Characteristics of Kelvin waves and gravity waves

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D22, PAGES 26,159-26,171, NOVEMBER 27, 1997 Characteristics of Kelvin waves and gravity waves observed with radiosondes over Indonesia Atsushi Shimizu and Toshitaka. Tsuda Radio Atmospheric Science Center, Kyoto University Abstract. Profiles of wind velocity and temperature at 0-35 km were observed by means of radiosondes in west Java, Indonesia, during November 1992 and April 1993 and used to study the behavior of various atmospheric waves in the equatorial nfmncnhoro An n. cillnf. inn nœ nnn.1 wine]. wif, h a. riprifle] fif a.hf 11f, 27 da.va wa.a... _r....i...,/ found in the troposphere, which was associated with variations in humidity and cloud top height. Kelvin waves showed phase progression beginning at cloud top height (13-16 kin) and were particularly enhanced near the tropopause. The Kelvin waves strongly modulated the tropopause structure including the tropopause height, minimum temperature, and atmospheric stability. A hodograph analysis was applied to determine the propagation characteristics of inertial gravity waves. Height variation of the vertical group velocity suggests that the gravity waves were generated in the troposphere, while the horizontal phase velocity distribution suggests that they were interacting with the background mean zonal winds. These wave activities were enhanced when tall, convective clouds passed over the site, suggesting that cumulus convection seems to play a key role in generating these waves in the equatorial region. 1. Introduction spouse to the heating due to cumulus convection. However, most theories have not considered feedbacks from The theoretical study of equatorial waves was ini- the excited disturbances to the processes causing the tiated by Matsuno [1966], who demonstrated the ex- heating. Holton [1973] examined the atmospheric reistence of barotropic waves on an equatorial / plane. sponse to adiabatic heating with a white frequency In the same period, Yanai and Maruyama [1966] made spectrum in the troposphere and found that discrete the first observation of westward moving waves in the frequency components of Kelvin waves existed accordstratosphere over the Pacific, while Wallace and Kousky ing to the height structure of the heat source. $alby and [1968] independently discovered eastward propagating Garcia [1987] examined the response of the atmosphere waves. These equatorial waves corresponded to Mat- to stochastic heating and obtained two different types suno's mixed Rossby-gravity wave and to the equatorial of wave generation. The first kind of wave corresponds Kelvin wave, respectively. They are of planetary scale to a normal mode that cannot propagate vertically but in a horizontal extent, and their wave periods range spreads horizontally. The second type of wave can propfrom several days to a few weeks. agate vertically, and is 'concentrated horizontally near Lateral forcing from midlatitudes could excite mixed the excitation field. Salby and Garcia [1987] concluded Rossby-gravity waves. A theoretical study of merid- that the dominant wavelength of the latter wave is deional propagation of large-scale waves in midlatitudes termined by the effective depth of heating and is insenrevealed that the disturbances with phase velocity less sitive to the height distribution of heat sources when the than the mean zonal velocity on the path can prop- depth of the excitation layer is the same. Although it agate and transport momentum toward the equator is rather difficult to observe an exact profile of the heat [Bennett and Young, Zangvil and Yanai [!980] source in the equatorial atmosphere, it was estimated studied equatorial wind disturbances by using a space- using mass continuity from the coordinated observation time spectrum analysis and found that a mixed Rossby- of horizontal wind velocity fields over several stations. gravity wave with a global zonal wave number s = 4 and Using results of radiosonde observation at the Marshall a 5-day period showed up as a distinct spectral peak in Islands, Nitta [1972] showed that the heating rate had the equatoward energy flux. Another excitation mechanism is a forced heating of the atmosphere. Equatorial waves are explained as a re- Copyright 1997 by the American Geophysical Union. Paper number 96JD / 97 / 96 J D ,159 a maximum around 500 hpa with a period of 5 days. Wave conditional instability of second kind (CISK) was also introduced as a generation mechanism for equatorial waves [Hayashi, 1970]. In this theory heat release from individual cumulus convection is parameterized by a large-scale wind field. The response with the maximum growth rate was dependent on the particular parameterization.

2 26,160 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES In the tropical troposphere, Madden and Julian [1971] detected a long-period (30-60 days) oscillation of zonal winds, which is often refered to as the "Madden-Julian oscillation" (MJO). The MJO has a standing phase structure in height, is confined within the troposphere, and is accompanied by a large-scale cloud cluster, advecting eastward over the Pacific. Analyzing cloud data with Geostationary Meteorological Satellite (GMS), Nakazawa [1988] showed that a super cloud cluster propagated eastward, however, it included westward propagating, smaller-scale cloud clusters. In the equatorial stratosphere the quasi-biennial oscillation (QBO), with a period of months, is the dominant motion [Naujokat, 1986]. Vertical transport of energy and momentum by equatorial waves is thought to be a primary mechanism maintaining the QBO. Holton and Lindzen [1972] further theorized that the wind reversal is caused by thermal damping of vertically propagating Kelvin waves and mixed Rossbygravity waves. Takahashi and Kumakura [1995] recently studied excitation of the QBO in a three-dimensional model, and found that westward propagating gravity waves, coupled with precipitation, are important in producing the easterly phase of QBO, while the westerly phase of QBO seemed to be maintained by both Kelvin waves and eastward propagating gravity waves. $ato and Dunkerton [this issue] estimated the upward momentum flux from equatorial Kelvin and gravity waves using radiosonde data from balloons launched at Singapore and concluded that the momentum flux of shortperiod (1-3 days) gravity waves was 3 times larger than that of Kelvin waves in the westerly shear of QBO. Although sutficient observational studies of inertial gravity waves have not been conducted so far in the equatorial region, some recent studies reported interesting characteristics of inertial gravity waves employing a hodograph analysis [ Tsuda et al., 1994b]; [Karoly et al., 1996]. The results reported in this paper are from the second of two campaigns in collaboration with the Indonesian National Institute of Aeronautics and Space (LAPAN) for intensive observations of equatorial atmosphere dynamics by means of a balloon-borne radiosonde. The first campaign was conducted in February-March 1990 [Tsuda et al., 1994a; b] and depicted the behavior of Kelvin waves, diurnal tides, and inertial gravity waves. However, the observation was continued for only 25 days, just one cycle of a typical Kelvin wave, and was too short for investigating interseasonal variations. We report here results of a second campaign, executed during November 16, 1992, to April 10, 1993, at the LAPAN observatory in Bandung, west Java, Indonesia (107.6øE, 6.9øS, 740 m MSL). The campaign period was mostly overlapping with the Coupling and Dynamics of the Regions Equatorial (CADRE) intensive observation period as well as the Tropical Ocean- Global Atmosphere Coupled Ocean-Atmosphere Re- sponse Experiment (TOGA COARE), scheduled during November 1, 1992, to February 28, TOGA COARE was intended to reveal the atmospheric process in the western Pacific warm pool region [Webster and Lukas, 1992], while the present study chiefly focused on the dynamics over the Indonesian maritime continent. 2. Radiosonde Campaigns in Indonesia We employed a V&isiilii RS-80 rawinsonde, with OMEGA navigation system, and TA/TX-1000 balloons, provided by TOTEX, where the TX-!000 was especially developed for a launch in a tropical region. We sampled data every 2 s during a balloon ascent, corresponding to a height spacing of about 10 m, and averaged in a layer with a thickness of 150 m. The 150-m height resolution for wind data is su cient to resolve the dominant atmospheric waves [Tsuda et al., 1992]. We launched balloons four times every day at 0500, 1100, 1700 and 2300 LT (Indonesian standard time, which precedes universal time by 7 hours). To avoid balloon bursts, caused by the cold temperature near the tropopause at night, the morning launching at 0500 LT was sometimes delayed. However, in this study we assume that data have an even time intervals of 6 hours during the whole observation period. A total of 410 balloons were launched in 5 months, with two major interruptions during November 20-29, 1992, and during December 16, 1992 to January 11, 1993, due to malfunctioning of the receiver. As a re- sult, 359 balloons (88%) observed the whole range of troposphere and 292 (71%) reached 30 km altitude. We also utilized the infrared radiation temperature data, collected by GMS 4 to investigate horizontal structure of tropical clouds. The GMS 4 instrument measures temperature eight times every day, with an accuracy of 1 K, at longitudes 80ø-200øE and latitudes 60øS to 60øN. The data had a resolution of about 0.2 ø x 0.5 ø in longitude and latitude. Note that we plotted GMS 4 data only in 1993, since the radiosonde data in 1992 was not long enough. We also used global objective analysis (GANAL) data, distributed by Japan Meteorological Agency, whose accuracy was especially improved during the TOGA COARE campaign, giving a horizontal resolution of ø at 15 height levels (from 1000 to 10 hpa) and time resolution of 12 hours. By comparing radiosonde data at Bandung with the GANAL data at øE,?.5øS, which is the closest grid point to Bandung, it is revealed that the GANAL data below 100 hpa had enough accuracy for the study of equatorial waves. However, in the lower stratosphere (at 70 hpa) the GANAL crepancy. data and radiosonde data showed some dis- 3. Fundamental Characteristics of Wind Velocity Profiles Here we first describe background conditions of the atmosphere during the observation period, present fre- quency spectra of wind velocity fluctuations, and investigate the long-period oscillation in the troposphere.

3 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES 26,161 NOV DEC JAN FEB )far APR T- F- i... -,o o 4o n 4o ', uon n.i Wind (m/,i 4O' (m/l) ' or weakly eastward, but it turned westward above this and attained a velocity in the range of-10 to -15 m/s at about 15 km. The zonal winds rapidly fluctuated between 17 and 23 km and became weakly eastward around 25 km. The zonal winds in the stratosphere were dominated by the QBO, with its phase descending in altitude during the observation period. The westward maximum was located around 20 km in November 1992 and moved down to around 17 km in April 1993, then another westward wind appeared above 30 km. Amplitudes of the monthly meridional winds in Figure 1 ranged from -5 to 5 m/s in the troposphere and were quite small in the stratosphere Ion Iefidlon.! Wind (n/i) (n/i) Figure 1. Monthly mean profiles of (top) zonal and (bottom) meridional wind velocity. Each profile is displaced by 20 m/s per month, and the right panel shows the average during the whole observational period. The horizontal dotted line indicates the mean tropopause height Monthly Mean Winds We present in Figure I monthly mean wind velocity profiles for eastward and northward components derived from the radiosonde observations. The uncertainty is greater for November 1992 due to a small number of launches. Because Kelvin waves with the period of 20 days had large amplitudes in the stratosphere, they may contaminate the monthly mean profiles, but fundamental structure of the background winds are well described. Below 5 km the zonal wind was nearly zero 3.2. Frequency Spectrum We analyzed the frequency spectrum of wind velocity fluctuations, as shown in Figure 2, for the observation period from January 12 to April 10, 1993, which did not include any major data gaps. However, there were a few short data gaps, therefore we binned the data into 12- hour averages by averaging pairs of data points where both were available. Then we applied the Blackman- Tukey method to the time series of zonal and merid- ional wind components at each altitude. The resulting spectra were averaged over 20 heights, corresponding to 3-km intervals in height. The spectral density of the low-frequency components was generally larger for the zonal wind than the meridional one. The zonal spectral density peaked at a period of about 40 days in the troposphere, except in the height range km. Note that the oscillations of meridional wind with a period of 5-10 days had fairly large amplitudes in the lower troposphere. Eastward Wind Northward Wind P e rio o (day) 100 I Periol (day) i... i... i A 1014? * 1012 a 0, , 1014? ** o a. 100, O. 100 Freqeney (hr-') r i,ney (hr- ) Figure 2. Frequency spectra of (left) zonal and (right) meridional wind velocity at eight layers with a thickness of 3 km, from km to km. Thin lines show w -5/3, and vertical dotted lines indicate the inertial period (100 hours) at the observatory. The spectra are displaced by a factor of 10.

4 26,162 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES The diurnal component had a distinct spectral peak in the lower troposphere and in the lower stratosphere, although it was not clear in the upper troposphere. The spectral density was similar between zonal and meridional wind for frequencies higher than the inertial frequency, corresponding to about 100 hours in wave period. A logarithmic slope of the spectra for high frequencies can be fitted by w -5/3, consistent with a prediction by a linear gravity wave model [VanZandt, 1985]. Doppler shifting effects due to background mean winds could be anticipated, particularly near the inertial frequency [Fritts and VanZandt, 1987]. However, the effects seemed to be small, since the intensity of background mean winds was fairly small during the campaign except for the zonal wind near the tropo- pause Long-Period Oscillations Time-height sections of zonal u and meridional v winds and temperature T are shown in Plates 1, 2, and 3, respectively. We first subtracted average values at each altitude during the whole observation period. Although slow downward procession of the QBO is still visible in the data, the QBO-related vertical shears are reduced. Then we applied a low-pass filter with a cutoff at 4 days so that the effects of gravity waves were also removed. It can easily be recognized that the characteristics of the perturbations were very different between the troposphere (below 17 km) and the stratosphere. In the troposphere, long-period oscillations, with an evanescent vertical structure, were dominant. In the stratosphere, vertically propagating waves became dominant in the u and T fields, with amplitudes maximizing near the tropopause. Time-height variations of relative humidity are shown in the bottom panel of Plate 4, which indicates that humidity was generally high during December 1992 and January 1993, but it became smaller after the end of January. This variation could be interpreted as a seasonal transition of the Asian monsoon. In a later section, we discuss a correlation between the humidity values which seem to represent activity of cloud convection and the behavior of waves in order to investigate wave generation mechanisms. In the troposphere, the low frequency oscillation of the zonal wind had quite large amplitudes, with a maximum at 4-5 km, while the corresponding oscillation was absent in the meridional component. The perturbations were greatly enhanced near the tropopause, particularly during January and February, when the humidity (shown in Plate 4) was also large. The analysis concentrated on this period because of the greater potential for vertical coupling of the waves. We fitted an individual sinusoidal function to the zonal wind perturbations at each altitude, changing the wave period between 4 and 40 days, which we refer to as a periodogram analysis. We detected the dominant wave period, having the maximum amplitudes, as 27 days at 0-15 km altitude, except at km. Figure 3 shows amplitudes and phases of the oscillation with a period of 27 days. The amplitudes were enhanced in two regions at 2-8 km and km, showing a large peak at 5 km and a secondary maximum at km. The zonal wind direction in the troposphere was reversed near 11-km altitude where the amplitude was quite small. Since the meridional wind was very small, these characteristics implies a passage of the zonal circulating cell along the equator. Actually, a time-longitude section of the lower tropospheric zonal wind derived from GANAL data (Figure 4) shows the eastward migration of the westward wind region preceding the eastward region. This indicates that convergence in the lower troposphere passed above the observatory at the end of January. The same characteristics are found in the upper troposphere with opposite wind direction. GMS 4 data (Figure 5) also show an eastward motion of the large-scale cloud system over Bandung in the end of January 1993, coinciding with the large humidity values obtained with radiosondes. At the same time, the zonal wind direction suddenly reversed in the troposphere. These characteristics were consistent with the day oscillation, or the Madden-Julian oscillation [Madden and Julian, 1972]. Eastward propagation of a cloud cluster seen in Figure 5 continued to near the date line, and the oscillation of zonal wind is also confirmed from result of TOGA COARE rawinsonde observation [Gutzler et al., 1994]. Gutzler et al. [1994] showed the time-height section of zonal wind observed at four stations located around 150øE. Their result is very consistent with our analysis. The oscillation we observed was probably a typical Madden-Julian oscillation. 4. Kelvin Waves We analyze here oscillations with periods of days, detected particularly in u and T. We first describe the vertically propagating Kelvin waves in the stratosphere, then discuss their behavior in the troposphere, investigating their excitation mechanisms Kelvin Waves in the Stratosphere In Plate 1 the phase progression of the waves was evident near and above the tropopause, having a dominant vertical wavelength of several kilometers and wave periods of days. It is noteworthy that these fluctuations in the stratosphere were smoothly connected to the standing perturbations in the troposphere. Similar fluctuations were seen in T in Plate 3. The phase of T variations preceded that of the zonal wind variations by about a quarter of the wave cycle, consistent with the polarization of Kelvin waves. We also investigated time-longitude distribution of zonal winds, using the GANAL data for January and February 1993, at latitude and height of 7.5øS and 100 hpa, respectively. We detected that a wave-like perturbation, with a zonal wavenumber I and 2, propagating from the Indian Ocean to the date line in the

5 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES 26,163 Zonal Wind(fil.ter: oo day) at Bandung(6.90S, 1,07.60E)-', 40,,. I,,, I,, t I, I... I (m/s) 1o 5 o ,,,...,...,...-,.. ;,,...,, Dec 03 Dec 23 Jan 12 Feb 01 Feb 21 Mar 13 Apr /1993 Plate 1. Time-height section of zonal wind velocity at Bandung. A low-pass filter with a cutoff at 4 days was applied after subtracting average values at each altitude during whole observation period. -1o 40_ Meridional Wind(filter: o day) at Bandung(6.90S, E) I I I ] I I I ]! ' ' I., I ' ' I,,!, I, (- / ) 10,t.,!: ' ' ' I ' ' ' I ' ' '" I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' Dec 03 Dec 23 Jan 12 Feb 01 Feb 21 Mar 13 Apr 02 ß 1992/1993-1o Plate 2. Same as Plate 1 except for the northward wind components.

6 26,164 SHIMIZU AND TSUDA' CHARACTERISTICS OF KELVIN AND GRAVITY WAVES 40 "t Temperature(filter: day) ak Bandung(6.90S, E) I I I ] I ' I a I., I, i i I,,!! I I,, I I (K) I I ' i, ' ' ' ' 10 ' ' ',,, i,,,,, i,,, i,,,,i,...,,, i,',, i,, Dec 03 Dec 23 Jan 12 Feb 01 Feb 21 Mar 13 Apr /1993 Plate 3. Same as Plate I except for the temperature. (Brunt-Vaisala Fre9.) 2 and Relative Humidity (x "" "":""'":,',"**,!: ;,"t,' ' 'i.,. :.' 'l, ', ', 1o Dec 03 Dec 23 Jan 12 Feb 01 Feb 21 Mar 13 Apr /1993 Plate 4. Time-height sections of (top) Brunt-Viiisiilii frequency squared (N2), and (bottom) relative humidity. Note that the vertical axis in both panels is continuous. The bold line in the bottom panel indicates the cloud top height averaged in a circular area with the radius of 500 km centered at Bandung, inferred from GMS 4 IR temperature data, while a square symbol indicates the highest cloud in this region.

7 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES 26,165 AMPUTUDF' PHA F' period (day) I I I WIND VELOCITY (m/.) TEMPERATURE PHASE (degree) Figure 3. (left) Amplitude and (right) relative phase of the Madden-Julian oscillation with a period of 27 days, determined by a periodogram analysis between January 12 and February 20, Solid and dashed lines correspond to zonal and temperature components, respectively. western Pacific. $hiotani et al. [this issue] analyzed the Another type of perturbation was observed in the global structure of temperature variations in the strato- stratosphere with a period of 4-5 days which was recsphere, obtained with the Cryogenic Limb Array Etalon ognized in both u and v components. For example, on Spectrometer (CLAES) onboard the Upper Atmosphere March 2-7, 1993, an upward moving pattern, like a wave Research Satellite (UARS), and detected a dominant packet, was detected in the meridional winds near 21 km Kelvin wave in the lower stratosphere in January altitude, as seen in Plate 2. In Figure 6, phase relations These results suggesthat the perturbations in Plate 1 among u, v, and T of this wave for 4 levels are shown. were not a local phenomenon but a manifestation of a The phase of u is in quadrature with that of v and is opglobally propagating Kelvin wave. posite from T. This pattern proceede downward with We employed a periodogram analysis to determine time consistent with upward energy propagation [e.g., the dominant period of Kelvin waves in the strato- Holton, 1992]. sphere, and found that it was days at kin, then decreased to days above 20-kin altitude. It was not possible, however, to examine the global structure of this wave, because it appeared at an alti- The vertical wavelengths were estimated as 5 and 12 tude above where the GANAL data are reliable. It is km at and km altitude, respectively, indi- noteworthy that in the lower troposphere the meridional cating that the vertical phase velocity became faster in wind component showed a significant spectral peak with the upper region. a period slightly longer than 4 days. However, this corn- 5O 40 Zonal wind at T _-I... _.!... I [ ::: ===============================.,.-.:.:. :.:.:.:..:.:. :.:.:.:.:.:.:.:.!!:..'-.. -:.:.:.:.::.:.:..:i :.:. : ================================= ::,, ':::::::::::::::f-::... *. ---:i: :i: :-- ¾'::z!:i:ii! ":.: ::::iiii:::.i ii::i i. o 30!:i':.-:.:.::i:i:i: :i:i:'"':: :!::: : 2O : 1o r... ß i- -- ß - -I - i i 0 30 eo Lonfltude Figure 4. Time-longitude section of zonal wind velocity at 7.5 øs, 500-hPa surface. A vertical line indicates the longitude of the radiosonde observatory.

8 26,166 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES GMS4 Cloudtop Temperature LATITUDE -- 7S (X) Apr Oa liar 13 Feb 21 Feb 01 Jan 12 oo loo teo t40 too Lonfltude(g) Figure 5. Time-longitude section of cloud top height estimated by GMS 4 IR temperature data at 7 øs. A vertical line indicates the longitude of the radiosonde observatory. ponent had smaller value as the altitude became higher in the stratosphere Modification of the Tropopause It has been reported that tropospheric air is uplifted into the middle atmosphere in the equatorial region. Therefore the tropical tropopause could influence the upward flux of minor constituents. For example, Tsuda et al. [1994a] suggested a possibility that the modification of the tropopause structure by Kelvin waves controls upward transport of water vapor, because the maximum water vapor mixing ratio in the uplifted tropospheric air is closely related to the minimum temperature near the tropopause. Thus the amount of water vapor can be modulated by the temperature variations near the tropical troposphere associated with Kelvin waves. The temperature variation amplitudes were greatly enhanced near the tropopause, and were generally larger in the stratosphere than in the troposphere, as shown in Plate 3. Figure 7 shows time variations of the minimum temperature of each T profile, and the corresponding altitude. During this campaign we again detected large variations of the minimum temperature near the tropopause as well as variations in the tropopause height, which were attributed to the Kelvin wave activity. The range of the minimum temperature perturbations, shown in Figure 7, was 5-10 K. Note that 24. Mixed Rossby-gravity wave (4-8day) :,-" UmttXON.

9 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES 26, M!n!m. u_m. Tern. p_er&tur. e... Jan 12 Feb 01 Feb 21 Mar 13 Apr 02 He.ight of Minimum Temperature re.6 t re.o-:! o o o o o o o 03 3 Jan 1 Fmb 01 Fmb 1 Mar 13 Apr 0 Figure 7. Time variations of (top) the minimum temperature and (bottom) the corresponding heights of the tropopause, determined every 6 hours. the altitude of the minimum temperature showed a downward phase progression between 18 km and 15.5 km with a period of days. We here investigate time-height variations of atmospheric stability. The top panel of Plate 4 shows Brunt- V/kis l frequency squared, N 2, calculated from radiosonde profiles of temperature and pressure. Between 13 km and 17 km, just below the mean tropopause height, N 2 showed large time variations. Plate 3 indicates that the Kelvin waves started to propagate vertically just above the region where the N values increased. We further analyzed profiles of amplitudes and phases of the Kelvin waves with the wave period of 20 days in two different observation periods, that is, during January 12-26, 1993, and January 27 to February 10, as shown in Figures 8a and 8b, respectively. These results were derived when the stability at km was rather low and high, respectively. From the fitting of sinusoidal curve with various period to time series of zonal wind and temperature, we find that the maximum amplitude of Kelvin waves near the tropopause was for a period around 20 days. The phase and amplitude were estimated for each time segment using the periodgram method and assuming a period of 20 days. The two profiles in Figure 8 had similar characteristics9n the troposphere, that is, the zonal phase was reversed in the middle of troposphere, consistent with the horizontal convergence-divergence structure of the day oscillation. In the stratosphere the phase showed a variation consistent with the theoretical model of Kelvin waves developed by Salby and Garcia [1987]. Salby and Garcia [1987] calculated the atmospheric response to heating in the cloud layer and showed vertical structure of amplitude and phase similar to our results. The vertical wavelength in the lower stratosphere in the two periods was almost the same, suggesting that the effective depth of the heat source in the troposphere did not greatly change. The altitudes where wave started to propagate were obviously different during the two time periods. This initial propagation altitude was around 16 km and 13 km in the first and second periods, respectively. In Plate 4 we also plotted the cloud top height inferred from satellite observations with GMS 4. It is clear that the mean cloud top height agreed well with the humidity variations, while the maximum cloud top height correlated well with the N structure. The inference is that the tropopause was lifted upward by a few kilometers in the presence of cloud clusters. Then atmospheric waves started to propagate vertically from the higher altitude. On the other hand, when cumulus convection was weak, the atmospheric region just below the tropopause became relatively stable, and atmospheric waves showed phase propagation even below the tropopause. 5. Inertial Gravity Waves We now focus on small scale perturbations which appeared in the radiosonde profiles, and investigate their characteristics. Wave-like disturbances with periods and vertical wavelengths less than a few days and 2-3 km, respectively, were recognized in Figure 9. Downward phase propagation of the waves was seen in all the u, v, and T components in the stratosphere. However, in the troposphere both upward and downward propagation coexisted, showing a complicated structure Hodograph Analysis A hodograph analysis is employed to estimate the propagation characteristics of gravity waves. We first applied a high-pass filter to the time series with a cut-

10 26,168 SHIMIZU AND TSUDA- CHARACTERISTICS OF KELVIN AND GRAVITY WAVES (ct) AMPLITUDE (ct) PHASE period = 20.00(day) Ze WIND VELOCITY TEMPERATURE (k) oo PHASE (degree) 360 (b) AMPLITUDE (b) PHASE period == 20.00(day) I, I I. 0 S 10 1S 20 0 O ;360 WIND VELOCITY (m/s) PHASE (degree) TEMPERATURE (k) Figure 8. (left) Amplitude and (right) relative phase of a Kelvin wave with a period of 20 days, determined by a periodogram analysis between (a) January 12-26, 1993, and (b) January 27 to February 10, Notations are the same as in Figure 3. off at 4 days, corresponding to the inertial period at the observatory. A band-pass filter was applied to the vertical profiles which passed vertical scales of 1-4 km and km the troposphere and the stratosphere, respectively. The pass bands were determined considering the dominant vertical wavelengths of gravity waves at each height range. A hodograph was drawn by combining the resultant profiles of zonal and meridional winds. Then an ellipse was least square fitted to the hodograph. Horizontal wave propagation direction is aligned to the major axis, although the exact direction must be determined, by considering a simultaneous temperature profile. The vertical propagating direction was determined by a rotation direction of the hodograph. An intrinsic wave period can be derived from a ratio between the major and minor axes, while the horizontal wavelength was estimated from a linear dispersion relation, by using the background N 2 value. We examined all the ellipses, and utilized only ellipses in which u, v, and T satisfied the theoretical polarization. We separated the observation height into three regions, that is, in the lower troposphere (1-5 km), the upper troposphere (8-13 km) and the lower stratosphere (20-25 km). We exclude km considering ill effects of the strong vertical shear of the zonal wind around the tropopause. We present in the following the height variations of the wave parameters Propagation Characteristics In the lower troposphere, about half of gravity waves showed downward phase progression, corresponding to the upward energy propagation. Two thirds of waves carried energy upward in the upper troposphere, and more than 95% of the waves tranaported energy up- ward in the lower stratosphere. Therefore it can be suggested that gravity waves were excited in the lower

11 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES 26,169 4O Zonal Wind(filter: day, krn) at Bandung(6.90S, 107:60E) (m/s),o 1o Jan 12 Jan 17 Jan 2 Jan 27 Feb 01 Feb ' Meridional Wind(filter: day, _ krn) at Bandung(6.90S, E) (m/s) 1 Jan 12 Jan 17 Jan 22 Jan 27 Feb 01 Feb 06 lg 3 Figure 9. Time-height section of short-period disturbances of (top) zonal and (bottom) meridional wind velocity. troposphere, and energy propagated upward into the middle atmosphere. Horizontal and vertical wavelengths of the gravity waves are shown in Figure 10. In the troposphere, gravity waves had horizontal wavelengths of less than about 1000 kin, but in the stratosphere they were distributed between 1000 and 4000 kin. The horizontal phase velocity vector was also estimated for individual gravity waves. In both the lower and upper troposphere the vectors were distributed uniformly in azimuth, as is shown in Figures 11a and 11b. In the lower stratosphere they were mainly concentrated into north-eastward direction, with very few directed westward, as shown in Figure 11c. Mean winds were westward near the tropopause during the observation period, with a magnitude larger than the phase velocity of gravity waves. The inhomogeneous distribution of the phase velocity vectors in the stratosphere in Figure 11 are the result of critical level interactions of the westward propagating waves. Since the mean wind was chiefly determined by the QBO, our results may support a recent theoretical prediction that inertial gravity waves are an essential driving force of the QBO [ Takahashi and Kumakura, 1995]. 6. Conclusion We conducted radiosonde campaign in Indonesia during November 1992 and April 1993 and observed the characteristics of various atmospheric waves in the equatorial atmosphere. The main results of this study are summarized as follows: 1. We found that in the equatorial atmosphere the Madden-Julian oscillation, Kelvin waves, and inertial gravity waves were dominant during the observation period, while the QBO was a key factor for the mean winds. 2. The 27-day oscillation (MJO) had large amplitudes only for the zonal winds in the troposphere, with a phase reversal around 11 kin. However, its amplitude

12 26,170 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES 4 enfth 7. Horizontal wavelengths of gravity waves were less than 1000 km in the troposphere and attained values between 1000 and 4000 km in the lower stratosphere. 8o The horizontal propagation direction of gravity waves was uniformly distributed in the troposphere. On [] [] 15 ^ - o o u x oo I... I... I Horizontal wevelenfth (km) Figure 10. Distributions of vertical and horizontal wavelengths of inertial gravity waves in the lower troposphere (1-5 km, crosses), upper troposphere (8-13 km, triangles), and lower stratosphere (20-25 km, squares) ,...,... {...,..., Eastward (m/s) became very small in the stratosphere. Time variations of humidity and cloud top heights correlated well with the direction of the horizontal wind. These characteris- tics are explained as the passage of the large-scale zonal circular cell over the observatory. 3. Kelvin waves with a period of days showed a vertical structure similar to that of the 27-day MJO in the troposphere, but they started to propagate upward above the tropopause. It was confirmed that Kelvin waves in the upper troposphere had a global structure by using GANAL and UARS satellite data. 4. Near the tropopause the Brunt-V//is//lg frequency squared, N 2, varied in association with the activity of cloud convection as well as Kelvin waves. The verti- cal propagation of Kelvin waves was affected by the N 2 variations. Although the behavior of convection varied, the vertical wavelengths of Kelvin waves, in a layer just above the excitation region, were almost independent of this behavior. This suggests that the effective depth of heating did not vary greatly. In the lower stratosphere, Kelvin waves showed relatively long vertical wavelengths and short wave periods. 5. Both the minimum temperature near the tropopause and the corresponding height fluctuated significantly, the former showed a sinusoidal variation with a period of about 20 days, and the latter indicated a periodic clear downward progression. It is most likely that these perturbations were attributed to Kelvin waves with enhanced activity near the tropopause. 6. Short-period (1-4 days) disturbances were detected in u, v, and T, whose characteristics are explained by inertial gravity waves. Using a hodograph analysis, it is suggested that the gravity waves were generated in the troposphere and propagated upward into the stratosphere (b) 8-13 km... i... i... i... i ,...,... i...,..., Eastward (m/s) Eastward (m/s) Figure 11. Distribution of horizontal phase velocity vector at (a) 1-5 km, (b) 8-13 km, and (c) km.

13 SHIMIZU AND TSUDA: CHARACTERISTICS OF KELVIN AND GRAVITY WAVES 26,171 the other hand, almost all waves propagated eastward in the stratosphere. This conspicuous difference in the horizontal propagation direction suggests an interaction between background wind and gravity waves. Considering background wind velocity, gravity waves which had westward momentum seemed to decay in the westward wind near the tropopause. Acknowledgments. We appreciate N. Adikusumah and H. Wiryosumarto of LAPAN for data collection in Indonesia. We are also deeply indebted to D. M. Riggin, I. Hirota, S. Yoden, M. Shiotani, T. Horinouchi, M. Nishi, R. A. Vincent, and D.C. Fritts for helpful comments and discussion. We thank greatly the Meteorological Satellite Center for providing GMS 4 satellite data and Japan Meteorological Agency for GANAL data. We acknowledge careful reading of the manuscript by S. P. Namboothiri. References Bennett, J. R., and J. A. 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