21B.1 An Examination of the 20 June 2015 Covective Initiation Event during PECAN

Thursday, 31 August 2017: 11:00 AM
Vevey (Swissotel Chicago)
Brianna M. Lund, Univ. of Alabama, Huntsville, AL; and K. Knupp

The timing and location of convective initiation (CI) represents a continued and important focus of severe weather phenomena. The poorly understood catalysts for CI motivate questions surrounding the timing, location and physical processes of CI, particularly within the boundary layer (BL) during the Afternoon-Evening Transition (AET). During the summer of 2015, the Plains Elevated Convection at Night (PECAN) field campaign took place to study nocturnal convection amongst a number of other phenomena. One of the objectives of PECAN was to examine the phenomenon of CI during the AET. During the late afternoon to early evening hours on 20 June 2015, a CI mission was conducted near Hays, KS. CI was observed near the intersection point of a horizontal convective roll and a weak cold front in close proximity to the University of Alabama in Huntsville (UAH) Mobile Integrated Profiling System (MIPS). Data from the S-PolKa radar, the UAH MIPS and mobile mesonet, and instruments at a fixed profiler site are utilized to characterize the BL evolution and BL convergent zone evolution. The present hypothesis of this study is that a closer investigation of the finer scale features found during the AET in the BL may shed light on the evolution of microscale mechanisms initiating convection, such as a reduction in turbulence (e.g., variance in vertical motion from the Doppler wind lidar and spectrum width from the 915 MHz wind profiler) and turbulent momentum fluxes, and a subsequent increase in boundary layer convergence as inferred from S-PolKa measurements. In this case, deep convective clouds rapidly developed along the cold front during the AET as the cold front passed over the MIPS. Analyses of radar and lidar observations illustrate that significant upward vertical motion associated with the collision of the boundaries was sustained for approximately 15 minutes, and was associated with an increase in the depth of water vapor. A characteristically turbulent boundary layer preceded initiation, and an unusually turbulent BL persisted into the early nocturnal boundary layer cycle after CI had occurred. A number of boundaries and linear features were present and observed by the S-PolKa radar and other mobile assets, making this a very complicated and unique case for study.
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