Seasonal and Interannual Variations of Diurnal Cycles of Wind and ...

Journal of the Meteorological Society of Japan, Vol. 84A, pp. 171--194, 2006

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Seasonal and Interannual Variations of Diurnal Cycles of Wind and Cloud Activity Observed at Serpong, West Jawa, Indonesia

Ryuzo ARAKI Graduate School of Science and Technology, Kobe University, Kobe, Japan

Manabu D. YAMANAKA Graduate School of Science and Technology, Kobe University, Kobe, Japan Institute of Observational Research for Global Change (IORGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan

Fumie MURATA Research Institute for Humanity and Nature, Kyoto, Japan

Hiroyuki HASHIGUCHI Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan

Yuichiro OKU 1 Disaster Prevention Research Institute, Kyoto University, Uji, Japan

Tien SRIBIMAWATI, Mahally KUDSY, and Findy RENGGONO Agency for the Assessment and Application of Technology (BPPT), Jakarta, Indonesia (Manuscript received 19 November 2005, in final form 20 March 2006)

Abstract 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 Corresponding author: Ryuzo Araki, Graduate School of Science and Technology, Kobe University, 1-1, Rokkoudaimachi, Nada-ku, Kobe, 6578501, Japan. E-mail: [email protected] ( 2006, Meteorological Society of Japan

1

Present affiliation: Kaijo Sonic Corporation, Hamura, Tokyo, Japan.

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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.

1. 1.1

Introduction

Diurnal variation of wind in the Indonesian Maritime Continent The Indonesian Maritime Continent is formed by many islands surrounded by seas with the warmest surface temperature in the world. In this region, diurnal variations of wind and convective cloud activity have been mainly studied. The investigation of the diurnal variation based on wind data with high time and height resolutions provides considerable information to clarify the existence and structure of local circulation. Diurnal variation of wind in and near Jakarta (Fig. 1) has been investigated since the early 20th century. The existence of the sea-land breeze circulation and its vertical structure in the dry season have been shown. Van Bemmelen (1922) investigated the diurnal variation of wind in Batavia (now Jakarta) from May to November (dry season) using pilot balloon observation data and showed the similar features as sea-land breeze circulation. A UHF-band wind profiler called ‘‘Boundary Layer Radar (BLR)’’ was installed at Serpong near Jakarta (6.4 E, 106.7 S, 50 m above the sea level) (Fig. 1) in 1992 (Hashiguchi et al. 1995a, b). The BLR provides a Doppler velocity of the atmosphere in the planetary boundary layer with high time and height resolutions. Hashiguchi et al. (1995a, b) showed the diurnal variation of wind similar to the sea-land breeze circulation at Serpong in the dry season based on BLR data. Hadi et al. (2000, 2002) showed a prominent decrease in the surface temperature and an increase in the relative humidity at Serpong simultaneously with the appearance of a vertical structure similar to a sea breeze in the BLR observation in the dry season. They investigated climatological seasonal changes in the features of wind in the daytime based on BLR data.

1.2

Diurnal variation of convective cloud activity in the Indonesian Maritime Continent Diurnal variation of convective cloud activity is assumed to be accompanied with local circulation, which is thermally induced. Houze et al. (1981) investigated diurnal variation of cloud activity off northeastern Borneo Island (Fig. 1)

Fig. 1. (Upper) A geographical map of the Indonesian Maritime Continent. (Lower) Geographical map around the Serpong radar observatory (6.4 S, 106.7 E, 50 m above the sea level). The cross sign indicates the location of the observatory. The data were downloaded from the Internet site of the U.S. Geological Survey (http://edcdaac.usgs.gov/ gtopo30/gtopo30.asp).

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and wind near the coastline and concluded that prominent convective cloud activity from midnight to morning was accompanied with the convergence of the northeasterly monsoon and land breeze. According to statistical studies based on satellite observations (Hendon and Woodberry 1993; Nitta and Sekine 1994; Ohsawa et al. 2001; Mori et al. 2004; Sakurai et al. 2005), diurnal variation of convective cloud activity is especially prominent over islands and the sea near the island coastline. Murata et al. (2002) showed that local cloud systems were prominent along the mountain range in Sumatera Island (Fig. 1) when the prevailing westerly wind was weak, and that such cloud systems were unclear when westerly winds were strong in September and October 1998. They considered that these local cloud systems developed from the convergence of the local circulation, which was suppressed when the prevailing westerly winds were strong. Wu et al. (2003) showed diurnal variations of precipitable water (water vapor) in the dry season based on GPS and rawinsonde data in Sumatera Island, and they suggested that the maximum of the precipitable water in the late evening in the mountain range was caused by a horizontal transportation of the water vapor by the local circulation. A project of ‘Coupling Processes in the Equatorial Atmosphere (CPEA)’ has been conducted since 2001 to study dynamical and electrodynamical coupling processes in the equatorial atmosphere (Fukao 2006). One of the scientific objectives of the CPEA project is to understand the coupling process of convective clouds from the meso- to the synoptic scale. Kawashima et al. (2006) investigated the precipitation cloud system over Sumatera Island using an X-band Doppler radar, which provided the information of precipitation and wind structure, from 10 April to 9 May, 2004, in which the first campaign of the CPEA (CPEA-I) was conducted. They suggested that a deep convection cloud was suppressed in the active phase of intraseasonal variation, which is one of the dominant modes in the equatorial region for a period of 30–60 days, because the ground was covered with a large cloud system and the development of the boundary layer and local circulation was suppressed. In studies on the diurnal variation of convec-

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tive cloud activity, the importance of the local circulation to the convective cloud activity has been suggested. However, the local circulation has not been examined in detail in these studies. To understand local cloud systems, which correspond to meso-scale cloud systems, it is important to clarify the relationship between local cloud systems and local circulation. 1.3 Objective of this paper The objective of this paper is to describe the climatological features of the diurnal variation of wind at Serpong in the rainy season in detail by using BLR data observed for a long period. It is important to investigate diurnal variation of wind in the rainy season to clarify the existence and structure of local circulation when diurnal variation of convective cloud activity is prominent. Local circulation is associated with the distribution and diurnal variation of surface temperature, and it dominates when the prevailing wind is weak. This paper focuses on (i) the features of the diurnal variations of wind and surface temperature in the rainy season in contrast to those in the dry season, (ii) the features of the diurnal variations of wind under various prevailing wind conditions in the rainy season, and (iii) the relationship between the diurnal variation of wind and that of convective cloud activity. This paper consists of six sections. In Section 2, the topography around Serpong, the BLR, and data used in this paper are described. The analysis results corresponding to (i), (ii), and (iii) mentioned above are described in Sections 3, 4, and 5, respectively. The conclusions are presented in Section 6. 2. 2.1

Observations

Serpong radar observatory and surrounding topography The Serpong radar observatory (6.4 S, 106.7 E, 50 m above sea level) was constructed in October 1992 in a broad plain (Serpong area) neighboring the southwestern side of Jakarta City, as shown in the lower panel of Fig. 1. This area was originally a natural plain until about 20 years ago and had developed as an artificial science city called PUSPITEK (Indonesian National Center for Research and Technology). This city is no so large. Therefore, the influence of the urban climate, such as heat

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islands, on the observatory is small, making the location suitable for observations of natural local climate, including the diurnal variation features studied here. The observatory is about 40 km south of the Jawa Sea coastline. The direction of the Jawa Sea coastline is roughly zonal (east–west). There are no high mountains between the coastline and Serpong. The mountainous area, which includes high mountains with elevations in excess of 3,000 m, exists in the south of Serpong. The Indonesian western standard time (GMT þ 7 hours) is used in this paper. 2.2 Boundary layer radar (BLR) The BLR is a UHF (L-band, 1.3 GHz) Doppler radar and has been operated at the Serpong radar observatory since November 1992 (Hashiguchi et al. 1995a). The BLR parameters are summarized in Table 1. The time and height resolutions of the BLR are about 1 minute and 100 m, respectively. The BLR receives a back-scattered echo from the atmospheric refractive fluctuations, which are mainly generated by fluctuations of the humidity, and atmospheric stability profiles associated with the atmospheric turbulence in and on top of the mixed layer on a clear day, and provides the vertical, meridional, and zonal wind velocities in the lower troposphere and thickness of the planetary boundary layer (PBL) on clear days. Since the operating frequency of the BLR is sensitive to raindrops, it observes the echo from raindrops during precipitation. In this

study, downward velocity above 1.0 ms1 was regarded as precipitation. Since the BLR often detects strong echoes caused by birds, bats, and insects, a Gaussian function was fitted to the atmospheric Doppler spectrum to reduce the contamination (Hashiguchi et al. 1995a). However, the contamination was sometimes left, in particular, in the evening and the nighttime. We reduced the contamination with a screening procedure developed for this study, as described in the Appendix. BLR data from 1993 to 2002, except for 2000, were used to investigate the climatological seasonal change in the diurnal variation of wind at Serpong. Some periods without data due to the stoppage of the BLR were included in the analysis. Data from 2000 were not included because there was too much contamination in that year. Data were averaged every 30 minutes after the screening. For example, the data at 0000 LST represent the mean averages from 0000 to 0030 LST. This resolution is sufficient to investigate the diurnal variation of wind. We defined the ‘‘anomaly’’ by subtructing the daily mean from the observed wind at each height and investigated the features of diurnal variation of the anomaly in detail. The wind, which was actually observed, was reffered to as the ‘‘actual’’ in this paper to avoid a confusion of the anomaly with the observed wind. We used data of the days that satisfied the following conditions:

Table 1. Summary of BLR parameters at the Serpong observatory. Operating frequency

1357.5 MHz

Antenna

3 parabolic antennas

Aperture

3.1 m 2

Beamwidth

7.6

Beam direction (Azimuth, zenith)

ð0; 0Þ, ð0; 15Þ, ð90; 15Þ

Peak power

1 kW (maximum)

Height range

0.3–6.4 km

Height resolution

about 100 m

Time resolution

about 1 minute

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There is at least one piece of data from 0000 to 0530 LST and 1800 to 2330 LST, respectively, at each height. Every data interval is less than 6 hours long.

2.3 Surface meteorological observation We used surface wind (OGASAWARA, WSA54), temperature (VAISALA, HMP-133Y) and solar radiation (EKO, MS-42) data at the Serpong radar observatory to investigate the diurnal variations of surface wind, temperature, and solar radiation. The original time resolutions of these data were 2 minutes but they were averaged every 30 minutes. We used surface rainfall data (IKEDA, RT-5) at the observatory to select the daytime-clearday defined in the next subsection. The sensitivity of the rain gauge was 0.5 mm, and its time resolution was 1 minute.

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2.4 Definition of daytime-clear-day The diurnal variations of wind, PBL, surface temperature, and solar radiation are quite different on no-rain and rainy days. It would be desirable to use data from no-rain days for the comparison of diurnal variations of wind, PBL, surface temperature, and solar radiation. However, the number of no-rain days was few at the observatory in the rainy season, in particular in the evening. Therefore, the following definition of ‘‘daytime-clear-day’’ was used: 1. Definition of daytime-no-rain-day at the Serpong radar observatory The daytime-no-rain-day at the observatory is defined as a day in which there is no rainfall greater than 0.5 mm detected by the rain gauge at the observatory from 0600 to 1730 LST. 2. Definition of average solar radiation of the daytime-no-rain-day at each time The average solar radiation of the daytimeno-rain-day is obtained by averaging the solar radiation data for the daytime-no-rainday every 30 minutes from 0700 to 1630 LST. 3. Definition of daytime-clear-day The number of solar radiation data averaged every 30 min is 20 from 0700 to 1630 LST. The daytime-clear-day is defined as a day in which the number of solar radiation data, of which value is greater than half of the average solar radiation of a daytime-no-rain-day, is 10 or more from 0700 to 1630 LST. Precipitation observed at the Serpong radar observatory is accompanied with a local cloud system or large-scale cloud system. When the observatory is clear, other area may be covered with the local cloud systems, but no large-scale cloud systems always exists in the vicinity. The number of daytime-clear-days and all days used in this analysis are 224 and 510 from July to September (dry season) and 116 and 533 from December to February (rainy season), respectively. 2.5 Satellite data We used hourly data on Black Body Temperature ðTBB Þ based on Geostationaly Meteorological Satellite (GMS) infrared 1 channel (11 mm) (IR1) data from January to May in 1996–2002, June to October and December in 1995–2002,

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and November in 1994–2002 to investigate the diurnal variation of cloud convective activity. The TBB corresponds to the cloud top temperature when the ground (or sea) is covered with clouds and to the surface temperature when the sky is clear. In previous studies, threshold techniques were used to distinguish between the TBB corresponding to the cloud top temperature and that corresponding to the surface one. Hendon and Woodberry (1993) determined the threshold TBB colder than 230 K to pick up deep convective activity. Ohsawa et al. (2001) used not only TBB based on IR1 channel data but also that based on water vapor channel (6.7 mm) data to define the index of convective activity in order to obtain the good correspondence with the surface rainfall. Their index corresponds to cloud top temperatures below approximately 230 K. In this study, the rate of the number of appearances of TBB below 230 K for the number of observation days at each time was used as the index of deep convective activity. 230 K corresponds to the air temperature at around 11 km height. The spatial resolution of the index is 0.1 0:1 . 3.

Seasonal change of the diurnal variation of wind

3.1 Diurnal variation of wind in each month Jawa Island is characterized by two seasons, namely dry and rainy seasons. Hamada et al. (2002) statistically showed that the climatological rainy season was from November to April based on rainfall data in Halim (6.16 S, 106.53 E) which is about 20 km east of the Serpong radar observatory. Figure 2 shows a climatological seasonal change of diurnal variation of actual meridional wind, whose direction is roughly perpendicular to the Jawa Sea coastline, and daily-mean wind based on BLR data at the Serpong radar observatory for 9 years from 1993 to 2002 except for 2000. The daily-mean zonal wind (see Fig. 2c) is easterly from May to October (dry season) and westerly from November to April (rainy season), which is consistent with the features of the monsoon around Jawa Island (Okamoto et al. 2003). The daily-mean zonal wind is stronger in the rainy than in the dry season. The speed of an easterly is 3–4 ms1 from May to August and the speed of a westerly is larger

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Fig. 2. (a) Time-height cross sections of the 9-year-mean and monthly-mean actual meridional wind and vertical profiles of the 9-year-mean, and daily-mean (b) meridional wind and (c) zonal wind components observed by the BLR from 1993 to 2002, except for 2000. The vertical broken line and the horizontal bars in (b) and (c) indicate 0 ms1 and the standard deviations of daily-mean wind in each month for 9 years, respectively.

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than 5 ms1 from December to February. The standard deviations of daily-mean wind in each month for 9 years are larger in the rainy season. This result suggests that the amplitude of a large-scale variation with a time scale longer than 1 day is larger in the rainy season. In March and April, the daily-mean zonal wind velocity is weak, but standard deviations due to large-scale variation are large. The diurnal variation which is consistent with the sea-land breeze circulation dominates from March to October. This variation is particularly clear in August and September (dry season). Northerly and southerly are prominent in 0.4–1.2 km and 1.2–2.0 km height ranges, respectively, from 1100 to 2230 LST. This feature is consistent with the sea breeze circulation shown in previous studies (e.g., Van Bemmelen 1922). This is particularly clear from 1400 to 1800 LST. On the other hand, an opposite vertical structure is prominent from 0600 to 0900 LST. This feature is consistent with that of the land breeze circulation. The consistent vertical structure with sealand breeze circulation is unclear in the rainy season (see Nov.–Feb. in Fig. 2). In November and December, southerly dominates in the 0.4–1.5 km height range. The consistent vertical structure with the sea breeze circulation is observed around noon. In January and February, northerly dominates through the day in the 0.4–1.5 km height range. Figure 3 shows the diurnal variation of meridional wind anomaly, which is defined in Section 2 to make clearer of the diurnal component of the wind, in each month. The diurnal variation which is consistent with sea-land breeze circulation is observed not only in the dry but also in the rainy season. However, this variation is not so clear in January and February. The southerly anomaly in the higher height range, which is consistent with the return flow above the sea breeze, is very weak in the daytime. The vertical structure is consistent with that of the land breeze circulation in the early morning, but the southerly layer is thinner than that in the dry season. The peak time of the northerly anomaly in the 0.4–1.2 km height range in the daytime is earlier in the rainy than in the dry season. The peak time of the northerly anomaly at this layer becomes earlier from November to Janu-

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ary, and it is around noon in January and February. Another feature of the diurnal variation of meridional wind anomaly in the rainy season is the southerly anomaly in the 0.4–2.0 km height range from 1600 to 2130 LST (particularly, at about a height of 1.0 km from 1700 to 1800 LST). Figure 4a is the same as Fig. 2a, except for actual zonal wind, whose direction is roughly parallel to the Jawa Sea coastline. An easterly dominates through the day from May to October (dry season). On the other hand, a westerly dominates through the day from November to March (rainy season). The zonal wind velocity is larger in the rainy than in the dry season. In the dry season, the speed of the easterly is weak below the 2.0 km height in the daytime. Figure 5a is the same as Fig. 2a, except for zonal wind anomaly. A westerly anomaly is prominent in the 0.4–2.0 km height range in the daytime from March to November and the easterly anomaly is prominent in January and February. This shows that the zonal wind velocity is weak below the 2.0 km height in the daytime not only in the dry but also in the rainy season in the diurnal variation of the actual zonal wind. The diurnal variation of the zonal wind anomaly is observed in the 1.0–2.0 km height range. The easterly and westerly anomalies are prominent from 0800 to 0900 LST and 1500 to 1600 LST, respectively. They are the clearest in September and unclear in January and February. 3.2

Diurnal variations of wind anomaly, PBL, surface temperature, and solar radiation in a daytime-clear-day We compare the diurnal variation of wind anomaly with that of the top height of the mixed layer, surface temperature, and solar radiation at the Serpong radar observatory. The data in the daytime-clear-day are used for this analysis because of the reason as mentioned in Section 2. Figures 6a and d show averaged diurnal variations of a meridional wind anomaly and vertical structures of the daily-mean meridional wind only for daytime-clear-days from 1993 to 2002, except for 2000. Averaged diurnal variations of the surface (10 m above the ground) meridional wind anomaly based on an-

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Fig. 3. Same as Fig. 2, except for (a) the meridional wind anomaly from the daily-mean wind at each height.

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Fig. 4. Same as Fig. 2, except for (a) the actual zonal wind.

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Fig. 5. Same as Fig. 2, except for (a) the zonal wind anomaly from the daily-mean wind at each height.

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Fig. 6. (a) (d) Time-height cross sections of the meridional wind anomaly from the daily-mean wind at each height (contour-plotted panel) and vertical profiles of the daily-mean meridional wind. (b) (e) Time series of surface (10 m above the ground) meridional wind anomaly from the daily mean. Vertical lines indicate standard deviations. (c) (f ) The time-height cross sections of averaged echo power observed by the BLR. Averages for the daytime-clear-day from July to September ((a)–(c)) and December to February ((d)–(f )) from 1993 to 2002, except for 2000, are shown.

emometer data are shown in Figs. 6b and e. The diurnal variation which is consistent with the sea-land breeze circulation is clear below the 2.0 km height from July to September (JAS) (see Figs. 6a, b). In the daytime, the peak time of the northerly anomaly in the 0.4– 1.2 km height range is around 1700 LST, which is delayed from that of the northerly anomaly on the surface around 1600 LST during JAS. The peak time of the southerly anomaly in the 1.2–2.0 km height range in the daytime (1500–1600 LST) is close to that of the northerly anomaly on the surface during JAS. Figure 6c shows the diurnal variation of the echo power averaged for a daytime-clear-day in the dry season (JAS) on the basis of BLR data. The echoes from raindrops appearing mainly in the nighttime are removed before averaging. The diurnal variation of the echo power corre-

sponding to the mixed layer top in the dry season, as shown by a case study in Hashiguchi et al. (1995a), appears climatologically throughout the dry season. The strong echo gradually ascends from a 0.4 km height from 0900 to 1200 LST during JAS. The mixed layer top reaches a 2.5 km height at 1200 LST. In a comparison of the diurnal variation of the mixed layer and that of a meridional wind anomaly, the development of the mixed layer becomes prominent when a southerly anomaly below a 0.5 km height becomes weak. A vertical structure, which is consistent with that of sea breeze circulation, appears after the development of the mixed layer (at around 1400 LST). The echo power becomes rapidly weakened at around 1500 LST, and it is the weakest at around 1700 LST at about a 1.0 km height when the northerly anomaly in the 0.4–1.2 km

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Fig. 7.

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Same as Figs. 6a and d, except for the zonal wind anomaly.

height range is the most prominent. This change in the features of PBL is consistent with the results shown by Hadi et al. (2000), who considered that a Kelvin-Helmholtz instability (KHI) billow was induced with a strong shear layer between the sea breeze and its return flow and the echo from the layer was very weak because of the existence of erosion of the potential temperature gradient in this shear layer. From December to February (DJF) (rainy season), the features of the diurnal variation of meridional wind anomaly averaged only for daytime-clear-days (see Fig. 6d) are not significantly different from the results shown in Fig. 3 on the basis of data including those obtained during precipitation. During DJF, the vertical structure is consistent with the sea breeze circulation from 1100 to 1400 LST, in which a prominent northerly anomaly is observed near the surface, but the southerly anomaly in a 1.5–2.0 km height range is very weak. The development of a mixed layer similar to that in the dry season is observed from 0900 to 1200 LST even in the rainy season, as shown in Fig. 6f. However, the height of the strong echo (about 1.0 km) is lower than that in the dry season. The mixed layer top height should be lower than that in the dry season. This is consistent with the results shown by Sugimoto et al. (2000) on the basis of lidar data at Jakarta. The period of the development of the mixed layer is close to the peak time of the northerly anomaly. The seasonal change in the diurnal variation of the zonal wind anomaly described in the previous subsection is confirmed in Fig. 7, which is the same as Figs. 6a and d except for zonal

wind anomaly. In the comparison of the diurnal variation of the zonal wind anomaly with that of the mixed layer shown in the Figs. 6c and f, a westerly (easterly) anomaly is observed inside the mixed layer during JAS (DJF). In the comparison of the diurnal variation of zonal wind anomaly during JAS, shown in Fig. 7a, and that of meridional wind anomaly, shown in Fig. 6a, when a vertical structure consistent with that of the sea (land) breeze circulation is prominent, a westerly (easterly) anomaly is prominent between the northerly (southerly) anomaly layer (0.4–1.2 km) and southerly (northerly) anomaly layer (1.2– 2.0 km). This shows that the direction of the anomaly of wind changes anticlockwise with increasing height. Figure 8a shows diurnal variations of the temperature averaged in the daytime-clear-day on the surface. The time of maximum temperature in the rainy season (1230 LST) is earlier than that in the dry season (1300–1330 LST). The amplitude of the diurnal variation of surface temperature in the rainy season (7.7 C) is smaller than that in the dry season (11.7 C). The maximum surface temperature at the observatory is 33.4 C in the dry season and 30.9 C in the rainy season. The minimum surface temperature at the observatory is 21.7 C in the dry season and 23.2 C in the rainy season. The sea surface temperature (SST) off the Jakarta coast averaged over the 1986–1994 period based on Fig. 10 of Hadi et al. (2002) is 28.0 C during JAS and 28.8 C during DJF. Assuming that the amplitude of the diurnal variation of SST is very small (less than about 4 C), the difference of surface temperature between at the observatory and the sea off the Jakarta

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and in the rainy seasons is consistent with the difference of the peak time of the northerly anomaly below the 1.2 km height in the daytime. Figure 8b shows diurnal variations of the solar radiation averaged in the daytime-clearday on the surface. The maxima of solar radiation occur at around 1100 LST in both the dry and rainy seasons. This does not explain the difference of the peak time of the northerly anomaly between the dry and rainy seasons. 4.

Prevailing wind and diurnal variation in the rainy season

4.1 General features We considered that the variation of the dailymean wind corresponds to the wind variation prevailing over West Jawa and investigated the diurnal variation of meridional wind anomaly in each prevailing wind direction and velocity in the rainy season. From December to February, there were many days in which the daily-mean meridional ðvd Þ and zonal ðud Þ components of the wind velocities averaged over a 0.4–1.0 km height range satisfying 2 a vd < 2 ms1 and 2 a ud < 8 ms1 (Table 2). Figure 9 shows the diurnal variation of meridional wind anomaly in each class of vd shown in Table 2. The typical diurnal variation of meridional wind anomaly in the rainy season, shown in Section 3, becomes unclear as vd is negatively (northerly) stronger. The northerly anomaly is prominent from 0100 to 1300 LST and southerly one from 1330 to 0030 LST in the 0.4–1.7 km height range in the days with

Fig. 8. Time series of (a) temperature and (b) solar radiation averaged for the daytime-clear-day at the 1.5 m height above the ground at the Serpong radar observatory. Solid and broken lines indicate averages from July to September and December to February, respectively. Vertical bars are the standard deviations.

coast is larger in the dry than in the rainy season in both the daytime and the nighttime. This feature is consistent with the results that the diurnal variation in the meridional wind anomaly, which is consistent with the sea-land breeze circulation, is clearer in the dry than in the rainy season. The difference of the peak time of surface temperature between in the dry

Table 2. The number of days in each class of daily-mean meridional ðvd Þ and zonal ðud Þ components of wind averaged over 0.4–1.0 km height range during December–February during 1993–2002, except for 2000. ms1

vd < 4

4 a vd < 2

2 a vd < 0

0 a vd < 2

2 a vd < 4

4 a vd

Total

ud < 2

0

0

1

0

0

0

1

2 a ud < 0

0

2

15

2

0

0

19

0 a ud < 2

2

11

44

19

6

0

82

2 a ud < 4

10

31

43

25

7

0

116

4 a ud < 8

36

30

54

61

22

2

205

8 a ud < 12

2

3

15

37

14

2

73

12 a ud

0

2

3

11

4

0

20

Total

50

79

175

155

53

4

516

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Fig. 9. (a) Time-height cross sections of the meridional wind anomaly from the daily mean at each height, and vertical profiles of (b) meridional and (c) zonal components of daily-mean wind in each class of daily-mean meridional component of wind ðvd Þ averaged over a 0.4–1.0 km height range from December to February from 1993 to 2002, except for 2000. Each class of vd is indicated above each panel in (a).

vd satisfying vd < 4 ms1 . In this diurnal variation, a change in the wind direction is not clearly observed in the vertical structure, and the amplitude is the largest at about a 1.0 km height. The northerly anomaly peaks (2 ms1

or larger) from 0600 to 0900 LST, and the southerly one peaks from 1700 to 1900 LST. The typical diurnal variation of meridional wind anomaly is prominent in the days with small vd . We selected days with vd satisfying

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Fig. 10. Same as Fig. 9, but in the class of 2 a vd < 4 ms1 . The diurnal variation of meridional wind anomaly and daily-mean wind in each class of daily-mean zonal component of wind ðud Þ averaged over a 0.4–1.0 km height range are indicated.

2 a vd < 4 ms1 and investigated the diurnal variation in each class of ud . From December to February, the ud is westerly on most days (Table 2). Figure 10 shows the diurnal variation of meridional wind anomaly for cases satisfying 2 a vd < 4 ms1 . An enhancement of the northerly anomaly around noon in the 0.4– 1.0 km height range is observed in each class of ud . The vertical structure consistent with that of sea breeze circulation is clearer as the ud is weaker. This is consistent with the feature that the local circulation is prominent when the prevailing wind is weak. When the ud is

strong westerly, a northerly anomaly around the 1.0 km height appears from 0230 to 1330 LST, and a southerly one appears from 1400 to 0200 LST. The feature is similar to the case with the vd of a strong northerly (see Fig. 9). The feature in the days satisfying vd < 4 ms1 is confirmed in the days, of which daily mean wind velocites are satisfying both vd < 4 ms1 and 2 a ud < 4 ms1 , and both vd < 4 ms1 and 4 a ud < 8 ms1 (not shown). The feature is prominent in the days in which the prevailing wind is a strong northwesterly to westerly.

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4.2

Interannual variation of the diurnal variation of meridional wind anomaly We investigated the interannual variation of the westerly monsoon and the diurnal variation of meridional wind anomaly. According to Hamada et al. (2002), an interannual variation of the onset of the rainy season in Jawa Island is dominant, and the onset varies by a few pentads. In this study, the monthly mean of the daily-mean zonal wind is used to observe the monsoon around West Jawa. Figure 11 shows monthly mean of zonal wind over West Jawa. The periods of transitions from easterly to westerly and from westerly to easterly vary annually. The westerly monsoon from 1993 to 1994 is shorter and weaker than that from 1998 to 1999. The easterly monsoon is prominent in November and April from 1993 to 1994, but the westerly is prominent in those months from 1998 to 1999. Figures 12 and 13 show the interannual variations of meridional wind anomalies in November and April, respectively. The feature consistent with the sea-land breeze circulation is clearer in November of 1993 and 1994 and in April of 1994, 1996, and 2002, which are the easterly monsoon shown in Fig. 11. The climatological feature that the peak time of northerly anomaly in the 0.4–1.2 km height range in November and April is 1300–1500 LST, which is earlier than that during JAS and later than that during DJF (Fig. 3), dose not have a clear interannual variation. This suggests that the seasonal variation of the peak time of the northerly anomaly in the daytime does not depend on the direction of the monsoon and is consistent with the result described in the previous subsection, that is, the enhancement of a northerly below the 1.0 km height is not related with the velocity of the westerly monsoon. The southerly anomaly from the evening to the midnight in the 0.4–2.0 km height range, which is another feature of diurnal variation in the rainy season, is observed in November with the westerly monsoon, especially in 1998. Such an interannual variation is not clear in April. 5.

Diurnal variation of convective cloud activity

We compared the diurnal variation of meridional wind anomaly with that of convective cloud activity in the western part of Jawa Is-

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land. Figure 14 shows the diurnal variation of the frequency of the appearance of TBB < 230 K described in Section 2 in each month along the longitude (106.7 E) passing near the observatory. The frequency is high in the evening from November to April over the island, particularly above the mountainous area located to the south of the observatory (see Fig. 1). The frequency of the appearance of TBB < 230 K in November and March, which correspond to the beginning and the end of the rainy season, is higher than that from December to February. From December to February, the convective activity is also prominent over the Jawa Sea. It is active over the 4–5 S range in December and over the 5–6 S range in January and February. Figure 15 shows the diurnal variations of meridional wind anomaly at the observatory (averaged over the 0.4–1.0 km height range shown in Fig. 3), and the frequencies of the appearance of TBB < 230 K near the observatory and in the mountainous area (106.7 E, 6.7 S, 1450 m above the sea level). The peak time of the frequency of TBB < 230 K is from 18 to 19 LST in the mountainous area. As a reference, at the Serpong radar observatory and a mountainous site in Sumatera Island, Renggono et al. (2001) showed that deep convection clouds appeared most frequently from 15 to 16 LST and stratiform clouds were observed from 17 to 19 LST. The frequency peak of TBB < 230 K from 18 to 19 LST in this paper should be accompanied with an anvil, which spread from the top of developed cloud in the mature stage. From November to March, northerly anomalies in a 0.4–1.0 km height range at the observatory are the strongest when the increase of the frequency of TBB < 230 K begins and become weaker as the frequency increases (Figs. 15a, b and c). The meridional wind anomaly in 0.4– 1.0 km height is very weak in November (Fig. 15a) and southerly anomaly from December to March (Figs. 15b and c) at the peak time of the frequency of TBB < 230 K, corresponding to the mature stage of the deep convection cloud. These results suggest that a northerly anomaly in a 0.4–1.0 km height range is prominent when the convective cloud activity is active in the mountainous area. This is consistent with a feature in which the development of a local cloud system is accompanied with local circulation.

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Fig. 11. Monthly-mean zonal wind averaged over a 0.4–1.0 km height range from July to June (þ1 year). Dashed line indicates 0 ms1 . The month in which the number of the daily mean is more than 15 is indicated. The period from July to December 1992 and January to June 2003 is not included in the analysis period.

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Fig. 12. (a) Time-height cross sections of the monthly-mean meridional wind anomaly from the daily mean at each height and vertical profiles of the monthly-mean and daily-mean (b) meridional and (c) zonal wind components in November 1993, 1994, 1995, 1996, 1998, and 2001.

6.

Conclusions

In this paper, the climatological seasonal changes of the diurnal variations of wind below a 2.5 km height, the surface temperature, solar radiation, PBL, and convective cloud activity were described. The features of the diurnal variation of meridional wind, which is perpendicular to the coastline between Jawa Island and the Jawa Sea, under different prevailing wind conditions and the interannual variation were also investigated. We mainly used BLR data from 1993 to 2002, except for 2000, and

defined the anomaly, which means that the daily-mean wind subtracted from the observed wind, for the analysis. The results of this paper are summarized as follows: 1. In the dry season, a diurnal variation which is consistent with sea-land breeze circulation dominates in meridional wind in Serpong. Meridional wind variation at the Serpong radar observatory accompanied with the sea-land breeze circulation has been shown in previous studies. We assume that the diurnal variation shown in

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Fig. 13. Same as Fig. 12, except in April 1993, 1994, 1995, 1996, 1999, 2001, and 2002.

this paper is also accompanied with the sea-land breeze circulation. In the daytime, the peak time of a sea breeze in the 0.4– 1.2 km height range (1700 LST) is later than that of the return flow above the sea breeze and the sea breeze on the surface (1500–1600 LST). 2. In the rainy season, a diurnal variation which is consistent with sea-land breeze circulation is unclear in actual meridional wind variation. In the diurnal variation of

meridional wind anomaly, the consistent vertical structure with the sea-land breeze circulation is observed, but it is not so clear. The enhancement of a northerly anomaly, of which direction is the same as that of the sea breeze, below the 1.0 km height is prominent around noon, which is earlier than the peak time of the sea breeze in a 0.4–1.2 km height range in the dry season. 3. Another feature of the diurnal variation of meridional wind anomaly in the rainy sea-

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Fig. 14. Time-latitude cross sections of the frequency of the appearance of TBB < 230 K along the longitude (106.7 E) passing near the Serpong radar observatory in each month from 1996 to 2002 (November is from 1994, June to December, from 1995).

son is the southerly anomaly below the 2.0 km height from 1600 to 2130 LST. This feature is particularly prominent from 1700 to 1800 LST in January and February. 4. The interannual variation of the diurnal variation of meridional wind anomaly is observed in November and April, correspond-

ing to the beginning and the end of the rainy season. In a year when the easterly monsoon is prominent, the feature which is consistent with sea-land breeze circulation is relatively clear. On the other hand, it is not clear in a year when the westerly monsoon is prominent. However, the inter-

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Fig. 15. Diurnal variation of (upper) meridional wind anomaly at the Serpong radar observatory averaged over a 0.4–1.0 km height range and (lower) frequency of the appearance of TBB < 230 K in the observatory (broke line) and mountainous range (solid line) (106.7 E, 6.7 S, 1450 m above sea level). The vertical bars in the upper panel are the standard deviations. (a), (b), and (c) indicate November, from December to February, and March, respectively.

annual variation of the peak time of the northerly anomaly below 1.0 km height in the daytime is not observed. We consider that the peak time of the northerly anomaly does not depend on the monsoon variation. 5. In the diurnal variation of surface temperature, the amplitude of the diurnal variation in the rainy season is smaller than that in the dry season. The time of the maximum temperature in the rainy season is earlier than that in the dry season. Assuming that the enhancement of the northerly anomaly

below a 1.0 km height around noon in the rainy season is accompanied with local circulation that is thermally driven, these results are consistent with the features of the peak time of the northerly anomaly mentioned in item 2. 6. The fluctuation of the prevailing wind in the rainy season is larger than that in the dry season. As the prevailing wind is weaker, the consistent vertical structure with sea breeze circulation is clearer in the rainy season. This result is consistent with the feature that local circulation is promi-

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7.

8.

9.

10.


nent when the prevailing wind is weak. When the prevailing wind is active northwesterly to westerly in the rainy season, the diurnal variation of meridional wind is different from the typical one mentioned in items 2 and 3. The northerly anomaly is prominent in the morning, and the southerly one is prominent in the afternoon. The wind direction does not change in the 0.4– 2.0 km height range, and the amplitude of the diurnal variation is the largest at about a 1.0 km height. A mixed layer develops from 0900 to 1200 LST in both the dry and rainy seasons. In the dry season, it develops in the weakening of a land breeze and before the peak of a sea breeze. In the rainy season, the development of a mixed layer and prominent northerly anomaly below 1.0 km height appear almost simultaneously. Convective cloud activity is prominent from 15 to 19 LST in the rainy season (November–April) over land, especially in the mountainous area located to the south of the observatory. When convective cloud activity is strong in the mountainous area, the northerly anomaly is prominent below 1.0 km height at the observatory. This is consistent with the feature that the development of a local cloud system is accompanied with local circulation. The diurnal variation of the zonal wind anomaly (almost parallel to the coastline), which accompanied the development of mixed layer, is prominent in both the dry and rainy seasons. In the actual wind variation, the zonal wind velocity is weak in the mixed layer. Diurnal variation exists in the zonal wind over a 1.0–2.0 km height range. It is the clearest in September and the least clear in January and February. In the dry season, the easterly (westerly) anomaly is prominent from 0800 to 0900 (1500 to 1600) LST in the 1.0–2.0 km height range, which corresponds to the prominent time of the land (sea) breeze circulation and the layer between land (sea) breeze and return flow, respectively.

Items 3–10 are newly obtained in this study. Concerning the diurnal variation of the meri-

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dional wind in the rainy season, although the results of this study are based on data at only one observatory, except for TBB data, features that are consistent with those of the local circulation are observed (items 5, 6, and 8). We believe that a more detailed investigation, for example, analyses of observational data at plural locations and numerical calculation data, is needed to clarify the existence of local circulation, its structure, and the relationship between the local circulation and convective cloud activity. We are now planning an intense observation campaign (similar to those carried out in the CPEA project in Sumatera) by using meteorological radars in coming years. Acknowledgements We thank our colleagues at Kobe University, RISH of Kyoto University, and the Institute of Observational Research for Global Change, JAMSTEC for their participation in helpful discussions. We also thank Professors Shoichiro Fukao and Toshitaka Tsuda with RISH of Kyoto University and all operators who maintain the Serpong radar observatory and the colleagues of BPPT. We are grateful to the editor (Dr. Shuichi Mori) and two anonymous reviewers for their positive comments. GMS data from 1995 to 2002 were provided by the Japan Meteorological Agency (JMA) through the Internet site of Kochi University Weather Home (http://weather.is.kochi-u.ac.jp/), and the Institute of Industrial Science, University of Tokyo (http://www.tkl.iis.u-tokyo.ac.jp/SatIAN/), except for the data of November 1994, through the Disaster Prevention Research Institute, Kyoto University. Appendix Procedure of contamination screening BLR data at Serpong has the following features of the contaminations (Hashiguchi et al. 1995a; Hadi et al. 2000):

Although the development of a mixed layer by thermal convection should be suppressed in the evening and night, the observed echo power is as strong as (or stronger than) that from the refractive fluctuations in the top of the mixed layer in the daytime. Doppler velocity discontinuously varies in time and height.

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We developed a procedure of contamination screening in consideration of these features. The processes are shown as follows: 1. The monthly mean and standard deviation of the echo power are calculated monthly. Echoes from raindrops are omitted from the calculation. When the echo power at a given height and time is not from the raindrops and is more (less) than the monthly mean plus (minus) twice the standard deviation, data is regarded as contamination, and eliminated. When vertical velocity data is removed in this process, horizontal velocity data at the height and the time is also removed. 2. In Process 1, the contamination echo is not reduced if its power is roughly equal to that of the strong backscattered echo from the atmospheric strong refractive fluctuation at the top of the mixed layer. After Process 1, we calculate a 121 minute running mean and standard deviation by using data at each height. When the standard deviation of the vertical velocity from the running mean is 0.5 ms1 or more and the vertical velocity at the center of the period of the running mean is more (less) than the running mean plus (minus) twice the standard deviation, the vertical and horizontal velocity data at the time are removed. Data during precipitation is not used in this process. 3. The process is same as that in Process 2 for horizontal velocity data, but the data during precipitation is included. Furthermore, the comparison between data and the running mean is performed when the standard deviation of the running mean is 2.5 ms1 or more. 4. In the night, the effect of the contamination on the 121-minute running mean is sometimes large because the echoes from the contaminations are often observed through 121 minutes. The Processes are the same as Processes 2 and 3, but the period of the running mean is 1441 minutes, are done after Process 3. The data during precipitation are not used in this process. When the standard deviation is 0.5 ms1 (2.0 ms1 ) or more in vertical (horizontal) velocity, the data at the time are removed if the velocity at the time is more (less) than the running mean plus (minus) twice the standard deviation.

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