Journal of the Meteorological Society of Japan. Ser. II
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Volume 79 , Issue 1B
Special Issue: GEWEX Asian Monsoon Experiment
Showing 1-23 articles out of 23 articles from the selected issue
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Papers
  • Jianjun Xu, Johnny C L Chan
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 241-253
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    The Asian summer monsoon (ASM) in 1998 first broke out over the Bay of Bengal (BoB) on May 15, then the South China Sea (SCS) on May 25, and South Asia (the Indian summer monsoon) around June 10. However, the meridional temperature gradient in the upper troposphere (200-500 hPa) south of the Tibetan Plateau did not reverse its sign until the onset of the Indian summer monsoon. This suggests that some other mechanism must be responsible for the monsoon onset over the BoB, which is often referred to as the first transition of the ASM. The main objective of this study is to explore such a mechanism through an observational analysis. The thermal condition over the Tibetan Plateau was dominated by both sensible and latent heat fluxes, but the sensible heat flux (SHF) shows a substantial increase at the end of April, which is much earlier than the time of the first transition of the ASM. In contrast, changes in the thermal condition over the tropical Indian Ocean is mainly contributed by latent heat fluxes (LHF), the drastic increase of which in mid May coincided with the onset of the ASM. Such an increase occurred concomitantly with a migration of warm water from the southern Indian Ocean and across the Arabian Sea, as well as a strengthening of westerlies associated with the development of a tropical cyclone over the Bay of Bengal. The dynamics of the tropical cyclone allowed the latent heat to be realized through deep convection, which led to the onset of the ASM over the BoB.
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  • Ding Yihui, Liu Yanju
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 255-276
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    The South China Sea Monsoon Experiment (SCSMEX), which was carried out from 1 May to 31 August 1998, is a multi-endeavor that is closely linked to and coordinated with activities of national weather services and oceanographic bodies of different nations and regions. The scientific goal of the SCSMEX is to provide a better understanding of the key physical processes for the onset, maintenance and variability of the summer monsoon over Southeast Asia and the South China Sea (SCS), leading to improved predictions of monsoons. Based on the datasets obtained from the SCSMEX Intensive Observation Period (IOP), the characteristics of upper and low-level circulation, outgoing long-wave radiation (OLR) and precipitation patterns have been analyzed. The obtained results have shown that (1) the onset of the SCS summer monsoon consists of two stages: the first onset on the northern SCS and the full onset over the whole SCS. The summer monsoon onset over the northern part of the SCS occurred in the fourth pentad of May while for the whole area of the SCS the onset occurred in the fifth pentad of May. The date of the onset of the SCS summer monsoon for 1998 was overall about 1-2 pentads later than normal condition (May 15); (2) the monsoon onset over the northern part of the SCS was not a localized phenomenon. It broke out simultaneously with the monsoon over the Bay of Bengal and Indo-China Peninsula. From a viewpoint of synoptic process, its onset is related to the early rapid development of a twin cyclone to the east of Sri Lanka and subsequently a monsoon depression over the Bay of Bengal, two stepwise significant retreats of the subtropical high from the SCS and possible triggering effect of a strong cold wave from mid-latitude; (3) activities of the SCS monsoon were mainly affected by activities of the Indian monsoon, and its enhancement and northward advances were greatly influenced by acceleration and eastward extending of the near equatorial westerly wind from the Indian Ocean.
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  • Ding Yihui, Zhang Yan, Ma Qiang, Hu Guoquan
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 277-300
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    An analysis of the large-scale circulation features, major synoptic systems, precipitation, and atmospheric heating fields during the intensive observation period (IOP) of the GAME/HUBEX identified eight features. 1) The beginning and maintenance of the Meiyu season over the Yangtze-Huaihe River Basin in both 1998 and 1999 occurred under the condition of the northward advance and enhancement of the East Asian summer monsoon. 2) The circulation features characterized with two ridges in the region to west of the Baikal Lake and the Sea of Okhotsk, respectively, and one trough in between (in the region of the Baikal Lake) in the middle and high latitudes set a stage for the development of Meiyu seasons. Especially, the quasi-stationary Okhotsk blocking high and the upper-level vortex over East China/the Sea of Japan was a couplet of circulation systems under which the Meiyu front over the Yangtze-Huaihe River Basins was stably maintained. 3) The subtropical high in the western Pacific assumed a significant westward and northward shift before arrival of Meiyu seasons in 1998 and 1999. During the concentrating period of Meiyu precipitation for 1998, its meridional oscillation was closely related to variations of episodes of heavy rainfalls in this region. 4) The low-level easterly aircurrent to the north of the low-level southerly jet played an important role in maintenance of the frontal zone, which is also one of major factors leading to the persistent heavy rainfalls in middle and lower reaches of the Yangtze River. 5) The formation of the low-level jets is different in the synoptic conditions for 1998 and 1999. This difference mainly resulted from the different zonal positions of the subtropical high and activities of the summer monsoon. 6) There existed a succession of meso-scale disturbances (at least 2 and 3 meso-scale vortices for 1998 and 1999, respectively) along the Meiyu frontal zones, which mostly originated in the eastern flank of the Tibetan plateau and migrated eastward, with vortex centers located to the north of the low-level jet axis and heavy rainfalls in the southeast of the disturbances. 7) Along the Meiyu front, there was a high humidity and lower temperature conditions, while relatively dry and higher temperature conditions existed to south and north of the Meiyu front. This kind of sandwich-like temperature and humidity condition is important for the Meiyu maintenance; Zonally, the temperature field was characterized by lower temperature in the west and higher temperature in the east along the Meiyu frontal zone. 8) During the processes of heavy rains in 1998 and 1999, the patterns and the magnitudes of Q1 and Q2 are nearly similar to each other, indicating that the heating over the heavy rain regions was created by the moisture condensation process, due mainly to convective activities. However, it is now not clear how this prosess of energy and water cycle may in turn affect events of heavy rainfalls through feedback effect. The above indications of the large-scale circulation features and synoptic systems provide a starting point for further investigation of the energy and water cycle over the Huaihe River Basin.
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  • Bolin Zhao, Zhanyu Yao, Wanbiao Li, Jian Yuan, Yong Chen, Huilin Gao, ...
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 301-315
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    The ability of the Tropical Rainfall Measuring Mission Microwave Imager (TRMM/TMI) for rainfall and flooding detection has been demonstrated in this study. A dynamic cluster K-mean method was presented to classify surface types and rain areas.Based on the TMI measurements and surface observations during the Huaihe River Basin Energy and Water Cycle Experiment, the scattering index over land (SIL) was developed and a rain rate (RR) retrieval algorithm of TMI was established, in which the RR is the function of SIL and PCT85 (polarization corrected temperature at 85 GHz). Compared with the rainfall derived from the weather radar and the products of TMI(2A12) and TRMM/PR(2A25), the given surface classification method and the rain rate retrieval algorithm can provide reasonable estimates of rain areas and rain rates.On the basis of TMI measurements, for methods, the classification method, the soil wetness index (SWI) method, the polarization difference index (PDI) method, and the polarization ratio index (PRI) method, were utilized to monitor flooding and study soil wetness in the Yangtze River basin and the Huaihe River basin during summer 1998. Compared with the images provided by L-band Synthetic Aperture Radar (L-SAR) and Radar Satellite (Radarsat) and the figures derived from daily rain gauge data based on the Z-index method, the four methods of flooding detection were proved to be successful, among which the PRI method was best.
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  • Ipuk Widiyatmi, Hiroyuki Hashiguchi, Shoichiro Fukao, Manabu D. Yamana ...
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 317-331
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    Spectral features of lower tropospheric horizontal wind observed during 1996-1999 by Bukittinggi, Serpong and Biak boundary layer radars show 3-6 day periodicities in zonal and meridional components and longer period zonal wind oscillations. Using bandpass filtering analysis the 3-6 day modes show a westward phase speed of about 700 km day-1 and a zonal wavelength of about 3500 km, and propagate through the Indonesian maritime continent when the westerly wind is prevailing at Biak.
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  • Teruo Ohsawa, Hiromasa Ueda, Taiichi Hayashi, Akira Watanabe, Jun Mats ...
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 333-352
    Released: April 26, 2002
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    The diurnal variations of convective activity and rainfall in tropical Asia are investigated using hourly equivalent black body temperature (TBB) data from the Japanese Geostationary Meteorological Satellite (GMS-5), and hourly (or 3-hourly) rainfall data from Bangladesh, Thailand, Vietnam and Malaysia. As an index of convective activity, we use the frequency of occurrence of the difference in TBB between the infrared-1 and the water vapor channel ΔTBB (= TBB(IR1) − TBB(WV)) of less than 3K. By using this index, the diurnal variations of convective activity and rainfall have approximately the same phase. The time when convective activity reaches its maximum and minimum is examined in the domain of 80-120°E and 0-30°N. As a result, it is found that the largest number of grids exhibits the maximum at 17LT (local time) and 14LT, and the minimum at 11LT and 21LT over land and sea, respectively. Moreover, without using the harmonic analysis, which is a conventional method of analyzing the diurnal variation, areas with the maximum during the late night-early morning hours are clearly separated from those with the afternoon-early evening maximum. This late night-early morning maximum is mostly found in the windward areas of mountains, in basins and valleys, and in coastal areas. Therefore, this kind of maximum is most likely associated with terrain or its induced local circulations such as mountain and land breezes. From the analysis of rainfall data, it is also shown that the late night-early morning maximum is found at stations with high rainfall. The number of stations with the late night-early morning maximum is less than that with the afternoon-early evening maximum. But, the mean daily rainfall at the former stations is on average a few times greater than that at the latter stations. Thus, the diurnal variation averaged for all stations in the four countries has two nearly equal maxima at 05LT and 16LT. This result suggests a strong possibility that the late night-early morning maxima of convective activity and rainfall have a great efffect on energy and water cycles in tropical Asia.
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  • Koichiro Kuraji, Kowit Punyatrong, Masakazu Suzuki
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 353-363
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    To determine the characteristics of altitudinal dependence in rainfall in the mountainous area of the upper Chao Phraya river basin in northern Thailand, one and a half years of rainfall data from 13 sites, installed by the GAME-T project, distributed throughout the 3, 853 km2 of the Mae Chaem watershed were analyzed. It was found that the altitudinal increase in rainfall was obvious in the two wet seasons in 1998 and 1999 with the increment in 1999 being more than 2.5 times greater than that in 1998. The altitudinal increase in rainfall was also obvious in the dry season from 1998 to 1999, but the increment was smaller than in the two wet seasons. It is not the rainfall intensity, but the rainfall observed hours which causes the difference of altitudinal increment. These relationships were analyzed over a range of coefficients of variation (CV) for daily rainfall to understand how the spatial scale of the rain event affects the altitudinal increase in the rainfall. It was found that the altitudinal increase in the mean daily rainfall (MDR) was detected at both the high and low CV ranges. The increment for MDR with a low CV, which corresponds to a relatively larger spatial scale rainfall, is greater than that for high CV rainfall events for two wet and one dry seasons. For the large spatial scale rainfall, this tendency was due to the altitudinal increase in the duration for the large spatial scale rain in the wet seasons. In the dry season, however, this tendency was due to the altitudinal increase in the mean rainfall intensity.
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  • Tomoyoshi Hirota
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 365-371
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    Seasonal evaporation on bare soil and lawn was estimated using only routine meteorological data at Bangkok and Chiang Mai in the Chao Phraya river basin in Thailand by using two methods. One method is that evaporation was estimated by the heat balance model using daily mean ground surface temperature (0 cm depth), (hereafter GST), The second is that it was estimated by using the Force-Restore Method for estimating daily mean soil temperature (hereafter FRM). Seasonal and annual evaporation were estimated by using agrometeorological station data of the Thai Meteorological Department in 1989 at Chiang Mai (lawn) and Bangkok (bare soil). At Chiang Mai, annual precipitation was 1000 mm, annual pan evaporation was 1540 mm and estimated value of annual evaporation (lawn) was 760 mm by the FRM (using soil temperature at 5 cm depth) and 815 mm by the GST method. At Bangkok, annual precipitation was 1320 mm, annual pan evaporation was 1560 mm and the estimated value of annual evaporation (bare soil) was 1080 mm by the FRM (using soil temperature at 5 cm depth) and 1140 mm by the GST method. The results of these two methods were comparatively close to each other, however the estimated evaporation by the GST method was slightly larger than that by the FRM. This difference was caused by the differences between the ground surface temperature and soil temperature at 5 cm depth. The reason considered was the difficulty of accurate ground surface temperature measurement by the stem thermometer. The estimated evaporation at Chiang Mai was comparatively close to the results of the complex area of vegetation obtained by Toda et al. (2000 b) and paddy field obtained by Toda et al. (2000 a).
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  • Dawen Yang, Srikantha Herath, Taikan Oki, Katumi Musiake
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 373-385
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    Prediction of variations of global and regional hydrological cycles and their response to changes of climate and environment is an important topic. For this purpose, improving the ability of hydrological models has been in the direction of ever-increasing spatial scales. Owing to the highly nonlinear nature of hydrological processes and the high degree of variability of the spatial heterogeneity, distributed representation of spatial information and physical description of hydrological processes are necessary. It is also important to incorporate the regulation of water by infrastructures (e.g., large reservoirs) with the hydrological modeling to verify direct human influence on water resources. The GBHM (geomorphology based hydrological model) employs a flow interval-hillslope discretization scheme in the representation of hydrological processes using catchment geomorphologic properties. The GBHM can represent the characteristics of catchment landform properly and is flexible enough to describe other spatial variability, such as land cover and soil type. In the present study, GBHM was applied to the Chao Phraya basin in Thailand, in which the GEWEX Asian Monsoon Experiment in a tropic region (GAME-T) has been carried out since 1995, for investigating the applicability of this model to regional scale. The hydrological simulation has run from 1994 through 1995 with a drainage area of 110, 000 km2. The reservoir (Bhumibol and Sirikit) operations were incorporated with the hydrological modeling. The simulated hydrographs at 16 locations, including two reservoirs, were compared with the river flow observations for evaluation of the model performance. The possibility of coupling GBHM with the atmospheric model is discussed in the paper.
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  • Wonsik Kim, Takashi Arai, Shinjiro Kanae, Taikan Oki, Katumi Musiake
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 387-400
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    The simple biosphere model (SiB2) of Sellers et al. and the revised SiB2 (SiB2-Paddy) incorporated with a paddy model are compared and evaluated using micrometeorological data measured in the paddy field that is one of the GAME-Tropics experimental sites in Thailand. In terms of the diurnal cycle, simulated net radiation, and latent heat flux, the two models are in good agreement with the observation, except for the diurnal variation of latent heat flux simulated by SiB2. Sensible and soil heat fluxes, and assimilation rate by SiB2-Paddy agree well with the observation; however, those simulated by SiB2 are found to be biased. Canopy, water, and soil temperature are also well simulated by SiB-Paddy with parameter adjustment. In terms of the total energy and water balance, net radiation, latent heat flux, and assimilation rate do not differ much between the simulations and the observation. This is partly because the values of sensible heat and soil heat fluxes are too small compared to net radiation and latent heat flux, and the mean biases are not significant. However, SiB2-Paddy is required for realistic simulations of the diurnal cycles of latent heat flux and surface temperature.
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  • Tsuneo Kuwagata, Atusi Numaguti, Nobuhiko Endo
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 401-418
    Released: April 26, 2002
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    The diurnal variation of water vapor over the central Tibetan Plateau during summer was investigated using various observational data and a numerical model. A systematic diurnal cycle of the precipitable water was found in the rawinsonde data. That is, the precipitable water decreased during daytime at the valley area, while it increased over the mountain range. This diurnal cycle is most significant in the pre-monsoon season but it is not clear in the middle of the monsoon season. The horizontal distribution of this diurnal cycle was examined by global objective analysis datasets and GMS-5 satellite data, and the results were found to be consistent with the topographic dependence of the diurnal cycle described above. The systematic diurnal cycle of the water vapor amount is interpreted to be caused by water vapor transport due to the thermally-induced circulation over complex terrain. This diurnal cycle of water vapor can be qualitatively simulated by a two-dimensional numerical model. The dominant horizontal scale of topography of the central Tibetan Plateau is 100 to 300 km, which is close to the most effective scale for water vapor transport as predicted by the simple two-dimensional model.
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  • Kenichi Ueno, Hideyuki Fujii, Hiroyuki Yamada, Liping Liu
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 419-434
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    This study analyzed the temporal and spatial scales, types, and intensity of precipitation with their temporal variability by using the precipitation data, intensively observed as part of the GAME-Tibet international project, at the Naqu basin (4500-5000 m) in the Tibetan Plateau from May to September, 1998. The monsoon period at the basin was defined from the middle of June to the beginning of September by the precipitation and GMS/IR convective index variation. Basin averaged monsoon precipitation was 336 mm, and most of the precipitation were provided as rain or hail. The lowest altitude for a 50 % possibility of snowfall was estimated at almost around 5000 m above sea level. Daily precipitation data records from June to August in the the Asian monsoon region highlighted frequent precipitation with weak intensity in the Tibetan Plateau. In the Naqu basin, the percentage of days with precipitation of more than 0.25 mm/d was 87 % during the monsoon season, and the average hourly precipitation intensity was 1.27 mm/h. Difference in the accumulated precipitation amount between daytime and nighttime was not obvious, but the clear diurnal change existed in the precipitation intensity, i.e., weak precipitation events below 1 mm/h in the daytime and rather heavy precipitation events above 3 mm/h at night. Most of the events are associated with the passing of synoptic (plateau) scale cloud area over the plateau. The daytime events consisted of isolated convective echoes with a small spatial scale, the nighttime events consisted of widespread stratiform echoes. According to these observational results, conceptual mechanisms underlying the daytime frequent precipitation with weak intensity are discussed.
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  • Shuji Shimizu, Ken’ichi Ueno, Hideyuki Fujii, Hiroyuki Yamada, R ...
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 435-461
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    During the GAME-Tibet Intensive Observation Period (IOP), the precipitation radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) satellite detected a diurnal cycle of rainfall in Tibet. Much rainfall was brought both by convection in the daytime and by stratiform precipitation over a wide rain area in the evening and night. Two case studies were conducted to clarify the structure of stratiform rainfall in the nighttime. In Case 1 (7 and 8 July 1998), stratiform precipitation was observed in the evening and night. On 7 July, a synoptic convergence area developed in the southern part of the Plateau in the evening. Southwesterly wind dominated above the 6 km ASL, but the wind direction below 6 km ASL was variable during the rainfall. When the mesoscale convergence strengthened, the rainfall amount became large. On 8 July, a convergence zone between southwesterly wind and northerly wind was detected and passed over the radar site in the evening. A sudden wind direction change from westerly to northwesterly was observed below 8 km ASL from the vertical profiles at the radar site. The boundary between the two wind directions may correspond to the cold frontal surface. However, the precipitation with the frontal passage was stratiform and had a low echo top. The variation of rainfall on the two days was consistent with the diurnal variation revealed by TRMM PR.In Case 2 (1 and 2 August 1998), there was no remarkable disturbance in the synoptic field, but much rainfall (exceeding 4 mm/h) was observed in the midnight. A 90 km-diameter stratiform echo stayed over the radar site for seven hours. Below 6 km ASL, the wind direction varied from northerly to northeasterly, then to easterly. When northeasterly wind dominated, the convergence in the lower layer strengthened and stratiform precipitation was intensified. Mesoscale convergence of moist air in the lower layer effectively contributes to development and maintenance of stratiform rainfall.
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  • Hiroshi Uyeda, Hiroyuki Yamada, Junichi Horikomi, Ryuichi Shirooka, Sh ...
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 463-474
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    In order to reveal the characteristics of convective clouds over the Tibetan Plateau, we carried out Doppler radar observations of the clouds in the suburbs of Naqu city (4500 m ASL), which is located in the central part of the Tibetan Plateau, from 27 May to 19 September 1998, during the intensive observation period of the GEWEX Asian Monsoon Experiment in Tibet. Diurnal variation of the radar echo are and echo-top height were remarkable throughout the intensive observation period indicating the formation and development of convective clouds in the daytime and their decaying at night. The echo-top height and convective area, extracted objectively from grid data of radar reflectivity, were larger after the monsoon onset than before the onset. The maximum echo-top height of convective clouds (> 10 dBZ) was about 17 km ASL. Furthermore, many vortices accompanied by the clouds, identified on Doppler velocity fields, were observed in the daytime. The vortices generated in nearly all regions of the radar observation area (r = 64 km). The strongest vorticity of a given day with vortices during the observation period showed a positive correlation with the appearance of the maximum echo-top height (> 30 dBZ) of the convective clouds. The daily maximum vorticity after the monsoon onset was larger than that of the pre-monsoon period. A comparison of features of radar reflectivity and vorticity of convective clouds with sounding data indicates the presence of the following processes: formation of a strong updraft by solar heating, descending of westerly wind in the upper troposphere to the lower altitude, increase in strength of the vertical shear in low altitudes, tilting of horizontal vortex tube, and intensification of vortex in the clouds. Convective clouds developed above the melting layer at Naqu on the Tibetan Plateau were characterized by their rapid development from solar heating, by the formation of graupel particles and by the association of a strong vortex in the clouds.
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  • Hideyuki Fujii, Toshio Koike
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 475-483
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    This paper proposes a new algorithm for precipitation over land by deriving the optical thickness from the brightness temperature of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). The effect of land surface controlled by soil moisture emissivity on radiation transfer is taken into account in this algorithm. This means that soil moisture can be estimated at the same time in addition to precipitation. Based on a microwave radiative transfer equation, two indices, Index of Soil Wetness (ISW) and Polarization Index (PI), which remove the effect of land surface physical temperature, are introduced into the algorithm. Surface roughness effects on land surface emissivity are included by using the polarization mixing ratio and the surface roughness. The application of the algorithm to the GAME-Tibet meso-scale experimental field shows a good agreement with the spatial patterns of precipitation observed by the 3D Doppler radar. The estimated soil moisture shows good correspondence with the observed one. The relationship between the optical thickness and precipitation was also investigated.
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  • Jianqing Xu, Shigenori Haginoya
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 485-504
    Released: April 26, 2002
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    A model considering soil-water transportation through vapor and liquid phases under the ground surface has been used for estimating heat and water balances in the Tibetan Autonomous Region of the Tibetan Plateau. The input data is derived from routine meteorological observations in the 14 stations of the Tibetan region. Calculated results were verified by the heat fluxes, and soil-water observation results from JEXAM. Daily variations of the heat fluxes, and seasonal variations of the soil-water content and solar radiation show good or reasonable agreements between the calculated and observed results. Daily and seasonal variations have been estimated for these 14 areas in 1997. Net radiation fluxes show relatively large values (annual mean 55-79 W m-2) over the Plateau where the elevation is above 3000 m. Most (80-90 %) of the net radiation is accounted for by sensible heat flux due to low precipitation (e.g., 55 mm in Shiquanhe) in western Tibet (west of about 87 °E). Latent heat flux accounts for about 30-50 % of the net radiation in east Tibet. From west to east, latent heat flux increases gradually and even exceeds the sensible heat flux in the monsoon season. Latent heat flux (evaporation) is also limited by the relatively low surface temperature because of the high elevation, which indicates why the Plateau serves as the source of several large rivers despite its not so large precipitation. Relatively larger net radiation accompanied with limited evaporation suggests that the Tibetan Plateau plays a role in heating the atmosphere. Calculations have also been conducted for a climatic representative location, Lhasa, from 1979 to 1997.The average of net radiation was 62 W m-2 with a sensible heat flux of 38 W m-2 and a latent heat flux of 24 W m-2 for these 19 years. Sensible heat flux peaks in May or June at a range of 54-76 W m-2, and latent heat flux in July or August at 41-83 W m-2. The variations of annual precipitation are large, hence the annual total difference between the precipitation and evaporation varies from 8 mm in a low precipitation year to 217 mm in a high precipitation year.
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  • Kenji Tanaka, Hirohiko Ishikawa, Taiichi Hayashi, Ichiro Tamagawa, Yao ...
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 505-517
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    In the east of Tibetan Plateau, Amdo, the surface energy flux measurement with eddy correlation technique was conducted during the Intensive Observation Period (IOP) of GEWEX Asian Monsoon Experiment (GAME). This study is a preliminary analysis on surface energy budget at Amdo with these data. There is a remarkable change in sensible and latent heat fluxes between pre-monsoon season and summer monsoon season. In the pre-monsoon, the ground surface and the surface layer were very dry. The specific humidity was 2 to 4 g/kg and the sensible heat flux was dominant. As the monsoon progresses, the surface becomes wet due to almost daily precipitation. Accordingly, the sensible heat flux decreases and the latent heat flux increases. In the beginning of September, nearly the end of the summer monsoon, the latent heat flux exceeds the sensible heat flux. The ground heat flux at the surface was estimated using the observed soil temperature profile with the aid of the thermal conductive equation. The computed ground heat flux corresponds to the residual of the surface energy balance in the daytime. But disagreement occurred in the evening, when the surface temperature rapidly decreases. On the daily averaged bases, the surface energy balance is not well closed. In terms of closure ratio, the value in a typical clear day was 67%. Another rough estimation of soil heat flux is made, in which the heat needed to melt the soil water and to heat up the soil layer was calculated. According to this, it was estimated that about 30 W/m2 (20% of net radiation) of soil heat flux was required on average over April 20 to July 20, but the measured soil heat flux was only 5.7 W/m2.
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  • Takeshi Sato
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 519-534
    Released: April 26, 2002
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    The observations of frost depth and snow water equivalent were carried out at Lhasa, Rikeze and Nagqu in the eastern part of the Tibetan Plateau from July 1993 to March 1999. Also, the routine data at the ten meteorological stations in the same part was analyzed, which covers the observation period. All the stations are located in the seasonally frozen ground region. The main results are as follows: The development of seasonally frozen ground was most remarkable at Nagqu, where the maximum frost depth was 160-200 cm. At Nagqu, the frost penetration began in October and continued until the middle of March. During the frost penetration, except in the midwinter, the soil layer near the surface froze in the night and melted in the daytime. At the thawing stage from April to May, the frozen ground melted from the surface and the frozen soil layer left underground became thin. The efficiency of frost penetration, α, under certain temperature conditions tends to increase with the altitude and the precipitation in the preceding autumn, but α is not influenced by the snow cover. Exceptionally, α is large at Lingzi and Qamdo in spite of their low altitudes. This may be ascribed to the less solar radiation at these stations than other stations, which enhances the frost penetration. The maximum frost depth increases with the altitude except for Lingzi and Qamdo.The days of snow cover increases with the latitude, but long lasting snow cover is not usual. At Nagqu, where the days of snow cover is the second largest among the ten stations, the maximum snow water equivalent was 20 mm, the maximum snow depth was 9 cm and the mean snow density was about 0.2 g cm-3. inter-annual variation of the days of snow cover at Nagqu is almost similar to that at Sogxian. The localized negative correlation between the winter air temperature anomaly and the days of snow cover was found at Nagqu and Sogxian in the winter of 97/98, suggesting the albedo effect of snow cover. The correlation is positive between the precipitation is summer and the days of snow cover at Sogxian in the preceding winter, which apparently corresponds to the snow-hydrological effect. The warm (cold) winter tends to follow the wet (dry) summer with much (little) precipitation respectively. But the relationship between air temperature and precipitation in the summer is not clear.
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  • Izuru Takayabu, Kumiko Takata, Takeshi Yamazaki, Ken’ichi Ueno, ...
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 535-554
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    There is a need to compare and understand the characteristics of the land surface models used in the GAME (GEWEX Asian Monsoon Experiment) project with a common forcing data in order to integrate the various products of the project. We successfully drove the JMA-SiB, CCSR/NIES-LAND, MATSIRO and TOHOKU models, using the GAME/Tibet POP97 data (during about one month, with a time interval of one hour) at the Tanggula river basin (D105). This dataset also has some validation data (soil temperature and snow depth). All models succeeded in forecasting the soil freezing process, since there were no large differences in the ground surface flux. However, there were large differences in the Bowen ratio due to the differences in parametarization of stomatal resistance. It is expected that these differences in the Bowen ratio will cause a large difference when these land surface schemes are coupled with GCM or reg-CM. Large differences in snow amount are attributable to the differences in the estimation of albedo.
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  • Suxia Liu, Xingguo Mo, Haibin Li, Gongbing Peng, Alan Robock
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 555-574
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    We analyze the large-scale spatial variation of soil moisture in eastern China using geostatistical techniques with observations at 99 stations for the top 0.1 m and top 1 m from 1987 to 1989. Sample variograms are found to have a clear sill and a nugget in many cases. A spherical variogram model, including a nugget in some cases, fits the sample variograms closely. Using a quantitative method to select the separation interval for variogram analysis, we find that the average range is 200-400 km for the top 0.1 m and 400-700 km for the top 1 m. The averaged coefficient of variation of soil moisture in the top 0.1 m is larger than for the top 1 m, showing that the range for the top 0.1 m is less than for the top 1 m. The range in summer is less than in winter. By calculating the ratio of the nugget effect to the spatial variance, we find that the ratios for the top 0.1 m data are smaller than that for the top 1 m data, showing that in most cases the spatial variation of the top 0.1 m is more strongly autocorrelated than of the top 1 m, and that the measurement errors are much larger in the top 1 m data than in the top 0.1 m. For more than half of the measured dates, the ratio of the nugget effect to the variance is less than 20 percent, indicating that the spatially correlated variation on those dates can explain more than 80 percent of the total variance.
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  • Yafei Wang, Bin Wang, Jai-Ho Oh
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 575-588
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    This paper examines the relationship between the East Asian summer monsoon and the El Niño Southern Oscillation (ENSO) during the period from 1958 to 1998, which is quite different from the link between the South Asian summer monsoon and ENSO.Major findings are: (1) A new index, called the East Asian Monsoon Index (EAMI), is defined for measuring the East Asian monsoon, which could extensively describe the south-north distribution of the East Asian summer monsoon’s activity. The interannual variability of the EAMI displays a significant negative correlation with the broad-scale Asian monsoon index proposed by Webster and Yang (1992) from 1976 to 1998. (2) A significant positive correlation between the summer 500 hPa geopotential height anomalies and the NINO-3 SST in the preceding fall and winter is found in the subtropical regions of East Asia and the western North Pacific, and in northeast Asia centered at 70°N, 137.5°E. A strong (weak) summer monsoon in the subtropical regions of East Asia tends to occur about two to three seasons after the NINO-3 SST anomalies exceed 1.5 °C (drop below −0.7 °C). (3) The above results suggest a delayed impact of the ENSO on the East Asian summer atmosphere circulation. During the summer after the El Niño reaches its mature phase, an anomalous blocking anticyclone tends to occur in northeast Asia. Meanwhile a subtropical high of the western North Pacific extends abnormally westward. This anomalous circulation pattern enhances the summer monsoon in subtropical East Asia. The abovementioned evolution of the circulation anomalies became more prominent in the unprecedented ’97/98 El Niño event, suggesting that the devastating 1998 flood in southern central China may be partially due to the delayed impact of the ’97/98 El Niño. The physical processes for the delayed impact besides the air-sea interaction in the tropical and subtropical western Pacific are discussed too.
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  • Akiyo Yatagai
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 589-598
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    The brightness temperature of the Geostationary Meteorological Satellite (GSM)-5 water vapor channel (Tb6.7) and relative humidity, precipitable water, specific humidity and temperature observed by radionsondes over the Tibetan Plateau are compared for the summer of 1995. A wavelength of 6.7 μm was employed, so most of the radiation sensed by the satellite comes from the atmospheric layer between 300 and 600 hPa. The plateau altitude is 3, 000 to 4, 000 m, and GMS-5 Tb6.7 over the Plateau represents the moisture information above that. The regression equations of upper tropospheric humidity (UTH) and precipitable water with Tb6.7 are presented in this paper. The relationships between Tb6.7 and observed temperature and specific humidity at the standard pressure levels are then compared to each other. Results show that, over the Plateau, Tb6.7 is sensitive to the moisture information around 200 to 300 hPa height. The results demonstrate that Tb6.7 is useful for not only qualitatively analyzing the moisture transporting field over and around the Tibetan Plateau, but also quantitatively inferring UTH and precipitable water above 400 hPa (W400). Estimates of W400 for July and August 1995 and its diurnal variability over and around the Tibetan Plateau are presented.
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  • Maki Tsujimura, Atusi Numaguti, Lide Tian, Shigemasa Hashimoto, Atsuko ...
    Type: Original Paper
    Subject area: Others
    Volume 79 (2001) Issue 1B Pages 599-605
    Released: April 26, 2002
    JOURNALS FREE ACCESS
    Intensive observation of the pressure head in subsurface water and sampling of subsurface water were performed to investigate the subsurface flow process in the Tibetan plateau from July 24 to September 12-the monsoon season-1998. Pressure head of the subsurface water was monitored using tensiometers and subsurface water was sampled using suction lysimeters installed at multiple depths from 10 to 100 cm.The pressure head of subsurface water ranged from −10 to −100 cmH2O, and zero flux plane was often observed above the depth of 30 cm. The groundwater recharge was very active during this period, thus the groundwater table rose up to the depth of 55 cm in the beginning of September. The δD and δ18O of shallow subsurface water varied markedly with precipitation and evaporation, whereas those of groundwater were stable. The mean δ18O of groundwater was 3.4‰ higher than the volume weighted mean δ18O of precipitation. The difference of δ18O between the groundwater and the precipitation would be caused by isotopic enrichment along with evaporation from the soil surface, and 27% of precipitation might be lost by evaporation from the soil surface.
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