Researchers compare two model scenarios to assess the role of a “moist shell” in storm development
Convection organizing in several thunderstorms over the Maritime Continent. Credit: NASA
Source: Geophysical Research Letters

Just like people are often divided into two opposing categories—early birds and night owls, introverts and extroverts—meteorologists designate two main types of storms. Storms that form through organized convection are long-lived, cover a large area, generate and accumulate more clusters of clouds with time, take place in environments with large-scale circulation, and are triggered by uplifting air from a passing front or low-pressure system. Meanwhile, storms that form through unorganized convection are triggered by a temperature anomaly or change near Earth’s surface, take place in environments without large-scale circulation, and are more chaotic and unpredictable.

Of the two types, organized convection is better understood and more ubiquitous, especially in the tropics. But meteorologists still have much to learn about convective organization to better understand and predict the behavior of storms.

Here Becker et al. investigated the impact of convective organization on entrainment—a process in which warm, buoyant parcels of air become saturated with moisture; form cumulus clouds; and mix with cooler, drier parcels of air. This causes some cloud droplets to evaporate, cooling down the clouds and making them less buoyant. At this point, either the clouds can dissipate into a popcorn-like formation—which is called unaggregated convection—or other cumulus clouds in the vicinity will pile onto them, forming a larger cloud cluster (aggregated convection).

Using a numerical model developed by the Max Planck Institute for Meteorology and the German Weather Service, the team created two simulations of convectional organization over a 312,000-square-kilometer grid with 1-kilometer spacing. One simulation started out with unaggregated convection that remained unaggregated throughout. The other simulated the same conditions but started out with aggregated convection and stayed fully aggregated throughout the simulation.

The team found that in the lower levels of the troposphere, where our weather occurs, the rate of entrainment is higher when convection is aggregated. This is due to increased turbulence caused by updrafts. Meanwhile, more buoyancy is maintained during aggregated convection because of a moist layer, or shell, surrounding the convective cluster.

The researchers advise that future modeling of convective organization should include simulations of this moist shell. This study sheds light on how convective systems interact with their environments and their behavior (whether by aggregating or dissipating) as they move farther skyward. (Geophysical Research Letters,, 2018)

—Sarah Witman, Freelance Writer


Witman, S. (2018), Modeling storm evolution, Eos, 99, Published on 06 March 2018.

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