Mesosphere-lower Thermosphere Region Research Articles

Nighttime lower ionosphere height estimation from the VLF modal interference distance

We have studied the dynamics of the nighttime lower ionosphere height through continuous monitoring of the VLF modal interference distance (so-called distance D). Since the distance D is related to the nighttime propagation modes within the Earth-Ionosphere waveguide, it provides information of the nighttime reflection height (hN). We have used a long-term VLF narrowband database of almost 8 years (2006–2014) from a long transequatorial VLF propagation path between the transmitter NPM (Hawaii, 21.4 kHz) and the receiver ATI (Atibaia, Brazil). Our results show that hN assumes lower values during northern hemisphere wintertime as compared with summertime. By using the Lomb-Scargle periodogram, periodicities around 180 (SAO), 365 (AO) and 800 (QBO) days have been found, being the periodicity around 180 days stronger than all other oscillations. Since these large-scale oscillations are commonly observed in several measurable parameters of the mesosphere-lower thermosphere (MLT) region, our results suggest that the nighttime lower ionosphere can be strongly influenced by the dynamics of the MLT region. The effect of the long-term solar activity on hN is also studied, resulting in high negative correlation (R = −0.91). This effect makes hN decrease around 1.2 km from low to high solar activity. This result suggests a control of the solar radiation on the nighttime lower ionosphere, and hence, on the electron density at night.

The Enhancement of Neutral Metal Na Layer Above Thunderstorms

AbstractAtomic sodium (Na) is one of the major meteoric species existing as layers of atoms in the mesosphere/lower thermosphere (MLT) at altitudes of 80–105 km and atomic ions at higher altitudes. As the boundary between neutral atmosphere and ionosphere, the MLT region is of particular interest because it is influenced by the mesoscale convective weather and thunderstorms from below and exposed to solar photons from above. There has been great interest on the atmosphere‐ionosphere coupling, and numerous studies have been reported on the connection between thunderstorms and the ionosphere. However, the influence of thunderstorms on metallic species, which would significantly enhance our understanding of how Earth's atmosphere interacts with its ionosphere and studying the chemistry and physics of the MLT region, has rarely been studied. Here we present observational results on a statistical basis showing evidence that thunderstorm activities can affect the metal layer, by identifying a statistically significant enhancement of the neutral metal Na layer above thunderstorms at Haikou, China (20.0°N, 110.3°E). The thunderstorm‐generated gravity waves and electric field effects could be the mechanisms responsible for the lightning‐associated enhancement of Na layer.

Ozone climatology and variability over Irene, South Africa determined by ground based and satellite observations. Part 1: Vertical variations in the troposphere and stratosphere

In this investigation, the climatological characteristics of ozone over Irene (25.5o S, 28.1o E) are studied with the aid of both ground-based and satellite observations. The principle aim of this work is therefore to examine the variability of vertical ozone distribution over Irene. Ozonesonde measurements and satellite measurements in the mesosphere-lower thermosphere region from the Microwave Limb Sounder (MLS) onboard the Aura satellite and it measures for the period 2004-2007 are employed in this study. The MLS satellite measurements are collected when it overpasses Irene. The vertical ozone profiles are obtained from ozonesondes and the MLS and are found to be in good agreement from a 15 km altitude and above. Maximum ozone concentration is found in the altitude between 23-27 km. The monthly variation of ozone concentration measured by ozonesondes and MLS show maximum and minimum concentrations during spring and autumn, respectively. During spring, the maximum ozone concentration measured by ozonesonde is 4.5 × 10 12 molecules cm –3 while that measured by the MLS instrument is ~4.1 × 10 12 molecules cm –3 . A difference in the range of 0.4 × 10 12 to 0.8 × 10 12 molecules cm –3 was obtained for all seasons except during winter, when the difference was in the range 0.6 × 10 12 to 0.9 × 10 12 molecules cm –3 .

Ozone measurements with meteors: a revisit

Understanding the role of ozone in the Mesosphere/Lower Thermosphere (MLT) region is essential for understanding the atmospheric processes in the upper atmosphere. Earlier studies have shown that it is possible to use overdense meteor trails to measure ozone concentration in the meteor region. Here we revisit this topic by comparing a compilation of radar observations to satellite measurements. We observe a modest agreement between the values derived from these two methods, which confirm the usefulness of the meteor trail technique for measuring ozone content at certain heights in the MLT region. Future simultaneous measurements will help quantifying the performance of this technique.

Spatial and temporal variability in MLT turbulence inferred from in situ and ground-based observations during the WADIS-1 sounding rocket campaign

Abstract. In summer 2013 the WADIS-1 sounding rocket campaign was conducted at the Andøya Space Center (ACS) in northern Norway (69° N, 16° E). Among other things, it addressed the question of the variability in mesosphere/lower thermosphere (MLT) turbulence, both in time and space. A unique feature of the WADIS project was multi-point turbulence sounding applying different measurement techniques including rocket-borne ionization gauges, VHF MAARSY radar, and VHF EISCAT radar near Tromsø. This allowed for horizontal variability to be observed in the turbulence field in the MLT at scales from a few to 100 km. We found that the turbulence dissipation rate, ε varied in space in a wavelike manner both horizontally and in the vertical direction. This wavelike modulation reveals the same vertical wavelengths as those seen in gravity waves. We also found that the vertical mean value of radar observations of ε agrees reasonably with rocket-borne measurements. In this way defined 〈εradar〉 value reveals clear tidal modulation and results in variation by up to 2 orders of magnitude with periods of 24 h. The 〈εradar〉 value also shows 12 h and shorter (1 to a few hours) modulations resulting in one decade of variation in 〈εradar〉 magnitude. The 24 h modulation appeared to be in phase with tidal change of horizontal wind observed by SAURA-MF radar. Such wavelike and, in particular, tidal modulation of the turbulence dissipation field in the MLT region inferred from our analysis is a new finding of this work.

Measuring FeO variation using astronomical spectroscopic observations

Abstract. Airglow emission lines of OH, O2, O and Na are commonly used to probe the MLT (mesosphere–lower thermosphere) region of the atmosphere. Furthermore, molecules like electronically excited NO, NiO and FeO emit a (pseudo-) continuum. These continua are harder to investigate than atomic emission lines. So far, limb-sounding from space and a small number of ground-based low-to-medium resolution spectra have been used to measure FeO emission in the MLT. In this study the medium-to-high resolution echelle spectrograph X-shooter at the Very Large Telescope (VLT) in the Chilean Atacama Desert (24°37′ S, 70°24′ W; 2635 m) is used to study the FeO pseudo-continuum in the range from 0.5 to 0.72 µm based on 3662 spectra. Variations of the FeO spectrum itself, as well as the diurnal and seasonal behaviour of the FeO and Na emission intensities, are reported. These airglow emissions are linked by their common origin, meteoric ablation, and they share O3 as a common reactant. Major differences are found in the main emission peak of the FeO airglow spectrum between 0.58 and 0.61 µm, compared with a theoretical spectrum. The FeO and Na airglow intensities exhibit a similar nocturnal variation and a semi-annual seasonal variation with equinoctial maxima. This is satisfactorily reproduced by a whole atmosphere chemistry climate model, if the quantum yields for the reactions of Fe and Na with O3 are 13 ± 3 and 11 ± 2 % respectively. However, a comparison between the modelled O3 in the upper mesosphere and measurements of O3 made with the SABER satellite instrument suggests that these quantum yields may be a factor of ∼ 2 smaller.

Modelling the descent of nitric oxide during the elevated stratopause event of January 2013

Using simulations with a whole-atmosphere chemistry-climate model nudged by meteorological analyses, global satellite observations of nitrogen oxide (NO) and water vapour by the Sub-Millimetre Radiometer instrument (SMR), of temperature by the Microwave Limb Sounder (MLS), as well as local radar observations, this study examines the recent major stratospheric sudden warming accompanied by an elevated stratopause event (ESE) that occurred in January 2013. We examine dynamical processes during the ESE, including the role of planetary wave, gravity wave and tidal forcing on the initiation of the descent in the mesosphere-lower thermosphere (MLT) and its continuation throughout the mesosphere and stratosphere, as well as the impact of model eddy diffusion. We analyse the transport of NO and find the model underestimates the large descent of NO compared to SMR observations. We demonstrate that the discrepancy arises abruptly in the MLT region at a time when the resolved wave forcing and the planetary wave activity increase, just before the elevated stratopause reforms. The discrepancy persists despite doubling the model eddy diffusion. While the simulations reproduce an enhancement of the semi-diurnal tide following the onset of the 2013 SSW, corroborating new meteor radar observations at high northern latitudes over Trondheim (63.4°N), the modelled tidal contribution to the forcing of the mean meridional circulation and to the descent is a small portion of the resolved wave forcing, and lags it by about ten days.

Sensitivity of equatorial atomic oxygen in the MLT region to the 11-year and 27-day solar cycles

We report on 27-day and 11-year solar cycle signatures in atomic oxygen (O) concentrations ([O]) in the MLT (Mesosphere/Lower Thermosphere) region of the terrestrial atmosphere. MLT [O] profiles were retrieved on the base of green line (557.7nm) nightglow data sets provided by the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) onboard Envisat from 2002 to 2012. A statistically significant solar 27-day signature was identified (and then quantified with respect to the sensitivity and phase relationship to solar forcing) in time series of MLT [O] profiles with use of cross-correlation and superposed epoch analysis techniques. It was the first identification of the solar 27-day signature in MLT atomic oxygen on the base of such experimental data sets. The sensitivity of [O] to solar cycle variability at the 11-year time scale was quantified with use of cross-correlation and multiple-linear regression analysis techniques, which yield results consistent with known studies and, particularly, indicate that the sensitivity of [O] to solar forcing increases with increasing altitude. A comparison of obtained values of atomic oxygen sensitivity in response to solar forcing at the 27-day and 11-year time scales reveals the fact that the sensitivities agree well to each other within their uncertainties during the descending phase of the last (23rd) 11-year cycle of solar activity, whereas the [O] sensitivity values at the 27-day time scale during the last solar minimum phase were lower than those ones during the descending phase. It was also determined that atomic oxygen is in-phase with the solar forcing (in agreement with model results) at the 11-year time scale, whereas the time lag of the 27-day signature in response to solar forcing was about 12 – 14 days.

On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

The ionospheric D-region (~60 km up to ~95 km) and the corresponding neutral atmosphere, often referred to as the mesosphere–lower thermosphere (MLT), are challenging and costly to probe in situ. Therefore, remote sensing techniques have been developed over the years. One of these is based on very low frequency (VLF, 3–30 kHz) electromagnetic waves generated by various natural and man-made sources. VLF waves propagate within the Earth–ionosphere waveguide and are extremely sensitive to perturbations occurring in the D-region along their propagation path. Hence, measurements of these signals serve as an inexpensive remote sensing technique for probing the lower ionosphere and the MLT region. This paper reviews the use of VLF narrowband (NB) signals (generated by man-made transmitters) in the study of the D-region and the MLT for over 90 years. The fields of research span time scales from microseconds to decadal variability and incorporate lightning-induced short-term perturbations; extraterrestrial radiation bursts; energetic particle precipitation events; solar eclipses; lower atmospheric waves penetrating into the D-region; sudden stratospheric warming events; the annual oscillation; the solar cycle; and, finally, the potential use of VLF NB measurements as an anthropogenic climate change monitoring technique.

First ground-based observations of mesopause temperatures above the Eastern-Mediterranean Part I: Multi-day oscillations and tides

The mesopause region (~90km altitude) is the coldest region of our atmosphere, and is found at the boundary between the upper mesosphere and lower thermosphere. Ground-based spectrometers, which are sensitive to the emissions from the hydroxyl (OH*) airglow layer (lying at ~87km altitude), are used to monitor the temperature variability within the mesosphere-lower-thermosphere (MLT), at high temporal resolution. The variability of the MLT region of the atmosphere is driven by momentum deposition from gravity waves, atmospheric tides and planetary waves. The displacement of air caused by these waves can produce strong temperature, wind and species concentration perturbations.In this study we present an analysis of 4-years of OH* rotational temperature data, acquired with the German Aerospace Center (DLR) GRIPS-10 (Ground Based Infrared P-branch Spectrometer) instrument, which was installed in Israel in November 2011. This instrument provided the first long-term ground-based observations of airglow emissions in the Eastern Mediterranean. We show the nocturnal mean temperature analysis, which includes time series as well as spectral analysis of the data. In addition, we obtain (migrating) tidal oscillation estimates from the high resolution (1min) data, by using harmonic fitting, and we analyze the variability of planetary wave signatures in the residual temperature data, which are retrieved after the removal of the tidal harmonic fits from the data. In this analysis of the residual data we find a dominant quasi-5–7 day planetary wave influence on the mesopause temperatures above the Eastern Mediterranean.

Global normal mode planetary wave activity: a study using TIMED/SABER observations from the stratosphere to the mesosphere-lower thermosphere

A comprehensive study of three normal mode travelling planetary waves, namely the quasi-16, -10 and -5 day waves, is carried out globally using 5 years (2003–2007) of TIMED/SABER temperature measurements from the stratosphere to the mesosphere-lower thermosphere (MLT) by employing the two dimensional Fourier decomposition technique. From preliminary analysis, it is found that significant amplitudes of normal modes are confined to wave numbers-2 (westward propagating modes) to 2 (eastward propagating modes). The westward propagating quasi 16-day waves with zonal wave number 1 (W1; W1 refers to westward propagating wave with zonal wave number 1) peaks over winter-hemispheric high latitudes with northern hemisphere (NH) having higher amplitudes as compared to their southern hemispheric (SH) counterpart. The W1 quasi 16-day waves exhibit a double peak structure in altitude over winter hemispheric high latitudes. The eastward propagating quasi 16-day waves with wave number 1 (E1; E1 refers to eastward propagating wave with zonal wave number 1) exhibits similar features as that of W1 waves in the NH. In contrast, the E1 quasi 16-day waves in the SH show larger amplitudes as compared to the W1 waves and they do not exhibit double peak structure in altitude. Similar to the quasi 16-day waves, the quasi 10- and 5-day wave amplitudes with respect to their wavenumbers are delineated. Unlike quasi-16 and -10 day waves, quasi-5 day waves peak during vernal equinox both in the SH and NH. The peak activity of the W1 quasi-5 day wave is centered around 40°N and 40°S exhibiting symmetry with respect to the equator. A detailed discussion on the height-latitude structure, interannual variability and inter-hemispheric propagation of quasi 16-, 10- and 5-day waves are discussed. The significance of the present study lies in establishing the 5-year climatology of normal mode planetary waves from the stratosphere to the MLT region including their spatial–temporal evolution, which are very important from the middle atmospheric dynamics standpoint.

Lidar observations of gravity wave activity in the middle atmosphere over Davis (69°S, 78°E), Antarctica

AbstractA 16 month series of lidar measurements in the stratosphere and mesosphere‐lower thermosphere (MLT) region over Davis Station (69°S, 78°E) in Antarctica is used to study gravity waves. The unprecedentedly large number of observations totaling 2310 h allows us to investigate seasonal variations in gravity wave activity in great detail. In the stratosphere the gravity wave potential energy density (GWPED) is shown to have a large seasonal variation with a double peak in winter and minimum in summer. We find conservative wave propagation to occur between 29 and 41 km altitude in winter as well as in summer, whereas smaller energy growth rates were observed in spring and autumn. These results are consistent with selective critical‐level filtering of gravity waves in the lower stratosphere. In the MLT region the GWPED is found to have a semiannual oscillation with maxima in winter and summer. The structure of the winter peak is identical to that in the stratosphere, suggesting that the gravity wave flux reaching the MLT region is controlled by the wind field near the tropopause level.

Gravity wave momentum fluxes from MF and meteor radar measurements in the polar MLT region

AbstractAnnual cycles of horizontal winds and gravity wave (GW) momentum fluxes in the mesosphere/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. Four 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 colocated 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 provide 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.

Does sudden stratospheric warming induce meridional circulation in the mesosphere thermosphere system?

AbstractOxygen dayglow emissions at multiple wavelengths that emanate from different heights (from around 130 km to peak altitude of the ionospheric F region) over a low‐latitude location showed systematic enhancements in intensities throughout the daytime hours during four sudden stratospheric warming (SSW) events that occurred in the years 2010–2013. The lower thermospheric temperatures at 120 km obtained from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument are found to be enhanced during SSW events at arctic latitudes and show a gradient with a decrease toward low‐latitudes. During these events, the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics Doppler Interferometer measurements showed equatorward winds in the mesosphere lower thermosphere (MLT) altitudes over high‐latitudes. Both the high‐latitude lower thermospheric temperature enhancements and the MLT region equatorward winds occur nearly simultaneously with the observed enhancements in the atomic oxygen dayglow emission intensities at all the wavelengths over low‐latitudes. Based on these measurements and other supporting information, it is proposed that a new cell of meridional circulation in the MLT winds is set up during SSW events, which enables transport of atomic oxygen from high‐to‐low latitudes. Such an additional contribution of oxygen density over low‐latitudes interacts with lower thermospheric daytime dynamics in that region and is attributed to be the cause for the observed enhancement in the oxygen daytime optical emission intensities over low‐latitudes. The results presented here provide experimental evidence to such circulation alluded to by earlier simulation studies.

The year-to-year variability of the autumn transition dates in the mesosphere/lower thermosphere wind regime and its coupling with the dynamics of the stratosphere and troposphere

The behaviour of the zonal prevailing wind in the autumn mesosphere/lower thermosphere (MLT) and its correlations with dynamical processes in the lower atmosphere is considered in terms of the autumn transition date (ATD) variability. The ATD is defined as the day when the rate of zonal wind decrease in the MLT region reaches its maximum in September–October. The method is applied to MLT radar wind data obtained at Collm (52°N, 15°E) and Obninsk (55°N, 37°E) during 1979–2007 and 1979–2011, respectively. The interannual variability of the ATDs depends on the direction of the stratospheric equatorial zonal winds at 30hPa, i.e. the phase of the equatorial quasi-biennial oscillation (QBO). There is also a statistically significant correlation between the ATDs and the Northern Annular Mode (NAM) index taken at upper tropospheric/lower stratospheric heights during the westerly phase of the QBO. Numerical simulations with a mechanistic global circulation model demonstrate that high NAM indices correspond to later ATD and that the ATD, defined from the zonal wind characteristics, depends on the gravity wave flux.

Daytime wave characteristics in the mesosphere lower thermosphere region: Results from the Balloon‐borne Investigations of Regional‐atmospheric Dynamics experiment

AbstractResults obtained from a joint INDO‐US experiment on the investigations of mesosphere/lower thermosphere wave dynamics using balloon‐borne optical dayglow measurements in combination with ground‐based optical, radio, and magnetometer data are presented. Ultraviolet OI 297.2 nm dayglow emissions that originate at ~ 120 km were measured from low‐magnetic latitudes from onboard a balloon on 8 March 2010. This paper describes the details of a new spectrograph that is capable of making high spectral resolution (0.2 nm at 297.2 nm) and large (80°) field of view ultraviolet dayglow emission measurements and presents the first results obtained from its operation onboard a high‐altitude balloon. Waves of scale sizes ranging from 40 to 80 km in the zonal direction were observed in OI 297.2 nm emissions. Meridional scale sizes of similar waves were found to be 200 km as observed in the OI 557.7 nm emissions that originate from ~ 100 km. Periodicities were also derived from the variations of equatorial electrojet strength and ionospheric height on that day. Common periodicities of waves (in optical, magnetic, and radio measurements) were in the range of 16 to 30 min, which result in intrinsic horizontal wave speeds in the range of 21 to 77 m s−1. It is argued that gravity waves of such scale sizes and speeds at these heights are capable of propagating well into the thermosphere because the background wind directions were favorable. These waves were potentially capable of forming the seeds for the generation of equatorial plasma irregularities which did occur on that night.

Simulations of gravity wave‐induced variations of the OH(8,3), O2(0,1), and O(1S) airglow emissions in the MLT region

AbstractWe investigate gravity wave‐induced airglow intensity variations of the OH(8,3), O2(0,1) atmospheric band, and O(1S) greenline emissions in the mesosphere/lower thermosphere (MLT) region with two two‐dimensional, time‐dependent, nonlinear models—an OH Chemistry‐Dynamics model and a Multiple Airglow Chemistry‐Dynamics model. Our simulation results of the wave effects by a small‐scale 30 km wave packet show that it induces an ~22% secular increase in the OH(8,3) airglow intensity, a 30% increase in the O2(0,1) atmospheric band, and a 33% increase in the O(1S) greenline. The largest wave‐induced airglow intensity fluctuation amplitude is seen in the OH(8,3) emission (~2.4%), followed by the O2(0,1) atmospheric band and O(1S) greenline with comparable maximum wave‐induced airglow intensity fluctuation amplitudes of 1%. Our study also shows that the production of atmospheric bands and O(1S) strongly depends on the rate coefficients in the three‐body recombination reactions.

Ionospheric response to the ultrafast Kelvin wave in the MLT region

AbstractThe modulation of the ultrafast Kelvin wave (UFKW) on the equatorial ionosphere is investigated, using satellite neutral wind and temperature observations in the mesosphere/lower thermosphere (MLT) and ground‐based global maps of total electron content (TEC) observations. The UFKW signatures are identified in the least squares fitting spectra for MLT zonal wind, temperature, TEC, and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) electron density profiles, providing strong evidence for the neutral‐ion coupling through the UFKW. The periods of the UFKW are mostly confined within 2–5.5 days. The UFKW in zonal wind and temperature maximize in the equatorial regions, whereas the UFKW band‐pass perturbations in electron density peak at the equatorial ionization anomaly (EIA) crests with minima at the equator. The UFKW band‐pass perturbations in TEC show consistent seasonal variability with the UFKW in the MLT region, including the intraseasonal oscillations with periods of 20–70 days. The long‐term variability of the absolute UFKW band‐pass amplitudes in TEC is in‐phase with solar cycle with maximum amplitude of ~7–8 (~1–2) total electron content unit (TECU; 1 TECU = 1016 el m‐2) during solar maximum (minimum) years, while the relative amplitudes show little solar activity dependence. The COSMIC electron density profiles show that the UFKW band‐pass perturbations maximize at ~300–400 km with fountain‐like spatial distributions, indicating the modulation of the UFKW on the EIA through electrodynamics.

Studies on planetary waves and tide interaction in the mesosphere/lower thermosphere region using meteor RADAR data from Rothera (68°S, 68°W), Antarctica

In this study, we report the interaction of planetary waves and tides observed in the mesosphere/lower thermosphere (MLT) region using a meteor radar located at Rothera (68°S, 68°W) Antarctica. The study has been conducted using wind data collected over the year 2005. The obtained data are subjected to wavelet analysis to determine the spectral content of the data and it was found that the MLT region is dominated by semidiurnal tide (SDT) and planetary waves with periods around 5, 10, 16 and 23 days. The behaviour of the semidiurnal tide was investigated and found to be very variable and modulated at the period of ∼23 days in the zonal component and at ∼5 and 16 days in meridional component. Non-linear interaction between the SDT and 16-day planetary wave in the meridional component was found to be mostly responsible for the variability of the SDT than the interaction between the SDT and 5 days in the meridional component and between the SDT and 23-day planetary wave in the zonal component. This was established by investigating the validity of frequency, phase and wavenumber relationship.

Composite analysis of the temporal development of waves in the polar MLT region during stratospheric warmings

During winter the wind field in the mesosphere/lower thermosphere (MLT) at middle and polar latitudes is characterized by a strong variability due to enhanced planetary wave activity and related stratospheric sudden warming (SSW) events. Such events are considered as distinct vertical coupling processes influencing the atmosphere below and above the stratosphere. In the last 12 years, an enhanced number of SSW, compared to the period from 1989 to 1998, has been observed in the northern hemisphere. Every SSW is connected with different effects in the MLT (strength and temporal development of wind reversals, temperature changes, wave activity, longitudinal dependence). To characterize the average behavior of the mesospheric response to strong SSWs, we combine high-resolution wind measurements from MF- and meteor radar at Andenes (69°N, 16°E) with global temperature observations from MLS aboard the Aura satellite for SSW events with a return to the middle atmosphere normal winter condition afterwards. Our aim is to identify characteristic wave patterns which are common to the majority of these events and to define the average characteristics of the SSW-related wave activity in the MLT. These will be compared to the relatively quiet winter 2011 with only a short minor warming without a wind reversal and to the wave activity in 2009 and 2010. The results show clear signatures of enhanced mesospheric planetary wave activity before and during the SSW and an earlier onset of the short term wind reversal in the mesosphere compared to wind and temperature changes in the stratosphere. The strong eastward winds at altitudes below 80km after SSW are connected with an enhanced gravity wave activity caused by changed filter conditions. This provides evidence for a strong modulation of semidiurnal tidal amplitudes before and during SSW by planetary waves. However, no clear relation has been found in the temporal development of tides relative to the onset of the selected SSW events.