Observations and numerical simulations of intense atmospheric subrotor vortices during T-REX

High-resolution observations from scanning Doppler and aerosol lidars as well as wind profiler radars obtained during Terrain-Induced Rotors Experiment (T-REX) provide documentation of small-scale intense vortices associated with atmospheric rotors that commonly form in the lee of mountainous terrain. Although rotors are already recognized as potential hazards for aircraft, it is proposed that these small-scale vortices, or subrotors, are the most dangerous features because of strong wind shear and the transient nature of the vortices.

During T-REX IOP-13, the DLR Doppler lidar documented an intense shear layer associated with strong downslope flow along the Sierra slopes that transitioned to much weaker cross-valley valley flow beneath a lee wave crest. We focus on the evolution of a subrotor vortex that develops and intensifies along a shear layer at the leading edge of the larger-scale rotor. The NCAR Raman-shifted Eye-safe Aerosol Lidar (REAL) indicates that plumes of aerosols and clouds are advected along the lee slopes prior to flow separation and are lofted and advected downstream in a layer of elevated strong westerly flow, consistent with the Doppler lidar observations. The REAL documents overturning of the aerosol layer as a result of the sub-rotor amplification and associated circulation. Vertical velocity maxima and minima derived from the NCAR Multiple Antenna Profiler Radar (MAPR) are consistent with the presence of transient subrotor circulations that are generated along the shear zone near the leading edge of the rotor and subsequently advected downstream. The DRI surface network indicates high-frequency oscillations in the 10-m pressure perturbation during this time period, which may be a signature of transient subrotors.

High-resolution eddy-resolving numerical simulations (grid increment of 60 m) reveal that subrotors are generated along the leading edge of an elevated sheet of horizontal vorticity that is a manifestation of boundary layer shear and separation along the lee slopes. The suite of ground based instruments and numerical simulations suggest that subrotors may be common during conditions that feature strong downslope windstorms, mountain waves, and rotors in the presence of strong vertical wind shear.