Dry Intrusion observed at Sumatera Island
Fumie Murata,
Research Institute for Humanity and Nature, Kyoto 602-0878, Japan
Manabu D. Yamanaka,
Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan
Hiroyuki Hashiguchi,
Research Institute of Sustainable and Humanosphere, Kyoto University, Uji 611-0011, Japan
Tien Sribimawati and Mahally Kudsy,
Agency for the Assessment and Application of Technology (BPPT), Jakarta Pusat, Indonesia

e-mail: murataf@chikyu.ac.jp

vective activities were strongly suppressed during
the periods during dry intrusions. These dry events
are sometimes expressed as drought periods (Brownand Zhang, 1997). Short-wave radiation into theground increases under dry air events. Inversion
layers are often observed at the lower part of drylayers. Mapes and Zuidema(1995) suggested thatthe inversion layers are formed by radiative heating of longwave radiation. It implies that budget ofenergy and water cycle must be different between
dry air events and other conditions (Parsons et al.,2000).
Yoneyama (2003) analyzed rawinsonde observations based on nine-years research vessel journeys
in the western Pacific since 1993, and reported
many dry events. Yatagai and Sumi(1997) utilized
NASA Water Vapor Project data and ECMWF
data during 1988–1994, and reported some horizontal structures similar to dry intrusions observed
during TOGA-COARE in the Pacific Ocean region
and the Indian Ocean region. However, there arestill few reports for dry intrusions except for thewestern Pacific. In this paper, dry intrusion observed by intensive rawinsonde observation in Sumatera Island is described.

Abstract
A dry intrusion observed at 2–4 km MSL altitude at
Kototabang (100.32◦ E, 0.20◦ S, 865m MSL), West
Sumatera, Indonesia during 6–7 Oct., 1998, is decsribed. The horizontal distribution of total precipitable water showed that the low precipitable waterwith narrow band was intruded from Central Indian Ocean to Sumatera Island. A back trajectory
analysis supported that the dry air intruded fromthe central Indian Ocean. The different featuresfrom the dry intrusion in the western Pacific is twopoints: 1) the dry layer is ascent with time, and 2)
the origin is not traced back to subtropical region
but equatorial western–central Indian Ocean.
1. Introduction
During Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment(TOGA-COARE) intensive observation period(Nov. 1992–Feb. 1993) extremely dry layers weresometimes observed in the lower–middle troposphere of equatorial western Pacific, where it had
been thought to be always wet (Parsons et al.,1994). The air was incredibly too dry not to be
explained by diabatic vertical motion. They werealso found that the dry air event was visualized
by distribution of total precipitable water, derived
from special sensor microwave/imager (SSM/I). It
was concluded that the extremely dry air intruded
by lateral advection (e.g., Numaguti et al., 1995;
Yoneyama and Fujitani, 1995; Mapes and Zuidema,
1996). These dry air events have been termed “dryintrusions”.
In general, convections in the warm pool region
from the Indonesian maritime continent to equatorial western Pacific are the most active in the world,
and they drive global-scale circulation. But, con-

2. Data Description
An intensive rawinsonde observation was conducted
for comparison with a new boundary layer radar
during 00UTC 29 Sep–06 UTC 7 Oct, 1998 (Renggono et al., 2001; Murata et al., 2002) at Kototabang, West Sumatera, Indonesia (100.32◦ E,0.20◦ S, 865m MSL)(Fig. 1).
The observation
was conducted 4 times / day (0000,0600,1200,1800
UTC, LST = UTC + 7 hours). 4-times per
daily NCEP/NCAR reanalysis data (Kalnay et al.
1996) were utilized to obtain horizontal structureof dynamical condition, and to do back trajec1

tory analysis. The 0.05x0.05 GMS-5 black bodytemperature(TBB ) data were provided by Kochi
University. The total precipitable water (TPW)
was obtained from the dataset provided by JAXA
retrieved from Tropical Rainfall Measurement Mission Microwave Imager (TRMM/TMI).

Figure 2: Longitude–time cross-section of GMS
TBB (left), 700 hPa zonal wind anomaly (center)and 700 hPa geopotential height anomaly (right)
from 1 Sep.–31 Oct. 1998 average.
4. Observational results
Figure 3 is a vertical profile of temperature, andspecific humidity on Oct. 6 of 18UTC (Oct. 7 of01LST), 1998. The vertical axis is a height from theground (865m MSL). There is a extremely dry layer
around 2 km height (around 3 km above MSL).
The weak inversion layers appear at the bottom andthe upper dry layer (1.5 and 2.5 km above groundlevel). Figure 4 is time-height cross-sections of relative humidity and specific humidity. An extremely
air with less than 60–70 % of relative humidity andless than 6–8 g/kg specific humidity appears at thealtitude of 2–4 km MSL on Oct 6–7. The dry layer
is ascent with time by about 500 m/day.
Figure 5(a) is the horizontal distribution of TPW
retrieved from TRMM/TMI, and Fig.5(b) is distributions of NCEP 700 hPa geopotential height andhorizontal wind. The difference of the TPW withFig. 1 is the narrow band of low TPW along theequator between 80–100◦ E. The dynamical structure (Fig. 3b) at the level of dry layer represents
that the almost symmetric two depressions to theequator located in the central Indian Ocean, andstrong westerly blows between the depressions.

Figure 1: Horizontal distribution of total precipitable water (unit is mm) and NCEP 1000 hPa wind(unit is m/s) averaged for 29 Sep.–7Oct., 1998. The
location of observation site is shown as a smallsquare.

3. Synoptic condition
The average map of TPW and NCEP 1000 hPa
wind during the campaign period is shown in Fig. 1.
The period corresponded to the end of northernsummer monsoon, but monsoon westerly still appear between 0–10◦ N. The amount of TPW is muchdifferent between south-western part and northeastern part. Abundant moisture more than 50 mm
of precipitable water exist over Bengal Bay, off thewest coast of Sumatera and the western Pacific.
Figure 2 shows time-height cross-sections of GMSTBB , NCEP 700 hPa geopotential height anomaly
and 700 hPa zonal wind anomaly during Sep. 1–Oct. 31, 1998. Active convections correspond toMJO active phase are present in 70–100◦ E during
10–25 Sep. The organized cloud system propagatedeastward, but it was unclear in the maritime continent. The strong westerly (westerly burst) begins
to occur in the lower troposphere of equatorial eastern Indian Ocean on around 15 Sep. The westerly
varies with quasi-10-day periodicity in the westerly
burst. Low pressures correspond to strong westerlies, and each pressure anomaly seems to propagate
westward. A dry intrusion described in this paper
is a part of this strong westerly periods during 6–7Oct., 1998.

5. Discussion
The dry intrusion observed at Sumatera Island
is compared with that observed during TOGACOARE IOP. The dry layer is observed at theheight of 3 km MSL with around 1 km thickness,
and an inversion layer appears at the bottom of thedry layer. These thermodynamic structure is wellsimilar to that observed during dry air events in thewestern Pacific. Numaguti et al.(1995) and Mapes2

Figure 3: Vertical profiles of temperature and specific humidity on Oct 6 of 18UTC (Oct 7 of 01LST),
1998

Figure 5: Horizontal distributions of total precipitable water and 700 hPa geopotential height andwind on Oct 7, 1998
equatorial convective disturbance (e.g., KemballCook and Wang, 2001) concerning to Madden Julian Oscillation (Madden and Julian, 1994) by
the Rossby-wave response (Matsuno, 1966; Gill,1980). Compared two figures (Figs. 3a and 3b),
it looks that the dry air was intruded from the central Indian Ocean to Sumatera by strong westerlyby Rossby-wave disturbance.
To confirm the origin of the dry air observed at
Kototabang, back trajectory analysis (Fig. 4) isconducted. Each trajectories are calculated from(100◦ E, 0◦ ) at 700 hPa, and the start times are
00UTC on 3 Oct (triangle), 00UTC on 4 Oct(square), 00UTC on 5 Oct (cross), 00UTC on 6 Oct(circle), and 00UTC 7 on Oct (plus). The trajectories of 3–4 Oct came from near sea level of south–
southeastern part of Sumatera. The trajectoriesof 5–7 Oct was almost along 5◦ N and came fromabove 700 hPa level of the eastern Indian Ocean bystrong westerlies. It indicates that the origin of 700
hPa level air above Kototanbang change on 5 Oct.
The results of calculated back trajectories and themap of precipitable water during TOGA-COARE
showed that the dry airs intruded from subtropics.
In contrast, dry air above Kototabang comes fromthe equatorial central Indian Ocean. The precipitable water in the Indian Ocean is relatively low
in the western side and high in the eastern side.

Figure 4: Time-height cross-section of (a)relative
humidity and (b) specific humidity. The period is
29 September–07 October, 1998.
and Zuidema(1996) considered the effect of the inversion layers at the bottom of the dry layer as acause of convectively suppressed condition.
The dry layer in the western Pacific tended to
be descent with time. It was considered a result ofatmospheric subsidence by radiative cooling. The
dry layer observed at Sumatera is ascent with time.
It is a different point from that observed in thewestern Pacific.
The distribution of total precipitable water onOct. 7, 1998 indicates a narrow band of low precipitable water with several thousands km on longitude scale and several hundreds km on latitude
scale along the equator, and it extends from central
Indian Ocean to Sumatera Island. The horizontal
scale of this dry intrusion is almost same as thatobserved in the equatorial western Pacific during
TOGA-COARE.
This disturbance can be regarded as a part of theequatorial Rossby-wave disturbance emenated from3

Therefore, it is considered that mid troposheric relatively dry air above the eastern Indian Ocean advected into Sumatera Island by strong westerlies.

Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins,
D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White,
J. Woolen, Y. Zhu, M. Chelliah, W.Ebisuzaki, W. Higgins, J. Janowiak, K. C. Mo, C. Ropelewski, J. Wang,
A. Leetmaa, R. Reynolds, R. Jenne, and D. Joseph,1996: The NCEP/NCAR 40-year reanalysis project.
Bull. Amer. Meteor. Soc., 77, 437–471.
Kemball-Cook, S. and B. Wang, 2001: Equatorial waves
and air-sea interaction in the boreal summer intraseasonal oscillation. J. Climate, 14, 2923–2942.
Madden, R. A. and P. R. Julian, 1994: Observations of the40–50-Day tropical oscillation – a review. Mon. Wea.
Rev., 122, 814–837.
Mapes, B. E. and P. Zuidema, 1996: Radiative-dynamicalconsequences of dry tongues in the tropical troposphere. J. Atmos. Sci., 53, 620–638.
Matsuno T., 1966: Quasi-geostrophic motions in the equatorial area. J. Meteor. Soc. Japan, 44, 25–43.
Murata, F., M. D. Yamanaka, M. Fujiwara, S. -Y. Ogino,
H. Hashiguchi, S. Fukao, M. Kudsy, T. Sribimawati,
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UHF radar, GPS rawinsondes, and surface meteorological instruments at Kototabang, west Sumatera during September–October 1998. J. Meteor. Soc. Japan,
80(3), 347–360.

Figure 6: Results of a back trajectory analysis for(a) longitude–latitude and (b) time–pressure crosssections, calculated from 100.◦ E, 0◦ N, 700 hPa.
Start times of each line are 00UTC of 3 Oct (triangle), 4 Oct (square), 5 Oct (cross), 6 Oct (circle)
and 7 Oct (plus).

Numaguti, A., R. Oki, K. Nakamura, K. Tsuboki, N. Misawa, T. Asai, and Y.-M. Kodama, 1995: 4–5-day-periodvariation and low-level dry air observed in the equatorial western Pacific during the TOGA-COARE IOP. J.
Meteor. Soc. Japan, 73, 267–290.

6. Summary

Parsons, D., W. Dabberdt, H. Cole, T. Hock, C. Martin, A,L. Barrett, E. Miller, M. Spowart, M. Howard, W. Ecklund, D. Carter, K, Gage and J. Wilson, 1994: The integrated sounding system: description and preliminary
observations from TOGA COARE. Bull. Am. Meteorol. Soc., 75, 553–567.

An extremely dry air was observed in 2–4 km MSL
altitude at Kototabang (100.32◦ E, 0.20◦ S, 865m
MSL), West Sumatera, Indonesia during 6–7 Oct,
1998. It occurred after the passage of active convections of MJO, and in the folloing westerly burst.
The horizontal distribution of TPW showed thatthe low precipitable water with narrow band wasintruded from the Central Indian Ocean to Sumatera Island. The strong westerly was caused between
nearly symmetric two depressions to the equator. Aback trajectory analysis supported that the dry airintruded from the central Indian Ocean. The different features from the dry intrusion in the western
Pacific is two points: 1) the dry layer is ascent withtime, and 2) the origin is traced back to equatorial
western–central Indian Ocean.

Parsons, D. B., K. Yoneyama and J.-L. Redelsperger,
2000: The evolution of the tropical western Pacificatmosphere–ocean system following the arrival of a dry
intrusion. Q.J.R.Meteorol.Soc., 126, 517–548.
Renggono, F., H. Hashiguchi, S. Fukao, M. D. Yamanaka,
S. -Y. Ogino, N. Okamoto, F. Murata, B. P. Sitorus,M. Kudsy, M. Kartasasmita, and G. Ibrahim, 2001:
Precipitating clouds observed by 1.3-GHz boundarylayer radars in equatorial Indonesia. Annales Geophysicae, 19, 889–897.
Yatagai, A. and A. Sumi, 1997: Statistical features of dry
air intrusions into the equatorial ocean. Proceedings ofGEWEX COARE 98 Conference, 161–162.
Yoneyama, K. and T. Fujitani, 1995: The behavior of dry
westerly air associated with convection observed during the TOGA COARE R/V Natsushima cruise. J.
Meteor. Soc. Japan, 73, 291–304.

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