GEWEX Asian Monsoon Experiment Research Articles

Theory and Application of the Numerical Simulation in the Frozen Soil Problems

The freezing-thawing processes in soils are important components of terrestrial hydrology, which significantly influence energy and water exchanges between land surface and sub-surface. Long-term changes in frost and thaw depths are also an important indicator of climate change. A water-heat coupled movements model is established with frozen soil in this paper, which treats the freezing/thawing front as a moving interface governed by some Stefan problems with two free boundaries. The numerical simulation is conducted by using the modified finite difference method. The model is validated to compare its predictions with GEWEX Asian Monsoon Experiment(GAME)-Tibet observations at D66 site in Tibetan Plateau. The results show that the simulated soil temperature, soil water content and frost/thaw depth are in excellent agreement with the measured values. Finally, optimal error estimation for L^∞ norm is derived on the model problem by using coordinate transformation method. The numerical simulation system is established on the basis of rigorous mathematics and mechanics, which successfully solved the important and difficult problems of environmental science.

Energy partitioning and environmental influence factors in different vegetation types in the GEWEX Asian Monsoon Experiment

Environmental influences upon energy balance in areas of different vegetation types (i.e., forest at Kog-Ma in Thailand and at Yakutsk in Russia, grassland at Amdo in Chinese Tibet and at Arvaikheer in Mongolia, and mixed farmland at Tak in Thailand) in the GEWEX Asian Monsoon Experiment were investigated. The sites we investigated are geographically and climatologically different; and consequently had quite large variations in temperature (T), water vapor pressure deficit (VPD), soil moisture (SM), and precipitation (PPT). During May-October, the net radiation flux (Rn) (in W·m−2) was 406.21 at Tak, 365.57 at Kog-Ma, 390.97 at Amdo, 316.65 at Arvaikheer, and 287.10 at Yakutsk. During the growing period, the Rn partitioned into latent heat flux (λE/Rn) was greater than that partitioned into sensible heat flux (H/Rn) at Tak and at Kog-Ma. In contrast, λE/Rn was lower than H/Rn at Arvaikheer, H/Rn was less than λE/Rn between DOY 149 and DOY 270 at Amdo, and between DOY 165 and DOY 235 at Yakutsk. The Rn partitioned into ground heat flux was generally less than 0.15. The short-wave albedo was 0.12, 0.18, and 0.20 at the forest, mixed land, and grass sites, respectively.

구름-장파복사 상호작용이 아시아 몬순에 미치는 영향

Three-dimensional distributions of longwave radiation flux for the April-September 1998 period are generated from radiative transfer calculations using the GEWEX Asian Monsoon Experiment (GAME) reanalysis temperature and humidity profiles and International Satellite Cloud Climatology Project (ISCCP) cloudiness as inputs to understand the effect of cloud radiative forcing in the monsoon season. By subtracting the heating of the clear atmosphere from the cloudy radiative heating, cloud-induced atmospheric radiative heating has been obtained. Emphasis is placed on the impact of horizontal gradients of the cloud-generated radiative heating on the Asian monsoon. Cloud-induced heating exhibits its maximum heating areas within the Indian Ocean and minimum heating over the Tibetan Plateau, which establishes the north-south oriented differential heating gradient. Considering that the differential heating is a ultimate source generating the atmospheric circulation, the cloud-induced heating gradient established between the Indian Ocean and the Plateau can enhance the strength of the north-south Hadley-type monsoon circulation. Cooling at cloud top and warming at cloud bottom, which are the vertical distributions of cloud-induced heating, can exert on the monsoon circulation by altering the atmospheric stability.

Nighttime Precipitation Induced by a Synoptic-scale Convergence in the Central Tibetan Plateau

In the 1998 monsoon season, precipitation from midnight through early morning was frequently observed in the central Tibetan Plateau (TP). It occurred on several successive days and tended to be accompanied by low-level easterly winds. Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis data also showed easterly component winds as a part of anti-cyclonic circulations in the northeastern plateau systematically occurring with a prevailing synoptic scale trough at around 100°Einthe mid-latitudes. A numerical simulation by the Weather Research and Forecasting (WRF) model with a resolution of 15 km reproduced the development of a convergence flow over the central TP, and corresponding zonal cloud formation was confirmed by METEOSAT-IR images. The simulation results indicated that the precipitation system was developed through with dissipating nighttime low-level convective instability by low-level convergence flows, and sonde observations confirmed the existence of an instability layer. Sensitivity experiments revealed that the convergence pattern over the central TP was produced by the dynamical stagnation effect of the plateau topography to the northwesterly general flows from the mid-latitudes. Many former studies have explained nocturnal precipitation in monsoon Asia as a part of an orographically induced diurnal variation. However, this study characterized the nighttime precipitation over the central TP due to synoptic-scale convergence. Two reasons are given to explain the substantial precipitation enhancement in the night. One is the daytime thermal convections that depend strongly on the topography, which dissipate synoptic-scale convergences zone, and the other is the low-level moistening after the maturing of the monsoon season to increase nighttime convective instability.

Snow Cover and Depth of Freeze-Thaw on the Tibetan Plateau: A Case Study from 1997 to 1998

An increase in snow cover on the Tibetan Plateau has been observed over the past 50 years. The frequency of snow disasters on the Plateau has also increased. Much of the Qinghai-Xizang (Tibetan) Plateau is underlain by permafrost. Some observation sites of the GEWEX Asian Monsoon Experiment (GAME)-Tibet are located in the Naqu-Anduo area, and have been operating since 1997. A data set obtained from the GAME-Tibet measurement network was used to examine variations in the active layer, with emphasis on the heavy snow cover in 1997/1998. Analysis in this study is focused on air and soil temperature, soil moisture, freeze-thaw depths, and snow depth in years of normal (1998/1999) and abnormally heavy (1997/1998) snow cover. Results indicate that anomalously deep snow cover influenced the pattern of soil freeze-thaw depth through its control over ground temperature. Although air temperature was lower than normal during this period, ground temperatures were appreciably warmer under the heavy snow cover, which retarded frost penetration at depth. Snow cover can also contribute to the cooling process through its high albedo. Because the anomalously heavy snow load lasted well beyond the normal date of meltout, soil thaw developed later than usual during the summer of 1998. Interannual variations in snow depth have significant implications for the maintenance of Tibetan permafrost.

Water Recycling between the Land Surface and Atmosphere on the Northern Tibetan Plateau—A Case Study at Flat Observation Sites

ABSTRACT High-resolution soil moisture, temperature, and precipitation data from the northern part of the Tibetan Plateau provide the basis for analyzing the cycling of water between the land surface and atmosphere. Data analyzed come from the Intensive Observation Period (IOP) of the GEWEX (Global Energy and Water Cycle Experiment) Asian Monsoon Experiment (GAME) on the Tibetan Plateau (GAME-Tibet). Observations from July to August 1998 show that evaporation from flat land surfaces was 177 mm on the south side of the Tanggula Mountains, and 73 mm on the north side. These represent about 73% and 58%, respectively, of the precipitation in the same period. Evaporation not only transports considerable water but also considerable energy from the land surface to the atmosphere, which can slow the rising of soil temperatures. Differences in the evaporation between the south and north sides of Tanggula Mountains is mainly caused by differences in precipitation.

The products and validation of GAME reanalysis and JRA‐25 part 1: surface fluxes

AbstractThe Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis datasets are the products of four‐dimensional data assimilations (4DDA) of meteorological observations made from April 1998 to October 1998. These datasets were created using the most recently updated 4DDA schemes of the Japan Meteorological Agency (JMA) during the GAME reanalysis project (1999–2001) as well as radiosonde observations made during the intensive observation period of the GAME project (April 1998–October 1998). Two versions of reanalyses products have been released officially: Ver. 1.1 and Ver. 1.5.One of the greatest differences between the two versions is the representation of snow cover: Ver. 1.1 used climatological snow data, while Ver. 1.5 used satellite‐derived snow information. Here, we describe how snow cover modifications and land surface schemes differ between the two versions and how changes in surface processes affect surface fluxes. The use of satellite‐derived snow information produced a difference in surface fluxes over the Tibetan Plateau, especially for May.We also compare surface fluxes estimated by the long‐term reanalysis (JRA25) product of the JMA as well as GAME reanalysis product with in situ observations by the Asian Automatic Weather Station (AWS) Network (AAN). Radiosonde observations made 4 times daily were incorporated into the atmospheric field of the 4DDA GAME reanalysis and resulted in a good correspondence between the reanalyses products and surface observations, especially for July. The use of satellite‐derived snow information as a boundary condition also had improved flux results over the Tibetan Plateau in May. Copyright © 2007 John Wiley & Sons, Ltd.

Distributed tank model and GAME reanalysis data applied to the simulation of runoff within the Chao Phraya River Basin, Thailand

AbstractThe tank model is well known as a lumped conceptual hydrological model. In contrast with the large number of physical‐based models developed over the past 30 years, only a few distributed conceptual hydrological models are available. In this study, a high‐resolution distributed tank model is used to simulate river flow within a continental river basin, the Chao Phraya River basin, Thailand, using GEWEX Asian Monsoon Experiment (GAME) Reanalysis data. The model has a spatial resolution of 0·1 by 0·1 degrees and a temporal resolution of 1 h. Four terrain types and ten types of land use are identified in the model, and the model parameters were found to vary with the terrain type and land use. The model parameters were first determined in the 4841 km2 Khwae Noi River basin as a prototype area before being generalized to the entire 160 000 km2 of the Chao Phraya River basin. Simulation results at various sites indicate that the distributed tank model provides a sound approximation of the observed runoff data and is therefore a useful and reliable tool for the runoff simulation in this tropical continental river basin. Our results also indicated that after assimilating observational data, it was possible to use GAME Reanalysis data for such a distributed hydrological simulation. Copyright © 2007 John Wiley & Sons, Ltd.

GEWEX Asian Monsoon Experiment–Hydrological Aspects

GEWEX Asian Monsoon Experiment–Hydrological Aspects

Nocturnal Evolution of Cloud Clusters over Eastern China during the Intensive Observation Periods of GAME/HUBEX in 1998 and 1999

Global Water Cycle Experiment (GEWEX)-Asian Monsoon Experiment (GAME)/Huaihe River Basin Energy Water Cycle Experiments (GAME/HUBEX) were conducted during the Meiyu period in 1998 and 1999. Using infrared brightness temperature (TBB) data of the Geostationary Meteorological Satellite (GMS)-5, we investigated the diurnal variation of 61 long-lasting cloud clusters that developed during GAME/HUBEX Intensive Observation Periods (IOPs). More than two-thirds of the cloud clusters, named nocturnal-type clusters, attained a convective peak between midnight and early morning, with most peaking between 00 and 02 LST. Almost all of these nocturnal-type cloud clusters developed in, or south of the Meiyu frontal zone. The other clusters, named evening-type cloud clusters, peaked from late afternoon to evening, and were less intense than the nocturnal clusters.GAME-Reanalysis (Version 1.5) data showed that the ageostrophic wind component over a large domain to the south of the Meiyu front shifted from easterly to southerly, and increased the velocity of the southwesterly airflow largely at low levels at nighttime (02 LST). This southwesterly ageostrophic synoptic-scale low-level jet (S-LLJ) transported large amounts of water vapor to the Meiyu front, forcing large moisture convergence within, and immediately south of the Meiyu frontal zone. The low-level moisture convergence, and the S-LLJ itself are proposed to support the nocturnal evolution or redevelopment of cloud clusters.

The relationship between Indian monsoon precipitation along the Qinghai–Tibet Highway and differences in sensible heat flux

AbstractSensible heat flux greatly influences the Indian monsoon. In this study, we calculated sensible heat flux time‐series for 12 sites over the western Tibetan Plateau using Price and Dunne's formula and adjusting the stability function. The time‐series were derived from the field observations from the GEWEX Asian Monsoon Experiment (GAME)/Tibet programme under the Global Energy and Water Cycle Experiment (GEWEX). This paper demonstrates that monthly sensible heat fluxes show strong correlations with corresponding precipitation, and that the correlation coefficients increase with precipitation amount. The preceding winter and spring solid precipitation (snowfall and resulting snowpack) can also influence sensible heat flux in May, but the situation is complex. The correlations between heat flux and snowfall at Amdo, Naqu, and Lhasa are negative, but they are positive at Gaize (also known as Gerze) and Dingri. There is a significant relationship between how the variations from the mean calculated heat fluxes at Amdo differ from those at Rikaze, or Dingri, Cuona and Longzi, and their respective June–September precipitation amounts. This phenomenon may result from changes in circulation. When the sensible heat fluxes are above average north of the influence of the Indian monsoon and below average to the south, the summer monsoon circulation develops early and with greater intensity and precipitation, and vice versa. Copyright © 2006 John Wiley & Sons, Ltd.

Comparison analysis of the summer monsoon precipitation between northern and southern slopes of Tanggula Mountains, Qinghai–Xizang (Tibetan) Plateau: a case study in summer 1998

AbstractBased on the precipitation data obtained through GEWEX Asian Monsoon Experiment–Tibet fieldwork from May to September 1998, this study investigated the features of the summer monsoon precipitation on the northern and southern slopes of the huge Tanggula Mountains in the Qinghai–Xizang (Tibetan) Plateau. The results show that the precipitation on the southern slope is about 50% higher than on the northern slope, whereas the frequency and diurnal pattern of the precipitation are very similar. The mean precipitation intensity on the southern slope is larger than on the northern slope. In most cases, the daily precipitation showed similar variation on both slopes, demonstrating that the precipitation processes might be similar. In the summer monsoon period, the local convective precipitation contributed to the total precipitation on both slopes and such a contribution on the southern slope is larger. Copyright © 2006 John Wiley & Sons, Ltd.

Dimension Characteristics and Precipitation Efficiency of Cumulonimbus Clouds in the Region Far South from the Mei-Yu Front over the Eastern Asian Continent

Abstract Dimension characteristics in precipitation properties of cumulonimbus clouds are basic parameters in understanding the vertical transport of water vapor in the atmosphere. In this study, the dimension characteristics and precipitation efficiency of cumulonimbus clouds observed in the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) Huaihe River Basin Experiment (HUBEX) are studied using data from X-band Doppler radars and upper-air soundings. The maximum echo area (EAmax) of the cumulonimbus clouds ranged from 0.5 to 470 km2, and the maximum echo top (ETmax) ranged from 2 to 19 km. The total number of cells (TNC) within the cumulonimbus clouds over their lifetime was from 1 to 25. The ETmax, TNC, area time integral (ATI), and total rainfall amount (Rtot) strongly correlate with the EAmax of the cumulonimbus clouds. The cell-averaged ATI (ATIcell = ATI/TNC), maximum rainfall intensity (RImax), and cell-averaged rainfall amount (Rcell = Rtot/TNC) increase when the EAmax is smaller than 100 km2. On the other hand, they are almost constant when the EAmax is larger than 100 km2. The rain productivity of small clouds (<100 km2 in EAmax) increases not only by the increase of the TNC but also by the intensification of cells, while that of large cumulonimbus clouds (>100 km2 in EAmax) increases by the increase of the TNC rather than by the intensification of cells. In the present study, precipitation efficiency (ɛp) is defined as the ratio of the total rainfall amount (Rtot) to the total water vapor amount ingested into the cloud through the cloud base (Vtot). The ɛp was calculated for six clouds whose vertical velocity data at the cloud-base level were deduced by dual-Doppler analysis throughout their lifetime. The ɛp ranged from 0.03% to 9.31% and exhibited a strong positive correlation with the EAmax. This indicates that more than 90% of the water vapor that enters the clouds through the cloud base is consumed to moisten the atmosphere and less than 10% is converted to precipitation and returned to the ground. The cumulonimbus clouds in the region far south from the mei-yu front over the eastern Asian continent efficiently transport water vertically and humidify the upper troposphere.

A climatological‐dynamical analysis associated with precipitation around the southern part of the Himalayas

The precipitation variability and circulation characteristics around the Himalayas are examined using the Tropical Rainfall Measuring Mission (TRMM) data, the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis, National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP‐NCAR) reanalysis, and some of the Indian radiosonde data sets. The observation by precipitation radar on board the TRMM satellite reveals an afternoon maximum of precipitation during the premonsoon season and midnight–early morning maximum during the summer monsoon season over the southern slopes of the Himalayas. The data also shows that the morning precipitation moves southward in the mature monsoon season. The GAME reanalysis reveals a robust diurnal cycle of the atmospheric system that is coherent with the diurnal cycle of precipitation around the southern slopes of the Himalayas. The significant increase of moisture due to the southeasterly wind in the mature monsoon season seems to produce favorable conditions for the midnight–early morning rain. The down‐valley wind at midnight probably triggers moist convection. The radiative cooling at the top of clouds may enhance the convection. The precipitation associated with the moist convection seems to generate a cold pool, which results in a density current. The downslope movement of the density current probably induces the southward movement of the precipitation system in the morning.

A Diagnostic Study on Interactions Between Atmospheric Thermodynamic Structure and Cumulus Convection over the Tropical Western Pacific Ocean and over the Indochina Peninsula

Interactions between the convective activity and the atmospheric thermodynamic structures are analyzed utilizing upper-air rawinsonde observations obtained by R/V Mirai, R/V Kaiyo, R/V Natsushima, of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) over the western tropical Pacific Ocean, and those over three GEWEX Asian Monsoon Experiment (GAME) stations: Chiang Mai, Non Khai, and Ubon Ratchathani. Special emphases are placed on understanding the correlation between convection and the atmospheric thermodynamic structures in relation to the recent findings of trimodal cloud levels over the warm ocean (e.g., Johnson et al. 1999) and to the cloud diagnostics proposed by Raymond and Blyth (1992). We first examine the relationships between a convection index and thermodynamic structure indices. A large correlation is found between the convective activity and lower-tropospheric (600-800 hPa) humidity, while there is no significant correlation between the convective activity and Convective Available Potential Energy (CAPE) or Convective Inhibition (CIN). Next, we apply a cloud diagnostic model introduced by Raymond and Blyth (1992) (referred to as RB92) to the observed profiles. As a result, it is shown that there are fundamentally 3 peaks of detrain-ment levels, which are lower-troposphere (near 900 hPa), mid-troposphere (near 450 hPa), and upper-troposphere (near 150 hPa), over ocean as well as over land. In the soundings over ocean, when the lower-troposphere (600-800 hPa) is dry, there is a tendency for simultaneous existence of stable layers both in the lower-troposphere and in the mid-troposphere. Such atmospheric thermodynamic structure is diagnosed as favorable for strengthened detrainments in the low- and mid-troposphere and weakened in the upper-troposphere. Finally, meridional winds are composited to the north and to the south of the maximum convective activity in the Inter Tropical Convergence Zone (ITCZ) region, respectively, over the tropical western Pacific Ocean. It is confirmed with upper-air soundings that there is a significant meridional divergence near the melting layer level in the mid-troposhere around 500-600 hPa and a significant meridional convergece near 350-400 hPa, in addition to the lower-tropospheric convergence and the upper-tropospheric divergence of the local Hadley Circulation. These additional circulations in the mid-troposphere are consistent with detrainment profiles diagnosed for observed atmospheric profiles utilizing RB92 cloud model. After all, it is strongly suggested that the cloud microphysics, such as melting and freezing, play significant roles in determining the large-scale circulation.

Convective Cloud Systems over the Tibetan Plateau and Their Impact on Meso-Scale Disturbances in the Meiyu/Baiu Frontal Zone

The relationship between convective activity over the Tibetan Plateau (TP) and meso-scale cloud systems in the Baiu/Meiyu frontal zone, are examined for the 1998 summer monsoon season by using GEWEX Asian Monsoon Experiment (GAME) reanalysis data and Geostationary Meteorological Satellite (GMS)-IR Tbb data. Diurnal cycle of the plateau-scale heat low associated with convective clouds is dominated, forming a plateau-scale convergence line. Moisture transport from south of TP is essential for developing convective clouds, and the convergence line. Occasionally, the convergence line extends to the eastern edge of the TP, which triggers a meso-scale cyclonic vortex over the head of the Yangtze River basin through the plateau edge cyclogenesis (PEC). This vortex induces a strong low level jet with moisture inflow to the east of it, which facilitates further development of the vortex to a meso-α scale cloud system embedded in the Baiu/Meiyu front (BMF) over China. The occurrence of PEC is likely to be modulated by an interaction between mid-latitude westerly waves, with a quasi-biweekly time scale, passing over the TP, and a shorter-period oscillation of the monsoon trough over the Indian sub-continent, with around a 4-7 day period.

Effect of heavy snowfall on ground temperature, northern Tibetan Plateau

AbstractVery heavy snowfall occurred in the Amdo-Nagqu region during winter 1997/98, and enormous numbers of sheep and yaks died due to starvation and low temperatures. Some observation sites of the GEWEX (Global Energy and Water Cycle Experiment) Asian Monsoon Experiment (GAME)-Tibet are located in this area. In this paper, the variation of the ground temperature (GT) on the northern part of the Tibetan Plateau and its relationship with the heavy snow cover is analyzed based on the GAME-Tibet in situ observational data at several sites. The temporal and spatial differences of the variations of the daily maximum, daily minimum and range in GT are significant in 1997/98 in the northern part of the Tibetan Plateau. For example, at site D110, the daily range in GT fluctuated only 0.2˚C from the end of December 1997 to mid-April 1998, but in the north, at site D66, the daily range in GT fluctuated between 5˚C and ∼20˚C at the same depth and during the same period. At the southernmost site, MS3637, the daily range in GT fluctuated within 1.0˚C from mid-November to early February. From mid-February to mid-March, the daily range in GT increased and the peak was 8.1 ˚C. The temperature variation was related to the heavy snowfall that occurred on the northern Tibetan Plateau in winter 1997/98. The snow-cover conditions at different sites on the northern Tibetan Plateau were evaluated quantitatively from the variation of the GT at shallow depths.

Estimating the criterion for determining water vapour sources of summer precipitation on the northern Tibetan Plateau

AbstractAccording to the precipitation and δ18O data obtained during the GEWEX Asian Monsoon Experiment–Tibet Intensive Observation Period, based on the knowledge that δ18O is lower in precipitation formed from ocean air mass vapour than that from local evaporation vapour, the water vapour sources can be identified from the δ18O values in precipitation. We attempt to give the identification criterion of δ18O values in precipitation. The threshold values chosen to distinguish between ocean and local sources are δ18O < −20‰ and δ18O > −13‰ respectively. According to this criterion, the proportion of local evaporation‐formed precipitation and ocean air‐mass‐formed precipitation in total precipitation was estimated. The average value of precipitation at three sites (NODA, Amdo and AQB) is 249·76 mm. Among this, precipitation formed directly by the ocean air mass vapour is 80·08 mm at most. Precipitation formed by water vapour evaporated from local places is 117·05 mm at least. That is to say that precipitation formed directly by the ocean air mass vapour accounts for 32·06% of the total precipitation at most. Precipitation formed by water vapour evaporated from local places accounts for 46·86% of the total precipitation at least. At least 21·8% of the total precipitation came from water vapour that was evaporated on the way and transported by the monsoon circulation. Copyright © 2005 John Wiley & Sons, Ltd.

An Observational Study of Intraseasonal Variations over Southeast Asia during the 1998 Rainy Season

Abstract In the present study, intraseasonal variations of precipitation over Thailand [Thailand daily rainfall (TDR)] during 1998 and the accompanying variations of the tropospheric circulation are analyzed, mainly with the use of rain gauge data and the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis version 1.5. Wavelet analysis reveals that the amplitudes of the intraseasonal variations of the 1998 TDR exhibit two peaks. One peak is categorized as a 30–60-day variation that appeared in the early half of the rainy season, while the other peak is categorized as a 10–20-day variation that appears in the later half of the rainy season. The variation of atmospheric circulation associated with the latter peak of the TDR is analyzed. When the 10–20-day variation of the TDR is in the most active phase, a negative streamfunction anomaly at the 850-hPa level exists over the Bay of Bengal, and a positive anomaly is found in the Southern Hemisphere to the south of the negative anomaly. Both anomalies are associated with cyclonic vortices and are accompanied by negative height anomalies at the same level. Such horizontal structures resemble the horizontal structure of an equatorial Rossby wave of the gravest meridional mode (n1ERW). The cyclonic disturbances move westward with speeds of 6–6.4 m s−1. In the vertical, the disturbances have a first-baroclinic structure in the troposphere, with a node at the 300–400-hPa levels. In addition, the amplitude of the streamfunction anomaly in the lower troposphere is smaller than that in the upper troposphere over the western Pacific but larger than that in the upper troposphere over the Bay of Bengal. The mechanism for the longitudinal difference in the vertical structure and comparison of the analyzed disturbance with n1ERW-type disturbances over the Pacific are also discussed.

Structure of Moist Layer and Sources of Water over the Southern Region Far from the Meiyu/Baiu Front

We investigate the structure of a moist layer and sources of water in the southern region far from the Meiyu/Baiu front using GAME (GEWEX Asian Monsoon Experiment, GEWEX: Global Energy and Water Cycle Experiment) reanalysis data. In the period from July 4 to 15, 1998, the Meiyu/Baiu front was located over northern China (the Yellow River Valley) and the Korean Peninsula. Both regions over the plain area of mainland China (the eastern region of 112°E line) and the East China Sea have very moist air masses in this period; the total precipitable water over the regions is about 45 kgm−2. However, the structure of these moist layers is quite different. The moist layer over mainland China is deep, reaching 600 hPa. On the other hand, the moist layer over the East China Sea is shallow, with a thickness of up to 800 hPa. This difference in the structure of the moist layers can be attributed to the development of a moist convective mixing layer and the generation of shallow convective clouds in the daytime over mainland China, as a result of the supply of abundant latent and moderate sensible heat fluxes from the land surface.We also examine the sources of water supplied into the Meiyu front using Colored Moisture Analysis (CMA). Though water vapor to generate the deep moist layer over mainland China is mainly transported from the upstream regions of the Asian summer monsoon, which include the Indian Ocean, the Indochina Peninsula, and the South China Sea, the rate of the contribution of water from the southeastern region of mainland China is over 15% of the total precipitable water, when the Meiyu front is located over northern China. This shows that the land surface over southeastern China is regarded as a major source of water to the Meiyu front.