On the temporal evolution of turbopause altitude, 1996–2021, 70°N, 19°E

Abstract

Medium frequency radars with multiple receivers are able to track the movement of the interference pattern on the ground from echoes from irregularities in refractive index. In particular, refractive index in the mesosphere is determined by electron density – commonly known as the ionospheric D-region. Thus using this technique it is possible to determine winds in the height regime 70-90 km, depending on the degree of ionization throughout the year. In addition, by examining the fading times of the passage of these structures, it is possible to deduce metrics pertaining to neutral air turbulence. Here, we employ a well-established method to this effect. Thereafter, comparing the turbulent intensity to the kinematic viscosity of the neutral atmosphere, we determine the turbopause altitude. Above this height, atmospheric constituents behave independently, whereas below, all components are mixed. Contrary to earlier analyses, we present evidence the turbopause altitude has been constant since approximately 2004.

1. Introduction

The turbopause is a demarcation between regions in the atmosphere characterised by different mixing processes. Below, the homosphere, turbulence dominates, whereas above, the individual constituents are distributed according to their respective scale heights. Since ionisation creates different species (such as atomic oxygen) at different heights, an elevated turbopause will result in mixing of these components downwards affecting the mesospheric chemistry such as hydroxyl. In turn, species such as water vapour and hydroxyl can be mixed to similarly elevated heights affecting the ion chemistry. For an introduction to the basics of turbulence applicable to this study, the reader is referred to Batchelor (1953). The method for extracting turbulence parameters from ground-based radar is described by Briggs (1980), and to facilitate this we employ data from the 2.78 MHz medium frequency radar (hereafter MFR) at Ramfjordmoen in northern Norway at 70°N, 19°E. The system has been described fully by Hall (2001). Hitherto, the most recent study of the available time-series is described by Hall et al. (2016); this furthermore contains an exhaustive reference list and is the de facto reference for the method and description of underlying physics for this study; Weinstock (1978) is an underlying study and deserves specific mention.

2. Method and results

The 2.78 MHz medium frequency radar MFR) at Ramfjordmoen (70°N, 19°E, Hall, 2001) transmits through a 4x4 crossed dipole array, and therefore illuminates structures in refractive index, dominated above the radar by corresponding structures in electron density (i.e. the ionospheric D-region). The geographic location is beneath the auroral oval and therefore a degree of ionisation extending to as low as 70 km altitude is not uncommon. Reflections from these structures give rise to an interference pattern on the ground which is subsequently recorded by simple equi-spaced inverted-V antennas connected to the receivers. Cross-correlations between signals from these receivers provide information on horizontal winds that cause the reflecting structures to drift through the radar beam. Autocorrelations indicate backscattered power. The backscattered power, however, fades with time as the scatterers change shape and/or become less distinct as they drift through the radar beam. Several mechanisms might cause this fading, but a common prime one is considered to be neutral turbulence (e.g. Briggs, 1980). Thus, by determining the characteristic fading times of the autocorrelations, it is possible to estimate turbulent intensity - or in fact an upper limit for it (since other mechanisms may be present) – which is parametrised by the energy dissipation rate, ε´ (the prime being used to differentiate between this determination and the actual dissipation rate, ε). At the same time, kinematic viscosity increases with altitude due to decreasing density, and at a certain height, energy dissipation due to viscosity, ε_min, equals that due to neutral turbulence; this can be thought of when inertial forces equal viscous forces, i.e. when the Reynolds number R_e is unity. This condition, the intersection of the ε_min, and ε, profiles defines the turbopause. The reader is referred to Fukao et al. (1994), Hall et al. (2016), Weinstock (1984) and references therein for an in-depth description of turbulent energy dissipation rate estimation and thereafter turbopause identification.

The results are shown in Figure 1. Turbulent intensity is indicated below the turbopause in the vicinity of 105 km. The distinct seasonal variation is due to gravity wave saturation and breaking being more evenly distributed though the mesosphere in winter, whereas breaking is more concentrated immediately above the mesopause in summer when temperature gradients are steep. Again the atmospheric physics is fully described in references to be found in Hall et al. (2016). Figure 2. Illustrates the turbopause altitude alone, here with a 30-day smoothing applied to enhance clarity. Earlier analyses of this dataset had identified possible trends; two possible candidates, identified by simple inspection are highlighted (1996–2001 and 2001–2005), but if we consider the entire series from 1996 to 2021, there is no convincing trend to be seen, although the determined value of 0.68 km decade⁻¹ does indeed exceed the 2-σ uncertainty. For the individual segments and the entire series, the 95% confidence limits (Working & Hotelling, 1929) are indicated by dotted hyperbolae.

Figure 1. Turbulent intensity from Northern Norway (70°N, 19°E) (upper limits – see main text). Thick line shows turbopause.

Figure. 2. Turbopause estimate over Northern Norway (70°N, 19°E). Purely from visual evaluation, two periods of change are identified: 1996–2001 and 2001 to 2005, and trendlines are fitted to these and to the entire period. Trends are indicated in the annotation on the plot and dotted hyperbolae indicate the 95% confidence limits.

3. Conclusion

While earlier examinations of data from the Ramfjordmoen MFR suggested trends which could be attributed to climate change in the underlying neutral atmosphere, this latest review of the entire timeseries from 1996 to 2021 (time of writing) suggests stability of the turbopause altitude with little significant variation, viz. 1.7 km over the last quarter of a century. In some ways, the result is surprising: kinematic viscosity and therefore the minimum energy dissipation sustained by the atmosphere, ε_min, is proportional to temperature^2/3 (e.g. https://en.wikipedia.org/wiki/Viscosity and Fukao et al., 1994); ε´ is proportional to tempertature^-1/2 (Weinstock, 1978). Combined, these dependencies imply that increasing temperatures will tend to depress the turbopause and vice versa. However, the turbopause is located in a region of transition between summer and winter mesopauses, and therefore steep negative and positive temperature gradients. The recent stability of the turbopause is therefore simply reported here and relationships to upper atmosphere temperature trends left to deeper investigation.