The evolution and sensitivity of katabatic flow dynamics to external influences through the evening transition

Data collected over an arid shallow slope (2–4°) during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program are used to study the katabatic structure and onset of katabatic flow through the evening transition. An unprecedented suite of instrumentation, including a transect of five turbulence towers with 29 sonic anemometers, is used for the investigation. Fifteen transition periods with well-defined katabatic flow and relatively little synoptic forcing are used in the study. The katabatic onset, jet velocity and jet height all show a large degree of interdiurnal and intersite variance. The slope-aligned budgets of momentum and potential temperature are used to define time-scales that describe the evolution of the katabatic flow. Composite wind velocity time series are used to show that ≈30 min elapses from the time when the katabatic flow initializes at 0.5 m to the point of initialization at 20 m. A simple katabatic model utilizing surface energy-budget modelling is developed and used to model the interdiurnal katabatic variance. Finally, uni- and multi-variate statistical analyses are used to diagnose the influence of specific external variables. Valley wind speed, turbulence structure, soil moisture, and shadow front speed are all found to influence the katabatic dynamics to varying degrees.