Radiosonde observations of equatorial atmosphere dynamics

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. DS, PAGES 1,491-1,55, MAY 2, 1994 Radiosonde observations of equatorial atmosphere dynamics over Indonesia 1. Equatorial waves and diurnal tides Toshitaka Tsuda, Yasuhiro Murayama,, 2 Harsono Wiryosumarto, 3 Sri Woro B. Harijono, 4 and Susumu Kato,5 Abstract. This paper describes the characteristics of the mean winds, equatorial waves with periods ranging from 4 to 2 days, and diurnal tides determined by analyzing the profiles of wind velocity, temperature, and humidity obtained every 5-7 hours in the height range up to about 35 km with a height resolution of 15 rn during an observation campaign conducted February 27 to March 22, 199, in East Java, Indonesia. The structures of the mean winds in the troposphere and lower stratosphere seemed to be affected by the Australian monsoon and the quasi-biennial oscillation, respectively. Frequency spectra indicated that the equatorial waves as well as the diurnal tides were dominant below about 25 km, while gravity waves with periods shorter than 4 days became more significant above 25 km. A 7-day oscillation showing an antiphase relation between the eastward and northward components and exhibiting large amplitudes was observed in the lower troposphere. The time-height variations of the activity of this 7-day oscillation were clearly correlated with a region of high relative humidity. Perturbations in the zonal wind and temperature with wave periods varying from 15 to 17 days were also enhanced in the troposphere, while Kelvin waves with periods of about 7 and 2 days were detected in the lower stratosphere, and activity near the tropopause was conspicuously enhanced. We found that the 2-day Kelvin wave greatly modified the structure of the tropopause, such as the minimum temperature, the tropopause height, and the values of the Brunt-Vfiisfilfi frequency squared N 2, which further suggests the effects of Kelvin waves on the transportation of tropospheric water vapor into the stratosphere and on the downward mixing of stratospheric minor constituents into the troposphere. The observed profiles of the diurnal oscillation were compared with those of a numerical model assuming only migrating tides, which reasonably agreed above about 25 km. Below 25 km, however, the observed amplitudes were m/s, e zceeding those of the model about 1 times. Moreover, the phase profiles involved fluctuations with small vertical scales, suggesting interference by many nonmigrating tides with short vertical wavelengths. 1. Introduction The equatorial atmosphere is characterized by the active generation of atmospheric disturbances due to the large solar heat input, which further generates various atmospheric waves. Because an inertial period rapidly increases near the equator, an intrinsic frequency range of waves that could propagate upward without vertical trapping becomes increasingly wider as well. It is well known that in the equatorial region there are peculiar waves with longitudinal I 1Radio Atmospheric Science Center, Kyoto University, Uji, Kyoto, Japan. 2Now at Communications Research Laboratory, Ministry of Posts and Telecommunications, Tokyo. 3Indonesian National Institute of Aeronautics and Space (LA- PAN), Jakarta Timur, Indonesia. 4Agency for the Assessment and Application of Technology (BPPT), Jakarta, Indonesia. 5Now at Japan Indonesia Science and Technology Forum, Tokyo. Copyright 1994 by the American Geophysical Union. Paper number 94JD /94/94JD ,491 scales of over several thousands kilometers and wave periods ranging from a few to a few tens of days which have been clarified by means of radiosonde and rocketsonde measurements as well as satellite remote sensing. In particular, the Kelvin wave and the Yanai-Maruyama wave (or the mixed Rossby-gravity wave) have been extensively studied for many years since their initial discovery by Wallace and Kousky [19681, and Yanai and Maruyama [1966], respectively, as reviewed, for instance, by Andrews et al. [1987]. Theoretical studies highlighted the important roles of these waves in generating a quasi-biennial oscillation (QBO) [e.g., Holton and Lindzen, 1972; Plumb, 1982], which further stimulated experimental studies and numerical modeling of the equatorial waves [e.g., Coy and Hitchman, 1984; Salby and Garcia, 1987; Garcia and Salby, 1987; Itoh and Ghil, 1988]. It is suggested that cumulus convection, which is most active in the equatorial region, is closely related to the generation of these waves. Furthermore, a 3- to 6-day oscillation propagating eastward was recently found to be maintained by a convectively active super cloud cluster [Hayashi and Sumi, 1986]; furthermore, the inner structure of a super cluster was associ-

2 1,492 TSUDA ET AL.: EQUATORIAL ATMOSPHERIC DYNAMICS, 1 ated with westward propagating waves [e.g., Takayabu and Murakami, 1991]. The diurnal variation of the cumulonimbus activity, accompanied by latent heat release, could be an additional excitation source of atmospheric tides, which are expected to produce localized nonmigrating tides in the troposphere and lower stratosphere. Gravity waves with much smaller horizontal scales as well as shorter wave periods seem also to be generated. The large energy thus transferred to various atmospheric waves in the equatorial region is transported vertically as well as horizontally for long distances and is finally dissipated far away from the excitation area. Yet the detailed structure and underlying physical mechanisms of the coop- erative dynamical-heating processes occurring exclusively in the equatorial region have not been entirely clarified because of a lack of comprehensive observations. For the global transport of atmospheric minor constitu- tion of 15 m. Since there are not many previous works of balloon soundings with a good altitude resolution and a wide observation height range exceeding 3 km, except for some campaigns [e.g., Cadet and Teitelbaum, 1979], it can be expected that the present study will provide new information on the equatorial atmosphere dynamics over Indonesia. The preliminary results, including details of the experimental setup as well as of the background conditions during the campaign, have already been reported by Tsuda et al. [ 1992]. This is the first of two papers concerned with analysis of the equatorial atmosphere dynamics on the basis of radiosonde profiles. We describe in this paper the characteristics of equatorial waves with wave periods longer than 4 days and diurnal atmospheric tides, while the companion paper presents the behavior of gravity waves with wave periods shorter than the inertial period (about 4 days) [Tsuda et al., this issue]. ents, the behavior of the equatorial atmosphere is supposed to be very important, since the tropospheric air is uplifted into the stratosphere only through the tropopause in the 2. General Characteristics of the Wind Fields equatorial region. In particular, equatorial Indonesia is now 2.1. Outline of the Experiment known to be a "fountain" region for the minor constituents [Newell and Gould-Stewart, 1981]; that is, tall cumulus clouds exclusively generated near Indonesia sometimes penetrate above the tropopause, transporting the tropospheric air into the stratosphere [Danielsen, 1982; Kley et al., 1982]. Moreover, it has been reported that the column amount of ozone in the equatorial stratosphere, where the production of ozone is most effective in the Earth's atmosphere, varies according to the tropopause height [Hasebe, 1993; Shiotani, 1992]. Since the tropopause structure could be affected by various dynamical effects, it can be suggested that equatorial atmosphere dynamics is also important to the distribution of minor constituents. We conducted an observation campaign by means of a portable radiosonde system in East Java with collaboration between Japan and Indonesia. We launched a total of 1 radiosondes, one every 5-7 hours, which measured wind velocity, temperature, pressure, and humidity in the height range from the ground to about 35 km with a height resolu- We launched four radiosondes per day at intervals of 5-7 hours from February 27 to March 22, 199, from the Watukosek stratospheric balloon observatory of LAPAN in East Java, Indonesia (7.57øS, øE; 5 m above mean sea level (MSL)). Figure 1 shows the observed wind velocity profiles for the eastward and northward wind velocities. During the campaign period, the tropopause, with a minimum temperature ranging from 186 to 194 K, was located at km altitude. Note that on each day two or three balloons launched during the daytime penetrated into the stratosphere, with maximum heights exceeding 3 km, while others at night reached only up to about 18 km [Tsuda et al., 1992]. Figure 1 also exhibits various wave activities with different time and vertical scales. In particular, at km, a wave propagating downward with a vertical scale of 5-6 km can be clearly recognized only for the eastward component. In addition to the dominant component, many wave activities can be recognized in the wind velocity fluctuations, EASTWARD b NORTHWARD 55 E O ' 2 "1 '"'1'"'""'1'""'"'1"'"'"'1""'""1"'"'"'1 1 O 2, O 2,5 4 WIND VELOCITY (m/s) WIND VELOCITY (m/s) Figure l. Profiles of the (a) eastward and (b) northward wind velocities observed every 5-7 hours with radiosondes launched from Watukosek in East Java from February 27 to March 22, 199. Successive profiles are displaced by 5 m/s. 1; 5OO

3 -- TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, 1 1,493 4O EASTWARD I I I. I o I I I I I I WIND VELOCITY (m/s) I I z z z NORTHWARD I I I ß. I I I. I i I i i i WIND VELOCITY Figure 2. Mean profiles of the (left) eastward and (right) northward wind velocities for the Watukosek (solid line) and Singapore (dashed line) results collected in the period February 27 to March 22, 199. although the amplitudes of the perturbations were generally smaller in the troposphere. Above about 22 km, short-period waves were sometimes evident for both the eastward and northward wind components, being characterized by downward phase progression with vertical scales of 2-3 km. According to the wave periods and vertical scales, these fluctuations can be decomposed into equatorial waves, atmospheric tides, and gravity waves, the first two components being described in the following sections Vertical Structures of Mean Winds We here briefly review the structure of the mean wind velocity during the campaign, which has already been described by Tsuda et al. [1992]. Since the entire period of the campaign (25 days) was only slightly longer than that of a fundamental Kelvin wave with a period of about 2 days, it is rather difficult to accurately delineate the background mean winds by using only the campaign results. Therefore, we also employed data collected on daily radiosonde observations at the Singapore meteorological station (Singapore: 1.33øN, 13.9øE; 32 m above MSL). We present in Figure 2 the wind velocity profiles averaged for all the results collected at Watukosek in comparison with the mean profiles obtained at Singapore in the corresponding observation periods. Below 9 km altitude the mean zonal wind was eastward with amplitudes of -5 m/s at Watukosek, while it was weakly westward at Singapore. The difference in the zonal winds between the two stations seems to be due to longitudinal variation of the wind fields affected by the Australian monsoon [e.g., Johnson, 1992]. In the 1- to 25-km region, the zonal winds at Watukosek were westward with a double-hump vertical structure, peaks of 19 and 27 m/s being recognized at 15 and 21 km, respectively, which was quite similar to the Singapore results. Analysis of the Singapore data showed that the QBO could be clearly recognized in the zonal winds in the stratosphere, and furthermore, the observation campaign coincided with the transition from the end of the westward phase to the beginning of the eastward phase [Tsuda et al., 1992]. Therefore the background conditions of the zonal wind fields at 1-25 km seemed to be mainly determined by large-scale wind fields. However, above 25 km the structures of the mean zonal winds were greatly different; i.e., they became almost m/s at 25 km and linearly increased westward at Watukosek, while the zonal wind direction was the reverse, i.e., eastward, at Singapore. The discrepancy above 25 km could be attributed to (1) the latitudinal dependence of the QBO evolution, which produced the delay at higher latitudes [Dunkerton and Delisi, 1985]; (2) the semiannual oscillation; or (3) waves with periods much longer than the observation period at Watukosek. The exact reason for the discrepancy, however, is not within the scope of this paper. The vertical structures of the meridional winds shown in Figure 2 agreed fairly well between the two locations, with the wind directions changing about every 8 km. For the meridional component, the annual oscillation with amplitudes of about 1 m/s was normally dominant in the equatorial troposphere, as inferred by using the Singapore observations [Tsuda et al., 1992]. However, the meridional winds at Watukosek shown in Figure 2 were as small as 1-2 m/s in the entire height range, probably because the campaign was carried out at the equinox, which corresponds to a transition period of the annual oscillation Comparison of Time Series Between Watukosek and Singapore In Figure 3 we compare the time series of the wind velocity between Watukosek and Singapore at six standard pressure levels (7, 5, 2, 1, 25, and 1 hpa), with the mean value at each altitude being subtracted. Oscillations with periods ranging from 5 to 2 days can be recognized for both the zonal and meridional components, showing a fairly good correlation between the two locations except in the case of the eastward component at the 1-hPa level (3.9 km). However, waves with periods shorter than a few days were not clearly recognized at Singapore. It can be suggested that such short-period oscillations were due to waves with short vertical scales, which were probably smoothed out by the coarse height resolution of the soundings at Singapore. In the troposphere, perturbations with periods of 5-7 days were detected for both the eastward and the northward components. In addition, an oscillation with a period longer than about 15 days was detected only for the zonal component. The eastward winds also involved waves with periods of about 2 and 7-8 days which were conspicuously enhanced at 16.4 and 24.6 km, respectively, while no significant fluctuations were found for the meridional component, suggesting the manifestation of Kelvin waves in the stratosphere. It is noteworthy that at 3.9 km the amplitudes of the waves with periods longer than several days became fairly small for both the zonal and meridional components Frequency Spectra From the time series of the wind velocity, frequency spectra were determined at each altitude with 15-m spacing and then were averaged in nine height ranges, each with a thickness of about 3 km, as shown in Figure 4. Below 12 km, the spectral shapes were fairly similar between the zonal and meridional components, showing a logarithmic slope of -5/3 to -2 in the entire frequency range, where the spectral amplitudes were slightly larger for the meridional compo-

4 1,494 TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, 1 EASTWARD NORTI-1W D o o 15 1o o -2s TIME (day) TIME (day) Figure 3. Time variations of (left) eastward and (fight) northward wind velocities observed at 6 standard pressure levels (7, 5, 2, 1, 25, and 1 hpa) approximately corresponding to 3., 5.9, 11., 16.4, 24.6, and 3.9 km, respectively. Note that the mean value at each altitude was subtracted. The solid lines present the campaign results at Watukosek, while the circles and dashed lines indicate results of the daily routine radiosonde soundings at Singapore. nent. Above 15.1 km the spectral amplitudes were signifi- first two-thirds of the entire observation period but had cantly larger at low frequencies for the zonal component, dissipated by the end of the campaign. probably owing to the effects of Kelvin waves. Because the observation period was not long enough, it In the highest two regions, components with wave periods was difficult to clearly define a dominant period of equatorial shorter than the inertial period (91.1 hours, or 3.8 days) waves from frequency spectra in Figure 4. Therefore, we exhibited large amounts of energy, implying the predomi- employed a periodogram analysis, in which a sinusoidal nance of gravity waves there. The slopes of the zonal spectra function was least squares fitted to time series of wind in the stratosphere were sometimes as steep as -3, while the velocity and temperature variations at each altitude, the meridional spectra had slopes ranging from -5/3 to -2. wave period for the harmonic fitting being changed from 1 to It is noteworthy that the amplitude of the highest- 25 days. frequency component (corresponding to a wave period of 24 Figure 6 shows the amplitudes of wave components for the hours) of the spectra was enhanced at altitudes below 24 km, zonal and meridional wind velocities determined by periodwhich does not seem to be caused by noise but can be ogram analysis at three height ranges, i.e., in the lower attributed to diurnal tides. In a later section, we discuss the troposphere ( km), near the tropopause ( vertical structure of the 24-hour component in terms of km), and in the stratosphere ( km). A dominant diurnal nonmigrating tides. wave component can be detected when the amplitude in Figure 6 becomes the maximum in a specified wave period 2.5. Analysis of Dominant Wave Periods With a Periodogram We removed the gravity wave component by applying a low-pass filter with a cutoff at 96 hours (4 days) for the time series at each height. The resultant profiles of wind velocity are shown in Figure 5, which demonstrates fluctuations caused by waves with periods ranging from 4 to 2 days. For the eastward component there appeared a downward propagating wave with a vertical wavelength of 5-6 km at km altitude. In contrast, the northward component did not include clear wavelike structures in the corresponding altitude region. The downward phase progression of the wind perturbations at km altitude was fairly smooth, although the observed profiles in Figure 5 suggest interference by more than two waves. In the troposphere, both the eastward and northward components showed an oscillation with a period of several days, where the vertical phase velocity was fairly fast. It is noteworthy that this perturbation had large amplitudes in the range. At km altitude an oscillation with a period of 7 days with an amplitude of about 5 m/s was recognized for both the zonal and meridional components. Moreover, only the zonal component showed a broad enhancement, centered at about days, whose amplitude was about 8 m/s for the zonal component, while it was as small as 1 m/s for the northward component. Near the tropopause at km, the amplitudes of the zonal component became large in three wave period ranges, i.e., 3-5, 7, and days, with the maximum amplitude at a period of 2 days. On the other hand, the amplitude of the meridional component was less than about 2 m/s in the entire wave period range of the analysis. It is noteworthy that at km altitude, the amplitude of the 2-day wave was reduced to about 5 m/s, and there were no other dominant components with periods longer than 5 days. In contrast, the amplitudes of short-period components centered at 3 days, being shorter than the

5 -- TSUDA ET AL.: EQUATORIAL ATMOSPHERIC DYNAMICS, I 1,495 inertial period, were increased in the stratosphere. These results suggest that a major part of the fluctuating components in the stratosphere was due to gravity waves. To summarize the results so far, in different regions of the atmosphere peculiar perturbations were dominant during the campaign observations, that is, an oscillation with periods of 5-7 days in the troposphere below about 1 km, waves with periods of 2 and 7 days at 15-2 km, and gravity waves superimposed on each other in the stratosphere. WATUKOSEK, 27 FEB - 22 MAR 199 EASTWARD 35-4j,,I,,,,,,,,,I,,,,,,,,,I,,I,,,,... 'J,,,,k,k,\\\\\..N\\\\... '25 3. An Oscillation With a Period of 7 Days 3.1. Analysis With a Periodogram and a Hodograph We now describe the detailed analysis of oscillations with a period of about 7 days which Figure 6 revealed in two regions, the troposphere and lower stratosphere. We examined height variations of the 7-day wave by a periodogram analysis, the period being fixed at 7 days, and here present the amplitudes and phases in Figure 7, the latter referring to the relative phase of the maximum wind velocity in the eastward or northward direction measured from the begin WIND VELOCITY (m/s) NORTHWARD 4 11Illlllllllllllllll,,',llllllllllllllllllllllll 1( krn iiii krn krn 2-i - ' lo- - lc lc... I... N -2 T krn - 1( 17 ld u 1( 1 1( ld km krn FREQUENCY Figure 4. Frequency spectra of wind velocity fluctuations in the nine height ranges, with a thickness of about 3 km. Solid and dashed lines represent eastward and northward components, respectively. The thin straight lines are a reference, having a logarithmic slope of-5/3. o "1"'"'"'1'"'"'"1'"'"" I"" ""1'"" o 1 oo 2 3oo 4 5 WIND VELOCITY (m/s) Figure 5. The same as Figure 1 except that a low-pass filter with a cutoff period of 4 days was applied to the time series at each altitude. ning of the time series. Large amplitudes can be seen in two different regions, -15 and km. Below 7 km altitude the amplitudes of the zonal and meridional components were similar, with maximum values of about 5 m/s at 5 km, and then the meridional component became larger than the zonal component at 8-15 km. The phase profiles indicated fairly slow downward progression, showing a 6 ø lag at 1 km, and were quite similar between the two components. Phase difference of nearly 18 ø existed between the zonal and meridional components below about 11 km altitude. These characteristics are quite different from the behavior of a mixed Rossby-gravity wave, which is normally the most predominant wave component with a period of several days; therefore, other mechanisms should be considered to explain the observed phenomena. On the other hand, in the stratosphere large amplitudes were detected only for the zonal components, being enhanced in fairly narrow regions near the tropopause, with a peak value of 7-8 m/s at 17 km and a secondary peak at km. Therefore, it is more likely that the 7-day oscillation in the stratosphere can be explained by a Kelvin wave, which

6 1,496 TSUDA ET AL' EQUATORIAL ATMOSPHERIC DYNAMICS, 1 HEIGHT' km I... I,,,,I,,,,I,,,,I lo... --TTT, ' HEIGHT: :krn r 15 i i i, i,, i,, i i i i i i i i G-, lo õ HEIGHT: km 15,,,,I... I... I,,,I,,,,I 1 tudes in the first two-thirds of the observation period. It is noteworthy that the hodographs were sometimes aligned in the southeast to northwest direction, such as on February 27 and March 3 and 7, 199, when the large amplitudes of wind velocity were detected. The hodographs obtained on other days during the first half of the observation period were round with a clockwise rotation. In the last one-third of the observation period, the hodographs in Figure 8a showed a rather complicated structure with smaller amplitudes and sometimes indicated counterclockwise rotation. The hodographs in the lower stratosphere (see Figure 8b) were rather elongated along the east-west axis, being consistent with the behavior of Kelvin waves, although interference by other waves can be suggested. We have seen two independent oscillations with a period of 7 days in two different regions, -15 and km, the latter being interpreted as a Kelvin wave, while the former was not identified as a specific wave defined in earlier studies. I e,mo Figure 6. Amplitudes of the wave components in the three height ranges determined by periodogram analysis of wave periods ranging from 1 to 25 days. Circles and triangles denote eastward and northward components, respectively. seemed to be independent from the other wave simultaneously detected in the troposphere. We applied a band-pass filter with cutoffs at 5 and 9 days in order to further extract only the 7-day wave from the profiles in Figure 5. Here we present in Figure 8 hodographs which illustrate the height variations of the horizontal wind vector for about every 2 days in height ranges -15 and 15-2 km. At -15 km altitude the hodographs exhibited great variability during the campaign period, showing larger ampli Correlation Between the 7-Day Oscillation and Cloud Amount We are interested in the time-height variations of the activity of the 7-day oscillation in the troposphere, which is discussed here in comparison with the cloud amount. Figure 9 shows horizontal distribution of the cloud top temperature, determined every 3 days from the GMS 4 satellite observations of infrared radiation. Near the Watu- kosek radiosonde station, the cloud amount was large in the first five panels, slightly decreased on March 15, and then fairly small on March 18 and 21. The contour plots of the northward and eastward wind velocities and the temperature in Figure 1 clearly indicate that the amplitudes of the 7-day oscillation in the troposphere were larger in the first two-thirds of the observation period and then decreased at the end of the observation period, showing a good correlation with the time variation of cloud amount in Figure 9. Moreover, the height extent of the effects of the oscillation also decreased. It is noteworthy that the temperature fluctuations did not correlate well with the variations in the wind velocity. 4 I I I 35 3 E 25 2 ' WIND VELOCITY (m/s) PHASE (degree) Figure 7. Height profiles of the amplitude and phase of a wave with a period of 7 days determined by means of periodogram analysis. Solid and dashed lines represent eastward and northward components, respectively.

7 TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, 1 1,497 a FEB/27 12 LT MAR/ 1 12 LT MAR/ 3 12 LT MAR/ 5 15 LT '' ''' 4. o - o.!l. o b FEB/27 12 LT MAR/ 1 12 LT MAR/ 3 12 LT MAR/ 5 15 LT ;h, o,.1 o -,, o MAR/ 7 1 LT MAR/ 9 1 LT MAR/11 1 LT MAR/13 1 LT MAR/ 7 1 LT MAR/ 9 1 LT MAR/11 1 LT MAR/13 1 LT ' o o. o -5 o o ß MAR/15 1 LT z 4 MAR/17 1 LT MAR/19 1 LT MAR/21 1 LT MAR/15 1 LT MAR/17 1 LT MAR/19 1 LT MAR/21 1 LT z EASTWARD WIND -5 z EASTWARD WIND Figure 8. Hodographs for wind velocity fluctuations in the (a) - to 15-km and (b) 15- to 2-km regions after a band-pass filter with a pass band of 5-9 days had been applied. Note that the results are for 12 cases at intervals of about 2 days. Triangles and circles indicate the top and bottom of the height range in each panel. Figure 11 shows contour plots of the relative humidity obtained with radiosondes, where a low-pass filter with a cutoff at 1.5 days was applied for the time series at each altitude in order to remove the dominant diurnal variations [Tsuda et al., 1992]. Large humidity values at 4-8 km altitudes were mainly detected in the first two-thirds of the campaign period, from February 27 to March 13, 199, with time intervals ranging from 3 to 4 days, which was generally consistent with the satellite observations in Figure 9. Figure 11 also shows contours of the square of the amplitudes of the northward wind velocity v '2 after a low-pass filter with a cutoff at 4 days had been applied. We chose the meridional component to represent the activity of the 7-day oscillation because it is not normally contaminated by Kelvin waves. Large values of v' appeared every 3-4 days, corresponding to half of the oscillation cycle. It is surprising that large humidity values were always associated with large v' values in Figure 11, suggesting a close relation between the enhancement of the 7-day oscillation and the time-height structure of the region of moist air. By comparing the humidity profiles with satellite measurements of the height of the cloud top, they were found to coincide with tall cumulonimbus clouds. The appearance of the region of moist air was related to wave activity by Nitta [197]. Takayabu and Murakami [1991] further proposed a mechanism to explain the relation between convective activity and easterly waves in the western Pacific region. If these explanations are applied to the present case, a region of high humidity should be detected once per wave cycle (that is, every 7 days), but it appeared every 3-4 days in Figure 11, corresponding to half of the wave period. Therefore we might need another physical picture to explain the observed correlation. 4. Kelvin Waves With Periods of 15-2 Days 4.1. Height Profiles We describe in this section the characteristics of fluctua- tions with periods longer than 12 days in the troposphere and lower stratosphere as detected in Figure 6. Periodogram analysis was applied to the zonal wind speed at each altitude with a vertical spacing of 15 m for a wave period between 12 and 24 days, where the resolution of the wave period determination is 1 day. Then we plotted in Figure 12 the wave period and the corresponding amplitude for the component with the maximum zonal amplitude at each altitude. The amplitudes of the zonal winds were enhanced in two separate height ranges, -13 and km, with maximum values of 8 and 15 m/s at 3 and 16 km, respectively. On the other hand, the amplitudes of the meridional component were generally much smaller in the entire height range. In the -13 km region, the dominant period increased almost linearly from about 15 to 17 days; then a discontinuity in the profile of the oscillation period was detected at km altitude, and then it became nearly constant at about 2 days at km. Above 18 km, fluctuations of the periods were very large, indicating that the periodogram analysis was unable to reveal a dominant component within the analyzed range of wave periods. We analyzed a hodograph after applying a low-pass filter with a cutoff at 1 days. The results, determined every 4 days, are shown in Figure 13 for the two separate height regions. The figure generally shows a linearly polarized structure with an axis aligned in the east-west direction, which is consistent with the behavior of Kelvin waves. However, smaller-scale perturbations seemed to be superimposed. Although the hodograph was a very flat ellipse, it was rotated counterclockwise, as expected in the southern hemisphere. In the troposphere the amplitudes of the hodographs were larger at the beginning of the campaign and then decreased at the end of the observations, while in the stratosphere the amplitudes were the smallest at the beginning of the observation period. It is noteworthy that the vertical structure of the amplitudes in the troposphere shown in Figure 12 resembled that for the 7-day oscillations, as shown in Figure 7. Moreover, the time variation of the amplitudes in the troposphere, being

8 ... 1,498 TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, I MAR ::'...- i: i? ibi i i i:! :.. i!i :::'?: :.: i i : ',,, ß...,,,,, :::::::::::::::::::::::: ' '%iii!i.e&:::':'.i:; :iiie.::."-'::i ii! ::i:: ::.' -...?::::: :. :.;::.... "::. i s.i:..."..3..: :...'-.-:.i -. :!:i. -i :-:. "i:.-:..%:. 7'.' ':':' i-. -i'i*: -:!:: MAR 3, ""',' %,";"'.,'..., '..: :' a ' ": %'""'". i'; '... :i "?'-,,, ';. M 1 :::?...:,,:i... -::a E½5 i 5 : :- :, El : ', ::555:...,' :. 4:. -* * ---. a.. :::..-::::: ::u.: ::::::.:. -::n :...:::::. ' a,,--..,, ', i!!e} ;i i2:: ::fifi k::::'fa IE '"!lli!(: ::.:.'.'::':':...i.'fs : :':'"" :' 5:':'L :. :i.. :: ':! :::.':' --2 *:! %i.-'.i.. :.,.:::- :"::i '" MAR 15,199, :!j.r z, - "i:>,,:::::::::::::::::::::::::...;.'l..a '.¾ - 'i!!!i ai a i, -... a % "::i:.:.i..a ::% I i ;... ;:'",, C-. ' :' " :i :.'--.-.:.:!...,. ':5 z:... " 5:*-':".'"'a::U' :5 ir "5i 5% 2/" 5'5 1!5T55:- '... *.'- ':' :::... :. :::$:..z :l :.. ' ::-... ' :E E :" - a.:,.. 5.?: ß ".: %.-.Eii! : y, ' MAR 6, MAR 18,199..d. - r '% -x.? -.- '. :: e",.i ;:;iik :i.i:iii:i: 41 i,,% :;., t%. :. ½ ' --::-Li: rea :. : ::..:---?:..: :*il!!ii>ih$ {llt:{)iifuiii:i :) ' ':... HL i '-"':':½ :, 1: %.. '... :"'; : : ":".::... ; : : 1 :2:iE:::i :<':<'k:::::e j 3 ( [' f ;;;;j;.i..... a.:..'. '.4 '. ": : ::; :':$t:,. 'ii i:i i!:::::!:::}i!::: i i ' "' J /.4 2:: '.... i : i i{ : la '!8:::- '">. ' ::... a,, : E;?l :5 SES-":: ::' ', ::. '::-...+ '%:.::.: - a iif:.::.::.::i t ' ': ::i :z::-i-::'.'-... :':... =========================================================== - 1 '?. : i F. i " *:'.. :':':':-Ei;:':.:.:.:.: ;E!i 5: ::, ' :::: :... lo... } E4 b::.-.-.::::-r:, ' " 5 :.-.-." :a 5 :::- :?... '..' i i... ' ''-:... MAR 21,199 2., ii;;., a...,,.:.:.-:: :.,:...,,...,.v,,.!,,,.,..:i { <...,,: {? q, : '.'... '.....a,.,z%. '.' ;'... 7:? : : :: ::. : ;; '" j... : SL,?% &i ': : 'E' ' a'?f:{ 9 ' LONGITUDE (deg) LONGITUDE (deg) T (K) 3OO 28O 26O 24O 22O 2OO Figure 9. Horizontal distribution of cloud top temperatures derived from the GMS 4 satellite observations of outgoing infrared radiation at GMT each day. The Watukosek observatory (7.6øS, 112.7øE) is indicated by a dot. large at the beginning and small at the end of the campaign, was quite similar to that for the 7-day oscillation in the same height range. A similar coincidence in the vertical structure can be inferred between the 2-day and 7-day waves in the region near the tropopause, showing large enhancements in the narrow height ranges, as shown in Figures 7 and 12, respectively. We present in Figure 14 a time series of the eastward wind ve19city at 16.4 km after a low-pass filter with a cutoff at 1 days had been applied for comparison with the results obtained at Singapore. Note that a filter was not applied for the Singapore data, but daily raw values are plotted in Figure 14, since quite a few data values were lacking for the campaign period at Singapore. A cross-correlation analysis between the two time series showed a peak with the maximum cross-correlation function (CCF) value of.9 at a lag of approximately -1 day, which implies a slight phase delay for the Watukosek results. For an eastward propagating Kelvin wave with period and zonal wavenumber of 2 days and 1, respectively, the phase delay should be about -.5 day, since the longitudinal difference between Watukosek and Singapore is 8.8 ø. By considering

9 TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, I 1,499 EASTWARD NORTHWARD i i i i i i i i i I I I I I i I i I i I I I I 2o iiii... """':':':':':':::::::'::.'...'"::::!!!ii!!iiiiiiiii :: =======================.:i i i!i i ii::'"" ::' "'/--' :::... ½:'"':' ':'"::..--..::...-i TEMPERATURE 2o DAY Figure 1. Contour plots of the (top) eastward and (middle) northward wind velocity components and (bottom) temperature after application of a band-pass filter with cutoffs at 5 and 9 days to the time series at each altitude. the short longitudinal separation of these stations, the observed phase delay seems to be reasonably consistent with the theoretical prediction. Note that the time variations in the wave amplitudes were quite large, as revealed by the Singapore results. That is, only one wave cycle was clearly recognized in the 9-day period including the campaign at Watukosek, suggesting that the detected Kelvin wave was not long-lasting but more like a short burst of disturbances, which was also found with satellite observations [e.g., Coy and Hitchman, 1984]. The downward phase progression of the fluctuations was clearly detected in the wind velocity, as in Figures la and 5, indicating a vertical wavelength of about 5 km. A recent numerical simulation (S. Yoden, private communication, 1993) revealed that a Kelvin wave can be confined in a relatively thin height range near a tropopause, which describes fairly well the observed characteristics of the wind velocity perturbations. The results given above suggesthat the waves in the stratosphere with a period of about 2 days should be interpreted as Kelvin waves. However, the perturbations with similar oscillation periods in the troposphere may not be directly related to the

10 1,5 TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, 1 NORTHWARD m2s RELATIVE HUMIDITY I I I I I I I!!! I I I I! I I I! I I I I oo.o - - o ;j: 9. o I ? DATE Figure 11. Contour plots of the square of the amplitudes of (top) the northward wind velocity (a low-pass filter with a cutoff at 4 days was applied to the time series at each altitude) and (bottom) the relative humidity (a low-pass filter with a cutoff of 1.5 days was applied to the time series at each altitude). waves in the stratosphere, because their vertical structures as well as dominant periods were significantly different in the two height ranges. However, they could be related to, for instance, variations in convective activity Effect of Kelvin Waves on Tropopause Structure The activity of Kelvin waves can be expected to cause corresponding temperature fluctuations. Figure 15 shows the amplitudes of temperature fluctuations determined on periodogram analysis with the period fixed at 2 days. The vertical structures of the amplitudes were quite similar for temperature and eastward winds, having enhanced peaks near the tropopause with maximum amplitudes of 5 K at 17 km. The phase difference between the temperature and the eastward winds was roughly 9 ø at km, being consistent with the behavior of a Kelvin wave. The other Kelvin wave with a period of 7 days also seems to produce temperature fluctuations near the tropopause, although the results are not presented here. We discuss in the following the effects of the Kelvin waves on the time-height structure of the tropopause. All the temperature profiles collected during the campaign are simultaneously drawn in Figure 16, where the stratosphere is clearly separated from the troposphere in each profile. In the troposphere, temperature fluctuations were suppressed, while they became large just above the tropopause. The effects of the Kelvin waves on the tropopause structure can be clearly recognized in Figure 16, such that the tropopause showed downward progression from 17 km at the beginning of the campaign to 15.5 km near the center of the observation period and then suddenly jumped upward by about 1.5 km. Figure 17 shows the time variation of the minimum temperature determined in each profile in Figure 16, which clearly exhibits sinusoidal variation, producing large temperature fluctuations in the range from 186 to 194 K. The period of oscillation was about 2 days, coinciding with that for the Kelvin wave described in the previous subsection. The height of the minimum temperature, also shown in Figure 17, clearly indicates a discontinuity in the time variation of the tropopause height. It has been reported that the freeze-dry effect near the cold equatorial tropopause removes water vapor contained in the tropospheric air; therefore, only dry air can penetrate into the stratosphere. This is called the dehydration effect [Danielsen, 1982; Holton, 1982]. Thus the tropopause in the equatorial region acts as a barrier for the upward transportation of water vapor. A necessary condition for the manifestation of the freeze-dry effect was experimentally determined to be that at the altitude corresponding to 1 hpa in atmospheric pressure, the minimum temperature should be less than 191 K, or equivalently, the minimum volume mixing ratio of water vapor should be less than 3 parts per million by volume. The results shown in Figure 17 indicate that this condition seemed to be marginally satisfied in the equatorial tropopause over Indonesia; moreover, it was

11 TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, 1 1,51 4 D IlllJllllJllllJllil 3 i HEIGHT= 16.4 km EASTW D I I ' I I ' I I I ' I I,,, I I,, t::: 25, I ,o-=- o-- - o //,,/ /\\el\\ I t\ I / I\ \ r/' I i\ / \ I i ' \.-. ""\ ' ' J II 1 / / ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' 4 8 IGHT= 1.4 km NOR D loo I. - 5 lo o lo PERIOD (day) AMPLITUDE (m/s) o ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' TIME (day) ' Figure 12. Periodogram analysis of a wave with periods ranging from 12 to 24 days. (left) Period of the wave with the largest amplitude observed on periodogram analysis, and (right) amplitudes of the corresponding waves for the eastward (solid line) and northward (dashed line) components. Figure 14. Comparison of the time series of the (top) eastward and (bottom) northward wind velocity between Watukosek (solid line) and Singapore (dashed line) results. Note that a low-pass filter with a cutoff of 1 days was applied to the Watukosek results. largely affected by the activity of a Kelvin wave. Therefore, it can further be suggested that upward flux of water vapor transported from the troposphere into the equatorial stratosphere can also be modulated by the activity of Kelvin waves. Considering the proposed mechanism of QBO generation through interaction of mean winds with Kelvin and mixed Rossby gravity waves, the major components of wave activities around the tropopause seem to be variable, depending on the QBO phase. So, although the effect of a Kelvin wave was dominant in the present case, in other observation periods the tropopause structure could include variations caused by mixed Rossby-gravity waves or other kinds of wave activities. Figure 18 shows a contour of Brunt-Vfiisfilfi frequency squared N 2, which more clearly exhibits the modification of the tropopause structure due to the Kelvin wave activity. That is, the downward motion of the tropopause and its discontinuity, appearing on March 12-17, can be more easily recognized. It can be suggested from Figure 18 that the bottom part of the stratosphere intruded into the troposphere, although it was not so deep as in the case of the tropopause folding at middle latitudes. $hiotani [1992] and Hasebe [1993] showed that annual and long-term variations of total ozone in the tropics were related to variation of the tropopause height, assuming the difference in diffusion processes between troposphere and stratosphere. If a similar mechanism can be expected for a motion of the tropopause due to Kelvin waves, the air parcels near the deformed tropopause can be bitten out and mixed with the tropospheric air. Thus the downward transport of minor constituents could occur. Therefore minor constituents in the stratosphere, represented by ozone, could be periodically transported downward into the troposphere with an interval determined by the wave activity. a 2O FEB/27 12 LT MAR/ 3 12 LT MAR/ 7 1 LT b 2O FEB/27 12 LT MAR/ 3 12 LT MAR/ 7 1 LT o '-', o z 2 MAR/11 1 LT MAR/15 1 LT MAR/19 1 LT 2O 2O MAR/11 1 LT MAR/15 1 LT MAR/19 1 LT < o -2-2 o -2 z Figure 13. days. EASTWARD WIND EASTWARD WIND The same as Figure 8 but for low-pass-filtered wind velocity fluctuations with a cutoff at 1

12 1,52 TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, 1 4O I I I E E < ' 'l'''l'''l''' WIND VELOCITY (m/s) PHASE (degree) 36O Figure 15. The same as Figure 7 but for eastward wind velocity (solid line) and temperature (dashed line) fluctuations with a wave period of 2 days. Note that the horizontal axis in the left panel also corresponds to the amplitudes of temperature fluctuations. 5. Diurnal Atmospheric Tides We found that a diurnal cycle of the cumulus convection which could cause excitation of atmospheric tides was dominant in the equatorial region [Tsuda et al., 1992]. However, such heat sources are thought to be localized in their horizontal extent, which is largely affected by the land-sea distribution. So, they may not generate a global tide but may instead excite nonmigrating tides, which consist of various longitudinal wavenumbers and high-order Hough modes with small vertical scales [Tsuda and Kate, 1989]. The tidal theory also suggests that the vertically propagat- ing modes of diurnal tides have large amplitudes in the latitude range equatorward of 3ø; therefore, nonmigrating diurnal tides are expected to be significant only in the equatorial region. As a matter of fact, a numerical model predicted that nonmigrating diurnal tides could have even larger amplitudes than the migrating component in the equatorial region [Tsuda and Kate, 1989]. v WATUKOSEK, 27 FEB - 22 MAR 199 ß ß 4o "1'"'1'"'1'"'1""1'"'1'"'1'"'1""1'"'1'",1'1111 %%ß 35 3 T 2 The amplitudes and phases of a 24-hour oscillation in the wind velocity fluctuations were determined by periodogram analysis, as shown in Figure 19, where the phases correspond to the local times of the maximum amplitude of eastward and northward wind velocities. It is noteworthy that the analyzed results could be affected by the gravity waves with a period of 24 hours. However, gravity waves, which are normally not synchronous with local time, seemed to be smoothed out during the analysis, since the periodogram was calculated using data extending for 25 days. Figure 19 also shows the results of a numerical model assuming only migrating tides generated by global heat sources as a result of absorption of solar radiation by water vapor in the lower troposphere and ozone in the middle i. - a: 19-- I-- - I ,,.. 18 i- E I- - o - '"16 - ß % e e ße I I I I [ I I I I I I I I I [ I I I [ ] I I I I TIME (DAY) ß ß ß ß ß ß eeo TEMPERATURE (K) Figure 16. The same as Figure 1 but for temperature profiles. Successive profiles are displaced by 5 K nmr (D^¾) Figure 17. Time variations of (top) the minimum temperature values detected in each profile in Figure 16 and (bottom) the corresponding altitudes.

13 TSUDA ET AL.' EQUATORIAL ATMOSPHERIC DYNAMICS, 1 1,53 4 WATUKOSEK 27 FEB - 22 MAR 199 I i ::::::::::::::::::::::::::::::' - "' '"" x i [ :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: " ::,::[: :: ":' -. H i : " '" /: : i : o: :: :::::::::: u::+: ::': :: DAY Figure 18. Contour plots of the Brunt-Vfiisfilfi frequency squared. atmosphere, which are assumed to be distributed without longitudinal variations [Forbes, 1982]. Below about 25 km the disagreement between the observations and the model was very large, such that the observed amplitudes were generally in the range of m/s, which was more than 1 times larger than the model prediction assuming only migrating.tides. Both the eastward and northward components showed fairly large variations in the amplitude profiles, suggesting that many wave components were superimposed on each other. Moreover, the dominant vertical scale was as short as 1-2 km below 2 km altitude, suggesting the appearance of higher-order modes, which are most likely caused by nonmigrating tides. Above about 25 km, both the amplitude and the phase profiles generally approached the model values, although the phase profiles still included fluctuations with small vertical scales. It might be that the nonmigrating tides rapidly dissipated in the course of upward propagation, because they were characterized by high-order modes with small vertical scales, as predicted by a numerical model [Tsuda and Kato, 1989]. Therefore in the equatorial region the nonmigrating tides seemed to be dominant in the tropo- 4O 35 3O E 25 H- 2 " 15 1 i WIND VELOCITY PHASE (degree) Figure 19. Amplitudes and phases of the diurnal oscillations for the eastward (solid line) and northward (dashed line) wind velocity components. The model results, assuming only migrating tides, are indicated by circles and triangles for the eastward and northward components, respectively.

14 1,54 TSUDA ET AL.: EQUATORIAL ATMOSPHERIC DYNAMICS, 1 Table 1. Characteristics of Equatorial Waves Amplitude Period, Height, Component day km Zonal Meridional 7-Day oscillation m/s at 5 km 5 m/s; dominant below 5 km Kelvin wavelike m/s at 3 km -<2 m/s; wave period was oscillation variable Kelvin wave m/s at 16 km -<4 m/s Kelvin wave m/s at 17 km -<2 m/s sphere, but they were unable to propagate deeply into the stratosphere above about 25 km. Note that the nonmigrating tides are composed of both eastward and westward propagating waves, while the migrating tides propagate toward the west in synchrony with the movement of the sun. The role of the nonmigrating tides in transporting momentum and energy from near the ground to the lower stratosphere seems to be an important future research subject. 6. Concluding Remarks Using profiles of wind velocity, temperature, and humidity collected in the period February 27 to March 22, 199, through frequent launching of radiosondes at intervals of 5-7 hours, we have analyzed the behavior of the mean winds, equatorial waves with periods from 4 to 2 days, and diurnal atmospheric tides. The main conclusions can be summarized as follows. 1. The mean winds in the troposphere seemed to be affected by the Australian monsoon, while in the stratosphere they were mainly determined by QBO, although some discrepancy from the Singapore results was found above about 25 km. 2. In the troposphere and lower stratosphere, equatorial waves with periods longer than about 4 days exhibited large amounts of energy, while gravity waves became dominant above about 25 km. The asymptotic shapes of the frequency spectra for wind velocity were approximated by a logarithmic slope of-5/3 to -2. However, the frequency component, corresponding to the diurnal tides, was enhanced below about 24 km altitude. 3. On periodogram and hodograph analysis, we detected various equatorial waves with periods from 4 to 2 days, as summarized in Table 1. In the stratosphere above about 2 km, waves with periods longer than 4 days generally had small amplitudes, indicating that equatorial waves were not a dominant component there. 4. In the troposphere we detected an oscillation with a period of 7 days which showed fairly slow downward phase progression with a phase difference of 18 ø between the eastward and northward components. Its amplitudes were about 5 m/s below 5 km altitude for both the zonal and meridional components and rapidly decreased above 5 km. Its activity was largely enhanced in the first two thirds of the observation period and then diminished at the end of the campaign. The time-height variations of the regions with high relative humidity, simultaneously measured with radiosondes, clearly coincided with the large amplitudes of the meridional components of the oscillation. 5. Perturbations with periods of days were also detected in the troposphere. They showed large amplitudes of up to 8 m/s for the zonal component only. The dominant period of the oscillations clearly increased from 15 days at 2-4 km to 17 days at 1-12 km. The general structure of their amplitude profile was fairly similar to that for the 7-day oscillation detected in the same altitude region. In the lower stratosphere, fluctuations with periods of 7 and 2 days having large amplitudes of up to 15 and 7 m/s, respectively, only for zonal winds, were most likely Kelvin waves. In particular, the 2-day Kelvin wave was conspicuously enhanced only in a narrow region with a thickness of about 5 km that was centered at the tropopause. 6. We further found that the 2-day Kelvin wave also caused temperature fluctuations which modified the structure of the tropopause such that the minimum temperature varied in the range of K, showing sinusoidal time variation. Moreover, the tropopause height and the N 2 values were found to exhibit discontinuous time variations. It was suggested that the upward transport of water vapor from the troposphere into the stratosphere in the equatorial region might be controlled by the activity of equatorial waves. It is also anticipated that stratospheric air can intrude into the troposphere because of deformation of the tropopause by Kelvin wave activity, which might result in the downward transport of minor constituents through the tropopause. 7. Profiles of amplitudes and phases for a 24-hour oscillation were compared with those of a numerical model assuming only migrating tides, it being indicated that the model agreed reasonably well with the observed profiles above about 25 km. Below 25 km the observed amplitudes exceeded those in the model about 1 times, and the phases indicated fluctuations with small vertical scales, which suggested the manifestation of nonmigrating tides. Acknowledgments. The data used in this study were collected through a collaborative project between RASC, Kyoto University, BPPT, and LAPAN. We deeply appreciate the extensive collaboration of the staffs of LAPAN and BPPT during the radiosonde experiment at the Watukosek stratospheric balloon observatory of LAPAN. We also gratefully acknowledge the assistance of T. Tanaka in data analysis. We also thank M. Shiotani for providing the database of radiosonde observations collected at the Singapore meteorological station in and the Meteorological Satellite Center for the VISSR database with the GMS 4 satellite. We are also deeply indebted to I. Hirota, H. Itoh, M. Shiotani, S. Yoden, N. Nishi, T. Iwasaki, Y. Takayabu, M. Yanai, T. Nitta, T. Maruyama, and D.C. Fritts for helpful comments and suggestions. This study was financially supported by the Japanese Ministry of Education, Science and Culture under grants , 2NP21, 3NP21, and 4NP21.

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