The global-scale atmospheric motions are originally generated by the strongest convective motions in the equatorial region caused by absorption of strong solar radiations. Specifically, the western Pacific region called the Indonesian Archipelago is known for its convections, which are the most active and the highest all over the globe due to its warmest ocean water. Therefore, through these global-scale motions, the atmospheric dynamics over the Indonesian equator result in the most significant influences to global atmospheric changes. The mechanisms of these atmospheric changes and fluctuations, however, have not yet been made clear due to the sparseness of observational data in that region. The Coupling Processes in the Equatorial Atmosphere (CPEA) is a six-year research project of Japan to study dynamical and electrodynamical coupling processes in the equatorial atmosphere by conducting various observations in the Indonesian equatorial region. In the present paper we describe the outline of this project and show preliminary results from its first campaign conducted from March to May 2004.
A special tropospheric observation was conducted during 10 April-09 May 2004 as a part of the first campaign of the Coupling Processes in the Equatorial Atmosphere (CPEA-I), using upper soundings at seven stations in/around Sumatera, and weather radar and wind profilers at Kototabang (KT; 100.32°E, 0.20°S, 865 m above mean sea level), West Sumatera. A super cloud cluster (SCC) with a westerly wind burst (WWB) propagated eastward over the Indonesian Maritime Continent during 04-07 May. In the present report, we examine the evolution of that SCC and the associated wind behavior in detail, using Geostationary Operational Environmental Satellite (GOES-9) Infrared (IR) data and CPEA observations. During the analysis period, the SCC developed over the eastern Indian Ocean, decayed rapidly as it reached Sumatera, and re-developed over Kalimantan. The eastward propagation of SCC resulted from the successive formation of meso-scale cloud clusters (CCs) with westward propagation. The transition of the SCC was related to the evolution of CCs. A CC generated over Sumatera began to diminish as the WWB arrives at a mountain range in western Sumatera, but it dominated in/around the mountain range for ∼9 hours. From upper sounding data aligned along the equator, it was found that the migration speed of the WWB over Sumatera was approximately half that over the sea region between Sumatera and Kalimantan, due to the orography of the Indonesian Maritime Continent. In western Sumatera, the peak height of the WWB at the mountain range ascended 1.5 km from that on the windward side. This shows that the eastward migration of the WWB was intercepted by the mountain range. The orographic influence on the WWB is considered to persist during the retention of the CC in its vicinity. Further, we reveal features of the orographic precipitation associated with the WWB and the detailed wind structure inside the SCC over the mountain range, from radar observations at KT.
During the latter half of the first CPEA campaign (CPEA-I), X-band Doppler radar (XDR) observation was carried out from 10 April to 9 May 2004 in west Sumatera. In this paper, characteristics of precipitating systems and their relation to the Madden-Julian Oscillation (MJO) are investigated based on the analysis of the XDR data. Significant diurnal variations of precipitation were observed both in the convectively inactive and active phases of MJO, in which the area of intense echoes with reflectivity greater than 40 dBZ attained a maximum around 16 LST, whereas the area of weaker echoes attained a maximum in the evening. However, while the area of weaker echoes showed significant drop in the evening (21 LST) in the inactive phase, such a drop occurred in the early morning in the active phase. During the convectively inactive phase from 10 to 22 April, the precipitation was caused by systems that formed within the observation area. Shallow convective cells appeared in the mountain range around midday, which subsequently developed into deep convective systems. These convective systems then migrated southwestward in many cases, but in some cases they were observed to split and then migrated both southwestward and northeastward. During the convectively active phase from 23 April to 6 May, much of the precipitation occurred associated with the development of pre-existing cloud systems within super cloud clusters (SCCs). While the precipitation area with weak to moderate reflectivity became considerably larger than that in the convectively inactive phase, the area and the top heights of intense echoes were generally suppressed. Environmental factors responsible for these modulations in precipitation are also discussed. The structure and evolution of precipitation systems were further investigated for some selected cases. The precipitation system observed on 17 April showed typical evolution during the convectively inactive phase, whereas the precipitation system on 11 Apri1 was observed to split into two systems that migrated southwestward and northeastward. The northeastward migrations of the precipitation systems occurred as a result of successive generation of new convective elements on the northeastern side of old ones. The precipitation systems observed on 23-24 April and 5-6 May occurred during passages of SCCs, and moved eastward associated with intrusions of low-level westerlies. These systems were composed of convective regions around the leading portions of the low-level westerlies, and stratiform regions behind. Analyses of reflectivity and Doppler velocity fields for the latter case suggested that the elevated orography in the west Sumatera temporarily blocked the eastward propagation of the system and the intrusion of the low-level westerly.
Data from the Equatorial Atmosphere Radar (EAR) were analysed during the Coupling Processes in the Equatorial Atmosphere (CPEA) campaign of Apri1-May 2004. Statistical averages of the daily perturbations of wind velocity, temperature and humidity were examined. In the lower stratosphere, horizontal wind variances of up to 1.5 m2s−2 were visible above the afternoon tropospheric convection. Vertical wind variances of up to 0.03 m2s−2 occurred at the same time. The average variances of temperature and humidity in the lower troposphere also increased in the afternoon. Estimates of average momentum flux were made. In the lower stratosphere (18.0-20.0 km), the average magnitudes were 0.7-0.9 m2s−2 for both |u´w´| and |v´w´|. These fluxes were observed above the convective cells and indicate the average momentum emitted by them. Residuals of the momentum flux components showed a small westward preference and very small northward directional preference for wave propagation during CPEA. Four individual convective events and three Super Cloud Clusters (SCCs) were examined in detail. Tropospheric horizontal and vertical wind variances increased by 5 to 10 times during intense convection. Directly above this convection, in the lower stratosphere, increases in variance were also recorded. Vertical wind velocity fluctuation amplitudes in the lower stratosphere increased from 0.05-0.1 ms−1 away from convection, to be 0.1-0.4 ms−1 during convection. The amplitudes decreased to their back ground levels as soon as the convection had passed the EAR site. In the lower troposphere, the virtual temperature perturbations increased to 1.0-2.0°C during convection, an increase from a background value of about 0.5°C. During two of the four individual events, the amplitude of the temperature perturbations stayed enhanced for several hours after the end of convection. This contrasts with the vertical velocities in the lower troposphere, which quickly decreased following the passage of the storms. Specific humidity profiles in most cases showed large increases in the amount of water vapour below 4 km during times of convection. At certain heights and times during intense convection, the specific humidity increased by up to 50% from its background level.
This study focuses on features of vertical wind and cloud distributions in Sumatra during the initial phase of a westerly wind burst (WWB) associated with a synoptic-scale super cloud cluster (SCC), by mainly using radar, radiosonde, and lidar data from 5 to 9 May 2004. The convective envelope of the SCC reached Sumatra from the Indian Ocean on 5 May, passing over Sumatra on 7 May. Intensification of the westerly wind occurred over Sumatra below 5.5-6.0 km as the SCC passed over it. On 7 May, the 2.5-4.0 km westerly wind at Kototabang (KT; 0.2°S, 100.32°E, 865 m MSL) was identified as a WWB. Precipitating clouds around KT were suppressed after 7 May, as drier air (lower than 60% relative humidity) was transported from the Indian Ocean over Sumatra at 2.5-6.0 km. Non-precipitation clouds were observed at 5-8 km by the lidar after 7 May. After 7 May, the vertical wind at 2.5-5.5 km showed the oscillatory motion with a timescale of about 12 hours. Similar oscillatory motion was found in the 1.5-2.5 km zonal wind. Contrary to the radiosonde-derived downward wind with a horizontal scale of several hundred km, daily-averaged vertical wind at KT showed upward motion of 0.07-0.08 m s−1 on 7 and 8 May, when westerly winds larger than 10 m s−1 prevailed at 2.5-4.0 km. These facts imply that the topography around KT, which has steep mountains to the west, modulates the behaviors of the vertical wind. The vertical wind oscillation was suppressed above 3.0-5.5 km, where the Richardson number (Ri) was smaller than 0.45 and westerly wind changed to easterly wind. The small Ri was brought about by strong vertical wind shear (larger than 10 m s−1 km−1) and/or weak vertical gradient of potential temperature (smaller than 3 K km−1). Both regions appeared at the upper part of the westerly wind region. This fact implies that shear instability and horizontal wind change inhibit upward propagation of vertical wind oscillations.
Characteristics of vertical wind profiles (VWPs) in precipitating clouds were studied over western Sumatera (or Sumatra) Island of the Indonesian maritime continent during the first Coupling Process of Equatorial Atmosphere (CPEA) campaign period (10 April-09 May 2004) using Equatorial Atmosphere Radar (EAR) and X-band Doppler radar (XDR) to examine the dynamic and thermodynamic structures of various types of tropical precipitating systems. VWPs were precisely measured with the EAR directly in precipitating clouds, which were identified using simultaneous XDR observation. A super-cloud cluster (SCC) system of the intraseasonal variation (ISV) onset phase was examined as a case study. The SCC consisted of three cloud clusters (CCs), which were further partitioned into convective and stratiform cloud regions based on the XDR reflectivity fields. Vertical winds varied greatly in time and height according to variations in the reflectivity fields. Although the VWPs for each CC showed quite scattered variations in time and height, they had significant dependence on the corresponding 10 dBZ radar echo top heights (H10dBzs). Spectral representations of vertical wind and reflectivity profiles, which were stratified by H10dBz heights, showed the following distinct characteristics: 1) Convective spectra were divided into two groups by H10dBz = 10 km. The shorter spectra had massive reflectivity, especially in the lower height with upward currents; the taller spectra had a gentle reflectivity profile with significant upward currents around H10dBz and apparent downward currents below 8 km in height. 2) Spectra for stratiform clouds were partitioned into three groups by H10dBz = 6 and 8 km. Beside the shortest group of shallow stratiform clouds, bright band signs were intensified as the H10dBz increased in the taller two groups. The heights of the maximum upward currents above their bright bands also increased too with the heights of H10dBz. The reflectivity spectra and VWPs are discussed in comparison to global precipitation spectra observed by the Tropical Rainfall Measuring Mission (TRMM) satellite and VWPs calculated by rawinsonde sounding array data, respectively.
Lightning activity observed by the Tropical Rainfall Measuring Mission-Lightning Image Sensor (TRMM-LIS) and aerological data from the first campaign of the Coupled Process in the Equatorial Atmosphere (CPEA-I) were used to diagnose convective activity over the western part of the Indonesian maritime continent (MC) for the period between 10 April and 9 May 2004. Lightning and many deep but small convective clouds associated with low outgoing longwave radiation (OLR) dominated over large islands of the MC before the onset of a Madden Julian Oscillation (MJO) active period. During the MJO active period, a MJO cloud system composed of three super-cloud clusters (SCCs) passed eastwardly over the study area. The first SCC (SCCI) was accompanied by active lightning. After SCCI passed, lower OLR extended over a wide area including both islands and ocean, and lightning was suppressed over the large islands. Lightning activity was weaker in subsequent SCCs. Atmospheric stratification, vertical winds, the apparent heat source (Q1), and apparent moisture sink (Q2) were studied using rawinsonde observations for two areas: one over Sumatera and the other over the ocean between Sumatera and Kalimantan. Differences in vertical wind profiles between land and ocean suggest stronger onshore land-sea circulations in the break period than in the active period. Convective instability was greatest before the onset of the MJO active period over land and then weakened after onset. Over land, height of the positive peak was much higher in Q1 than in Q2 before onset. The height of the peak was comparable after onset; this change suggests that subarea-scale deep convective rain (stratiform rain) dominated before (after) the onset. These observations are consistent with variations in lightning activity. Previous observations showed that MJO cloud systems over the western Pacific have enhanced convection in their eastern portions and stratiform rain in western parts. A similar large-scale structure was maintained in the MJO cloud system over the western part of the MC, despite blocking effects by the MC on the MJO cloud system, especially in mesoscale and synoptic-scale structures.
Interactions between the convective activity and the atmospheric thermodynamic structures are analyzed utilizing upper-air rawinsonde observations obtained by R/V Mirai, R/V Kaiyo, R/V Natsushima, of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) over the western tropical Pacific Ocean, and those over three GEWEX Asian Monsoon Experiment (GAME) stations: Chiang Mai, Non Khai, and Ubon Ratchathani. Special emphases are placed on understanding the correlation between convection and the atmospheric thermodynamic structures in relation to the recent findings of tri-modal cloud levels over the warm ocean (e.g., Johnson et al. 1999) and to the cloud diagnostics proposed by Raymond and Blyth (1992). We first examine the relationships between a convection index and thermodynamic structure indices. A large correlation is found between the convective activity and lower-tropospheric (600-800 hPa) humidity, while there is no significant correlation between the convective activity and Convective Available Potential Energy (CAPE) or Convective Inhibition (CIN). Next, we apply a cloud diagnostic model introduced by Raymond and Blyth (1992) (referred to as RB92) to the observed profiles. As a result, it is shown that there are fundamentally 3 peaks of detrainment levels, which are lower-troposphere (near 900 hPa), mid-troposphere (near 450 hPa), and upper-troposphere (near 150 hPa), over ocean as well as over land. In the soundings over ocean, when the lower-troposphere (600-800 hPa) is dry, there is a tendency for simultaneous existence of stable layers both in the lower-troposphere and in the mid-troposphere. Such atmospheric thermodynamic structure is diagnosed as favorable for strengthened detrainments in the low- and mid-troposphere and weakened in the upper-troposphere. Finally, meridional winds are composited to the north and to the south of the maximum convective activity in the Inter Tropical Convergence Zone (ITCZ) region, respectively, over the tropical western Pacific Ocean. It is confirmed with upper-air soundings that there is a significant meridional divergence near the melting layer level in the mid-troposhere around 500-600 hPa and a significant meridional convergece near 350-400 hPa, in addition to the lower-tropospheric convergence and the upper-tropospheric divergence of the local Hadley Circulation. These additional circulations in the mid-troposphere are consistent with detrainment profiles diagnosed for observed atmospheric profiles utilizing RB92 cloud model. After all, it is strongly suggested that the cloud microphysics, such as melting and freezing, play significant roles in determining the large-scale circulation.
In this paper, the seasonal changes in the diurnal variations of wind and the cloud activity at Serpong (106.7°E, 6.4°S), near Jakarta, are climatologically described. In the dry season (May-October), diurnal variation of wind accompanied with sea-land breeze circulation was prominent. In the rainy season (November-April), the diurnal variation was consistent with sea-land breeze circulation, but was not as clear as that in the dry season. The peak time of the northerly in the rainy season, similarly to that of the sea breezes at Serpong in the low level (below 1.0 km height), was earlier than that in the dry season. The maximum time in the climatological diurnal variation of the surface temperature at Serpong in the rainy season was earlier than that in the dry season. The vertical structure which was consistent with sea breeze circulation was clearer when the prevailing (daily-mean) wind was weaker in the rainy season. These results are consistent with the features of the local circulation; in other words, the local circulation depends on the diurnal variation of surface temperature and is prominent when the prevailing wind is weak. The typical diurnal variation of the wind in the rainy season was unclear when the prevailing northwesterly to westerly was strong around Serpong. Interannual variation of the diurnal variation could be detected in the transitional period from the dry to the rainy season. Cloud activity had prominent diurnal variation over West Jawa in the rainy season and was active in the early evening over land, particularly, in the mountainous area in the south of Serpong. When cloud activity was active over the mountainous area, the northerly below 1.0 km in height was prominent at Serpong, which is consistent with the feature that the development of a local cloud system is accompanied with local circulation.
Diurnal and seasonal variations of raindrop size distribution (DSD) at Gadanki (GD), Singapore (SG) and Kototabang (KT) are studied to elucidate characteristics of DSD in the Asian monsoon region. It is found that DSDs are affected by diurnal convective cycles and seasonal variations in precipitation characteristics. GD has the most significant seasonal variation in DSD. Clear difference in rainfall characteristics between the Southwest and Northeast monsoon seasons is considered to be the main cause of such clear seasonal variation. KT has the most significant diurnal variation of DSD, which is probably caused by the fact that KT is greatly affected by ocean-land contrast and mountain effects to generate local convection in the afternoon. SG has less diurnal and seasonal variations compared with the other two locations, which is related to the fact that SG is affected both by land and oceanic rainfall. Z-R relations applicable to radar rainfall measurement in these areas are derived. It is shown that the use of the Marshall-Palmer Z-R relation (Z = 200R1.6) gives bias errors of about 1.5 dB or less in rain rate estimation except for the northeast monsoon season in GD, for 12∼18 local time during pre-southwest monsoon season in GD, and for 06∼12 local time during some monsoon seasons in KT.
This study investigates the effects of tropical rainfall on the Ku-band satellite communications links that connect Research Institute for Sustainable Humanosphere (RISH), Kyoto University in Japan to Equatorial Atmosphere Radar Observatory (EAR; 0.2°S, 100.3°E) in Indonesia, using the satellite Super bird C (144°E in orbit). Rain attenuation of the up- and down-link radio wave signals is, for the first time, obtained at the same time in the tropics, monitoring each signal level that has been received at both stations in Japan and Indonesia for the past three years from 2003 to 2005. The up-link attenuation at each station can be estimated from the down-link signal level measured at its opposite station, because SCPC (Single Channel Per Carrier) signals used in this experiment are linearly amplified without saturation of the satellite transponders. At EAR in Indonesia, a slightly larger attenuation ratio between up and down links is statistically presented for the attenuation range of higher that 10 dB, suggesting the effects of smaller raindrop size distributions (DSD) than observed at RISH in Japan. This tendency is more conspicuous in the rainfall events when the observed attenuation shows only one peak in its time series, indicating the effects of simple convective precipitating clouds with one single cell. At RISH in Japan, a larger difference between worst month and yearly average statistics is found, due to a larger variation of the ground temperature that affects the slant-path length during the seasons, although the yearly average time percentages as such are larger at EAR than RISH up to the up-link attenuation of 15 dB. Using time percentages of their local rainfall rates, fairly good agreement is found between the observations and the ITU-R (International Telecommunication Union—Radiocommunication Sector) predictions for both locations. At EAR in Indonesia, however, the time percentages of the attenuation of more than 10 dB become significantly smaller than those predicted by the ITU-R methods for both up and down links. This indicates the remarkable reduction of equivalent path lengths down to about 2 km, caused by a fairly localized structure of convective precipitating clouds. Simultaneous X band radar observations have revealed that intense echo cores of typical rain cells causing severe attenuation are confined to about 2 km along the propagation path. This result, which seems rather small even compared with those obtained in other tropical observation sites, may be attributed to unique features of the EAR site that is located in a highland basin 865 m above the mean sea level and frequently observes simple precipitating clouds with a single cell.
Radar observation is widely recognized as an essential technique to study the three-dimensional dynamics of the atmosphere with a high temporal resolution. Conventional monostatic radar with a single aperture or array, however, has rather limited spatial resolution. This is because this radar can only obtain the radial component of the wind velocity, and thus it must observe at least three spatially separate target volumes to estimate the three components of the wind vector. In this paper, a multistatic radar observation technique, which uses two receiver arrays together with a high-gain rapid scanning mesosphere-stratosphere-troposphere (MST) radar, is presented. Multistatic receiver arrays obtain non-radial components of the wind velocity; hence, the technique enables us to determine 3D wind velocity at each minimum resolution volume. Consequently, it eliminates estimation error caused by horizontal inhomogeneity of the wind field and improves spatial resolution. Applying this technique, we made a series of tropospheric observations in September 2004, utilizing a newly developed digital receiver system at the Equatorial Atmosphere Radar, West Sumatra, Indonesia. Each receiver antenna is equipped with this digital receiver and recording system, which is constructed with a cost-effective ready-made digital receiver PCI board and a PC. This structure enables us to swing the receiving beam after an observation using digital beamforming techniques. First, the importance and the effectiveness of ground clutter rejection using an adaptive spatial filter, which is another advantage of digital receiver systems, is demonstrated. Then for the first time an example profile of a 3D wind velocity field with 1-km horizontal resolution at 3.8 km altitude is presented. Considering the accuracy of the multistatic radar system, the resulting wind field shows the existence of significant perturbation that previously would have been averaged in conventional monostatic radar observations.
The vertical and temporal variations of inertia-gravity waves are studied by means of an intensive radiosonde campaign conducted from 10 Apri1 to 9 May, 2004 at five sites, including the Equatorial Atmosphere Radar (EAR) site at Koto Tabang (0.2°S, 100.32°E) in west Sumatra, Indonesia. The four other balloon sounding sites are located about 75-400 km away from EAR. Dominant gravity waves with periods of 2-3 days and vertical wavelengths ofapproximately 3-5 km showing clear downward phase propagation were detected, particularly in the upper troposphere and lower stratosphere (UTLS) region. The gravity wave energy is found to become the largest at an altitude of approximately 20 km, although the enhancement was not continuous, but intermittent. The wave activity was similar at all five sites, having only a slight phase shift, which suggests that the horizontal scale of the wave is larger than the distance between the sites. We have applied a correlative analysis to delineate the horizontal propagation characteristics of gravity waves, and estimated the horizontal wavelength (λh) to be approximately 1,700 km propagating toward 30° south from the east from 26-30 April, 2004, which is further verified by hodograph analysis for individual profiles. From 10-14 April, 2004 and 5-9 May, 2004, λh and the direction of the propagation were found to be 2,700 km and 3,250 km, and 26° and 3° north from the east, respectively. The spatial and temporal variations in the convection, which is thought to be a major source in the generation of gravity waves, is also studied using satellite data of outgoing long-wave radiation (OLR). We noticed clear eastward advection of large super cloud clusters (SCCs) from the Indian Ocean to the maritime continent, with occasional movement towards the observational sites. The source of the gravity waves is strongly related to this slowly eastward-advecting tropospheric convection, implying that the wave activity observed in the UTLS region was generated by far distant sources located west of the EAR. In addition, we present a case study in which large wave activity did not correspond to the particular cloud convection.
Radiosonde observations of winds and temperature over several sites in Southeast Asia during the CPEA radiosonde observation period (10 Apri1-9 May, 2004) reveal the presence of a 7-day wave in the upper troposphere and lower stratosphere (UTLS), especially during the first half of the radiosonde observation period. Many of the characteristics of the wave resemble those of Kelvin waves. The wave amplitude peak is observed at altitudes of 20-21 km. The vertical phase structure of the wave is consistent with altitude over the radiosonde sites considered. The vertical wavelength of the wave is found to be in the range of 5.5-6.5 km. The correlative analysis among the different radiosonde sites and analysis of TIMED-SABER data sets reveals that the wave is more active in the longitude band of 0-180°E and has a zonal wave number of 3. A wave of similar periodicity and zonal structure observed in OLR suggests that tropical convection could be a source for these waves. A slight shift is observed in the peak of wave amplitude toward the northern hemisphere. Moreover, the latitudinal width of the wave is narrower than that predicted by theory. This is discussed based on the existing model results, which indicate that the latitudinal structure of the wave may be affected by strong vertical shear in zonal wind, such as that reported in the present work.
As a part of the Coupling Processes in Equatorial Atmosphere (CPEA) program, an intensive radiosonde sounding was conducted for 30 days from April 10 to May 9, 2004, coinciding with the eastward phase of QBO, at seven stations over the Indonesian maritime-continent, including the Equatorial Atmosphere Radar (EAR) site at Koto Tabang (0.2°S, 100.32°E), west Sumatra. Using radiosonde profiles, we studied the behavior of equatorial Kelvin waves with periods of 10-12 days and vertical wavelengths of 6-7 km. The global feature of the Kelvin wave was also analyzed with simultaneous CHAMP/GPS (CHAllenging Mini satellite Payload/Global Positioning System) radio occultation (RO) data. The Kelvin wave characteristics delineated by the GPS RO analysis, assuming only zonal wave number 1 and 2 components, shows good agreement with radiosonde results in the stratosphere, although the wave amplitudes were estimated to be somewhat smaller for GPS RO. However, a discrepancy in the Kelvin wave characteristics was recognized around and below the tropopause. This discrepancy is likely due to higher zonal wave number (>2) components being dominant in the troposphere. The Kelvin wave amplitudes were enhanced around the tropopause and in the lower stratosphere between 17 and 25 km and dissipated above approximately 25 km, which is consistent with earlier results. GPS RO results show eastward phase propagation of wave number 1 during the first half of the CPEA campaign, while a mixture of wave numbers 1 and 2 appeared during the second half of the campaign. We also report the modulation of the tropopause structure by these Kelvin waves.
The MF and Meteor radar observations of horizontal winds over Koto Tabang (0.2°S, 100.3°E), Pontianak (0°, 109.3°E), and Pameungpeuk (7.5°S, 107.5°E) are utilized to study the characteristics of 5-8-day waves in the mesosphere and lower thermosphere (MLT) region over Indonesia. The wave activity is larger in zonal wind than in meridional wind and in the year 2003, it maximizes before and after northern hemispheric spring equinox. In the year 2004, the wave activity in zonal wind is larger during June-August 2004. During this period, the phase of the wave over Pontianak leads that over Koto Tabang, indicating that the wave is westward propagating with zonal wave number 1. The wave period ranges from 6.3 to 7.0 days over the equatorial sites Koto Tabang and Pontianak, whereas over the southern hemispheric site, Pameungpeuk, it is in the range 6.2-6.4 days. This shows that the dominant period of the wave changes with latitude. The amplitude and phase structures of the wave with altitude are similar among the three sites considered. The wave dominates the MLT dynamics over Pameungpeuk even during second half of CPEA (Coupling Processes in the Equatorial Atmosphere) campaign period (10 April-9 May 2004) with similar dominant wave period near 6.4-days. The wave amplitude is larger during last quarter of the campaign period.
We have made a comprehensive measurement of a front-like structure in the mesosphere at the equator at Kototabang, Indonesia (0.2°S, 100.3°E), using an airglow imager, an airglow temperature photometer, a meteor radar, and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The event was detected in airglow images of both OH-band (peak emission altitude: 87 km) and 557.7-nm (96-km) emissions, as an east-west front-like structure propagating northward with a velocity of 52-58 m/s. Wave trains with a horizontal wavelength of 30-70 km were observed after the passage of the front. The airglow intensity decreased for all the mesospheric emissions of OI (557.7 nm), OH-band, O2-band (altitude: 94 km), and Na (589.3 nm) (90 km) after the front passage. The rotational temperatures of both the OH-band and the O2-band also decreased ∼10 K. An intense shear in northward wind velocity of 80 m/s/6 km was observed at altitudes of 84-90 km by the meteor radar. The kinetic temperature profile at altitudes of 20-120 km was observed near Kototabang by TIMED/SABER. The front-like structure and trailing waves were similar to those of the mesospheric tidal bore. However, we found that the ducting condition, which is necessary to form a bore, was not satisfied for the observed wave parameters. We speculate that the intense wind shear may play some role for the generation of the front-like structure.
Characteristics of sodium and iron layers in the mesopause region over the equator observed with the resonance scattering lidars installed at Kototabang, Indonesia (0.2°S, 100.3°E) are reported. These lidars were operated during the night. The sporadic sodium layer (Nas) and the sporadic iron layer (Fes) were detected in almost every observing opportunity other latitude regions. Nas and Fes layers appeared almost at the same time and the heights above 90 km. However, Fes layers appeared below 90 km as well. The occurrence of the Nas layer correlated well with that ofsporadic E layer which was observed simultaneously by the ionosonde at Kototabang. On the other hand, the occurrences of the Nas and Fes layers does not correlate with that of the wind shear which has been observed simultaneously by the meteor radar at Kototabang. This result is not in agreement with the result of simultaneous observations by the Na lidar and the MU radar at Hachioji (35.6°N) and Shigaraki (34.9°N) in mid-latitude.
As part of the CPEA (Coupling Processes in the Equatorial Atmosphere) project, we have been conducting ground-based optical and radio observations of the ionosphere and thermosphere at Kototabang in Indonesia, Japan, and Australia. First, this paper gives a brief overview of some results, paying special attention to 100-1000 km scale plasma disturbances in the F region ionosphere, i.e., medium-scale traveling ionospheric disturbances (MSTIDs) and plasma bubbles. MSTIDs over the equator are observed within and in the south of the F region equatorial anomaly crest and have, on the average, a phase velocity of 300 m s−1 toward the south, a period of 40 min, and a wavelength of 700 km. Plasma bubbles move to the east at ∼ 100 m s−1, have a scale of about 100 km with spacings of 200-250 km, and are embedded within plasma structures with a scale of about 1000 km. Interestingly, giant plasma bubbles can be simultaneously detected at lower midlatitudes in southern and northern hemispheres that are connected by the geomagnetic field line, and are very identical in appearance in the both hemispheres. Then, we present newly-obtained characteristics of equatorial ionospheric scintillations of 1.6 GHz GPS radio waves associated with plasma bubbles. Continuous scintillation observations for two years at Kototabang indicate that the scintillations appear predominantly from sunset to midnight in equinoctial months. Such a seasonal variation is also recognized from a statistical study of bubble occurrences over the Philippines, Singapore, and Indonesia. To investigate possible dynamical coupling between the ionosphere/thermosphere and troposphere over the equator, we compare the scintillation (bubble) activity and Earth’s brightness temperature variation over the Indian Ocean measured by meteorological satellites. The results indicate that there can exist meaningful correlations between the scintillation occurrence and tropospheric disturbance at 80°-95°E longitudes, i.e. 5°-20° west of Kototabang. Possible processes to seed plasma bubbles are discussed.
To investigate drift velocities of a few hundred meter-scale irregularities associated with equatorial plasma bubbles, we used three single-frequency GPS receivers at the Equatorial Atmosphere Radar (EAR) site at Kototabang, Indonesia (0.20°S, 100.32°E; geomagnetic latitude 10.6°S), since January 2003. The GPS receivers sampled GPS signal intensity at a rate of 20 Hz. Distances between the receivers were 116, 127, and 152 m. An analysis of scintillation index (S4) in two years (2003-2004) revealed that the scintillations often occurred between 2000-0100 LT at equinoxes and that their occurrence rate was higher during March-April than during September-October. Drift velocities of irregularities were measured using cross-correlation analysis with the time series of the GPS signal intensity obtained from the three receivers. From a statistical analysis of the drift velocities, the eastward component of drift velocity just after sunset is found to be greater during March-April than during September-October. Based on these results, for the first time, we suggest that the east-west component of plasma drift velocity (or vertical electric field) may be related to the evolution of plasma irregularities causing scintillations throughout the mechanism causing the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocity could be attributed to the equinoctial asymmetry of neutral winds in the thermosphere.