A Case Study of a High-Precipitation Supercell-Bow Echo Transition on 4-5 June 2008
Dillingham, Sarah Marie
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On 4-5 June 2008, the Texas Tech StickNet teams deployed an array of instruments to make measurements from a high-precipitation (HP) supercell-bow echo (SBE) transition during Project MOBILE. Prior to crossing the StickNet array, this HP supercell interacted and merged with a nascent supercell before transitioning into a bow echo (BE). Surface kinematic and thermodynamic observations were coupled with WSR-88D radar data in an effort to diagnose key features of the SBE transition. Prior to its BE transition, the parent HP supercell produced wind gusts of over 50 knots, hail up to 2.75” in diameter, and four EF1 tornadoes. One of the tornado-producing low-level mesocyclones was sampled by the StickNet array and is compared to previous observations of tornadic and non-tornadic low-level mesocyclones. It is hypothesized that the merger of the nascent supercell with a HP supercell initiated its SBE transition. This merger resulted in the development of an updraft bridge which led to an increase in precipitation rates along the leading edge of the storm. Heavy precipitation rates systematically increased the strength of the storm-scale cold pool and resulted in the rearward tilting of thunderstorm updrafts, subsequently forcing the development of a mid-level rear inflow jet (RIJ). Intensification of the RIJ lead to the parent HP supercell’s outflow dominance and transition into a BE. As the SBE progressed east of the StickNet array, large thermodynamic deficits were observed under a large rear inflow notch associated with the RIJ. These observations appeared to be consistent with the descent of branches of the RIJ to the surface. Observations of this HP-SBE transition will also be compared to previous conceptual models developed by Moller et al. (1990) and Hastings and Richardson (2016) to identify characteristics consistent with HP-SBE transitions. These evolutions can occur in a variety of ways and over various timescales. The observations documented in this case study will be used to offer recommendations to aid in the improvement of parameterizations in future HP-SBE simulations.