Midlevel Cloud-Base Turbulence: Radar Observations and Models

Midlevel Cloud-base Turbulence (MCT) caused by ice/snow precipitation from midlevel clouds falling into dry air below the cloud base and sublimating is investigated. MCT phenomena in the lower troposphere as revealed by the Middle and Upper atmosphere (MU) radar in Shigaraki, Japan, during the Shigaraki unmanned aerial vehicle and radar Experiment campaigns in the spring-summer of 2015 and 2016 are described. The MU radar was operated in a high-resolution (similar to 20-m) range imaging mode and hence revealed the structure of these MCTs in great detail. These MCT layers grew to hundreds of meters in thickness, often reaching nearly 2,000m in depth, lasting often for as much as 24hr. A simple analytical model and a second-moment closure-based turbulent mixing model are used to estimate the levels of turbulence kinetic energy and its dissipation rate in the MCT layer and compare them to those measured by the MU radar. The study shows that MCT can give rise to moderate levels of turbulence of potential interest to aviation, confirming conclusions reached by earlier pioneering studies. However, to our knowledge, this important process has not received the attention it deserves by the atmospheric community so far, and therefore, it is the principal goal of this paper to not only present some observations and preliminary modeling results, but also redraw the attention of atmospheric scientists to this important process. Plain Language Summary The free atmosphere above the planetary boundary layer is stably stratified, and therefore, turbulent mixing there is shear driven and occurs generally under the influence of stable density stratification. However, convective mixing can also occur there under certain conditions. When precipitating snow/ice particles fall below the cloud base of midlevel clouds, they can sublimate if the air column below the cloud is dry enough. The resulting cooling of the air column drives vigorous convection. This midlevel cloud-base turbulence process is the subject of this article.