Midlevel Cloud‐Base Turbulence: Radar Observations and Models

Abstract

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 (~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 2,000 m in depth, lasting often for as much as 24 hr. 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.

Polarimetric Observations and Simulations of Sublimating Snow: Implications for Nowcasting

“…Many theoretical (e.g., Klett 1995, and references therein) and observational (Westbrook et al 2010) studies have concluded that the velocity fluctuations imposed on particles by turbulent eddies at scales comparable to their size are insufficient to cause appreciable canting. However, other studies (Kantha et al 2019) have reported eddy dissipation rates beneath sublimating snow up to three orders of magnitude higher than those observed in Westbrook et al (2010). Garrett et al (2015) also reported increasingly broad snowflake canting angle distributions with increasing turbulence, although uncertainty remains as disturbance of particle orientations due to turbulence generated by the sensor cannot be ruled out (Jiang et al 2019).…”

“…First, owing to the large enthalpy of sublimation and the relatively slow terminal velocity of snowflakes, sublimation may rapidly cool and moisten dry environments, allowing snow to reach the surface unexpectedly (Market et al 2006). In addition, this cooling can force a dynamic response that promotes frontogenesis and generates convective instability and/or turbulence below cloud base, all of which can have a positive feedback on the production of snow (e.g., Harris 1977;Auria and Campistron 1987;Clough and Franks 1991;Parker and Thorpe 1995;Clough et al 2000;Kudo 2013;Kudo et al 2015;Kantha et al 2019). Sublimation has also been proposed as a possible secondary ice production (SIP) mechanism for dendritic and irregularly shaped ice crystals (e.g., Oraltay and Hallett 1989;Korolev et al 2020), as sublimation diminishes and weakens the supporting structure of the narrow branches and subbranches and makes them more prone to fracturing.…”

Atmospheric structures in the troposphere as revealed by high-resolution backscatter images from MU radar operating in range-imaging mode

“…Around Japan, it occurs typically during the rainy season (Kudo 2013). In spite of the fact that the first article on MCT was published in 2010 (Luce et al 2010) and was followed up by Kudo (2013), Wilson et al (2014), and Kudo et al (2015), very little additional work has been done on MCT until Kantha et al (2019) cataloged and presented MCT events observed during the 2015 and 2016 campaigns. They also present a one-dimensional, second moment closure-based model of turbulent mixing in the MCT layer, with the event observed on June 13, 2015 (Fig.…”