NONLINEAR-INTERACTION OF THE QUASI 2-DAY WAVE AND LONG-TERM OSCILLATIONS IN THE SUMMER MIDLATITUDE MESOPAUSE REGION AS SEEN FROM LF D1 WINDMEASUREMENTS OVER CENTRAL-EUROPE (COLLM, 52-DEGREES-N, 15-DEGREES-E)

From night-time mesopause region wind measurements over Central Europe , the day-to-day variability of the measured quasi 2-day wave is inves tigated with respect to its correlation with summer planetary wave eve nts. A 14-year dataset of summer winds is used. The results indicate t hat the 2-day wave is modulated by planetary waves, especially by the 16-day wave. Secondary frequencies that are expected, if a non-linear interaction between the waves is assumed, are found from regression an alysis. Additionally, these secondary oscillations are modulated by th e duration of the quasi 2-day wave event itself. Tertiary frequencies that belong to; this modulation are also found from periodogram analys is. The long-term dataset analysis shows that the non-linear interacti on is a regular phenomenon that is frequently found in connection with planetary wave activity in the summer upper mesosphere and lower ther mosphere. (C) 1998 Elsevier Science Ltd. All rights reserved.