Abstract. We use measurements by the 52 MHz windprofiling radar ESRAD, situated near Kiruna in Arctic Sweden, and simulations using the Advanced Research and Weather Forecasting model, WRF, to study vertical winds and turbulence in the troposphere in mountain-wave conditions on 23, 24 and 25 January 2003. We find that WRF can accurately match the vertical wind signatures at the radar site when the spatial resolution for the simulations is 1 km. The horizontal and vertical wavelengths of the dominating mountain-waves are ∼10-20 km and the amplitudes in vertical wind 1-2 m/s. Turbulence below 5500 m height, is seen by ESRAD about 40% of the time. This is a much higher rate than WRF predictions for conditions of Richardson number (R i ) <1 but similar to WRF predictions of R i <2. WRF predicts that air crossing the 100 km wide model domain centred on ESRAD has a ∼10% chance of encountering convective instabilities (R i <0) somewhere along the path. The cause of low R i is a combination of wind-shear at synoptic upper-level fronts and perturbations in static stability due to the mountain-waves. Comparison with radiosondes suggests that WRF underestimates wind-shear and the occurrence of thin layers with very low static stability, so that vertical mixing by turbulence associated with mountain waves may be significantly more than suggested by the model.

The relationship between polar mesosphere summer echoes (PMSE) and geomagnetic disturbances (represented by magnetic <I>K</I> indices) is examined. Calibrated PMSE reflectivities for the period May 2006–February 2012 are used from two 52.0/54.5 MHz radars located in Arctic Sweden (68° N, geomagnetic latitude 65°) and at two different sites in Queen Maud Land, Antarctica (73°/72° S, geomagnetic latitudes 62°/63°). In both the Northern Hemisphere (NH) and the Southern Hemisphere (SH) there is a strong increase in mean PMSE reflectivity between quiet and disturbed geomagnetic conditions. Mean volume reflectivities are slightly lower at the SH locations compared to the NH, but the position of the peak in the lognormal distribution of PMSE reflectivities is close to the same at both NH and SH locations, and varies only slightly with magnetic disturbance level. Differences between the sites, and between geomagnetic disturbance levels, are primarily due to differences in the high-reflectivity tail of the distribution. PMSE occurrence rates are essentially the same at both NH and SH locations during most of the PMSE season when a sufficiently low detection threshold is used so that the peak in the lognormal distribution is included. When the local-time dependence of the PMSE response to geomagnetic disturbance level is considered, the response in the NH is found to be immediate at most local times, but delayed by several hours in the afternoon sector and absent in the early evening. At the SH sites, at lower magnetic latitude, there is a delayed response (by several hours) at almost all local times. At the NH (auroral zone) site, the dependence on magnetic disturbance is highest during evening-to-morning hours. At the SH (sub-auroral) sites the response to magnetic disturbance is weaker but persists throughout the day. While the immediate response to magnetic activity can be qualitatively explained by changes in electron density resulting from energetic particle precipitation, the delayed response can largely be explained by changes in nitric oxide concentrations. Observations of nitric oxide concentration at PMSE heights by the Odin satellite support this hypothesis. Sensitivity to geomagnetic disturbances, including nitric oxide produced during these disturbances, can explain previously reported differences between sites in the auroral zone and those at higher or lower magnetic latitudes. The several-day lifetime of nitric oxide can also explain earlier reported discrepancies between high correlations for average conditions (year-by-year PMSE reflectivities and <I>K</I> indices) and low correlations for minute-to-day timescales

Abstract. Tropopause folds are one of the mechanisms of stratosphere-troposphere exchange, which can bring ozone rich stratospheric air to low altitudes in the extra-tropical regions. They have been widely studied at northern mid-or high latitudes, but so far almost no studies have been made at mid-or high southern latitudes. The Moveable Atmospheric Radar for Antarctica (MARA), a 54.5 MHz wind-profiler radar, has operated at the Swedish summer station Wasa, Antarctica (73 • S, 13.5 • W) during austral summer seasons from 2007 to 2011 and has observed on several occasions signatures similar to those caused by tropopause folds at comparable Arctic latitudes. Here a case study is presented of one of these events when an ozonesonde successfully sampled the fold. Analysis from European Center for Medium Range Weather Forecasting (ECMWF) is used to study the circumstances surrounding the event, and as boundary conditions for a mesoscale simulation using the Weather Research and Forecasting (WRF) model. The fold is well resolved by the WRF simulation, and occurs on the poleward side of the polar jet stream. However, MARA resolves fine-scale layering associated with the fold better than the WRF simulation.

Abstract. Inertia-gravity waves (IGWs) are an important component for the dynamics of the middle atmosphere. However, observational studies needed to constrain their forcing are still insufficient especially in the remote areas of the Antarctic region. One year of observational data (January to December 2013) by the PANSY radar of the wind components (vertical resolution of 150 m and temporal resolution of 30 min) are used to derive statistical analysis of the properties of IGWs with short vertical wavelengths ( ≤ 4 km) and ground-based periods longer than 4 h in the lowermost stratosphere (height range 10 to 12 km) with the help of the hodograph method. The annual change of the IGWs parameters are inspected but no pronounced year cycle is found. The year is divided into two seasons (summer and winter) based on the most prominent difference in the ratio of Coriolis parameter (f ) to intrinsic frequency (ω) distribution. Average of f/ω for the winter season is 0.40 and for the summer season 0.45 and the average horizontal wavelengths are 140 and 160 km respectively. Vertical wavelengths have an average of 1.85 km through the year. For both seasons the properties of IGWs with upward and downward propagation of the energy are also derived and compared. The percentage of downward propagating waves is 10.7 and 18.4 % in the summer and winter season respectively. This seasonal change is more than the one previously reported in the studies from mid-latitudes and model-based studies. It is in agreement with the findings of past radiosonde data-based studies from the Antarctic region. In addition, using the so-called dualbeam technique, vertical momentum flux and the variance of the horizontal perturbation velocities of IGWs are examined. Tropospheric disturbances of synoptic-scale are suggested as a source of episodes of IGWs with large variance of horizontal perturbation velocities, and this is shown in a number of cases.