| 1. |
- Dalin, Peter A., et al.
(författare)
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Wave influence on polar mesosphere summer echoes above Wasa : experimental and model studies
- 2012
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 30:8, s. 1143-1157
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Tidskriftsartikel (refereegranskat)abstract
- Comprehensive analysis of the wave activity in the Antarctic summer mesopause is performed using polar mesospheric summer echoes (PMSE) measurements for December 2010–January 2011. The 2-day planetary wave is a statistically significant periodic oscillation in the power spectrum density of PMSE power. The strongest periodic oscillation in the power spectrum belongs to the diurnal solar tide; the semi-diurnal solar tide is found to be a highly significant harmonic oscillation as well. The inertial-gravity waves are extensively studied by means of PMSE power and wind components. The strongest gravity waves are observed at periods of about 1, 1.4, 2.5 and 4 h, with characteristic horizontal wavelengths of 28, 36, 157 and 252 km, respectively. The gravity waves propagate approximately in the west-east direction over Wasa (Antarctica). A detailed comparison between theoretical and experimental volume reflectivity of PMSE, measured at Wasa, is made. It is demonstrated that a new expression for PMSE reflectivity derived by Varney et al. (2011) is able to adequately describe PMSE profiles both in the magnitude and in height variations. The best agreement, within 30%, is achieved when mean values of neutral atmospheric parameters are utilized. The largest contribution to the formation and variability of the PMSE layer is explained by the ice number density and its height gradient, followed by wave-induced perturbations in buoyancy period and the turbulent energy dissipation rate
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| 2. |
- Kirkwood, Sheila, et al.
(författare)
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Independent Calibration of Radar Reflectivities Using Radiosondes: Application To ESRAD
- 2011
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Ingår i: Proceedings of the ‘20th ESA Symposium on European Rocket and Balloon Programmes and Related Research’. ; , s. 425-429
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Konferensbidrag (refereegranskat)abstract
- A large number of empirical and theoretical studies haveshown that radar reflectivity from the atmosphere at 50MHz is proportional to the mean vertical gradient of therefractive index. Up to 30 km height the refractive indexis determined by temperature, pressure and humidityprofiles, which can readily be measured by radiosondes.In practice, humidity becomes unimportant abovethe mid-troposhere. The coefficient of proportionality betweenradar reflectivity and mean refractive index gradientshould, in principle, depend on the fine-scale structureof refractive index fluctuations. However, recent empiricalevidence shows that the coefficient varies very littlebetween widely different meteorological conditions andbetween radars in very different locations (Esrange, tropicalIndia, Antarctica). This means we can use meanprofiles of refractive index, measured by radiosondes, asan independent method to provide continuous calibrationof radar reflectivity and to cross-calibrate between differentradars without the need to interrupt operations for thekind of engineering tests which are usually used for calibration.We show how this can be applied for long-termcalibration of the 52 MHz atmospheric radar at Esrange,ESRAD.The observed invariability of the coefficient of proportionalityalso poses an intriguing question as to how it canbe explained. High-resolution sondes, constant-heightsondes, and UAV’s are suggested as suitable platformsfor further study of the radar scattering mechanisms.
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| 3. |
- Kirkwood, Sheila, et al.
(författare)
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Response of polar mesosphere summer echoes to geomagnetic disturbances in the Southern and Northern Hemispheres : The importance of nitric oxide
- 2013
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 31:2, s. 333-347
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Tidskriftsartikel (refereegranskat)abstract
- The relationship between polar mesosphere summer echoes (PMSE) and geomagnetic disturbances (represented by magnetic I K 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 indices) and low correlations for minute-to-day timescales
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| 4. |
- Kirkwood, Sheila, et al.
(författare)
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Turbulence associated with mountain waves over Northern Scandinavia : a case study using the ESRAD VHF radar and the WRF mesoscale model
- 2010
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 10, s. 3583-3599
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Tidskriftsartikel (refereegranskat)abstract
- We use measurements by the 52 MHz wind-profiling 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 (Ri) <1 but similar to WRF predictions of Ri<2. WRF predicts that air crossing the 100 km wide model domain centred on ESRAD has a ~10% chance of encountering convective instabilities (Ri<0) somewhere along the path. The cause of low Ri 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
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| 5. |
- Mihalikova, Maria
(författare)
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Mesoscale processes in the polar atmosphere : radar remote sensing, balloon-borne in situ measurements and modelling
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mesoscale processes (atmospheric phenomena with horizontal scales ranging from a few tens to several hundred kilometres and lasting from a few tens of minutes to a few days) have the potential to influence the chemical composition of the troposphere. Tropopause folds and mountain waves are two important types of mesoscale processes. Concentrations and gradients of trace gases like ozone (O3) can be influenced by these processes. Tropopause folds bring ozone-rich stratospheric air to lower altitudes. Mountain waves and turbulence associated with them influence O3 gradients in the troposphere. Tropospheric O3 is a toxic pollutant and a short-lived greenhouse gas with an influence on the lifetime of many other trace gases. Understanding of its long-term development and budgets are important. For this, better understanding, generalization and representation of mesoscale processes are necessary. Observations made by the 52MHz wind-profiler radar ESRAD (ESrange RADar) and the 54.5MHz wind-profiler radar MARA (Movable Atmospheric Radar for Antarctica) served as the basis for this study. ESRAD is located close to Kiruna in arctic Sweden and has been in operation since July 1996. This is a site with frequent mountain wave activity. By analysis of ESRAD and sonde data we have studied vertical mixing and turbulence associated with mountain waves. An attempt was made to show the influence of these processes on relaxation of the O3 gradient in the lower troposphere. Additional balloon-borne in situ measurements of vertical profiles of atmospheric characteristics (temperature, humidity, O3 mixing ratio) complement the radar measurements and aid in correct identification and improved understanding of the observed processes as well as of the radar backscatter signal itself. MARA was operated at the Swedish summer station Wasa (73°S, 13.5°W) during austral summer 2010/2011 and at the Norwegian year-round station Troll (72°S, 2.5°E) nonstop since December 2011. During its operation at the Wasa station, ozonesonde measurements were successfully undertaken during the passage of a tropopause fold. These provided validity to the radar measurements and proved them to be a useful tool for tropopause fold studies, for the first time at Antarctic latitudes. Data gathered at the Troll station exhibit signs of an annual cycle of tropopause folds with winter maximum and summer minimum in their occurrence rate which is similar to the observed behaviour in the northern hemisphere. Comparisons with ECMWF (European Centre for Medium-Range Weather Forecasts) model data and the WRF model (Advanced Research and Weather Forecasting) show that higher resolution models such as WRF are needed for more adequate representation of these processes. High resolution models can in return serve as a basis for studies of areas that are not at all or only partially covered by measurement networks, as well as for global studies. Thus they can provide useful information about atmospheric transport and the state of trace gases like O3.
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| 6. |
- Mihalikova, Maria, et al.
(författare)
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Observation of a tropopause fold by MARA VHF wind-profiler radar and ozonesonde at Wasa, Antarctica : Comparison with ECMWF analysis and a WRF model simulation
- 2012
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 30:9, s. 1411-1421
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Tidskriftsartikel (refereegranskat)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 co
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| 7. |
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| 8. |
- Mihalikova, Maria, et al.
(författare)
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Tropopause fold occurrence rates over the Antarctic station Troll (72 degrees S, 2.5 degrees E)
- 2013
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 31:4, s. 591-598
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Tidskriftsartikel (refereegranskat)abstract
- One of the important mechanisms of stratosphere-troposphere exchange, which brings ozone-rich stratospheric air to low altitudes in extratropical regions, is transport related to tropopause folds. The climatology of folds has been studied at high latitudes of the Northern Hemisphere with the help of radars and global models. Global models supply information about fold occurrence rates at high latitudes of the Southern Hemisphere as well, but so far comparisons with direct measurements are rare. The Moveable Atmospheric Radar for Antarctica (MARA), a 54.5 MHz wind-profiler radar, has been operated at the Norwegian year-round station Troll, Antarctica (72 degrees S, 2.5 degrees E) since December 2011. Frequent tropopause fold signatures have been observed. In this study, based on MARA observations, an occurrence rate statistics of tropopause folds from December 2011 until November 2012 has been made, and radar data have been compared with the analysis from the European Center for Medium-Range Weather Forecasting (ECMWF). The fold occurrence rates exhibit an annual cycle with winter maximum and summer minimum and suggest significantly higher occurrence rates for the given location than those obtained previously by global model studies.
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| 9. |
- Mihalikova, Maria, et al.
(författare)
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Vertical mixing in the lower troposphere by mountain waves over Arctic Scandinavia
- 2011
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 11, s. 31475-31493
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Tidskriftsartikel (refereegranskat)abstract
- Measurements made by ozonesondes and by a 52 MHz wind-profiling radar during February and March 1997 are studied. The radar is located at Esrange, near Kiruna in Arctic Sweden, on the eastern flank of the Scandinavian mountains. Daily ozonesondes were launched from the same site. The radar vertical and horizontal wind measurements are used to identify times when mountain waves were present. Mean vertical gradients in ozone mixing ratio in the lower troposphere are determined in conditions with mountain waves present and when they were absent. Back-trajectories were calculated so that only air-masses with their origin to the west of the mountains were included in the final averages. The vertical gradient in ozone mixing ratio is found to be about twice as steep outside wave conditions as it is during mountain waves. This suggests a very high rate of vertical mixing, with an average eddy diffusivity of order 5000 m2 s−1. This is consistent with an earlier estimate of the occurrence rate of complete mixing by wave breaking over the mountain range.
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