Residual Layer turbulence and Nocturnal Boundary Layer dynamics at the Boulder Atmospheric Observatory using the high-resolution turbulence sensors of the CIRES Tethered Lifting System

Observations in the Stable Boundary Layer (SBL) and Residual Layer (RL) are difficult because of 1) the typical variability and intermittency of the turbulent processes driving the bulk of the mixing, 2) the high resolutions required to correctly capture and interpret the physics of the processes driving its structure and mixing properties, and 3) the limited vertical extent of instrumented towers which are only sometimes sufficiently high to reach into the Residual Layer.

Recent studies have shown that an accurate description of the SBL requires about one meter vertical resolution and sensitive instruments that can accurately estimate the low-turbulence statistics, especially in the residual layer. Typical 10-20 Hz measurements from sonic anemometers have difficulty resolving the small-scale turbulence statistics because of the lack of a spectral gap in the nocturnal regime, and fixed-level instrumentation on meteorological towers provide too coarse of a vertical resolution. Improving our understanding of the SBL/RL dynamics and turbulent processes requires higher resolution and direct measurements of the small-scale turbulence statistics, more studies of the climatology of the nocturnal mixing events, investigations of SBL-RL interactions, and a better documentation of the effects of complex terrain.

The CIRES TLS was developed to meet those requirements. The new sensors provide high-rate (1kHz) insitu measurements of temperature and wind speed that permit accurate measurements of small-scale turbulence (energy dissipation rate ε for velocity and temperature structure constant C_T²) at sub-meter vertical resolutions, with an accuracy better than 15% (based on 1-second spectra), and down to extremely small turbulence levels as low as ε=10^-8 m²s^-3. The combination of direct measurements of vertical gradients of potential temperature and velocity, turbulence energy dissipation rate, as well as turbulence length scales such as the Kolmogorov and Taylor microscales, Ozmidov and Corrsin scales, and non-dimensional parameters such as the Gradient Richardson number, Froude number, and Reynolds number to name a few, provide the essential parameters for comparisons with turbulence theory and for descriptions of the vertical structure of the SBL and RL.

We will present and comment on recent measurements acquired with the TLS at the Boulder Atmospheric Observatory during the testing of new and improved instrumented payloads. These data, although explorative in nature, provide qualitative information about the scales, intensities, and duration of the large number of turbulent processes populating the residual layer (active, passive, and fossil turbulence). Additionally, continuous profiling of the first 300 meters of the atmosphere provides insights on NBL dynamics as well SBL/RL interactions.