What is Cumulus Cloud Turret Succession?

Beauty and education collide with these two insightful cumulus cloud timelapses! I love this footage captured during the monsoon by the University of Arizona (UofA) on July 17th, 2023. The two timelapses showcase the daytime evolution, struggle, and ultimate success of thunderstorms reaching maturity in proximity to both the Santa Catalina and Rincon Mountains near Tucson, Arizona (top and bottom videos, respectively). There are numerous meteorological concepts on display worthwhile to dive into, but I want to focus this discussion on how fully fledged thunderstorm development in the late afternoon hours required a sequence of events traced all the way back to sunrise that day!

Let's get to it! A transformation on the local environment is jumpstarted after sunrise. Critically, heating of the terrain generates positive buoyancy at the surface, whether abundant moisture is present or not. You have likely noticed bubbles eventually developing at the bottom of a heated pot of water and then breaking off surging to the top. You are witnessing convection in action! This convective process is happening on a grand scale across the landscape as sunlight energy is absorbed by the ground and reradiated as heat to the air in contact. Essentially, the layer of air in contact with the ground gains enough positive buoyancy relative to the cooler air above and wants to rise in response. Within these rising currents of hotter air, also called thermals, available water vapor at the surface now can hitch a ride, so to speak, higher up in the atmosphere.

At what altitude water vapor starts to condense into cloud material is dependent on the surface's dewpoint temperature. Moist environments have higher dewpoint temperatures, which means water vapor can condense into cloud droplets at lower altitudes after ascending thermals are forced to lose heat due to air parcel pressure-temperature relationships (i.e., higher altitudes = lower pressure = greater air parcel expansion = air parcel cooling). 

Thermals may still initiate clouds in drier environments; however, lower dewpoints mean thermals have to reach ever higher altitudes to achieve a temperature cool enough to trigger condensation of the limited amount of water vapor content it possesses. Sometimes there just isn't enough surface heating to translate to strong thermals rising high enough before losing their positive buoyancy in an unsaturated state. This is a key reason why mountains have an edge versus surrounding valleys for witnessing cumulus buildup to start the day (this is clearly evident in the timelapse videos). In other words, rising surface thermals from heated higher terrain tend to have a greater initial buoyancy contrast compared to the surrounding environment, which better promotes lifting of these mountain air parcels more quickly to cooler altitudes above.

Assuming thermals start to trigger vertical cumulus cloud formation, there is another inherent source of lift aiding rising motion. The physical phase change of water vapor condensing to a liquid releases additional heat into the environment. This latent heat release aspect is vital to sustain deep moist updrafts feeding developing thunderstorms. Thermals may lift moisture off the ground, but the positive feedback of latent heat release once clouds deepen serve as the "rocket boosters" for updrafts. Water vapor's latent heat storage is an essential driver for our weather once tapped! As you might expect then, limited moisture not only has less potential latent heat to contribute to rising motion, but also is harder to unleash due to lower dewpoints in general. Drier air near the surface is simply detrimental to thunderstorm chances in this manner. 

Additionally, dry air entrainment aloft is a significant limiting factor for cloud development. Specifically, air near edges of cloudy parcels of air start to cool and sink because existing cloud material gets converted back to a water vapor from persistent dry air mixing and subsequent evaporative cooling. Evaporative cooling and removal of heat energy from the air parcel occurs during the conversion of liquid back to water vapor. It is then necessary to have a constant moisture feed carried upwards by established thermals to counter dry entrainment effects experienced aloft. Otherwise, the air parcel no longer maintains positive buoyancy and stalls. At this point the hopes of generating a deeper cumulus cloud is lost in the absence of another source of lift. You can witness this effect when growth of cumulus clouds in the sky appear to have hit a barrier and start to evaporate away. 

Wait, in the timelapse videos it wasn't a one and done attempt for cumulus cloud development that day. Indeed, vertical clouds came and went , but ultimately reappeared. There is more to the story!

Tying it all together, now we can talk about the importance of cumulus cloud turret succession leading to an active monsoon day. As you can see in the timelapse videos, cumulus clouds in the morning hours are short-lived and smaller in stature, but during peak heating of the afternoon they get taller, wider, more widespread, and survive longer in the environment. The key here is that early day cumulus updrafts, although initially weak, are repeatedly injecting more water vapor content higher and higher into the atmosphere. This in turn diminishes the amount of dry air entrainment and evaporative cooling affecting subsequent generations of building cumulus clouds should the ground continue to be strongly heated. Eventually, ongoing thermals lifting off the surface finally reach a tipping point to sustain deep, warm, and moist updrafts making robust thunderstorms possible!

Without water vapor condensing to leave visible cloud material as tracers, the chaotic convective atmospheric symphony discussed during this monsoon day would largely go unnoticed at ground level. The exception, of course, would be turbulence experienced by passing aircraft!

-JWM

Photo: Jonny William Malloy

Timelapse Video: University of Arizona (UofA)