On the life cycle of a shallow cumulus cloud: Is it a bubble or plume, active or forced?

Science

The shallow cumulus – the puffball of a cloud with a flat bottom and cauliflower top – is a common sight overhead in summertime. They are often welcomed for the patches of shade they provide, which is generated by the cloud reflecting sunlight back to space. By reflecting that sunlight to space, shallow cumuli keep Earth cooler than it would be in their absence. In fact, if shallow cumuli did not exist, the temperatures over land would be nearly 10 ^oF hotter than they are today.

A shallow cumulus is fed at its base by ascending air and the fate of that cloudy air determines how big the cloud becomes and how long it lasts, both of which are critical for determining its overall climate impact. The cloudy air will eventually convert to clear air by one of two mechanisms: either by drying out above the cloud base (by mixing with clear air, which evaporates its drops) or by descending back down through the cloud base (the increase in pressure compresses the air, heats it up, and dries out the drops). A cloud whose drops evaporate above the cloud base is called "active" and a cloud whose drops evaporate as the air sinks back down through the cloud base is called "forced". Clouds can also behave as either long-lived fountains of air (like a "plume") or as an isolated packet of air (like a "bubble"). In this paper, we give rigorous measures of these properties as a step towards quantifying shallow-cumulus life cycles and understanding why they generate 10 ^oF of cooling and not 5 ^oF or 20 ^oF.

Impact

Using stereo cameras and a Doppler lidar at the Department of Energy's Atmospheric Radiation Measurement (ARM) user facility Southern Great Plains (SGP) observatory in Oklahoma, we find that shallow cumuli are intermediate between active and forced, and are much more bubble-like than plume-like. In contrast, large-eddy simulations that are designed to model the SGP atmosphere generate clouds that are more active and plume-like than the observations, suggesting room for improvement.

Summary

The climate impact of shallow cumuli is determined by aspects of their life cycle, which connect cloud-base mass fluxes (their birth) to their eventual size and longevity. As a first step towards quantifying the life-cycle characteristics of shallow cumuli, two metrics are proposed to measure the degree to which they are forced (or active) and bubble-like (or plume-like). Stereo and lidar data reveal clouds to be intermediate between active and forced, and much more bubble-like than plume-like.