Observational Study of the Entrainment-Mixing Process in Warm Convective Clouds

Abstract

Thermodynamical and microphysical measurements collected in convective clouds are examined within the frame of the homogeneous/inhomogeneous mixing concept, to determine how entrainment-mixing processes affect cloud droplets, their number concentration, and their mean size. The three selected case studies-one stratocumulus layer and two cumulus clouds-exhibit very different values of the cloud updraft intensity, of the adiabatic droplet mean volume diameter, and of the saturation deficit in the environment, all three parameters that are expected to govern the microphysical response to entrainment mixing. The results confirm that the observed microphysical features are sensitive to the droplet response time to evaporation and to the turbulent homogenization time scale, as suggested by the inhomogeneous mixing concept. They also reveal that an instrumental artifact due to the heterogeneous spatial droplet distribution may be partly responsible for the observed heterogeneous mixing features. The challenge remains, however, to understand why spatially homogeneous cloud volumes larger than the instrument resolution scale (10 m) are so rarely observed. The analysis of the buoyancy of the cloud and clear air mixtures suggests that dynamical sorting could also be efficient for the selection, among all possible mixing scenarios, of those that minimize the local buoyancy production.

Droplet growth in warm turbulent clouds

“…Warner, 1955, 1969a, 1969b, Blyth et al, 1988Gerber et al, 2005Gerber et al, , 2008Burnet and Brenguier, 2007;Haman et al, 2007) tend to show that stratocumulus is only slightly diluted by entrainment, whereas cumulus is much more strongly diluted. In cumulus clouds the droplet number concentration is less reduced by entrainment than the liquid water content because the majority of droplets exhibit some evaporation.…”

“…In stratocumulus, on the other hand, entrainment dilutes the droplet number concentration and the liquid water content by a similar amount, while the size of the droplets is relatively unaffected compared with cumulus clouds (e.g. Burnet and Brenguier, 2007). Latham and Reed (1977), Baker and Latham (1979) and Baker et al (1980) characterized mixing in terms of the relative time-scales of droplet evaporation and turbulent homogenization of a given control volume.…”

“…A further factor concerns the point in the evolution of the cloud at which the measurements are taken: intuitively one may expect mixing to become more inhomogeneous as the cloud becomes less vigorous. As the liquid water content is proportional to na 3 , the microphysical properties of mixing can be examined on a mixing diagram as shown schematically in Figure 3, in which the abscissa is n and the ordinate is proportional to the mean droplet volume, both normalized by their respective adiabatic values (Burnet and Brenguier, 2007). The inhomogeneous limit then appears as a horizontal line on this diagram, while homogeneous mixing is represented by curved lines for different values of the relative humidity, or saturation deficit (negative s), of the entrained air (note that the final mixture returns to equilibrium i.e.…”

“…s = 0). Different points along these lines represent different fractions of entrained and cloudy air (lines of constant mass fraction of cloudy air would appear as vertical lines on the mixing diagram in Figure 3; see also Figures 1 and 2 of Burnet and Brenguier, 2007). When the entrained air is close to saturation, i.e.…”

“…In general, though, one would expect that over a sufficiently long time period ε would decrease as a result of mixing. The type of mixing near the cloud edge also depends on the amount of dilution of the cloud due to entrainment of dry air, with the mixing in cumulus being homogeneous when the dilution is low but becoming more inhomogeneous as the dilution increases (Burnet and Brenguier, 2007). Comparisons of stratocumulus and cumulus by Burnet and Brenguier (2007) (in which the turbulent homogenization time-scale, τ r , and the evaporation time-scale, τ e are calculated for a length scale of 10 m) show that mixing in stratocumulus appears more inhomogeneous than in cumulus: values of ε tend to be lower in stratocumulus compared with cumulus, hence the value of l c is, on average, smaller in stratocumulus than in cumulus, which explains why inhomogeneous mixing is more likely in stratocumulus compared with cumulus.…”