JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D22, PAGES 26,217-26,224, NOVEMBER 27, 1997

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D22, PAGES 26,217-26,224, NOVEMBER 27, 1997 Observations of diurnal oscillations with a meteor wind radar and radiosondes in Indonesia Toshitaka Tsuda, Takuji Nakamura, Atsushi Shimizu, and Tetsuo Yoshino Radio Atmospheric Science Center, Kyoto University, Kyoto, Japan Sri Woro B. Harijono and Tien Sribimawati Agency for the Assessment and Application of Technology, Jakarta, Indonesia Harsono Wiryosumarto Indonesian National Institute of Aeronautics and Space, Jakarta, Indonesia Abstract. We constructed a meteor wind radar in an observatory near Jakarta, Indonesia, through collaboration between Japan and Indonesia, which has been operating continuously since November Horizontal wind velocity fluctuations were determined for altitudes between 75 and 100 km with time and height resolutions of 1 hour and 4 km, respectively. We also launched radiosondes from Bandung, four times a day from November 1992 to April 1993, and obtained profiles of horizontal wind velocity and temperature at 0-35 km with a height resolution of 150 m. Further, the data collected during the Tropical Ocean and Global Atmosphere/Coupled Ocean-Atmosphere Response Eperiment (TOGA/COARE) campaign have been particularly used to investigate the horizontal variations of diurnal oscillations. Overall structure of the observed wind velocity profiles agreed well with the model diurnal winds above 20 km, with a dominant vertical wavelength of km. However, disagreements appeared below 20 km, showing significantly larger amplitudes and complicated phase structures, which may suggesthe effects of nonmigrating tides. Diurnal oscillations of the temperature in the lower atmosphere were described very well by a numerical model. 1. Introduction Diurnal oscillations of wind velocity in the troposphere and middle atmosphere are primarily described in terms of global atmospheric tides, which are chiefly generated by thermal forcing due to absorption of solar radiation by water vapor or ozone [e.g., Chapman and Lindzen, 1970; Kato, 1980]. The global diurnal tides migrate westward, synchronizing with an apparent motion of the Sun, so their time variations are the same when they are plotted as a function of local time. Forbes [1982] developed a numerical model for the migrating diurnal tides and provided their amplitudes and phases for solstice and equino conditions. Hagan et al. [1995] further improved the model, incorporating a more realistic dissipation process, and consequently, the delineated tidal amplitudes became smaller. In addition to the global tides, localized ecitation sources, such as diurnal activity of cloud convection or heat echange near the Earth's surface, could also produce diurnal oscillations, which were theoretically treated as nonmigrating tides with various zonal wavenumbers; therefore they could propagate both westward and eastward or be standing. A classical tidal theory was applied to describe the nonmigrating tides by assuming an idealized heat source [McKenzie, 1968; Karo er al., 1982]. Tsuda and Karo [1989] studied nonmigrating tides due to the land-sea difference of the upward heat Copyright 1997 by the American Geophysical Union. Paper number 96JD /97/96JD ,217 transfer within the planetary boundary layer. The general circulation model (GCM) was recently used for determining nonmigrating tides [Tokioka and Yagai, 1987; Yagai, 1989; Miyahara et al., 1993]. In particular, Williams and Avery [1995] studied tidal ecitation by latent heat release within convective clouds in an equatorial region and compared with observations with wind profiler radars and a satellite. Statistical analysis of nonmigrating tides was etensively conducted by Haurwitz [1965] by using large amount of surface data. Wallace and Tadd [1974] employed 12 hourly radiosonde data for detecting horizontal distribution of diurnal tides, and found significant regional deviations. Hsu and Hoskins [1989] suggested effects of the land-sea distribution on the diurnal tides, analyzing the European Centre for Medium-Range Weather Forecasts (ECMWF) results. Lieberman [1991] determined the behavior of diurnal tides from satellite observations in the middle atmosphere, reported that nonmigrating tides could have larger amplitudes than migrating components and that the nonmigrating tides could cause significantime varia- tions of diurnal tides. The general characteristics of diurnal tides indicates that vertically propagating tides are restricted in the low-latitude region. Accordingly, we concentrated the observations of diurnal tides near the equator. A radiosonde campaign was carried out in Indonesia in February-March 1990, and based on these observations, Tsuda et al. [1994] reported results of diurnal wind oscillations at 0-35 km. The observed amplitudes were much larger than a model prediction in the troposphere, while the behavior of diurnal wind oscillation above 20 km was consistent with the Forbes [1982] model.

2 26,218 TSUDA ET AL.' DIURNAL OSCILLATIONS OVER INDONESIA EASTWARD WIND NORTHWARD WIND TEMPERATURE 3O lo ! AMPLITUDE(m/a) PHASE (hr) AMPLITUDE(m/a) PHASE (hr) AIdPUTUDE(k) PHASE (hr) Figure 1. Amplitudes and maimum phases in local time of diurnal oscillations for the (left) eastward and (middle) northward wind velocity, and (right) temperature from radiosonde observations from November 1992 to April 1993 at Bandung. Numerical models by Forbes [1982] for 6øS are also presented by circles and squares with a dashed line in December (circles) and March (squares) conditions, respectively, while crosses indicate a model by Ekanayake [1994]. We have again conducted a similaradiosonde campaign for sonde measurements were conducted four times a day at 0500, a longer observation period in in Indonesia, to- 1100, 1600, and 2300 LT (or 2200, 0400, 0900, and 1600 UT) gether with a meteor wind radar (MWR) observation of wind from November 16, 1992, through April 10, 1993 [Tsuda et al., velocity in the mesosphere. Using these observations, in this paper we analyze the profiles of diurnal wind oscillations 1995]. The sounding was, however, interrupted on November 20-30, 1992, and from December 17 to January 11, 1993, 0-35 km and km and compare the results with various because of a system failure. Therefore the tidal characteristics numerical models. during January 12 and April 10 were mainly reflected in the observed results, although in the following, we present the 2. Observations With MWR and Radiosondes mean profiles delineated for the whole observation period. We launched a total of 411 balloons, where 362 (88%) and We started collaboration with Indon½sian research institutes 293 (71%) of them reached 18 and 30 km, respectively. We on equatorial atmosphere dynamics by means of a ground- obtained profiles of horizontal wind velocity and temperature based radar and radiosond½s. A radar observatory was estab- T with a height resolution of 150 m. A 24-hour oscillation was lished in the Indon½sian National Center for Research, Science detected at 0-35 km by fitting a sinusoidal curve to the entire and Technology (PUSPIPTEK)(6.4%, 106.7øE, 50 m mean sea time series of u, v, and T, sampled every 6 hours. 1½v½1 (MSL)), located about 27 km southwest of Jakarta. We It is noteworthy that the intensive observation period of constructed a meteor wind radar (MWR) in the observatory Tropical Ocean and Global Atmosphere/Coupled Oceanand started operation since November The system de- Atmosphere Response Eperiment (TOGA/COARE) overscription of MWR and the preliminary results are reported by lapped with the campaign of the present study. Radiosonde Tsuda et al. [1995]. sounding was continued four times a day at a number of sta- For tidal analysis we first determined castward, u, and northward,, wind velocity at km with time and height tions spread in the west Pacific. The height coverage was, resolution of 1 hour and 4 km, respectively. Then, tidal amplihowever, limited to a region below about 20 km. tudes and maimum phases in local time are determined by In the following sections we first discuss diurnal oscillations using a least squares fitting on the time series at each altitude. of u, v, and T at 0-35 km, using the radiosonde profiles at We also launched radiosond½s from Bandung (6.9%, Bandung. Then we investigate horizontal variations of diurnal 107.6øE, 740 m MSL), located about 100 km east of the MWR oscillations, referring to radiosonde profiles collected through the site, in cooperation with an international campaign of Cou- TOGA/COARE project. We further presenthe diurnal wind pling and Dynamics in Regions Equatorial (CADRE). Radio- velocity fluctuations km using MWR data over Jakarta.

3 TSUDA ET AL.: DIURNAL OSCILLATIONS OVER INDONESIA 26,219 MANUS 2.03'$ 'E 3m(MSL) KAVIENG KAPINGA 2.35'S 'E 3m(MSL) 1.00'N 154.BO'E 3rn(MSL) NAURU 0.50'S 'E 30rn(MSL) ½ O AMPLITUDE(m/s) PHASE(hr) AMPLITUDE(m/s) PHASE(hr) AMPLITUDE(m/s) PHASE(hr) AMPLITUDE(m/s) PHASE(hr) Figure 2. Diurnal oscillations for eastward wind velocity determined at Manus, Kavieng, Kapinga, and Nauru during January and February Model profiles by Forbes [1982] are shown by circles and squares with dashed lines for December and March conditions, respectively Diurnal Oscillations in the Troposphere and Lower Stratosphere 3.1. Radiosonde Observations Over Bandung, Indonesia By using all the radiosonde profiles obtained during November 1992 and April 1993, mean amplitudes and phases of diurnal oscillations were determined, as shown in Figure 1 in comparison with numerical models by Forbes [1982] and Ekanayake et al. [1997]. Although the Forbes models are illustrated for December and March conditions, they are almost identical, indicating small seasonal variations. Note that the Forbes model assumed only migrating tides, while the Ekanayake et al. model included nonmigrating components as well. The amplitudes monotonously increase with altitude in the model by Forbes [1982], while the other model by Ekanayake et al. [1997] suggestsmaller amplitudes above 15 km with considerable height variations, probably due to an interference of several modes. Although the zonal phases are different by auuut u nouns, uutn liiou lb lnu cate downward ase progression in the stratosphere, corresponding to an upward ener propagating wave. Below about 15 km altitude the obse ed zonal amplitudes in Figure 1 ranged from 0.3 to 1.0 m/s, which were several times larger than predicted by the o models. In the lower stratosphere the amplitudes became rather smaller than those in the troposphere, showing a minimum near the tropopause. The obse ed amplitude profile agreed well with the model by Ekanayake et al. [1997], while the phases show significant resemblance to both models. The v component also showed similar characteristics to u. The amplitudes for v were, however, generally smaller than u :--., u,c... uupu p,c c. n tn um nanu, ti cir am plit udes were larger than the zonal values in the stratosphere. The phase profile agreed very well with the models above about 20 km. Amplitudes of T were as large as 1.5 K at 1 km altitude, decreased rapidly at 1-2 km, then they became K in the troposphere, which was fairly consistent with the model. Although the model indicates a linear increase of the amplitudes above 15 km, the observed values did not grow, showing fluctuations at K. While the phase profile of T agreed very well with the model in the entire height range, the phases were fairly constant at 1800 LT at 2-15 km, then showed phase pro- gression in the stratosphere. Note that the maimum phase appeared at around 1400 LT below 2 km due to the effects of upward heat transfer from the ground to the planetary boundary layer Horizontal Variations by TOGA-COARE Data In order to study spatial variation of tidal winds, we also refer to radiosonde profiles, collected through the TOGA/ COARE project. Diurnal oscillations of u, v, and 1' are determined at standard pressure levels for January-February Figure 2 shows the results for u at Manus (2.0øS, 147.3øE, 3 m MSL), Kavieng (2.4øS, 150.5øE, 3 m MSL), Kapinga (1.0øN, 154.8øE, 3 m MSL), and Nauru (0.5øS, 166.9øE, 30 m MSL). In the troposphere, observed amplitudes largely eceeded the model prediction, while the phase structure agreed well with the model at all of four sites, ecept in the boundary layer. It can be suggested that the maimum phase of u in local time was almost the same in the lower troposphere for the stations spread over a large spatial etent in an equatorial region. Above about 15 km, the observed phases showed downward progression, although they were out of phase to the modei vaiues.

4 26,220 TSUDA ET AL.: DIURNAL OSCILLATIONS OVER INDONESIA MANUS 2.03'S 'E 3m(MSL) KAVIENG KAPINGA 2.35"S 'E 3m(MSL) 1.00'N 'E 3m(MSL) NAURU 0.50'S 'E 30m(MSL) AMPLITUDE(m/s) PHASE(hr) AMPUTUDE(rn/s) PHASE(hr) AMPLITUDE(m/s) PHASE(hr) AMPLITUDE(m/s) PHASE(hr) Figure 3. The same as Figure 2 ecept for northward wind velocity. 12 Results for v are plotted in Figure 3. The model predicted processes [Forbes and Hagan, 1988], or they may indicat every small amplitudes for v, since principal modes of migrating istence of nonmigrating tides that are not necessarily symmettides are basically symmetrical relative to the equator. Obser- rical about the equator. Although the model phases are convations indicate, however, that v had significant amplitudes, stant at 1200 LT, observations indicated a miture of both although they were smaller than u. This could be eplained by propagating and evanescent (constant phase) structures. the asymmetry in the background mean winds or dissipation Phases for T, shown in Figure 4, agreed very well with the MANUS 2.03 'S 'E 3m(MSL) KAVIENG KAPINGA 2.,35'S 'E 3m(MSL) 1.00 *N 154. BO*E 3m(MSL) NAURU 0.50'S 'E 30m(MSL) AMPLITUDE(K) PHASE(hr) AMPLITUDE(K) PHASE(hr) AMPLITUDE(K) PHASE(hr) Figure 4. The same as Figure 2 ecept for temperature AMPLITUDE(K) PHASE(hr)

5 TSUDA ET AL.: DIURNAL OSCILLATIONS OVER INDONESIA 26, I... I ' ß o 5 iiiiiiiiii -15 I... I... I... I... I... h, I I ß 5 J_ I? so --I-- o -s, I I I-- -- _g I--- i_' _ HEIGHT = 9.38(km ' -10 lo "-I- -T-F I... I... I... I... I... I... I...,... I I... I,,,,... I... I...,... I...,I... I... I I...,,I... I...,,,I... I...,,I... I,,... I ' j-_ J_ _ _i_4_ ; l_ -,,,,,,... I,... I,,,:R,,,E,,...,,7 $,,,,,,,,',,,2,,,,:,i,:,,,,,,:,,T,,,,,,,,...,,,,...,......, HEIGHT = 3.38(km) h,,,,,,,,i,,,,,,,,,i,,,,...,i,,,,,,,,, h,,,,,,,,i,,, 15 I I ',,o -T-,L-I o -,,,: 2...,,,24 -, i i i HEIGHT = :,, (kin) ' o T I t... j_ I I,, I I "r T -I,-T-F-I; o - -I --'-i- -,, :,,',,Z, _,o ', -': ' ',", i,, " ' " ' ' ' " ' ""'""l'"'""'l'""""'""'"'l'""'"l"! '" "1'"'""' $0 40 so so so LONGITUDE(' E) LONGITUDE(' E) LONGITUDE:(" E) Figur. Harmonic dial of diurnal oscillations for (left) eastward and (middle) northward wind velocity, and (right) temperature determined at Bandung and the TOGA/COARE stations. Results are shown for 15.4, 12.4, 9.4, 6.4, 3.4, and 0.4 model at all stations, although small-scale perturbations were superimposed. The amplitudes were similar to the model below about 5 km, but they became several times larger than the prediction. Ponape (7.0øN, 158.2øE, 39 m MSL), and Honiara (9.4øS, 160.0øE, 56 m MSL). Harmonic dials for u were fairly random at 0.4 km, but they became more organized above 3.4 km. Some similarity Figure 5 illustrates a harmonic dial of diurnal oscillations, can be recognized among stations with similar latitudes. In whose length and direction indicate tidal amplitude and maimum phase in local time. Results for u, v, and T are determined at Bandung and the TOGA/COARE stations at si altitudes from 0.4 to 15.4 km with a height spacing of 3 km. particular, at 3.4, 9.4, and 12.4 km altitudes the harmonic dials at Bandung were reasonably consistent with those at Misima and Honiara, although their longitudes are separated by 40o-50 ø. Note that we also refer to the radiosonde observations at For T fluctuations harmonic dials in Figure 5 looked alike four additional TOGA/COARE sites, i.e., at Truk (7.5øN, 151.9øE, 3 m MSL), Misima (10.7øS, 152.8øE, 7 m MSL), for all the stations ecept at 3.4 km altitude. However, those for v did not clearly show any consistency. It can be suggested

6 26,222 TSUDA ET AL.' DIURNAL OSCILLATIONS OVER INDONESIA EASTWARD 130 '"'E "... ' '... WIND NORTHWARD WIND 110, -?,E 100 ', : 90 " I I 8O /,I 7O 60 ' I,,, I I I $ AMPLITUDE(m/s) PHASE (hr) AMPLITUDE(m/s) PHASE (hr) Figure 6. Amplitudes and phases of (left) eastward and (right) northward wind velocity at km, observed with MWR in Indonesia during November 1992 and April Dashed lines with circles or squares show the model by Forbes [1982] for 6øS in December and March, respectively. Dot-dashed lines are the corresponding models by M. Hagan (personal communication, 1994), while the crosshows another model by Ekanayake [ 1994]. that horizontal winds were more affected by smaller-scale nonmigrating (or local) tides comparing with the temperature oscillation; which is reasonable, because they are proportional to the horizontal pressure gradient, while the temperature is related to pressure itself. phase structure of v could be eplained by an interference of more than two modes. 5. Concluding Remarks By means of MWR we observed u and v at km with time and height resolution of 1 hour and 4 km, respectively. 4. Diurnal Tides in the Mesosphere We also sampled u, v, and T at 0-35 km every 6 hours during Figure 6 shows the MWR results for u and v at km between November 1992 and April 1993 in comparison with various models [Forbes, 1982; Hagan et al., 1995; Ekanayaket al., 1997]. Amplitude of u was about 4 m/s at 80 km, and increased to 8 m/s at 95 km, while v was 8 m/s at 76 km, increased to 18 m/s near 88 km, then decreased to 5 m/s at 100 km. That is, v was November 1992 and April 1993, with some data gaps, at Bandung, Indonesia. We analyzed diurnal wind oscillations in the troposphere, lower stratosphere, and mesosphere. We also used a radiosonde database of TOGA/COARE, collected at a number of tropical stations in west Pacific, determining diurnal oscillations of u, v, and T below 15 km. We discussed the behavior of the observed diurnal oscillations in about twice larger than u, consistent with a general tendency comparison with various numerical models. of the model. Although the observed amplitudes were much smaller than the Forbes model, they were described fairly well with the models by Hagan et al. [1995] or Ekanayake et al. [1997]. The phase profile for u agreed well with the models. Although Forbes [1982] and Hagan et al. [1995] predicted a dom- 1. Below about 15 km the amplitudes of u were several times larger than the model predictions, while in the lower stratosphere they could be described by the models. The v had smaller and larger amplitudes than u in the troposphere and the stratosphere, respectively, although its characteristics are similar to u. Phases of u and v agreed well with the models inant vertical wavelength, X z, as 30 km for the fundamental above about 20 km. Both amplitudes and phases of T agreed (1,1) mode, Ekanayaket al. [1997] obtained Xz = 23 km, including nonmigrating tides. The observed X for zonal winds was about 25 km, which was more consistent with the latter model. On the other hand, the phase profile for v was fairly gradual, very well with the model at 2-35 km. 2. The behavior of diurnal tides was similar in the lower troposphere over a large area in the west Pacific. We detected finite amplitudes of v even near the equator, which was predicted to be very small by the model. although it intersected the model near 85 km. The complicated 3. The diurnal oscillations of the temperature in the lower

7 TSUDA ET AL.: DIURNAL OSCILLATIONS OVER INDONESIA 26,223 EASTWARD WIND NORTHWARD WIND o h t 1 oo 9o o 70 6O 5O 4O,3O 2O 10 o O.Ol o.1 1 lo loo ^UmTUDœ(m/,) PHASE (h 0 AMPLITUDE(m/a) PHASE (hr) Figure 7. Mean profiles of amplitudes and phases of diurnal tides observed from November 1992 to April 1993 at 0-35 km and km with radiosondes and MWR, respectively. (left) Eastward and (right) northward components are shown. Lines with squares and circles correspond to a numerical model in March and December for 6øS [Forbes, 1982]. atmosphere were described very well by the Forbes model suggestions and careful reading of the manuscript by P.S. Namboothiri. ecept below 2 km where the effects of a planetary boundary layer are dominant. The phase structure suggested that evanescentidal modes were dominant. The inconsistency be- References tween wind velocity and temperature results suggests that the Chapman, S., and R. S. Lindzen, Atmospheric Tides, D. Reidel, Noreffects of nonmigrating tides could be different between hori- well, Mass., zontal winds and temperature. Ekanayake, E. M.P., Atmospheric tides in a realistic atmosphere: A 4. In the mesosphere, v was about twice larger than u, numerical simulation, Ph.D. Thesis, Kyushu Univ., Japan, consistent with the models. The phase profile for u agreed Ekanayake, E. M.P., T. Aso, and S. Miyahara, Background wind effect on propagation of nonmigrating diurnal tides in the middle atmofairly well with the models, although the dominant vertical sphere, J. Atmos. Solar Terr. Phys., 59, , wavelength was estimated as about 25 km, which is slightly Forbes, J. M., Atmospheric tides, 1, Model description and results for shorter than the Forbes model. The phase profile for v was the solar diurnal component, J. Geophys. Res., 87, , fairly gradual, suggesting an interference of more than two Forbes, J. M., and M. E. Hagan, Diurnal propagating tide in the modes. presence of mean winds and dissipation: A numerical investigation, Planet. Space Sci., 36, , Figure 7 shows a combined profile between the radiosonde Hagan, M. E., J. M. Forbes, and F. Vial, On modeling migrating solar and MWR measurements during a period from November tides, Geophys. Res. Lett., 22, , to April Although considerable deviations may be Haurwitz, B., The diurnal surface-pressure oscillation, Arch. Meteorol. Geophys. Bioklimatol., Ser. A, 14, , found, overall structure of the observed wind velocity profiles Hsu, H. H., and B. J. Hoskins, Tidal fluctuations as seen in ECMWF agreed quite well with the models above about 20 km, showing data, Q. J. R. Meteorol. Soc., 115, , amplitude increase with a vertical wavelength of km. Kato, S., Dynamics of the UpperAtmosphere, D. Reidel, Norwell, Mass., However, disagreements appeared below 20 km, showing significantly larger amplitudes and complicated phase structures, Kato, S., T. Tsuda, and F. Watanabe, Thermal ecitation of nonmigrating tides, J. Atmos. Terr. Phys., 44, , which could be attributed to the effects of local (or nonmigrat- Lieberman, R. S., Non-migrating diurnal tides in the equatorial middle ing) tides. atmosphere, J. Atmos. Sci., 48, , McKenzie, D., The diurnal atmospheric tide with Newtonian cooling and longitudinally dependent drive, Ph.D. thesis, Univ. of Wash., Acknowledgments. The data used in this study were collected by a Seattle, collaborative project between RASC, Kyoto University, BPPT, and Miyahara, S., Y. Yoshida, and Y. Miyoshi, Dynami coupling between LAPAN. We wish to thank S. Miyahar and M. E. Hagan for providing the lower and upper atmosphere by tides and ravitv waves, J. _ the tabulations of their numerical tidal models. We appreciate the Atmos. Terr. Phys., 55, , 1993.

8 26,224 TSUDA ET AL.: DIURNAL OSCILLATIONS OVER INDONESIA Tokioka, T., and I. Yagai, Atmospheric tides appearing in a global atmospheric general circulation model, J. Meteorol. Soc. Jpn., 65, , Tsuda, T., and S. Kato, Diurnal non-migrating tides ecited by a differential heating due to land-sea distribution, J. Meteorol. Soc. Jpn., 67, 43-55, Tsuda, T., Y. Murayama, H. Wiryosumarto, S. W. B. Harijono, and S. Kato, Radiosonde observations of equatorial atmosphere dynamics over Indonesia, 1, Equatorial waves and diurnal tides, J. Geophys. Res., 99, 10,491-10,505, Tsuda, T., et al., A preliminary report on observations of equatorial atmosphere dynamics in Indonesia with radars and radiosondes, J. Meteorol. Soc. Jpn., 73, , Wallace, J. M., and R. F. Tadd, Some further results concerning the vertical structure of atmospheric tidal motions within the lower 30 kilometers, Mon. Weather. Rev., 102, , Williams, C. R., and S. K. Avery, Diurnal nonmigrating tidal oscillations forced by deep convective clouds, J. Geophys. Res., 101, , Yagai, I., Nonmigrating thermal tides detected in data analysis and a general circulation model simulation, J. Geophys. Res., 94, , S. W. B. Harijono and T. Sribimawati, Agency for the Assessment and Application of Technology (BPPT), J1. M. H. Thamrin No. 8, Jakarta 10340, Indonesia. T. Tsuda, T. Nakamura, A. Shimizu, and T. Yoshino, Radio Atmospheric Science Center (RASC), Kyoto University, Uji, Kyoto 611, Japan. ( tsuda@kurasc.kyoto-u.ac.jp; nakamura@kurasc.kyotou.ac.jp; H. Wiryosumarto, Indonesian National Institute of Aeronautics and Space (LAPAN), J1. Pemuda Persil No. 1, Jakarta Timor 13220, Indonesia. (Received March 20, 1996; revised July 15, 1996; accepted July 23, 1996.)