Abstract
Abstract. The data of the regular low-frequency D1 E-region observations at Collm, Germany (52°N, 15°E) in 1983–1999 are used for estimations of the intensity of short-period perturbations of the horizontal drift velocity at 85–110 km altitude. A simple half-hourly-difference numerical filter is used to extract perturbations with time scales of 0.7–3 h. The average monthly standard deviations of short-period perturbations of the zonal velocity near altitude 83 km have a main maximum in summer, a smaller maximum in winter, and minimum values at the equinoxes. At higher altitudes the summer maximum is shifted towards the spring months, and a second maximum of perturbation amplitudes appears in autumn at altitudes near and above 100 km. The seasonal changes in the standard deviations of meridional velocity show the maxima in spring and summer. A numerical model describing the propagation of a set of harmonics modeling a spectrum of internal gravity waves in the atmosphere is used for the interpretation of observed seasonal variations of wind perturbation intensity. Numerical modeling reveals that the observed altitude changes in the seasonal variations of the drift velocity standard deviations may be explained by a superposition of IGWs generated at different levels in the troposphere and middle atmosphere. IGWs generated in the stratospheric and mesospheric jet stream may have substantial amplitudes at altitudes near and above 100 km, where they may modify the seasonal variations, which are typical for IGWs propagating from the troposphere. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides) – Ionosphere (ionospheric irregularities)
Highlights
Internal gravity waves (IGWs) are supposed to be important for the formation of general circulation, thermal regime and composition of the middle and upper atmosphere
The equations are solved for a set of 50 × 50 × 12 IGW harmonics, where multipliers denote the numbers of wave harmonics with different frequencies, horizontal phase speed and azimuths, respectively
In the previous section we showed that our model gives the possibility to calculate seasonal variations of such important characteristics as turbulent viscosity, heating rates and accelerations of the mean wind produced by breaking IGWs
Summary
Internal gravity waves (IGWs) are supposed to be important for the formation of general circulation, thermal regime and composition of the middle and upper atmosphere. This determines an increased interest in the study of IGW climatology. One of the experimental methods, which can be used for IGW study at the altitudes 85–120 km is the D1 method observing drifts of ionospheric irregularities using low-frequency (LF) radio waves propagating from ground-based transmitters and reflecting from the lower ionosphere. The LF D1 method has been used for measuring ionosphere drift velocities at altitudes between 80 and 120 km at the Collm Observatory of the University of Leipzig, Germany (52◦ N, 15◦ E) since the International Geophysical Year, 1959 (Schminder and Kurschner, 1994). Calculated in the manner used here, the mean velocities better reflect the true background conditions corresponding to the drift velocity measurements
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