The Generation of Turbulence below Midlevel Cloud Bases: The Effect of Cooling due to Sublimation of Snow

Abstract

In the author's experience as a forecaster, commercial aircraft sometimes report turbulence beneath midlevel clouds that extend above upper frontal zones. Turbulence caused by Kelvin-Helmholtz instability occurs in upper frontal zones with strong vertical shear of horizontal winds. However, the turbulence seems to occur not only in the cloud bases (where upper frontal zones are) but also below the cloud bases where the vertical shear is not strong. Because those clouds are usually accompanied by precipitation that does not reach the ground, cooling by evaporation or sublimation seems to contribute to the generation of turbulence. In this paper, the mechanisms generating turbulence below midlevel cloud bases are examined by using observations and high-resolution three-dimensional numerical simulations with idealized initial conditions. The numerical simulations showed that the following sequence of events led to turbulence. Falling snow sublimated below cloud bases and cooled the air, which created absolute instability. This generated Rayleigh-Bé nard convection cells. The vertical motion caused turbulence. The horizontal scale of the convection was about 800-1000 m, and the variations of vertical wind velocity were up to about 7 m s 21 . The cloud base was accompanied by a virga-like distribution of snow. Sensitivity experiments showed the appropriate conditions to cause the turbulence: 1) the cloud-base temperature was between about 08 and 2158C, 2) the relative humidity in subcloud layers was sufficiently low, and 3) the stability in subcloud layers was weak. The results of the numerical simulations agreed well with the observations.

Convective Instability Underneath Midlevel Clouds: Comparisons between Numerical Simulations and VHF Radar Observations

“…The MU radar detected turbulence below a frontal zone (cloud base) where the atmosphere was convectively unstable and vertical shear was weak, consistent with MCT events described by Kudo (2013) on the basis of numerical simulations. To study the nature of the observed turbulence, we performed high-resolution 3D numerical simulations initialized with the balloon data and compared the results with those obtained from the MU radar observations.…”

“…The temperature at the cloud base (212.58C) and the weak stratification and low humidity below the frontal zone observed by T52 (Fig. 1) are also consistent with conditions related to the occurrence of MCT according to the simulations by Kudo (2013).…”

“…They found that inflowing air was destabilized by lifting and by the vertical profile of evaporation and melting, which helped to maintain the simulated FFLS systems; however, they did not mention turbulence in the destabilized layers. Turbulence led by Rayleigh-Bénard-like convection below the base of clouds extending above upper-level fronts is called midlevel cloud-base turbulence (MCT) by Kudo (2013). He showed the morphology, evolution, and occurrence conditions of MCT from 3D numerical simulations with idealized initial conditions.…”

Shigaraki UAV-Radar Experiment (ShUREX): overview of the campaign with some preliminary results

“…The billows lasted from 06:31 LT to 06:55 LT. Immediately below, mid-level cloud-base convection (see Kudo 2013;Wilson et al 2014;Kudo et al 2015 for a description of MCT) was initiated at a height of~5.5 km at around 06:16 LT. Incidentally, the signature of another UAV (UAV 16) launched prior to UAV 17 can be seen from~06:20 LT to 06:50 LT but UAV 16 did not fly high enough to reach the deep turbulent layers. The MCT layer expanded rapidly to span 5 to 5.5 km by 07:30 LT and descended to lower altitudes and thickened.…”

Comparisons between high-resolution profiles of squared refractive index gradient <i>M</i><sup>2</sup> measured by the Middle and Upper Atmosphere Radar and unmanned aerial vehicles (UAVs) during the Shigaraki UAV-Radar Experiment 2015 campaign

“…A proxy of the cloud base around 14:30 LT is given by the dashed line (7.0 km) for easy reference. A radar echo layer of 1.0-1.5 km in depth can be noted at the cloud base and is a signature of a mid-level cloud-base turbulence (MCT) layer developing due to a convective instability below a cloudy frontal zone (e.g., Luce et al, 2010;Kudo, 2013;Kudo et al, 2015). The probable convective motions inside the layer are likely the cause of small-scale internal wave oscillations seen below the turbulent layer down to 2.0 km at least since they coincide with its passage (oscillations start from ∼ 14:15 LT).…”