Turbopause determination, climatology, and climatic trends using medium frequency radars at 52°N and 70°N

Abstract

We explore methods for identifying the turbopause in its various guises using results from the medium frequency (MF) radars at Saskatoon (52°N) and Tromsø (70°N) and using data from 1999 to 2007 inclusive. The classical radar turbopause, identified as the altitude at which molecular viscosity inhibits turbulence, is determined and exhibits clear annual variation at both latitudes, occurring slightly lower down at 52°N as compared with 70°N. Second, we determine the altitude at which the zonal wind in the mesosphere undergoes a zero crossing as a possible height regime for gravity wave breaking. Third, we then investigate the altitude at which the mean square velocity fluctuation undergoes a transition from height independence to a steep increase with height. This altitude has recently been proposed as a "wave" turbopause, and our method and determination are in good agreement with those of other authors. This wave turbopause lies somewhat under the classical radar turbopause and exhibits annual and semiannual variations at 70°N and 52°N, respectively. The relationships between these different transition altitudes are discussed, and we conclude that the recently defined wave turbopause may represent a useful parameterization of mesospheric dynamics for future research. Examining the classical radar turbopause time series we identify a trend in the 70°N results but none in those from 52°N. The possible reasons for the high-latitude finding are then discussed in the context of determinations of other features of the middle atmosphere and lower thermosphere. We conclude that mesospheric cooling in the underlying mesosphere is a probable cause. Copyright 2008 by the American Geophysical Union.