South China Sea Monsoon Experiment Research Articles

Organization of Oceanic Convection during the Onset of the 1998 East Asian Summer Monsoon

Abstract The organizational modes of convection over the northern South China Sea (SCS) during the onset of the summer monsoon are documented using radar and sounding data from the May–June 1998 South China Sea Monsoon Experiment (SCSMEX). The onset occurred in mid-May with a rapid increase in deep convection over a 10-day period, accompanied by a major shift in the circulation over the east Asian region. Analysis of Bureau of Meteorology Research Centre (BMRC) radar data from Dongsha Island reveals a wide range of organizational modes of convection over the northern SCS. Proximity sounding data indicate that lower- and middle-level vertical wind shears exerted a dominant control over the orientation of convective lines within mesoscale convective systems in this region, as has been found in the Australian monsoon region and the equatorial western Pacific. The results are consistent with the conceptual model of LeMone et al. based on the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), except two new organizational modes have been identified: shear-parallel bands for strong low-level shear and weak midlevel shear when there is weak instability and the air is dry aloft, and shear-parallel bands for strong shears in both layers when the shear vectors are in the same direction. Midlatitude influences, namely, the passage of troughs over southern China, likely contributed to these two additional modes. The stratiform rain fraction from the convective systems during the monsoon onset period was relatively small (26%) compared to the estimated average of about 40% for the entire Tropics. This small fraction is attributed to the weak instability during the onset period and relatively dry air in the upper troposphere.

Evaluation of the U.S. Navy’s Modular Ocean Data Assimilation System (MODAS) Using South China Sea Monsoon Experiment (SCSMEX) Data

The Navy’s Modular Ocean Data Assimilation System (MODAS) is an oceanographic tool to create high-resolution temperature and salinity on three-dimensional grids, by assimilating a wide range of ocean observations into a starting field. The MODAS products are used to generate the sound speed for ocean acoustic modeling applications. Hydrographic data acquired from the South China Sea Monsoon Experiment (SCSMEX) from April through June 1998 are used to verify the MODAS model. MODAS has the capability to provide reasonably good temperature and salinity nowcast fields. The errors have a Gaussian-type distribution with mean temperature nearly zero and mean salinity of −0.2 ppt. The standard deviations of temperature and salinity errors are 0.98°C and 0.22 ppt, respectively. The skill score of the temperature nowcast is positive, except at depth between 1750 and 2250 m. The skill score of the salinity nowcast is less than that of the temperature nowcast, especially at depth between 300 and 400, where the skill score is negative. Thermocline and halocline identified from the MODAS temperature and salinity fields are weaker than those based on SCSMEX data. The maximum discrepancy between the two is in the thermocline and halocline. The thermocline depth estimated from the MODAS temperature field is 10–40 m shallower than that from the SCSMEX data. The vertical temperature gradient across the thermocline computed from the MODAS field is around 0.14°C/m, weaker than that calculated from the SCSMEX data (0.19°–0.27 °C/m). The thermocline thickness computed from the MODAS field has less temporal variation than that calculated from the SCSMEX data (40–100 m). The halocline depth estimated from the MODAS salinity field is always deeper than that from the SCSMEX data. Its thickness computed from the MODAS field varies slowly around 30 m, which is generally thinner than that calculated from the SCSMEX data (28–46 m).

Overview of the South China sea monsoon experiment

The present paper gives an overview of the key project “ South China Sea Monsoon Experiment (SCSMEX)” operated by the Ministry of Science and Technology of China during the period of 1996–2001. The SCSMEX is a joint atmospheric and oceanic field experiment which aims to better understand the onset, maintenance, and variability of the summer monsoon over the South China Sea (SCS). It is a large-scale international effort with many participating countries and regions cooperatively involved in this experiment. With the field observation in May–August 1998, a large amount of meteorological and oceanic data was acquired, which provides excellent datasets for the study of the SCS monsoon and the East Asian monsoon and their interaction with the ocean. The preliminary research achievements are as follows. (1) The earliest onset of the Asian monsoon over the SCS and Indo-China Peninsula has been well documented. From the viewpoint of the synoptic process, its onset is closely related to the early rapid development of a twin cyclone to the east of Sri Lanka. The conceptual model of the SCS monsoon onset in 1998 was put forward. The 50-year time series of the SCS monsoon onset date was also made. (2) Two major modes, namely the 30–60-day and 10–20-day oscillations were ascertained. The influences of the abnormal SCS monsoon on the precipitation over eastern China and its modes were identified. A strong (weak) monsoon over the SCS usually leads to less (more) precipitation over the middle and lower reaches of the Yangtze River basin, and more (less) precipitation in North China. (3) During the monsoon onset over the SCS, a wide variety of organized mesoscale convective systems (MCSs) were observed by a Doppler radar array deployed over the northern SCS. The relationship between large-scale circulations and MCSs during the monsoon onset process in 1998 was clearly revealed. It was suggested that there is a kind of positive feedback mechanism between large-scale circulations and MCSs. (4) The SST over the SCS during the early period influences the timing of the monsoon onset date and the monsoon’s intensity. During the monsoon onset, the ocean undergoes a process of energy release through air-sea interaction. During the break phase of the SCS monsoon, the ocean demonstrates the process of energy re-accumulation. Obvious differences in the air-sea turbulent flux exchange between the southern and northern parts of the SCS due to different characteristic features of the atmosphere and sea structure were observed in those regions. (5) The verification of impact of intensive observations on the predictive performance is made by the use of regional models. The air-sea coupled regional climate model (CRCM) was also developed under the SCSMEX Project. The simulation of the oceanic circulation in 1998 produced with the model was well compared with the observations.

Evolution and Structure of the Mesoscale Convection and Its Environment: A Case Study during the Early Onset of the Southeast Asian Summer Monsoon

Abstract The evolution and structure of mesoscale convection in the South China Sea (SCS) region are documented for the first time mainly using the dual-Doppler radar dataset collected during the South China Sea Monsoon Experiment (SCSMEX) in 1998. In particular, this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast Asian monsoon (SEAM). For the case of 15 May 1998, interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection. In the prefrontal region, the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast-to-southwest-oriented convection. In the postfrontal region, the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east-to-west orientation. In addition, the convergence between the southerly monsoon flow and the northerly postfrontal flow gener...

Estimation of Latent Heating of Rainfall during the Onset of the Indian Monsoon Using TRMM PR and Radiosonde Data

Abstract The objective of this study is to estimate the vertical structure of the latent heating of precipitation in the vicinity of the Himalayas. Based on a cloud physics parameterization and the thermodynamic equilibrium equation, a simple algorithm is proposed to estimate latent heating from a combination of radiosonde and Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) data, specifically, the radar reflectivity and the rain-rate estimates. An evaluation of the algorithm against 6-hourly areal averages from diagnostic budget studies during the South China Sea Monsoon Experiment (SCMEX) suggests that the algorithm captures well the vertical structure of latent heating between the top of the moist layer and the cloud-top detrainment layer. The retrieval algorithm was applied systematically over the Indian subcontinent and Tibetan plateau within a region comprising 15°–32°N and 70°–95°E during June, the month of monsoon onset, for three different years (1999, 2000, and 2001). The esti...

Convective Systems over the South China Sea: Cloud-Resolving Model Simulations

The two-dimensional version of the Goddard Cumulus Ensemble (GCE) model is used to simulate two South China Sea Monsoon Experiment (SCSMEX) convective periods [18–26 May (prior to and during the monsoon onset) and 2–11 June (after the onset of the monsoon) 1998]. Observed large-scale advective tendencies for potential temperature, water vapor mixing ratio, and horizontal momentum are used as the main forcing in governing the GCE model in a semiprognostic manner. The June SCSMEX case has stronger forcing in both temperature and water vapor, stronger low-level vertical shear of the horizontal wind, and larger convective available potential energy (CAPE). The temporal variation of the model-simulated rainfall, time- and domain-averaged heating, and moisture budgets compares well to those diagnostically determined from soundings. However, the model results have a higher temporal variability. The model underestimates the rainfall by 17% to 20% compared to that based on soundings. The GCE model-simulated rainfall for June is in very good agreement with the Tropical Rainfall Measuring Mission (TRMM), precipitation radar (PR), and the Global Precipitation Climatology Project (GPCP). Overall, the model agrees better with observations for the June case rather than the May case. The model-simulated energy budgets indicate that the two largest terms for both cases are net condensation (heating/drying) and imposed large-scale forcing (cooling/moistening). These two terms are opposite in sign, however. The model results also show that there are more latent heat fluxes for the May case. However, more rainfall is simulated for the June case. Net radiation (solar heating and longwave cooling) are about 34% and 25%, respectively, of the net condensation (condensation minus evaporation) for the May and June cases. Sensible heat fluxes do not contribute to rainfall in either of the SCSMEX cases. Two types of organized convective systems, unicell (May case) and multicell (June case), are simulated by the model. They are determined by the observed mean U wind shear (unidirectional versus reverse shear profiles above midlevels). Several sensitivity tests are performed to examine the impact of the radiation, microphysics, and large-scale mean horizontal wind on the organization and intensity of the SCSMEX convective systems.

Mechanism of formation of low level jets in the South China Sea during spring and summer of 1998

The mechanism responsible for the formation of the lowlevel jet (LLJ) over the South China Sea during spring and summer of 1998 was analyzed through using the assimilated grid data provided by the South China Sea monsoon experiment (SCSMEX). More analyses were emphasized on a heavy rainfall process in the Zhujiang Delta area between 8th and 10th June 1998 in which a LLJ played a key role. Furthermore, the MM5 meso-scale model was utilized in studying the effects of the weather systems on the LLJ. The results showed that the kinetic energy associated with the LLJ transferred upward when the LLJ formed, which results in the fact that the big velocity of the LLJ expanded upward to higher level. Simulation verifies that a synopticscale low pressure in the low troposphere of the west of South China moved toward the east, with a lowtroposphere synoptic-scale high pressure in the western Pacific moving toward the west. In this process, the approach of both systems leads to the reinforcement of pressure gradient force between them, making the kinetic energy needed by the formation of the LLJ increase. Through this process the LLJ formed and developed over the South China Sea between the low and high pressures. Because the formation of the LLJ indicates the enhancement of summer monsoon in the South China Sea, the mechanism responsible for formation of the LLJ could be used in explaining an enhancement of summer monsoon in the South China Sea.

The Role of Bay of Bengal Convection in the Onset of the 1998 South China Sea Summer Monsoon

Assimilated analysis fields from the South China Sea Monsoon Experiment and the outgoing longwave radiation data from the National Center for Atmospheric Research (NCAR) have been employed to describe the large-scale and synoptic features of the subtropical circulation during the Bay of Bengal (BOB; 6°–20°N, 80°–100°E) and South China Sea (SCS; 7°–20°N, 110°–120°E) monsoon onsets in 1998. The results show that the Asian monsoon onset during May 1998 exhibited a typical eastward development from the BOB region to the SCS domain. The weakening and retreat of the subtropical anticyclone from the SCS were preceded by the intrusion of westerlies and the development of convective activities over the northern part of the SCS (NSCS; 15°–20°N, 110°–120°E). As the vertical shear of zonal wind changes in sign, the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the SCS. Based on these observational results, version 4 of the NCAR climate model (CCM3) is used to investigate the physical link between the convection associated with the BOB monsoon vortex and the SCS summer monsoon onset, as well as the mechanism of the evolution of the low-level subtropical anticyclone over the SCS. Introduction of heating over the BOB results in vigorous convection over the BOB, and the BOB monsoon onset, as well as the development of westerlies and vertical ascent over the NSCS region due to an asymmetric Rossby wave response. Together with the low-level moisture advection, convection is induced over the NSCS. It is the condensation heating over the NSCS that causes the overturning of the meridional gradient of temperature over the SCS. Consequently the subtropical anticyclone in the lower troposphere over the SCS weakened gradually. Eventually as convection develops over the entire SCS domain, the subtropical anticyclone moves out of the region.

Mechanism of thermal features over the Indo-China peninsula and possible effects on the onset of the South China sea monsoon

The thermal characteristics during the South China Sea (SCS) summer monsoon onset period near the Indo-China Peninsula are analyzed by using the South China Sea Monsoon Experiment (SCSMEX) reanalysis data from 1 May to 31 August 1998 and the NCEP/ NCAR pentad-mean reanalysis data from January 1980 to December 1995. The possible relationships between the anomaly of thermal features near the Indo-China Peninsula and the SCS monsoon onset are investigated, and the mechanism causing the SCS summer monsoon onset is also discussed. Results from the 1998 SCSMEX reanalysis data show that there exists a strong persistent surface sensible heating near the Indo-China Peninsula prior to the SCS monsoon onset, which has apparent low frequency oscillation features. This sensible healing leads lo a warmer center in the lower atmosphere near the Indo-China Peninsula and strong local horizontal temperature and geopotential height gradients which are favorable to strengthening the southwest wind over the Indo-China Peninsula. It is also found that stronger convergent winds at lower levels and stronger divergent winds at high levels appear, which provide a favorable configuration for the development of vertical motion, enhancement of precipitation, and onset of the SCS monsoon. These results can be verified by analysis of the multi-year mean data. Additionally, it is found that the temperature at 850 hPa increases more rapidly over the Indo-China Peninsula than the South China Sea prior to the SCS monsoon onset, which leads to a strengthening of the temperature difference between the Indo-China Peninsula and the South China Sea. Moreover, results from the analysis of the longitudinal temperature and geopotential height differences show that the eastern retreat of the subtropical high over the Indo-China Peninsula during the period of SCS monsoon onset is associated with the temperature increase over the Indo-China Peninsula and the eastern extension of low trough over the Bay of Bengal.

Clear-Sky Surface Solar Radiation during the South China Sea Monsoon Experiment

Downward total solar fluxes measured at Dungsha coral island (20°42'N, 116°43'E) during the South China Sea Monsoon Experiment (May-June 1998) have been calibrated and compared with radiative transfer calculations for three clear-sky days. Model calculations use water vapor and temperature profiles from radiosonde measurements, and the aerosol optical thickness is derived from sunphotometric radiance measurements. Results show that the difference between observed and model-calculated downward surface fluxes is ＜ 3 % of the daily mean. Averaged over the three clear days, the difference reduces to 1 %. The averaged downward surface fluxes from observations and calculations for the three selected dates are 314 and 317 W/m2, respectively. This result is consistent with a previous study using TOGA CO ARE measurements, which found good agreement between observations and model calculations. This study provides an extra piece of useful information for the modeling of radiative transfer, which helps to fill in the puzzle of the absorption of solar radiation in the atmosphere.

Intraseasonal variability of the subsurface temperature ob-served in the South China Sea (SCS)

Using Morlet wavelet transform we investigated the intraseasonal variability of the subsurface temperature in the SCS by analyzing ATLAS mooring data obtained during the South China Sea Monsoon Experiment (SCSMEX). It is found for the first time that subsurface temperature in the SCS exists intraseasonal variability, particularly in winter. The strongest intraseasonal variability ranges from 50 to 100 m with its maximum amplitude about 1.0–2.0°C. The subsurface temperature intraseasonal variability in the SCS is primarily induced by local wind stress curl via adjusting the vertical displacement of thermocline. It reveals the important characteristic of intraseasonal variations in the SCS upper ocean vertically.

Rain retrievals using Tropical Rainfall Measuring Mission and Geostationary Meteorological Satellite 5 data obtained during the SCSMEX

An accurate technique for tracking heavy rains induced by mesoscale convective systems occurring over the northern South China Sea during the Mei-Yu season is urgently needed by nearby countries. By using data observed during the period of the South China Sea monsoon experiment, a simple technique was developed for combining passive microwave and infrared rainfall observations for retrieving oceanic rain rates. The equations for estimating oceanic rain rates were developed using the Tropical Rainfall Measuring Mission Microwave Imager data and the corresponding island rain rates with the linear regression method. By using the Visible and Infrared Scanner 11 w m infrared data, a calibrated infrared rain rate estimation technique (GOES Precipitation Index) used the microwave rain retrievals as the ground truth for estimating the best infrared rain threshold. At times when the Tropical Rainfall Measuring Mission's observations were not available, the following infrared rain retrievals were made using the 11 w m infrared data from the Geostationary Meteorological Satellite 5. The microwave/infrared technique presented the ability to track heavy rains induced by mesoscale convective systems. Though applications in other regions and different seasons have yet to be tested, the procedures followed in this study could be utilized for universal applications, especially for those oceanic regions lacking conventional stations.

Mechanisms Responsible for the Maintenance of the 1998 South China Sea Summer Monsoon.

This study examines the dynamic and thermodynamic characteristics associated with the evolution of the SCSSM of 1998 using the data from the South China Sea Monsoon Experiment (SCSMEX). The objective is to identify the physical processes that determined the active and inactive periods of the 1998 SCSSM. The intraseasonal variations in the activity of the 1998 SCSSM are found to be mainly controlled by the 30-60-day oscillation, but modified by the 10-20-day mode. Definition of the active and inactive period is thus based on the OLR anomaly and its 30-60-day filtered component, giving two active (25-30 May, 23 June-5 July) and two inactive periods (6-18 June, 13-26 July). An examination of the circulation features suggests that the two modes operated very differently. After the onset of the SCSSM, a northward propagation of the 30-60-day mode into the SCS dictates the active and inactive phases of the SCSSM. During an active (inactive) phase, positive (negative) relative vorticity and convergence (divergence) associated with a low-level trough (ridge) propagated into the SCS, initiating (suppressing) the convection. The divergence field at the upper level was in phase with the low-level relative vorticity. However, the direction of propagation associated with the 10-20-day mode was different. A westward propagation of this mode into the SCS caused changes in the convection over the region during the first half of the 30-60-day mode. A second oscillation apparently propagated from the midlatitudes into the SCS to initiate or suppress convection. The dynamic conditions in terms of the vorticity and divergence are similar to those for the 30-60-day mode. The atmosphere over the SCS was also found to be generally favourable for the development of deep convection after the onset of the SCSSM.

Characteristics of the 1998 Summer Monsoon Onset over the Northern South China Sea.

Observations from the May–June 1998 South China Sea Monsoon Experiment (SCSMEX) have been used to determine the characteristics of the onset of the summer monsoon over the northern South China Sea (SCS). The onset occurred in mid-May with a rapid increase in deep convection over a weekto-ten-day period, followed by a ~ten-day break, and then a resumption of deep convection for a week-toten-day period in early June. The temporal and spatial variabilities of rainfall rates from the atmospheric moisture budget, satellite, and model estimates generally agree well, although the mean rainfall rate from the models for the 47-day Intensive Observing Period (IOP) is about 25% greater than the other estimates. The SST gradually increased over the northern SCS during the IOP, interrupted by slight cooling following monsoon onset until early June when the warming resumed. Surface sensible and latent heat fluxes increased after onset, but then decreased in June as a result of warm, moist air advecting over cooler water near the south China coast. The mean apparent heat source Q1 during convective periods resembled that determined for other tropical oceanic regions with a peak near 400 hPa. The apparent moisture sink Q2 also resembled profiles for other tropical regions, except for larger values below 900 hPa. The heating and moistening rates and vertical eddy flux of total heat during the early-June active period were greater than those observed during the May monsoon onset active period, indicating more vigorous deep convection as the monsoon ensued, a finding supported by Tropical Rainfall Measuring Mission (TRMM) precipitation radar data.

Evolution of the Large Scale Circulation, Cloud Structure and Regional Water Cycle Associated with the South China Sea Monsoon during May-June, 1998.

In this paper, changes in the large-scale circulation, cloud structures and regional water cycle associated with the evolution of the South China Sea (SCS) monsoon in May-June 1998 were investigated using data from the Tropical Rainfall Measuring Mission (TRMM) and field data from the South China Sea Monsoon Experiment (SCSMEX). Results showed that both tropical and extratropical processes strongly influenced the onset and evolution of the SCS monsoon. Prior to the onset of the SCS monsoon, enhanced convective activities associated with the Madden and Julian Oscillation were detected over the Indian Ocean, and the SCS was under the influence of the West Pacific Anticyclone (WPA) with prevailing low level easterlies and suppressed convection. Establishment of low-level westerlies across Indo-China, following the development of a Bay of Bengal depression played an important role in building up convective available potential energy over the SCS. The onset of SCS monsoon appeared to be triggered by the equatorward penetration of extratropical frontal system, which was established over the coastal region of southern China and Taiwan in early May. Convective activities over the SCS were found to vary inversely with those over the Yangtze River Valley (YRV). Analysis of TRMM microwave and precipitation radar data revealed that during the onset phase, convection over the northern SCS consisted of squall-type rain cell embedded in meso-scale complexes similar to extratropical systems. The radar Z-factor intensity indicated that SCS clouds possessed a bimodal distribution, with a pronounced signal (less than 30dBz) at a height of 2-3 km, and another one (less than 25 dBz) at the 8-10 km level, separated by a well-defined melting level indicated by a bright band at around 5-km level. The stratiform-to-convective cloud ratio was approximately 1:1 in the pre-onset phase, but increased to 5:1 in the active phase. Regional water budget calculations indicated that during the active phase, the SCS was a strong sink (E-P much less than 0) of atmospheric moisture, with the primary source of moisture coming from regions further west over Indo-China and the eastern Indian Ocean. Before onset and during the break, the SCS was a moisture source (E-P greater than ) to the overlying atmosphere. In particular, the SCS provided the bulk of moisture to the torrential rain over the YRV in mid-June 1998.

Evaluation of the Princeton Ocean Model Using South China Sea Monsoon Experiment (SCSMEX) Data

The Princeton Ocean Model (POM) has been implemented in the South China Sea for hindcast of circulation and thermohaline structure. A two-step technique is used to initialize POM with temperature, salinity, and velocity for 1 April 1998 and integrate it from 1 April 1998 with synoptic surface forcing for 3 months with and without data assimilation. Hydrographic and current data acquired from the South China Sea Monsoon Experiment (SCSMEX) from April through June 1998 are used to verify, and to assimilate into, POM. The mean SCSMEX data (Apr‐Jun 1998) are about 0.58C warmer than the mean climatological data above the 50-m depth, and slightly cooler than the mean climatological data below the 50-m depth, and are fresher than the climatological data at all depths and with the maximum bias (0.2‐0.25 ppt) at 75-m depth. POM without data assimilation has the capability to predict the circulation pattern and the temperature field reasonably well, but has no capability to predict the salinity field. The model errors have Gaussian-type distribution for temperature hindcast, and non-Gaussian distribution for salinity hindcast with six to eight times more frequencies of occurrence on the negative side than on the positive side. Data assimilation enhances the model capability for ocean hindcast, if even only conductivity‐temperature‐depth (CTD) data are assimilated. When the model is reinitialized using the assimilated data at the end of a month (30 Apr; 31 May 1998) and the model is run for a month without data assimilation (hindcast capability test), the model errors for both temperature and salinity hindcast are greatly reduced, and they have Gaussian-type distributions for both temperature and salinity hindcast. Hence, POM gains capability in salinity hindcast when CTD data are assimilated.

Analysis of the characteristics of 30– 60 day low-Frequency oscillation over asia during 1998 SCSMEX

The wavelet analysis is performed of the mid- and low-latitude circulation index at 850 hPa over East Asia, the East Asian monsoon index and the precipitation over the middle and lower reaches of the Yangtze River during 1998 South China Sea Monsoon Experiment (SCSMEX) from May to August. Analysis shows that distinct 30–60 day low-frequency oscillation (LFO) exists in all of the above elements during the exper-iment period. Analysis of low-frequency wind field at 850 hPa from May to August with 5 days interval is performed in this paper. Analysis results reveal that: (1) A low-frequency monsoon circulation system over East Asia, characterized by distinct 30–60 day low-frequency oscillation, exists over 100°-150°E of East Asian area from the middle and eastern parts of China continent and the South China Sea to the western Pacific in both the Northern and Southern Hemisphere. The activity of East Asian monsoon is mainly af-fected by the low-frequency systems in it; (2) All of the tropical monsoon onset over the South China Sea in the fifth pentad of May, the beginning of the Meiyu period and heavy rainfall over the middle and lower reaches of the Yangtze River in mid-June and the heavy rainfall after mid-July are related to the activity of low-frequency cyclone belt over the region, whereas the torrential rainfall over the upper reaches of the Yangtze River in August is associated with the westward propagation of low-frequency anticyclone into the mainland; (3) There are two sources of low-frequency oscillation system over East Asia during SCSMEX. i.e. the equatorial South China Sea (SCS) and mid-high latitudes of the middle Pacific in the Northern Hemisphere. The low-frequency system over SCS propagates northward while that in mid-high latitudes mainly propagates from northeast to southwest. Both of the heavy rainfall over the middle and lower reaches of the Yangtze River in June and July are associated with the northward propagation of the above-mentioned SCS low-frequency systems from the tropical region and the southwestward propagation from mid-high latitudes respectively and their convergence in the middle and lower reaches of the Yangtze River; (4) There are two activities of low-frequency cyclone and anticyclone belt each in the East Asian monsoon system during May to August. However the activity of these low-frequency circulation systems is not clearly relevant to the low-frequency circulation system in the Indian monsoon system. This means that the low-frequency circulation systems in Indian monsoon and East Asian monsoon are independent of each other. The concept previously put forward by Chinese scholars that the East Asian monsoon circulation sys-tem (EAMCS) is relatively independent monsoon circulation system is testified once more in the summer 1998.

A sea surface radiation data set for climate applications in the tropical western Pacific and South China Sea

The sea surface shortwave and longwave radiative fluxes have been retrieved from the radiances measured by Japan's Geostationary Meteorological Satellite 5. The surface radiation data set covers the domain 40°S‐40°N and 90°E–170°W and a period starting from January 1998. The temporal resolution is 1 day, and the spatial resolution is 0.5°×0.5° latitude‐longitude. The retrieved surface radiation has been validated with the radiometric measurements at the Atmospheric Radiation Measurement (ARM) site on Manus Island in the equatorial western Pacific. It has also been validated with the measurements at the radiation site on Dungsha Island during the South China Sea Monsoon Experiment (SCSMEX) (May and June 1998). The data set is used to study the effect of El Niño and East Asian summer monsoon on the heating of the ocean. Interannual variations of clouds associated with El Niño and the East Asian summer monsoon have a large impact on the radiative heating of the ocean, exceeding 40 W m−2 in seasonal mean over large areas. Together with the Clouds and the Earth's Radiant Energy System (CERES) shortwave fluxes at the top of the atmosphere and the radiative transfer calculations of clear‐sky fluxes, this surface radiation data set is also used to study the impact of clouds on the solar heating of the atmosphere. It is found that clouds enhance the atmospheric solar heating by ∼21 W m−2 in the tropical western Pacific and the South China Sea.

Modeling of the 1998 East Asian Summer Monsoon by a limited area model with p—σ incorporated coordinate

The 1998 East Asian Summer Monsoon is simulated by use of an improved nine—level p—σ model, the boundary forcing is the South China Sea Monsoon Experiment (SCSMEX) reanalysis data from May 1 to August 31, 1998. It is found that basic features of the atmospheric circulation (such as the South Asia high and the West Pacific subtropical high) can be simulated fairly. However the South Asia high is a little stronger than the observed, while the West Pacific subtropical high a little weaker. Seen from variations of the time correlation coefficient, this model is good for the short-time climate simulation (less than two months), while for the long—time simulation, its climate drift is a little obvious. It can be also seen from the spatial distribution of correlation coefficient that the worse simulation areas of the model are located in the Tibetan Plateau and the adjacent northwest Indo—China Peninsula. For the simulation of precipitation, the movement of rain belt from May to June can be simulated, but the simulation of July and August precipitation shifts obviously to north of the observed. It is also found from the analysis of sensitive experiment that the improvement of the nested boundary condition has a great impact on the simulation results, especially on the precipitation, so the model and the nesting technique need further improvements.

A possible mechanism effecting the earlier onset of southwesterlymonsoon in the South China Sea compared to the Indian monsoon

A possible mechanism effecting the earlier onset of the South China Sea (SCS) southwesterly (summer) monsoon compared to the Indian monsoon is revealed by use of the NCAR/NCEP reanalysis data during the 1998 South China Sea Monsoon Experiment (SCSMEX). It is found that the fundamental cause might be the different spatial distributions and time variations of the surface sensible heat (SH) fluxes in the two areas. In the SCS area, positive surface sensible heating exists all the time in both the tropics and the subtropics with the latter being dominant after March. While in the India Peninsula (IP) area, the tropical heating is the dominant one at the early period before May and the subtropical heating is smaller and even negative. It is further found that the west-east contrasts of the SH fluxes at middle latitudes are also favorable to the earlier onset of the SCS monsoon.