Observations and Modeling of the Effects of Waves and Rotors on Submeso and Turbulence Variability within the Stable Boundary Layer over Central Pennsylvania

Open Access
Suarez Mullins, Astrid
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
October 22, 2015
Committee Members:
  • David R Stauffer, Dissertation Advisor
  • George Spencer Young, Committee Member
  • Fuqing Zhang, Committee Member
  • Jacob Willem Langelaan, Committee Member
  • high-resolution modeling
  • wavelets
  • model verification
  • waves
  • rotors
  • submeso
  • turbulence variability
  • stable boundary layer
Terrain-induced gravity waves and rotor circulations have been hypothesized to enhance the generation of submeso motions (i.e., nonstationary shear events with spatial and temporal scales greater than the turbulence scale and smaller than the meso-gamma scale) and to modulate low-level intermittency in the stable boundary layer (SBL). Intermittent turbulence, generated by submeso motions and/or the waves, can affect the atmospheric transport and dispersion of pollutants and hazardous materials. Thus, the study of these motions and the mechanisms through which they impact the weakly to very stable SBL is crucial for improving air quality modeling and hazard predictions. It is important to note that while gravity waves and rotors have been thoroughly studied through theoretical, observational, and idealized modeling studies, little still is known about the response of nonstationary and nonlinear waves for real cases under typical SBL conditions, and about the impact of different wave behaviors on rotor development and evolution, and the generation of intermittent low-level turbulence. Consequently in this thesis, the effects of waves and rotor circulations on submeso and turbulence variability within the SBL is investigated over the moderate terrain of central Pennsylvania using special observations from a network deployed at Rock Springs, PA and high-resolution Weather Research and Forecasting (WRF) model forecasts. The investigation of waves and rotors over central PA is important because 1) the moderate topography of this region is common to most of the eastern US and thus the knowledge acquired from this study can be of significance to a large population, 2) there have been little evidence of complex wave structures and rotors reported for this region, and 3) little is known about the waves and rotors generated by smaller and more moderate topographies. Six case studies exhibiting an array of wave and rotor structures are analyzed. Observational evidence of the presence of complex wave structures, resembling nonstationary trapped gravity waves and downslope windstorms, and complex rotor circulations, resembling trapped and jump-type rotors, is presented. These motions and the mechanisms through which they modulate the SBL are further investigated using high-resolution WRF forecasts. First, the efficacy of the 0.444-km horizontal grid spacing WRF model to reproduce submeso and meso-gamma motions, generated by waves and rotors and hypothesized to impact the SBL, is investigated using a new wavelet-based verification methodology for assessing non-deterministic model skill in the submeso and meso-gamma range to complement standard deterministic measures. This technique allows the verification and/or intercomparison of any two nonstationary stochastic systems without many of the limitations of typical wavelet-based verification approaches (e.g., selection of noise models, testing for significance, etc.). Through this analysis, it is shown that the WRF model largely underestimates the number of small amplitude fluctuations in the small submeso range, as expected; and it overestimates the number of small amplitude fluctuations in the meso-gamma range, generally resulting in forecasts that are too smooth. Investigation of the variability for different initialization strategies shows that deterministic wind speed predictions are less sensitive to the choice of initialization strategy than temperature forecasts. Similarly, investigation of the variability for various planetary boundary layer (PBL) parameterizations reveals that turbulent kinetic energy (TKE)-based schemes have an advantage over the non-local schemes for non-deterministic motions. The larger spread in the verification scores for various PBL parameterizations than initialization strategies indicates that PBL parameterization may play a larger role modulating the variability of non-deterministic motions in the SBL for these cases. These results confirm previous findings that have shown WRF to have limited skill forecasting submeso variability for periods greater than ~20 min. The limited skill of the WRF at these scales in these cases is related to the systematic underestimation of the amplitude of observed fluctuations. These results are implemented in the model design and configuration for the investigation of nonstationary waves and rotor structures modulating submeso and meso-gamma motions and the SBL. Observations and WRF forecasts of two wave cases characterized by nonstationary waves and rotors are investigated to show the WRF model to have reasonable accuracy forecasting low-level temperature and wind speed in the SBL and to qualitatively produce rotors, similar to those observed, as well as some of the mechanisms modulating their development and evolution. Here, it is demonstrated that for these cases modest changes in the background wind speed appears to play a more crucial role modulating the wavelength of waves within the valley than the low-level stratification. Generally decreasing wind speeds through the night for both cases produce average wavelength rates of change ranging from -15 to -8 % h-1, similar to those observed near large mountain ranges. These nonstationary wave motions result in highly nonlinear wave-wave interactions that lead to wave amplification and/or intensification of low-level rotors. Rotors are shown to persist for extended periods throughout the night and to be highly coupled to the wave structure. The rotor propagation speed is similar to that of the transitioning modes, and the strongest wind reversal regions typically occur during periods of the largest wavelength rate of change and/or wave amplification. It is demonstrated using real data conditions that rotor intensification is likely under weakening environmental wind speeds, increasing environmental stratification, and transient waves due to the combined effects from 1) wave amplification and 2) rotor nonstationarity. Overall, the nonstationarity of the waves and rotors due to moderate chnages in upstream background conditions must be recognized as an additional source of intermittency in the SBL. Finally, observations and high-resolution WRF forecasts under different environmental conditions using various initialization strategies are used to investigate the impact of nonlinear gravity waves and rotor structures on the generation of intermittent turbulence and valley transport in the SBL. Evidence of the presence of elevated regions of TKE generated by the complex waves and rotors is presented and investigated using an additional four case studies, exhibiting two synoptic flow regimes and different wave and rotor structures. TKE budget analysis shows that wave and rotor activity can enhance turbulence intermittency in the nighttime through the development of shear and convective instabilities and elevated regions of TKE. Turbulent transport is shown to play as much of a role in the local change of TKE as shear production; however advective processes might dominate the low-level TKE intermittency for these cases. The model is shown to reproduce some of the hypothesized nonlinear dynamics and downslope-wind response for one of the cases. However, very high horizontal resolutions (0.148-km grid spacing) are needed in order to better resolve wave amplitudes, rotor structures, and near-surface interactions. It is shown that the presence of strong wind speed and negative vertical shear near ridge top is important for a downslope-wind response over the network. The non-dimensional mountain height, a type of Froude number, is shown to be an effective parameter to identify downslope wind behaviors. However, it is not a good indicator of the wave amplitude (and rotor intensity). The presence of rotors can enhance horizontal thermal gradients and create regions of convergence with highly variable flow regimes and little spatial coherence. Overall, waves and rotors can significantly affect the transport within the valley, enhancing the downward transport of elevated materials and leading to locally higher concentrations near the surface. Throughout this thesis, terrain-induced gravity waves and rotors in the SBL are shown to synergistically interact with the surface cold pool and to enhance low-level turbulence intermittency through the development of submeso and meso-gamma motions. These motions are shown to be an important source of uncertainty for the atmospheric transport and dispersion of pollutants and hazardous materials under very stable conditions.