Structure function analysis of chemical tracer trails in the mesosphere‐lower thermosphere region

Abstract

AbstractTrimethyl aluminum (TMA) trails released from sounding rockets have been used extensively as a tracer of the neutral atmospheric motions in the mesosphere/lower thermosphere region and serve as a tracer of the atmospheric structure in the altitude range where the trails are released. In addition to the bulk motion corresponding to the winds, smaller‐scale structure is also evident in the trail images, which can be an indication of turbulence or small‐scale wave behavior. In the 90 to 100 km altitude range, the trails are often visible for periods of 20 to 30 min or even longer, so that the horizontal scale sizes spanned by the trails increase significantly as the trails expand due to the combined effects of turbulent diffusion and horizontal displacements due to vertical shears in the winds. We describe a study involving data from high‐, middle‐, and low‐latitude TMA releases. In each case the structure function for the fluctuation structure within the trails was calculated for a sequence of trail images. Each image in the sequence corresponds to an outer horizontal spatial scale that increases with time. By analyzing the time evolution of the structure function, changes in the dynamical processes associated with the form of the structure function have been identified as a function of time and thus also of the increasing horizontal spatial scale size. A consistent pattern is found for all locations, namely a transition in fitted exponent for the structure function that suggests a change from more isotropic turbulence immediately after the trail is released to stratified turbulence at the later times when the horizontal scale size has increased to several hundred kilometers. The transition is found to occur near a horizontal spatial scale in the range between 100 and 200 km.