Diurnal tides at low latitudes: Radar, satellite, and model results

Journal of Atmospheric and Solar-Terrestrial Physics 118 (2014) 96–105

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Diurnal tides at low latitudes: Radar, satellite, and model results

G. Kishore Kumar a,n, W. Singer a, J. Oberheide b, N. Grieger a, P.P. Batista c,D.M. Riggin d, H. Schmidt e, B.R. Clemesha c

a Leibniz-Institute of Atmospheric Physics e.V. at the Rostock University, 18225 Kühlungsborn, Germanyb Department of Physics and Astronomy, Clemson University, Clemson, SC, USAc INPE, São José dos Campos, 12201-790 SP, Brazild GATS Inc., Boulder, CO, USAe Max Planck Institute for Meteorology, Hamburg, Germany

a r t i c l e i n f o

Article history:Received 31 December 2012Received in revised form7 July 2013Accepted 10 July 2013Available online 27 July 2013

Keywords:Migrating and non-migrating tidesMLTMeteor radarTIDI/TIMEDHAMMONIA

26/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.jastp.2013.07.005

esponding author. Tel.: +49 38293 68245; faxail addresses: [email protected],[email protected] (G. Kishore Kumar).

a b s t r a c t

Mean winds and tidal signatures in the mesosphere and lower thermosphere (MLT) region are derivedfrom meteor radar observations at three sites around 221S acquired in 2005. The observed differences ofmean winds and tides are discussed in relation to the meteorological situation in the lower atmosphereand the possible generation of non-migrating tides. The longitudinally well separated radar sites allowedthe evaluation of the migrating tidal component. The seasonal variation of signatures of the diurnal tidederived from ground-based radar observations, TIDI measurements aboard TIMED satellite, and modelresults obtained with HAMMONIA (Hamburg Model of the Neutral and Ionized Atmosphere) arecompared. The ground-based, satellite, and model results of the total diurnal tide are in good agreement.The same is true for the migrating diurnal tide obtained from the radar observations, TIDI observationsand from the model studies of HAMMONIA and GSWM00 (Global Scale Wave Model).

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1. Introduction

Atmospheric tides are global-scale oscillations with periods atharmonics of the solar day, which are excited by periodic absorptionof IR radiation by water vapor and latent heat release in thetroposphere (dominating forcing mechanism), as well as UV absorp-tion by ozone (secondary forcing mechanism) in the stratosphere(Hagan, 1996). Even though the source of these tides in troposphereand stratosphere, they will attain large amplitudes in the mesospherewhere they represent a major part of the atmospheric variability.Thermal tides are an essential coupling process in the atmosphere.A detailed analysis of the mechanisms controlling the diurnal tidehas been done by Achatz et al. (2008) interpreting results of thegeneral circulation and chemistry model HAMMONIA. Talaat andLieberman (2010) reported that tides in the thermosphere aredirectly linked to the troposphere and observed the DE3 non-migrating tidal mode in satellite winds between 90 km and270 km. Goncharenko et al. (2012) studied the vertical couplingduring a stratospheric warming event and found that the non-linearinteraction of planetary waves and tides plays a more complex role instratosphere and ionosphere coupling than expected earlier.

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Based on the period, the tides are categorized mainly intodiurnal, semidiurnal and terdiurnal with periods of 24 h, 12 h and8 h, respectively. These tides can be further divided into migratingand non-migrating tides based on their characteristic zonal wavenumber and phase speed. Migrating tides are sun-synchronousand propagate westward, whereas the non-migrating tides are notsun-synchronous and can propagate both eastward and westward.These tidal components grow in amplitude with increasing alti-tude since the atmospheric density decreases and energy must beconserved. The main source for migrating tides is solar forcing.Migrating tides in the MLT region can also be excited locallythrough the absorption of ultraviolet radiation by oxygen atoms inthe thermosphere (Forbes, 1995). For non-migrating tides thesources are like non-linear interaction of migrating tides withlongitudinally inhomogeneous heating agents (Chapman andLindzen, 1970) and latent heat release in the tropical troposphere(e.g., Hagan and Forbes, 2002). McLandress and Ward (1994)identified localized gravity wave (GW) drag, which occurs as aresult of GW-tide interactions as a plausible non-migrating tidalsource. The non-linear interaction between planetary waves andtides also produce the non-migrating tides (Pedatella et al., 2012).Detailed information about the source mechanism for the non-migrating tides can be found in (Forbes et al., 2006). Since themigrating tides are sun-synchronous the migrating tidal ampli-tudes and phases are uniform throughout all longitudes at fixedlatitudes, which is not true for the non-migrating component.

G. Kishore Kumar et al. / Journal of Atmospheric and Solar-Terrestrial Physics 118 (2014) 96–105 97

Non-migrating tides have characteristics which are varying withlongitude. Especially at low latitudes the contributions of the non-migrating tides to the observed total tide can be substantial.On the one hand, tides are modulated by the atmosphericpropagation conditions between the source regions and the meso-sphere. On the other hand, the tidal wind field on his partinfluences the upward propagation of smaller-scale gravity waves.To understand the short-term variability as well as the long-termvariations in the MLT region a detailed understanding of theprocesses controlling the generation and propagation of solartides is necessary.

There are many observations of diurnal tides over tropical latitudes.The observations include both single point observations and globalsatellite measurements. Global satellite measurements allow thediscrimination between migrating and non-migrating tides in themesosphere with a low time resolution of one or two months. Pointmeasurements by radars sample the total tide (sum of migrating andnon-migrating tide) only, but with much higher time resolution in theorder of days. Over southern subtropics, the observations of diurnaltides are limited (e.g., Vincent et al., 1998; Batista et al., 2004).In general, the observations describe selected locations or latitudinalvariations. For the first time, we are reporting the longitudinalvariation of the diurnal tide over southern subtropics around 221S in2005—the year of the first Global Campaign on Tides of the CAWSESprogram in September–October 2005. CAWSES stands for “Climateand Weather of the Sun–Earth System” and is an internationalprogram sponsored by the Scientific Committee on Solar–TerrestrialPhysics. The initial results of the first CAWSES tidal campaign inSeptember-October 2005 are discussed in detail by Ward et al. (2010)showing the consistency of the extended Canadian Middle Atmo-sphere Model, satellite, and ground-based observations of tidalsignatures for the first time. In addition, Chang et al. (2012) presenteda comparison between various general circulationmodels and ground-based observations of the diurnal tide during the 2005 equinox tidalcampaign.

In the present study, the complete seasonal cycle of meanwinds and diurnal tides observed with meteor radars around 221Sin the year 2005 are presented. The data is compared with TIDImeasurements on the TIMED satellite and model results ofHAMMONIA and GSWM00. The main objectives are:

(i)
Exploring the capabilities of meteor radars to describe theshort term variability of the tides through different compositeanalyses to demonstrate the higher time resolution of theground based observations compared to the satellite observa-tions which require �60 days to cover one diurnal cycle.
(ii)
The comparison of the total tide observed by ground basedmeteor radars, satellite, and model simulations—one of themain objectives of the CAWSES global tidal campaigns.
(iii)
The estimation of the migrating tide from longitudinallyseparated radar observations at a fixed latitude.
The paper is organized as follows. The meteor radar data andthe data analysis are described in Section 2. The results arepresented in Section 3 with subsections describing (i) the meanbackground winds and the background meteorological conditionsover the radar sites, (ii) seasonal variation of the total diurnal tideat the three radar sites in comparison with TIDI and HAMMONIAresults, and finally (iii) comparisons of migrating tides derivedfrom radar observations with HAMMONIA, GSWM00 and TIDImigrating tides. Section 4 summarizes the obtained results.

2. Data and analysis procedure

The MLT winds used in this study were acquired using meteorradars located at Learmonth, Australia (22.21S, 1141E), Rarotonga,

Cook Island (21.21S, 2001E), and Cachoeira Paulista, Brazil (22.71S,3151E) in 2005. Hereafter we used notations L, R, and C forLearmonth, Rarotonga, and Cachoeira Paulista, respectively. Thethree radars use identical hardware as well as standard softwarefor meteor detection and analysis allowing observations withminimal instrumental bias. The basic system parameters are:operating frequency 35.24 MHz, pulse repetition frequency2144 Hz, range resolution 2 km, peak power 12 kW (6 kW overLearmonth). A detailed system description can be found inHocking (2001).

Time series of hourly horizontal winds are derived for eachradar site for an altitude range from 82 km to 98 km. Meteorsdetected at zenith angles between 101 and 601 are binned intoheight slabs of 3 km width whereas a minimum number of 10meteors per hour is required. The tidal analysis has been carriedout with simple least square fitting procedure considering themean wind, 24-h, 12-h, and 8-h harmonic components for thezonal and meridional wind components, separately. The datapoints were weighted in the fitting processes according to thenumber of meteors composing each hourly mean. In general,monthly composite tidal analysis is used to study the seasonaland inter annual variability of tides. Here we estimated the tidalcharacteristics using composite day analyses with different lengthswhich are appropriate for the comparisons with satellite measure-ments and model results—namely 60-d composite analysis shiftedby one day (60-d/1-d) and 10-d composite analysis shifted by5 days (10-d/5-d). 60 days of TIDI observations are required tocover 24 h of local time. The HAMMONIA results are provided as10-d analyses. In addition, for studies of the short-term variability a4-d composite analysis shifted by one day (4-d/1-d) is performed.

2.1. Comparison between different composite analyses

The short term variability of tides is always an important issuesince the tides will interact with the GWs and PWs and hencealtering the dynamics. Observational studies (Pancheva, 2001;Pancheva et al., 2002) and model studies (Miyoshi, 2006;Pedatella et al., 2012) revealed the influence of the planetarywaves on tidal variability through non-linear interaction. Here weare not concentrating on the tidal variability issue, but we tried toexplain how the short term variability of tides is present on thedifferent scales of composite analyses.

To demonstrate the differences between the various compositeanalyses, we derived the monthly mean tidal parameters for allcomposite analyses (4-d/1-d, 10-d/5-d, 60-d/1-d) using vectoraverage (Grieger et al., 2002). As a typical example, the monthlymean diurnal tidal amplitudes for the above mentioned compositeanalyses over Cachoeira Paulista in 2005 are shown in Fig. 1. Zonaland meridional components show similar variations of the diurnalamplitude for the 4-d/1-d and 10-d/5-d analyses with slightdifferences in magnitude. The 60-d/1-d analysis reveals smalleramplitudes compared to other analyses mainly during the seaso-nal transition periods due to the long averaging period. The phasecomparison (not shown here) also shows similar behavior in allanalyses with negligible phase differences (�10 min) with excep-tions for 60-d/1-d analysis at particular altitudes (82 km and98 km during equinox months). In addition, the monthly meansof the various composite analyses are compared with the tradi-tional monthly (30-d) composite analysis by correlation. Thecorrelation coefficients are significant at the 99% level and sum-marized in Table 1.

Mean height profiles present in Fig. 2 give a more detailedpicture for amplitude and phase of the diurnal meridional tideover Cachoeira Paulista for selected months (March, July, andSeptember). The amplitude profiles of the 4-d/1-d and 10-d/5-danalyses are in good agreement whereas the amplitudes of the

Fig. 1. Monthly variation of the diurnal tidal amplitude of zonal and meridional components for various composite analyses over Cachoeira Paulista: traditional monthlyanalysis, 4-d analysis shifted by 1 day (4-d/1-d), 60-d analysis shifted by 1 day (60-d/1-d), and 10-d analysis shifted by 5 days (10-d/5-d).

Table 1Correlation coefficients of the diurnal tide for various composite analyses versus monthly composite analysis. Note coefficients for both zonal and meridional componentsand for amplitude (phase) are listed.

Station\Combination Zonal component amplitude (Phase) Meridional component amplitude (Phase)

4-d/1-d 60-d/1-d 10-d/5-d 4-d/1-d 60-d/1-d 10-d/5-d

Learmonth 0.98(0.78) 0.81(0.59) 0.97(0.85) 0.98(1.00) 0.77(0.76) 0.98(1.00)Rarotonga 0.91(0.55) 0.26(0.31) 0.94(0.66) 0.88(0.83) 0.65(0.79) 0.86(0.84)Cachoeira Paulista 0.96(0.56) 0.65(0.53) 0.97(0.69) 0.98(0.93) 0.42(0.48) 0.98(0.93)

Fig. 2. Height profiles of meridional diurnal tidal amplitude (top) and phase(bottom) for various composite analyses over Cachoeira Paulista for selectedmonths in 2005: March, July, and September.

G. Kishore Kumar et al. / Journal of Atmospheric and Solar-Terrestrial Physics 118 (2014) 96–10598

60-d/1-d analysis are remarkably reduced during the spring andautumn equinox. The phase profiles are in excellent agreement,only the 60-d component differs by up to 3 h from the 4-d/1-d or10-d/5-d components. The temporal variability of the meridionaldiurnal tide is illustrated in Fig. 3 for the different compositeanalyses for the period mid-July to mid-October. Both the 4-d/1-dand 10-d/5-d tidal amplitudes are in good agreement for themonthly means (dashed lines in Fig. 3). Compared with the 4-d/10-d results the 60-d/1-d monthly means are substantial less by5 to 8 m/s. Larger variations are observed for the daily resultswhich may be attributed to the influence of planetary waves asobserved by Pancheva et al. (2002).

3. Results

In this section, we present the background conditions over theradar sites. The observed seasonal variability of the total diurnaltide over the radar sites is compared with the total diurnal tideextracted from TIDI/TIMED measurements and HAMMONIAresults. Finally, we estimate the migrating diurnal tide from

Fig. 3. Meridional diurnal tidal amplitudes at 85 km over Cachoeira Paulista forthe period mid-July to mid-October 2005 for 4-d/1-d, 10-d/5-d, and 60-d/1-dcomposite analyses represented by black, blue, and red lines. The horizontal dashedlines indicate the corresponding monthly means. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web versionof this article.)

Fig. 4. Monthly mean zonal (left panel) and meridional (right panel) winds ov


longitudinally well distributed radar sites and compare theseresults with the diurnal migrating tide from HAMMONIA andGlobal Scale Wave Model (GSWM00).

3.1. Background atmosphere

Before discussing the details of the tidal results, we brieflydescribe the background prevailing horizontal winds in the sub-tropics around 221S. Fig. 4 illustrates the horizontal winds, derivedfrom 10-d/5-d analysis over the radar sites L, R, and C. Here thepositive zonal (meridional) wind values represent the eastward(northward or equatorward) winds, while the negative zonal(meridional) wind values represent the westward (southward orpoleward) wind. The prevailing horizontal wind flow shows similarpatterns over three sites with differences in the magnitude. Thehorizontal winds are in good agreement with the earlier observa-tions made over the southern subtropics (e.g., Batista et al., 2004).

The prevailing zonal winds are dominantly eastward withexceptions around February and September/October. A strongeastward jet is observed in June/July peaking below 82 km withdifferent magnitudes between 40 m/s and about 60 m/s over the

er Learmonth (L), Rarotonga (R), and Cachoeira Paulista (C), respectively.


three sites. In summer, westward jet peaks above 90 km withmagnitudes close to 20 m/s. The prevailing meridional winds arepoleward directed between March and September and equator-ward directed during the rest of the year. Only over CachoeiraPaulista a southward jet at 85 km is well established in June/Julyreaching magnitudes up to 20 m/s.

The observed differences in the horizontal winds over the threesites can be attributed to longitudinal differences of the back-ground atmosphere. It is well known that planetary waves con-tribute significantly to the variability of the MLT winds (e.g.,Pancheva et al., 2009). So differences in the horizontal windscould be due to differences in the wave activity over these siteswhich is controlled by the local meteorological conditions.

3.1.1. Outgoing long wave radiation (OLR)In order to explain the local meteorological condition like

convection, we present the variations of the outgoing long waveradiation (OLR) for the year 2005 in Fig. 5. OLR is an indicator of

Fig. 5. Longitudinal variation of the un-interpolated OLR (Outgoing Long-waveRadiation) from NOAA at 22.51S. The notations L, R, and C indicate the locations ofthe radar sites.

Fig. 6. Meridional diurnal tide over Learmonth (L), Rarotonga (R), and Cachoeira Paulismean data.

cloud top height. Low OLR indicates low cloud top temperaturesand thereby high cloud tops. Such high cloud tops are indicative ofstrong convective activity and therefore of strong condensation,precipitation and latent heat release. The latent heat released inthe lower atmosphere is one of the main source mechanisms forthe generation of non-migrating tides (Hagan and Forbes, 2002).The OLR data at the top of the atmosphere are observed from theAVHRR (Advanced Very High Resolution Radiometer) instrumentaboard the NOAA polar orbiting spacecraft. The AVHRR is aradiation-detection imager that can be used for remotely deter-mining cloud cover and the surface (either surface of the earth orthe upper surface of clouds or the surface of a body of water)temperature. Here we illustrate the longitudinal variability of dailymean values of the un-interpolated OLR at 22.51S (the OLR atadjacent circles of latitude 201/251S is marginally different).Un-interpolated OLR data is taken from http://www.esrl.noaa.gov/psd. The locations of the meteor radars are indicated bydotted lines. In generally, the OLR values less than 200–220 Wm�2 are considered as convection and OLR values less than180 Wm�2 are considered as deep convection. The differences ofOLR over the meteor radar locations represent the variability ofconvection over the sites. Less OLR values are observed overRarotonga, medium values over Cachoeira Paulista, and largevalues over Learmonth indicating deep convection over Rarotonga.

3.2. Total diurnal tides

In this section we compare the observed total diurnal tides overthe radar sites with TIDI/TIMED measurements and with resultsfrom HAMMONIA. Here onwards we represent the tides by theso-called “cosine term”. The cosine term is a function of amplitudeand phase of the tide and can be defined for the diurnal tide as“A cos((2π/24) (t�T))”. Here ‘A’ and ‘T’ are amplitude and phase ofthe diurnal tide, and ‘t’ is the universal time. The cosine termrepresentation has some advantages like it prevents the confusionwith sudden jumps in the phase profiles and the vertical wave-length can easily be derived. The value of the cosine term at 00UTwill be negative if the phase is equal or in between 6 h and 18 h.

ta (C) from TIDI (top) and radar (bottom) observations at 00UT based on monthly

Fig. 8. Height profiles (April 2005) of the meridional component of the diurnal tidefor the stations Learmonth (L), Rarotonga (R), and Cachoeira Paulista (C) and TIDI(dashed line) at different longitudes near to 201S (top: amplitude, bottom: phase,universal time of maximum).


3.2.1. Comparison with TIDI/TIMED measurementsThe TIDI total (net) wind field from 80 km to 105 km is

constructed from 60 day running TIDI observations of non-migrating tides and migrating tidal winds derived from SABER(version 1.07) tidal temperature observations using tidal theorybased Hough Mode Extensions (Oberheide et al., 2006; Oberheideand Forbes, 2008). SABER based migrating tidal winds have beenproven to be reliable, e.g., during the CAWSES tidal campaigns(Ward et al., 2010).The time resolution for the TIDI results islimited to 60 days because of the precession rate of the TIMEDsatellite. Monthly means of the radar observations are formed asvector means of the 60-d/1-d composites. Vector means of thecosine terms represent monthly means in the following. The totaldiurnal tides obtained from radar and TIDI/TIMED measurementsover L, R, and C are depicted in Fig. 6 for the meridional component.

Both the radar and the TIDI total diurnal tides agree in generalat the three sites with slight differences in magnitude. A weakertidal signal was found at R compared to L and C. In general, themeridional component has greater values than the zonal compo-nent (not shown here). A clear semi-annual oscillation is evidentover C and L in both the zonal and the meridional component. Theresults obtained for the whole year 2005 confirm the firstcomparison of radar and satellite tidal observations provided byWard et al. (2010) for the period September-October 2005. Thedifferences between L, R, and C from both radar and satelliteobservations indicate the presence of non-migrating tides, ofcourse the influence of planetary waves cannot be ruled out.In general, the non-migrating tidal influence could be larger thanthe planetary waves influence as supported by the OLR observa-tions discussed in Section 3.1.1. To illustrate this we presentlatitude versus longitude cross section of the net meridionalcomponent of the diurnal tide at 90 km in April 2005 in Fig. 7.Particularly striking feature from the figure is the longitudinalvariability in the amplitude of the diurnal tide. The longitudinalvariability in amplitude indicates the presence of significant non-migrating tidal components. Height profiles of amplitude andphase of the meridional diurnal tide over L, R, and C during April2005 provide a more detailed view of the differences betweenradar and TIDI/TIMED observations (Fig. 8). The tidal phases agreevery well with differences in general less than 4 h, in most casesthe differences are less than 2 h. Greater differences are found forthe amplitudes.

3.2.2. Comparisons with HAMMONIATides are estimated from a 20 year run of the general circula-

tion model HAMMONIA (Hamburg Model of the Neutral andIonized Atmosphere) for solar-minimum conditions sampled every3 h. HAMMONIA (Schmidt et al., 2006) is a general circulationmodel (GCM) covering the atmosphere up to an altitude of about

Fig. 7. Latitude versus longitude cross section of the TIDI observations of the netmeridional component of the diurnal tide at 90 km in April 2005.

250 km. It is based on the ECHAM5 GCM (Roeckner et al., 2006)but accounts additionally for physical processes relevant in themesosphere and lower thermosphere, and is interactively coupledto the MOZART3 chemistry code (Kinnison et al., 2007). It has beenused in studies of diurnal and semidiurnal tides by Achatz et al.(2008) and Yuan et al. (2008), respectively.

Observations at L, R, and C together with the HAMMONIAresults obtained at these locations are shown in Fig. 9. Theseasonal variation of the meridional tidal component (‘cosineterm’) based on 10-d/5-d analysis is shown. The presentation oftides at 00 UT allows an easy estimation of the vertical wavelength. At subtropical latitudes the total diurnal tide appears witha vertical wave length of about 18 km around the equinoxes. Thethree radar sites, well distributed in respect of geographicallongitude at nearly the same latitudinal circle, give us thepossibility to estimate the migrating tidal component as describedin Section 3.3. The total tide reaches as well for the meridionalwind as for the zonal wind (not shown here) its largest amplitudesin the MLT region at low subtropical latitudes. For the three radarsites the meridional components of the diurnal tide derived fromthe radar observations agree well with the corresponding HAM-MONIA data. Model and radar tidal fields at R differ remarkablyfrom the other two sites with considerably smaller magnitudes.The reduced total tidal amplitudes over R correspond well withthe deep convection over the radar site (Fig. 5) where migratingand non-migrating components cancel each other. The semi-annual oscillation is well expressed at L and C. The tidal activityin the upper mesosphere shows its maximum over L duringMarch–April–May whereas it appears over C during September–October–November. Similar patterns are also observed at othertimes. The strong semi-annual variation of the diurnal tide is aninteresting aspect of the tidal variability in the MLT region. Modelstudies by Achatz et al. (2008) showed that it is caused by theseasonal variation of zonal mean winds in the middle atmosphere.Especially for the migrating tidal components the different zonalwind fields are of larger influence on the seasonal tidal structurethan the variation in the forcings.

3.3. Migrating diurnal tides

The longitudinally well separated radar sites near to 221S putus in a position to estimate the migrating tidal component. Themigrating tide has the same phase and amplitude along a circle of

Fig. 9. Seasonal dependence of the meridional component of the total diurnal tide based on monthly means at Learmonth (L, 1141 E), Rarotonga (R, 2001 E), CachoeiraPaulista (C, 3151E), and from HAMMONIA at 00 UT.

v24mig

OBS

HAM

Fig. 10. Meridional component of the migrating diurnal tide at 00 LT from theHAMMONIA model (HAM) and from the combination L-C (OBS) at 00 LT.


longitude at a fixed latitude and the same local time. Averaging ofthe total tidal components of longitudinally separated observa-tions for a fixed local time will provide an estimate of themigrating component by canceling of non-migrating tidal compo-nents. Note, the estimated migrating tide can still be contaminatedby non-migrating tides, if the non-migrating tides are remarkablydifferent at the sites under study. In addition, this procedure willnot produce correct migrating tides if a standing tide (D0 tide)exists over the selected latitude circle, but Grieger et al. (2004)found that the stationary tide is negligible in the subtropics. TheTIDI/TIMED observations of D0 also reveal similar signature (notshown here). Based on monthly vector means of the 10-d/5-danalysis at 00LT the migrating tidal amplitudes and phases wereestimated from different combinations of the observed tides viz.,L–R, L–C, R–C and L–R–C (for three stations).

The site combination L–C provides the best agreement forboth the migrating tides derived from the HAMMONIA data aswell as from the radar observations. The observations and themodel results of the migrating tide are shown in Fig. 10 for themeridional component. The semi-annual oscillation is wellexpressed in both data sets, and also the peak values are inexcellent agreement regarding peak height and time of occur-rence in March and September. The peak values of the migratingtide are slightly weaker but comparable with the total tidalsignals at L and C and remarkably stronger than the total tideat Rarotonga. The OLR over R shows deep convection and shallowconvection over L and C. It is well known that the deep convec-tion release large amount of latent heat and hence triggering thenon-migrating tides.

In addition, we used the Global Scale Wave Model (GSWM00)to compare model results with observations. The GSWM is amechanistic model providing diurnal and semi-diurnal tides. It isa two-dimensional linearized model using solutions of the Navier–Stokes equations to determine wind and temperature perturba-tions due to tides and planetary waves as a function of height,latitude, wave periodicity, and zonal wave number. Non-linearinteractions between tides and planetary waves are not consid-ered. The GSWM has been described in detail by Hagan et al.(1995). Results are available at http://www.hao.ucar.edu/modeling/gswm/gswm.html#ASC24. The GSWM00 migrating tidal com-ponents at 211S are presented in the Fig. 11 together with themigrating tide derived from the site combination L–C as used forthe HAMMONIA comparison.

Observations and model results are in general agreement forthe meridional component showing again a semi-annual

oscillation. The model results show greater peak values and thetimes of the peak occurrence are shifted by about one month toApril and October. The zonal component from the model presentsagain the semi-annual oscillation with one month shifted times ofpeak occurrence. The observed migrating tidal signal (zonalcomponent) is weaker but with maxima in April and August.

Fig. 11. Zonal and meridional component of the migrating tide around 211S from GSWM00 (top) and from the combination L-C (bottom).


Observed migrating tides and the model tides are well correlatedwith correlation coefficients of 0.75 for the meridional componentand 0.65 for the zonal component. This high correlation wasobtained for the site combination L–C.

Finally, a comparison of the zonal and meridional migratingdiurnal tides of satellite-based and radar-based tidal analysis ispresented in Fig. 12. The migrating tidal fields derived from theradar observations at Learmonth and Cachoeira Paulista andestimated from the migrating diurnal tidal analysis based onSABER temperatures and the HME approach for conversion intotidal winds (for details see Section 3.2.1) represent monthly meanvalues of 60-d composite analysis. The structure of the tidal fieldsfrom both observations is in general agreement, little differencesare found in the tidal amplitudes. So, the satellite-based ampli-tudes are about 4 m/s larger than the radar-based amplitudes forthe meridional diurnal tide whereas the opposite holds for thezonal diurnal tide.

The selection of the radar sites L and C for the estimation ofmigrating tides is also supported by the observed vertical wave-lengths. According to the classical tidal theory (Chapman andLindzen, 1970), the tides can be defined as a function of Houghmodes and vertical wavelengths. Each Hough mode has a uniqueequivalent depth and hence unique vertical wavelength. Forexample, the vertical wavelength for the diurnal tide with zonalwave number 1 is about 28 km at equator (Andrews et al., 1987;Forbes, 1995). The vertical wavelengths are derived from thevertical phase gradient for each individual height profile. Thedistributions for various combinations of radar sites are presentedas box car plots in Fig. 13 separately for the zonal and meridionalmigrating tide. The broader distributions indicate the presence ofdifferent tidal modes. Different factors influence the distribution ofvertical wave lengths such as superposition of non-migratingtides, mode coupling, and wave activity. Based on simulationswith a mechanistic model, Ortland and Alexander (2006) proposedthat the GW source in the MLT region will influence the diurnaltide by enhancing the amplitude and reducing the vertical wave-length. Narrow distributions for both migrating components are

found only for the site combination L–C with a mean value close tothe theoretical value supporting our assumption that the non-migrating tides over site ‘R’ are contribute to more to the total tidethan over ‘L’ and ‘C’.

4. Summary and conclusions

Mean winds and signatures of the total diurnal tide and themigrating diurnal tide are derived from meteor radar observationsat 221S well distributed in longitude (Learmonth 1141E, Rarotonga2001E, and Cachoeira Paulista 3151E). The mean zonal wind flowover the three sites is similar with slight differences in magnitude.A strong eastward directed jet appears in summer peaking ataltitudes below 82 km. The mean meridional flow is directedpoleward in summer with about two times larger amplitudes overCachoeira Paulista. The meridional flow over Cachoeira Paulista isweaker in winter compared with the other two sites. The long-itudinal variations of the prevailing winds are mainly related tothe generation of non-migrating tides (Mayr et al., 2005a, 2005b)and the differences in planetary wave activity.

The seasonal variation of diurnal tides observed by meteorradars located at Learmonth (L), Rarotonga (R), and CachoeiraPaulista (C) at latitudes around 221S is consistent with the totaldiurnal tides derived from TIDI/TIMED satellite measurements andmodel results obtained with HAMMONIA above the radar sites.The total tidal amplitudes observed over ‘L’ and ‘C’ are consider-able larger than the tidal amplitudes over ‘R’ applying for the radarobservations as well as for the satellite and the HAMMONIAresults. A well-established semi-annual oscillation is evident atthe sites ‘L’ and ‘C’. The weaker tidal signal at Rarotonga isprobably caused by destructive superposition of strong non-migrating and migrating tidal components. The differencesbetween the total tides observed at the radar sites seem to berelated to the differences in generation of non-migrating tides.Enhanced release of latent heat over the Rarotonga site could beidentified to be the cause of larger non-migrating tidal

Fig. 12. Zonal and meridional component of the migrating diurnal tide around 221S at 00 LT from TIDI observations (top) and from the radar combination L-C (bottom).

Fig. 13. Distribution of vertical wavelengths of the diurnal tide for total tides over L,R, and C and estimated migrating tides over L-C,L-R, R-C, and L-R-C (a) zonalcomponent and (b) meridional component. Note that the dotted line indicates thetheoretical vertical wavelength of diurnal tide with zonal wave number 1.


components. Increased diurnal latent heat release due to the deepconvection, results in forcing of non-migrating modes as shown byLieberman et al. (2007).

The longitudinally well separated radar sites allowed theestimation of the seasonal variation of the migrating diurnal tide.It is dominated by a semi-annual oscillation comparable with theseasonal variation of the total tide. The amplitude of the migratingdiurnal tide reaches comparable amplitudes with the total tide atthe sites ‘L’ and ‘C’. The radar based estimates of the zonal andmeridional component of the diurnal migrating tide are in goodagreement with the migrating diurnal tides derived from TIDI/SABER satellite measurements as well as with correspondingmodel results of HAMMONIA and GSWM00. Ekanayake et al.

(1997) found that the westward (eastward) propagating diurnalnon-migrating tides undergo significant amplification comparedto migrating tides at MLT where westerlies (easterlies) are domi-nated. The differences in the prevailing winds also support theargument about the differences in the non-migrating tides overthe region under study.

Acknowledgments

The authors are grateful to Peter Hoffmann and Markus Rappfor helpful discussions. We also thank Genesis Software for theirsupport to running the radar at Learmonth. G. Kishore Kumar andWerner Singer were supported by DFG in the frame of the CAWSESpriority program SPP 1176 under Grants SI 501/5-1 and SI 501/5-2.We wish to thank two anonymous reviewers and the editor fortheir critical comments and suggestions that helped in bringingout this manuscript to the present stage.

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Documents

Diurnal tides at low latitudes: Radar, satellite, and model results