Horizontal Correlation Functions of Wind Fluctuations in the Mesosphere and Lower Thermosphere

Measurements of kinetic energy in vortical and divergent fluctuations in the mesosphere and lower thermosphere can be used to study stratified turbulence (ST) and gravity waves. This can be done using horizontal correlation functions of the fluctuating component of velocity. This study introduces a novel method for estimating these correlation functions using radars that observe Doppler shifts of ionized specular meteor trails. The technique solves the correlation functions directly on a longitudinal-transverse-up coordinate system, assuming axial symmetry. This procedure is more efficient and leads to smaller uncertainties than a previous approach. The new technique is applied to a year-long data set from a multistatic specular meteor radar network in Germany, to study the annual variability of kinetic energy within turbulent fluctuations at 87-93 km of altitude. In monthly averages, the kinetic energy is found to be nearly equipartitioned between vortical and divergent modes. Turbulent fluctuations maximize during the winter months with approximately 25% more energy in these months than at other times. The horizontal correlation functions are in agreement with the inertial subrange of ST, exhibiting a 2/3 power law in the horizontal lag direction, with an outermost scale of ST to be about 380 km. This suggests that horizontal correlation functions could be used to estimate turbulent energy transfer rates.

Automated summary

Measurements of kinetic energy in vortical and divergent fluctuations in the mesosphere and lower thermosphere using radar observations of Doppler shifts of specular meteor trails are introduced. Monthly averages show that the kinetic energy is almost evenly distributed between vortical and divergent modes, with higher turbulence during winter months. The horizontal correlation functions exhibit a 2/3 power law in the horizontal lag direction and suggest the potential use of these functions in estimating turbulent energy transfer rates.