Prelude:
For the benefit of new subscribers/followers and a reminder to others, I use four sections, namely weather, climate, beyond and case studies, and two threads, one relating to droughts, the other to intercessory prayer, for this Substack. When possible I try to at least rotate around the first three sections, and update the last thread, each month. This current post is the next instalment in the weather section. Finally, I have recently updated my about information.
In the last weather post I introduced one of two major theories that can explain how minute cloud droplets develop into precipitation, namely, the Bergeron-Findeisen ice-crystal mechanism1. I will now briefly look at the second theory, called Longmuir's/ Langmuir's collision and coalescence theory (see ‘The formation of precipitation’, and find section 10.2 at here):
This applies to 'warm' clouds i.e. those without large numbers of ice crystals. Instead, they contain water droplets of many differing sizes, which are swept upwards at different velocities so that they collide and combine with other droplets. It is thought that when the droplets have a radius of 3 mm, their movement causes them to splinter and disintegrate, forming a fresh supply of water droplets.
This coalescence ‘scavenging’ is found to be an important sink of cloud condensation nuclei (CCN) in both liquid and mixed-phase precipitating stratocumulus, thus the ‘just-right’ fine-tuning of CCN (e.g. here).
I would now like to mention a few referred papers. Firstly, the paper (here), by Peter Barnet (2011), reported that by analysing the ‘collision-kernel’, which governs the chance of cloud droplets colliding and coalescing, it was found that the chance of two droplets colliding and coalescing is extremely sensitive to the size of the colliding drops. Further, the relationship between the time taken to produce rain (t) and the total water content (w) for drops of constant size, was found to be (t ∝ w−1). It was suggested that turbulence, especially when convective clouds have substantial updraughts and downdraughts, could be incorporated into the model, and as a result the time taken to produce rain could be quicker, but in some cases prevent collisions occurring between certain drops entirely.
This collision prevention may be a contributing factor to why pyrocumulonimbus (pyroCb) clouds, though directly injected with smoke particles, produce a vigorous convective column devoid of appreciable precipitation (e.g. see here)? (Here) explains the difference between pyrocumulus (pyroCu) and pyroCb clouds. The article also cites information that a single pyroCb cloud can send particles as high as 10 miles (around 16 km) into the lower stratosphere. Clearly more research2 is needed, such as looking at the effects of turbulence on rain production, to hopefully one day provide practical solutions to enable these ‘fire-breathing dragon of clouds’ to self-extinguish (even better, not even form in the first place, e.g. see here), and prevent/reduce the spreading of their diabolical consequences3.
A somewhat related example is this 2014 paper (here, and summarised here), which provides details of a study to understand the role of turbulence on rain formation through its influence on drop collision coalescence in cumulus clouds. Their summary explains the impacts of turbulence on drop coalescence by combining observations of drop size distributions with a ‘super-droplet method’ and theoretical scaling analysis. This provided substantial evidence for the critical impacts of turbulence on rain initiation and growth. They found that turbulent coalescence considerably enhances rain initiation and needs to be included in their model to accurately simulate the ‘tail’ of drop size distributions, especially near the cloud base. Further, large aerosols, serving as giant CCN, had little impact on rain formation in cumulus clouds when turbulence effects were considered.
Finally, coalescence activity can be quantified to assist with decisions in whether to proceed with specific cloud seeding programs in some parts of the world (e.g. see information here). This paper explains that ‘soundings’ conducted just before cloud seeding operations provides information about the thermodynamic and microphysical state of the cloud. Further, when the Index of Coalescence Activity (ICA) is used as an indicator of the microphysical state of the cloud, it is useful for providing guidelines for various cloud seeding programs.
In conclusion, this process (coalescence), may also be considered as ‘amplification’4, in this case due to the collisions between (just-right) resulting cloud droplets (with or without turbulent effects), leading to rain droplets, followed by larger rain drops. When these rain drops, or via melted ice crystals (see previous post) have formed, the ‘elephant-in-the-sky’ is then - how do these rain drops reach the ground as, well, rain, including their ability to eventually infiltrate into the soil for agricultural purposes? This will be the topic of the final weather post.
An example of what can be achieved is this new airborne research facility, primarily for research on clouds over the great barrier reef described (here).
Margi Prideaux presents some life changing information about these clouds in her book ‘FIRE - A Message from the Edge of Climate Catastrophe’.
Where the probability distribution (outcomes) of the event changes, increasing the likelihood of occurrence of very low probability (outcomes).