Small scale turbulence modulation by ducted gravity waves above the stable nocturnal boundary layer

High-frequency temperature and velocity data collected by the turbulence probes of the CIRES Tethered Lifting System (TLS) during the CASES99 field campaign in Wichita, Kansas, are used to investigate the possibility of small-scale turbulence modulation by internal gravity waves in the residual layer above the stable nocturnal boundary layer. During the night of October 14, 1999, propagating gravity waves of small amplitude (~0.2 m/s; 0.2C) were observed below a steep temperature inversion. These observations agree well with predictions made from ducted gravity waves theory. Spectra of the TLS 200Hz temperature and velocity fluctuations exhibit a strong wave-like signature at roughly 0.003Hz, a frequency that matches well with the frequency of the ducted wave's dominant mode. In an attempt to connect the small-scale turbulence fluctuations to the presence of the ducted gravity waves, we have computed the time series of the fluctuating component of the background Richardson number (Ri). These variations arise as a direct consequence of the oscillations of the local temperature and velocity gradients induced by the ducted wave. Comparisons of the fluctuating Ri with the time series of the measured turbulent quantities, specifically the temperature structure constant (Ct2) and the energy dissipation rate (epsilon), show good agreement, with regions of enhanced turbulence matching the troughs of Ri, and quieter regions matching Ri maxima. These observations support the proposed theory of "deterministic approach of turbulence intermittency" introduced by previous theoretical studies that focus on the topic of "wave-turbulence interactions".