Physical Interpretation of Mixing Diagrams

Abstract

Type of mixing at cloud edges is often determined by means of mixing diagrams showing the dependence of normalized cube of the mean volume radius on the dilution level. The mixing diagrams correspond to the final equilibrium state of mixing between two air volumes. While interpreting in situ measurements, scattering diagrams are plotted in which normalized droplet concentration is used instead of dilution level. Utilization of such scattering diagrams for interpretation of in situ observations faces significant difficulties and often leads to misinterpretation of the mixing process and to uncertain conclusions concerning the mixing type. In this study we analyze the scattering diagrams obtained by means of a Lagrangian‐Eulerian model of a stratocumulus cloud. The model consists of 2,000 interacting Largangian parcels which mix with their neighbors during their motion in the atmospheric boundary layer. In the diagram, each parcel is denoted by a point. Changes of microphysical parameters of the parcel are represented by movements of the point in the scattering diagram. The method of plotting the scattering diagrams using the model is in many aspects similar to that used in in situ measurements. It is shown that a scattering diagram shows snapshots of a transient mixing process. The location of points in the scattering diagrams reflects largely the history and the origin of air parcels. Location of points on scattering diagram characterizes intensity of entrainment, and different parameters of droplet size distributions (DSDs) like concentration, mean volume (or effective) radius, and DSD width.

Theoretical analysis of mixing in liquid clouds – Part IV: DSD evolution and mixing diagrams

“…In the present study as well as in Pt3 and modeling studies performed by Andrejczuk et al (2006Andrejczuk et al ( , 2009 and Khain et al (2018), it is shown that time needed to establish equilibrium is either quite long or even never reached. This means that the scattering diagrams observed in situ are just snapshots of the transient mixing process.…”

“…At the same time, scattering diagrams may contain useful information concerning intensity of mixing, the DSD width and other parameters of DSDs (see Khain et al, 2018 …”

“…Processes of mixing are investigated in observations (Yum et al, 2015;Bera at al., 2016a, b), large eddy simulations (Andrejczuk et al, 2009;Khain et al, 2018) and direct numerical simulations (Kumar et al, 2014(Kumar et al, , 2017. Processes of mixing and their effects on droplet size distributions were recently investigated in a set of theoretical studies (Yang et al, 2016;Korolev et al, 2016 (hereafter, Pt1); Pinsky et al, 2016a, b).…”

response to reviewer 1 and to Dr. Kumar

“…To summarize, our general conclusion is that the simplifications underlying the classical concept of mixing are too crude, making it impossible to use scattering diagrams for comprehensive analysis of mixing and especially for determination of mixing types. At the same time, scattering diagrams may contain useful information concerning intensity of mixing, the DSD width and other parameters of DSDs (see Khain et al, 2018…”

“…The effects of mixing of cloudy air with surrounding dry air on cloud microphysics are still the focus of many studies (see overview by Devenish et al, 2012). Processes of mixing are investigated in observations (Yum et al, 2015;Bera at al., 2016a, b), large eddy simulations (Andrejczuk et al, 2009;Khain et al, 2018) and direct numerical simulations (Kumar et al, 2014(Kumar et al, , 2017. Processes of mixing and their effects on droplet size distributions were recently investigated in a set of theoretical studies (Yang et al, 2016;Korolev et al, 2016 (hereafter, Pt1); Pinsky et al, 2016a, b).…”