Thermal tides in the middle atmosphere at mid-latitudes measured with a ground-based microwave radiometer

In recent decades, theoretical studies and numerical models of thermal tides have gained attention. It has been recognized that tides have a significant influence on the dynamics of the middle and upper atmosphere; as they grow in amplitude and propagate upward, they transport energy and momentum from the lower to the upper atmosphere, contributing to the vertical coupling between atmospheric layers. The superposition of tides with other atmospheric waves leads to non-linear wave-wave interactions. However, direct measurements of thermal tides in the middle atmosphere are challenging and are often limited to satellite measurements in the tropics and at low latitudes. Due to orbit geometry, such observations provide only a reduced insight into the short-term variability in atmospheric tides. In this paper, we present tidal analysis from 5 years of continuous observations of middle-atmospheric temperatures. The measurements were performed with the ground-based temperature radiometer TEMPERA (TEMPErature RAdiometer), which was developed at the University of Bern in 2013 and was located in Bern (46.95 degrees N, 7.45 degrees E) and Payerne (46.82 degrees N, 6.94 degrees E). TEMPERA achieves a temporal resolution of 1-3 h and covers the altitude range between 25-50 km. Using an adaptive spectral filter with a vertical regularization (ASF2D) for the tidal analysis, we found maximum amplitudes for the diurnal tide of approximately 2.4 K, accompanied by seasonal variability. The maximum amplitude was reached on average at an altitude of 43 km, which also reflected some seasonal characteristics. We demonstrate that TEMPERA is suitable for providing continuous temperature soundings in the stratosphere and lower mesosphere with a sufficient cadence to infer tidal amplitudes and phases for the dominating tidal modes. Furthermore, our measurements exhibit a dominating diurnal tide and smaller amplitudes for the semidiurnal and terdiurnal tides in the stratosphere.