Seasonal characteristics of gravity waves over Kühlungsborn

As described in the objective of the project the magnitude of the temperature fluctuations caused by waves and their vertical wavelengths are determined by from the lidar temperature measurements between 1 and 105 km. Some results of these investigations are presented in the following.

1. Seasonal variation of temperature fluctuations

The temperature fluctuations restricted to 3-5 h duration are shown in Figure 1 depending on the season. The fluctuations are calculated as the mean of the absolute values of the deviations. For a continuous distribution throughout the year the data of all 236 measurements are smoothed over ±15 days und ±2 km. Below 35 km and above 95 km no seasonal variation is observed. In the remaining altitude range the large difference between winter and summer values are remarkable. In winter the temperature fluctuations are more than twice as large as they are in summer. In contrast the values in spring and autumn are similar and the values lie between the temperature fluctuations of winter and summer. 

2. Winter-summer-differences

2a Temperature fluctuations

The large winter-summer-differences described above are also visible in Figure 2. To characterise more precisely the amplitudes all measurements in winter (November, December, January) and summer (June, July), which are longer than 3 h, are used. The mean temperature fluctuations of every night are shown as black lines. The red lines are the seasonal means and their variances. Between 38 und 45 km the fluctuations have the same magnitude as the statistical uncertainty of the measurements. Therefore the values are linearly interpolated. As described above the seasonal mean of temperature fluctuations between 20 and 90 km are always larger in winter than in summer. Consequently, the winter-summer-ratio is larger than 1 and increase up to 2.5 in the mesosphere. Furthermore the profiles in summer show a smaller night-to-night variability than in winter.

Because of the large observed altitude range the measurements are suitable for studying the changes of wave amplitudes with height. In this way the influence of wave breaking and filtering are investigated by the comparison of the observed fluctuations (red line) with the theoretically expected growth of an undisturbed propagating wave (green line). In winter the energy dissipation is evenly distributed at all altitudes above 45 km. In contrast in summer the wave damping is very pronounced around 90 km. The reason is the dramatic change of the temperature gradient around the summer mesopause which is accompanied by a significant change of the static stability. Furthermore in two altitude ranges (20 – 40 km und 76 – 89 km) the wave propagation is nearly undisturbed in summer. Generally, the analysis shows the different wave filtering and damping in summer and winter.

Further investigations have shown that, apart from the different wave filtering and breaking, the temperature and density background also play an important role in creating the winter-summer-differences of the temperature fluctuations.

2b vertical wavelengths

In addition to the temperature fluctuations the vertical wavelengths also are an important characteristic of the observed waves. By means of wavelet spectra up to three dominating vertical wavelengths have been considered at each height. In Figure 3 the distribution of dominating vertical wavelengths in the altitude range 40-60 km is shown. The dominating vertical wavelengths do not show any large differences between winter and summer. In both seasons smaller vertical wavelengths (< 22 km) are more frequent compared to larger wavelengths. It is assumed that smaller wavelengths are associated with inertial gravity waves, whereas large wavelengths are from both inertial gravity waves and tides.    

temperature fluctuations

Figure 1: Seasonal variation of temperature fluctuations of all measurements restricted to 3-5 h duration. The data are smoothed over ±15 days und ±2 km.

Mean temperature fluctuations

Figure 2: Mean temperature fluctuations of all measurements a) in winter b) in summer. The red line and the bars indicate the mean and the standard deviations, respectively. The green line shows an undisturbed exponential growth of amplitudes with exp (z/2H), H =7 km.

vertical wavelengths

Figure 3: Distribution of the dominating vertical wavelengths for a) winter and b) summer measurements in the altitude range 40-60 km. The range without exact identification of the vertical wavelength is indicated by the hatched area.