2344 MONTHLY WEATHER REVIEW VOLUME 136 Surface Analysis of the Rear-Flank Downdraft Outflow in Two Tornadic Supercells BRIAN D. HIRTH,JOHN L. SCHROEDER, AND CHRISTOPHER C. WEISS Atmospheric Science Group, Texas Tech University, Lubbock, Texas (Manuscript received 15 June 2007, in final form 15 November 2007) ABSTRACT The rear-flank downdraft regions of two tornadic supercells were sampled on 12 June 2004 and 9 June 2005 using four “mobile mesonet” probes. These rear-flank downdraft outflows were sampled employing two different data collection routines; therefore, each case is described from a different perspective. The data samples were examined to identify variations in measured surface equivalent potential temperature, virtual potential temperature, and kinematics. In the 12 June 2004 case, the tornadic circulation was accompanied by small equivalent potential temperature deficits within the rear-flank downdraft outflow early in its life followed by increasing deficits with time. Virtual potential temperature deficits modestly increased through the duration of the sample as well. The 9 June 2005 case was highlighted by heavy precipitation near the tornado itself and relatively small negative, or even positive, equivalent and virtual potential temperature perturbations. Large horizontal variations of surface thermodynamic properties were also noted within several regions of this rear-flank downdraft outflow. 1. Introduction small (large) equivalent potential temperature and vir- tual potential temperature deficits as compared to The existence of hook echoes and rear-flank down- those found in the storm inflow environment are drafts (RFDs) in supercell thunderstorms has been well thought to be associated with tornadogenesis success documented over the past several decades (Markowski (failure). These observational findings were all focused 2002b; Stout and Huff 1953; van Tassel 1955; Fujita on time periods very near tornadogenesis (Ϯ5 min). 1958, 1973, 1975; Browning and Donaldson 1963; These conclusions were further reinforced through a Browning 1964, 1965; Lemon 1977; Burgess et al. 1977; numerical simulation conducted by Markowski et al. Brandes 1978; Barnes 1978a,b). Initial investigations at- (2003). The present study attempts to examine the tem- tempted to directly link hook echoes with tornado oc- poral and spatial variability of the thermodynamic and currences, but recent research has confirmed that su- kinematic properties within RFDs while a tornado is percell thunderstorms possessing hook echoes often fail ongoing. to produce tornadoes (Markowski et al. 2002, hereafter Four “mobile mesonet” instrumented vehicles, mod- MSR02). Using direct surface measurements from the eled after those designed by Straka et al. (1996), were Verification of the Origins of Rotation in Tornadoes used by the Texas Tech University (TTU) Atmospheric Experiment (VORTEX; Rasmussen et al. 1994) and Science Group and the TTU Wind Science and Engi- subsequent endeavors, general conclusions have been neering Research Center to collect data during the made relating RFD thermodynamics (equivalent po- Wheeled Investigation of Rear-Flank Downdraft Life- tential temperature and virtual potential temperature) cycles (WIRL) project in May and June of 2004 and to tornadogenesis success and failure (MSR02; 2005. Project WIRL’s efforts yielded two RFD outflow Markowski 2002a, hereafter M02; Grzych et al. 2007, samples suitable for analysis. The first dataset was col- hereafter GLF07). RFD outflow parcels that possess lected from a supercell located west of Lehigh, Iowa, on 12 June 2004 (case 1). The second dataset was acquired from the 9 June 2005 supercell, which passed south of Corresponding author address: Brian D. Hirth, Atmospheric Science Group, Department of Geosciences, Texas Tech Univer- Hill City, Kansas (case 2). These cases offer differing sity, Box 42101, Lubbock, TX 79409. perspectives of the thermodynamic and kinematic vari- E-mail: [email protected] ability within tornadic RFDs. DOI: 10.1175/2007MWR2285.1 © 2008 American Meteorological Society Unauthenticated | Downloaded 10/02/21 06:08 AM UTC MWR2285 JULY 2008 HIRTHETAL. 2345 TABLE 1. Instrumentation and accessories used on the MM platform. Instrument Model Output Comment Wind monitor RM Young 05103 Wind Speed (m sϪ1) and wind direction (°) Temperature–relative Vaisala HMP45A Temperature (°C) and humidity relative humidity (%) Fast-response Campbell Scientific Temperature (°C) Susceptible to radio frequency interference temperature Thermistor 107-L5 resulting in data spikes. Aluminum flashing was added to the PVC housing to help alleviate this interference. Pressure sensor Vaisala PTB220B Pressure (hPa) Static pressure port RM Young 61002 GPS antenna Motorola ANT97-PGS 24 dB BNC-6 GPS receiver Motorola ONCORE Vehicle heading (°) and Vehicle must be in motion 8-channel vehicle speed (m sϪ1) Datalogger Campbell Scientific CR23X Enclosure Hoffman A-1214CHAL Houses the datalogger, pressure sensor, and GPS receiver Vehicle mounts Yakima Q-Towers and Q-Clips Electric fan Radio Shack 6250 Provides continuous aspiration to the PVC housing for the temperature–relative humidity and fast temperature probes Laptop computer Dell Latitude D505 Running LabVIEW software (Intel Celeron) HF radio ICOM IC-F320S Power inverter Tripp-Lite PV-5002 2. Methodology VIEW software. This real-time display allowed team members to better identify the RFD gust front and a. Data collection other noteworthy features. The entire system, including Mobile mesonet (MM) data were collected utilizing radio communications, was powered by the vehicle’s the instrumentation presented in Table 1. Instrumenta- battery. tion and hardware were mounted to an aluminum rack following the design of Straka et al. (1996) to limit the effects of vehicle motion on wind and pressure mea- surements. Consistent with the original design, a PVC housing was mounted to the rack mast to protect both temperature and relative humidity sensors from water, debris, and direct contact with sunlight. A sheet of alu- minum flashing was added surrounding the PVC hous- ing to shield the interior sensors from radio frequency (RF) noise. An electric fan was inserted into the bot- tom of the PVC housing to allow for a continuous as- pirated airflow through the entire enclosure. Because of erroneous measurements made by the flux gate com- pass as a result of RF noise, the compass was removed from the instrumentation suite. Data were collected at 0.5 Hz and were continuously processed and stored by a Campbell Scientific CR23X datalogger. The datalog- FIG. 1. TTU MM platform including wind monitor (A), static ger, pressure sensor, and GPS receiver were housed in pressure port (B), GPS antenna (C), PVC housing (D) containing a hail-resistant enclosure mounted to the mast just “fast” temperature and “slow” temperature–relative humidity probes, and hail-resistant enclosure (E) containing pressure sen- above the vehicle’s roof (Fig. 1). Data were simulta- sor, datalogger, and GPS receiver. The background tornado oc- neously transferred via serial interface from the data- curred in rural Kent County, TX, on 12 Jun 2005. Photo taken by logger to an on-board laptop computer running Lab- Ian Giammanco. Unauthenticated | Downloaded 10/02/21 06:08 AM UTC 2346 MONTHLY WEATHER REVIEW VOLUME 136 FIG. 2. Schematic diagram of the life cycle data collection rou- tine. The RFD boundary is indicated by a solid black line while individual MM teams are marked with an M. Location of the FIG. 3. Same as Fig. 2, but for the snapshot data collection low-level mesocyclone or tornado is denoted with a T. The storm routine. motion vector is provided. Vehicles were to utilize the road net- work to maintain their respective mesocyclone or tornado-relative positions as much as possible as the storm moved eastward. The sample was obtained, the probes attempted to reposi- ϫ idealized road grid spacing is approximately 1.6 km 1.6 km. tion themselves to repeat the procedure as roads al- lowed. In all cases during Project WIRL, the RFD was ini- Two primary data collection routines were executed tially identified visually through the manifestation of a through the duration of Project WIRL. The life cycle clear slot (Beebe 1959). Because of logistical chal- routine was executed for slow-moving supercells or lenges, the team was unable to conduct successful sam- storms following a densely gridded road network. The pling prior to the development of this feature, suggest- goal of this routine was to sample various regions of the ing that the processes driving the RFD were already RFD outflow for as long as logistically permitted. ongoing when the team made its initial measurements. Three individual MM probes attempted to position themselves with a 1- to 2-km spacing in the western, b. Analysis methodologies and techniques southern, and eastern portions of the RFD outflow. The fourth MM probe was dedicated to continuous To ensure data consistency among the four MM sampling of the inflow environment so direct thermo- probes, parameter value comparisons were made while dynamic comparisons could be made between the vari- the team was traveling in a caravan and in quiescent ous RFD sectors and the inflow “base state” (Fig. 2). conditions. Instrument offsets for each probe were The inflow probe was charged with helping to identify computed to remove any individual instrument biases potential low-level mesocyclogenesis, storm cycling, relative to the overall team mean calculated using data and storm transition