Diagnosis of Severe Wind Processes in Two Nocturnal MCSs Observed during PECAN

Severe wind processes in nocturnal mesoscale convective systems (MCSs) are not well understood since historically it has been assumed that these systems are elevated due to a stable surface layer. To further investigate these processes, two MCSs from the PECAN field project were selected for analysis due to their different evolution to cover a broad range of severe wind processes: the 25-26 June 2015 Kansas MCS and the 5-6 July 2015 South Dakota MCS. Results will be presented from WRF simulations with three nested grids utilizing a novel vertically integrated ice (VII) data assimilation (DA) scheme. This method is similar to the DA technique discussed in Fierro et al. (2012), wherein lightning data is used to nudge a column towards saturation by increasing water vapor mixing ratio. VII derived from the Multi-Radar Multi-Sensor (MRMS) product is used in lieu of lightning to effectively introduce virtual thermal buoyancy in the updraft column. VII nudging should introduce buoyant updrafts before the appearance of lightning while also decreasing errors in timing and location of convection initiation. Once the DA WRF simulations produced simulated reflectivity that best resembled the observed reflectivity during the severe phases of the MCSs, backwards trajectories were calculated using WRF output from the inner-most nest with 333-m grid spacing every 5 seconds. While most of the typical WRF output variables were unable to discern between severe and sub-severe trajectory processes, the integrated acceleration from the horizontal buoyancy and dynamic forcing along selected trajectories showed that within the last 5 and/or 2 minutes of the trajectory the increase in wind speed due to the dynamic forcing became larger than impacts from buoyancy. Over the entire trajectory time period, buoyancy forcing is the largest forcing term and explains increases in horizontal and vertical wind speed. This suggests that while all surface-based MCSs with an established cold pool will have buoyancy contributing the most to the wind speed, an extra contribution from some dynamic process is necessary for those winds to exceed the severe threshold. For a more strongly forced system such as the 5-6 July MCS, the enhanced dynamic push came from leading line mesovortices. For a more marginal MCS such as the 25-26 June MCS, an updraft/downdraft couplet associated with horizontal vorticity would enhance the horizontal acceleration.