[Show abstract][Hide abstract] ABSTRACT: The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent scatter (IS) radar facilities on the mainland of northern Scandinavia (the EISCAT UHF and VHF radar systems) and on Svalbard (the electronically scanning radar ESR (EISCAT Svalbard Radar) for studies of the high-latitude ionised upper atmosphere (the ionosphere). The mainland radars were constructed about 30 years ago, based on technological solutions of that time. The science drivers of today, however, require a more flexible instrument, which allows measurements to be made from the troposphere to the topside ionosphere and gives the measured parameters in three dimensions, not just along a single radar beam. The possibility for continuous operation is also an essential feature. To facilitatefuture science work with a world-leading IS radar facility, planning of a new radar system started first with an EU-funded Design Study (2005-2009) and has continued with a follow-up EU FP7 EISCAT_3D Preparatory Phase project (2010-2014). The radar facility will be realised by using phased arrays, and a key aspect is the use of advanced software and data processing techniques. This type of software radar will act as a pathfinder for other facilities worldwide. The new radar facility will enable the EISCAT_3D science community to address new, significant science questions as well as to serve society, which is increasingly dependent on space-based technology and issues related to space weather. The location of the radar within the auroral oval and at the edge of the stratospheric polar vortex is also ideal for studies of the long-term variability in the atmosphere and global change. This paper is a summary of the EISCAT_3D science case, which was prepared as part of the EU-funded Preparatory Phase project for the new facility. Three science working groups, drawn from the EISCAT user community, participated in preparing this document. In addition to these working group members, who are listed as authors, thanks are due to many others in the EISCAT scientific community for useful contributions, discussions, and support.
[Show abstract][Hide abstract] ABSTRACT: We present atmospheric gravity wave (GW) measurements obtained by a Rayleigh/Raman lidar at Lauder, New Zealand, (45°S, 170°E) during and after the Deep Propagating Gravity Wave Experiment campaign. GW activity and characteristics are derived from 557 h of high-resolution lidar data recorded between June and November 2014 in an altitude range between 28 and 76 km. In this period, strong GW activity occurred in sporadic intervals lasting a few days. Enhanced stratospheric GW potential energy density is detected during periods with high tropospheric wind speeds perpendicular to New Zealand's Southern Alps. These enhancements are associated with the occurrence of quasi-stationary GW (mountain waves). Surprisingly, the largest response in the mesosphere is observed for conditions with low to moderate lower tropospheric wind speeds (2-12 m/s). On the other hand, large-amplitude mountain waves excited by strong tropospheric forcings often do not reach mesospheric altitudes, either due to wave breaking and dissipation in the stratosphere or refraction away from New Zealand.
No preview · Article · Oct 2015 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: Gravity waves (GWs) greatly influence the background state of the middle atmosphere by imposing their momentum on the mean flow upon breaking and by thus driving, e.g., the upper mesospheric summer zonal wind reversal. In this situation momentum is conserved by a balance between the vertical divergence of GW momentum flux (the so-called GW drag) and the Coriolis acceleration of the mean meridional wind. In this study, we present first quantitative mean annual cycles of these two balancing quantities from the medium frequency Doppler radar at the polar site Saura (SMF radar, 69° N, 16° E). Three-year means for 2009 through 2011 clearly show that the observed zonal momentum balance between 70 and 100 km with contributions from GWs only is fulfilled during summer when GW activity is strongest and more stable than in winter. During winter, the balance between GW drag and Coriolis acceleration of the mean meridional wind is not existent, which is likely due to the additional contribution from planetary waves, which are not considered by the present investigation. The differences in the momentum balance between summer and winter conditions are additionally clarified by 3-month mean vertical profiles for summer 2010 and winter 2010/2011.
No preview · Article · Sep 2015 · Annales Geophysicae
[Show abstract][Hide abstract] ABSTRACT: The paper presents a feasible method to complement ground-based middle atmospheric Rayleigh lidar temperature observations with numerical simulations in the lower stratosphere and troposphere to study gravity waves. Validated mesoscale numerical simulations are utilized to complement the temperature below 30 km altitude. For this purpose, high-temporal resolution output of the numerical results was interpolated on the position of the lidar in the lee of the Scandinavian mountain range. Two wintertime cases of orographically induced gravity waves are analysed. Wave parameters are derived using a wavelet analysis of the combined data set throughout the entire altitude range from the troposphere to the mesosphere. Although similar in the tropospheric forcings, both cases differ in vertical propagation. The combined data set reveals stratospheric wave breaking for one case whereas the mountain waves in the other case could propagate up to about 40 km altitude. The lidar observations reveal an interaction of the vertically propagating gravity waves with the stratopause leading to a stratopause descent in both cases.
[Show abstract][Hide abstract] ABSTRACT: This study evaluates commonly used methods of extracting gravity wave induced temperature perturbations from lidar measurements. The spectral response of these methods is characterized with the help of a synthetic dataset with known temperature perturbations added to a realistic background temperature profile. The simulations are carried out with the background temperature being either constant or varying in time to evaluate the sensitivity to temperature perturbations not caused by gravity waves. The different methods are applied to lidar measurements over new Zealand and the performance of the algorithms is evaluated. We find that the Butterworth filter performs best if gravity waves over a wide range of periods are to be extracted from lidar temperature measurements. The running mean method gives good results if only gravity waves with short periods are to be analyzed.
[Show abstract][Hide abstract] ABSTRACT: Sudden stratospheric warmings (SSWs) are the most prominent vertical coupling process in the middle atmosphere, which occur during winter and are caused by the interaction of planetary waves (PWs) with the zonal mean flow. Vertical coupling has also been identified during the equinox transitions, and is similarly associated with PWs. We argue that there is a characteristic aspect of the autumn transition in northern high latitudes, which we call the "hiccup", and which acts like a "mini SSW", i.e. like a small minor warming. We study the average characteristics of the hiccup based on a superimposed epoch analysis using a nudged version of the Canadian Middle Atmosphere Model, representing 30 years of historical data. Hiccups can be identified in about half the years studied. The mesospheric zonal wind results are compared to radar observations over Andenes (69 degrees N, 16 degrees E) for the years 2000-2013. A comparison of the average characteristics of hiccups and SSWs shows both similarities and differences between the two vertical coupling processes.
No preview · Article · Feb 2015 · Annales Geophysicae
[Show abstract][Hide abstract] ABSTRACT: Annual cycles of horizontal winds and gravity wave (GW) momentum fluxes in the mesosphere and lower thermosphere (MLT) are presented for the medium frequency Doppler radar at Saura (SMF radar, located at 69°N, 16°E) for the first time. 4-year mean wind and momentum flux fields for 2008 through 2011 clearly show the coupling and interactions between GWs and the mean flow especially in the summer months. GW breaking at mesopause heights results in momentum flux divergence and affects the wind field by forcing a reversal of the wind profile in summer. Height-time cross-sections for the individual years (2008 to 2011) illustrate the year-to-year variation of horizontal winds and the vertical fluxes of zonal and meridional momentum. They show similar annual patterns from year to year which are more consistent in the summer months than during winter and have maximum absolute values in 2009. Furthermore, the precise SMF radar measurements give an excellent possibility to evaluate momentum flux estimates from the co-located meteor radar at Andenes. Both radars have different capabilities, and different techniques are applied to derive momentum fluxes. They show comparable results for the 4-year mean annual cycles of horizontal winds and momentum fluxes especially in summer. This holds for both structure and magnitudes in the overlapping heights, where the SMF radar data provides a wider vertical coverage. The best agreement is found for the zonal components of both radars whereas there are some larger discrepancies in the meridional components, especially in the vertical flux of meridional momentum.
No preview · Article · Dec 2014 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract] ABSTRACT: Meteor smoke particles (MSP), which are thought to be the nucleation germs for mesospheric ice are currently discussed to consist of highly absorbing materials such as magnesiowüstite, hematite or magnesium-iron-silicates and may therefore be warmer than the ambient atmosphere. In order to quantify the temperature difference between MSPs and the atmosphere we developed a model to calculate the MSP equilibrium temperature in radiational and collisonal balance. The temperature difference between MSP and the surrounding atmosphere strongly depends on the composition of the MSP, especially on the relative iron content, where a higher iron content leads to warmer MSP. We then derive an expression of the nucleation rate of mesospheric ice particles which explicitly accounts for this temperature difference. We find that the nucleation rate is strongly reduced by several orders of magnitude if the germ temperature is increased by only a few Kelvin. Implementing this nucleation rate depending on the germ temperature into CARMA, the Community Aerosol and Radiation Model for Atmospheres, we find that fewer but larger ice particles are formed compared to a reference scenario with no temperature difference between MSP and ambient atmosphere. This may indicate that iron-rich MSP are not ideal ice nuclei and that either other MSP-types or other nucleation pathways (e.g. wave induced heterogeneous nucleation or even homogeneous nucleation) are responsible for ice formation at the mesopause.
No preview · Article · Oct 2014 · Journal of Atmospheric and Solar-Terrestrial Physics
[Show abstract][Hide abstract] ABSTRACT: Long-term observations from medium-frequency and meteor radars (1993–2012) and rocket soundings (1979–1990 and 2002–2007) are used to study mesosphere and lower thermosphere (MLT) zonal wind variations in relation to the stratospheric winds over northern low latitudes. The combined data set provides a complete height profile of amplitude of semiannual oscillation (SAO) up to 100 km, with an exception around 75–80 km. The SAO signal has maxima around 50 km and 82 km and a minimum around 65 km. The MLT zonal winds show remarkable interannual variability during northern hemispheric spring equinox and much less during fall equinox. Zonal wind mesospheric spring equinox enhancements (MSEE) appear with a periodicity of 2–3 years, suggesting a modulation by the quasi-biennial oscillation, which we identified with the strength of stratospheric westward winds. Out of 20 years of observations, the stratospheric westward winds are strong during 11 years (non-MSEE) and weak during 9 years. Six of these 9 years show large MLT winds (MSEE), and 3 years (1999, 2004, and 2006) show small MLT winds (missing MSEE). These unexpected small winds occur in years with global circulation anomalies associated with strong sudden stratospheric warmings and an early spring transition of zonal winds. With the proposed three MSEE classes, we take into account local and global forcing factors.
Full-text · Article · May 2014 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: The influence of gravity waves on noctilucent
clouds (NLC) at ALOMAR (69 N) is analysed by relating
gravity wave activity to NLC occurrence from commonvolume
measurements. Gravity wave kinetic energies are derived
from MF-radar wind data and filtered into different period
ranges by wavelet transformation. From the dataset covering
the years 1999–2011, a direct correlation between gravity
wave kinetic energy and NLC occurrence is not found,
i.e., NLC appear independently of the simultaneously measured
gravity wave kinetic energy. In addition, gravity wave
activity is divided into weak and strong activity as compared
to a 13 yr mean. The NLC occurrence rates during strong and
weak activity are calculated separately for a given wave period
and compared to each other. Again, for the full dataset
no dependence of NLC occurrence on relative gravity wave
activity is found. However, concentrating on 12 h of NLC
detections during 2008, we do find an NLC-amplification
with strong long-period gravity wave occurrence. Our analysis
hence confirms previous findings that in general NLC
at ALOMAR are not predominantly driven by gravity waves
while exceptions to this rule are at least possible.
Preview · Article · Dec 2013 · ATMOSPHERIC CHEMISTRY AND PHYSICS
[Show abstract][Hide abstract] ABSTRACT: This work investigates the influence of meteoric
smoke particles (MSP) on the charge balance in the D-region
ionosphere. Both experimental in situ measurements and a
one-dimensional ionospheric model reveal a clear impact of
MSP on the ionospheric composition of the D-region. The
study reviews rocket-borne in situ measurements of electron
and positive ion density, which show a distinct deficit
of electrons in comparison to positive ions between 80 and
95 km. This deficit can be explained by the ambient negatively
chargedMSP measured simultaneously with a Faraday
cup. The influence of MSP on the D-region charge balance
is addressed with a simplified ionospheric model with only
six components, i.e. electrons, positive and negative ions and
neutral and charged MSP (both signs). The scheme includes
reactions of plasma captured by MSP and MSP photo reactions
as well as the standard ionospheric processes, e.g. ionion
recombination. The model shows that the capture of
plasma constituents by MSP is an important process leading
to scavenging of electrons. Since Faraday cup measurements
are biased towards heavy MSP because of aerodynamical filtering, we have applied an estimate of this filter on the modelled MSP densities. By doing that, we find good qualitative agreement between the experimental data and our model results. In addition, the model study reveals an increase of positive ions in the presence of MSP. That is primarily caused by the reduced dissociative recombination with electrons which have been removed from the gas phase by the MSP.
Preview · Article · Nov 2013 · Annales Geophysicae
[Show abstract][Hide abstract] ABSTRACT: It is now well understood that the occurrence of PMSE is closely connected to the presence of ice particles. These ice particles modify the ambient electron density by electron attachment which occasionally leads to large electron density depletions which have also been called ‘biteouts’. There has been some debate in the literature regarding the relative depth of such depletions which is usually expressed by the parameter Λ=|ZA|NA/neΛ=|ZA|NA/ne. Here, |ZA|NA|ZA|NA is the charge number density of ice particles and ne is the electron density. In this paper, we present, for the first time, the statistical distribution of ΛΛ using measurements with the EISCAT VHF- and UHF-radars. Based on 25 h of simultaneous observations, we derived a total of 757 ΛΛ values based on 15 min of data each. In each of these cases, PMSE were observed with the EISCAT VHF-radar but not with the UHF-radar and the UHF-measurement were hence used to determine the electron density profile. From these 757 cases, there are 699 cases with Λ⪡1Λ⪡1, and only 33 cases with Λ>0.5Λ>0.5 (21 cases with Λ>1Λ>1). A correlation analysis of ΛΛ versus PMSE volume reflectivities further reveals that there is no strong dependence between the two parameters. This is in accordance with current PMSE-theory based on turbulence in combination with a large Schmidt-number. The maxima of ΛΛ from each profile show a negative relationship with the undisturbed electron densities deduced at the same altitudes. This reveals that the variability of ΛΛ mainly depends on the variability of the electron densities. In addition, variations of aerosol number densities may also play a role. Although part of the observations were conducted during the HF heating experiments, the so-called overshoot effects did not significantly bias our statistical results. In order to avoid missing biteouts because of a superposition of coherent and incoherent scatter in the UHF-data, we finally calculated spectral parameters n by applying a simple fit to auto-correlation functions as introduced by Strelnikova and Rapp (2010). Corresponding statistical results of the parameter n indicate that charged ice particles do exist in the vicinity of PMSE (i.e., n<1n<1) but they did not efficiently modify ambient electron densities so that clear ‘biteouts’ are observed.
No preview · Article · Nov 2013 · Journal of Atmospheric and Solar-Terrestrial Physics
[Show abstract][Hide abstract] ABSTRACT: The Middle Atmosphere Alomar Radar System
(MAARSY) on the island of Andøya in Northern Norway
(69.3� N, 16.0� E) observes polar mesospheric summer
echoes (PMSE). These echoes are used as tracers of atmospheric dynamics to investigate the horizontal wind variability at high temporal and spatial resolution. MAARSY has the capability of pulse-to-pulse beam steering allowing for systematic scanning experiments to study the horizontal structure of the backscatterers as well as to measure the radial velocities for each beam direction. Here we present a method to retrieve gravity wave parameters from these horizontally resolved radial wind variations by applying velocity azimuth display and volume velocity processing. Based on the observations a detailed comparison of the two wind analysis techniques is carried out in order to determine the zonal and meridional wind as well as to measure first-order inhomogeneities. Further, we demonstrate the possibility to resolve the horizontal wave properties, e.g., horizontal wavelength, phase velocity and propagation direction. The robustness of the estimated gravity wave parameters is tested by a simple atmospheric model.
Full-text · Article · Oct 2013 · Atmospheric Measurement Techniques