DRY INTRUSION FROM THE INDIAN OCEAN OBSERVED AT SUMATERA ISLAND ON OCTOBER 6-7, 1998

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1 DRY INTRUSION FROM THE INDIAN OCEAN OBSERVED AT SUMATERA ISLAND ON OCTOBER 6-7, 1998 FUMIE MURATA Research Institute for Humanity and Nature, 335 Takashima-cho, Kamigyo, Kyoto, , Japan MANABU D. YAMANAKA Gradiuate School of Science and Technology, Kobe University, Nada, Kobe, , Japan HIROYUKI HASHIGUCHI Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto, , Japan, MAHALLY KUDSY, TIEN SRIBIMAWATI Agency for the Assessment and Application of Technology, Jakarta Pusat, Indonesia, A dry layer was observed in 2-4 km above mean surface level (MSL) at Sumatera Island during Oct 6-7, Total precipitable water (TPW) distributions obtained by TRMM/TMI implied that low TPW horizontally intruded from the central Indian Ocean to Sumatera Island in a short-time scale (around a few days). This phenomena is similar to dry intrusion event observed in the equatorial western Pacific during TOGA-COARE. We have also observed some another cases of similar phenomena in this region. It implies that daily variability of water vapor distributions is generally important for convective activities in tropical region INTRODUCTION Understandings of variability of convective activities are important to improve predictability of rainfall and to develop management of water resources. Water vapor are crucial for development of convection. We can now obtain total precipitable water (TPW) distributions by SSM/I or TRMM/TMI on satellites with high time resolution (around 1 day). Vertical distributions of water vapor are able to be derived by rawinsonde observations with high spatial resolution (around 100 m). Variability of water vapor distributions in short time-scale may be important for the tropical convective activities. During the intensive observation period(iop) of he Tropical Ocean and Global Atmosphere Program Coupled Ocean--Atmosphere Response Experiment(TOGA-COARE) in some rawinsondes observed inevitably dry layer in the middle troposphere. TPW distributions obtained by satellite clearly indicated that the dry air was advected from higher latitude (Numaguti et al., 1995). The 1

2 phenomena is generally called as "dry intrusion". However, most of studies concerning dry intrusion base on the observations during TOGA-COARE and the studied area is limited in the equatorial western Pacific. In this paper an observational evidence of dry intrusion at Sumatera Island is reported. DATA DESCRIPTION An four-times par day (00, 06, 12, 18 UTC) rawinsonde observation was carried out during 00GMT on 29 September--06GMT on 7 October 1998 at Kototabang (0.20\degr S, \degr E, 865m MSL), West Sumatera, Indonesia (marked as a square in Fig.~1; see Murata et al., 2002 for detailed description). Figure~1 shows an average distribution of TPW and 1000 hpa wind field during observing period. In this paper TPW retrieved daily by TRMM/TMI are utilized. NCEP/NCAR reanalysis is utilized to grasp threedimensional synoptic condition. In addition GMS Tbb is utilized to analyzed horizontal distribution of convective activity. The large TPW value distributes in the northern hemisphere. Westerly monsoon flow appears between the equator and 10 N. It represents that summer monsoon system still continues during this period. In the Indian Ocean there is large difference of TPW between the southwestern part (less than 40 mm) and the northeastern part (more than 50 mm). This observing period corresponds to one of two rainy seasons during September-- November and March--May in the central part of Sumatera (Hamada et al. 2002), which appear due to the annual variation (north-south movement around the equator) of the inter--tropical convergence zone (ITCZ). 2 Figure 1. Horizontal distribution of IOP average of total precipitable water (unit is mm) and NCEP reanalysis 1000 hpa wind (unit is m/s). The location of observation site, Kototabang ( degree E, 0.2 degree E, 865 m MSL) is shown as a small square.

3 3 SYNOPTIC CONDITION Figure 2 represents longitude-time cross-sections of (a) GMS Tbb, (b) 700 hpa zonal wind anomaly and (c) 700 hpa geopotential height anomaly during 1 Sep--31 Oct Two large cloud disturbances appear in the Indian Ocean during Sep and Oct. These correspond to active phase of Madden--Julian Oscillation (MJO) and propagate eastward by around 5 m/s. Eastward propagating westerly anomalies follow the active MJO disturbances with time lag of several days. Geopotential height anomaly is relatively low during active MJO passages. On the latter phase of an active MJO (25 Sep--11 Oct) quasi-10-day disturbances appear clearly in geopotential anomaly field of the Indian Ocean. This disturbances propagate westward with around 10 m/s and the low pressures correspond to westerly anomaly and vice vasa. Our observing period was in this latter phase of an active MJO period. Quasi-10-day variation was also observed at Kototabang (Murata et al., 2002). The phase speed, period and horizontal scale(wave length is around 8000 km ) of the disturbances satisfy a dispersion relation of theoretical n=1 equatorial Rossby wave. It is considered that the Rossby wave was emitted by Rossby wave response of equatorial disturbances related to MJO. Figure 2. Longitude-time cross-section of (a) GMS Tbb, (b) 700 hpa zonal wind anomaly from 1 Sep 31 Oct 1998 average, and (c) 700 hpa geopotential height anomaly from 1 Sep 31 Oct 1998 average. DRY INTRUSION Figure 3 is time-height cross-sections of (a)relative humidity and (b) specific humidity observed by rawinsondes at Kototabang. Relative humidity shows clear diurnal variation

4 on which increase in the afternoon or night time associated with the afternoon convection and/or radiative cooling. There are sometimes reduction of relative humidity below 80% at 1--4 km height. Similar distribution of humidity was obtained in the equatorial western Pacific(Zuidema, 1998; Yoneyama, 2003). Distribution of specific humidity (Fig~3(b)) shows 1--2 days periodicity in the low troposphere. Though the humidity is usually more than 10 g/kg in the low troposphere (until 2 km), very low humidity is observed during 6--7 Oct in the height of 1--3 km above ground level (AGL) or 2--4 km mean surface level (MSL). The minimum relative humidity is less than 50% and the minimum specific humidity is less than 4 g/kg. This low humidity layer corresponded to layer of strong westerlies of more than 10 m/s and we observed almost no rainfall during the period (Murata et al., 2002). Figure 4(a) is a horizontal distribution of geopotential height and wind at 700 hpa, which correspond to the layer observed dry air at Kototabang. Double depressions appear in the latitudes of around 12 degree N and 7 degree S in the eastern Indian Ocean. Between the double depressions strong westerlies blows along the equator. This horizontal structure correspond to theoretical n=1 equatorial Rossby wave. Figure 4(b) is a distribution of TPW on Oct 6. Each grid value is the average of observation during Oct The general distribution is the same as Figure 1. However, low TPW (less than 50 mm) is appears along the equator from central Indian Ocean to Sumatera Island by strong westerly. The large difference of TPW in the Indian Ocean implies that the low TPW is intruded horizontally from central Indian Ocean to Sumatera Island. But to confirm the implication we need to show results of trajectory analysis. CONCLUSION A rawinsonde observation campaign by 4-times per day was conducted at Kototabang, West Sumatera, Indonesia during Sep 29--Oct 7, The results were analyzed regarding water vapor distributions.the wind system characterized by summer monsoon were still kept in this period. It also corresponded to the latter period of active MJO phase and quasi-10-day Rossby wave disturbances emitted by Rossby wave response of active MJO convections were presented. The vertical distribution of water vapor at Kototabang showed very dry layer at 2--4 km MSL on Oct 6-7. This period corresponded to a low TPW extended from central Indian Ocean to Sumatera Island in the horizontal TPW distributions by TRMM/TMI. During this period convective activity was suppressed at Kototabang. This condition was very similar to the 'dry intrusion' observed during TOGA-COARE. A plausible mechanism for the change of vertical water vapor distribution at Kototabang is horizontal advection of relatively dry air from the Indian Ocean to Sumatera Island. Three-dimensional water vapor distribution have large variations in a short-time scale (around few days). The variations of water vapor distribution may greatly affect the convective activity in the tropics. 4

5 5 Figure 3. Time-height cross-section of (a) relative humidity and (b) specific humidity. The period is Sep 29 Oct Figure 4. Time-height cross-section of (a) relative humidity and (b) specific humidity. The period is Sep 29 Oct

6 6 ACKNOWLEDGEMENT TMISST (Ver. 2.0) was produced and supplied by the Earth Observation Research and application Center, Japan Aerospace Exploration Agency. REFERENCES [1] Hamada J.-I., Yamanaka M.D., Matsumoto J., Fukao S., Winarso P.A. and Sribimawati T., Spatial and temporal variations of the rainy season over Indonesia and their link to ENSO., J. Meteor. Soc. Japan, Vol. 80, pp [2] Murata F., Yamanaka M.D., Fujiwara M., Ogino S.-Y., Hashiguchi H., Fukao S., Kudsy M., Sribimawati T., Harijono S.W.B. and Kelana E., Relationship between wind and precipitation observed with a UHF radar, GPS rawinsondes and surface meteorological instruments at Kototabang, West Sumatera during September- October 1998., J. Meteor. Soc. Japan, Vol. 80(3), pp [3] Numaguti A., Oki R., Nakamura K., Tsuboki K., Misawa N., Asai T. and Kodama Y.-M., 1995, 4-5-day-period variation and low-level dry air observed in the equatorial western Pacific during the TOGA-COARE IOP., J.Meteor.Soc.Japan, Vol. 73, (1995), pp