Research by Pieter Smets of TU Delft and the KNMI shows that infrasound can be used for weather and climate forecasts. These inaudible low sound waves can be used to gain a better picture of the stratosphere, which can barely be measured in any other way. On Wednesday 28 March, Smets will be awarded his Ph.D. at TU Delft for his work on this subject.

Inaudible

Infrasound is comprised of inaudible, low-frequency sound waves (under 20 Hz). The waves are able to travel efficiently over long distances through the atmosphere because they undergo little dissipation. The measuring method is passive: you set up a microphone somewhere on Earth and all you have to do is listen.麓

The microphones used to measure the sound are part of the worldwide International Monitoring System (IMS) that was implemented for the verification of the Comprehensive Nuclear Test-Ban Treaty (CTBT). Although the primary purpose of the IMS is to detect any nuclear testing, it can also be used to gather long series of climatological data through deeper ocean monitoring, or for monitoring rumbling volcanoes.

Weather predictions

"A new application for infrasound is gradually emerging: weather forecasts," says doctoral candidate Pieter Smets. "Measuring infrasound as part of the ARISE project (Atmospheric Dynamics Research Infrastructure in Europe), provides opportunities for examining the higher atmosphere 鈥� the stratosphere 鈥� the layer of air at an altitude of around 10 to 50 kilometres. A vital advantage of infrasound is its sensitivity to both wind and temperature in that part of the atmosphere where observations are limited."

Sudden stratospheric warming

This is of particular importance during a particular weather phenomenon, the sudden stratospheric warming (SSW), explains Smets. In mid-February such an event was responsible for the coldest week of this winter in the Netherlands. "Such a sudden warming of the stratosphere is an important characteristic of the winter atmosphere in the northern hemisphere. During this short-lived phenomenon the stratosphere exerts a strong influence on the layer beneath it, the troposphere. This has consequences for the weather and for weather forecasts. In recent years attempts have been made to improve predicting the stratospheric variability using numerical weather forecasts. However, this requires extra independent observations of the upper atmosphere, and this is an area that is extremely difficult to observe. Wind observations, for example, are lacking in weather models past the middle of the stratosphere, higher than around 30 km."

"The capacities of infrasound have already been known for many years. Yet there is still no prospect of them being used in the short term in weather and climate models. My dissertation will contribute to the development of new methods for promoting the use of infrasound in current weather and climate models, as a first step. In any case, my research shows that infrasound can give new insights into the atmosphere in those parts where measurements are limited, particularly during an SSW," says Smets.