D. Keuer

Variability of the mesospheric wind field at middle and Arctic latitudes in winter and its relation to stratospheric circulation disturbances

Abstract Continuous wind observations allow detailed investigations of the upper mesosphere circulation in winter and its coupling with the lower atmosphere. During winter the mesospheric/lower thermospheric wind field is characterized by a strong variability. Causes of this behaviour are planetary wave activity and related stratospheric warming events. Reversals of the dominating eastward directed mean zonal winds in winter to summerly westward directed winds are often observed in connection with stratospheric warmings. In particular, the amplitude and duration of these wind reversals are closely related to disturbances of the dynamical regime of the upper stratosphere. The occurrence of long-period wind oscillations and wind reversals in the mesosphere and lower thermosphere in relation to planetary wave activity and circulation disturbances in the stratosphere has been studied for 12 winters covering the years 1989–2000 on the basis of MF radar wind observations at Juliusruh (55°N, since 1989) and Andenes (69°N, since 1998). Mesospheric wind oscillations with long-periods between 10 and 18 days are observed during the presence of enhanced planetary wave activity in the stratosphere and are combined with a reversal of the meridional temperature gradient of the stratosphere or with upper stratospheric warmings.

Scattering properties of PMSE irregularities and refinement of velocity estimates

Abstract Observations of polar mesosphere summer echoes (PMSE) were performed with the ALOMAR SOUSY VHF radar in the Doppler beam swinging (DBS) and the spaced antenna (SA) mode. This radar is located in polar regions at 69°N, 16°E in Andenes, Norway. After a summary of the basic features of PMSE and the system lay-out, we will come to the analysis of PMSE irregularity structures. Differences of the signal power observed with the DBS and the smaller SA antenna configuration point to a dominance of a scattering process of PMSE irregularities rather than to specular reflection. The SA method yields a more pronounced aspect sensitivity than the DBS method which points towards fairly anisotropic PMSE irregularities in the smaller volume sensed by the SA set-up. These anisotropic irregularity layers are frequently inclined towards the horizontal by a few degrees. There are wave and instability events which change the layer structure, their inclination and also the velocity components. When the layer inclination is not horizontal, a leakage of the horizontal velocity into the vertical velocity is manifest. It is shown that this can be corrected by using the measured zenith and azimuth angles of signal arrival and by calculating the leakage of the horizontal velocity components. This procedure yields improved estimates of the velocity components in vertical direction. Finally, some thoughts how to explain such observations of highly anisotropic and inhomogeneous PMSE irregularity structures are summarized.

Seasonal variation of middle latitudes wind fields of the mesopause region – A comparison between observation and model calculation

Mean MF-radar prevailing mesospheric wind data over Juliusruh (54.6°N, 13.4°E) for 8 years (1990–1997) have been compared with daytime averaged winds obtained from calculations using the Cologne global three-dimensional dynamic model of the middle atmosphere COMMA. The altitude-seasonal cross section shows that the model reflects the seasonal zonal wind pattern fairly well. The mesospheric summer and especially the winter winds are somewhat weaker in the observations. The model results are better reflected by measurements over Saskatoon (52°N, 107°W). The height of the observed maximum of the summer jet occurs more than 5 km above that one inferred from the model calculations. The long-term averages of the zonal winds will not be significantly changed if the periods with stratospheric warming events, which can lead to short term wind reversals at mesospheric heights during the winter months, are eliminated from the observational basis. The daytime averaged meridional wind component matches the observations over Juliusruh partly and agrees also with the published results obtained from measurements over Saskatoon.

Representative height-time cross-sections of the upper atmosphere wind field over Central Europe 1990–1996

Abstract A joint analysis of LF and MF wind profiler data of Central Europe for the height range from 75 to 105 km for 7 years, obtained from daily measurements and overlapping in height and time, makes it possible to represent reliable height-time cross-sections for some wind field parameters. Interesting details are the great summer zonal prevailing wind gradient which reaches, on average, 3 m s−1 km−1 and a relatively height-invariable vertical wave length (about 50 km) of the semidiurnal tidal wind component in winter, seen clearly in the vertical phase gradient.