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Measuring eddy heat, constituent, and momentum fluxes with high‐resolution Na and Fe Doppler lidars

Vertical transport by turbulent mixing plays a fundamental role in establishing the thermal and constituent structure of the upper mesosphere and lower thermosphere (MLT). Because of observational challenges, eddy heat, constituent, and momentum fluxes, and the associated coefficients for thermal (kH), constituent (kzz), and momentum (kM), diffusion have not been well characterized in the MLT. We show that properly configured Na and Fe Doppler lidars, with sufficient resolution to observe the turbulence‐induced wind, temperature, and density fluctuations, can make direct measurements of eddy fluxes throughout the mesopause region. When the horizontal (zθL with z = altitude and θL = full width at half maximum laser divergence), vertical (Δz), and temporal (Δt) resolutions of the lidar satisfy the condition LC≃3zθL2+0.6u¯Δt2+1.5Δz2 ≤ 270 m (125 m), where u¯ is the mean horizontal wind velocity, the observations include more than 80% (90%) of the energy in the turbulence fluctuations, and the observed fluxes and derived diffusivities will be highly representative of the actual values. For existing Na and Fe Doppler lidars, which have modest power‐aperture products of about 1 W m2, long averaging times (5–20 h) are required to obtain statistically significant estimates of the eddy fluxes, kzz, kH, and kM profiles, and the turbulent Prandtl number (Pr = kM/kH) between about 85 and 100 km. These systems are capable of measuring the weekly or monthly mean flux and diffusivity profiles. Systems with power‐aperture products of 5–100 W m2 or larger could be used to study the eddy fluxes generated by the dissipation and breaking of individual gravity waves to altitudes as high as the turbopause (~110 km).