Wind Amplitudes Research Articles

Nonlinear interactions between gravity waves with different wavelengths and diurnal tide

By using a compressible nonlinear two‐dimensional gravity wave model, we simulate the nonlinear interactions between gravity waves (GWs) with three different vertical wavelengths and a meridional component of the diurnal tidal wind and temperature (30°N, September) calculated from GSWM‐00. We also compare the results with the simulation of GWs propagation in a background with zero wind. Consistent with the dispersion relation, the numerical experiments show that tidal wind reduces the vertical wavelengths of the GWs when it is in the same direction as the wave propagation, and thus increases the perturbative shear and the likelihood of instability and wave breaking, especially for waves with shorter vertical wavelengths. The breaking of GWs below the critical level can increase the amplitude of diurnal tidal wind due to the momentum deposition. Because the GW penetration height increases with vertical wavelength, the amplitude of diurnal tidal wave at higher altitudes is more likely to be affected by GWs with large vertical wavelengths. Therefore gravity wave breaking not only accelerates the mean winds, but also increases the amplitudes of the diurnal tide at various altitudes.

Constraints on a Non-orographic Gravity Wave Drag Parameterization Using a Gravity Wave Resolving General Circulation Model

The results of a T213L250 gravity wave (GW) resolving general circulation model (GWR-GCM) are used to constrain the GW source spectra of a non-orographic GW drag parameterization (GWDP) proposed by Hines. In this study, the following two constraints were placed on Hines’s GWDP: 1) the launch level at which GW source spectra are specified, and 2) the GW-source spectra, that is, the seasonally varying geographical and azimuthal distribution of GW momentum flux and horizontal wind amplitude. Considering the importance of the lateral propagation of GWs, which is ignored in Hines’s GWDP, the GW source spectra are prescribed using information from the GWR-GCM at 70 hPa, where the GWs have already propagated laterally some distance from their source regions. The GW-source spectra have significant geographical variations and anisotropy, reflecting source distribution and the effects of critical level filtering due to the background flows. Although the effects of the lateral propagation and intermittency of GWs are ignored, a T42L80 chemistry coupled climate model using the GWDP with the constraints developed in this study realistically reproduced the meridional structures of the zonal wind jets in the stratosphere and mesosphere.

Sodium lidar–observed strong inertia‐gravity wave activities in the mesopause region over Fort Collins, Colorado (41°N, 105°W)

In December 2004, the Colorado State University sodium lidar system at Fort Collins, Colorado (41°N, 105°W), conducted an ∼80‐hour continuous campaign for the simultaneous observations of mesopause region sodium density, temperature, and zonal and meridional winds. This data set reveals the significant inertia‐gravity wave activities with a period of ∼18 hours, which are strong in both wind components since UT day 338 (second day of the campaign), and weak in temperature and sodium density. The considerable variability of wave activities was observed with both wind amplitudes growing up to ∼40 m/s at 95–100 km in day 339 and then decreasing dramatically in day 340. We also found that the sodium density wave perturbation is correlated in phase with temperature perturbation below 90 km, and ∼180° out of phase above. Applying the linear wave theory, we estimated the wave horizontal propagation direction, horizontal wavelength, and apparent horizontal phase speed to be ∼25° south of west, ∼1800 ± 150 km, and ∼28 ± 2 m/s, respectively. The vertical profiles of wave intrinsic period, intrinsic phase speed, and vertical wavelength were also estimated. While the onset of enhanced inertia‐gravity wave amplitude in the night of 338 was observed to be in coincidence with short‐period gravity wave breaking via convective instability, the decrease of inertia‐gravity wave amplitude after noon of day 339 was also observed to coincide with the development of atmospheric dynamical instability layers with downward phase progression clearly correlated with the 18‐hour inertia‐gravity wave, suggesting likely breaking of this inertia‐gravity wave via dynamical (shear) instability.

Variability of mesospheric diurnal tides and tropospheric diurnal heating during 1997–1998

This study focuses on interannual variations of diurnal tropospheric heating and the response in the mesosphere observed by radars and predicted by a model. The work is prompted by reports of interannual variability in amplitudes of tidal variables at low latitudes. Diurnal tides observed at Hawaii and Christmas Island exhibit a pronounced “spike” in amplitude from late 1997 to early 1998. It has been speculated that this variability may be linked to the El Niño–Southern Oscillation phenomenon. We examine diurnal solar heating due to water vapor absorption, and diurnal latent heat release due to deep convection between 1988 and 2005. Both of these heating drives exhibit anomalously higher amplitudes in the tropical central and eastern Pacific during 1997–1998. The altered heating patterns result in a stronger forcing of the migrating diurnal tide by water vapor heating, and excitation of several weaker nonmigrating modes by latent heating. A primitive equation model is used to evaluate how these drives contribute to diurnal winds in the mesosphere. Anomalous water vapor heating results in about 15% increases in model meridional wind amplitudes over climatological values at subtropical latitudes between 300°E and the Greenwich meridian. While the timing of the model amplitude enhancements is consistent with observations at Hawaii, the observed increases are significantly stronger. Our study indicates that water vapor heating is the larger contributor to tidal enhancement observed during 1997–1998.

Convectively Coupled Equatorial Waves. Part I: Horizontal and Vertical Structures

Abstract Multilevel 15-yr ECMWF Re-Analysis (ERA-15) and satellite-observed brightness temperature (Tb) data for the period May–October 1992 are used to examine the horizontal and vertical structures of convectively coupled equatorial waves. Dynamical waves are isolated using a methodology developed previously. Composite structures of convectively coupled equatorial waves are obtained using linear regression/correlation between convection (Tb) and dynamical structures. It is found that the relationship depends on the ambient flow and the nature of the convective coupling, and varies between off-equatorial- and equatorial-centered convection, different hemispheres, and seasons. The Kelvin wave structure in the Western Hemisphere is generally consistent with classic equatorial wave theory and has its convection located in the region of low-level convergence. In the Eastern Hemisphere the Kelvin wave tends to have convection in the region of enhanced lower-tropospheric westerlies and a tilted vertical structure. The Kelvin wave also tends to have a third peak in zonal wind amplitude at 500 hPa and exhibits upward propagation into the lower stratosphere. Lower-tropospheric westward-moving mixed Rossby–gravity (WMRG) and n = 1 Rossby (R1) wave structures and their relationship with convection are consistent with classic equatorial wave theory and the implied lower-tropospheric convergences. In the Eastern Hemisphere the WMRG and R1 waves have first baroclinic mode structures in the vertical. However, in the Western Hemisphere, the R1 wave has a barotropic structure. In the Eastern Hemisphere the R1 wave, like the Kelvin wave, tends to have equatorial convection in the region of enhanced lower-level westerlies, suggesting that enhanced surface energy fluxes associated with these waves may play an important organizing role for equatorial convection in this warm-water hemisphere. In the upper troposphere, eastward-moving Rossby–gravity (EMRG) and n = 1 gravity waves are found in the Eastern Hemisphere, and eastward-moving WMRG and R1 waves are found in the Western Hemisphere, suggestive of Doppler shifting of waves by the ambient flow.

Interannual variability of the stratospheric wave driving during northern winter

Abstract. The strength of the stratospheric wave driving during northern winter is often quantified by the January–February mean poleward eddy heat flux at 100 hPa, averaged over 40°–80° N (or a similar area and period). Despite the dynamical and chemical relevance of the wave driving, the causes for its variability are still not well understood. In this study, ERA-40 reanalysis data for the period 1979–2002 are used to examine several factors that significantly affect the interannual variability of the wave driving. The total poleward heat flux at 100 hPa is poorly correlated with that in the troposphere, suggesting a decoupling between 100 hPa and the troposphere. However, the individual zonal wave-1 and wave-2 contributions to the wave driving at 100 hPa do exhibit a significant coupling with the troposphere, predominantly their stationary components. The stationary wave-1 contribution to the total wave driving significantly depends on the latitude of the stationary wave-1 source in the troposphere. The results suggest that this dependence is associated with the varying ability of stationary wave-1 activity to enter the tropospheric waveguide at mid-latitudes. The wave driving anomalies are separated into three parts: one part due to anomalies in the zonal correlation coefficient between the eddy temperature and eddy meridional wind, another part due to anomalies in the zonal eddy temperature amplitude, and a third part due to anomalies in the zonal eddy meridional wind amplitude. It is found that year-to-year variability in the zonal correlation coefficient between the eddy temperature and the eddy meridional wind is the most dominant factor in explaining the year-to-year variability of the poleward eddy heat flux.

A statistical analysis of longitudinal dependences of upper thermospheric zonal winds at dip equator latitudes derived from CHAMP

New observations, obtained by the accelerometer onboard the CHAMP satellite, reveal a detailed picture of the thermospheric zonal wind. Based on three years of data (2002–2004) we have studied the longitudinal dependence of the zonal delta wind (deviations from the zonal average) at the dip equator. The large number of passes ( ∼ 33 750) allows to consider several aspects of the wind characteristics at the same time. For this analysis we derived the longitudinal variation of the zonal delta wind at about 400 km altitude and investigated its dependence on solar flux, magnetic activity, and season. Major longitudinal dependences are confined to the morning hours, 03-09 local time (LT). The amplitude of the delta wind is approximately proportional to the latitudinal displacement of the magnetic dip equator from the geographic equator. The direction of the delta wind reverses sign between the June and December Solstices. During Equinox seasons these large scale features are almost absent. The flux level of solar EUV has no significant influence on the longitudinal variations. A dependence on magnetic activity could only be found during the post-sunset hours, 18-21 LT. Performing a Fourier transform of our delta wind velocities revealed a dominance of the wavenumber 4 in the Equinox data at some LT sectors. The wave-4 structure is a prevailing feature in the slowly precessing satellite frame, which has been recently reported, e.g. in nonmigrating tidal temperature measurements of the SABER instrument on the TIMED satellite in the Mesosphere Lower Thermosphere (MLT) region. Therefore, this statistical study of zonal wind longitudinal dependences provides new observational evidence for the coupling of the various atmospheric layers by nonmigrating tides.

Ekman Layer Rectification

Abstract The phenomenon of oceanic Ekman layer rectification refers to how the time-mean, Ekman layer velocity profile with depth differs as a consequence of variability in the surface wind in addition to the time-mean wind. This study investigates rectification using the K-Profile Parameterization (KPP) model for the turbulent surface boundary layer under simple conditions of uniform density and no surface buoyancy flux or surface wave influences. The rectification magnitude is found to be significant under typical conditions. Its primary effects are to extend the depth profile deeper into the interior, reduce the mean shear, increase the effective eddy viscosity due to turbulent momentum mixing, and rotate slightly the surface velocity farther away from the mean wind direction. These effects are partly due to the increase in mean stress because of its quadratic dependence on wind speed but also are due to the nonlinearity of the turbulent mixing efficiency. The strongest influence on the rectification magnitude is the ratio of transient wind amplitude to mean wind speed. It is found that an accurate estimate of the mean current usually can be obtained by using a quasi-stationary approximation that is a weighted integral of the steady Ekman layer response over the probability density function for the wind, independent of the detailed wind history. Rectification occurs even for very high frequency wind fluctuations, though the accuracy of the quasi-steady approximation degrades in this limit (as does the validity of the KPP model). This theory is extended to include the effects of the horizontal component of the Coriolis frequency, f y. Based on published computational turbulence solutions, a simple parameterization is proposed that amplifies the turbulent eddy diffusivity in KPP by a factor that decreases with latitude and depends on the wind orientation. The effect of f y ≠ 0 is to increase both the shear and the surface speed in the time-mean Ekman current for winds directed to the northeast and decrease both quantities for winds to the southwest, with weaker influences on these properties for the orthogonal directions of southeast and northwest. Furthermore, with transient winds there is significant coupling between f y ≠ 0 and the rectification effect; for example, the mean surface current direction, relative to the mean wind, is significantly changed for these orthogonal directions.

Solar Semidiurnal Tide in the Dusty Atmosphere of Mars

Abstract Vertical coupling due to the solar semidiurnal tide in Mars's atmosphere, and effects on zonal mean temperature and wind structures, are investigated using a numerical model. The model provides self-consistent solutions to the coupled zonal mean and tidal equations from the surface to 250 km. Breaking (convective instability) of the semidiurnal tide is parameterized using a linear saturation scheme with associated eddy diffusivities. Thermal forcing in the model gives rise to surface pressure perturbations and middle-atmosphere zonal mean winds and temperatures that are consistent with available measurements and general circulation models. Results presented here primarily focus on globally elevated dust levels during Southern Hemisphere summer, conditions similar to those experienced by the Viking 1 and Viking 2 landers during the 1977 global dust storms. Semidiurnal temperature and wind amplitudes maximize in the winter hemisphere and exceed 50 K and 100 m s−1 above 150 km and are typically 10–20 K and 10–20 m s−1 at 50 km. Perturbation densities are of order 50%–70% between 90 and 150 km, and thus contribute significantly to variability of the aerobraking regime in Mars's atmosphere. Eddy diffusivities associated with the breaking parameterization reach values of order 103–104 m2 s−1 between 100 and 150 km, and can be of order 1–10 m2 s−1 between 0 and 50 km. Dissipation of the semidiurnal tide induces zonal mean westward winds of order 10–30 m s−1 below 100 km, and in excess of 200 m s−1 above 150 km. The corresponding temperature perturbations range between −20 and −70 K over most of the thermosphere, with 10–20-K increases in temperature at high winter latitudes between 50 and 100 km. All of the wave and zonal mean perturbations noted above represent very significant modifications to the thermal and dynamical structure of Mars's atmosphere. Estimates are also provided for the eastward-propagating diurnal tides with zonal wavenumbers s = −1 and s = −2. These waves also have long vertical wavelengths and hence are capable of effectively coupling the lower and upper atmospheres of Mars. However, the perturbation and zonal mean effects of these waves are a factor of 2 or more smaller than those cited above for the semidiurnal tide under dusty conditions.

The two‐day wave in EOS MLS temperature and wind measurements during 2004–2005 winter

Two‐day wave observations during January–March 2005 are reported using the recently launched Microwave Limb Sounder (MLS) aboard NASA's Earth Observing System Aura mission. Wave‐induced disturbances in temperature, water vapor, carbon monoxide, and MLS line‐of‐sight wind appear in early January, peak near the end of January, and persist until late February. Temperature and wind amplitudes as large as 9 K and 50 m/s are observed near 90 km. The wave disturbance is initially confined in the mid to low summer latitudes where the climatological summer easterly jet exhibits strong shear. The wave then develops features akin to the third Rossby‐gravity global normal mode, with a weak temperature disturbance in the winter hemisphere (anti‐symmetric about the equator) and wind disturbance over the equator. Strong wind perturbation episode around 17–23 January 2005 in the mid‐latitude Southern hemisphere coincides with a particularly intense solar proton event.

Interannual variations of the diurnal tide in the mesosphere generated by the quasi‐biennial oscillation

We report results from a study with the numerical spectral model (NSM), which produces significant interannual variations in the diurnal tide. Applying Hines's Doppler spread parameterization, small‐scale gravity waves (GWs) drive the quasi‐biennial oscillation (QBO) and semiannual oscillation. With a GW source that peaks at the equator and is taken to be isotropic and independent of season, the NSM generates a QBO with variable periods around 26 months and zonal wind amplitudes close to 25 m s−1 at 30 km. As reported earlier, the NSM reproduces the observed equinoctial maxima in the diurnal tide at altitudes around 95 km. In the present paper it is shown that the QBO also modulates the tide such that the seasonal amplitude maxima can vary from one year to another by as much as 30%. To shed light on the underlying mechanisms, the relative importance of the linearized advection terms is discussed, which involve the meridional and vertical winds of the diurnal tide. These interactions generate pronounced QBO modulations of the tide at altitudes below 50 km. In the mesosphere between 50 and 80 km the signature is not well defined because of interference from the seasonal variations. At altitudes above 80 km, however, the QBO‐related interannual variations of the tide are again relatively large, but they are generated primarily by GW momentum deposition. Since the period of the QBO is variable and its phase is relative to the seasonal cycle changes, the magnitude of the QBO modulation of the tide in the upper mesosphere varies considerably as our long‐term model simulation shows. This intermittency must be attributed to nonlinear interactions, and GW drag (momentum deposition) is discussed as a potential candidate.

Origins and Dynamics of the 90-Day and 30–60-Day Variations in the Equatorial Indian Ocean

Abstract Sea level observations in the equatorial Indian Ocean show a dominant spectral peak at 90 days and secondary peaks at 30–60 days over an intraseasonal period (20–90 days). A detailed investigation of the origins and dynamics of these variations is carried out using an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Two parallel experiments are performed in the tropical Indian Ocean basin for the period 1988–2001: one is forced by NCEP 3-day mean forcing fields together with the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) pentad precipitation, and the other is forced by monthly mean fields. To help to understand the role played by the wind-driven equatorial wave dynamics, a linear continuously stratified ocean model is also used. Both the observed and modeled 90-day sea level anomaly fields and HYCOM surface current clearly show equatorial Kelvin and first-meridional-mode Rossby wave structures that are forced by the 90-day winds. The wind amplitude at the 90-day period, however, is weaker than that for the 30–60-day period, suggesting that the equatorial Indian Ocean selectively responds to the 90-day winds. This selective response arises mainly from the resonant excitation of the second-baroclinic-mode (n = 2) waves by the 90-day winds. In this case, Rossby waves reflected from the eastern ocean boundary enhance the directly forced response in the ocean interior, strengthening the 90-day peak. In addition, the directly forced response increases monotonically with the increase of forcing period, contributing to the larger variances of currents and sea level at 90 days. Two factors account for this monotonic increase in directly forced response. First, at lower frequency, both Rossby and Kelvin waves associated with the low-order baroclinic modes have longer wavelengths, which are more efficiently excited by the larger-scale winds. Second, responses of the high-order modes directly follow the local winds, and their amplitudes are proportional to both forcing period and wind strength. Although most energy is surface trapped, there is a significant amount that propagates through the pycnocline into the deep ocean. The dominance of the 90-day peak occurs not only at the surface but also in the deeper layers down to 600 m. In the deeper ocean, both the directly forced response and reflected waves associated with the first two baroclinic modes contribute to the 90-day variation. Spectra of the observed sea surface temperature (SST) also show a 90-day peak, likely a result of the selective response of the equatorial Indian Ocean at the 90-day period. Near the surface, the spectral peaks of currents and sea level at the 30–60-day period are directly forced by winds that peak at 30–60 days. In the deeper layers, both directly forced and reflected waves associated with the first two baroclinic modes contribute. Oceanic instabilities can have significant contributions only near the western boundary and near 5°N south of Sri Lanka.

Solar activity variations of equivalent winds derived from global ionosonde data

The equivalent winds at the F layer peak are derived from global ionosonde data to investigate their solar activity variations. With increasing solar activity, the derived equivalent winds are found of nonlinearly decreased diurnal amplitudes in all seasons at most stations. This implies that the increase in ion drag more than compensates for pressure gradients and thus restrains the diurnal amplitude at high solar activity. The diurnal phase of the derived equivalent winds generally shifts later at higher solar activity. It is the first time to explicitly report this striking feature that emerged at so many stations. Another pronounced feature is that the diurnal phase has a summer‐winter difference. The diurnal phases at most stations in the Northern Hemisphere are later in winter than in summer at higher solar activity. Furthermore, a decrease in the semidiurnal amplitudes of equivalent winds with increasing solar activity is evident in winter over most stations considered and in other seasons at stations with a lower dip, but the decrease trend becomes weak in other seasons at stations with a larger dip. However, complicated dependences on solar activity can be found in the diurnal mean and the semidiurnal phases of equivalent winds at stations considered.

Modeling study of mesospheric planetary waves: genesis and characteristics

Abstract. The Numerical Spectral Model (NSM) extends from the ground into the thermosphere and incorporates Hines' Doppler Spread Parameterization for small-scale gravity waves (GWs). In the present version of the model we account for a tropospheric heat source in the zonal mean (m=0), which reproduces qualitatively the observed zonal jets near the tropopause and the accompanying reversal in the latitudinal temperature variations. In the study presented here, we discuss the planetary waves (PWs) that are solely generated internally, i.e. without the explicit excitation sources related to tropospheric convection or topography. Our analysis shows that PWs are not produced when the zonally averaged heat source into the atmosphere is artificially suppressed, and that the PWs are generally weaker when the tropospheric source is not applied. Instabilities associated with the zonal mean temperature, pressure and wind fields, which still need to be explored, are exciting PWs that have amplitudes in the mesosphere comparable to those observed. Three classes of PWs are generated in the NSM. (1) Rossby type PWs, which slowly propagate westward relative to the mean zonal flow, are carried by the winds so that they appear (from the ground) to propagate, respectively, eastward and westward in the winter and summer hemispheres below 80km. Depending on the zonal wave number and magnitudes of the zonal winds, and under the influence of the equatorial oscillations, these PWs typically have periods between 2 and 20 days. Their horizontal wind amplitudes can exceed 40 m/s in the lower mesosphere. (2) Rossby-gravity waves, which propagate westward at low latitudes and have periods around 2 days for zonal wave numbers m=2 to 4. (3) Eastward propagating equatorial Kelvin waves, which are generated in the upper mesosphere with periods between 1 and 3 days depending on m. A survey of the PWs reveals that the largest wind amplitudes tend to occur below 80km in the winter hemisphere; but above that altitude the amplitudes are larger in the summer hemisphere where the winds can approach 50m/s. This pattern in the seasonal variations also appears in the baroclinity of the zonal mean (m=0). The nonmigrating tides in the mesosphere are significantly larger for the model with the tropospheric heat source, in which PWs are apparently generated by the instabilities that arise around the tropopause.

Initial full-diurnal-cycle mesopause region lidar observations: diurnal-means and tidal perturbations of temperature and winds over Fort Collins, CO (41°N,105°W)

The Colorado State Sodium lidar has been upgraded to a two-beam system capable of simultaneous measurements of mesopause region temperature and winds, over full diurnal-cycles, weather permitting. Though our lidar is a modest system with a power-aperture product of only 0.06 W m 2 , good data quality is demonstrated by means of contour plots depicting a 80-h continuous observation between August 9th and 12th, showing the existence of atmospheric waves with different periods along with their coherence and interactions. The salient feature of data with full-diurnal-cycle coverage lies in its ability to describe the vertical profiles of dynamical fields (temperature, zonal and meridional winds) as a unique linear superposition of diurnal-mean and oscillations with different tidal periods, plus a residual term. In this manner, we investigate diurnal-means and oscillations with diurnal and semidiurnal periods. Using 6 data sets between July 17 and August 12, each covering a full-diurnal-cycle as a case study, we found considerable day-to-day variability, as much as 20 K , 35 and 75 m/s for diurnal-mean temperature, zonal wind and meridional wind, respectively, and as 15 K , and 50 m/s , for the diurnal and semidiurnal tidal temperature and wind amplitudes, respectively. While a minimum of 3 full diurnal cycles appears to be adequate in the case studied here, the 6-day composite yields diurnal-means and diurnal tides in agreement with model predictions very well. Since the resulting amplitudes and phases of the observed diurnal oscillations agree well with the global scale wave model, GSWM00 predictions, we conclude that the migrating diurnal tide contributes significantly to the observed oscillations with diurnal period over Fort Collins, CO (41°N, 105°W). Unlike the diurnal perturbations, the observed semidiurnal amplitudes and phases differ from the GSWM00 predictions with considerably smaller model amplitudes. The coherence of solar forcing is found to prevail over the variability for diurnal-mean as well as both tidal oscillations, leading to expected convergence to climatology in a multi-day composite observation. The possible causes for the observed variability, and the role of planetary waves and nonlinear interactions are discussed, along with future studies to assess the extent of local perturbations that may exist in the observed data.

Unexpectedly small semidiurnal tidal wind amplitudes in the mid-latitudemesopause region during September 2002

Abstract. The mesopause region monthly mean winds and semidiurnal tidal amplitudes and phases over Central Europe have been measured at Collm Observatory since September 1982. The regular annual cycle of the semidiurnal tidal amplitudes show maximum values during late August and September. In contrast to that, in autumn 2002 no enhancement of the tidal amplitudes was measured, while the autumn tidal phase transition occurred unusually early. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides; climatology)

Gravity wave generation in the lower stratosphere due to passage of the typhoon 9426 (Orchid) observed by the MU radar at Shigaraki (34.85°N, 136.10°E)

Intense gravity wave activities were investigated in the lower stratosphere during the typhoon 9426. Strong vertical winds were observed just a few hours before the arrival of the typhoon‐center at the MU radar site. About 30 min to 1 hour after the typhoon‐center had passed, a considerable reduction in vertical wind amplitude was detected. Dominant gravity waves showed time period in the range of 7–8 min, 15 min, and 40–60 min in the upper troposphere and lower stratosphere. In the vicinity of the central region of the typhoon, a gravity wave was observed, which was monochromatic in nature with a vertical wavelength ∼3 km between 1.5 km and 23 km height. In the lower stratosphere, the horizontal wavelength for the prominent period was detected in the range of 10–15 km (for 15 min wave period) and 25–50 km (for 40–60 min wave period). The vertical wavelength of these waves was examined from 2.5 km to 4.0 km. In the horizontal direction, the intrinsic group velocity was estimated between 9 ± 2 and 11 ± 2 m/s. Near the tropopause, the average direction of group velocity was assessed at about 20° ± 3° from the horizontal. The generation of gravity wave like features, in the lower stratosphere, is believed induced by convection, as the low temperature of the clouds indicates a deep penetration over the radar region as seen in the satellite GMS images.

Longitude variability of the solar semidiurnal tide in the lower thermosphere through assimilation of ground‐ and space‐based wind measurements

Wind measurements from the Upper Atmosphere Research Satellite (UARS) and model output from the Middle Atmosphere General Circulation Model (GCM) at Kyushu University are used to investigate the nature of nonmigrating semidiurnal tides between 50–55°N using combined space‐based (SBM) and ground‐based (GBM) wind measurements at 95 km. The GCM is used to create a mock database to test the effects of various sampling scenarios, data gaps, and relative weighting between SBM and GBM, on retrieval of the longitude structure of the semidiurnal tide. SB sampling is based upon orbital characteristics of UARS. GB sampling corresponds to hourly radar measurements from Saskatoon (52°N, 107°W), Sheffield (53°N, 4°W), Collm (52°N, 15°E), Obninsk (55°N, 37°E), and Kazan (56°N, 49°E). Results are presented for the month of August when semidiurnal amplitudes are large and sampling by UARS instruments is good. By compositing over a 5–10 day “fit span,” it is found that the combination of temporal coverage by GB radars and spatial sampling by the satellite is sufficient to allow reasonable recovery of the zonal wave number s = 1, 2, 3 components of the semidiurnal tide. Over significantly longer fit spans, the contributions of GBM become less critical. Using actual UARS and GBM during 1–20 August 1993, the semidiurnal amplitude of eastward wind is found to vary from a minimum value (12 ms−1) at 20°E, to a maximum of 45 ms−1 near 160°E, and a secondary maximum (29 ms−1) at 300°E. The zonal wave number components corresponding to this longitude variation in the semidiurnal tide are 7.7 ± 1.9 ms−1, 19.8 ± 1.5 ms−1 and 13.0 ± 1.3 ms−1 for s = 1, 2, 3 (westward), respectively where ±1−σ uncertainties are indicated. These results are in reasonable agreement with those simulated within the Kyushu GCM. However, there is roughly a four‐ to five‐hour phase offset between the phases recovered from the observational data and from the Kyushu GCM, possibly connected with strong model phase gradients in this atmospheric regime.

Lunar tidal winds in the upper atmosphere over Jakarta

Winds have been measured at heights from 70 to 120 km by means of a Meteor Wind Radar near Jakarta (6.4S, 106.7E) from 1993 to 1997. These data have been analysed for the lunar semidiurnal tide using a least squares fitting method. Generally lunar tidal amplitudes in the winds are about 2 m s−1 though the northward wind amplitude reaches 6 m s−1 in southern summer. Variations with height and season are shown and compared with the Global Scale Wave Model (GSWM). Often the agreement with the model is good but there is a difference in phase of the eastward wind in southern winter. The GSWM predicts a more rapid increase in northward wind magnitude with height than is observed. Year‐to‐year changes are examined for different seasons and these reveal sometimes consistent results and sometimes a variation in phase from year to year. Often the phase relations between eastward and northward winds at Jakarta are more characteristic of a Northern Hemisphere tide, as predicted by classical tidal theory, particularly in southern spring and summer.

Nonmigrating diurnal tides in the thermosphere

Horizontal wind measurements from the HRDI and WINDII instruments on the Upper Atmosphere Research Satellite are analyzed to reveal the most prominent nonmigrating diurnal tidal components at 95 km: the eastward propagating diurnal tide with zonal wave number s = 3 (DE3), the standing (s = 0) diurnal oscillation (D0), and the westward propagating diurnal tide with s = 2 (DW2). The strongest DE3 occurs primarily during Northern Hemisphere summer/fall with maximum eastward winds near the equator of order 15 ms−1 and is a vertical extension of the s = 3 Kelvin wave. The first antisymmetric mode of DE3 dominates during October–April, with maximum meridional winds near the equator of order 8 ms−1. D0 exists during nearly all months and is nonsymmetric about the equator with maximum northward wind amplitudes in the Southern Hemisphere of order 7–10 ms−1. DW2 closely resembles the first symmetric propagating mode from classical tidal theory, with maximum northward wind amplitudes of order 10–12 ms−1 during September through February. The combination of DE3, D0, and DW2 with DW1 gives rise to significant longitude variations in the diurnal tide between ±40° latitude. Forcing in the Global Scale Wave Model (GSWM) is calibrated according to the above observations, thus enabling global estimates of nonmigrating tidal temperatures and other fields. For instance, it is estimated that the temperature and eastward wind perturbations associated with DE3 may be as large as 20K and 35 ms−1 at 145 km and 115 km, respectively, over the equatorial region during July.