GEWEX Asian Monsoon Experiment Research Articles

Diurnal and inter-monthly variation of land surface heat fluxes over the central Tibetan Plateau area

The energy and water cycle over the Tibetan Plateau play an important role in the Asian monsoon system, which in turn is a major component of both the energy and water cycles of the global climate system. Using field observational data observed from the GAME/Tibet (GEWEX (Global Energy and Water cycle Experiment) Asian Monsoon Experiment on the Tibetan Plateau) and the CAMP/Tibet (CEOP (Coordinated Enhanced Observing Period) Asia-Australia Monsoon Project (CAMP) on the Tibetan Plateau), some results on the local surface energy partitioning (diurnal variation, inter-monthly variation and vertical variation etc.) are presented in this study. The study on the regional surface energy partitioning is of paramount importance over heterogeneous landscape of the Tibetan Plateau and it is also one of the main scientific objectives of the GAME/Tibet and the CAMP/Tibet. Therefore, the regional distributions and their inter-monthly variations of surface heat fluxes (net radiation flux, soil heat flux, sensible heat flux and latent heat flux) are also derived by combining NOAA-14/AVHRR data with field observations. The derived results were validated by using the ground truth, and it shows that the derived regional distributions and their inter-monthly variations of land surface heat fluxes are reasonable by using the method proposed in this study. Further improvement of the method and its applying field were also discussed.

Turbulent exchange of heat, water vapor, and momentum over a Tibetan prairie by eddy covariance and flux variance measurements

Land‐atmosphere interactions on the Tibetan Plateau are important because of their influence on energy and water cycles on both regional and global scales. Flux variance and eddy covariance methods were used to measure turbulent fluxes of heat, water vapor, and momentum over a Tibetan shortgrass prairie during the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) in 1998. Under unstable conditions during the monsoon period (July–September), the observed standard deviations of temperature and specific humidity (normalized by appropriate scaling parameters) followed the Monin‐Obukhov theory. The similarity constants for heat CT and water vapor Cq in their dimensionless functions of stability under a free convection limit were both 1.1, unlike the differences (i.e., CT ≤ Cq) reported in other studies. While the transfer efficiency of heat and water vapor exchange generally agreed with the prediction from the Monin‐Obukhov theory, momentum exchange was less efficient than predicted. In comparison with the eddy covariance data, the flux variance method (with CT = Cq = 1.1) underestimated both heat and water vapor fluxes by <5%. When the eddy covariance data were absent, the flux variance method was used for gap filling the seasonal flux database. To estimate latent heat flux during the premonsoon period in June, CT/Cq was approximated as rTq (where rTq is a correlation coefficient for the fluctuations of temperature and water vapor) because of the sensitivity of Cq to changes in soil moisture conditions. The dramatic changes in the Bowen ratio from 9.0 to 0.4 indicate the shift of energy sources for atmospheric heating over the plateau, which, in turn, resulted in the shift of turbulent exchange mechanisms for heat and water vapor.

Impact of surface meteorological observations on RAMS forecast of monsoon weather systems over the Indian region

An attempt has been made to study the impact of surface meteorological observations on the Regional Atmospheric Modelling System (RAMS) simulation of a monsoon depression and two low pressure systems. The surface observations are blended with the GEWEX Asian Monsoon Experiment (GAME) gridded analyses for these cases. In one set of experiments the model is run in 12 hour nudging mode initially and then in forecast mode using GAME gridded data without incorporating surface observations. In another set of experiments surface data are incorporated to enhance the signature of the systems in the large scale GAME analyses and nudging is applied initially for twelve hours. Subsequently the model is run in forecast mode to see the temporal and spatial evolution of different meteorological features associated with the systems. It is found that inclusion of the surface data has in general enhanced the signature of the systems in the analysis and subsequently shows improvement in the forecast of sea-level pressure, geopotential, wind field, etc. and the associated forecast of heavy rainfall, in particular. To make a quantitative comparison of the predicted rainfall with the observed one, equitable threat score and bias are calculated for different threshold values of rainfall. It is clearly noted that inclusion of surface data has improved the precipitation forecast over the Indian land mass as indicated by the equitable threat score and bias for all the threshold rainfall categories.

Modeling of surface energy partitioning, surface temperature, and soil wetness in the Tibetan prairie using the Simple Biosphere Model 2 (SiB2)

This paper examines the performance of a stand‐alone version of the Simple Biosphere Model 2 (SiB2) to investigate the accuracy of the modeled surface energy components, surface effective radiative temperature, and soil wetness in a Tibetan short grass prairie from 15 July to 14 September 1998. During this monsoon period the mean canopy height and the leaf area index were 0.05 m and 0.5, respectively. The study site represented the prevailing conditions of the high Tibetan plateau with an average elevation of 4500 m. The model was initialized using in situ measurements and was driven by half‐hourly atmospheric observations. The modeled values for the two months period were compared with micrometeorological field measurements made through the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME)‐Tibet project. Results show that (1) when underestimating net radiation by 11%, the model overestimated sensible, latent, and soil heat fluxes by 8, 3, and 13% respectively; (2) the model generated warmer (colder) ground surface in daytime (nighttime); and (3) soil wetness was estimated reasonably.

OBSERVATION AND NUMERICAL EXPERIMENTS OF PRECIPITATION IN A TROPICAL MOUNTAIN IN SOUTHEAST ASIA

Since 1997, rainfall has been measured by using tipping-bucket rain gauges in a mountainous area as a part of GAME-Tropics (the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME)). Orographic precipitation tends to occur when easterly or southeasterly wind is predominated, wind speed is relatively strong (Average: 4.6m/s, S. D.: 2.1m/s), and the lifted condensation level is low. By investigating orographic precipitation using a regional atmospheric model under various atmospheric conditions, 1) comparatively strong monsoon westerly in the lower troposphere and complex terrain and 2) moist condition in the lower atmosphere including evapotranspiration from the wet ground are significant factors to the spatial pattern of precipitation in the mountainous region.

Surface energy budget and closure of the eastern Tibetan Plateau during the GAME-Tibet IOP 1998

Turbulent flux measurements based on the eddy correlation technique were conducted on the Tibetan Plateau during the Intensive Observation Period (IOP) of the GEWEX Asian Monsoon Experiment (GAME) in 1998. This paper presents on analysis of the surface energy budget and its closure at the Amdo planetary boundary layer site in the eastern Tibetan Plateau using GAME-Tibet IOP data. A seasonal variation in the surface energy closure ratio (CR) was seen. CR was higher than 0.8 in the pre-monsoon period and after DOY 233, when the infrared hygrometer was performing satisfactorily. However, CR was lower than 0.7 throughout most of the summer monsoon, due at least in part to degraded performance of the infrared hygrometer under heavy precipitation. In addition, through detail analysis of the diurnal variations of surface energy flux for the best-CR period of September 4–6, it was found that the melting and freezing of near-surface soil moisture plays a significant role in the variation of surface energy fluxes, particularly in terms of latent heat flux. The calculated effective mass of melting and freezing water in one day was 3.3–3.9 kg/m 2, which is comparable to the daily total.

Seasonal Contrasting Features of Heat and Moisture Budgets between the Eastern and Western Tibetan Plateau during the GAME IOP

Abstract Using the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) four-dimensional data assimilation (4DDA) upper-air data, the large-scale heat source (Q1) and moisture sink (Q2) over the western and eastern Tibetan Plateau are examined for a 4-month period from 1 May to 31 August 1998. The computations were performed on the sigma–pressure hybrid coordinate, named η-coordinate, since the analysis area includes high-elevation mountains. Over the western Tibetan Plateau in May, there is a deep layer of heating occupying the whole troposphere with the maximum value exceeding 3 K day−1 around 400–600 hPa. The smaller magnitude of the apparent moisture sink is confined in the lower troposphere 1 km above the ground surface. Vertically integrated heat sources of 103 W m−2 over the western Tibetan Plateau are accompanied by a moisture sink of about half that (60 W m−2). These results indicate that the latent heat release associated with condensation plays an important role in the total heating besides the sensible heat supply from the land surface. Later in July, the moisture sink over the eastern Tibetan Plateau nearly equals the heat source indicating the dominance of moist processes associated with summertime monsoon rains. The contrasting features of the heat source and moisture sink are closely related to the circulation fields. Throughout May and June, we observe strong upward motion along the western and southwestern slopes of the western Tibetan Plateau, while there is salient subsidence motion over the eastern Tibetan Plateau. The analyses of static stability and lifting condensation level indicate that the release of latent heat relevant to moist convection is a dominant factor for tropospheric heating after the monsoon onset, while the premonsoon period (May) is composed of both convective rainfall and dry thermal convection. Thus, the heating mechanism prior to the onset of the monsoon, especially over the western Tibetan Plateau, can be characterized by the hybrid nature of “wet” processes due to condensation heating and “dry” processes associated with the sensible heat flux from the elevated mountain surface.

Frozen soil parameterization in SiB2 and its validation with GAME-Tibet observations

This paper presents a new frozen soil parameterization in the land surface scheme (LSS). A modified approximation Stefan solution is incorporated in the framework of the land surface model-SiB2 to calculate the frost/thaw depth over time and to estimate the soil moisture and temperature profiles during the freeze/thaw cycle. The structure of the soil model in Simple Biosphere Model 2 (SiB2) is kept, but the governing equations of water balance and surface heat balance are modified to account for soil freezing/thawing. The model is calibrated and validated using the GEWEX Asian Monsoon Experiment (GAME)-Tibet observations at MS3608 site, Tibetan Plateau. First, the original SiB2 without frozen soil parameterization is calibrated using the observations in summer; an optimized parameter set for the short grassland in the Tibetan Plateau was obtained. Then, the modified SiB2 with frozen soil parameterization is validated using the soil moisture and temperature observations in winter. The results show that the new model estimates the frost depth precisely with an error less than 9% and predicts the soil temperature reasonably and phase transition time realistically. It also improves the soil moisture estimation in the surface layer and the root zone significantly. The mean absolute errors of simulated soil moisture using the new model are 0.020 and 0.013 in the surface layer and root zone, respectively, far below than those without using the frozen soil parameterization.

AN INVESTIGATION OF MONSOON RAINFALL OVER A TROPICAL MOUNTAIN IN SOUTHEAST ASIA USING REGIONAL CLIMATE MODEL

Since 1997, rainfall has been measured by using tipping-bucket rain gauges in a mountainous area as a part of GAME-Tropics (the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME)). The fact that a larger amount of rainfall was recorded at a high altitude was attributed to duration and frequency rather than intensity. After investigating orographic precipitation using a regional climate model, the duration and frequency of rainfall at high altitudes were found to be caused by two mechanisms: 1) convective clouds over mountains are activated in the evening, and 2) stratus formed by radiative cooling or heating at the tops of clouds and low-level orographically triggered convective clouds cause a seeder-feeder mechanism. The results of a numerical simulation were quite consistent with the measurements acquired from the tipping-bucket rainfall gauges.

On measuring and remote sensing surface energy partitioning over the Tibetan Plateau––from GAME/Tibet to CAMP/Tibet

The energy and water cycle over the Tibetan Plateau play an important role in the Asian monsoon system, which in turn is a major component of both the energy and water cycles of the global climate system. The intensive observation period and long-term observation of the GEWEX (global energy and water cycle experiment) Asian monsoon experiment on the Tibetan Plateau (GAME/Tibet) and CEOP (coordinated enhanced observing period) Asia–Australia monsoon project (CAMP) on the Tibetan Plateau (CAMP/Tibet) have been done successfully in the past five years. A large amount of data has been collected, which is the best data set so far for the study of energy and water cycle over the Tibetan Plateau. The field experiments of GAME/Tibet and CAMP/Tibet are introduced and some results on the local surface energy partitioning (imbalance, diurnal variation, inter-monthly variation and inter-yearly variation etc.) are presented by using the field observational data in this study. The study on the regional surface energy partitioning is of paramount importance over heterogeneous landscape of the Tibetan Plateau and it is also one of the main scientific objectives of GAME/Tibet and CAMP/Tibet. Therefore, the regional distributions of surface variables (surface reflectance and surface temperature), vegetation variables (NDVI, MSAVI, vegetation coverage and LAI) and surface heat fluxes (net radiation flux, soil heat flux, sensible and latent heat flux) are also derived by combining NOAA-14 AVHRR data with field observations in this study.

Observations of Energy Fluxes and Evapotranspiration over Terrestrial Complex Land Covers in the Tropical Monsoon Environment

The surface energy flux balance and total evapotranspiration were estimated using the eddy correlation, and bandpass covariance technique over a tropical monsoon environment within the framework of GEWEX (Global Energy and Water cycle Experiment) Asian Monsoon Experiment (GAME). The aim of this present study is to obtain information on the seasonal variation of heat and water vapor exchanges between the atmosphere and terrestrial land cover (complex area) in tropical monsoon environment. The result indicated the daily integrated values of net radiation, sensible heat, latent heat and ground heat flux during the observation period from July 1998 to February 1999 were 10.76 MJ m -2 , 2.32 MJ m -2 , 5.18 MJ m -2 and 0.03 MJ m -2 , respectively. Sensible and latent heat fluxes were the dominant energy partitioning components throughout the year. The seasonal difference in surface fluxes between wet and dry seasons was seen, and the latent heat flux was dominant in the monsoon, corresponding with the increase of specific humidity after frequent precipitation. Whereas the sensible heat flux increased as the surface temperature increased in the absence of rainfall during the dry season. However, the closure of energy balance remained unresolved as with the foregoing experimental studies. The estimated amount to evapotranspiration was 526 mm versus 641 mm of actual precipitation, and accounted for about 80% of the precipitation during this period.

Comparison of the Four Land Surface Models Driven by a Common Forcing Data Prepared from GAME/Tibet POP'97 Products. Snow Accumulation and Soil Freezing Processes.

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.

Application of Distributed Hydrological Model in the Asian Monsoon Tropic Region with a Perspective of Coupling with Atmospheric Models.

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.

Surface Energy Budget at Amdo on the Tibetan Plateau using GAME/Tibet IOP98 Data.

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.

Characteristics of Convective Clouds Observed by a Doppler Radar at Naqu on Tibetan Plateau during the GAME-Tibet IOP.

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.

Third International Scientific Conference on the Global Energy and Water Cycle, Jointly with the Fourth Study Conference on GEWEX Asian Monsoon Experiment (GAME)

Third International Scientific Conference on the Global Energy and Water Cycle, Jointly with the Fourth Study Conference on GEWEX Asian Monsoon Experiment (GAME)

Evaluation of Surface Fluxes Over a Paddy Field in Tropical Environment: Some findings from a preliminary observation of GAME.

A preliminary observation for the GEWEX Asian Monsoon Experiment (GAME) was carried out at one of the observation sites of GAME near Sukhothai, Thailand in the summer of 1996, in order to assess the performance of prototype stations for measuring surface fluxes and soil moisture as well as to evaluate relative merits of the methods of determining fluxes. The data sets from three observation systems yielded turbulence statistics and the surface flux values by applying the eddy correlation method, the bandpass covariance method, the Bowen ratio method, the profile method and the bulk method. Intercomparisons of the resulting values indicate that the turbulence statistics are of high quality but the algorithm for evaluating bandpass covariance needs further improvement. It was also found that an accurate evaluation of energy storage in water body is essential in the application of energy balance approaches at paddy field. Overall, all stations performed satisfactory during the experiment in a hot, humid environment. However, a better protection against a strong rainfall appears to be needed for long term observations in this area.