Picture (above): Lightning over Sydney. Credit: Mariano Carpentier (Unsplash).

When organized groups of storms redevelop over or repeatedly pass over the same location for an extended period of time, they can lead to extreme rainfall and flash flooding. Existing explanations of a commonly seen, but poorly understood example of this rely on a region of air, cooled by the evaporation of raindrops and its interaction with the wind. Most existing studies have focused on how this evaporatively-cooled air behaves when it reaches the surface, as something called a cold pool.

However, it’s been suggested that in some cases, especially at night, the evaporatively-cooled air from the thunderstorm may not be as cool as (i.e. it’s less dense than) the air near the surface, which means no surface cold pool. The goal of CLEX researchers was to try to understand how storms could have both no cold pool, and this particular organisation.

They used 3-dimensional computer models to try to understand both where the air that enters these storms comes from and what happens to the rain-cooled air that exits them. They also used a simplified 2-dimensional model to try to break down how the rain-cooled air interacts with the environment around it in the most fundamental ways.

The researchers discovered that in some cases, the rain-cooled air might actually move through a layer above the ground, as something called an intrusion. Think of it as like oil flowing over water–the oil is less dense than the water but denser than the air (It’s not quite the same, but it is a helpful visual).

The researchers also showed how, like a surface cold pool, wind can change the behaviour of the intrusion, and its ability to support storms. The researchers hope this helps broaden the way in which we think about organized nighttime thunderstorms and how rain-cooled air interacts with the environment near the surface.

  • Paper: Hitchcock, S. M., and R. S. Schumacher, Analysis of Back-Building Convection in Simulations with a Strong Low-Level Stable Layer. Mon. Wea. Rev., doi: https://doi.org/10.1175/MWR-D-19-0246.1.