Atmospheric conditions during convective storms over the tropical rainforest

Convective storms often occur the tropics and have the potential to disturb the lower part of the atmosphere. They might even improve the venting of trace gases out of the forest canopy into the atmosphere above. To better understand these processes, Maurício Oliveira and co-authors used the infrastructure at ATTO to study storm outflows during nighttime. They published the results in a new paper in the Open Access Journal Atmospheric Chemistry and Physics.

Why does it rain so frequently in the tropics? The reason is a mixture of many factors, but most importantly it’s very warm and very humid most of the time. Over the course of the day, it gets hotter and warm air rises. Further up in the atmosphere it slowly cools down. Thus water vapor condenses into cloud droplets and subsequently ice particles and convective storms form. A convective storm is per definition one that forms because of latent heat, i.e. the process just described. These storms often produce rain and strong winds. Once the winds hit the ground at high speeds, they spread out into all directions and may continue to flow over considerable distances. That’s why we sometimes experience strong winds (called outflow) before thunderstorms reach us.

But convective storms are associated with another important process that we can’t experience ourselves. These strong storms disturb the lower part of the atmosphere. This may affect the exchange of chemicals between the forest and the atmosphere. For example, the outflow winds associated with convective storms might improve the venting of trace gases out of the forest canopy into the atmosphere above. This is why our team wants to learn more details about convective storms in tropical forests.

convective storm over the forest
© Steffen Schmidt / MPI-BGC
Schematic illustrating the effects of a gust front passing over ATTO during nighttime. Figure from Oliveira et al. (2020)

Maurício Oliveira and co-authors used the infrastructure at ATTO to study storm outflows during nighttime, when stable atmospheric layers are usually established. They measured at several heights on our 80-meter tower during several separate storm events. They found that the storm events all had well-defined gust fronts. When they passed over the ATTO site, temperatures quickly dropped while wind speeds picked up. In addition, the sensible heat flux reversed. Before the storm arrived, warmer air was heating the colder ground below (=negative heat flux) as is typical at night. When the gust front brought cooler air to the site, the ground became warmer in comparison and was now transmitting heat back into the air (=positive heat flux) in a short, abrupt burst. Such a positive flux is typical for daytime, when solar radiation warms up the ground. However, it does not usually occur at night, especially in such a transient fashion. Our team observed this effect both above and below the canopy, although it was more pronounced above the treetops. Finally, they noticed that the air from the outflow was very turbulent. It was also drier, despite some rain occurring in the center of the storm. This led to an increase in the latent heat flux, meaning an increase in heat loss due to evaporation and transpiration.

Such land-atmosphere exchanges can have a significant impact on weather-simulations and predictions. And until we know more about them, they might be misrepresented in climate models. With this study, our team made a step toward bettering understanding the atmospheric dynamics within the storms and their interaction with the forest. Most importantly they learned that convective storms can create significant and abrupt changes in atmospheric conditions. To detect them was only possible with the high-frequency, multi-level measurements.