An Observational and Numerical Modeling Study of Rapid Changes in the Pre-Storm Boundary Layer of a Severe Nocturnal QLCS during VORTEX-SE on 9-10 March 2017

The Weather Forecasting Model (WRF) was used to study the boundary layer evolution prior to the passage of a severe nocturnal QLCS that moved through northern Alabama on March 9, 2017 during the 2017 VORTEX-SE field campaign. 45 storm reports were tallied in Alabama, including 35 wind, 10 hail and 2 tornado reports in southern Tennessee within 100 km of the Mobile Integrated Profiling System (MIPS) at UAH. The MIPS consists of a suite of remote sensing instruments including a 915 MHz Doppler wind profiler (DWL), a Doppler mini-sodar, a 35-ch microwave profiling radiometer (MPR), and a sensitive lidar ceilometer. The Mobile Doppler Lidar and Sounding (MoDLS) system with a Doppler Wind Lidar (DWL) was co-located with the MIPS during this event. The NOAA/NSSL Collaborative Lower Atmospheric Mobile Profiling System (CLAMPS), consisting of a DWL, AERI, and a surface station, was deployed 59 km east of the MIPS within the complex terrain system of the Tennessee River Valley and the adjacent Sand Mountain plateau. These profiling systems were utilized to evaluate the evolution and structure of the simulated boundary layer characteristics from the midafternoon hours up to the arrival of the QLCS shortly before midnight LST. The main focus is placed on the rapidly evolving nocturnal boundary layer (NBL), simulated vs observed, over both profiling sites. High resolution vertical profiles of wind within the NBL and lower troposphere are compared with high-resolution observations from the combined remote sensing instruments which extend from 25 m to about 2 km AGL. Thermodynamic profiles are compared with a balloon sounding released at 0515 UTC to assess the model performance.

Multiple simulations were conducted with varying parameterizations for the PBL (YSU, MYJ, and ACM2), and Microphysics (Morrison Double-Moment, and WSM6) schemes. It is well known that accurate simulations of the NBL wind and stability profiles present significant challenges to NWP models, particularly during the cold season low cape, high shear severe weather events in the Southeast. Results show the model being too stable (low level inversion present), the simulated winds above the surface layer (> 100 m) generally exceed observed winds, and that winds within the NBL surface layer are generally lower in the model compared to observations. In addition, the wind direction matches up well with the observations, and the temperature drop during QLCS passage is not as substantial as was observed. This is one of the first studies to compare high-resolution WRF simulations to comprehensive profiling observations.