Realistic modelling of climate change requires an appropriate description of energy fluxes in the atmosphere. Some of the energy is transported via so-called gravity waves, which progress through the atmosphere similar to the movement of water waves. They can break, thereby releasing energy. An important parameter for determining the energy transported by gravity waves is the so-called Brunt-Väisälä frequency. This value is now available as part of a climatology for every day of the year in the Alpine region.
The atmosphere is a strongly coupled system, so processes occurring at various altitude levels cannot be studied in isolation. According to current understanding, atmospheric gravity waves deposit their energy primarily at high altitudes, between around 80 and 100 kilometres. They usually collect it from lower atmospheric levels. For example, gravity waves can arise when air flows over mountain ranges. The air diverted upward flows down again after passing the mountain obstacle and begins to oscillate around its equilibrium position. These oscillations continue to spread out horizontally as well as vertically: a wave is generated. The impulse from this wave is transmitted to higher layers of the atmosphere, where it can be measured.
At present, the assumption is that the energy transferred by gravity waves upholds a global north-south atmospheric circulation. This exchange process is mirrored in an annual temperature curve at about 80 to 100 km altitude, where there are high temperatures in winter and low ones in summer. EOC uses GRIPS infrared spectrometers to measure the temperature of the atmosphere in the realm of the upper mesosphere/mesopause. The measurements are part of the international “Network for the Detection of Mesospheric Change” (NDMC) and can be obtained from EOC’s World Data Center for Remote Sensing of the Atmosphere (WDC-RSAT). They are updated daily.
Also temperature fluctuations at this altitude are partially caused by gravity waves. Continuous measurement help to better understanding them. In order to estimate the potential energy transported by gravity waves it is necessary to extract from the measurements the temperature fluctuations they cause. It is also essential to know a basic geophysical value, the so-called Brunt-Väisälä frequency, which is the highest frequency at which a gravity wave can oscillate. The reciprocal of the time interval between two wave peaks measured at a given location has to have a value smaller or equal to the Brunt-Väisälä frequency. Climate data for deriving the Brunt-Väisälä frequency have now been published for the Alpine region using measurements from the USA’s SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on board the TIMED satellite.
With their own measurements and the Brunt-Väisälä frequency derived from TIMED-SABER data, EOC scientists can now estimate the energy stored in gravity waves. This makes it possible to state how much energy is liberated by these waves when it is released, thereby heating up the atmosphere. With the help of NDMC measurements for the Alpine region it could be shown that the energy content of the particularly energetic gravity waves clearly follows an annual cycle at the observed altitude. This is an important clue for deciphering atmospheric energy exchange processes.
Information about the potential energy transported by gravity waves can now be routinely derived for the Alpine area. This possibility is to be extended by including additional measurements made in the Antarctic, Israel, Georgia, Sicily, and later also in the Czech Republic and Chile. The required measuring equipment is being operated in cooperation with the Alfred Wegener Institute at its Neumayer III station, the University of Tel Aviv, Tiflis University, Italian disaster control authorities, the Czech Academy of Sciences, and the European Southern Observatory. Little by little other stations in the global NDMC network will be involved.
The Brunt-Väisälä frequency (N) is lower in winter than in summer. This reflects the annual temperature progression at 80-90 km altitude. The Brunt-Väisälä frequency limits the period of a gravity wave: there can be no gravity waves that require less time for a complete oscillation (e.g., the time between two maxima) than specified by the Brunt-Väisälä frequency. © Atmospheric Measurement Techniques Journal