The structure, variability, and mean‐flow interactions of the quasi‐2‐day wave (Q2DW) in the mesosphere and lower thermosphere during January 2015 were studied employing meteor and medium‐frequency radar winds at eight sites from 23°S to 76°S and Microwave Limb Sounder (MLS) temperature and geopotential height measurements from 30°S to 80°S. The event had a duration of ~20–25 days, dominant periods of ~44–52 hr, temperature amplitudes as large as ~16 K, and zonal and meridional wind amplitudes as high as ~40 and 80 m/s, respectively, at middle and lower latitudes. MLS measurements enabled definition of balance winds that agreed well with radar wind amplitudes and phases at middle latitudes where amplitudes were large and quantification of the various Q2DW modes contributing to the full wave field. The Q2DW event was composed primarily of the westward zonal wavenumber 3 (W3) mode but also had measurable amplitudes in other westward modes W1, W2, and W4; eastward modes E1 and E2; and stationary mode S0. Of the secondary modes, W1, W2, and E2 had the larger amplitudes. Inferred MLS balance winds enabled estimates of the Eliassen‐Palm fluxes for each mode, and cumulative zonal accelerations that were found to be in reasonable agreement with radar estimates from ~35°S to 70°S at the lower altitudes at which radar winds were available.

A minor stratospheric sudden warming (SSW) event was noticed in the southern hemisphere (SH) during September (day 259) 2010 along with two episodic warmings in early August (day 212) and late October (day 300) 2010. Among the three warming events, the signature of mesosphere response was detected only for the September event in the mesospheric wind dataset from both meteor radar and MF radar located at King Sejong Station (62°S, 59°W) and Rothera (68°S, 68°W), Antarctica, respectively. The zonal winds in the mesosphere reversed approximately a week before the September SSW event, as has been observed in the 2002 major SSW. Signatures of mesospheric cooling (MC) in association with stratospheric warmings are found in temperatures measured by the Microwave Limb Sounder (MLS). Simulations of specified dynamics version of Whole Atmosphere Community Climate Model (SD-WACCM) are able to reproduce these observed features. The mesospheric wind field was found to differ significantly from that of normal years probably due to enhanced planetary wave (PW) activity before the SSW. From the wavelet analysis of wind data of both stations, we find that strong 14–16 day PWs prevailed prior to the SSW and disappeared suddenly after the SSW in the mesosphere. Our study provides evidence that minor SSWs in SH can result in significant effects on the mesospheric dynamics as in the northern hemisphere.

The statistics of gravity wave momentum flux estimation are investigated using data from the MU radar at Shigariki, Japan (136°E, 35°N). The radar has been operating during campaign periods since 1986. The first part of the paper focuses on a multi-day campaign during October 13–31, 1986. The second part of the paper investigates data after 2006 when the radar was operated in a meteor scatter mode. Momentum fluxes are derived from both the turbulent scatter and the meteor scatter measurements, but the techniques are quite different. Probability Distribution Functions are formed using turbulent scatter data. These show that wave packets sometimes have momentum flux magnitudes in excess of 100m2s−2. The technique for meteor radars, introduced by Hocking (2005), has been widely adopted by the radar community in recent years. The momentum flux estimated using this technique is found to be anti-correlated with the background tidal winds. A validation investigation is carried out for periods with a high meteor echo data rate. The conclusion was that the method can be used to calculate the sign of momentum flux, but does not accurately specify the magnitude.

Two nearly identical meteor radars were operated at Koto Tabang (0.20° S, 100.32° E), West Sumatra, and Biak (1.17° S, 136.10° E), West Papua, in Indonesia, separated by approximately 4000 km in longitude on the Equator. The zonal and meridional momentum flux, u′w′ and v′w′, where u, v, and w are the eastward, northward, and vertical wind velocity components, respectively, were estimated at 86 to 94 km altitudes using the meteor radar data by applying a method proposed by Hocking (2005). The observed u′w′ at the two sites agreed reasonably well at 86, 90, and 94 km during the observation periods when the data acquisition rate was sufficiently large enough. Variations in v′w′ were consistent between 86, 90, and 94 km altitudes at both sites. The climatological variation in the monthly averaged u′w′ and v′w′ was investigated using the long-term radar data at Koto Tabang from November 2002 to November 2013. The seasonal variations in u′w′ and v′w′ showed a repeatable semiannual and annual cycles, respectively. u′w′ showed eastward values in February–April and July–September and v′w′ was northward in June to August at 90–94 km, both of which were generally anti-phase with the mean zonal and meridional winds, having the same periodicity. Our results suggest the usefulness of the Hocking method.

Observations of winds and gravity waves (GW) by MF radars from the Arctic to the Equator are used to provide frequency spectra and spectral variances of horizontal motions, and information on the predominant azimuthal directions of propagation for the waves. The years used are mainly 1993/4; the height layer 76–88 km; and the GW bands 10 100 min. and 1–6 hrs. The high/mid-latitude locations of Tromsø, Saskatoon, London/Urbana, Yamagawa, generally demonstrate similar behaviour: the monthly spectra have slopes near −5/3 in winter months, but smaller (absolute) slopes at higher frequencies (<2 hrs.) in summer. Corresponding to this, the spectral densities (10–100 min.) are larger for conditions of higher mean background windspeed—this is related by means of a new correlation-vector technique to GW propagating anti-parallel to the mean zonal winds, and the closure of the solstitial mesospheric jets. Also consistent with this, the sizes and orientations of perturbation ovals (fitted to the wind variations), demonstrate strong semi-annual-oscillations (SAO), and generally similar monthly and latitudinal directions. This suggests strong control, especially of the high-frequency GW band, by the dominant zonal wind-structures of the mesosphere. In contrast the low-latitude locations of Hawaii and Christmas Island demonstrate uniquely different behaviours, with indications of significant inter-annual variability. The frequency spectra for all months tend to have smaller slopes at higher frequencies. Also the dependence of spectral density in both GW bands, upon background wind speed, is negative rather than positive, and is shown to be generally consistent with GW propagating parallel to the mean-global winds. This is consistent with weaker vertical shears in the zonal winds (76–88 km), and lower GW momentum depositions. The perturbation ovals reveal much weaker SAO, and more variable orientations, consistent with more dependency upon GW sources, and less control by the mean winds of the mesosphere.

A mesospheric horizontal wind data set measured during 1991-2006 by the medium frequency (MF) radar at Kauai, Hawaii (22 degrees N, 160 degrees W) is analyzed to examine the long-term variability of the quasi two-day wave (QTDW). The QTDW over Hawaii is amplified twice a year, with the January and July events most likely being the representation of zonal wave numbers 3 and 4 modes, respectively. The amplitudes of the January monthly mean QTDW in both meridional and zonal winds and the July monthly mean QTDW in meridional component are nearly in phase with the solar cycle but with the solar maxima leading the QTDW maxima by 1 or 2 years. However, the July monthly mean QTDW in zonal wind is more antiphase with the solar cycle. Enhanced QTDW oscillations are evident in both wind components in January 1998, which is likely related to the strong El Nino event during the winter of 1997/1998. The enhanced gravity wave activity and the increased barotropic/baroclinic/inertial instability related to the strengthened stratosphere summer easterly jet might provide additional forcing to amplify the QTDW. Moreover, the enhanced migrating diurnal tide during warm El Nino-Southern Oscillation events could also contribute to the abnormally strong QTDW by increasing the refractive index and thus the growth rate of the QTDW. Additional enhancement of the QTDW with a short period of similar to 43h is observed during the major sudden stratospheric warming in January 2006.

For many years the amplitude of the diurnal tide in the mesosphere, lower thermosphere (MLT) has been known to be deeply modulated over a wide range of time scales. These time scales range from a few days, to seasonal dependencies, and up to interannual time scales. The causes of the variability have remained elusive, but can fall into two main categories. The first is source variability, the variability arising from variations in water vapor heating, latent heating, and ozone heating. The second potential cause is interaction of the diurnal tide with the background, including the effects of the zonal mean structure, planetary waves, gravity waves and turbulence. In this paper we focus on the second category of tidal variability. Measurements at the equator allow us to focus on particular mechanisms of tidal variability. We are close to the main sources driving the tide, equatorial convection and water vapor heating. The equatorial mean zonal winds are relatively weak so we also avoid the filtering of the tide due to the midlatitude jets. Although we cannot observe wave–mean and wave–wave interactions directly, a decadal climatology allows to diagnose how much tidal–background interaction might contribute to tidal variability. We make use of data from the SABER instrument, providing continuous temperature measurements and augment these with some equatorial radar wind observations. Two-month averages of SABER temperatures are required to unambiguously define migrating tidal temperature fields which confine our study mainly for variability on a seasonal time scale or longer. The temperature tide has a repeatable seasonal variation that is shown to stem directly from the sources and which can be observed as low as tropopause heights. Refraction and reflection are found to play an important role in the vertical structure of the tide and found to influence temporal variability in both amplitude and phase when the tide propagates into the mesosphere. An anti-symmetric migrating tidal mode was found to be present during some seasons of the year above 80km.

The gravity wave activities in the stratosphere and mesosphere of subtropics during the sudden stratospheric warming were studied using the temperature profiles measured by the Jet Propulsion Laboratory (JPL) Rayleigh lidar at Mauna Loa Observatory (19.5N, 195.6W), Hawaii, and horizontal wind profiles measured by the MF radar at Kauai (22°N, 200.2°W), Hawaii . We found that the significant enhancement of gravity wave activities was observed before the sudden stratospheric warming in winter 2005/2006, followed by the decrease of activity during and after the warming. The significant change of GW activities during the warming will be discussed together with the ECMWF wind in the stratosphere, MF radar and TIMED/TIDI mean winds in the mesosphere.

Abstract. To investigate possible couplings between planetary waves and the semidiurnal tide (SDT), this work examines the statistical correlations between the SDT amplitudes observed in the Northern Hemisphere (NH) mesosphere and stationary planetary wave (SPW) with wavenumber S=1 (SPW1) amplitudes throughout the global stratosphere and mesosphere. The latter are derived from the Aura-MLS temperature measurements. During NH summer-fall (July–October), the mesospheric SDT amplitudes observed at Svalbard (78° N) and Eureka (80° N) usually do not show persistent correlations with the SPW1 amplitudes in the opposite hemisphere. Although the SDT amplitudes observed at lower latitudes (~50–70° N), especially at Saskatoon (52° N), are often shown to be highly and positively correlated with the SPW1 amplitudes in high southern latitudes, these correlations cannot be sufficiently explained as evidence for a direct physical link between the Southern Hemisphere (SH) winter-early spring SPW and NH summer-early fall mesospheric SDT. This is because the migrating tide's contribution is usually dominant in the mid-high latitude (~50–70° N) NH mesosphere during the local late summer-early fall (July–September). The numerical correlation is dominated by similar low-frequency variability or trends between the amplitudes of the NH SDT and SH SPW1 during the respective equinoctial transitions. In contradistinction, during NH winter (November–February), the mesospheric SDT amplitudes at northern mid-high latitudes (~50–80° N) are observed to be significantly and positively correlated with the SPW1 amplitudes in the same hemisphere in most cases. Because both the SPW and migrating SDT are large in the NH during the local winter, a non-linear interaction between SPW and migrating SDT probably occurs, thus providing a global non-migrating SDT. This is consistent with observations of SDT in Antarctica that are large in summer than in winter. It is suggested that climatological hemispheric asymmetry, e.g. the SH and NH winter characteristics are substantially different, lead to differences in the inter-hemispheric SPW-tide physical links.

Significant attention is being paid in recent times by several observational and modeling studies to quantify the spatial and temporal variabilities of diurnal tide in the mesosphere and lower thermosphere (MLT) region. These variabilities are ascribed to spatial and temporal variations in the tidal forcing or interactions between the propagating tides and background wind, planetary waves or gravity waves. The present work makes use of simultaneous ground-based radar wind observations of different durations from five equatorial/low latitude sites in the Indian, Indonesian and Pacific sectors: Tirunelveli (8.7°N, 77.8°E), Jakarta (6.4°S, 106.7°E), Pontianak (0.03°N, 109°E), Kauai (22°N, 160°W) and Christmas Island (2°N, 157°W). This study delineates the longitudinal differences in the tidal characteristics in (i) interannual time scales over Tirunelveli and Kauai during 1993–2002, (ii) seasonal time scales over Christmas Island, Jakarta and Tirunelveli for the years 1993–1997 and (iii) shorter than seasonal time scales over Christmas Island, Pontianak and Tirunelveli during 1996–1997. An important observational feature noticed in this work is the differing behavior of the long-term tidal fields over Tirunelveli and Kauai. The monthly tidal amplitudes over Tirunelveli reveal a strong QBO signature whereas a similar, strong QBO signal could not be traced in the long-term observations from Kauai.

The coupling of the dynamical regimes in the high- and low-latitude stratosphere and mesosphere during the major SSW in the Arctic winter of 2003/2004 has been studied. The UKMO zonal wind data were used to explore the latitudinal coupling in the stratosphere, while the coupling in the mesosphere was investigated by neutral wind measurements from eleven radars situated at high, high-middle and tropical latitudes. It was found that the inverse relationship between the variability of the zonal mean flows at high- and low-latitude stratosphere related to the SSW is produced by global-scale zonally symmetric waves. Their origin and other main features have been investigated in detail. Similar latitudinal dynamical coupling has been found for the mesosphere as well. Indirect evidence for the presence of zonally symmetric waves in the mesosphere has been found.

As part of an ongoing effort to understand the migrating diurnal tide generated by the NCAR Whole Atmosphere Community Climate Model, version 3 (WACCM3), we compare the WACCM3 migrating diurnal tide in the horizontal wind and temperature fields to similar results from the Global Scale Wave Model (GSWM). The WACCM3 diurnal tidal wind fields are also compared to tropical radar measurements at Kauai (22°N, 200.2°E) and Rarotonga (21.3°S, 199.7°E). The large-scale features of the WACCM3 results, such as the global spatial structure and the semiannual amplitude variation are in general agreement with past tidal studies; however, several differences do exist. WACCM3 exhibits a much higher degree of hemispheric asymmetry, lower overall amplitudes around the equinoxes, and peaks which are more confined in latitude when compared with the GSWM. Factors which may contribute to such differences between WACCM3 and GSWM are the solar heating profiles from ozone and water vapor, dissipation, and the zonal mean zonal winds. We find that the internally generated heating in WACCM3 and eddy dissipation values are both smaller than the values specified in the GSWM; the eddy dissipation fields and zonal mean zonal winds of the two models also display measurable differences in spatial structure. Comparisons with radar data show several differences in spatial and seasonal structure. In particular, the diurnal tide zonal winds in WACCM3 above Kauai are considerably larger in amplitude than those observed in the radar data, due to contributions from nonmigrating tidal components including wave numbers eastward 1 through 3, westward 2, and stationary components, which interfere constructively with the migrating component around equinox in WACCM3.

We report here and in a companion paper by Fritts et al. (2006a) on a new use of the UHF radar at the Arecibo Observatory in Puerto Rico. We have employed the 430 MHz radar for incoherent scatter measurements of radial wind spectra at altitudes from ∼71 to 95 km using the Gregorian and line‐feed antennas to define beam angles inclined 15° to the east and west of zenith. We find that the two beams define radial velocities with sufficient accuracy to characterize both the gravity waves and the momentum fluxes due to these waves over the majority of the observed altitude range during daylight hours. The characteristics of the gravity waves inferred from these measurements include (1) vertical scales ranging from ∼2 to 20 km, (2) downward phase progression of the dominant gravity waves up to ∼5 ms −1 , and (3) vertical wave number spectra having slopes near a value (−3) expected for saturated gravity waves. Gravity wave frequency spectra and momentum fluxes are addressed in the companion paper.

Janches et al. (2006) described a new dual‐beam use of the 430 MHz incoherent scatter radar at the Arecibo Observatory in Puerto Rico. We found the technique to define the radial wind field in the mesosphere and lower thermosphere with sufficient accuracy to characterize gravity waves occurring at high frequencies and small spatial scales over an extended altitude range. The coplanar, dual‐beam experiment was also designed to test the ability of the system to measure gravity wave momentum fluxes and their frequency distributions, and we report here on those results. Initial measurements were of limited duration and necessarily represent a case study, but they demonstrate the value of such measurements for studies of GW variability and large‐scale interactions. Radial velocity variances reveal preferential eastward propagation for most intervals and altitudes, with the greatest propagation bias at lower altitudes and later times on 11 September when strong westward mean winds favor strong gravity filtering. The momentum fluxes observed during this experiment had ∼50‐min averages that were often near zero, occasionally achieved amplitudes of ∼20 to 50 m2s−2, displayed significant consistency in altitude, and exhibited an approximate anticorrelation with the zonal wind field in cases with significant momentum fluxes. Frequency spectra defined the major contributions to the momentum fluxes, while S transforms were employed to examine the temporal variability of the GWs and momentum fluxes in greater detail.

At Rothera Research Station (67.3°S, 68.1°W), Antarctica, 296h of day and nighttime Fe-Boltzmann temperature lidar data were accumulated in 2003. During this time, sporadic iron layers (FeS) were observed with an annual average occurrence probability of 14%. The peak altitude of the FeS layers was highest during the summer, with a fitted value of 103km while during the winter the layer decreased in height to 90km with an annual average of 97km. The atmospheric temperature perturbations and potential energy density profiles computed from the same lidar data exhibited increased temperature but constant-with-height potential energy density when sporadic Fe layer occurred. Once sporadic Fe layers disappeared, the potential energy density decreased with height, indicating an energy loss due to atmospheric gravity wave breaking. These results suggest a link between FeS and atmospheric dynamics.

The mesosphere/lower thermosphere (MLT) wind data from the 46 ground-based (GB) MF and meteor radar (MR) stations, located at the different latitudes over the globe, and the space-based (SB) HRDI data were used for constructing of the empirical global climatic 2-D prevailing wind model at 80–100 km heights for all months of the year. The main data set is obtained during 1990–2001 period. It is shown that the three datasets (MF, MR, HRDI) are mainly well correlated. However, a certain systematic bias between the GB and SB data at 96 km exists, as well as that between the MF and MR data higher 88 km. Simple correction factors are proposed to minimize these biases. The 2-D distant-weighted least-square interpolation procedure for some arbitrary collection of points was used for drawing model contour plots. The model is available in the computer readable form and may be used for construction of the new CIRA model.

Tropospheric and stratospheric radar wind data obtained at Jicamarca, Per5

In an earlier study based upon medium frequency radar (MFR) data from Saskatoon (52degreesN) the variability of time-sequences of gravity wave (GW) variances was linked to tidal (12,24 h) and planetary wave (2 d) oscillations of the wind. Fifty days of data were chosen from each of winter, spring and autumn seasons for this most comprehensive assessment of wave interactions. While modulations of the GW variances were observed, the results indicated considerable intermittency in the strength and direction of waves from the GW sources.Here we extend the study to other sites in the MLT-MFR (mesosphere, lower-thennosphere) network, and focus upon the planetary waves (PWs) and their modulating influences upon the GW variances. The PW events include the 2 d waves as seen at Saskatoon, London, Hawaii and Christmas Island during the summer of 1994; and 16 d wave activity evidenced at Saskatoon and London throughout 1994, and also during 12 and 7 years of observations, respectively. The modulations of the sequences of GW variances (10-100/150 min, 2-6 h periods) are significant, and the phase-differences between the PW (2 d, 16 d) oscillations in the time-sequences of the winds and of the GW variances allow the propagation directions for the GW fluxes to be inferred. These are eastward in summer months and westward in winter at MLT (60-90 km) altitudes. (C) 2002 Elsevier Science Ltd. All rights reserved.