Vertical Variation of Turbulent Entrainment Mixing Processes in Marine Stratocumulus Clouds Using High‐Resolution Digital Holography

Abstract

Marine stratocumulus clouds contribute significantly to the Earth's radiation budget due to their extensive coverage and high albedo. Yet, subgrid variability in cloud properties, such as aerosol concentration, droplet number, and precipitation rates, lead to considerable errors in global climate models. While these clouds usually have small vertical extent, turbulent entrainment‐mixing and precipitation can generate significant variations in droplet number, size, and relative dispersion with altitude. Here, we analyze turbulent entrainment‐mixing processes and the variability in cloud microphysical properties as a function of height within a warm marine stratocumulus cloud layer over the Eastern North Atlantic. We use high‐resolution airborne holographic measurements and compare them with local turbulence measurements. We find that entrainment‐mixing is primarily inhomogeneous near cloud top and homogeneous near cloud base. Further analysis of Damköhler number and transition scale number are able to explain the mixing mechanisms at different cloud heights using phase relaxation but not droplet evaporation as the microphysical time scale. A modified droplet evaporation time scale that considers local saturation deficit using a simple linear mixing model is developed, and it is able to reliably explain the observed mixing mechanisms. This study reinforces the importance of turbulent mixing and the use of appropriate microphysical time scales in determining cloud microphysical processes.

Comprehensive quantification of height dependence of entrainment mixing between stratiform cloud top and environment

“…In this scenario, number concentration decreases but droplet size remains unchanged. Some observational studies support the extreme IM concept (Burnet and Brenguier, 2007;Lu et al, 2011;Freud et al, 2011Freud et al, , 2008Pawlowska et al, 2000;Haman et al, 2007), while some others indicate that the HM mixing dominates (Gerber et al, 2008;Lu et al, 2013c;Burnet and Brenguier, 2007;Jensen et al, 1985), and still some others find intermediate features fall in between the HM and IM mixing (Lehmann et al, 2009;Lu et al, 2014a;Kumar et al, 2018).…”

Cloud Top Radiative Cooling Rate Drives Non‐Precipitating Stratiform Cloud Responses to Aerosol Concentration

“…It is also worthwhile to briefly discuss why the cloud top evaporation rate is not directly impacted by the droplet concentration (as seen in Figure 4). Recent studies have found that at the tops of stratocumulus clouds the timescale for mixing is much larger than the timescale for evaporation leading to inhomogeneous mixing (Desai et al, 2021;Gao et al, 2021). This result suggests that the bulk evaporation rate at cloud top is controlled primarily by how fast dry air can be entrained into the cloud rather than by the microphysical properties of the cloud droplets.…”

A Study on the Wide Range of Relative Humidity in Cirrus Clouds Using Large-Ensemble Parcel Model Simulations

“…The entrainment and mixing process on cloud edges can affect the in-cloud water through mass exchange [25][26][27]. It is noteworthy that the entrainment and mixing process remains poorly understood, and that its measurement and quantification are extremely difficult [28][29][30][31]. Thus, this study only focuses on the mechanisms unassociated with the surrounding ambiance.…”