Coordinated global radar observations of tidal and planetary waves in the mesosphere and lower thermosphere during January 20-30, 1993

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. A4, PAGES , APRIL 1, 1997 Coordinated global radar observations of tidal and planetary waves in the mesosphere and lower thermosphere during January 20-30, 1993 W. Deng, J. E. Salah, R. R. Clark, 2 S. J. Franke, 3 D.C. Fritts, 4 P. Hoffmann, 5 D. Kuerschner, A. H. Manson, 7 C. E. Meek, 7 D. Murphy, T. Nakamura, 9 S. E. Palo, ø D. M. Riggin, 4 R. G. Roble, ø R. Schminder, W. Singer, 5 T. Tsuda, 9 R. A. Vincent, and Q. Zhou 2 Abstract. A multi-instrument global campaign involving incoherent scatter, medium frequency, and meteor wind radars was conducteduring the period of January 20-30, 1993, to study the dynamics of the mesosphere and lower thermosphere. Data obtained from 15 radar stations covering a wide latitude range have been used to determine the global distribution of planetary and tidal waves during this 1 O-day campaign. Spectral analysis of the neutral winds measured by the radars in the altitude range from 80 to 130 km indicates the existence of a strong 48-hour wave near 90 km at latitudes between 40øN and 40øS that is present up to 108 km at 18øN. The semidiurnal tide is large at middle and high latitudes near 90 km and is predominant above 110 km, while the diurnal tide is observed to be particularly important in the upper mesosphere near 40 ø latitude. A least squares fit to the radar data is performed to obtain the amplitudes and phases of the tidal and 48-hour waves. Comparison with National Center for Atmospheric Research thermosphere-ionospheremesosphere general circulation model shows that the predictions from the model agree reasonably well with the observed global morphology of tidal wave amplitudes. 1. Introduction Energy Program (STEP), under the Mesosphere-Lower Thermosphere Coupling Study [Forbes, 1990; Manson et al., Characterization of the global distribution of tidal and 1994]. A series of global multiday observational campaigns planetary waves in the Earth's mesosphere and lower have been organized and conducted to study the mesosphereatmosphere has been a prime objective of the U.S. program lower thermosphere coupling through tidal and planetary on Coupling, Energetics and Dynamics of Atmospheric waves, and the January 20-30, 1993, period was one such Regions (CEDAR), and the international Solar-Terrestrial campaign. This was the longest campaign conducted to date and one of its primary objectives was to delineate the global characteristics of these waves in the km region. Haystack Observatory, Massachusetts Institute of Technology, Westford. Illustrative results from previous global coordinated studies 2Department of Electrical and Computer Engineering, University of New have been reported by Forbes and Salah [ 1991 ], Manson et Hampshire, Durham. 3Space Science and Remote Sensing Laboratory, University of Illinois, al. [ 1990, 1991 ], Johnson and Virdi [ 1991 ], Salah et al. Urbana. [1991,1994], Fesen and Roble [1991], Fesen et al. [1993], ndepartment of Electrical and Computer Engineering University of Colorado, Fuller-Rowell et al. [ 1991 ], and Hagan and Salah [ 1995]. Boulder. For the January 1993 campaign, a large number of 5Institut far Atmospharenphysik, Khlangsborn, Germany. 6Collm Geophysical Observatory, Leipzip University, Col!m, Germany. globally distributed ground-based instruments participated in 7Institute of Space and Atmospheric Studies, University of Saskatchewan, the observations, including incoherent scatter radars, Saskatoon, Canada. medium- and low-frequency radars, and meteor wind radars. gaustralian Antarctic Division, Kingston. 9Radio Atmospheric Science Centre, Kyoto University, Kyoto, Japan. A special CEDAR working group was organized to øhigh Altitude Observatory, National Center for Atmospheric Research, coordinate the analysis of these data, and this paper reports Boulder, Colorado. the results obtained to date using some of the data available Department of Physics, University of Adelaide, Adelaide, Australia. from 15 of these radars. The prime emphasis of this narecibo Observatory, Arecibo, Puerto Rico. working group study has been the spectral analysis of the Copyright 1996 by the American Geophysical Union. data to determine the various dominantidal and longerperiod waves observed at each geographic location and the Paper number 96JA /96/96JA comparison of the tidal parameters with results available from general circulation models. 7307

2 7308 DENG ET AL.' GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES i i i i the range of 100-i 10 x Wm-2Hz -l, and the Kp index generally varied in the range from 0 to 3 + during most of the interval, with an excursion to Kp=5 for about 12 hours on January 25, A plot of the Fl0.7 n flux and the Kp index for the campaign period is given in F gure ,. 115[ Day number 100' i Day number 2. Observations Experimental data gathered during the 1 O-day campaign in January 1993 were obtained from a wide range instruments at multiple geographic locations. The data used in this study were obtained from the 15 radars listed in Table 1, which provides the location of the radars and their type. These include incoherent scatter radars (ISR), medium-frequency (MF) and low-frequency (LF) radars, and meteor wind (MW) radars. The time series of neutral winds obtained at various Figure 1. Variation of the solar flux Fl0.7cmparameter and the altitudes in the mesosphere and lower thermosphere by these Kp geomagnetic index for the January 20-30, 1993, period. radars were analyzed to obtain harmonic parameters, and the resultant parameters derived from the measurements at each The geophysical conditions prevalent during the station are also listed in Table 1. January 20-30, 1993, period may be described as pertaining Figure 2 shows the latitude and altitude distribution of the to near solar minimum, moderately quiet geomagnetic data gathered from various radars during this campaign. conditions. The Flo.7cm solar flux during the period was in While reasonably goodata coverage for the mesosphere and Table 1. A List of the Radars Whose Data Gathereduring January 20-30, 1993, Were Utilized in the Analysis Presented in this Paper Juliusruh (JS) 55 N, 13E MF x x Saskatoon (ST) 52 N, 107 W h/if x x x x Sylvan Lake (SL) 52 N, 114W h/if x x x x Collm (CL) 52 N, 15 E LF x x Robsart (RS) 49 N, 109W MF x x x x Millstone Hill (MH) 42 N, 72 W IS x x Durham (DH) 42 N, 71 W MW x x x x Urbana (UB) 40 N, 88 W MF x x x x Kauai (KI) 22 N, 160W MF x x x x Arecibo (AC) 18 N, 67 W IS x x x x Christmas I. (XI) 2 N, 158 W MF x x x x Jakarta (JK) 6 S, 107 E MW x x x x Jicamarca (JM) 12 S, 77 W MST x x Adelaide (AL) 35 S, 138 E MF x x x x Mawson (MA) 68 S, 63 E MF x x x x

3 DENG ET AL.: GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES ' 120 ' i,, i,, i,, i,, MH AC RS SL observed over the km altitude range, and a mean altitude of 95 km is ascribed to the results since this represents the expected peak of the meteor echo distribution with altitude. For this campaign, the Durham radar interferometer system was not operating properly and height I AL J i K i CL UB H Juliusruh 55øN 60, -90 I,, I I i,, i Figure 2. and altitude distribution of data set used in this study during the 10-day campaign between January 20-30, The code names of the stations are listed in Table 1. lower thermosphere is found in the northern hemisphere, only a few data sets are available in the southern hemisphere and their altitude range is limited to the mesosphere. We have selected a subset of the radar data for the analysis presented here, concentrating primarily on the spectra near 90 km where there is a preponderance of observations, and on a select set of observations at higher altitudes from the incoherent scatter radars. 3. Spectral Analysis Spectral analysis techniques can be applied to the neutral wind measurements to determine the periodicities of tides and planetary waves. Most spectral analysis techniques require a uniformly sampled time series. However, data obtained from some of the radars are often unevenly spaced in time and have gaps, necessitating the use of special spectral tools to analyze the observations, with less restriction on sampling uniformity. One of methods used to estimate the power spectrum of unevenly spaced data is based on the least squares fitting technique developed by Lomb [ 1976] and Scargle [1982]. In addition to the ability of this technique to treat unevenly space data, the Lomb-Scargle method also,provides an estimate of the significance of the height of a spectral peak. Estimates of the power spectrum, computed using the Lomb-Scargle (LS) periodogram, are shown in Figure 3, and Frequency (cph) illustrate the power spectral density in the meridional neutral wind field for four northern hemisphere sites. These Figure 3. Periodograms of the northward neutral wind estimates are obtained by applying the LS-periodogram to observed at four radar stations. The Lomb-Scargle method is neutral wind data at a particular altitude in the upper used to calculate the periodograms from neutral wind data mesosphere. The actual altitude of the data selected in this selected at an altitude in the mesosphere, which is 89 km for calculation varies for each of the radars represented the Juliusruh radar, 91 km for the Sylvan Lake and Robsart depending on the availability of the data. This altitude is radars, and an equivalent altitude of 95 km for the Durham 89 km for Juliusruh and 91 km for Sylvan Lake and Robsart. radar. A 90% significance level (or 10% false alarm The LS-periodogram calculated at Durham is based on northward neutral winds derived from the meteor echoes probability) of a peak in the periodograms is indicated by a horizontal dashed line Sylvan Lake 52øN Robsart 49øN Durham 20 t 42 0 N

4 7310 DENG ET AL.' GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES discrimination for the meteor echoes was not possible. For each LS-periodogram, an estimate of the 90% significance level (or 10% false alarm probability) of a peak in the power spectral distribution calculated using an algorithm provided by Home and Baliunas [1986]. This estimate is indicated the periodogram by a horizontal dashed line. A prominent peak with a period of 12 hours is observed in the data from the radars at Robsart, Sylvan Lake, and Ju!iusruh, all located above 49øN latitude. This is the only peak whose magnitude is well above the 90% significance level at a frequency lower than 0.25 cycle per hour (cph), or a period longer than 4 hours. This result indicates that the dynamic structure of the upper mesosphere at these latitudes is dominated by the semidiurnal tides. In contrast, a diurnal tide becomes increasingly important at Durham's latitude of 42øN. Two significant peaks exist in the periodogram at this latitude, a 24-hour and a larger 12-hour component, and their magnitudes are well above the 90% significance level. This suggests a transition region that lies between the higherlatitude region where the semidiurnal tide dominates and the lower-latitude region where the diurnal tide becomes important, as expected from classical tidal theory. To illustrate further the variation of spectral content with latitude, LS periodograms of the northward neutral wind at other radar stations are presented in Figure 4 for Adelaide, Kauai, and Christmas Island and in Figure 5 for Urbana. These periodograms are obtained by averaging the individual periodograms over the entire altitude range covered by the MF radars, nominally from 82 to 98 km. A log scale is used for the plots to satisfy other applications of the analysis, namely, the study of waves with shorter period than the tidal components. The main observation about the tidal components from this comparison that the amplitude of the semidiumal tide is seen to decrease with decreasing latitude. The diumal tide is prominent at Adelaide and Urbana, as was observed at Durham, and it is the relatively largest tidal component at Christmas Island. The results thus indicate the dominance of the semidiumal tide above about 45øN and the : presence of both diumal and semidiurnal components around 22ø-42øN and similarly at 35øS. Near the equator, although lo 6 the diurnal component is larger than the semidiurnal component, it is quite weak in this January period. However, the most dominant feature of the periodograms : in Figures 4 and 5 is the emergence of a quasi 2-day planetary wave at a period close to 48 hours at latitudes between 40øN and 40øS. This component is seen clearly in the Urbana data and is quite large at Adelaide. The 104,:,:,,,,:,J i ' I I I f i iii amplitude of the 2-day wave is so large at Kauai and O.Ol 0.10 I.oo Christmas Island that it dominates the dynamics of upper Frequency (el>h) mesosphere in this region. Quasi 2-day waves in the Figure 4. A collection of averaged Lomb-Scargle periodomesosphere have been found in many ground-based radar grams of northward wind at three radar stations over an measurements [e.g., Manson and Meek, 1986; Clark, 1989; altitude range of MF radar measurement (about km). Fritts and Isler, 1992; Harris and Vincent, 1993; Palo and Avery, 1995] and in satellite observations [Wu et al., 1993]. In general, the 2-day wave tends to be more pronounced at results in January 1993 agree well with previous equatorial regions of the mesosphere in the solstices and the observations. summer hemisphere at extratropical latitudes. This behavior It is somewhat surprising that the relatively strong 2-day has been modeled successfully by Hagan et al. [1993]. The wave seen in the periodogram at Urbana (40øN), Figure 5, is :: 36 0 S 10 5 Io 5 104, lo : ß :. ß. : : i : ß. Kauai 22øN : Xmas Island ß : 2øN :

5 ,,,,.,, DENG ET AL.: GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES 7311 ',48 '2.4!12,:8 6 8 '2.4!12, ', lo lo 5 lo a.' i t.' i i ] i i i i i iii 104,,,,,,,,J,:, c,,ic,l ' ', ' ' ' O. 1 O O. 1 O Frequency (cph) Frequency (cph) Figure 5. The averaged Lomb-Scargle periodogram of northward neutral wind at Urbana (40øN) over an altitude range of km: (lei ) zonal; (right) meridional. not seen at Durham (42øN), Figure 3, where only diurnal and wind at Arecibo, and the 90% significance level is again semidiurnal components are observed. We have therefore indicated as a horizontal dashed line. examined the manner in which the data are processed for the At the lower altitudes, below about 104 km, two strong two sites to determine whether that could explain this responses exist with periods of 48 and 24 hours. The 2-day apparent difference. An examination of the profile of the wave is the dominant component of the motion field since its 2-day wave amplitude at Urbana in the range km magnitude is much greater than that of the diurnal tide. The shows that this wave has a peak at about 85 km and 2-day wave is then rapidly damped out above about 108 km. decreases sharply above this altitude. The averaging of the In a transition altitude range starting at about 108 km, the periodograms with altitude maintains a relatively strong semidiurnal tide becomes important and predominates above component in the average spectrum compared to the diurnal 111 kin. The semidiurnal tide in this region has a longer and semidiurnal components. It was noted previously that vertical wavelength than that of the diurnal tide, and it the Durham radar was unable to provide altitude propagates more effectively at altitudes above 110 km. discrimination, and the overall altitude ascribed to the winds Overall, the existence of various wave modes in this region is near 95 km, weighted by the preponderance of meteor provides an excellent example of the propagation of tides and echoes at that altitude. This suggests that the processing planetary waves in the mesosphere and their ultimate performed on the Durham data without altitude dissipation in the lower thermosphere. discrimination may have suppressed the 2-day wave component which is contained in the region well below 4. Harmonic Analysis In order to study the altitude variations of various spectral In the previous section, the major spectral components of components in the mesosphere and lower thermosphere, the winds in the mesosphere and lower thermosphere neutral wind data obtained from the Arecibo incoherent observeduring the 10-day campaign of January 1993 were scatter radar are used to calculate LS periodograms at discussed in terms of their estimated power spectral density. different altitudes from 97 to 125 km. A continuous high- In this section, a more quantitative assessment of the quality data set covering more than 12 hours of local time amplitudes and phases of these waves will be presented. To each day was obtained from the Arecibo radar during this obtain the tidal and planetary wave parameters, a least 10-day campaign. This allows semidiurnal, diurnal, and squares firing technique is used to describe the neutral winds longer-period waves to be resolved from the periodograms in terms of a model containing the mean component, 12-, 24- and minimizes aliasing problems. Figure 6 shows the and 48-hour harmonic components at stations where altitude variation of a LS periodogram for the northward continuous data sets with 24-hour local time coverage are

6 ß 7312 DENG ET AL.' GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES Periodo m {M} 122k m,o 122 lcm O20 O25 P_e (V 111k m 111 km 0 ' O25 P er x:logr m (V) loskin :I-,, ' 108 lcm 0, peliodo r l? (V) 101k m : i : 101 km ß -_,_.'-'-,,, _. ß... o.oo 0.05 O. lo o.1: Frequency (cph) Figure 6. Variation of the Lomb-Scargle periodograms of northward wind over Arecibo (18øN) as a function of altitude from 97 to 125 km. A 90% significance level (or 10% false alarm probability) of a peak of the periodograms is indicated by a horizontal dashed line.

7 DENG ET AL.: GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES 7313 available. At some stations, such as Millstone Hill and Juliusruh, where the observations in the mesosphere and lower thermosphere are limited to the daytime, a simplified model with mean and 12-hour harmonics is used to fit the daytime data. The limitation of observations to the daytime observed downward phase velocity indicates the upward propagation of the tide from below the mesosphere. The northward phase is seen to lead the eastward phase by about 3 hours below 110 km, and the vertical wavelength derived from the phase measurements in the km altitude introducesome uncertainty in the determination of the tidal region is about km. These characteristics are components [Crary and Forbes, 1983]. To assess the effect of this limitation and the validity of the fitting approach, as well as to examine the internal consistency of tidal parameters obtained from multiple radars which are nearly colocated at one latitude, the amplitude and phase of semidiurnal tides obtained from the measurements by the IS radar at Millstone (42øN), the MW radar at Durham (42øN), and the MF radar at Urbana (40øb0 are presented in Figure 7. The comparison demonstrates an excellent continuity with consistent with the dominance or at least strong influence of the semidiurnal (2,4) mode. At altitudes above 110 km, however, a reversal in the direction of phase propagation is seen in the northward component, and the eastward phase does not advance with altitude at the same rate as that below 110 km. The change in the rate of phase progression at about 110 km was also observed in previous experiments combining data from the Millstone IS and Durham MW radars [Salah et al., 1994]. This altitude appears to mark a altitude in the mean, amplitude, and phase of the semidiurnal transition region that lies between a tidally dominated tide derived from the IS and MF radars and shows very good agreement between the measurements of MF and MW radar at 95 km, where the Durham data are available. This result gives us confidence about the validity of our assumption of a 12-hour wave to fit the daytime data at Millstone Hill and lends credibility to the overall consistency among the methodologies used to analyze the data obtained from the different techniques. This is particularly important for the MF radar since winds obtained by this technique have been a subject of recent concern and discussion [Hines et al., 1993; Roper et al., 1993]. For tidal wave analysis, the mean winds and tidal amplitudes from the MF radar are seen to be propagation region and a region influenced by diffusive forces where shorter-wavelength propagation modes are more susceptible to dissipation and longer-wavelength evanescent modes prevail. Thus the observations above 110 km indicate the interference between other semidiurnal modes and the (2,4) mode seen below. Figure 8 shows the latitude variations of the amplitudes of the tidal and planetary waves for the eastward and northward winds at 90 km during this 10-day campaign. Similar analyses were developed for altitudes of 85 and 95 km which resulted in similar although not identical overall variation with latitude. For the eastward component at 90 km, the in good agreement with those from the IS and MW radars. semidiurnal, diurnal and 2-day wave have comparable The only apparent discrepancy is in the northward wind phase, where a difference of 2 hours is seen between the MF amplitudes, but a strong 48-hour component dominates northward of about 7øS to 40øN, and the semidiurnal and IS radar near 103 km. Such a difference has been seen component dominates above that latitude. For the northward previously in comparisons between the Millstone Hill IS and Durham MW radar [Salah et al., 1994] and is the subject of special common volume experiments planned to delineate the source of this discrepancy. It is possible that the clutter effects at the lowest altitudes of the Millstone Hill data, the paucity of meteor echoes for the MW radar above 100 km, and the uncertainties in the MF radar data due to groupretardation effects above 95 km, all contribute to this discrepancy [Namboothiri et al., 1993]. From Figure 7, the magnitude of the mean zonal winds at midlatitudes is in the range of m/s, with a shift in wind, a strong 48-hour component prevails from Adelaide (36øS) to Kauai (22øN), except at one station, Jicamarca (12øS), where it is a minimum. No diurnal and semidiurnal components were derived from the Jicamarca data for comparison because such components were considered to be unreliable as a result of the absence of nighttime data. The diurnal and semidiurnal components are of relatively comparable amplitude, with the diurnal tide dominating near the equator and southward, and the semidiurnal tide dominating northward of 45øN. These results are in general agreement with those obtained from the spectral analysis direction from eastward below about 100 km to westward described earlier. above that altitude. The variation of the mean meridional wind with altitude is very small with a prevalent northward 5. Comparison with Model Predictions component below 95 km and southward above 95 km. The amplitude of the semidiurnal tide shows a systematic increase Substantial progress has been made in the modeling of of eastward winds with increasing altitude reaching 60 m/s tidal wave propagation from the lower atmosphere into the at 120 km. The northward semidiurnal amplitude is also thermosphere. The thermosphere-ionosphere-mesosphere seen to increase with altitude, but oscillates around general circulation model (TIM-GCM), developed at the m/s at altitudes above 100 km. The phase variation National Center for Atmospheric Research (NCAR), is the of semidiurnal wind shows the general characteristic of the most recent three-dimensional presentation of the dynamics semidiurnal tide observed at these altitudes, namely, of the middle and upper atmosphere. The model, which downward phase propagation with altitude in both originated from the thermosphere general circulation model components below 110 km. Since the energy for such waves and is under continued development, has been recently propagates at the group velocity which, for the vertical expanded to cover the height range from 30 to 500 km and component, is opposite in direction to the phase velocity, the represents the important coupling phenomena that

8 7314 DENG ET AL.' GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES... I... $... i o'... '... i... o. o o Ig o... i... i... i,... i... 1 'I::: o z E E o o (un4) epntplv (unl) epnl!tlv

9 ß DENG ET AL.' GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES O 4O I I,., 12hr. o o 24 hr. _._._ 48 hr. 3O 2O 10 ß O 4O i i i i i i i i i i i i i i,., 12 hr. c o 24 hr. -._._+ 48 hr. ---._,. '!..., /'. I \ E 3O 20! _ 1, \ ß, I \, /' \ I \ /' / ß /' /' -/' Figure 8. Variation oœ the northward amplitude oœ the semidiurnal (dashed line), diurnal (solid line), and the 2-day wave (dotted-dashed line) with latitude at 90 km for (top) eastward and (bottom) northward winds during the 1 O-day campaign. characterize the various regions in this large altitude band. a particular latitude and longitude. In other words, we This includes the interesting middle atmosphere phenomena neglect the effects of nonmigrating tides that may be such as the diurnal and semidiurnal tidal waves [Roble and included in this model. Ridley, 1994] but does not yet include the planetary 48-hour Figure 9 shows the comparison between model predictions wave. and radar measurements of the eastward and northward wind A TIM-GCM simulation was conducted at NCAR to study the dynamics and energetics of the mesosphere and lower thermosphere region during the January 20-30, 1993, campaign, and the results have been made available to the CEDAR working group for comparison with the at 90 km as a function of latitude. Excellent agreement is found in the mean component for both eastward and northward winds. The model successfully predicts the eastward jet at midlatitudes of the mesosphere in winter and the westward jet in the equatorial region, in agreement with observations. To facilitate this comparison, a least squares the radar observations. While an overall agreement is found fitting procedure was used to obtain tidal wave parameters in the northward mean wind, the model underestimates its and was applied to the winds obtained from the model magnitude at equatorialatitudes. Excellent agreement is simulation, in a similar way as was derived from the radar also seen in the semidiurnal component for both eastward observations. The amplitude and phase of semidiurnal and and northward winds. Both the model and observations diurnal tides are obtained from this fit, utilizing the time reveal a seasonal difference of tidal amplitude, with a larger series of model output at various latitudes. The longitude peak found in the northern winter hemisphere and a weaker location of the time series is set at 0 ø. By assuming one in the summer. There is a clear indication of a longitude independence of these tidal parameters, we extended these parameters obtained at each latitude to represent the simulation for all the radar stations located at longitudinal difference in the amplitudes, such as between the stations near 50øN, most probably because of the nonmigrating components. Most of discrepancies between

10 . ß 7316 DENG ET AL.: GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES Mean Eastward Winds at 90 Km 40 i I i i I i i i I ' i i i i Mean Northward Winds at 90 Km' 40 [ [ [, [ [ [ ] [ [ [ [ ß ß ß ø20 i, - o,,,,,, ' 'o 'o4'o ' Semidiurnal Eastward Winds at 90 Km -20..o , -50, -40, -30-, ; 0-0 0, 1 'o 'o 'o4'o' ' 70 Semidiurnal Northward Winds at 90 Km 40,,,,,,,,,,,,, ' 20 lo $ I I I I 1 0 ' 10 I 20 ' 30 I 40 '5 )6 370 Diumal Eastward Winds at 90 Km I I I I I I I I' I I I I I Diurnal Northward Winds at 90 Km 40 i i i! i i i i i i i i i + ' 20.;, lo lo ß o o , i i i, i 011(31 I 20 30, 4 50 i 60 i 70 Figure 9. Comparison of observed amplitude of semidiurnal tide with the prediction of the TIM-GCM at 90 km for both eastward and northward winds during the 10-day campaign. model results and radar measurements are found in the current model due to the wide latitude coverage of the diurnal components. The model underestimates the experimental data and the use of the same least squares magnitude of diurnal tide at most latitudes. fitting procedures on both the radar data and the model The above comparison of tidal amplitude in the output to extract the tidal parameters. The results of this test mesosphere region is perhaps the most rigorous test of the are very encouraging and can be used to guide further

11 DENG ET AL.: GLOBAL OBSERVATIONS OF TIDAL AND PLANETARY WAVES 7317 refinements of the model in order to improve the agreement Information on the Jakarta meteor radar observations is available with the experimental data at various locations, particularly from Tsuda et al. [1995]. W.D. is grateful to the NSF CEDAR for the diurnal tide. program for support of his postdoctoral research appointment at the Millstone Hill Observatory, where this work was carried out. 6. Conclusions Millstone Hill is operated by the Massachusetts Institute of Technology under a cooperative agreement with the NSF. The Editor thanks G. R. Swenson and another referee for their The prime objective of this study of globally coordinated assistance in evaluating this paper. observations of winds in the mesosphere and lower thermosphere over a 10-day winter campaign period, References January 20-30, 1993, has been to delineate the latitudinal variation of the spectral content of the time series of winds Clark, R.R., The quasi 2-day wave at Durham: Solar magnetic measured at various radars stations. The conclusions derived effects, J..dtmos. Terr. Phys., 51, , from this analysis indicate the following: Crary, D.J., and J.M. Forbes, On the extraction of tidal information 1. A semidiurnal tide dominates the dynamic structure from measurements covering a fraction of a day, Geophys. Res. of the upper mesosphere (- 90 km altitude) at latitudes above about 45øN, and a diurnal tide is found to have a large and Lett., 10, , Fesen, C.G., and R.G. Roble, Simulations of the September 1987 comparable amplitude to the semidiurnal component near lower thermospheric tides with the National Center for 40 ø latitude. Atmospheric Research thermosphere-ionosphere general circulation model, J. Geophys. Res., 96, , A strong 2-day wave is seen at latitudes between Fesen, C.G., R.G. Roble, and E. C. Ridley, Thermospheric tides 40øN and 40øS, dominating the dynamics of the upper simulated by the National Center for Atmospheric Research mesosphere (- 90 km altitude) at equatorial latitudes, where thermosphere-ionosphere general circulation model at equinox, the diurnal and semidiurnal tides are found to be much J. Geophys. Res., 98, , smaller for this observing period. At the latitude of Forbes, J.M., The Lower Thermosphere Coupling Study of the Jicamarca (12øS), a small 48-hour wave is observed. CEDAR and WITS programs,.ddv. Space Res., 0(6), , 3. The variation of the tides and planetary waves is seen most remarkably in the spectral results of winds observed at Arecibo (18øN). A strong 2-day wave is seen below 108 km, damping in favor of a combination of diurnal and Forbes, J.M., and J.E. Salah, Mesosphere-thermosphere tidal coupling during the September 21-25, 1987 LTCS-1 campaign, J. Geophys. Res., 96, , semidiurnal waves in the region up to 111 km, above which Fritts, D.C., and J.R. Islet, First observations of mesospheric a semidiurnal wave is seen to dominate up to 125 km. dynamics with a partial reflection radar in Hawaii, Geophys. 4. At three midlatitude stations near 40ø-42øN Res. Lett., 19, , Fuller-Rowell, T.J., D.' Rees, H.F. Parish, T.S. Virdi, (Millstone Hill, Durham, and Urbana), there is excellent P.J.S. Williams, and R.M. Johnson, Lower thermosphere agreement in the variation of the semidiurnal tide with coupling study: Comparison of observations with predictions of altitude, indicating the overall consistency of the the University College London-Sheffield thermospheremeasurements amongst the three techniques (incoherent ionosphere model, J. Geophys. Res., 96, , scatter, meteor wind, and medium-frequency radars). Hagan, M.E., and J.E. Salah, Upper thermospheric variability over 5. Mean zonal winds at midlatitude are seen to reverse Millstone Hill during the LTCS-2 and LTCS-6 campaigns, from eastward to westward above 100 km, the amplitude of the semidiurnal tide is seen to increase steadily with altitude, with the Zonal component exceeding the meridional J. Geophys. Res., 100, , Hagan, M.E., J.M. Forbes, and F. Vial, Numerical investigation of the propagation of the quasi-two-day wave into the lower component and maximizing near 120 km. Phase variations thermosphere, J. Geophys. Res., 98, , Harris, T.J., and R.A. Vincent, The quasi-two-day wave observed indicate a propagating tide from below with a wavelength of 40 km and suggesthe mixing with other modes above 110 km. in the equatorial middle atmosphere, J. Geophys. Res., 98, , Hines, C.O., G.W. Adams, J.W. Brosnahan, F.T. Djuth, 6. Comparison of the observations with predictions M.P. Sulzer, C.A. Tepley, and J.S. Van Baelen, Multi-instrument from the NCAR/TIM-GCM analyzed in the same manner as observations of mesospheric motions over Arecibo: Comparisons the radar data shows very good agreement, as the model and interpretations, J..dtmos. Terr. Phys., 55, , succeeds in reproducing the zonal mean wind seasonal Home, J.H., and S.L. Baliunas, A prescription for period analysis variation and the latitude variation of the semidiurnal tide. of unevenly sampled time series,.dstrophys. J., 302, , The mean meridional component is underestimated by the model, as is the diurnal tidal component. Johnson, R.M., and T.S. 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