Abstract
Abstract. The turbopause is the demarcation between atmospheric mixing by turbulence (below) and molecular diffusion (above). When studying concentrations of trace species in the atmosphere, and particularly long-term change, it may be important to understand processes present, together with their temporal evolution that may be responsible for redistribution of atmospheric constituents. The general region of transition between turbulent and molecular mixing coincides with the base of the ionosphere, the lower region in which molecular oxygen is dissociated, and, at high latitude in summer, the coldest part of the whole atmosphere. This study updates previous reports of turbopause altitude, extending the time series by half a decade, and thus shedding new light on the nature of change over solar-cycle timescales. Assuming there is no trend in temperature, at 70° N there is evidence for a summer trend of ∼ 1.6 km decade−1, but for winter and at 52° N there is no significant evidence for change at all. If the temperature at 90 km is estimated using meteor trail data, it is possible to estimate a cooling rate, which, if applied to the turbopause altitude estimation, fails to alter the trend significantly irrespective of season. The observed increase in turbopause height supports a hypothesis of corresponding negative trends in atomic oxygen density, [O]. This supports independent studies of atomic oxygen density, [O], using mid-latitude time series dating from 1975, which show negative trends since 2002.
Highlights
The upper mesosphere and lower thermosphere (UMLT) regime of the atmosphere exhibits a number of features, the underlying physics of which are interlinked and, relative to processes at other altitudes, little understood
An effort has been made to demonstrate that conceivable temperature trends are unable to alter the overall results, viz. that there is evidence of increasing turbopause altitude at 70◦ N, 19◦ E in summer, but otherwise no significant change during the period 2001 to 2014
Updated temporal evolutions of the turbopause altitude have been presented for two locations: 70◦ N, 19◦ E (Tromsø) and 52◦ N, 107◦ W (Saskatoon), the time interval spanning 1999 to 2015
Summary
The upper mesosphere and lower thermosphere (UMLT) regime of the atmosphere exhibits a number of features, the underlying physics of which are interlinked and, relative to processes at other altitudes, little understood. The UMLT is, inter alia, characterised by the base of the ionosphere, dissociation of molecular species (for example oxygen) by sunlight, and, the focus in this study, the transition from turbulent mixing to distribution of constituents by molecular diffusion. The altitude at which transition turbulence-dominated mixing gives way to molecular diffusion is known as the turbopause, and typically occurs around 100 km, but displaying a seasonal variation, being lower in summer Prevailing winds filter or even inhibit propagation of gravity waves generated in the lower atmosphere, and the static stability (or lack of it) of the atmosphere dictates the vertical distribution of gravity wave saturation and breaking. The generation of turbulence and its height distribution vary with season and affect the turbopause altitude
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