Effects of Nonstationary Wave Structures on Rotor Development and Evolution

The impact of nonstationary mountain waves due to mean flow variability and nonlinearity on rotor development and evolution is investigated using The Weather Research and Forecasting model (WRF) and tower and SODAR observations from a special network located at Rock Springs, PA. To the authors' knowledge, this is the first study to examine these circulations for the smaller-scale (~300 m valley to mountain-top height), Appalachian Mountains of Central, PA. High-resolution (i.e., 0.444-km horizontal grid spacing and 12 vertical levels below 100 m) forecasts are examined for two cases with nonstationary wave motions. The first case exhibits transient wave structures that result in an overall wavelength reduction from ~7 km to ~3 km through the nighttime period. North American Regional Reanalysis (NARR) data suggest the presence of wind ducting regions decreasing in height through the night. These ducting regions act to trap the smaller horizontal wavelengths near the surface. Network observations and model simulations support the existence of a rotor circulation, resembling that of Type 1. The second case study features greater nonlinearity due to the presence of breaking waves and a downslope windstorm event with Type 2 rotor over the network. NARR analyses for this case suggest the presence of upstream critical levels and weak shear regions through the near-mountain top inversion. Tower and SODAR measurements support the presence of a large-amplitude breaking wave that results in larger downward momentum flux, abrupt and long-lasting wind speed and temperature increases, and large near-surface turbulent kinetic energy. WRF forecasts for this case show a large-amplitude breaking wave that oscillates backwards and forwards through the night creating brief weak-wind and cooling periods at the surface. For both cases, model forecasts suggest a direct correlation between periods of wavelength transition (from longer to shorter) and rotor strength. Decreases in horizontal wavelength are associated with periods of wave amplification and rotor intensification. Overall, rotors are shown to be transient features that are highly coupled to the evolution of the wave structures.