Scale dependence of entrainment‐mixing mechanisms in cumulus clouds

Abstract

This work empirically examines the dependence of entrainment-mixing mechanisms on the averaging scale in cumulus clouds using in situ aircraft observations during the Routine Atmospheric Radiation Measurement Aerial Facility Clouds with Low Optical Water Depths Optical Radiative Observations (RACORO) field campaign. A new measure of homogeneous mixing degree is defined that can encompass all types of mixing mechanisms. Analysis of the dependence of the homogenous mixing degree on the averaging scale shows that, on average, the homogenous mixing degree decreases with increasing averaging scales, suggesting that apparent mixing mechanisms gradually approach from homogeneous mixing to extreme inhomogeneous mixing with increasing scales. The scale dependence can be well quantified by an exponential function, providing first attempt at developing a scale-dependent parameterization for the entrainment-mixing mechanism. The influences of three factors on the scale dependence are further examined: droplet-free filament properties (size and fraction), microphysical properties (mean volume radius and liquid water content of cloud droplet size distributions adjacent to droplet-free filaments), and relative humidity of entrained dry air. It is found that the decreasing rate of homogeneous mixing degree with increasing averaging scales becomes larger with larger droplet-free filament size and fraction, larger mean volume radius and liquid water content, or higher relative humidity. The results underscore the necessity and possibility of considering averaging scale in representation of entrainment-mixing processes in atmospheric models.

On Which Microphysical Time Scales to Use in Studies of Entrainment‐Mixing Mechanisms in Clouds

“…The criterion of the in‐cloud drizzle water content (radius > 25 μm) averaged over the observation period of the flight smaller than 0.005 g m −3 is used to select the nondrizzling cumulus clouds. In total, there are 186 nondrizzling growing cumulus clouds from eight flights (22–24 May, 11 June, 19 June, 23 June, 24 June, and 26 June 2009); the same data set was used in Cheng et al () and Lu et al (). As shown in Table , the total number of cloud droplet size distributions was 13,652 from 186 clouds.…”

Scale Dependence of Cloud Microphysical Response to Turbulent Entrainment and Mixing

“…The existence of the turbulent energy cascade implies that there is a range of turbulence time scales, from the large‐scale eddy turnover time (short: large‐eddy time) T L to the dissipation (or Kolmogorov) time τ η . It is thus possible that within the turbulence cascade, both D a > 1 and D a < 1 can exist at different spatial scales, so that one can also expect a different microphysical response at different scales (Andrejczuk et al, ; Beals et al, ; Korolev & Mazin, ; Lehmann et al, ; Lu et al, , ; Siebert et al, ). For example, some DNS results suggested that a transition from inhomogeneous to homogeneous mixing could lead to an inhomogeneous offset in the microphysical trajectory in a mixing diagram (Kumar et al, ).…”

Theoretical study of mixing in liquid clouds – Part 1: Classical concepts

“…There are a number of numerical simulations and theoretical efforts on studying different aspects of mixing and its effect on cloud microphysics (e.g., Latham, 1979, 1982;Jensen and Baker, 1989;Su et al, 1998;Lasher-Trapp et al, 2005;Jeffery, 2007;Andrejczuk et al, 2009;Kumar et al, 2013;Jarecka et al, 2013;Lu et al, 2011Lu et al, , 2014Tolle and Krueger, 2014, and many others). A comprehensive review of the works on the effect of turbulence and mixing on cloud droplet formation can be found in Devenish et al (2012).…”