A contribution to the occurrence of undular bores and solitary waves in the lower atmosphere, especially over North Germany

Abstract

In a stably stratified atmosphere the propagation of small amplitude internal gravity waves is rather common but often not noticed. The appearance of a group of large amplitude, nonlinear gravity waves in the lower atmosphere is, in comparison, a special event. These waves with periods of several minutes, pressure amplitudes partly more than 1 hPa may cause periodic wind changes up to 10 m/s (speed) and more than 100 degrees (direction). They appear as a packet of a few amplitude-ordered single waves: as an internal undular bore or as an internal solitary wave group. Their occurrence in North Germany was investigated. The results of former gravity wave investigations were used for the area. For other parts use was made of published data. A detailed analysis is made with the Hamburg Morning Wave of 7 May 1986. In a period of 7 years 6 clear events passed over Hamburg. Most of them happened in spring, and together with a nocturnal temperature inversion in the lower atmosphere. Coherent small amplitude gravity wave activity occurred in about 10 % of the time. Examples of recordings at an 250 m instrumented (wind, temperature) antenna mast up to 250 m are presented. Evident are the often strong changes of the wind direction during these disturbances. The horizontal wind has been decomposed into a mean back-ground wind, an additional mean wind (adm) during the passage of the group and a quasi sinusoidal periodic part. The adm points into the direction of the wave propagation, indicating a mean mass flux together with the waves.