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Recent observational and modeling evidence has demonstrated that planetary waves can modulate atmospheric tides, and secondary waves arising from their nonlinear interactions are an important source of both temporal and longitude variability in the thermosphere. While significant progress has been made on understanding how this form of vertical coupling occurs, uncertainty still exists on how the horizontal structures of primary and secondary waves evolve with height and the processes responsible for this evolution, in part due to lack of global observations between 120 km and 260 km. In this work we employ a Thermosphere Ionosphere Mesosphere Electrodynamics general circulation model simulation covering all of 2009 that is forced by Modern-Era Retrospective Analysis for Research and Applications dynamical fields, to assess the relative contribution of zonal mean winds and molecular dissipation on the vertical coupling of the eastward propagating diurnal tide with zonal wave number 3 (DE3), the 3 day ultrafast Kelvin wave, and the secondary waves arising from their nonlinear interaction. By developing and applying a new analytic formulation describing the latitudinal structure of an equatorially trapped wave subject to dissipation and background winds, we show that dissipation is the primary contributor to the broadening of the latitudinal structures with height, while asymmetries in the background wind field are responsible for the distortion of the height-latitude structures.

In this paper we demonstrate that dissipation of upward propagating tides produces significant changes in the mean temperature of the thermosphere, ranging from +19 K at solar minimum to −15 K at solar maximum in the equatorial region. Our methodology consists of measuring the differential response of the thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM) under solar minimum and solar maximum conditions to constant tidal forcing at its 97 km lower boundary, as specified by the observationally based Climatological Tidal Model of the Thermosphere. Diagnosis of the model reveals that these changes are mainly driven by 5.3 μm nitric oxide (NO) cooling, which more efficiently cools the thermosphere at solar maximum. The main role of the tides is to modify the mean molecular oxygen densities ([O2]) through tidal-induced advective transport, which then lead to changes in NO densities through oxygen-nitrogen chemistry. Through tidal-induced changes in temperature and O, O2, and N2 densities, effects on the ionosphere are also quite substantial; tidal-induced modifications to zonal-mean F region peak electron densities (NmF2) are of order −10% at solar maximum and −30% at solar minimum in the equatorial region. Our results introduce an additional consideration when attributing long-term changes in thermospheric temperature and electron densities to CO2 cooling effects alone; that is, dissipation of upward propagating tides may constitute an additional element of global change in the ionosphere-thermosphere (IT) system.

We explore tropospheric latent heat release as a source of variability of the diurnal tide in the mesosphere and lower thermosphere (MLT) in two ways. First, we present analyses of the UARS WINDII horizontal wind data, which reveal signatures of nonmigrating tidal effects as large as 25 m/s during both vernal equinox and boreal winter. These effects are of greater relative importance during the latter season. Complementary global-scale wave model (GSWM) results which account for a tropospheric latent heat source generally underestimate the observed nonmigrating tidal effects but capture the seasonal variability that is observed. Second, we pursue a new parameterization scheme to investigate seasonal variability of the migrating diurnal tidal component of the latent heat source with GSWM. These results confirm previously reported seasonal trends, but suggest that the MLT effects may be as much as an order of magnitude larger than earlier predictions.

Latent heat release associated with tropical deep convective activity is investigated as a source for migrating (sun-synchronous) diurnal and semidiurnal tidal oscillations in the 80–150-km height region. Satellite-based cloud brightness temperature measurements made between 1988 and 1994 and averaged into 3-h bins are used to determine the annual- and longitude-average local-time distribution of rainfall rate, and hence latent heating, between ±40° latitude. Regional average rainfall rates are shown to be in good agreement with climatological values derived from surface rain gauge data. A global linearized wave model is used to estimate the corresponding atmospheric perturbations in the mesosphere/lower thermosphere (80–150 km) resulting from upward-propagating tidal components excited by the latent heating. The annual-average migrating diurnal and semidiurnal components achieve velocity and temperature amplitudes of order 10–20 m s–1 and 5–10 K, respectively, which represent substantial contributions to the dynamics of the region. The latent heat forcing also shifts the phase (local solar time of maximum) of the semidiurnal surface pressure oscillation from 0912 to 0936 h, much closer to the observed value of 0944 h.

During stratospheric sudden warming (SSW) periods large changes in the low-latitude vertical drift have been observed at Jicamarca as well as in other longitudinal sectors. In general, a strengthening of the daytime maximum vertical drift with a shift from prenoon to the afternoon is observed. During the January 2013 stratospheric warming significant longitudinal differences in the equatorial vertical drift were observed. At Jicamarca the previously reported SSW behavior prevails; however, no shift of the daytime maximum drift was exhibited in the African sector. Using the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere electrodynamics general circulation model (TIME-GCM) the possible causes for the longitudinal difference are examined. The timing of the strong SSW effect in the vertical drift (15–20 January) coincides with moderate geomagnetic activity. The simulation indicates that approximately half of the daytime vertical drift increase in the American sector may be related to the moderate geophysical conditions (Kp = 4) with the effect being negligible in the African sector. The simulation suggests that the wind dynamo accounts for approximately 50% of the daytime vertical drift in the American sector and almost 100% in the African sector. The simulation agrees with previous findings that the migrating solar tides and the semidiurnal westward propagating tide with zonal wave number 1 (SW1) mainly contribute to the daytime wind dynamo and vertical drift. Numerical experiments suggest that the neutral wind and the geomagnetic main field contribute to the presence (absence) of a local time shift in the daytime maximum drift in the American (African) sector.

In this report we examine the spatial and temporal variability of the quiescent thermosphere leading up to and after the 5 April 2010 geomagnetic disturbance. We attribute the dominant driver of this variability to a combination of tides generated in situ and in the troposphere, stratosphere, and mesosphere. We identify nonmigrating tidal signatures attributable to the latter source that are ubiquitous, persistent, and significant at all thermospheric latitudes. Further, these perturbations underlie the upper atmospheric response to solar geomagnetic disturbances and are measurably altered, along with their migrating counterparts, during the storm. Our investigation is centered on a series of National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model simulations during 1–10 April 2010, including an optimal simulation with lower boundary forcing based on Modern-Era Retrospective Analysis for Research and Application reanalysis data and upper boundary forcing based on satellite and ground magnetometer measurements and the Assimilative Mapping of Ionospheric Electrodynamics procedure and three diagnostic simulations. The differences between the optimal and diagnostic simulations allow us to quantify thermospheric variability attributable to solar geomagnetic forcing and dynamical effects propagating into the thermosphere from below. We find that they can be comparable at high latitudes for some nonmigrating tidal components.

In the last decade evidence demonstrated that terrestrial weather greatly impacts the dynamics and mean state of the thermosphere via small-scale gravity waves and global-scale solar tidal propagation and dissipation effects. While observations have shown significant intra-seasonal variability in the upper mesospheric mean winds, relatively little is known about this variability at satellite altitudes (c.a., 250-400 km). Using cross-track wind measurements from the CHAMP and GOCE satellites, winds from a MERRA/TIME-CGM simulation, and outgoing long-wave radiation (OLR) data, we demonstrate the existence of a prominent and global-scale 90-day oscillation in the thermospheric zonal mean winds and in the diurnal eastward-propagating tide with zonal wavenumber 3 (DE3) during 2009-2010 and present evidence of its connection to variability in tropospheric convective activity. This study suggests that strong coupling between the troposphere and the thermosphere occurs on intra-seasonal time scales.

We report on a new source of tidal variability in the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM). Lower boundary forcing of the TIME-GCM for a simulation of November–December 2009 based on 3-hourly Modern-Era Retrospective Analysis for Research and Application (MERRA) reanalysis data includes day-to-day variations in both diurnal and semidiurnal tides of tropospheric origin. Comparison with TIME-GCM results from a heretofore standard simulation that includes climatological tropospheric tides from the global-scale wave model reveal evidence of the impacts of MERRA forcing throughout the model domain, including measurable tidal variability in the TIME-GCM upper thermosphere. Additional comparisons with measurements made by the Gravity field and steady-state Ocean Circulation Explorer satellite show improved TIME-GCM capability to capture day-to-day variations in thermospheric density for the November–December 2009 period with the new MERRA lower boundary forcing.

The National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) is utilized to understand the role that upward propagating tides play in determining the zonal mean state of the ionosphere-thermosphere system. A sensitivity assessment of the TIE-GCM shows that TIE-GCM solutions greatly depend on the lower boundary conditions. We also establish the veracity of our TIE-GCM solutions within and above the dynamo region. To isolate the mean effects of tidal dissipation, differences between TIE-GCM simulations with and without lower boundary tidal forcing as specified by the Climatological Tidal Model of the Thermosphere are investigated. Dissipation of the DW1, (diurnal westward propagating tide with zonal wave number 1), diurnal eastward propagating tide with zonal wave number 3, and SW2 (semidiurnal tide with zonal wave number 2) explains most of ∼10–30 m s−1 seasonal and latitudinal variability in zonal winds within the dynamo region, with SW2 playing a greater role than ascribed in previous studies. Tidal dissipation at low latitudes causes a 9% decrease (30% increase) in [O] ([O2]) number densities near the F2 layer peak, leading to at least a 9% decrease in peak electron density (NmF2) throughout the year. F2 layer peak height (hmF2) differences of -4 to 2 km at low latitudes are explained by variations in the field-aligned plasma motion driven by meridional wind differences induced by tidal dissipation. Compositional effects are mainly driven by DW1 and SW2, which differs from previous interpretations of tidal-driven composition changes by DW1 “tidal mixing” exclusively. We suggest that tides may produce a net transport of constituents in the thermosphere similar to the way that, e.g., gravity waves can drive net transport of sodium in the mesosphere.

This modeling study quantifies the daytime low-latitude vertical E×B drift changes in the longitudinal wave number 1 (wn1) to wn4 during the major extended January 2006 stratospheric sudden warming (SSW) period as simulated by the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere electrodynamics general circulation model (TIME-GCM), and attributes the drift changes to specific tides and planetary waves (PWs). The largest drift amplitude change (approximately 5 m/s) is seen in wn1 with a strong temporal correlation to the SSW. The wn1 drift is primarily caused by the semidiurnal westward propagating tide with zonal wave number 1 (SW1), and secondarily by a stationary planetary wave with zonal wave number 1 (PW1). SW1 is generated by the nonlinear interaction of PW1 and the migrating semidiurnal tide (SW2) at high latitude around 90–100 km. The simulations suggest that the E region PW1 around 100–130 km at the different latitudes has different origins: at high latitudes, the PW1 is related to the original stratospheric PW1; at midlatitudes, the model indicates PW1 is due to the nonlinear interaction of SW1 and SW2 around 95–105 km; and at low latitudes, the PW1 might be caused by the nonlinear interaction between DE2 and DE3. The time evolution of the simulated wn4 in the vertical E×B drift amplitude shows no temporal correlation with the SSW. The wn4 in the low-latitude vertical drift is attributed to the diurnal eastward propagating tide with zonal wave number 3 (DE3), and the contributions from SE2, TE1, and PW4 are negligible.

This study demonstrates how the mean zonal wind affects the propagation of tidal energy from the troposphere to the mesosphere-lower thermosphere (MLT) region (60–120 km). We diagnose the seasonal-latitudinal variation of the eastward-propagating diurnal tide with zonal wavenumber 3 (DE3), which is a prominent tidal component and agrees well between model and observation in recent studies. It is shown that during the course of a year the seasonal-latitudinal variation of mean zonal wind modulates and filters DE3 energy propagating upwards from tropospheric radiative and latent heating sources. Our results are based upon ISCCP and TRMM troposphere heating rates, SABER-derived zonal mean gradient wind, SABER tidal signatures and GSWM (the global-scale wave model) diagnoses. The tropospheric tidal heating rates are derived from ISCCP (International Satellite Cloud Climatology Project) radiative fluxes and TRMM (Tropical Rainfall Measuring Mission) latent heating profiles together with TRMM rainfall rates. The zonal mean winds are derived from measurements of geopotential height from the SABER (sounding of the atmosphere with broadband emission radiometry) instrument on the TIMED (thermosphere, ionosphere, mesosphere energetics and dynamics) spacecraft, and the tidal signatures are determined from SABER temperature measurements.

In this study, ground-based observations of equinox diurnal tide wind fields from the first CAWSES Global Tidal Campaign are compared with results from five commonly used models, in order to identify systematic differences. WACCM3 and Extended CMAM are both self-consistent general circulation models, which resolve general climatological features, while TIME-GCM can be forced to approximate specific conditions using reanalysis fields. GSWM is a linear mechanistic model; while GEWM is an empirical model derived from ground-based and satellite observations. The models resolve diurnal tides consistent in latitudinal structure with observations, dominated by the upward propagating (1,1) mode. There is disagreement in the magnitudes of the tidal amplitudes and vertical wavelengths, while differences in longitudinal tidal variability indicate differences in the nonmigrating tides in the models. These points suggest inconsistencies in model forcing, dissipation, and background winds that must be examined as part of a coordinated effort from the modeling community.

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.

With the launch of the TIMED satellite in December 2001, continuous temperature and wind data sets amenable to MLT tidal analyses became available. The wind measuring instrument, the TIMED Doppler Interferometer (TIDI), is operating since early 2002. Its day- and nighttime capability allows to derive tidal winds over a range of MLT altitudes. This paper presents climatologies (June 2002–June 2005) of monthly mean amplitudes and phases for six nonmigrating semidiurnal tidal components between 85 and 105 km altitude and between 45°S and 45°N latitude (westward propagating wave numbers 4, 3, 1; the standing oscillation s0; and eastward propagating wave numbers 1, 2) in the zonal and meridional wind directions. Amplitude errors are 15–20% (accuracy) and 0.8 m/s (precision). The phase error is 2 h. The TIDI analysis agrees well with 1991–1994 UARS results at 95 km. During boreal winter, amplitudes of a single component can reach 10 m/s at latitudes equatorward of 45°. Aggregate effects of nonmigrating tides can easily reach or exceed the amplitude of the migrating tide. Comparisons with the global scale wave model (GSWM) and the thermosphere–ionosphere–mesosphere–electrodynamics general circulation model (TIME-GCM) are partly inconclusive but they suggest that wave–wave interaction and latent heat release in the tropical troposphere both play an important role in forcing the semidiurnal westward 1, westward 3, and standing components. Latent heat release is the leading source of the eastward propagating components.

This article is a revision of the previous edition article by M Hagan, J Forbes, A Richmond, volume 1, pp 159–165, © 2003, Elsevier Ltd.

In this paper, we provide insights into limitations imposed by current satellite-based strategies to delineate tidal variability in the thermosphere, as well as the ability of a state-of-the-art model to replicate thermospheric tidal determinations. Toward this end, we conducted a year-long thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) simulation for 2009, which is characterized by low solar and geomagnetic activity. In order to account for tropospheric waves and tides propagating upward into the ?30–400 km model domain, we used 3-hourly MERRA (Modern-Era Retrospective Analysis for Research and Application) reanalysis data. We focus on exospheric tidal temperatures, which are also compared with 72 day mean determinations from combined Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellite observations to assess the model's capability to capture the observed tidal signatures and to quantify the uncertainties associated with the satellite exospheric temperature determination technique. We found strong day-to-day tidal variability in TIME-GCM that is smoothed out when averaged over as few as ten days. TIME-GCM notably overestimates the 72 day mean eastward propagating tides observed by CHAMP/GRACE, while capturing many of the salient features of other tidal components. However, the CHAMP/GRACE tidal determination technique only provides a gross climatological representation, underestimates the majority of the tidal components in the climatological spectrum, and moreover fails to characterize the extreme variability that drives the dynamics and electrodynamics of the ionosphere-thermosphere system. A multisatellite mission that samples at least six local times simultaneously is needed to provide this quantification.

Preliminary meridional wind data from the TIMED Doppler Interferometer (TIDI) onboard the TIMED satellite are analyzed for nonmigrating diurnal tides. Tidal definitions are given for the most pronounced westward, eastward and standing oscillations (w2, e3, s0). The analysis interval is March 2002 to June 2004 and covers the altitude range 85-105 km. Monthly tidal wind fields from 40 � S-40 � N are presented. TIDI tides compare favorably with previously reported 95 km HRDI results. Nonmigrating diurnal tidal wind speeds larger than 30 m/s are observed thus emphasizing the important role of nonmigrating tides in middle atmosphere coupling. A comparative analysis with the Global Scale Wave Model (GSWM) and the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) indicates that both latent heat release in the tropical troposphere and non-linear Planetary Wave / migrating tide interaction are important sources of nonmigrating tides in the mesosphere and lower thermosphere.

HRDI (High Resolution Doppler Interferometer-UARS) winds data have been analyzed in 4°-latitude by 10°-longitude cells at 96km to obtain the global distribution of the solar-tidal amplitudes and phases. The solstices June–July (1993), December–January (1993–1994), and one equinox (September–October, 1994) are analyzed. In an earlier paper (Manson et al., 2002b) the emphasis was solely upon the longitudinal and latitudinal variations of the amplitudes and phases of the semidiurnal (12h) and diurnal (24h) tides. The longitudinal structures were shown to be quite distinctive, and in the case of the EW component of the diurnal tide there were typically four maxima/perturbations of amplitudes or phases around a latitude circle. In this case they tended to be associated with the locations of the major oceans. Here, a spatial complex spectral analysis has been applied to the data set, to obtain the zonal wave numbers for the tides as functions of latitude. For the diurnal tide the dominant s=1 migrating component and nonmigrating tides with wave numbers s=–3, –2, 0, 2 are identified; and for the semidiurnal tide, as well as the dominant s=2 migrating component, the spectra indicate the presence of nonmigrating tides with wave numbers s=–2, 0, 4. These wave numbers are also simply related to the global longitudinal structures in the tidal amplitudes and phases. Comparisons are made with the Global Scale Wave Model (GSWM-02), which now incorporates migrating and nonmigrating tides associated with tropospheric latent heat processes, and offers monthly outputs. For the diurnal tide the dominant nonmigrating tidal spectral feature (94km) is for wave number s=–3; it is relatively stronger than in the HRDI winds, and produces quite consistent structures in the global tidal fields with four longitudinal maxima. Overall, the modelled 24-h tidal amplitudes are larger than observed during the equinox beyond 40° latitude. For the semidiurnal tide, nonmigrating tides are frequently indicated in the spectra with wave numbers s=–2, 0, 6; and there are complementary longitudinal structures in the global tidal fields with two and four maxima evident. Modelled 12-h tidal amplitudes are much smaller than observed during non-winter months beyond 30°. There is a detailed discussion of the spectral features, their seasonal variations, and the similarities with the HRDI tidal data. This discussion is in the context of the inherent limitations of the model.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; thermospheric dynamics; waves and tides)

Nonlinear interactions between stationary waves and the migrating tides have been proposed as possible sources of nonmigrating tides in the middle and upper atmosphere. The objective of this study is to increase observational support for these processes. We examine the evolution of stationary planetary waves and nonmigrating diurnal tides in the lower mesosphere during November 1978–May 1979, based on a newly released, version 6 of the Nimbus 7 LIMS dataset. Planetary wavenumber one is large and variable during the Northern hemisphere winter months, reaching peak amplitude in the lower mesosphere between 20 and 30 January. This behavior is accompanied by rapid amplification of nonmigrating diurnal tides with zonal wavenumbers zero and two. These components correspond to product waves generated by interaction between the migrating diurnal tide and the stationary wave. The westward traveling zonal wavenumber two diurnal tide is dominant at tropical latitudes, in accordance with theoretical studies. The correlation between the nonmigrating tide and stationary wavenumber one is highest when the stationary wave penetrates to subtropical latitudes.

Migrating and nonmigrating diurnal tides in the temperature data from the satellite borne, Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment are compared to model predictions from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) and from the Global Scale Wave Model (GSWM). The comparative model/observation analysis is performed between 75–120 km altitude and at 7.5°N. The results suggest that interactions between quasi-stationary planetary waves and radiative and latent heat forced tidal components may play an important role in generating the thermal tidal structure above the mesopause though more extended model studies are necessary.

Observations of mean winds and semidiurnal and diurnal tides in the mesosphere/lower-thermosphere (MLT) region were made during the 3-month Planetary-Scale Mesopause Observing System Summer 1999 campaign. Data from 22 ground-based radars (and from two other instruments with measurements for the same period but in 1998) allow us to investigate the ability of the GSWM-00 to simulate the solar tides in the mesopause region (90-95 km). Here we have found that the GSWM-00 provides an increasingly reasonable estimate of most of the tidal characteristics in the MLT region. However, the representation of the 24 h tide appears superior to that of the 12 h tide. Some of these discrepancies are studied in detail. In particular, the observations reveal significant 12 h tidal amplitudes at high latitudes in the Northern Hemisphere summer. There is evidence for relation between the longitudinal variability of the mean zonal wind and the tidal characteristics seen from the radar wind measurements in the summer middle latitudes and a quasi-stationary planetary wave with zonal wave number one.

We present a case study of the 5 April 2010 geomagnetic storm using observations and numerical simulations. The event was driven by a fast-moving coronal mass ejection and despite being a moderate storm with a minimum Dst near ?50 nT, the event exhibited elevated thermospheric density and surges of traveling atmospheric disturbances (TADs) more typically seen during major storms. The Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIMEGCM) was used to assess how these features were generated and developed during the storm. The model simulations gave rise to TADs that were highly nonuniform with strong latitude and longitude/local time dependence. The TAD phase speeds ranged from 640?m/s to 780?m/s at 400?km and were ~5% lower at 300?km and approximately 10–15% lower at 200?km. In the lower thermosphere around 100?km, the TAD signatures were nearly unrecognizable due to much stronger influence of upward propagating atmospheric tides. The thermosphere simulation results were compared to observations available from the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE), CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites. Comparison with GOCE data shows that the TIMEGCM reproduced the cross-track winds over the polar region very well. The model-data comparison also revealed some differences, specifically, the simulations underestimated neutral mass density in the upper thermosphere above ~300?km and overestimated the storm recovery tome by 6 h. These discrepancies indicate that some heating or circulation dynamics and potentially cooling processes are not fully represented in the simulations, and also that updates to some parameterization schemes in the TIMEGCM are warranted.

We used the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) to calculate the variability of the migrating diurnal tide in a January through December 1993 simulation. While TIME-GCM captures the salient features of the latitudinal, altitudinal, and seasonal variability of the migrating diurnal tide, upper mesospheric meridional wind amplitudes are somewhat smaller than those that have been observed from the ground and space. The discrepancies may be attributable to unresolved uncertainties in tidal forcing and/or dissipation in the TIME-GCM. However, our diagnostic simulation suggests that the nonlinear interactions between the migrating diurnal tide and stationary planetary wave 1 produce measurable nonmigrating diurnal tidal components that modulate migrating diurnal tidal amplitudes and account for significant variability in the upper mesosphere and lower thermosphere.

Diurnal tides are forced in a global-scale wave model using monthly mean tropospheric heating rates from the NCEP/NCAR Reanalysis Project. The heating rates give rise to a semiannual variation in the migrating (sun-synchronous) tidal wind response near 95 km altitude with equinoctial maxima (∼50–70 ms−1) similar in magnitude to those derived from observational data. Nonmigrating (longitude-dependent) tides with zonal wavenumbers s=−3 (eastward) to s=+5 (westward) are also generated. Many of these oscillations are characterized by wind and temperature amplitudes of order 20–30 ms−1 and 10–20 K in the lower thermosphere (100–130 km), and suggest that the tidal response in this regime varies considerably with longitude.

The propagation characteristics of a diurnal Kelvin wave (DKW) in the atmosphere of Mars and a 3-day Kelvin wave in the terrestrial atmosphere are compared and interpreted for Northern Hemisphere summer conditions (Ls = 90 and July, respectively). Significant horizontal wind and temperature perturbations are shown to accompany these oscillations in the thermospheres of both planets: ∼10–40 ms−1 and 10–25 K on Earth and 20–70 ms−1 and 10–30 K on Mars. Molecular dissipation on both planets serves to latitudinally broaden the thermospheric response, and to induce meridional wind maxima at the poles. On Earth, mean zonal winds asymmetric about the equator are found to locally (in altitude) distort the latitudinal shapes of the Kelvin wave fields by coupling into modes which tend to remain confined to the level of excitation, and which are equatorially trapped. On Mars the first asymmetric eastward propagating mode with vertical wavelength of about 60 km is very efficiently induced by the asymmetric zonal mean winds in the middle atmosphere; this wave component propagates well into the thermosphere and accounts for much global asymmetry seen in the wave fields.

During shuttle missions STS-66 (November, 1994) and STS-85 (August, 1997) the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) acquired temperature data with very high spatial resolution. These are analyzed for gravity waves (GW). The altitude range spans the whole middle atmosphere from the tropopause up to the mesopause. In the upper mesosphere tidal amplitudes exceed values of 10 K. Modulation of GW activity by the tides is observed and analyzed using CRISTA temperatures and tidal predictions of the Global Scale Wave Model (GSWM). The modulation process is identified as a tidally-induced change of the background buoyancy frequency. The findings agree well with the expectations for saturated GW and are the first global scale observations of this process.

We derive rates of energy deposition in the mesosphere due to the absorption of solar ultraviolet radiation by ozone. The rates are derived directly from measurements of the 1.27-microns oxygen dayglow emission, independent of knowledge of the ozone abundance, the ozone absorption cross sections, and the ultraviolet solar irradiance in the ozone Hartley band. Fifty-six months of airglow data taken between 1982 and 1986 by the near-infrared spectrometer on the Solar-Mesosphere Explorer satellite are analyzed. The energy deposition rates exhibit altitude-dependent annual and semi-annual variations. We also find a positive correlation between temperatures and energy deposition rates near 90 km at low latitudes. This correlation is largely due to the semiannual oscillation in temperature and ozone and is consistent with model calculations. There is also a suggestion of possible tidal enhancement of this correlation based on recent theoretical and observational analyses. The airglow-derived rates of energy deposition are then compared with those computed by multidimensional numerical models. The observed and modeled deposition rates typically agree to within 20%. This agreement in energy deposition rates implies the same agreement exists between measured and modeled ozone volume mixing ratios in the mesosphere. Only in the upper mesosphere at midlatitudes during winter do we derive energy deposition rates (and hence ozone mixing ratios) consistently and significantly larger than the model calculations. This result is contrary to previous studies that have shown a large model deficit in the ozone abundance throughout the mesosphere. The climatology of solar energy deposition and heating presented in this paper is available to the community at the Middle Atmosphere Energy Budget Project web site at http://heat-budget.gats-inc.com.

Continuous observations of the wind field have been made by six Medium Frequency Radars (MFRs), located between the equator and high northern latitudes: Christmas Islands (2°N), Hawaii (22°N), Urbana (40°N), London (43°N), Saskatoon (52°N) and Tromso (70°N). Data have been sought for the time interval 1990–1997, and typically 5 years of data have become available from each station, to demonstrate the level of annual consistency and variability. Common harmonic analysis is applied so that the monthly amplitudes and phases of the semi-diurnal (SD) and diurnal (D) wind oscillations are available in the height range of (typically) 75–95 km in the upper Middle Atmosphere. Comparisons are made with tides from the Global Scale Wave Model (GSWM), which are available for 3-month seasons. The emphasis is upon the monthly climatologies at each location based upon comparisons of profiles, and also latitudinal plots of amplitudes and phases at particular heights. For the diurnal tide, the agreement between observations and model is now quite excellent with modelled values frequently lying within the range of yearly values. Both observations and model demonstrate strong seasonal changes. This result is a striking improvement over the comparisons of 1989 (JATP, Special issue). In particular, the phases and phase-gradients for the non-winter months at mid- to high-latitudes are now in excellent agreement. Some of the low latitude discrepancies are attributed to the existence of non-migrating tidal components associated with tropospheric latent heat release. For the semi-diurnal tide, the observed strong transitions between clear solstitial states are less well captured by the model. There is little evidence for improvement over the promising comparisons of 1989. In particular, the late-summer/autumnal tidal maximum of mid-latitudes is observed to be larger, and with strong monthly variability. Also the summer modelled tide has unobserved short (20 km) wavelengths at high latitudes, and much smaller amplitudes than observed at all extratropical locations. Possible improvements for the GSWM’s simulations of the SD tide are discussed, which involve migrating tidal modes due to tropospheric latent heating.

We report on new global-scale wave model (GSWM) predictions for the migrating solar tide in the troposphere, stratosphere, mesosphere and lower thermosphere. The model revision, hereafter GSWM-98, includes an updated gravity wave (GW) stress parameterization and modifications to the background atmosphere based on 6-year monthly averaged Upper Atmosphere Research Satellite (UARS) climatologies. UARS Halogen Occultation Experiment and Microwave Limb Sounder ozone data are used to define the strato-mesospheric tidal source, while GSWM-98 background winds are based on UARS High Resolution Doppler Interferometer (HRDI) zonal mean zonal wind data. We quantify and interpret differences between previous diurnal and semidiurnal predictions, hereafter GSWM-95, and GSWM-98 results. The revised GW stress parameterization accounts for the most profound changes and leads to seasonal variability predictions that are consistent with diurnal amplitudes observed in the upper mesosphere and lower thermosphere. Unresolved differences between HRDI and other wind climatologies significantly affect MLT tidal predictions.

The effects of latent heat release due to deep tropical convection on solar tidal variability in the 80 – 150 km altitude region are examined. The latent heating estimates are based on 7-year mean monthly 3-hourly rainfall rates obtained through the Global Precipitation Climatology Project. The latent heating rates are used to force the Global Scale Wave Model (GSWM) to obtain monthly estimates of the migrating diurnal (zonal wavenumber s = 1), migrating semidiurnal (s = 2) and standing (s = 0) diurnal tidal oscillations. It is demonstrated that monthly variations of up to 20 ms −1 and 10 K are attributable to this excitation source, as well as longitudinal variations in the diurnal tide of order 15 – 25 ms −1 at peak amplitudes. Higher-order zonal wavenumber contributions are expected to be as important, but are not considered within the scope of the present work.

To understand how the quasi-two-day wave (QTDW) interacts with the mean flow and migrating tides we perturbed the lower boundary of the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) to excite the QTDW. Results from our initial model runs, computed for a perpetual January 15th, show wave amplitudes in wind and temperature that are in qualitative agreement with previous model results and observations. The wave period and amplitude change over the duration of the model run and it is expected that this results from changes in the zonal mean zonal winds. Evidence supporting nonlinear interactions between the QTDW and the migrating tides is presented.

Observations of sea surface mean square slope (mss) were made with a scanning radar altimeter (SRA) from a low-flying research aircraft on a light-wind day during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. Wind speed (measured at 60- to 90-m height and extrapolated to the 10-m level) and mss were found to be positively correlated, with correlation coefficients in some cases as high as those obtained for wind speed measurements on adjacent aircraft flying wingtip to wingtip with a 100-m lateral separation of their fuselages. The SRA measurements suggest a much larger azimuthal asymmetry in mss at 36 GHz than was observed in the seminal Cox and Munk optical experiment; the SRA measurements showed a crosswind-to-upwind ratio as low as of 0.26 at 1.8 m s -1 compared to the typical value of 0.8 for Cox and Munk. For higher wind speed the mss becomes more isotropic. The data suggest that waves generated by a wind speed less than 2 m s -1 are restricted to the downwind direction. The crosswind-to-upwind ratio would probably have been even lower had it not been for an isotropic background mss caused by an assortment of swell.

Using a helium plus reversible fluid balloon system as the observing platform, multiple profiles of shortwave irradiance between 4 and 10 km were recently obtained over the Los Angeles basin. Measurements of downwelling hemispheric broadband irradiance were made over a period of six hours in conditions that could be characterized by a mid-latitude, summer model atmosphere. These data are described and compared to model computations using a spectrally-resolving, plane parallel, multiple scattering model. The systematic difference between model and observations (10%) at high altitudes is discussed in terms of possible factors that affect the instrument measurement response under cold temperature conditions. The measurements and calculations of downwelling irradiance agree to better than 5% in the lower troposphere.

The global-scale wave model (GSWM) is used to investigate the effects of mean winds and realistic dissipation on upward propagating nonmigrating diurnal tidal components. We explore the signatures of two plausible tropospheric sources of these waves, namely, latent heat release associated with deep convective activity and the absorption of solar infrared radiation which varies zonally with longitudinal variations in tropospheric water vapor. Our calculations suggest that while nonmigrating components are up to a factor of 3 times smaller than the migrating diurnal tide, they modulate the latter and introduce measurable (∼10 m/s) longitudinal variability in the mesosphere and lower thermosphere. These effects are most pronounced in the GSWM for the latent heat source and during northern hemisphere winter.

A lunar tidal model similar to that of Forbes [1982] is used to investigate the effects of changes in background zonal winds on the structure of the lunar tide in the upper atmosphere. It is found that changes in the winds have a greater effect than changes in the background temperature profile with latitude. Observed changes in the structure of the lunar tide from year to year at Saskatoon may be explained by introducing stratospheric warnings into the model. The addition of an associated mesospheric cooling gives an even better fit to the observed phase data. It is also seen that northern hemisphere warnings are predicted to give considerable amplitude changes in southern hemisphere lunar tides. It is expected that much of the difficulty in measuring lunar tides in the upper atmosphere is due to changes in the tidal parameters at these levels brought about by variations in the underlying atmosphere.

We attempt to find the northern hemisphere zonal wavenumber for a striking quasi-2-day wave “event” or “burst” observed near 90 km altitude in the summer of 1992. A unique set of data on the upper atmosphere from nine radar sites is analysed (spacings ∼400– ∼ 12,000 km), and compared with expectations from models. The 2-day wave phase comparison, which finds zonal wavenumber m = 4, is conclusive. Determination of n, which defines the meridional wave amplitude structure, is not attempted, as the sites here have only a small latitude spread (21°N to 55°N). Also the amplitude seems to be unstable showing some sort of modulation which is not simultaneous at all sites. Finally, the radars have not been “calibrated” against each other in terms of wind speed. This calibration would have to be done before small differences in wave amplitude could be believed. A similar event in 1991 for which fewer sites are available is also discussed. Here the choice between m = 3 and 4 is not as clear.

A steady state two-dimensional linearized model that extends from the ground into the thermosphere and captures the salient features of migrating diurnal and semidiurnal tidal components is used to investigate the comparative importance of the principal sources of these waves. The results, which have previously gone unreported in the literature, demonstrate the nonnegligible effects of atmospheric absorption of solar radiation at infrared (in the troposphere) and ultraviolet (in the stratosphere) wavelengths on mesospheric and lower thermospheric semidiurnal and diurnal tidal fields, respectively. In addition, latent heat release associated with cloudiness or rainfall in the troposphere is shown to be another plausible source of semidiurnal variability in the upper atmosphere. The important effects of these sources on the dynamics of the mesosphere and lower thermosphere emphasize the need to include realistic parameterizations of migrating tides at the lower boundaries of middle and upper atmospheric general circulation models. The results of this investigation also suggest that updated parameterizations of tropospheric tidal forcing are needed to further current understanding of tidal variability in the upper atmosphere.

Observations of the mesosphere and lower thermosphere winds obtained by the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) during 1991 to 1995 reveal a semiannual variation in the amplitude of the (1,1) diurnal tide. The global-scale wave model (GSWM) represents the first numerical modeling attempt at simulating this seasonal variability, and a preliminary comparison of the GSWM tidal results with HRDI measurements is presented. The results of the comparison and of numerical tests point to some vital and unresolved questions regarding tidal dissipation and tropospheric forcing. In addition to the seasonal variability, HRDI has revealed a strong interannual modulation of the diurnal tide with amplitudes observed to change by nearly a factor of 2 from 1992 to 1994.

[1] We compare and examine diurnal temperature tides including their migrating component (DW1) from the troposphere to the lower mesosphere, using data from Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) and from six different reanalysis data sets: (1) the Modern Era Retrospective analysis for Research and Applications (MERRA), (2) the European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis (ERA-Interim) (3) the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), (4) the Japanese 25-year reanalysis by Japanese Meteorological Agency (JMA) and the Central Research Institute of Electric Power Industry (CRIEPI) (JRA25), (5) the NCEP/National Center for Atmospheric Research reanalysis (NCEP1), and (6) the NCEP and Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) reanalysis data (NCEP2). The horizontal and vertical structures of the diurnal tides in SABER and reanalyses reasonably agree, although the amplitudes are up to 30–50% smaller in the reanalyses than in the SABER in the upper stratosphere to lower mesosphere. Of all tidal components, the DW1 is dominant while a clear eastward propagating zonal wave number 3 component (DE3) is observed at midlatitudes of the Southern Hemisphere in winter. Among the six reanalyses, MERRA, ERA-Interim and CFSR are better at reproducing realistic diurnal tides. It is found that the diurnal tides extracted from SABER data in the winter-hemisphere stratosphere suffer from sampling issues that are caused by short-term variations of the background temperature. In addition, the GSWM underestimates the amplitude in the midlatitude upper stratosphere by about 50%.

More than 60 multiday incoherent scatter radar experiments conducted at Millstone Hill, Massachusetts, between 1984 and 1990 have been analyzed to determine neutral winds along the magnetic meridian. The wind determinations from geomagnetically undisturbed periods have been sorted into seasonal, solar cycle, and local time bins, and climatological averages have been calculated and compared with complementary empirical wind model predictions. The climatological wind averages have also been harmonically decomposed. Strong seasonal and solar cycle dependences characterize both the mean winds and diurnal wind amplitudes. This analysis suggests that the aurorally driven high-latitude circulation cell is important to the interpretation of variability in thermospheric circulation over Millstone Hill even during geomagnetically undisturbed periods. 40 refs., 3 figs., 3 tabs.

The study reports simultaneous observations of the neutral winds and temperatures and electron densities in the upper and lower thermosphere. The measurements were made at Arecibo and Millstone Hill during the first Lower Thermosphere Coupling Study (LTCS 1) campaign in September 1987. The observations show much day-to-day variability, possibly due to the geomagnetic activity which occurred during the period; storm effects are discernible in the data at low altitudes even at low latitudes. Simulations of the low and midlatitudes did not substantially improve with the use of lower boundary conditions derived from the measurements. The model predicts that diurnal waves are not negligible in analysis of lower thermosphere data; near 100 km the diurnal wave amplitudes of the meridional waves were predicted to be greater than the semidiurnal.

[1] In atmospheric and space environment studies it is key to understand and to quantify the coupling of atmospheric regions and the solar impacts on the whole atmosphere system. There is thus a need for a numerical model that encompasses the whole atmosphere and can self-consistently simulate the dynamic, physical, chemical, radiative, and electrodynamic processes that are important for the Sun-Earth system. This is the goal for developing the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM). In this work, we report the development and preliminary validation of the thermospheric extension of WACCM (WACCM-X), which extends from the Earth's surface to the upper thermosphere. The WACCM-X uses the finite volume dynamical core from the NCAR Community Atmosphere Model and includes an interactive chemistry module resolving most known neutral chemistry and major ion chemistry in the middle and upper atmosphere, and photolysis and photoionization. Upper atmosphere processes, such as nonlocal thermodynamic equilibrium, radiative transfer, auroral processes, ion drag, and molecular diffusion of major and minor species, have been included in the model. We evaluate the model performance by examining the quantities essential for the climate and weather of the upper atmosphere: the mean compositional, thermal, and wind structures from the troposphere to the upper thermosphere and their variability on interannual, seasonal, and daily scales. These quantities are compared with observational and previous model results.

[1] The latitudinal structure of the nighttime thermospheric mass density at 385 km has been investigated using observations made by the accelerometer onboard the CHAMP satellite between 2002 and 2007. The nighttime thermospheric mass density had a clear latitudinal variation. There was a local density peak around the geographic equator and two minima at about ±30°. This nighttime equatorial mass anomaly (NEMA) was opposite to the latitudinal variations of the daytime ionospheric equatorial anomaly and thermospheric mass density anomaly as both have minima at the magnetic dip equator and maxima around ±20° in magnetic latitudes. This anomalous behavior of the nighttime thermospheric mass density had strong local time, seasonal, hemispheric, and solar cycle dependences. The largest crest-to-trough ratio between the peak density at the equator and the minima at the anomaly latitudes (±30°) occurred between 0000 and 0200 local time. The NEMA appeared to be more pronounced during solstice seasons. It was also stronger during solar minimum than during solar maximum. In addition, under the same geophysical conditions, the empirical NRLMSISE-00 model shows similar but much weaker nighttime mass density anomaly features at low and middle latitudes. A high-resolution National Center for Atmospheric Research-Thermosphere Ionosphere Mesosphere Energetics Global Circulation Model (TIMEGCM) simulation reproduced most of the observed latitudinal variations of the nighttime mass density as well as the thermospheric midnight temperature maximum (MTM). Model results suggest that superposition of diurnal and semidiurnal migrating tides of modes up to wave number 6 is the likely cause of both the NEMA and MTM phenomena.

[1] Part 2 of our study examines the relative importance of radiative heating and latent heating in accounting for vertically propagating tides that impose longitude variability on mesosphere-lower thermosphere (MLT) winds, temperatures, and densities. Our results are based upon numerical simulations using the Global-Scale Wave Model (GSWM) and new tidal heating rates derived from International Satellite Cloud Climatology Project (ISCCP) radiative fluxes (see part 1), Tropical Rainfall Measuring Mission (TRMM) latent heating profiles, and TRMM rainfall rates. Contrary to previous results and general perceptions, we demonstrate that radiative heating is more important than latent heating in accounting for MLT longitude variability due to tides although latent heating causes some large nonmigrating tidal oscillations such as DE3. Through comparison with TIMED SABER temperature measurements, the model results are shown to approximate many observed features of this longitude variability.

[1] This study demonstrates that the diurnal cycle of net radiative heating in the troposphere accounts for considerable longitudinal variability of diurnal and semidiurnal tidal fields in the mesosphere and lower thermosphere (MLT) (∼80–120 km), whereas previously it was thought that latent heating associated with deep tropical convection is the predominant driver of this variability. The heating rates used for this study are derived from radiative flux products by NASA Goddard Institute for Space Studies (GISS), and the model employed to estimate the corresponding MLT tides is the Global-Scale Wave Model (GSWM). The radiative flux products by NASA GISS utilize improved International Satellite Cloud Climatology Project (ISCCP) cloud climatology and ancillary data sets and were validated by Earth radiation Budget Experiment (ERBE) and Clouds and the Earth's Radiant Energy System (CERES) radiative flux (0.2–200.0 microns) measurements at the top of the atmosphere and the Earth surface. Typical magnitudes of tidal temperature longitude variations at, e.g., 95 km or 110 km are 20 ± 5 K for the diurnal tide and 6 ± 2 K for the semidiurnal tide. The computed tides and their longitude variability are of comparable amplitude to those derived from TIMED SABER temperature measurements. Part 2 of this study provides new estimates of tidal forcing by latent heating and assesses the total MLT tidal response to these combined heat sources in comparison to tidal climatologies derived from TIMED SABER measurements.

[1] A number of recent studies have highlighted the observational evidence for a coupling between atmospheric tides in the thermosphere and the electron density structure of the ionosphere. The most commonly proposed mechanism to explain this is an electrodynamic coupling between tides at E region altitudes and ion drifts at F region altitudes. However, based on both the observational evidence from recent satellite missions such as those of the neutral winds associated with nonmigrating tides at F region altitudes, and considering the theoretical effects of atmospheric tides on the thermosphere and ionosphere, more than one coupling mechanism must be considered. We use Sami2 is Another Model of the Ionosphere to test a set of electrodynamic and chemical-dynamical coupling mechanisms that could explain the link between tides in the thermosphere and the low-latitude ionosphere. We investigate the possible role of the vertical drifts during the both the day and around sunset, perturbations to the thermospheric neutral density and thermospheric [O]/[N2], and tidal winds at F region altitudes. These simulations give an estimate of the sensitivity of the nighttime ionosphere to each of these coupling mechanisms. We then compare the results of these sensitivity tests with the effects of atmospheric tides on different thermospheric parameters as simulated by a self-consistent model of the atmosphere-ionosphere-electrodynamic system (thermosphere-ionosphere-mesosphere-electrodynamics general circulation model). This comparison shows that in addition to the E region dynamo modulation, the potential coupling between tides and the ionosphere via changes in thermospheric [O]/[N2], meridional winds at F region altitudes, and modification of the vertical drifts around sunset could play an important role and all require further study, both with models and new observations.

[1] Climatologies of gravity waves, quasi-stationary planetary waves, and tides are compared in the upper stratosphere, mesosphere, and lower thermosphere. Temperature standard deviations from zonal means are used as proxies for wave activity. The sum of the waves is compared to directly measured total temperature fluctuations. The resulting difference is used as a proxy for traveling planetary waves. A preliminary climatology for these waves is proposed. A ranking of the four wave types in terms of their impact on the total wave state of the atmosphere is achieved, which is dependent on altitude and latitude. At extratropical latitudes, gravity waves mostly play a major role. Traveling planetary waves are found to play a secondary role. Quasi-stationary planetary waves and tides yield a lesser contribution there. Vertical profiles of total temperature fluctuations show a sharp vertical gradient change (“kink” or “bend”) in the mesosphere. This is interpreted in terms of a change of wave damping, and the concept of a “wave turbopause” is suggested. The altitude of this wave turbopause is found to be mostly determined by the relative intensities of gravity waves and planetary waves. The turbopause is further analyzed, including earlier mass spectrometer data. It is found that the wave turbopause and the mass spectrometer turbopause occur rather close together. The turbopause forms a layer about 8 km thick, and the data suggest an additional 3 km mixing layer on top.

[1] Numerical experiments that explore the effects of tides of tropospheric origin on the upper and middle atmosphere reveal strong signatures of an eastward propagating zonal wave number 3 diurnal tide (DE3), which peaks near 110 km and penetrates into the upper thermosphere. We demonstrate that nonmigrating tidal dissipation dominated by DE3 effects in the upper mesosphere and lower thermosphere (MLT) strongly accelerates the zonal mean wind field, affecting both the altitude and magnitude of the low-latitude jets. We also quantify, for the first time, a stationary planetary wave 4 oscillation (sPW4) in the MLT, which is excited by nonlinear interaction between the DE3 and the migrating diurnal tide (DW1). Our results suggest that the sPW4 modulates the DE3 MLT response and may impact the E region dynamo as well as ionospheric and thermospheric signatures at much higher altitudes.

[1] Four years (2002–2005) of continuous accelerometer measurements taken onboard the CHAMP satellite (orbit altitude ∼400 km) offer a unique opportunity to investigate the thermospheric zonal wind on a global scale. Recently, we were able to relate the longitudinal wave-4 structure in the zonal wind at equatorial latitudes to the influence of nonmigrating tides and in particular to the eastward propagating diurnal tide with zonal wave number 3 (DE3). The DE3 tide is primarily excited by latent heat release in the tropical troposphere in deep convective clouds. In order to investigate the mechanisms that couple the tidal signals to the upper thermosphere, we undertook a comparison with the thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) developed at the National Center for Atmospheric Research (NCAR). We ran the model for a day in March, June, September, and December and applied the same processing steps to the model output as was done for the CHAMP tidal analysis. The main results of the comparison can be summarized as follows: (1) TIME-GCM simulations do not correctly reproduce the observed intra-annual variations of DE3 and the eastward propagating diurnal tide with zonal wave number 2 (DE2). (2) Simulations of DE3 for June are more successful. Both TIME-GCM and CHAMP show an increase in DE3 amplitudes with decreasing solar flux level. (3) The amplitudes of the simulated westward propagating diurnal tide with zonal wave number 2 (DW2) and the standing diurnal tide (D0) increase with increasing solar flux in June. The predicted dependence of DW2 and DO on solar flux is also observed by CHAMP.

[1] Using the TIMED Doppler interferometer (TIDI) mesospheric and lower thermospheric neutral-wind multiyear data set (2002–2007) and NCAR TIME General Circulation Models (GCM) 1.2 annual run results (2002–2005) at the TIDI sampling points, we study the migrating diurnal tide's global distribution, interannual, and seasonal variations in connection with the mean zonal wind interannual variations. A strong quasi-biennial oscillation (QBO) effect on the diurnal tide was observed in the TIDI data and reproduced to a lesser degree in the TIME-GCM run. The migrating diurnal tide amplitude is larger during the eastward phase of the stratospheric QBO and weaker during the westward phase. Westward mesospheric equatorial mean zonal winds appeared during the eastward phase of the stratospheric QBO (in 2002, 2004, and 2006). The strongest QBO effect on both the migrating diurnal tide and mean zonal winds was observed during the March equinox. The stronger tides may be related to the weaker gravity wave filtering in the stratosphere during the eastward phase stratospheric QBO. The TIDI data also exhibit large interhemispheric asymmetry. The westward mean zonal winds in the mesosphere appeared to be associated with the enhanced diurnal tide. The TIME-GCM 1.2 diurnal tide amplitudes are in general smaller than those observed by the TIDI instrument. Limited vertical spatial resolution for the TIME-CGM 1.2 is suggested as the cause. Future improvements are expected with a higher spatial resolution in the model.