C. M. Hall

Small scale structure and turbulence in the mesosphere and lower thermosphere at high latitudes in winter

Abstract The MAP/WINE campaign has yielded information on small scale structure and turbulence in the winter mesosphere and lower thermosphere by a number of very different remote and in situ techniques. We have assimilated the data from the various sources and thus attempted to present a coherent picture of the small scale dynamics of the atmosphere between 60 and 100 km. We review physical mechanisms which could be responsible for the observed effects, such as ion density fluctuations, radar echoes and wind corners. Evidence has been found for the existence of dynamic structures extending over distances of the order of 100 km; these may be turbulent or non-turbulent. The results indicate that gravity wave saturation is a plausible mechanism for the creation of turbulence and that laminar flows, sharply defined in height and widespread horizontally, may exist.

A comparison of mesosphere and lower thermosphere neutral winds as determined by meteor and medium‐frequency radar at 70°N

[1]xa0There has been much discussion as to the veracity of neutral wind measurements made using medium-frequency radar (MFR) employing the spaced-antenna technique. Such systems are able to operate continuously, providing information on mesosphere and lower thermosphere dynamics with typical resolutions of 3 km in altitude and 5 min in time, and thus represent a low-cost monitoring of the atmosphere. It is similarly important to be able to trust the results, and therefore we make a dedicated comparison between the Tromso MFR (70°N, 19°E) and the newly installed and colocated Nippon/Norway Tromso meteor radar. The agreement is particularly good between 75 and 85 km.

Improved estimates for neutral air temperatures at 90 km and 78°N using satellite and meteor radar data

[1]xa0A technique for using satellite-derived temperatures to calibrate initial estimates of 90 km temperatures measured by meteor wind radar is presented. Temperatures derived from the Nippon/Norway Svalbard Meteor Radar, situated on Svalbard at 78°N, 16°E, are calibrated using data from the Aura spacecrafts Microwave Limb Sounder (MLS) experiment. The calibration was performed in a two-step process: after an initial calibration of first-guess temperatures, results were used to adjust the MLS values to reflect daily means rather than the 0200–1100 UT observation period of the satellite instrument; thereafter the calibration was repeated with the revised MLS temperatures. The resulting temperature time series represents a marked improvement on earlier results calibrated using hydroxyl emission and potassium/K-Lidar observations, as the uncertainty is reduced from 17 to 7 K. These latest results represent a new step toward reliable and continual monitoring of upper mesosphere/lower thermosphere temperature.

The quasi 2‐day wave observed in the polar mesosphere: Comparison of the characteristics observed at Tromsø and Poker Flat

[1]xa0A comparison of the quasi 2-day wave (Q2DW) observed at Tromso (69.6°N, 19.2°E) and Poker Flat (65.2°, 147.6°W) is presented at four heights of 70, 76, 82, and 88 km using wind data taken for ∼4 years, from 1 November 1998 to 7 November 2002. The characteristics of the Q2DW such as seasonal variation, occurrence of period of maximum amplitude, ratio of meridional to zonal amplitudes, shape of altitude profile of phase, and modulation of amplitude at a 4–10 days rate found at Poker Flat are very similar to those found at Tromso reported by Nozawa et al. [2003]. The activity of the Q2DW is higher in winter than in summer for meridional and zonal components at the two sites. Long-term variation (i.e., seasonal variation) of the Q2DW is found to be similar between the two sites, while short-term variations (i.e., over several days) do not synchronize well with each other. The ratio of amplitudes of the Q2DW between the two sites varies mainly between 0.5 and 2, and there is no significant preference toward either site. Phase differences of Q2DWs between the two sites are examined, and it is found that in-phase-like events are more frequently seen than out-of-phase-like events at 76, 82, and 88 km in winter and at 88 km in summer. This can be interpreted to mean that the zonal wave number of the Q2DW appears to be 2 or 4 more often than 3 in the polar upper mesosphere. These results suggest that observed Q2DWs have features consistent with the Rossby gravity wave mode, but the amplitude and phase of the Q2DW are affected significantly by local sources. A possibility that the observed Q2DW is an eastward moving wave is also discussed.

Temperature trends at 90 km over Svalbard, Norway (78°N 16°E), seen in one decade of meteor radar observations

[1]xa0Temperatures at 90 km altitude above Svalbard (78°N, 16°E) have been determined using a meteor wind radar and subsequently calibrated by satellite measurements for the period autumn 2001 to present. The dependence of the temperatures on solar driving has been investigated using the Ottawa 10.7 cm flux as a proxy. Removing the response of the temperatures to the seasonal and solar cycle variations yields a residual time series which exhibits the negative trend of −4 ± 2 K decade−1. We indicate that, given the month-to-month variability and memory in the time series, for a 90% confidence in this trend, we require only 55 months of data – considerably less than the amount available. Cooling of the middle atmosphere, which would be strongly supported by these results, would result in contraction and subsequent lowering of pressure surfaces; we explain that including a negative trend in the pressure model used to obtain temperatures from meteor train echo fading times would also merely serve to augment the observed 90 km cooling.

High‐latitude mesospheric mean winds: A comparison between Tromsø (69°N) and Svalbard (78°N)

[1]xa0Daily mean winds in the upper mesosphere regions above Tromso (69°N, 19°E) and Svalbard (78°N, 16°E) are presented; these having been obtained from 6 years operation of the middle frequency radar at Tromso and almost 2 years operation of the Svalbard meteor radar. The findings depart somewhat from the predictions of current modeling, particularly in that outflow from the pole occurs almost all year-round. This picture is in accordance with a Ferrell cell circulation in summer but not in winter. Gravity-wave drag, parameterized by the Rayleigh drag coefficient, is suggested as a likely cause of the widespread ageostrophic motion characterizing the winter dynamics.

An examination of high latitude upper mesosphere dynamic stability using the Nippon/Norway Svalbard Meteor Radar

[1] Using wind measurements from the recently installed Nippon/Norway Svalbard Meteor Radar, (NSMR) at 78°N, 16°E, we have derived wind shears, and, combining these with model Brunt-Vaisala frequencies, have determined estimates of the gradient Richardson Number. These Richardson Number estimates parameterise the degree of stability of the upper mesosphere at a height of around 90 km. We find indications of dynamic instability in spring and autumn, with greater stability in summer.

A comparison of northern hemisphere winds using SuperDARN meteor trail and MF radar wind measurements

The main purpose of the Super Dual Auroral Radar Network (Super-DARN) is to use paired radars to deduce the F-region convection from Doppler measurements of backscatter seen at large ranges, typically beyond ∼900 km. Nearer to each HF radar, the nearest ranges at ∼165–400 km are dominated by meteor trail echoes. Once formed, the motion of these meteor trails is normally controlled by neutral winds in the 80–110 km altitude range. By combining the line-of-sight velocities from all 16 receiver beams (∼52° in azimuth) of a given SuperDARN radar, it is possible to determine the full horizontal wind vector field over the meteor trail height range. Elevation angles are also measured using an interferometer mode and as such height information can, in principle, be obtained from the combined range and elevation angle data. A comparison with neutral wind measurements from a colocated (Saskatoon, Canada) MF wind radar indicates good agreement between the two radar systems at heights of ∼95 km. Based on these detailed comparisons, a simple common method for determining two-dimensional winds for all SuperDARN radars, which have extensive longitudinal coverage, was developed. Comparisons with other systems used for dynamical studies of tides and planetary waves are desirable and prove to be essential to obtain a good SuperDARN neutral wind motion analysis. The MF radars at Saskatoon and Tromso, Norway, are located near the western and eastern ends of the Northern Hemisphere network of six SuperDARN radars. Comparisons between the two types of radars for two seasonal intervals (September and December) show that the SuperDARN radars provide good longitudinal coverage of tides in support of the more detailed MF radar data. The two systems complement each other effectively.

Seasonal variation of the turbopause: One year of turbulence investigation at 69°N by the joint University of Tromsø/University of Saskatchewan MF radar

Following upgrades to the University of Tromso/University of Saskatchewan MF radar, located in northern Norway at 69°N, 19°E, we have been able to complete a full calendar year of estimates of mesospheric turbulent intensity. The results represent the first such study using continuous measurements from this region, and temporal and height variations are satisfyingly in accordance with expectation. Since in the past there has been a degree of disagreement as to absolute intensities, we briefly compare our results with some from totally independent methods. The resulting dissipation rates, to be regarded as maxima for the turbulent dissipation, are used to identify an upper limit to the turbopause. The Arctic turbopause appears to exhibit an annual variation, being lower in the summer, in agreement with Danilov et al. [1979] but refuting Blum and Schuchard [1978].

Multi-instrument dérivation of 90 km temperatures over Svalbard (78°N 16°E)

[1]xa0For 90 km above Svalbard at 78°N, 16°E, neutral air temperatures derived from potassium lidar and OH spectrometer observations are combined and used to calibrate the corresponding estimates derived from meteor echo fading times. While lidar and spectrometer results depend heavily on observing conditions, a meteor radar can easily provide daily measurements; the three instruments thus complement each other to yield daily temperature estimates, hence providing a temporal coverage hitherto unprecedented at this location.