Publication Date:
2024-02-15
Description:
Entrainment and mixing play an essential role in shaping the droplet size distribution (DSD), with commensurate effects on cloud radiative properties or precipitation formation. In this paper, we use a model that considers all relevant scales related to entrainment and mixing by employing the linear eddy model (LEM) as a subgrid‐scale (SGS) mixing model, coupled with a large‐eddy simulation model and a Lagrangian cloud model (LCM) for a single cumulus congestus cloud. We confirm that the DSD is broadened toward small‐size droplets during homogeneous mixing. During inhomogeneous mixing, the DSD width remains almost unchanged. The DSD width can also be narrowed after mixing. We show that this happens when DSD is broadened toward small‐size droplets, which evaporate rapidly, while larger droplets are almost unaffected. In addition, when droplets ascend during mixing, DSD narrowing is caused when the adiabatic increase in supersaturation is slower than the average droplet evaporation, allowing only the largest droplets to benefit from the newly produced supersaturation. The narrowing mixing scenario prevents clouds from having too broad DSDs and causes the DSD relative dispersion to converge around 0.2 to 0.4. As this scenario is more frequent when the LEM SGS model is used, our results indicate that adequately modeling turbulent mixing is necessary to represent a realistic DSD shape.
Description:
Plain Language Summary:
Clouds are always in contact with the surrounding air. Because the air outside the cloud is drier than the cloud, cloud droplets tend to evaporate when it enters the cloud. The size of the cloud droplets after evaporation can vary depending on the timescales of turbulent mixing and droplet evaporation. If the dry air mixes quickly, all droplets evaporate simultaneously. If the dry air is mixed slowly, only the droplets exposed to the dry air evaporate. However, this mixing occurs on small scales that traditional cloud models cannot account for. To account for this, we use a special model capable of representing all relevant scales. We confirm previous theoretical work that when mixing is fast, all droplets evaporate and the mean droplet size decreases. When mixing is slow, some droplets evaporate completely, but the average droplet size remains constant. We also observe cases where only small droplets evaporate while large droplets barely change. This scenario happens when there are many small droplets to evaporate or when additional moisture from cloud motion prevents larger droplets from evaporating completely.
Description:
Key Points:
Changes in the droplet spectrum width under different mixing scenarios are investigated using a Lagrangian cloud model.
While droplet spectrum broadening is common, narrowing occurs when the droplet size relative dispersion is large, or when droplets ascend.
The interaction of these different mixing scenarios favors a relative dispersion of the droplet spectrum between 0.2 and 0.4.
Description:
Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659
Description:
http://rossby.msrc.sunysb.edu/SAM.html
Description:
https://doi.org/10.5281/zenodo.7120916
Keywords:
ddc:551.5
;
entrainment and mixing
;
cumulus clouds
;
droplet size distribution
;
Lagrangian cloud model
;
mixing scenarios
Language:
English
Type:
doc-type:article
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