Fine Structure, Instabilities, and Turbulence in the Lower Atmosphere: High-Resolution In Situ Slant-Path Measurements with the DataHawk UAV and Comparisons with Numerical Modeling

A new platform for high-resolution in situ measurements in the lower troposphere is described and its capabilities are demonstrated. The platform is the small GPS-controlled DataHawk unmanned aerial system (UAS), and measurements were performed under stratified atmospheric conditions at Dugway Proving Ground, Utah, on 11 October 2012. The measurements included spiraling vertical profiles of temperature and horizontal wind vectors, from which the potential temperature theta, mechanical energy dissipation rate epsilon, Brunt-Vasala frequency N, temperature structure parameter C-T(2), Thorpe and Ozmidov scales L-T and L-O, and Richardson number Ri were inferred. Profiles of these quantities from similar to 50 to 400 m reveal apparent gravity wave modulation at larger scales, persistent sheet-and-layer structures at scales of similar to 30-100 m, and several layers exhibiting significant correlations of large epsilon, C-T(2), L-T, and small Ri. Smaller-scale flow features suggest local gravity waves and Kelvin-Helmholtz instabilities exhibiting strong correlations, yielding significant vertical displacements and inducing turbulence and mixing at smaller scales. Comparisons of these results with a direct numerical simulation (DNS) of similar multiscale dynamics indicate close agreement between measured and modeled layer character and evolution, small-scale dynamics, and turbulence intensities. In particular, a detailed examination of the potential biases in inferred quantities and/or misinterpretation of the underlying dynamics as a result of the specific DataHawk sampling trajectory is carried out using virtual sampling paths through the DNS and comparing these with the DataHawk measurements.