4:15 PM - 4:30 PM
[PEM13-15] Atmospheric instabilities in the polar upper mesosphere
Keywords:Static stability, Dynamic stability, Polar upper mesosphere, sodium LIDAR, Tromsoe
We have studied instabilities of the atmosphere in the polar upper mesosphere (80-100 km) above Tromsoe (69.6 deg N, 19.2 deg E) using temperature and wind data obtained by the sodium LIDAR over 8 winter seasons from January 2012 to March 2019 (about 3100 hours). We have evaluated both the static (convective) and dynamic (shear) instabilities using the square of Brunt–Vaaisaalaa frequency (N2) and Richardson number (Ri), respectively. Furthermore, we have calculated probabilities of the instabilities defined as the percentage of an occurrence rate of unstable regions over the time interval. The probabilities of the static and dynamic instabilities are shown as P(N2 < 0) and P(0 < Ri < 0.25), respectively.
When we use data with 6 min temporal and 1 km height resolutions, we have found that probability of static stability varies from a maximum value of 21% to a minimum value of 1% with an average of 7%, while the dynamic stability varies from a maximum value of 20% to a minimum value of 2% with an average of 9%. Thus, we can say that the atmosphere is unstable for about 16% of time/height region. These instability probabilities do not show any prominent monthly variations from October to March, rather day-to-day variabilities are prominent. The averaged altitude profile of the probability of the static instability shows a minimum around 90 km between 80 and 100 km. One of causes would be due to dissipation of gravity waves. We have investigated an averaged altitude profile of the potential energy of gravity waves: it decreases between 81 and 85 km, is almost constant between 85 and 90 km, and increases above 90 km with height increasing.
To reveal possible causes of the instabilities, we have investigated relationship with geomagnetic disturbance (i.e. auroral heating), semidiurnal tidal amplitude, and solar activity (F10.7 index). We will discuss the causes that make the atmosphere in the polar upper mesosphere unstable.
When we use data with 6 min temporal and 1 km height resolutions, we have found that probability of static stability varies from a maximum value of 21% to a minimum value of 1% with an average of 7%, while the dynamic stability varies from a maximum value of 20% to a minimum value of 2% with an average of 9%. Thus, we can say that the atmosphere is unstable for about 16% of time/height region. These instability probabilities do not show any prominent monthly variations from October to March, rather day-to-day variabilities are prominent. The averaged altitude profile of the probability of the static instability shows a minimum around 90 km between 80 and 100 km. One of causes would be due to dissipation of gravity waves. We have investigated an averaged altitude profile of the potential energy of gravity waves: it decreases between 81 and 85 km, is almost constant between 85 and 90 km, and increases above 90 km with height increasing.
To reveal possible causes of the instabilities, we have investigated relationship with geomagnetic disturbance (i.e. auroral heating), semidiurnal tidal amplitude, and solar activity (F10.7 index). We will discuss the causes that make the atmosphere in the polar upper mesosphere unstable.