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The Turbopause Experiment: Atmospheric Stability and Turbulent Structure Spanning the Turbopause Altitude

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The Turbopause Experiment: Atmospheric Stability and Turbulent Structure Spanning the Turbopause Altitude Ann. Geophys., 29, 2327–2339, 2011 www.ann-geophys.net/29/2327/2011/ Annales doi:10.5194/angeo-29-2327-2011 Geophysicae © Author(s) 2011. CC Attribution 3.0 License. The Turbopause experiment: atmospheric stability and turbulent structure spanning the turbopause altitude G. A. Lehmacher1, T. D. Scott1, M. F. Larsen1, S. G. Bilen´ 2, C. L. Croskey2, J. D. Mitchell2, M. Rapp3, F.-J. Lubken¨ 3, and R. L. Collins4 1Department of Physics & Astronomy, Clemson University, Clemson, SC, USA 2Department of Electrical Engineering, The Pennsylvania State University, University Park, PA, USA 3Leibniz-Institute for Atmospheric Physics, Kuhlungsborn,¨ Germany 4Geophysical Institute, University of Alaska, Fairbanks, AK, USA Received: 18 May 2011 – Revised: 3 November 2011 – Accepted: 15 December 2011 – Published: 23 December 2011 Abstract. Very few sequences of high resolution wind and 103 km, trail diffusion was mainly laminar; however, unusual temperature measurements in the lower thermosphere are features and vortices in the trail diffusion were observed up available in the literature, which makes it difficult to verify to 118 km that have not been as prevalent or as clearly evi- the simulation results of models that would provide better dent in earlier trail releases. understanding of the complex dynamics of the region. To Keywords. Atmospheric composition and structure (Pres- address this problem the Turbopause experiment used four sure, density, and temperature) – Meteorology and atmo- rockets launched over a period of approximately two hours spheric dynamics (Turbulence; Waves and tides) from Poker Flat Research Range, Alaska (64◦ N, 147◦ W) on the night of 17–18 February 2009. All four rocket pay- loads released trimethyl aluminum trails for neutral wind and turbulence measurements, and two of the rockets car- 1 Introduction ried ionization gauges and fixed-bias Langmuir probes mea- suring neutral and electron densities, small-scale fluctuations and neutral temperatures. Two lidars monitored temperature The altitude range across the turbopause transition is one of structure and sodium densities. The observations were made the most dynamic regions in the Earth’s atmosphere. The tur- under quiet geomagnetic conditions and show persistence bopause boundary is generally clearly evident in photographs in the wind magnitudes and shears throughout the observ- of chemical tracer trails, which show a distinct transition ing period while being modulated by inertia-gravity waves. from three-dimensional turbulent structure within the trails High resolution temperature profiles show the winter polar to a laminar flow over a vertical distance of a few hundred mesosphere and lower thermosphere in a state of relatively meters or less and is often located around 103 km (e.g., Bla- low stability with several quasi-adiabatic layers between 74 mont, 1963). We may call this altitude the visual turbopause. and 103 km. Temperature and wind data were combined Detailed observations of chemical trails suggest that below to calculate Richardson number profiles. Evidence for tur- this level the atmosphere consists of layers and globules with bulence comes from simultaneous observations of density homogeneous and isotropic turbulence (Rees et al., 1972). fluctuations and downward transport of sodium in a mixed But even above the turbopause, diffusion may not be strictly layer near 75 km; the observation of turbulent fluctuations molecular. Layers of anomalous diffusion are observed, i.e., and energy dissipation from 87–90 km; and fast and irreg- the expansion is faster than from molecular diffusion alone ular trail expansion at 90–93 km, and especially between (e.g., Roper, 1996). Rees et al. (1972) suggested that larger 95 to 103 km. The regions of turbulent trails agree well scale gravity waves may provide conditions for quiet and tur- with regions of quasi-adiabatic temperature gradients. Above bulent layers and that small scale dynamics is thought to be anisotropic and indicative for viscous dissipation of smaller scale gravity waves. The altitude range between 100 and Correspondence to: G. A. Lehmacher 110 km is known to be a region where large wind speeds are ([email protected]) common, and the altitude immediately below the wind peak Published by Copernicus Publications on behalf of the European Geosciences Union. 2328 G. A. Lehmacher et al.: Turbopause experiment is known to have large and variable wind shears that are ei- pation rates (Lubken¨ et al., 1993), which can also only be ther unstable or close to instability (e.g., Larsen, 2002). accomplished by in situ rocket measurements. Finally, the The turbopause may also be defined as the altitude where combination of the simultaneous wind and temperature mea- the eddy diffusion coefficient is equal to the molecular diffu- surements provides context for the turbulent structure ob- sion coefficient, Ke ∼ Km. Profiles of Ke have been derived served in the trails. from mixing ratio profiles (e.g., Danilov et al., 1979; von The Turbopause experiment was a study of instability and Zahn et al., 1990), and the so-defined mixing turbopause is turbulence across turbopause altitudes. Four rockets were also called the homopause. Since Ke depends on eddy sizes launched during a two-hour period on 18 February 2009 from and time scales of mixing, Lubken¨ (1992) introduced a turbu- Poker Flat Research Range, Chatanika, Alaska. In Sect. 2 2 lent diffusion coefficient Kt = Ri(/N ) based on measured we describe the experiment and in Sect. 3 we present tem- energy dissipation rates, . This relationship assumes sta- perature and wind measurements, gravity waves, stability ble flows, with Brunt-Vaisal¨ a¨ (buoyancy) frequency squared, conditions, density fluctuations and energy dissipation rates. N 2 > 0, and uses often an average gradient Richardson num- Different regions of instability and mixing are discussed in ber Ri ∼ 0.81 based on Weinstock (1981). With Km ∼ ν, the Sect. 4 based on the observed density fluctuations and the kinematic viscosity, the turbopause criterion has also been morphology of the chemical trails, including above the nom- expressed as ∼ νN 2 (Lubken¨ , 1992). A similar expression inal turbopause. Section 5 summarizes our results. A sec- has been used to determine the classical radar turbopause ond paper by Collins et al. (2011) discusses the simultaneous (e.g., Hall et al., 2008). An alternative wave turbopause was Rayleigh and sodium lidar observations during the night. suggested by Offermann et al. (2006, 2007, 2009) based on the steep increase in the variance of satellite temperature pro- files near 95 km. It appears that different type of measure- 2 Experiment ments define their own turbopause. In addition, turbulence and turbopause height are variable in time and space and de- 2.1 Rocket measurements pend on background atmosphere, wave forcing and instabil- ity conditions. The region is of great interest, since grav- The criteria for launching the four-rocket sequence included ity waves and tides grow to very large amplitudes, dissipate clear skies over the northern part of Alaska and signifi- and deposit their momentum into the mean flow. This small- cant wave structure in the sodium density, in particular on scale and nonlinear dynamics is important for the global cir- the top side of the sodium layer. Clear skies were re- culation, and models must be able to parameterize sub-grid quired both for the lidar measurements and for the ground- processes efficiently and accurately using appropriate eddy based observations of the chemical trails. The conditions diffusion coefficients. were satisfied during the night of 18 February 2009 and A number of techniques provide wind measurements that the rockets were launched at 09:59, 10:29, 10:59, and extend up to altitudes close to the turbopause boundary, in- 11:49 UTC from Poker Flat Research Range, Chatanika, cluding medium-frequency (MF) radar measurements, me- Alaska (64.9◦ N, 147.8◦ W). The NASA mission numbers teor radar measurements, and resonance lidar measurements. were 41.077, 41.078, 41.079, and 41.076, respectively. More Measurements that span the turbopause range are difficult to details are given in Table 1. The apogee is determined from obtain, however, especially if good height resolution is re- the GPS data for the instrumented payload on 41.078 and quired. The chemical release technique is the only available 41.079, and the radar altitude for the other two launches. The option that provides direct measurements of the wind profiles flights were to follow the same nominal trajectory to obtain with good accuracy and height resolution. In addition to pro- observations near the same location. Flight 41.079 deviated viding wind measurements, the technique also provides in- slightly to the west, but its path differed only by ∼5 km at formation about the turbulent structure in the medium since 90 km altitude on upleg and by ∼17 km on downleg from this motion is directly visible as structure in the trails. The the other trajectories. For each flight, the upleg and down- disadvantage of the technique is that multiple measurements leg crossing of 90 km altitude was separated horizontally by are usually difficult to obtain since multiple rocket launches ∼90–100 km. are required. There have been only a handful of experiments All four payloads included an experiment for releasing that have provided wind measurements of this type over an chemoluminescent trails. Trimethyl aluminum (TMA) was extended time interval of several hours or longer. The data released from the payload as puffed trails, separately on up- described here represent one of the few sets of such mea- leg and downleg for a total of eight trails. Releases started surements. The same can be said for high-resolution neutral at T + 76 s, with 24 puffs, 2 s long and in 4-s intervals, un- density and temperature measurements, which are necessary til T + 170 s. The downleg trail started at T + 220 s, with to determine kinematic viscosity and buoyancy frequency.
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