Tidal Wind Research Articles

Evidence of Tropospheric Effects on the Ionosphere

A new paradigm in upper atmospheric and ionospheric physics has begun to emerge, starting with discoveries from observations taken with the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite in 2002. These discoveries [Immel et al., 2006] show that the entire ionosphere, sometimes referred to as “the inner edge of space,” regularly responds to the tropospheric weather systems below. The likely mechanism for effectively carrying tropospheric energy upward to the edge of space to modify the ionosphere is the generation and upward propagation of large‐scale waves, known as atmospheric tides. The coupling of these tides to the ionosphere is inferred from the strong similarity in longitudinal structure of the tidal winds and the ionospheric densities at low latitudes. However, the appearance of this behavior in the upper ionosphere is surprising because the tides that are forced by tropospheric weather are believed to dissipate at the base of the ionosphere and are expected to have little effect at higher altitudes.

Seasonal variability and descent of mid-latitude sporadic E layers at Arecibo

Abstract. Sporadic E layers (Es) follow regular daily patterns in variability and altitude descent, which are determined primarily by the vertical tidal wind shears in the lower thermosphere. In the present study a large set of sporadic E layer incoherent scatter radar (ISR) measurements are analyzed. These were made at Arecibo (Geog. Lat. ~18° N; Magnetic Dip ~50°) over many years with ISR runs lasting from several hours to several days, covering evenly all seasons. A new methodology is applied, in which both weak and strong layers are clearly traced by using the vertical electron density gradient as a function of altitude and time. Taking a time base equal to the 24-h local day, statistics were obtained on the seasonal behavior of the diurnal and semidiurnal tidal variability and altitude descent patterns of sporadic E at Arecibo. The diurnal tide, most likely the S(1,1) tide with a vertical wavelength around 25 km, controls fully the formation and descent of the metallic Es layers at low altitudes below 110 km. At higher altitudes, there are two prevailing layers formed presumably by vertical wind shears associated mainly with semidiurnal tides. These include: 1) a daytime layer starting at ~130 km around midday and descending down to 105 km by local midnight, and 2) a less frequent and weaker nighttime layer which starts prior to midnight at ~130 km, descending downwards at somewhat faster rate to reach 110 km by sunrise. The diurnal and semidiurnal-like pattern prevails, with some differences, in all seasons. The differences in occurrence, strength and descending speeds between the daytime and nighttime upper layers are not well understood from the present data alone and require further study.

Tropospheric tides from 80 to 400 km: Propagation, interannual variability, and solar cycle effects

Recent observations and model simulations demonstrate unequivocally that non‐Sun‐synchronous (nonmigrating) tides due to deep tropical convection produce large longitudinal and local time variations in bulk ionosphere‐thermosphere‐mesosphere properties. We thus stand at an exciting research frontier: understanding how persistent, large‐scale tropospheric weather systems affect the geospace environment. Science challenge questions include: (1) How much of the tropospheric influence is due to tidal propagation directly into the upper thermosphere? (2) How large is the interannual and the solar cycle variability of the tides and what causes them? These questions are addressed using solar maximum to solar minimum tidal wind and temperature analyses from the Thermosphere Ionosphere Mesosphere Electrodynamics and Dynamics (TIMED) satellite in the mesosphere/lower thermosphere (MLT), and from the Challenging Minisatellite Payload (CHAMP) satellite at ∼400 km. A physics‐based empirical fit model is used to connect the TIMED with the CHAMP tides, i.e., to close the “thermospheric gap” of current spaceborne observations. Temperature, density, and horizontal and vertical wind results are presented for the important diurnal, eastward, wave number 3 (DE3) tide and may be summarized as follows. (1) Upper thermospheric DE3 tidal winds and temperatures are fully attributable to troposphere forcing. (2) A quasi‐2‐year 15–20% amplitude modulation in the MLT is presumably caused by the QBO. No perceivable solar cycle dependence is found in the MLT region. DE3 amplitudes in the upper thermosphere can increase by a factor of 3 in the zonal wind, by ∼60% in temperature and by a factor of 5 in density, caused by reduced dissipation above 120 km during solar minimum.

Low‐altitude quasi‐periodic echoes studied using a large database of Gadanki radar observations

In this paper we present studies on low‐altitude quasiperiodic (LQP) echoes based on a large database of Gadanki radar observations. LQP echoes have been observed 33% of the time during daytime and 39% during nighttime. Their occurrence is found to be maximum in the summer (daytime, 58%; nighttime, 57%), followed by the September equinox (daytime, 32%; nighttime, 48%), the March equinox (daytime, 26%; nighttime, 36%), and minimum in the winter (daytime, 25%; nighttime, 26%). Height‐time occurrence of LQP echoes shows two local time maxima: one in the morning (0700–1100 LT) and another in the evening (1900–0000 LT). The most significant results not reported earlier are the large occurrence rate of LQP echoes and the height‐time occurrence maps showing a descending pattern with close resemblance to tidal wind behavior. The Doppler velocities are upward‐northward (downward‐southward) for positive‐ (negative‐) sloped LQP echoes. Also, we find the Doppler spread as high as 200 m s−1 at times underlining the presence of strong plasma turbulence in the collision‐dominated lower E region. These results are discussed in the light of the current understanding of the LQP echoes.

Diagnostic Analysis of Tidal Winds and the Eliassen–Palm Flux Divergence in the Mesosphere and Lower Thermosphere from TIMED/SABER Temperatures

Abstract For migrating tides or fast-moving planetary waves, polarization relations derived from the linear wave equations are required to accurately derive the wind components from the temperature field. A common problem in diagnosing winds from the measured temperature is the error amplification associated with apparent singularities in the wave polarization relations. The authors have developed a spectral module that accurately derives tidal winds from the measured tidal temperature field and effectively eliminates the error amplification near the apparent singularities. The algorithm is used to perform a diagnostic analysis of tidal winds and the Eliassen–Palm (EP) flux divergence in the mesosphere and lower thermosphere (MLT) based on the zonal mean and tidal temperature fields derived from 6 yr of temperature measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satellite. The derived zonal mean wind and diurnal tidal amplitude reveal new insights into the mesospheric biennial oscillation (MBO) that exists in the MLT at both equatorial and midlatitude regions. The equatorial MBO in the zonal mean wind is present in the entire mesosphere from 50 to 90 km. The equatorial MBO in the temperature amplitude of the diurnal tide occurs near the mesopause region between 80 and 90 km and is largely coincident with the downward phase propagation of the equatorial MBO in the zonal mean wind, indicating a possible mechanism of wave–mean flow interaction between the two. On the other hand, the newly discovered midlatitude MBOs in zonal mean wind and the meridional wind in diurnal tide occur at different altitudes, suggesting possibly a remote forcing–response relationship. The acceleration or deceleration of the zonal mean wind due to EP flux divergence that is contributed by the migrating tides peaks at midlatitudes with a typical value of 10–20 m s−1 day−1 around 95 km.

Gravity wave and tidal influences on equatorial spread F based on observations during the Spread F Experiment (SpreadFEx)

Abstract. The Spread F Experiment, or SpreadFEx, was performed from September to November 2005 to define the potential role of neutral atmosphere dynamics, primarily gravity waves propagating upward from the lower atmosphere, in seeding equatorial spread F (ESF) and plasma bubbles extending to higher altitudes. A description of the SpreadFEx campaign motivations, goals, instrumentation, and structure, and an overview of the results presented in this special issue, are provided by Fritts et al. (2008a). The various analyses of neutral atmosphere and ionosphere dynamics and structure described in this special issue provide enticing evidence of gravity waves arising from deep convection in plasma bubble seeding at the bottomside F layer. Our purpose here is to employ these results to estimate gravity wave characteristics at the bottomside F layer, and to assess their possible contributions to optimal seeding conditions for ESF and plasma instability growth rates. We also assess expected tidal influences on the environment in which plasma bubble seeding occurs, given their apparent large wind and temperature amplitudes at these altitudes. We conclude 1) that gravity waves can achieve large amplitudes at the bottomside F layer, 2) that tidal winds likely control the orientations of the gravity waves that attain the highest altitudes and have the greatest effects, 3) that the favored gravity wave orientations enhance most or all of the parameters influencing plasma instability growth rates, and 4) that gravity wave and tidal structures acting together have an even greater potential impact on plasma instability growth rates and plasma bubble seeding.

Morphology and seasonal characteristics of low latitude E‐region quasiperiodic echoes studied using large database of Gadanki radar observations

In this paper we study the morphology, seasonal features, and statistics of low latitude quasiperiodic (QP) echoes using an unprecedented data set of 147 nights of observations spanning over all seasons made using the Gadanki radar. The QP echoes have periods 2–20 min and occur on more than 50% nights. They are often found embedded in descending echoing layers whose characteristics resemble to those of tidal winds. Their occurrence shows strong seasonal dependence with 48% of the time in summer, 26–32% in equinox, and 14% in winter. The QP echoes have a tendency to appear first during 21–00 LT, which could last for 2–8 h, leading to their maximum occurrence rate during postmidnight hours. Height‐time occurrences of radar echoes and QP structures both show common descending behavior, which agree extremely well with the observed descending behavior of the Es layers. We also find that the seasonal variations of QP echo occurrence are in very close agreement with that of the Es activity, tidal activity in the same height region, and also gravity wave activity in the mesosphere. Interestingly, the occurrence statistics are much more than their midlatitude counterpart; and time of first appearance, time of maximum occurrence, and duration of QP echoes all strongly contrast the midlatitude features. These observations have been discussed in detail in the light of current understanding of QP echoes.

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.

New results on equatorial thermospheric winds and the midnight temperature maximum

Abstract. Optical observations of thermospheric winds and temperatures determined with high resolution measurements of Doppler shifts and Doppler widths of the OI 630-nm equatorial nightglow emission have been made with improved accuracy at Arequipa, Peru (16.4° S, 71.4° W) with an imaging Fabry-Perot interferometer. An observing procedure previously used at Arecibo Observatory was applied to achieve increased spatial and temporal sampling of the thermospheric wind and temperature with the selection of eight azimuthal directions, equally spaced from 0 to 360°, at a zenith angle of 60°. By assuming the equivalence of longitude and local time, the data obtained using this technique is analyzed to determine the mean neutral wind speeds and mean horizontal gradients of the wind field in the zonal and meridional directions. The new temperature measurements obtained with the improved instrumental accuracy clearly show the midnight temperature maximum (MTM) peak with amplitudes of 25 to 200 K in all directions observed for most nights. The horizontal wind field maps calculated from the mean winds and gradients show the MTM peak is always preceded by an equatorward wind surge lasting 1–2 h. The results also show for winter events a meridional wind abatement seen after the MTM peak. On one occasion, near the September equinox, a reversal was observed during the poleward transit of the MTM over Arequipa. Analysis inferring vertical winds from the observed convergence yielded inconsistent results, calling into question the validity of this calculation for the MTM structure at equatorial latitudes during solar minimum. Comparison of the observations with the predictions of the NCAR general circulation model indicates that the model fails to reproduce the observed amplitude by a factor of 5 or more. This is attributed in part to the lack of adequate spatial resolution in the model as the MTM phenomenon takes place within a scale of 300–500 km and ~45 min in local time. The model shortcoming is also attributed in part to the need for the model to include a hydrodynamical mechanism to describe the merging of the zonal wind with the meridional tidal winds that converge onto the geographical equator. Finally, a conclusion of this work is that the MTM compressional heating takes place along the perimeter of the pressure bulge rather than within the bulge, an issue previously not appreciated.

Satellite observations of mean winds and tides in the lower thermosphere: 1. Aliasing and sampling issues

The Canadian Middle Atmosphere Model, a general circulation model extending up to 200 km, is used to analyze the impact of aliasing and data gaps on satellite observations of zonal mean winds and migrating tides in the lower thermosphere. Focus is placed on measurements from the Wind Imaging Interferometer (WINDII). Mock data sets are constructed by sampling daily zonal mean and migrating tides from the model like the satellite. Previously unverified assumptions that satellite observations of the zonal mean zonal winds and the meridional component of the diurnal tide winds are largely unaffected by aliasing are confirmed. Of the two, the meridional component of the diurnal tide is more robust. The amplitude of the zonal component of the diurnal tide is not strongly affected by aliasing. Tidal phase is nearly unaffected. Gaps in the WINDII data do not adversely affect the results provided sufficient local time coverage is available. The analysis suggests that more subtle features in the WINDII observations, such as the tidally driven cell‐like structures in the zonal mean meridional wind at low latitudes, may be observable. Issues pertaining to the sampling interval and the use of daytime‐only data are also addressed. Sampling every second or third day provides nearly as good results as sampling every day, and that using daytime‐only data results in substantial biases in the zonal mean zonal winds in the lower thermosphere. The latter underscores the importance of the accurate removal of the migrating diurnal tide from climatologies generated from daytime‐only satellite data.

Satellite observations of mean winds and tides in the lower thermosphere: 2. Wind Imaging Interferometer monthly winds for 1992 and 1993

Satellite observations of monthly (i.e., 60‐d averages centered on the 15th of each month) zonal mean and migrating tide winds in the region of 90–120 km and 40°S–40°N from the Wind Imaging Interferometer are presented. The zonal mean zonal winds are characterized by annually varying eastward winds at midlatitudes with a maximum in summer and semiannually varying westward winds in the tropics with maxima at equinox below 105 km. The zonal mean meridional winds are characterized by a summer‐to‐winter flow below 100 km at solstice and by a cell‐like structure at equinox; the latter is the first such global observation of this tidally driven structure. Two distinct types of diurnal tide are observed: an upward propagating one and an evanescent one of comparable magnitude. The propagating tide, which is dominant in the subtropics below about 105 km, exhibits a semiannual variation with a primary maximum in March/April and a secondary maximum in September/October. The evanescent tide, which is dominant poleward of 20°N/S and above 100 km, has an annual variation with a maximum in the summer months. The coupling of the two produces a double‐peaked structure in the vertical profile of the diurnal amplitude. The semidiurnal tide is generally weaker than the diurnal tide. The maximum semidiurnal winds are at 30–40°N/S and at 105–110 km. Both zonal and meridional components are stronger in April–September than in October–March in both hemispheres, but in the equatorial region in April–September the meridional component has a maximum whereas the zonal component has a minimum.

A climatology of nonmigrating semidiurnal tides from TIMED Doppler Interferometer (TIDI) wind data

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.

Statistical characteristics of the day‐to‐day variability in the geomagnetic Sq field

Day‐to‐day variability in the geomagnetic Sq field is studied by using the magnetic data from a meridian chain of magnetometers along 120°E longitude. The method of natural orthogonal components (NOC or eigenmode analysis) is applied to separate the Sq variation from complicated disturbances. The first eigenmode with the largest eigenvalue represents fairly well the Sq variation with a conspicuous day‐to‐day variability in the daily range. For the stations on the same north‐ or south‐side of the Sq current system focus, the day‐to‐day variations show a positive correlation. In contrast, for the stations on the different sides of the Sq focus, they show a negative correlation, suggesting an important role of latitudinal shift of the Sq current system focus to the day‐to‐day variability in the Sq daily range. The Sq daily range is correlated with the magnetic indices Ap and Dst in a peculiar way: On some severe disturbed days, noticeable enhancements of the Sq are observed, implying increases in the ionospheric conductivities and/or tidal wind velocities; on other severe disturbed days, however, dramatically reduced Sq variations occur, suggesting dominant effects of the “disturbance dynamo” process.

Diurnally varying structure of stationary waves encircling the summer southern pole of Mars observed by MGS TES

The diurnal variation in the vertical structure of the wavenumber‐1 stationary planetary wave observed in the summer southern polar region of Mars was investigated by using temperature data on the dayside and nightside taken by Thermal Emission Spectrometer onboard Mars Global Surveyor. It was found that the westward phase tilt with height during this period tends to be obvious on the dayside, while it is much less on the nightside. A possible explanation for the diurnally‐varying wave structure is the advection of wave fields by the tidal wind: the wave fields away from the ground surface are carried westward by the tidal wind during the local time from midnight to noon, and the process is reversed from noon to midnight. The slow vertical propagation of the wave with a time constant much longer than a sol will allow the tidal wind to influence the wave structure.

The characteristics of the semi-diurnal tides in mesosphere/low-thermosphere (MLT) during 2002 at Wuhan (30.6°N, 114.4°E) – using canonical correlation analysis technique

In this paper, we use canonical correlation analysis (CCA) method to investigate the semi-diurnal tidal winds in mesosphere and low thermosphere (MLT) region, observed by a newly installed meteor radar at Wuhan (30.6°N, 114.4°E), during the year 2002. In general, 4(3) effective semi-diurnal tidal pairs of patterns are obtained, which represent ∼2/3 total variances of the origin data set. These patterns are expected to be corresponding to the atmospheric oscillations within the semi-diurnal frequency band excited or modulated by different sources, i.e., the seasonal variations, the modulations by the planetary wave oscillations or the solar 27-day activity. Among all the patterns, the 1st pattern, which represents ∼1/3 of total variances, is the most notable. Its amplitudes show maximum values in spring and autumn, and the vertical wavelengths are longer in summer and shorter in winter, which is in line with the results obtained from traditional harmonic analysis. The vertical wavelengths of the higher order patterns (∼50 km) suggest the classic semi-tidal mode S(2, 4)/S(2, 5) is dominant.

Gravity waves and wind-shear in the MLT at 23°S

We have used the technique suggested by Hocking [Hocking, W. A new approach to momentum flux determinations using SKiYMET meteor radars. Ann. Geophys. 23, 2005.] to derive short period wind variances in the 80–100 km region from meteor radar data. We find that these fluctuating winds, assumed to correspond to gravity waves and turbulence, are closely correlated with the vertical shear of the horizontal tidal winds. This close correlation suggests that in situ wind shear may be a major source of gravity waves and turbulence in the MLT. If this is the case, gravity waves generated in the troposphere and propagating up to the MLT region, generally assumed to constitute an important influence on the climatology of the region, may be a less important source of energy and momentum in the 80–100 km region than has been hitherto believed.

Lunar Tidal Winds at Four American Stations

Summary The semi-diurnal lunar-tidal surface winds have been determined at four stations in the northern hemisphere, supplementing an earlier determination of these winds by Chapman for Mauritius in the southern hemisphere. The series of data are in no case long enough to reduce the magnitude of the probable error circle sufficiently, but the amplitudes are found to be of the right order of magnitude, 1 cm/s, as predicted by the theory. The rotation of the wind vector with time is opposite in the two hemispheres, again in agreement with the theory. But in both hemispheres, the sense of rotation of the wind vector is contrary to that expected from theory. While this result cannot be regarded as firmly established because of the inaccuracy of the determinations, it strongly indicates the need for further empirical studies based on longer series of data.

Structure of the migrating diurnal tide in the Whole Atmosphere Community Climate Model (WACCM)

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.

Diurnal tides in mesosphere/low‐thermosphere during 2002 at Wuhan (30.6°N, 114.4°E) using canonical correlation analysis

In this paper, we use canonical correlation analysis (CCA) method to investigate the mesosphere/low thermosphere (MLT) diurnal tidal winds during the year 2002 observed by a newly installed meteor radar at Wuhan (30.6°N, 114.4°E). In general, six effective diurnal tidal pairs of patterns are obtained, which represent over 90% total variances of the origin data set. These patterns are expected to correspond to the atmospheric oscillations within diurnal frequency band excited or modulated by different sources, namely, the seasonal variations and the modulations by semiannual‐like variations, solar 27‐day rotation and the planetary wave oscillations. Among all of the patterns, the first pattern is the most notable, which represents ∼40% of total variances. Its amplitudes show maximum values in spring and autumn as well as sudden phase transit near equinox month, which is in line with the results obtained from traditional harmonic analysis. The vertical wavelengths (∼30 km) suggest the classic tidal mode S(1,1) is dominant, and the preceding phases (∼5–6 hours) of the meridional components of the diurnal tidal wind show right‐rotating circular polarization may be the main characteristic in tidal wind.

Doppler ducting of short‐period gravity waves by midlatitude tidal wind structure

Multiwavelength airglow image data depicting a short‐period (∼4.9 min) atmospheric gravity wave characterized by a sharp leading front have been analyzed together with synoptic meteor radar wind data recorded simultaneously from Bear Lake Observatory, Utah (41.6°N, 111.6°W). The wind data suggest the presence of a semidiurnal tide with horizontal winds peaking at around 60 m/s along the SSE direction of motion (170° from north) of this short‐period wave. It was found that the gravity wave was most probably ducted because of the Doppler shift imposed by this wind structure. A marked 180° phase shift was observed between the near‐infrared OH and the OI (557.7 nm) emissions. Numerical simulation results for similar ducted waves excited by idealized model sources suggest that the phase shift between the wave‐modulated airglow intensities may be explained simply by chemical processes rather than by wave dynamics. Phase velocities of simulated waves, however, appear higher than those of observed waves, suggesting the importance of tidal thermal structure in determining the Doppler‐ducted wave characteristics.