Mesospheric Wind Observations Research Articles

Day-to-day variability of the equatorial ionosphere in Asian sector during August–October 2019

This brief report examines the ground-based total electron content (TEC) in Asian sector during August–October 2019, covering the period of a stratosphere sudden warming (SSW) occurred in Antarctica. The analysis reveals pronounced ionospheric day-to-day variability with distinct periodicities. The most dominant and long-lasting periodicities are quasi-10 days and quasi 14-day during September and October, while a quasi 6-day also present in September. The 10-day and 6-day TEC oscillations were attributed by previous studies solely to the Antarctic SSW while assuming negligible geomagnetic effects. By comparing co-located ground mesospheric wind observations, along with the interplanetary electric field (IEF) and geomagnetic activity (Kp index), we demonstrate that the quasi 14-day oscillation is mainly driven by low-level geomagnetic activities, while quasi-6 days oscillation is driven by mesospheric wind changes during the SSW. The 10-day oscillation, on the other hand, is driven by both IEF and mesospheric wind in September, but by IEF in October. These results demonstrate that low-level geomagnetic activities traditionally classified as “quiet conditions” can induce significant day-to-day oscillations in TEC, and their impacts should not be ignored when studying meteorological (e.g., SSWs) impacts on the ionosphere.

Seasonal variability of atmospheric tides in the mesosphere and lower thermosphere: meteor radar data and simulations

Abstract. Thermal tides play an important role in the global atmospheric dynamics and provide a key mechanism for the forcing of thermosphere–ionosphere dynamics from below. A method for extracting tidal contributions, based on the adaptive filtering, is applied to analyse multi-year observations of mesospheric winds from ground-based meteor radars located in northern Germany and Norway. The observed seasonal variability of tides is compared to simulations with the Kühlungsborn Mechanistic Circulation Model (KMCM). It is demonstrated that the model provides reasonable representation of the tidal amplitudes, though substantial differences from observations are also noticed. The limitations of applying a conventionally coarse-resolution model in combination with parametrisation of gravity waves are discussed. The work is aimed towards the development of an ionospheric model driven by the dynamics of the KMCM.

Mesospheric wind disturbances due to gravity waves near the Antarctica Peninsula

Based on austral winter mesospheric wind observations from three closely deployed Antarctic Peninsula stations (King George Island, Palmer, and Rothera), mesospheric wind disturbances induced by gravity waves are examined. The mesospheric winds to the west of the Antarctic Peninsula below 88 km were affected by gravity waves, while the winds on the east side of the peninsula were unperturbed. The gravity waves are most likely generated by orographic features of the peninsula. Because the strong westerly stratospheric wind filtered out all eastward propagating waves, only westward propagating waves can reach the mesosphere on the west side of the peninsula. This data set shows the strong gravity wave effect on small scales. Hence, the mesospheric wind data at one station may not be representative of the region for global waves like tides. Small local features can greatly affect the mesospheric winds and may impact the interpretation of global waves. High‐density deployment of mesospheric wind instruments may be needed in some cases.

Noctilucent cloud variability and mean parameters from 15 years of lidar observations at a mid‐latitude site (54°N, 12°E)

Noctilucent clouds (NLC) are an important tracer of temperature and dynamics of the summer mesopause region. Our site at Kühlungsborn (Germany, 54°N) is at the equatorward edge of the NLC region and therefore of special interest for the understanding of these clouds. 41 nights (63 h) of NLC are observed since 1997. They form the largest lidar data set from mid‐latitudes. NLC are typically weak, with nearly 70% having a backscatter coefficient βmax,532nm<2 ⋅ 10−10 m−1 sr−1. The seasonal variation of NLC shows maximum occurrence around the temperature minimum (saturation maximum) but lower temperatures (higher saturation) at the beginning compared to the end of the season. Mean centroid altitude is 82.7 ± 0.03 km, with strong NLC being typically lower and vertically thinner compared to weak clouds. NLC occurrence was lowest in the years 2000–2002 and reached a maximum in 2009 with a rate of 19%. Overall, NLC are less frequent and dimmer compared to higher latitudes. The occurrence is highly anti‐correlated with solar activity. Beside NLC, we are measuring mesospheric temperatures since 2002 by lidars, complemented by microwave observations of water vapor (since October 2009) and radar observations of mesospheric winds. NLC occurrence is found anti‐correlated with ambient temperatures (r = −0.85 at 84 km), while low temperatures are necessary but not sufficient for individual events. Meridional winds at 84 km are weakly anti‐correlated with NLC occurrence (r = −0.58 at 84 km). Furthermore, we find some biennial variation of NLC occurrence in part of the time series. Any additional trend has not yet been detected.

Lunar and solar tidal variabilities in mesospheric winds and EEJ strength over Tirunelveli (8.7°N, 77.8°E) during the 2009 major stratospheric warming

Mesospheric wind observations by the medium frequency radar and geomagnetic field observations at Tirunelveli (8.7°N, 77.8°E, 1.75°N dip angle) are used to study the relative importance of solar and lunar influences in the variabilities of mesospheric tides and equatorial electrojet (EEJ) strength during the unprecedented major stratospheric sudden warming (SSW) of 2009. It is observed that the afternoon reversal in the EEJ, popularly known as counter electrojet, occurs consecutively for several days during the SSW event, when there is an enhancement of solar semidiurnal tide in both zonal wind at 90 km and EEJ strength over Tirunelveli. Although the amplitude of lunar tides also shows enhancement, it is much less than that of solar. The diurnal tidal amplitude in zonal wind and EEJ strength also shows large enhancement just before the onset of SSW. However, solar semidiurnal tide dominates diurnal tide during the SSW. The diurnal tidal phase in zonal wind shifts to a few hours earlier during the SSW. The lunar semidiurnal tidal phase shifts to later hours in both zonal wind and EEJ strength. The main observation of the present study is that the large semidiurnal tide observed during the SSW 2009 is mostly solar driven and only partly lunar driven, although tidal planetary wave interaction also may play a vital role. Although a similar behavior is noticed during the SSW 2006 also, the large lunar semidiurnal tide observed in the EEJ strength without having large lunar semidiurnal tide in the underlying mesospheric winds needs further investigation.

MAARSY – the new MST radar on Andøya: first results of spaced antenna and Doppler measurements of atmospheric winds in the troposphere and mesosphere using a partial array

Abstract. MST radars have been used to study the troposphere, stratosphere and mesosphere over decades. These radars have proven to be a valuable tool to investigate atmospheric dynamics. MAARSY, the new MST radar at the island of Andøya uses a phased array antenna and is able to perform spaced antenna and Doppler measurements at the same time with high temporal and spatial resolution. Here we present first wind observations using the initial expansion stage during summer 2010. The tropospheric spaced antenna and Doppler beam swinging experiments are compared to radiosonde measurements, which were launched at the nearby Andøya Rocket Range (ARR). The mesospheric wind observations are evaluated versus common volume meteor radar wind measurements. The beam steering capabilities of MAARSY are demonstrated by performing systematic scans of polar mesospheric summer echoes (PMSE) using 25 and 91 beam directions. These wind observations permit to evaluate the new radar against independent measurements from radiosondes and meteor radar measurements to demonstrate its capabilities to provide reliable wind data from the troposphere up to the mesosphere.

Global distribution, seasonal, and inter-annual variations of mesospheric semidiurnal tide observed by TIMED TIDI

Based on TIDI mesospheric wind observations, we analyzed the semidiurnal tide westward zonal wavenumber 1 and 2 (SW1 and SW2) component seasonal, inter-annual variations, and possible sudden stratospheric warming (SSW) related changes. Major findings are as follows: (1) The SW1 has a peak near the South Pole during the December solstice and near the North Pole during the March equinox. (2) The SW2 peaks at 60S and 60N mostly during winter solstices. The SW2 also peaks during late summer and early fall in the northern hemisphere. (3) The QBO effect on the semidiurnal tide is much weaker than that on the diurnal tide. The March equinox northern SW1 zonal amplitude appears to be stronger during the westward phase of the QBO, which is opposite of migrating diurnal tide QBO response. (4) Possible SSW event related changes in the semidiurnal tide are significant but not always consistent. Enhancements in the mid-latitude SW2 component during SSWs are observed, which may be related to the increase of total ozone at mid and high latitudes during SSW events. TIDI observations also show a decrease in the SW2 in the opposite hemisphere during a southern SSW event in 2002. Small increases in the high latitude SW1 in both hemispheres during the 2002 southern SSW event were recorded.

Trends of mesospheric gravity waves at northern middle latitudes during summer

[1] Recent investigations of the seasonal variation of the activity of gravity waves in the mesosphere/lower thermosphere (MLT) at middle and high latitudes suggest a semiannual variation with maxima during winter and summer and minima during the equinoxes. It is generally assumed that this annual cycle is determined by filtering processes due to the background winds in the stratosphere and lower mesosphere. On the other side, long-term observations of mesospheric winds at Juliusruh (55°N, 13°E) since 1990 indicate a stable increase of westward directed winds below 80 km (negative trends) during summer, as, e.g., clearly evident in monthly means in July. Here, we are studying how these long-term changes of winds are related to trends of the activity of gravity waves (GW) with periods between 3–6 hours. Our results show that the observed zonal wind trend at about 75 km during July goes along with an enhancement of the GW activity at altitudes above 80 km. Indeed, also the year-to-year variation of maxima of the observed westward directed winds at altitudes near 75 km and the GW activity at about 80 km are significantly correlated. Our results stimulate the further study of long-term wind changes and corresponding gravity wave trends.

Validation of imaging Doppler interferometer winds using meteor radar

There has been some debate over the years concerning the accuracy of mesospheric wind observations made using the imaging Doppler interferometer (IDI) technique. The high potential and increasing use of IDI wind data in joint studies with spaced‐antenna MF and meteor radar systems make it important to quantify the IDI results. This paper presents a novel comparison of wind measurements between a dynasonde implementation of IDI and winds derived from an all‐sky meteor radar system, a widely‐accepted standard for such measurements. Both radars were located at the USU Bear Lake Observatory and operated almost continuously for a four‐month period. The winds and tides derived from IDI were found to closely match those measured by meteor radar, not only during the day but also at night, and at all overlapping heights from 80–95 km.

Signatures of quasi-2-day planetary waves in the equatorial electrojet: results from simultaneous observations of mesospheric winds and geomagnetic field variations at low latitudes

Observations of mesospheric winds in the altitude region 84–98 km with the medium frequency (MF) radar located close to the magnetic equator at Tirunelveli ( 8.7° N, 77.8° E geographic, 0.35°N magnetic dip), India, and the ground-based geomagnetic data from the low latitude Indian sector, are used to identify the signatures of the quasi-2-day (QTD) planetary waves in the equatorial electrojet (EEJ). It is found that there is a reasonable correlation between the occurrence times of the quasi-2-day oscillation in the EEJ strength and the mesospheric winds as revealed by the observations.

Initial observations of mesospheric winds using IDI Radar measurements at the Bear Lake Observatory

The adaptation of Imaging Doppler Interferometry (IDI) to the dynasonde deployed at the Bear Lake Observatory (41.9°N, 111.4°W) in northeastern Utah enables the routine measurement of mesospheric dynamics. Influenced by the direction of the prevailing wind, a clear seasonal variation is seen in the mesospheric echo numbers. The technique also provides a monitor for the amplitude of the main tidal modes as well as for studying planetary waves with periods of several days. The observations are found to be consistent with those from different instruments based at the same site thus confirming the IDI characterisation of the mesospheric wind field.

Planetary scale and tidal perturbations in mesospheric temperature observed by WINDII

WINDII, the Wind Imaging Interferometer on the Upper Atmospheric Research Satellite measures winds and emission rates from selected excited metastable species. Here we report on measurements of the atmospheric Rayleigh scattering from the O(1S) background filter at 553 nm wavelength used to derive temperature profiles in the upper mesosphere from 70 km to 95 km, for solstice periods from December 1992/93 and January 1993/94. The data are first zonally averaged and then combined in local time over about one month. Based on these temperatures, an analysis of planetary wave structures and tidal perturbations employing least-mean-square (LMS) fits to the data has been conducted and the results are presented. The planetary wave structures observed were well described with a quasi two-day wave (QTDW). Amplitudes of 14 K and 10 K at 85 km height for downleg (descending) and upleg (ascending) sampling respectively at latitudes from 20°S to 40°S were found to be in good agreement with QTDW temperature results from the MLS/UARS experiment assuming a vertical amplitude structure of the type described by the HRDI/UARS mesospheric wind observations. It is shown that the diurnal tide amplitudes estimated from latitudes from 25°N to 35°S using the LMS fit maximize at the equator with an amplitude of about 6 K and decrease toward tropical latitudes, consistent with the classical tidal theory and predictions from the TIME-GCM model.

Mesospheric mean winds and tides observed by the Imaging Doppler Interferometer (IDI) at Halley, Antarctica

This paper describes the first ever mesospheric wind observations from Halley, Antarctica, over a full year. The recent implementation of an Imaging Doppler Interferometer at Halley is providing a new, high quality and continuous dataset to investigate the dynamics of the Antarctic mesosphere. The mean winds show clear seasonal variations, with reversals in both zonal and meridional components near the equinoxes. The dominant tidal modes have periods of 12 h and 24 h but with significant variations in amplitude during the year. Waves with longer periods are also apparent at certain times of year. The seasonal variations and amplitudes of the winds and tides are compared with other high-latitude sites in the southern and northern hemispheres. It is found that the overall pattern of winds at Halley is broadly similar to that seen at similar geographic latitudes, but with noticeable differences which may be related to it being a southern hemisphere site.

Mesospheric wind observations derived from Super Dual Auroral Radar Network (SuperDARN) HF radar meteor echoes at Halley, Antarctica: Preliminary results

The implementation of a technique to derive mesospheric winds from HF radar observations of meteor echoes at Halley (76°S, 27°W), Antarctica, is described. The meteor echoes are observed at near ranges (less than 400 km) and have characteristics distinctly different from echoes backscattered from plasma irregularities in the E and F regions of the ionosphere. A Lorentzian model is used to fit the echo spectrum. The echo occurrence rate has the diurnal variation expected of meteors with a minimum in the afternoon. There also appears to be an annual variation which may be related to seasonal changes in the atmosphere. At present, results are confined to a single beam; that directed toward the south geographic pole is presented here, as this will give the meridional component of the wind and can be compared with other studies. The meridional wind is found to be dominated by the semidiurnal tide most of the year, with maxima in spring and autumn. Data for May 1996 show that the semidiurnal tide is dominant, but there are significant contributions from the 24‐hour and 8‐hour tides. A moving window spectral analysis technique allows the temporal behavior of the waves over a 10‐day period to be studied. A quasi 2‐day wave is observed during this interval, and slight changes of period with time can be observed. Planetary waves with periods of 5 and 10 days are observed during the winter of 1996. The radar has been operating since 1988, and so these data form a substantial and valuable database of mesospheric observations in the high‐latitude southern hemisphere.

Seasonal variabilities of low-latitude mesospheric winds

Abstract. Observations of mesospheric winds over a period of four years with the partial reflection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70–98 km. The meridional winds exhibit overall transequatorial flow associated with differential heating in the Northern and Southern Hemispheres. At lower altitudes (70–80 km) the mean zonal winds reveal easterly flow during summer and westerly flow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80–98 km) and at all altitudes during equinox periods, the mean zonal flow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at specific harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation.Key words. Meteorological and atmospheric dynamics · General circulation · Middle atmosphere dynamics · waves and tides.

Seasonal variabilities of low-latitude mesospheric winds

Observations of mesospheric winds over a period of four years with the partial reflection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70–98 km. The meridional winds exhibit overall transequatorial flow associated with differential heating in the Northern and Southern Hemispheres. At lower altitudes (70–80 km) the mean zonal winds reveal easterly flow during summer and westerly flow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80–98 km) and at all altitudes during equinox periods, the mean zonal flow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at specific harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation.Key words. Meteorological and atmospheric dynamics · General circulation · Middle atmosphere dynamics · waves and tides.

Dynamics of the equatorial mesosphere: First results with a new generation partial reflection radar

The first observations of mesospheric winds made between January–August 1990 with an MF partial reflection radar located on Christmas Island (2°N, 157°W) in the central Pacific are described. The mean zonal winds are in general westward, but show clear evidence for a wave‐driven circulation. Power spectral studies indicate that waves are present over a wide range of periods. Ultra‐fast Kelvin waves are especially evident in January–March, with peak amplitudes ∼20 ms−1, and intrinsic phase speeds of ∼150 ms−1 indicated. The Kelvin waves are estimated to contribute an eastward acceleration of up to 10 ms−1 day−1. Gravity wave amplitudes are also found to be almost as large as those observed at mid‐latitude sites, which suggests that convection is a major source of gravity wave activity.

Tides and gravity waves in the mesosphere at mid- and low-altitudes

Observations of mesospheric winds made at Adelaide (35°S) are discussed and compared with wind measurements made at Jicamarca (12°S). In contrast to theoretical predictions it appears that the amplitudes of the diurnal tide at Adelaide are appreciably larger than the corresponding amplitudes observed at Jicamarca. Gravity wave motions are also evident at Adelaide but these motions are similar in magnitude and period to the wave motions observed at the tropical location.