylinski, R. J. Trapp, 13, 852-859. and G. Schmocker, Mon. 2005: Wea. Damaging Rev surface wind mechanisms within the 10 June 2003 Saint Louis bow Moller,and A.documentation. R., C. A. Doswell Preprints, III, and 16th R. Przybylinski,1990:Conf. Severe Local echo during BAMEX. .,133, 2275-2296. Storms,High precipitation supercells: A conceptual model Principles of Kinematics and Bluestein,Dynamics H. B., 1993: Synoptic–Dynamic Meteorology Kananaskis Park, Alberta, Canada, Amer. Meteor. in Midlatitudes. Vol. I, Soc., 52-57. , Oxford University Press, 431 pp. The operational recognition of supercell thunderstorm within severe local storms which travel to the right of ______,environments C. A. Doswell and III, storm M. P. Foster structures. and G. Wea. R. Woodall, Forecasting 1994:, Browning,the winds. K. A., J. 1964:Atmos Airflow and precipitation trajectories

. Sci., 21, 634-639. 9, 327–347.Storm Data

Burgess,radars. D. W., Preprints, and M. A.19th Magsig, Conf. 1998: on Severe Recent Local observations Storms, NCDC,28801-5001]. 1998. , Vol. 40. [Available from National of tornado development at near range to WSR-88D Climatic Data Center, 151 Patton Ave., Asheville, NC

Minneapolis, MN, Amer. Meteor. Soc.,756-759. Bull. Amer. Meteor. Soc evolution statistics, Preprints, 12th Conf. on Severe Local Nolen, R. H., 1959: A radar pattern associated with tornadoes. ______,Storms V. T. Wood, and R. A. Brown, 1982: Mesocyclone ., 40, 277-279. study of storm and vortex morphology during the , San Antonio, TX, Amer. Meteor. Soc., 422-424. Przybylinski,``intensifying R. W., stage’’ G. K. of Schmocker,severe wind mesoscaleand Y-J. Lin, convective 2000: A systems, Preprints, 20th Conf. on Severe Local Storms, Fujita, T., 1978: Manual of identification for project NIMROD. Satellite and Mesometeorology Research Paper No. 156, Dept. of Geophysical Sciences, Orlando, Florida, Amer. Meteor. Soc., 173-176. University of Chicago, 104 pp. Rasmussen E. N., and S. A. J. Rutledge, Atmos. Sci 1993: Evolution of Hamilton,Ohio. Preprints, R. E., 1970: 14th Radar Use of Meteorology detailed radar Conf., dataTucson, in quasi-two dimensional squall lines. Part 1: Kinematic mesoscale surface analysis of the July 4, 1969 storm in and reflectivity structure ., 50, 2584-2606. lines with trailing stratiform precipitation. Mon. Wea. AZ, Amer. Meteor. Soc., 339-342. Smull,Rev., B. F., and R. A. Houze Jr.,1987: Rear inflow in squall

Houze,of midlatitude R. A., S. A. Rutledge, mesoscale M. convective I. Biggerstaff, systems. and Bull.B. F. Amer.Smull, 115, 2689–2889. Meteor.1989: Interpretation Soc., of Doppler displays Trapp, R. J., E.D. Mitchell, G. A. Tipton, D. W. Effertz, A. I. 70, 608–619. Watson,Wea. D. L. Forecasting Andra, and M. A. Magsig, 1999: Descending Wea. and non-descending TVS signatures detected by WSR- Johns,Forecasting, R. H., 1993: Meteorological conditions associated 88Ds. , 14, 625-639. with bow echo development in convective storms. echoes. J. Atmos. 8, 294-299. Weisman, M. L., 1993: The genesis of severe, long-lived bow convectively induced wind-storms. Wea. Forecasting, 2, Sci.., 50, 645-670. ______, and W. D. Hirt, 1987: Derechos: Widespread numerically simulated convective storms on vertical ______, and J. B. Klemp, 1982:Mon. Wea.The dependence Rev of 32-49. 520. 166 National Weather Digest wind shear and buoyancy. ., 110, 504-