Contrasting Scale Dependence of Entrainment‐Mixing Mechanisms in Stratocumulus Clouds

Abstract

The apparent turbulent entrainment-mixing mechanism between clouds and surrounding air is scale dependent; however, such scale dependence has been rarely studied, hindering development of scale-aware entrainment-mixing parameterizations. Here we extend our previous study on cumulus clouds to investigate scale dependence of entrainment-mixing processes in stratocumulus clouds during Aerosol and Cloud Experiments in Eastern North Atlantic and Routine AAF (Atmospheric Radiation Measurement (ARM) Aerial Facility) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO). In contrast to previous studies, two opposite scale dependencies are found: Entrainment mixing can become more homogeneous or more inhomogeneous with increasing averaging scales, which is quantified by the difference between homogeneous mixing degree at the 100 and 10 m resolutions. A new heuristic model and two new quantities are introduced. The observations and model show that microphysical properties near and far away from droplet-free air and relative humidity of entrained air determine both the sign and strength of scale dependence, while droplet-free air fraction only affects the strength. The results shed new light on developing scale-aware parameterizations of entrainment-mixing mechanisms.Plain Language Summary Turbulent entrainment mixing between clouds and surrounding air significantly affects cloud microphysical properties over a wide range of scales, indicating that the apparent entrainment-mixing mechanism is scale dependent. Previous studies have shown that entrainment-mixing mechanisms tend to be more inhomogeneous when the sampling rate is lower. Surprisingly, the present study shows two opposite behaviors of scale dependence: The mechanisms can become more homogeneous or more inhomogeneous. To understand the physical mechanisms responsible for the scale dependence, a new heuristic model is established. Cloud microphysics and relative humidity of the entrained air determine both the sign and strength of scale dependence, while droplet-free air fraction only determines the strength. Two new quantities combining all the factors are defined, and they can better quantify the effects of the factors on scale dependence than each individual factor. This study finds a new phenomenon of scale dependence and improves our physical understanding of entrainment mixing.

Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA)

“…The effect of entrainment-mixing on droplet size spectrum appears to be dependent on the sampling scales (Burnet and Brenguier 2007;Lu et al 2014;Beals et al 2015;Kumar et al 2018). Gao et al (2020) analyzed in-situ ACE-ENA data and found two opposite trends of scale dependency: entrainment mixing can become more homogeneous or more inhomogeneous with increasing averaging scales, depending on the properties of the entrained dry air, cloud microphysics, and turbulence. Addressing this perplexing issue requires measurements at centimeter scales from the HOLODEC deployed during ACE-ENA (Fig.…”

Consideration of Initial Cloud Droplet Size Distribution Shapes in Quantifying Different Entrainment‐Mixing Mechanisms

“…From the microphysical perspective, our previous studies have measured the entrainment-mixing process through four homogeneous mixing degrees (ψ j , where j is from 1 to 4) (S. Gao et al, 2020;Z. Gao et al, 2018;Lu, Liu, Niu, & Endo, 2014;Lu, Liu, Niu, Krueger, & Wagner, 2013;Luo et al, 2020):…”

Parameterizations of Entrainment‐Mixing Mechanisms and Their Effects on Cloud Droplet Spectral Width Based on Numerical Simulations

“…Several conceptual models have been proposed for entrainment‐mixing mechanisms (Baker & Latham, 1979; Gao et al, 2020; Telford, 1996; Wang et al, 2009; Yang et al, 2016; Yeom et al, 2017; Yum et al, 2015). The most popular one is based on the concepts of homogeneous/inhomogeneous mixing mechanisms (e.g., Baker et al, 1980; Tölle & Krueger, 2014).…”