to nighttime is observed on days with nighttime rain. ... Jun Matsumoto1,2, and Fujio Kimura3,4 ... observed on 76 days, i.e., on about 2 out of 3 days in this.
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Diurnal Variation of Rainfall and Precipitable Water over Siberut Island off the Western Coast of Sumatra Island Peiming Wu1, Shuichi Mori1, Jun-Ichi Hamada1, Manabu D. Yamanaka1, Jun Matsumoto1,2, and Fujio Kimura3,4 1 Institute of Observational Research for Global Change, JAMSTEC, Yokosuka, Japan 2 Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, Hachioji, Japan 3 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan 4 Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan
Abstract Surface meteorological observations indicate that the diurnal cycle of rainfall over Siberut Island is characterized by dual maxima in the afternoon and after midnight, with the minima in the early morning and evening. The GPS-derived precipitable water (PW) over the island shows similar dual maxima in the afternoon and near midnight. The incident solar radiation during the morning hours is still sufficiently intense to generate local circulations on the island and to induce large diurnal variation of PW in the afternoon on days with daytime rain and days with nighttime rain. Meanwhile, a great increase of PW during the evening to nighttime is observed on days with nighttime rain. The results suggest a possibility that the increase of moisture during nighttime over Siberut Island, which is probably caused by the transport of water vapor from Sumatra Island, play a major role in formation of the nighttime rainfall over the island.
1. Introduction Diurnal cycle is one of the most significant variations of rainfall and convective activity in the tropics. The diurnal variation in convection over the Maritime Continent region was examined through analysis of infrared (IR) observations from Geostationary Meteorological Satellite (GMS) (e.g., Murakami 1983; Nitta and Sekine 1994; Ohsawa et al. 2001). Convection over large islands exhibits a late afternoon and evening maximum. Over sea areas in the vicinity of large islands, maximum convective activity generally occurs in the morning. Over coastal seas bordering land areas, thermallyand topographically-induced local circulation may have an important effect on convective activity. Houze et al. (1981) observed an extremely regular genesis of nocturnal convection near the coast in northwestern Borneo and propagating offshore in the morning in an analysis of radar observations during the Winter Monsoon Experiment (WMONEX). Land breeze interaction with the prevailing winter monsoon flow was documented as producing the offshore convection at night and early morning off the northern coasts of Borneo Island. Recently, Sakurai et al. (2005) investigated the diurnal cycle of convective system migration over Sumatra Island through analysis of infrared (IR) observations from GMS Satellite. Their results showed that the convective systems over Sumatra Island migrate westward during nighttime throughout the year, while they migCorresponding author: Peiming Wu, Japan Agency for MarineEarth Science and Technology, Natsushima-cho 2-15, Yokosuka-City, Kanagawa 237-0061, Japan. E-mail: pmwu@jamstec. go.jp. ©2008, the Meteorological Society of Japan.
Fig. 1. The topography of Siberut Island, Sumatra Island and location of Muarasiberut (cross sign), where surface observations were conducted. The shades of gray denote terrain elevation.
rate eastward only when the Intertropical Convergence Zone (ITCZ) is located over the island. Water vapor is essential for convection and rainfall. Knowledge of the distribution and temporal variation of atmospheric water vapor is therefore important for understanding of the diurnal cycle of convection and rainfall. However, because the lack of upper level sounding data, prior investigations of convection and rainfall in the Maritime Continent region were focused mainly on the diurnal cycle of convection at different locations with infrared (IR) observations from the GMS satellite. The diurnal variation of moisture in the region and its relation to precipitation still remain unclear. To obtain continuous moisture data in the region, we set up a GPS (Global Positioning System) receiving station and performed surface meteorological observations on Siberut Island. In the present paper, the diurnal variation of moisture over the island is examined by analyzing the GPS-derived precipitable water and surface meteorological observation data during July to October of 2003. The diurnal cycle of moisture and its relation to the occurrence of rainfall over the island will be discussed.
2. Atmospheric water vapor measurements with GPS and surface meteorological observations over Siberut Island Siberut Island is located in the Indian Ocean, about 150 kilometers from the western coast of Sumatra Island (Fig. 1). The size of the island is approximately 100 kilometers long by 40 kilometers wide. A GPS receiving station was set up in June 2003 on Siberut
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Island at Muarasiberut (1.582S, 99.200E), which is located near the seashore at the southeastern coast of the island. GPS is a satellite-based positioning and navigation system. By using precise GPS satellite orbits, accurate station positions and clocks, we can estimate accurately the precipitable water, that is, total atmospheric water vapor above the surface with a time resolution of 5 minutes (e.g., Duan et al. 1996). Simultaneously, surface meteorological variables, including pressure, temperature, relative humidity, wind direction and speed, rainfall and global solar radiation were measured with a time interval of 60 minutes by an Automatic Weather Station (Vaisala, MAWS 201).
Fig. 2. The time series of daily mean GPS-derived precipitable water (line) and daily total rainfall (bar) observed at Muarasiberut on Siberut Island for 1 July to 31 October 2003.
3. Results and discussion As Siberut Island is located near the equator, air temperature varies little throughout a year, and rainfall occurs perennially in the region. We performed observations during July to October 2003, within the season of relatively low rainfall and the transition period from the dry season to rainy season in the region over Sumatra Island (Wu et al. 2003). Figure 2 shows the time series of daily mean GPSderived precipitable water (PW) and daily total rainfall observed at Muarasiberut on Siberut Island during the period from 1 July to 31 October 2003. Rainfall is observed on 76 days, i.e., on about 2 out of 3 days in this period. Meanwhile, daily mean PW greater than 55 mm is observed on most days, although values of less than 50 mm were recorded occasionally. The four-monthly mean diurnal variations of the GPS-derived precipitable water and rainfall frequency observed at Muarasiberut on Siberut Island for 1 July to 31 October 2003 are shown in Fig. 3. Rainfall occurs through all hours of the day. However, it is obvious that the occurrence of rainfall events is characterized by dual maxima in the afternoon and after midnight, with the minima in the morning and evening. Similarly, the GPS-derived precipitable water exhibits a primary maximum in the afternoon and a secondary maximum near midnight in the diurnal variation of moisture over the island. The average increment of precipitable water during the period from 0700 LT in the morning to 1400 LT in the afternoon is 3.2 mm, and 1.5 mm during the period from 2000 LT in the evening to 0100 LT at night. To examine the diurnal variation of moisture and its relation to precipitation, the four-monthly mean diurnal variations of mean rainfall rate, GPS-derived precipitable water, surface specific humidity, wind roses and global solar radiation for days categorized by daytime rainy days, nighttime rainy days, and days without rain during the period from 1 July to 31 October 2003 are shown in Figs. 4a d, respectively. From Fig. 4a, it is evident that on days with nighttime maximum rainfall, a daytime maximum rainfall also occurs on the same days, the reverse situation is also true. Generally, the GPS-derived PW over the island for days with daytime rain and days with nighttime rain is high compared with that for days without rain (Fig. 4b). It is noteworthy that the diurnal patterns of the GPSderived PW in all of the three categories exhibit similar dual maxima in the afternoon and near midnight, with the minima in the early morning and evening. The average increments in precipitable water during the morning hours for days with daytime rain and days with nighttime rain are almost the same as that for days without rain. Meanwhile, a great increase of PW during the evening to nighttime occurs for days with nighttime rain.
Fig. 3. The four-monthly mean diurnal variations of the GPSderived precipitable water (line) and rainfall frequency (bar) observed at Muarasiberut on Siberut Island for 1 July to 31 October 2003. The rainfall frequency is the number of hours in which the amount of rainfall exceeded 1.0 mm.
The surface specific humidity exhibits a similar diurnal pattern in all of the three categories (Fig. 4c). It increases abruptly from about 0700 to 0900 LT in the morning. The increase in surface specific humidity occurs before the penetration of sea breeze, since the penetration of sea breeze at the observation site takes place after 0900 LT (Fig. 4c). The starting time of the increase is almost the same as that of the increase in surface air temperature (not shown). The increase in surface specific humidity is therefore attributable to solar heating of the ground, which causes evaporation of water from the surface. Except for the abrupt increase in the early morning, the surface specific humidity exhibits gradual changes in the other times of the day for days with daytime rain, days with nighttime and days without rain, whereas the GPS-derived PW shows apparent dual maxima in the diurnal patterns, which mean that the diurnal variations of PW over the island are caused mostly by variations of moisture at the layers above the surface. The mean global solar radiation exhibits a single maximum near local noon (Fig. 4d). Although it is lower than on days without rain, large global solar radiation is also observed on days with daytime rain and days with nighttime rain. The daily maxima of the global solar radiation for days with daytime rain, days with nighttime rain and days without rain are 570, 578 and 673 W m 2 , respectively. As a result, surface air temperature increases rapidly from 23.0°C to about 29.0°C during the period from 0600 LT to 1100 LT (not shown). Then, the air temperature decreases in the afternoon, with a daily maximum at 1100 LT. Even on days with daytime rain, the average increment of surface air temperature in the
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Fig. 4. The mean diurnal variations of a) mean rainfall rate, b) GPS-derived precipitable water, c) surface specific humidity, wind roses for every three-hour intervals and d) global solar radiation for days with daytime rain (defined as 0600 1800 LT rainfall exceeded 1.0 mm; 53 days), days with nighttime rain (as 1800 0600 LT rainfall exceeded 1.0 mm; 44 days), and days without rain (47 days) during the period from 1 July to 31 October 2003.
morning still reaches up to 5.0°C. The increments in PW during the period from 0700 to 1200 LT in the morning for days with daytime rain and days with nighttime rain are almost the same as that for days without rain (Fig. 4b). The results suggest that even on days with daytime rain the incident solar radiation during the morning hours is still sufficiently intense to generate local circulations and to induce large diurnal variation of PW over the island in the afternoon. The surface winds are weak in the observation period, as more than 40% of the time during 1900 to 2100 LT the winds are calm (not shown). During daytime diurnal heating associated with the strong solar irradiance causes a sea breeze circulation over the island. The four-monthly wind roses show a pronounced diurnal change in the surface winds at the observation site (Fig. 4c). The primary wind directions during night and early morning hours (1800 0900 LT) are the southwest, blowing from the island to the sea, while the primary wind directions for 1200 and 1500 LT are the northeast, blowing from the sea to the island. Consequently, a wind convergence is induced over the island, which in turn causes an enhancement of moisture, as shown by the increase of the GPS-derived PW in the morning and early afternoon. Meanwhile, the time of occurrence of the afternoon rainfall over the island is during 1100 to 1800 LT, which differs from that of the penetration of sea breezes at the observation site. The occurrence of the afternoon maximum rainfall coincides with the increase of PW (Figs. 3, 4a and b). Therefore, the frequent occurrences of afternoon rainfall over the island is considered to result from an increase of convective instability that caused by the enhancement
of moisture and development of the mixed layer. Siberut Island is located near the equator in the tropics. If the diurnal variations in moisture and rainfall over the island were caused by thermally induced local circulation alone, single maximum in the afternoon and minimum in the early morning in PW and rainfall would be expected, as observed on Sumatra Island, the neighboring island of Siberut Island (Wu et al. 2003). However, the GPS-derived precipitable water and rainfall from the surface observations in the present study show dual maxima in the afternoon and near midnight in the diurnal cycle of moisture and rainfall over the island (Figs. 3 and 4). Mori et al. (2004) examined the diurnal land-sea rainfall peak migration over Sumatra Island by using precipitation data from the TRMM satellite. Their results showed that a rainfall peak migrates from the southwestern coastline toward an offshore region of the Indian Ocean during 0000 1200 LT. In the present study, the results from field observations on Siberut Island show a late night maximum rainfall over the island, which is probably caused by the westward migration of precipitating systems from the western coast of Sumatra Island. Figure 5 shows the mean diurnal variations of the GPS-derived precipitable water and rainfall rate for days with heavy nighttime rain (rainfall exceeded 10.0 mm; 16 days) during the period from 1 July to 31 October 2003. Rainfall occurs beginning in the evening at about 2200 LT, with heavy rain during midnight to 0300 LT. At the same time, a rapid increase in the GPSderived PW occurs during the period from 1900 LT to 0100 LT. The great increase in PW during nighttime
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Fig. 5. The mean diurnal variations of the GPS-derived precipitable water (line) and rainfall rate (bar) for days with heavy nighttime rain (defined as 1800 0600 LT rainfall exceeded 10.0 mm; 16 days) during the period from 1 July to 31 October 2003.
island. The average increments in precipitable water during the morning hours for days with daytime rain and days with nighttime rain are almost the same as that for days without rain. Large global solar radiation is also observed even on days with daytime rain. The incident solar radiation during the morning hours is still sufficiently intense to generate local circulations on the island and to induce large diurnal variation of PW in the afternoon on days with daytime rain and days with nighttime rain. Meanwhile, a great increase of PW during nighttime is observed on days with nighttime rain, which is probably caused by the migration of precipitating systems from the western coast of Sumatra Island. The results also suggest a possibility that the increase in moisture during nighttime over Siberut Island, which is probably caused by the transport of water vapor from Sumatra Island, play a major role in formation of the nighttime rainfall over the island.
Acknowledgments typically observed on days with nighttime rain coincides with the occurrence of rainfall, which is probably caused by the migration of precipitating systems from the western coast of Sumatra Island. As shown previously in Fig. 4b, the GPS-derived PW over the island exhibits a secondary maximum near midnight, even on days without rain. Sasaki et al. (2004) investigated the moisture transport above the mixed layer around the mountains in western Sumatra Island by radio soundings and numerical experiments. They noted that ambient winds above the mountaintop advect the moist air that formed over the mountain ranges during daytime by convergence of thermallyinduced upslope winds, causing an increase of moisture at the lee-side terrain. As during the observation period the prevailing upper winds in the region are an easterly, the moisture transport mechanism provides a possible explanation for the variations of PW observed on Siberut Island, that is, the evening and nighttime increase of moisture over Siberut Island is probably caused by the transport of water vapor from Sumatra Island by upper easterly winds. Furthermore, Sato and Kimura (2003) investigated the diurnal cycle of precipitation by using a two dimensional model. They showed a moderate relationship between precipitation and convective instability, and noted that the moisture transport plays an important role in the diurnal cycle of rainfall. Therefore, we infer that the increase of moisture observed over Siberut Island during the evening to nighttime, which is probably caused by the transport of water vapor from Sumatra Island, is one of the major causes of the frequent occurrences of the late night rainfall over the island. To elucidate the transport process of water vapor from Sumatra Island and its role in the formation of the nighttime rainfall over Siberut Island, further study with numerical model is necessary.
4. Summary In this paper, the diurnal variations of rainfall and moisture were investigated at Muarasiberut on Siberut Island through analysis of the surface meteorological observation data and GPS-derived precipitable water (PW) during July to October 2003. The diurnal cycle of rainfall over the island is characterized by dual maxima in the afternoon and after midnight, with the minima in the early morning and evening. The GPS-derived PW shows similar dual maxima in the afternoon and near midnight in the diurnal variation of moisture over the
We wish to thank Professor T. Yasunari of Nagoya University, Japan and the anonymous reviewers for the many helpful comments. The observations of the present study were carried out at the office site of the National Park of Siberut, with the cooperation of the Agency for the Assessment and Application of Technology (BPPT), Indonesia and the Meteorological and Geophysical Agency of Indonesia (BMG).
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