Abstract
Small-scale mixing between cloudy air and unsaturated clear air is investigated in numerical simulations and in a laboratory cloud chamber. Despite substantial differences in physical conditions and some differences in resolved scales of motion, results of both studies indicate that small-scale turbulence generated through cloud–clear air interfacial mixing is highly anisotropic. For velocity fluctuations, numerical simulations and cloud chamber observations demonstrate that the vertical velocity variance is up to a factor of two larger than the horizontal velocity variance. The Taylor microscales calculated separately for the horizontal and vertical directions also indicate anisotropy of turbulent eddies. This anisotropy is attributed to production of turbulent kinetic energy (TKE) by buoyancy forces due to evaporative cooling of cloud droplets at the cloud–clear air interface. Numerical simulations quantify the effects of buoyancy oscillations relative to the values expected from adiabatic and isobaric mixing, standardly assumed in cloud physics. The buoyancy oscillations result from microscale transport of liquid water due to the gravitational sedimentation of cloud droplets. In the particular modeling setup considered here, these oscillations contribute to about a fifth of the total TKE production.
Highlights
Consider an isobaric entrainment event initiated at the top or near the edge of a convective cloud
The diagram shows that evaporation of liquid water due to mixing between cloudy air and the unsaturated environment results in a homogenized mixture that has lower density temperature than either of the elements
Results of laboratory experiments and numerical simulations show that evaporative cooling and droplet sedimentation in the vicinity of the cloud–clear air interface lead to the anisotropy of small-scale turbulence associated with cloud entrainment and mixing
Summary
Consider an isobaric entrainment event initiated at the top or near the edge of a convective cloud. Once a parcel of unsaturated environmental air becomes engulfed by saturated cloudy air containing small water droplets, both volumes undergo stirring and filamentation in the process of turbulent mixing [13], [19]–[21] This is illustrated in figure 1 that shows high-resolution temperature measurements taken by aircraft [22, 23]. As pointed out in [11] (see figure 3 therein and the accompanying discussion), sedimentation of droplets from cloudy to clear air filaments provides an additional transport mechanism for liquid water, which affects Tρ by changes of l (and q in the case of evaporation) increasing in consequence the range of buoyancy fluctuations beyond that resulting from the adiabatic isobaric mixing. Numerical simulations discussed assess the role of these oscillations in the TKE budget
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