Coupling Processes in the Equatorial Atmosphere (CPEA): a Project Overview

Journal of the Meteorological Society of Japan, Vol. 84A, pp. 1--18, 2006 1

Coupling Processes in the Equatorial Atmosphere (CPEA): A Project Overview

Shoichiro FUKAO

Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Uji, Japan

(Manuscript received 18 November 2005, in ﬁnal form 28 February 2006)

Abstract

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 Pro- cesses 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.

1. Introduction ward. The energy and momentum of these The equatorial region is the source of many waves are transported upward and deposited unique atmospheric processes that couple the at high altitudes. They interact with each other entire atmosphere vertically from bottom to or with the background flow at higher altitudes top and horizontally from equator to pole. De- and generate typical global-scale oscillations, spite its importance for global change, the such as quasi-biennial oscillation (QBO) (e.g., equatorial atmosphere is not well studied and Baldwin et al. 2001) and semi-annual oscilla- the coupling processes are poorly understood. tion (SAO) (e.g., Garcia et al. 1997). These Due to the equatorial region’s having the maxi- interactions are quite important because it is mum amount of solar radiation over the globe, the small-scale processes of atmospheric waves cumulus convection is most active there, gener- generated in the lower atmosphere that control ating a variety of atmospheric waves, such as the large-scale process of global circulation in the equatorial waves, atmospheric tides, and grav- middle atmosphere (Garcia and Solomon 1985). ity waves (e.g., Andrews et al. 1987). The mini- Electrodynamics shows unique features above mal Coriolis force effect present near the equa- 100 km altitude in the equatorial ionosphere, tor allows the widest range of periods for their where geomagnetic field lines are parallel to propagation over the globe. Their amplitudes the Earth’s surface (or perpendicular to the grow with altitude while they propagate up- Earth’s gravitational force) (e.g., Kelley 1989). Mesoscale atmospheric gravity waves exist at Corresponding author: Shoichiro Fukao, Research high altitudes and are believed to seed the Institute for Sustainable Humanosphere (RISH), plasma instability that generates equatorial Kyoto University, Gakasho, Uji, Kyoto 611-0011, plasma bubbles in the nighttime F region iono- Japan. E-mail: [email protected] sphere (e.g., Kelley et al. 1981; Hysell et al. ( 2006, Meteorological Society of Japan 1990). Thus, various dynamical and electrody- 2 Journal of the Meteorological Society of Japan Vol. 84A namical processes in the equatorial atmosphere observations were conducted with the deploy- are vertically coupled via convection-induced ment of Doppler radars (ship, land, air) in the atmospheric waves. A research project called Indonesian Archipelago (Webster and Lukas Coupling Processes in the Equatorial Atmo- 1992). Meanwhile endeavors have been made sphere (CPEA) has been initiated in Japan to for establishing core observatories and/or facili- understand the basic processes of the vertical ties in the equatorial region to integrate these coupling on various spatial and temporal scales investigations (e.g., Gage et al. 1991; Tsuda occurring in the equatorial low, middle, and et al. 1995). For instance, the Equatorial Atmo- upper atmosphere and ionosphere. CPEA is sphere Radar (EAR) was installed right at the funded as a Grant-in-Aid for Scientific Re- equator in Kototabang, West Sumatra, Indone- search on Priority Areas by the Ministry of sia (0.20S, 100.32E) in March 2001 (Fukao Education, Culture, Sports, Science and Tech- et al. 2003a). It started observations in July nology (MEXT) of Japan during the six-year pe- 2001 in collaboration between the Radio riod September 2001 to March 2007. Science Center for Space and Atmosphere (now In the equatorial region, cumulus convection Research Institute for Sustainable Humano- is a significant source of atmospheric gravity sphere), Kyoto University of Japan and the waves (e.g., Pfister et al. 1993; Alexander et al. National Institute of Aeronautics and Space 1995; Salby and Garcia 1987; Ricciardulli and (LAPAN) of Indonesia. CPEA is based on these Garcia 2000). It is well known from OLR (Out- developments, with the EAR site serving as a going Long Wave Radiation) satellite data that focus for its activities, and will encourage an the tallest clouds (deepest convection) appear extension of the network to incorporate as over the Indonesian Archipelago (or the Indo- many techniques as possible to ensure that nesian maritime continent) (e.g., Nishida et al. the equatorial atmosphere is characterized as 2000). The activity of mesoscale gravity waves broadly as possible. with vertical wavelengths shorter than 10 km 2. Scientific background of CPEA is estimated from potential energy per unit mass at 20–30 km altitude (Tsuda et al. 2000). 2.1 Dynamical coupling in the equatorial It is particularly enhanced over the Indonesian atmosphere Archipelago, where the cumulonimbus clouds The equatorial region features oscillations are tallest. The good correlation between these having temporal scales of about four years two types of data well demonstrates that atmo- (tropospheric El Nino-Southern Oscillation spheric waves are most significantly generated (ENSO)), about two years (stratospheric QBO), by convection there. Therefore, the Indonesian a half year (SAO in the middle atmosphere), Archipelago was selected as the main site for and shorter periods (tropospheric ISO and CPEA. equatorial waves) (e.g., Baldwin et al. 2001; The Scientific Committee on Solar- Garcia et al. 1997). Recently, with rapid prog- Terrestrial Physics (SCOSTEP) has sponsored ress in observational techniques, signatures of a series of previous international scientific pro- ENSO, QBO, etc. have been observed in the grams, among them the Solar Terrestrial En- mesosphere and lower thermosphere (MLT) ergy Program (STEP; 1990–97), and Energy too (e.g., Baldwin et al. 2001). Although theo- Processes Including Coupling (EPIC; 1998– retical principles for wave-wave and wave- 2002) was sponsored as one of four post-STEP mean flow interactions have been established, mini projects. The TOGA /COARE (Tropical identification of the actual mechanisms that Ocean Global Atmospheres/Coupled Ocean At- produce the oscillations seen is still controver- mosphere Response Experiment) was an inter- sial (Dunkerton 1997; Fritts and Alexander national research program promoted by the 2003). These mechanisms must be investigated World Climate Research Programme (WCRP) in detail by continuous observation with radio- that studied the interaction of the ocean and sondes and various ground-based instruments, atmosphere in the western Pacific. Several sci- including radars and lidars. The ten-to-twelve entific groups pioneered observational and the- year variability of the atmosphere and its rela- oretical investigations of coupling in the equa- tionship with solar activity (e.g., Kodera and torial atmosphere. Among them, extensive field Kuroda 2002) will be studied on the basis of July 2006 S. FUKAO 3 the maintenance of the observational network atmosphere? What are the principal wave established in the CPEA period. sources in the equatorial region? What are There are two transition regions in the the main dissipation mechanisms through Earth’s atmosphere where atmospheric proper- which atmospheric waves interact with ties show significant change across regional the mean flow, wave-breaking shear- boundaries; one is the tropopause and the other instabilities, inertial instabilities, or radia- is the mesopause-homopause region. Varieties tive cooling? of equatorial dynamics (such as tides and atmo- 2. Is development of cloud clusters associated spheric gravity waves) bring their signatures to with convective motion similar in the Indian upper atmospheric phenomena such as airglow Ocean and Indonesian Archipelago? and the equatorial electrojet (EEJ), beyond the 3. Which atmospheric waves drive the QBO mesopause-homopause region (see e.g., Bur- and SAO, and how do they affect atmo- rage et al. 1995 for tidal-origin airglow; Taylor spheric structure in general? How do they et al. 1987 for gravity-wave-origin airglow; Gel- affect meridional and zonal circulation? inas et al. 2002 for EEJ). Momentum transport 4. How do extratropical oscillations and plane- by equatorial waves appears to be particularly tary waves affect the equatorial lower, mid- important in producing the QBO and SAO in dle, and upper atmosphere? the stratosphere and mesosphere. However, 5. What is the mechanism that couples the the wave species responsible for the QBO and QBO to extratropical dynamics? SAO are not yet well understood (Baldwin et al. 6. What is the role of atmospheric waves, the 2001; Horinouchi et al. 2003; Mayr et al. 1998). diurnal tide in particular, in driving large- Momentum transport by the diurnal tide maxi- scale indirect circulation in the mesosphere/ mizes at the equator, and its dissipation in the lower-thermosphere (MLT) region? lower thermosphere produces indirect circula- 7. What is the role of small-scale waves in gen- tion, which transports neutral and ionized con- erating turbulence and associated mixing stituents both vertically and to higher latitudes processes? How do the large- and small-scale (Forbes and Roble 1983; Miyahara et al. 1993). processes affect the distribution of neutral Stratosphere-troposphere coupling at the tro- species, such as O and metallic atoms and popause appears crucially important to the ex- ions in the MLT region? What are the im- change of energy, momentum, and constituents pacts on the aeronomy there? such as H2O and O3 between the two regions (Holton et al. 1995). Also, it plays a key role in 2.2 Electrodynamical coupling in the the influence of stratospheric circulation on the equatorial ionosphere QBO in the troposphere (Yasunari 1990). Solar In the ionosphere, atmospheric wave motions activity effects, such as the 27-day variation in couple with plasma dynamics through electro- solar ultraviolet flux and hence ozone produc- dynamical processes. The ionospheric wind dy- tion, are most easily observed in equatorial re- namo generates large-scale electric fields that gion because solar flux maximizes there and cause plasma drifts, forming the equatorial dynamical interference on similar time scales (Appleton) anomaly in electron density (e.g., is less significant. Kelley 1989). Strong electric fields and plasma CPEA may not give comprehensive solutions density gradients, produced in part by atmo- to all important scientific questions relevant spheric gravity waves, cause instabilities that to dynamical coupling in the equatorial atmo- give rise to plasma irregularities on a wide sphere, but it is expected to give important range of spatial scales. Day-to-day variability clues to some of them by well-coordinated ob- of the winds and electric fields, associated with servations with various collocated instruments. variability in atmospheric tides, planetary Some of the specific scientific questions are as waves, and gravity waves, causes variability in follows: ionospheric structure, dynamics, and irregularities, which still remains poorly understood. Specific scientific questions: Electrodynamical effects are different in the dif- 1. What are the spatial and temporal variabil- ferent longitude sectors of the Earth, owing to ities of atmospheric waves in the equatorial the longitudinal variations in the latitude of 4 Journal of the Meteorological Society of Japan Vol. 84A the geomagnetic dip equator and magnetic field 5. What are the sources, transport mechanisms, strength, as well as to probable longitudinal and sinks of metallic ions in the equatorial variations in atmospheric wave motion. To the ionosphere, and what is their role in electro- extent that ionospheric properties like electron dynamics? density change, irregularities, and geomagnetic 6. How can observations of equatorial iono- variations can be simulated with physics-based spheric phenomena, like the strengths and models, observations of these quantities can diurnal variations of the equatorial electro- provide important information concerning the jet and the equatorial anomaly, be used to local and global properties of atmospheric provide information on the distribution and waves that penetrate into the ionosphere. variability of atmospheric tides and plane- The equatorial ionosphere is also coupled tary waves? with higher-latitude regions and with the plas- 7. What are the characteristics and sources of masphere and magnetosphere. Magnetospheric interannual variability in the equatorial electric fields and currents that couple with ionosphere? What are its relationships with those in the high-latitude ionosphere may pen- the solar-activity cycle, the stratospheric etrate to the equator, especially during mag- QBO, and global change in the atmosphere? netic storms, where they affect ionospheric 3. Outline of CPEA electron density, plasma irregularities, and the equatorial electrojet current (e.g., Kelley 1989). CPEA seeks pioneer research on the vertical Plasma mass and energy exchange between the coupling of the equatorial atmosphere by ionosphere and plasmasphere is important in means of (primarily) ground-based observa- determining ionospheric electron density, while tions. In the past four years of CPEA, develop- electrodynamical coupling of the ionosphere ment and installation of the instruments shown with the plasmasphere affects both large- and in Fig. 1 have been completed at the EAR site small-scale electric fields. These processes or its periphery. The altitude coverage of indi- need to be fully understood to clarify the rela- vidual instruments is limited, however. They tive roles of coupling from above and below for complement each other, and the whole atmo- the ionosphere, but they are beyond the scope sphere up to several hundred km altitude is of CPEA. covered by operating all instruments at the same time. Using all instruments available, we Scientific questions: 1. How do variations in planetary waves and tides influence the generation of large-scale electric fields, the development of the equatorial anomaly, and the occurrence of F- region irregularities? 2. What is the role of atmospheric gravity waves in the development of irregularities in the evening equatorial F region or of plasma bubbles? What are the principal sources of these waves? 3. What are the occurrence frequency and spatial distribution of F-region irregularities under different geophysical conditions? What are the characteristics and causes of longitudinal variability in the equatorial ionosphere? 4. What microphysical plasma processes are Fig. 1. Main instruments developed for involved in the development of equatorial CPEA and installed in the Equatorial ionospheric irregularities in the E and F re- Atmosphere Radar (EAR) site and its gions? How do these irregularities affect the periphery in Kototabang, West Suma- tra, Indonesia. The vertical bars show large-scale electrodynamics? height coverage. July 2006 S. FUKAO 5 conducted the first campaign, CPEA-1, in the kinds of lidar (PI: Chikao Nagasawa, Tokyo period March to May 2004 and plan another Metropolitan University). for the period November to December 2005. 6. Study group on the variation of the thermosphere and ionosphere owing to the energy CPEA consists of the following six study groups: of atmospheric waves: To investigate varia- 1. Study group on equatorial atmospheric pro- tions of the atmosphere above 80 km alti- cesses with EAR: To investigate various at- tude and the seeding of plasma instabilities mospheric processes, including convection, featured in the geomagnetic equatorial generation of gravity waves, and the tropical region due to atmospheric gravity waves tropopause layer (TTL) by means of long- coming from below by optical and radio tech- term observations with the EAR and other niques (PI: Tadahiko Ogawa, Nagoya Uni- instruments (Principal Investigator or PI: versity). Mamoru Yamamoto, Kyoto University). 4. Main instrumentation 2. Study group on small-scale wind and turbulence structure: To investigate small-scale The core instrument of CPEA is the Equato- three-dimensional air motion inside the rial Atmosphere Radar (EAR), shown in Fig. 2. volume illumined by the radar by adding It is located close to the geographic equator in two digital receiver sub-arrays as part of a West Sumatra, Indonesia (0.20S, 100.32E), multi-static radar system which utilizes the on the border between the Indian Ocean and EAR as the transmit-receive station (PI: the Indonesian Archipelago. EAR is a 47.0- Toru Sato, Kyoto University). MHz Doppler radar with a near-circular active 3. Study group on evolution of cumulus convec- phased array antenna system that enables fast tive clouds and their coupling with meso- to beam steering to any direction. The antenna synoptic-scale precipitation systems: To un- aperture is 110 m in diameter while the peak derstand the coupling process of convective power is only 100 kW, one tenth of that of the clouds from meso- to synoptic-scale and the MU radar (Fukao et al. 1985a, b), and accord- behavior of convection-induced atmospheric ingly the height coverage is limited. EAR can waves by establishing a radiosonde sounding observe atmospheric echoes up to 20 km in alti- network in Indonesia, Singapore, and Ma- tude and ionospheric irregularity echoes from laysia (PI: Toshiaki Kozu, Shimane Univer- the E- and F-regions. See Fukao et al. (2003a) sity). for more details. Observational data from EAR 4. Study group on the four-dimensional struc- are available at the following web site: http:// ture of atmospheric waves and energy trans- www.rish.kyoto-u.ac.jp/ear/data/ port: To determine the four-dimensional For meteorological observations, a boundary- structure of atmospheric waves in the tropo- layer radar and an X-band weather radar have sphere and stratosphere with radiosonde been installed at the EAR site and an X-band soundings, and that in the mesosphere and Doppler weather radar at Sungai Puar in West lower thermosphere (MLT; 60–100 km in al- Sumatra, Indonesia (0.36S, 100.41E). Surface titude) by establishing a regional radar net- meteorological instruments, such as a radiome- work in Indonesia. Satellite data are also ter, rain gauge, and disdrometer, have been in- utilized in order to supplement the ground- stalled at the EAR site. based and radiosonde observations (PI: A radiosonde-sounding network has been es- Toshitaka Tsuda, Kyoto University). tablished through collaboration between Indo- 5. Study group on the vertical structure of nesia, Singapore and Malaysia. Seven radio- the equatorial atmosphere with a multi-lidar sonde stations have joined the campaign; they system: To investigate the vertical struc- are Kototabang (EAR site; 0.20S, 100.32E), tures of temperature, clouds, and water Bandung (6.90S, 107.60E), Padang (0.88S, vapor in the equatorial atmosphere, and spo- 100.35E), Jambi (1.60S, 103.65E), Kuala radic metallic layers, mesospheric inversion Lumpur (2.73N, 101.70E), Kuching (1.48N, layers, and atmospheric gravity waves near 110.33E), and Singapore (1.22N, 103.59E). the mesopause, by means of a sophisticated At five stations, radiosondes were launched multi-lidar system consisting of several four times a day, and launching every 3 hours 6 Journal of the Meteorological Society of Japan Vol. 84A

Fig. 2. The Equatorial Atmosphere Radar (EAR) in Kototabang, West Sumatra, Indonesia (top) and 560 three-element Yagi antennas, each with a transmitter-receiver module mounted near the ground (bottom). The operational frequency is 47.0 MHz. The antenna is a near-circular array with the diameter of 110 m, and the peak transmit power is 100 kW. was conducted at four stations, Kototabang, Pa- ments, and Resonance lidar for measurements dang, Jambi, and Kuala Lumpur, for the five of the density and temperature of metallic spe- days of the intensive observation period (IOP) cies, such as Na, Fe, K, and Ca, in the meso- between April 18 and 22. The launching of pause region. The laser system consists of three radiosondes has been conducted in close collab- pulsed Nd:YAG lasers, a pulsed Ti:Sapphire oration with Frontier Observational Research laser seeded by a ring Ti:Sapphire laser and a Systems for Global Change (FORSGC) of Japan dye laser. The receiving system consists of a (PI: Manabu D. Yamanaka, FORSGC and Kobe Schmidt-Cassegrain telescope 20 cm in diame- University). ter, a Schmidt-Cassegrain telescope 35 cm in The regional radar network for MLT observa- diameter, and five Newtonian telescopes 45 cm tions is a quasi-equilateral triangle with one in diameter. Most of this multi-lidar system is side 700 km, consisting of an MF radar at Pon- remotely controlled via the Internet from Ja- tianak (0.0N, 109.3E), a meteor radar at Ko- pan. totabang (0.20S, 100.32E), and another MF For observation of the thermosphere and radar at Pameungpeuk (7.5S, 107.5E), Indo- ionosphere, GPS receivers, an all-sky CCD air- nesia. The first radar has been operational glow imager (ASI), and an airglow temperature since 1995 while the latter two were completed photometer have been in operation. Also, in March 2004. MLT winds have been mea- simultaneous mid-latitude ASI observations sured continuously with these three radars have been conducted at Sata, Japan (31.02N, ever since, with occasional interruptions due to 130.68E; geomagnetic latitude: 21.47N) and commercial power line failure, lightning dam- Darwin, Australia (12.44S, 130.96E; geomag- age, etc. netic latitude: 21.86S), which are nearly geo- The multi-lidar system consists of four lidars; magnetically conjugate. In addition, a VHF Mie and Raman lidars for measurements of (30.8 MHz) radar to detect ionospheric irregu- tropospheric aerosol /cirrus clouds and water larities was installed and began operation at vapor, respectively, Rayleigh lidar for strato- the EAR site in September 2005. FM-CW ionospheric and mesospheric temperature measure- spheric sounders were constructed at three July 2006 S. FUKAO 7

and other instruments for which non-attendant operation is possible are operated continuously on a routine basis while the remaining instruments operated only during CPEA-I. The period of CPEA-I was divided into two parts. The first half period, March 10 to April 3, placed emphasis on observations of the coupling between the middle and upper atmosphere. The latter half, April 10 to May 9, placed emphasis on observations of the coupling between the lower and middle atmosphere. An X-band Doppler weather radar, RASS, and radiosondes were deployed only during the latter period. RASS experiments were conducted with EAR to measure fine altitude profiles of temperature and humidity over Kototabang. As mentioned above, radiosondes were launched, on average, every six hours during the latter period. Among these days we set up an intensive observation period (IOP) of five days, from April 18 to 22, to catch heavy rain events, launching radiosondes every three hours. Fig. 3. Locations of major observational Figure 4 shows the longitude-time section of instruments for CPEA-I. The numbers cloud top temperature (or TBB). The vertical in the map correspond to those in Table line shows the longitude of EAR. Fortunately, 1. three super cloud clusters (SCC-1 to 3) passed over the EAR site during the campaign. Enor- places along the meridian of the EAR site; mous data sets are now being analyzed. The Kototabang (EAR site; dip latitude: 10.36S), second campaign (CPEA-II) is being prepared Chumphon in Thailand (10.72N, 99.37E; dip for a one-month period from November to De- latitude: 3.28N; close to the geomagnetic equa- cember 2005. Here we show highlights obtained tor) and Chiang Mai in Thailand (18.76N, primarily while drafting CPEA and from the 98.93E; dip latitude: 13.03N; close to the geo- first campaign, although they are still prelimi- magnetic conjugate point of Kototabang) by the nary and fragmental. National Institute of Information and Commu- 6. Principal research progress nication Technology (NICT) of Japan. 6.1 Cumulus convection over Sumatra 5. The first campaign (CPEA-I) The 30–60 day intraseasonal variations (ISV; Two periods are set up for CPEA campaigns. or intraseasonal oscillations, ISO) are featured The first campaign, CPEA-I, was conducted for by movement of super cloud clusters (SCC) [Na- about two months, from March 10 to May 9, kazawa 1988]. Hierarchical structures of SCC, 2004. The campaign was conducted as part of their relationship with El Nino-Southern Os- the CAWSES (Climate and Weather of the cillation (ENSO) (e.g., Nitta et al. 1986) and Sun-Earth System) campaign of SCOSTEP. equatorial waves (e.g., Takayabu and Nitta Various instruments, shown in Fig. 1, partici- 1993) are interesting subjects for CPEA to chal- pated in the campaign in order to cover a wide lenge. altitude range in the equatorial atmosphere, The top two panels of Fig. 5 show cloud top from the troposphere to the thermosphere/ temperature (TBB measured by GMS) and ionosphere. The sites of these instruments and NCEP low-level (850 hPa) zonal wind over the their operational modes and periods are pre- Indian Ocean. The high cloud tops are well cor- sented in Fig. 3 and Table 1, respectively. EAR related with a large westerly at 850 hPa, a 8 Journal of the Meteorological Society of Japan Vol. 84A

Table 1. Sites of the major instruments and their observational modes and periods. No. Instruments Location Observation mode and period 1 EAR Kototabang Standard ST/TR mode: routine observation (0.20S, 100.32E) FAI mode: Mar 10–Mar 31 RASS mode: Apr 10–May 4 Vertical wind mode: May 5–May 9 2 Boundary layer radar Kototabang Routine observation (0.20S, 100.32E) 3 Weather radar (1) Kototabang Routine observation (0.20S, 100.32E) Doppler weather radar (2) Sungai Puar Apr 10–May 9 (0.36S, 100.41E) 4 Radiosonde (1) Kototabang Apr 10–May 9 (0.20S, 100.32E) 6-hourly soundings (2) Padang Apr 10–May 9 (0.88S, 100.35E) 12-hourly soundings (3) Kuala Lumpur Apr 10–May 9 (2.73N, 101.70E) 6-hourly soundings (4) Singapore Apr 10–May 9 (1.22N, 103.59E) 4Z, 10Z, 23Z (12, 18, 07LST) (5) Jambi Apr 10–May 9 (1.60S, 103.65E) 6-hourly soundings (6) Bandung Apr 10–May 9 (6.90S, 107.60E) 6-hourly soundings (7) Kuching Apr 10–May 9 (1.48N, 110.33E) 6-hourly soundings 5 Radiometer and other Kototabang Routine observation surface instruments (0.20S, 100.32E) 6 Lidar Kototabang Troposphere observation mode (0.20S, 100.32E) Routine observation 7 Meteor Radar Kototabang Routine observation (0.20S, 100.32E) 8 MF Radar Tirunelveli Routine observation (8.7N, 77.8E) (1) Pontianak Routine observation (0.0N, 109.3E) (2) Pameungpeuk Routine observation (7.5S, 107.5E) 9 Ionosonde Kototabang Routine observation (0.20S, 100.32E) 10 GPS receivers (1) Kototabang Dual frequency observation (0.20S, 100.32E) Routine observation (2) Padang Single frequency observation (0.91S, 100.46E) Routine observation July 2006 S. FUKAO 9

Table 1 (continued) No. Instruments Location Observation mode and period VHF radar (30.8 MHz) Kototabang Routine observation (0.20S, 100.32E) 12 Magnetometer Kototabang Routine observation (0.20S, 100.32E) 13 Airglow imager (1) Kototabang Routine observation (0.20S, 100.32E) (2) Tanjungsari Routine observation (6.9S, 107.9E) 14 Airglow temperature Kototabang Routine observation photometer (0.20S, 100.32E) manifestation that cumulus convection is gen- having echo intensity above 40 dBZ was larger, erated by large-scale SCC over the Indian and the corresponding echo-top height was Ocean. On the other hand, the bottom two pan- higher, in inactive phase of ISV than in active els show that shorter period or one-day period phase. oscillations are predominant in both cloud top In Fig. 6, the two panels on the left show temperature and NCEP low-level zonal wind over Sumatra. Convective clouds here reached as high as those over the Indian Ocean but re- currently almost every afternoon. In addition, Kawashima et al. (2006) showed that echo-top heights are not correlated with ISV activities based on observations of the X-band Doppler weather radar at Sungai Puar. They demonstrated that the percentage of occupied area

Fig. 5. The top and second panels show cloud top temperature over Indian Ocean and NCEP 850 hPa zonal wind, Fig. 4. Longitude-time section of cloud respectively. The third and bottom pan- top temperature (or TBB). The vertical els are the same as the top and second line is the longitude of EAR. ones, respectively, except over Sumatra. 10 Journal of the Meteorological Society of Japan Vol. 84A

Fig. 6. (Left) OLR anomalies when cumulus convection is inactive (top) and active (bottom). (Right) Anomalies of lightning events observed by TRMM in the same periods shown in the left (Morita and Takayabu 2004).

OLR anomalies for the inactive and active (Dhaka et al. 2005). The X-band weather radar phases of SCC (Morita and Takayabu 2004). at Kototabang detected a strong convective Lightning-event anomalies observed by the cloud that moved from west to east over EAR, TRMM (Tropical Rainfall Measurement Mis- as shown in the bottom right panel. A high con- sion) satellite are shown on the right. Lightning vective cloud was observed by the boundary- events in the Indonesian Archipelago are more layer radar too. The vertical winds observed by frequent when ISV is inactive and less frequent EAR were significantly modulated just after when ISV is active. This is caused by a predom- passage of the convective cloud. The momen- inance of local convection over Sumatra during tum flux anomalies were upward on average, inactive ISV periods. and northwestward, opposite to the movement Kozu et al. (2005) discussed the intrasea- of the convective cloud. They are identified as sonal variation of raindrop size distribution convection-induced gravity waves with their (DSD) over EAR and demonstrated that DSD source located in the middle troposphere is broad in the inactive phase of SCC, as classi- (Dhaka et al. 2005). The wave amplitude was fied by GOES-9 IR TBB, but narrows as the 3–10 mPa. The period was about 1 hour, and phase of SCC shifts to active. Rain-top height the vertical wavelength was as short as 2 km derived from boundary layer radar data indi- above 13–14 km altitude, where an anvil cates that cumulus convection is more intense existed. in the inactive phase of SCC than in the active Regarding the stratosphere-troposphere ex- phase; this may cause the difference in DSD. change at the tropical tropopause, EAR de- All of these features seem opposed to our con- tected an important process of a Kelvin wave ventional understanding that cumulus convec- breaking at the tropopause and showed that tion is more active during the active phase of irreversible mixing of airmass between the tro- ISV as classified by TBB. This may come from posphere and the stratosphere occurs due to the fact that local convective processes are sig- enhanced wave-breaking turbulence (see Fuji- nificant in the inactive ISV phase due to the wara et al. 2003 for more details). topographic effect of Sumatra. If so, DSD response to ISV phase may be quite different 6.2 Impact of cumulus convection on the over the Indian Ocean. Such regional depend- mesosphere and lower thermosphere ences of convective activity should be investi- Several pieces of evidence have already been gated more extensively in the near future. provided to demonstrate that long-term vari- We are seeking direct evidence that atmo- ability in the troposphere influences the dy- spheric gravity waves are generated by cumu- namics of the mesopause-homopause region. lus convection. Figure 7 is one such example Figure 8 compares OLR over the Indonesian July 2006 S. FUKAO 11

Fig. 7. Momentum ﬂux anomalies observed by EAR (left panels), altitude-time intensity plot of convective cloud observed with the boundary layer radar (top right), and PPI plot obtained by the X- band weather radar (bottom right) (Dhaka et al. 2005).

Archipelago with diurnal atmospheric tides in sian Archipelago (120E) in response to El the MLT region as observed with radars over Nino-Southern Oscillation (ENSO). It has been the period 1993–1999 at Jakarta, Indonesia suggested that nonmigrating tides excited by (6S, 107E) and Tirunelveli, India (9N, 78E) large-scale convective systems in the tropo- (Gurubaran et al. 2005). OLR is used as an in- sphere propagate upward to the mesopause dex for wave excitation from large-scale convec- level and compete with the dominant migrating tive systems in the troposphere. The interan- tide in the mesopause region (Gurubaran et al. nual variation of the diurnal tide is likely 2005). caused by cumulus convection over the Indone- Intraseasonal oscillations (ISO) of zonal wind in the MLT region were studied using the same radar data at Jakarta, Indonesia during 1993– 99. Biennial variations are dominant in the ISO amplitude of the zonal wind and the zonal amplitude variation of the diurnal tide. Good correlation has been found between the ISO component in the OLR anomaly over the Indo- nesian Archipelago and that of the mean zonal wind and diurnal tide in the mesosphere (Isoda et al. 2004). These features show that signals from cumulus convection in the troposphere reach as high as the mesopause level. Small-scale gravity waves in the MLT region were observed by an OH CCD imager at Tan- jungsari in West Jawa (6.9S, 107.9E), and Fig. 8. Anomalies of diurnal tidal ampli- their horizontal propagation characteristics tude at 86 km in altitude over Tirunel- were analyzed in detail. This analysis sug- veli, India (9N, 78E) and Jakarta, In- donesia (6S, 107E) shown in black gested that horizontal propagation characteris- line. OLR anomalies at 120E are tics over the equatorial region are affected shown in red line at 10N and 5S for mainly by the distribution of cumulus convec- Tirunelveli and Jakarta, respectively tion in the troposphere and that the effect of (Gurubaran et al. 2005). the background mean wind in the middle atmo- 12 Journal of the Meteorological Society of Japan Vol. 84A sphere is not significant (Nakamura et al. 2001). The five panels (from bottom to top) indi- 2003). The vertical propagation of such gravity cate the number of data (or occultation events), waves observed in the MLT region was studied the geographical zonal cross sections of surface by using a reverse ray-tracing technique and topography at 5–25S, the water vapor pres- the difference between propagation in the sure in the middle troposphere (4–6 km in alti- tropics and at mid-latitudes was discussed in tude), gravity wave activity (relative tempera- detail (Wrasse et al. 2005). Three-dimensional ture variance) in the lower stratosphere (22– numerical simulations were conducted by using 28 km), and the activity of short-vertical-scale a vertically extended mesoscale meteorological (vertical scale less than 7 km) fluctuations of model, and breaking of convection-induced electron density associated with sporadic E gravity waves and airglow modulation induced layers in the MLT region, respectively. This re- by the breaking in the upper mesosphere were sult demonstrates that the cumulus convection revealed (Horinouchi et al. 2002). generated over the continents (including the Even ionospheric irregularities are well cor- maritime continent in the Indonesian Archipel- related with convective activity in the tropics. ago) excites gravity waves that enhance the Figure 9 shows one such result from the GPS/ activity of the sporadic E layer and other iono- MET (GPS Meteorology) experiment and spheric irregularities (with vertical scale less ECMWF reanalysis models (Hocke and Tsuda, than 7 km). Global and seasonal distributions of irregularity in the E-region ionosphere were clearly indicated through occultation measurement of CHAMP satellite GPS data too (Garcia and Tsuda 2005).

6.3 Vertical structure of the equatorial atmosphere revealed by multi-lidar system The multi-lidar system will be fully operational soon and dedicated to revealing the dynamical and compositional structure with high altitude resolution over a wide altitude range. Some preliminary results have already been provided with individual lidars. For instance, in November 2003, the Mie lidar started observation of subvisible cirrus (SVC) near the tropical tropopause layer. The cirrus is related to the freeze drying caused by sedimentation and lofting of air parcels affected through enhanced IR radiative heating of cloud particles that is thought to control water vapor entering the stratosphere. An example of the data obtained in December 2004 is shown every hour in the Fig. 9. GPS/MET observations during bottom panel of Fig. 10. Thick cirrus clouds February 2–16, 1977. Geographic zonal with 2 to 3 km thickness were observed (Naga- cross sections of (a) activity of spo- sawa, private communication). radic E layer with short-vertical scale Rayleigh and Raman lidars offer extremely fluctuations of electron density in the high-resolution profiles of temperature fluctu- MLT region, (b) gravity wave activity ations in the troposphere, stratosphere and in the lower stratosphere, (c) water vapor pressure in the middle troposphere, mesosphere. These observations can offer (d) surface topography at 5–25S, and unique information regarding how gravity (e) number of data (or occultation waves generated by deep convection in the events). Note that solid and doted lines tropical troposphere propagate into the strato- represent GPS/MET and ECMWF, re- sphere and mesosphere and influence the ther- spectively (Hocke and Tsuda 2001). mal, photo-chemical, and dynamical states of July 2006 S. FUKAO 13

borne lidar measurements by von Zahn et al. (1996) that covered equatorial and tropical latitudes, preceding lidar observations of the mesosphere have been conducted at mid-latitudes or near the poles. We successfully observed sodium (Na) layers in February 2005, and found more column abundance of Na than at mid- latitudes at midnight (Nagasawa, private communication). Time series of Na layers are shown at 30-min intervals in the top panel of Fig. 10.

6.4 Ionospheric irregularities To investigate the coupling between the lower and upper atmosphere, we have continued concurrent radio and optical observations of the equatorial ionosphere and thermosphere at the EAR site since October 2002. GPS receivers, an all-sky CCD imager (ASI), and an airglow temperature photometer are now in operation. Equatorial plasma bubbles are considered to be one of the most prominent features caused by electrodynamical coupling in the equatorial ionosphere. It has been conjectured that gravity waves seed the Rayleigh-Taylor instability in the nighttime F region (Kelley et al. 1981; Hysell et al. 1990). The locations of equatorial plasma bubbles Fig. 10. Examples of temporal variations imaged by ASI at the EAR site (dip latitude: of sodium density profiles observed in 10.36 S) are well coincident with those of radar the mesopause region by the resonance backscatter simultaneously detected by EAR, scattering lidar (top), temperature pro- suggesting that the 3-m irregularities responsi- files observed in the middle atmosphere ble for the radar echo collocate with plasma by the Rayleigh lidar (middle), and bubbles (Otsuka et al. 2004a). In addition, mid- cloud and aerosol (scattering ratio) pro- latitude ASI observations have also been con- files observed in the troposphere by the ducted at Sata, Japan and Darwin, Australia, Mie lidar over Kototabang (bottom). which are located at geomagnetic conjugate Note that observation dates are not the points. same. Dotted lines represent tempera- EAR observes the spatial structure of equato- ture profiles of MSISE-90 model in the middle panel (Courtesy of C. Nagasawa, rial plasma bubbles, utilizing its rapid beam Tokyo Metropolitan University). scanning capability over a wide azimuth sector, with a temporal resolution that has been here- tofore unachievable (Fukao et al. 2003a, 2003b, these regions. A series of temperature profiles 2004). Figure 11 shows a typical example of a observed in the 30 to 80 km altitude range in tiny bubble generated in EAR’s range of vision, May 2005 are shown one hour apart in the mid- a bubble that rapidly grew with time and grad- dle panel of Fig. 10 (Nagasawa, private commu- ually matured within 20 minutes or so (Fukao nication; similar diagram is presented in Naga- et al. 2004). The spatial extent of this bubble sawa et al. 2005). was approximately 300 km in the zonal direc- Resonance scattering lidar has been widely tion and 200 km in altitude. used to measure the atmospheric state in the Most notably, EAR can detect the generation mesopause region. With the exception of ship- of plasma bubbles, as shown in Fig. 11, discrim- 14 Journal of the Meteorological Society of Japan Vol. 84A

along the geomagnetic field lines. The occurrence of the bubbles is well correlated with large-scale (about 1000 km in longitude) electron density structures in the equatorial anomaly, as has been imaged by satellite (Ogawa et al. 2005). Two-year monitoring of GPS scin- tillations at the EAR site tells that the scintilla- tions occur in the postsunset-midnight sector in equinoctial months (i.e., maxima in March and September, minima in June and December); that is a characteristic of plasma bubble occurrence over the Indonesian Archipelago (e.g., Aarons 1993). The relationship between the mechanism that seeds the plasma instability and large-scale plasma structures is still un- Fig. 11. (Right) Beam directions with known, although atmospheric gravity waves beam numbers and corresponding azi- propagating from below are thought to play a muth angles. Beam 6 is directed due role. south. Thick lines indicate the altitude Two-year observations of ASI at the EAR range where some direction within site have revealed the existence of poleward- half-power beam width achieves per- moving, quasi-periodic waves in the equatorial pendicularity with the magnetic field anomaly region (Shiokawa et al. 2006). The direction. (Left) Azimuth-altitude sector waves have east-west phase fronts, propagate maps of backscatter echo intensity toward higher latitudes repeatedly at a speed from equatorial plasma bubbles during 1933–2000 LST on October 13, 2002. of about 310 m/s with a period of 40 min, and The ordinate and abscissa are the meri- appear most frequently in June. These features dional and zonal distances from the may suggest atmospheric gravity waves propa- EAR along the Earth’s surface. The five gating from below as the source. quasi-horizontal lines in each panel in- The huge earthquake that occurred in West dicate the altitudes (100 km to 500 km) Sumatra at 0058 UT on December 26, 2004 in- at which perpendicularity is achieved. duced big disturbances in ionospheric total Panels are displayed every 3 min in electron content (1.6 to 6.9 TECU), primarily, time from top left to bottom right. to the north of the epicenter. A model calcula- tion shows that such disturbances could be caused by an anisotropic response of the iono- inating them from fossil bubbles that were gen- sphere to acoustic waves launched from the erated in the western portion of EAR’s range of earthquake (Otsuka et al. 2005). beam scan and drifted eastward over EAR. Yo- The above results are manifestations that koyama et al. (2004) show that all bubbles ap- the equatorial ionosphere and thermosphere pear at or before sunset at the altitude of the are disturbed more or less by forcing from be- apex of the geomagnetic field line connected low. In addition, as shown in Fig. 12, spatial with the observed area in magnetically quiet structures of nighttime medium-scale traveling conditions. ionospheric disturbances (MSTIDs) simultane- Geomagnetic conjugate bubbles which extend ously imaged by ASIs at Sata and Darwin ex- to an apex altitude of 1800 km over the geo- hibit excellent geomagnetic conjugacy (Otsuka magnetic equator have been observed by ASIs et al. 2004b; see also Shiokawa et al. 2005). located at Sata and Darwin (Otsuka et al. This result strongly suggests that the origin 2002; Shiokawa et al. 2004). Their detailed fea- and temporal development of MSTIDs are con- tures and temporal evolutions are quite similar trolled by electric fields transported between at the two stations, suggesting that the small- the two hemispheres via the geomagnetic field to 100-km-scale electron density structures as- lines over the equator. This sort of electrical sociated with the plasma bubbles are elongated coupling makes it difficult to discriminate dy- July 2006 S. FUKAO 15

provided new results on the vertical coupling of the equatorial atmosphere. Peculiar features of cumulus convection over Sumatra, gravity wave excitation, ENSO and ISO signals in the mesopause-homopause region, and the conjugacy of equatorial plasma bubbles between two hemispheres are especially noteworthy results. More details on these individual topics and other new areas of progress are presented in ac- companying papers in this special issue. A number of the goals of CPEA overlap, to some extent, with other major research projects. Every effort should be made to minimize duplication of research efforts by maximizing Fig. 12. 630-nm all sky airglow images of cooperation with them. MSTIDs in geographic coordinates si- Finally, the principal scientific objective of multaneously observed at (left) Sata, CPEA is to promote and coordinate interdisci- Japan and (right) Darwin, Australia. The Darwin image is mapped in the plinary research on the dynamical and electro- northern hemisphere along the geomag- dynamical processes which couple the equato- netic field line. MSTID structures ex- rial atmosphere and ionosphere, but it is also tending in the NW-SE direction are intended, through the scientific activities of matched in detail on both images (Ot- CPEA, to promote and enhance the links be- suka et al. 2004b). tween research groups in the countries of the equatorial region. Acknowledgments namical coupling with the lower atmosphere The CPEA project has been conducted from electrical coupling with the upper atmo- through collaboration between RISH, Kyoto sphere: we must be cautious in interpreting ob- University, and LAPAN since 2001. The author servational results. deeply appreciates the staff of LAPAN for their extensive collaboration. The present project is 7. Conclusions supported by Grant-in-Aid for Scientific Re- The Japanese research project CPEA is based search on Priority Area—764 of the Ministry of on a unified viewpoint which integrates lower-, Education, Culture, Sports, Science and Tech- middle- and upper-atmosphere processes and nology (MEXT) of Japan. The author thanks combines dynamical and electrodynamical ap- the principal investigators of the six study proaches that have been typically studied inde- groups of CPEA, Dr. Mamoru Yamamoto, Prof. pendently. The Indonesian Archipelago, the Toru Sato, Prof. Toshiaki Kozu, Prof. Toshitaka center of the most active cumulus convection Tsuda, Prof. Chikao Nagasawa, and Prof. Tada- over the globe, is considered as the region hiko Ogawa, and all of their co-investigators. where this coupling is most conspicuous, and He thanks Prof. Manabu D. Yamanaka and consequently is set as the focus of CPEA activ- Drs. Takeshi Horinouchi and Akinori Saito for ity. After four years of endeavor, a new obser- valuable advice and Dr. W.L. Oliver (Boston vatory has been established for equatorial University) for careful editing of the original atmosphere research with EAR as the core fa- manuscript. The operation of EAR is based cility in Kototabang, West Sumatra, Indonesia. upon an Agreement between RISH and LAPAN CPEA has already initiated observational signed on September 8, 2000. studies on various types of dynamical and electrodynamical coupling in the equatorial atmo- References sphere over the Indonesian Archipelago. The Aarons, J., 1993: The longitudinal morphology of first CPEA campaign (CPEA-I) was conducted equatorial F-layer irregularities relative to in the period March 10 to May 9, 2004 and has their occurrence. Space Sci. Rev., 63, 209–243. 16 Journal of the Meteorological Society of Japan Vol. 84A

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