Jun Asanuma

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

[1] 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 C T and water vapor C q in their dimensionless functions of stability under a free convection limit were both 1.1, unlike the differences (i.e., C T ≤ C q ) 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 C T = 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, C T /C q was approximated as r Tq (where r Tq is a correlation coefficient for the fluctuations of temperature and water vapor) because of the sensitivity of C q 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.

An overview of the rangelands atmosphere–hydrosphere–biosphere interaction study experiment in northeastern Asia (RAISE)

Summary Intensive observations, analysis and modeling within the framework of the rangelands atmosphere–hydrosphere–biosphere interaction study experiment in northeastern Asia (RAISE) project, have allowed investigations into the hydrologic cycle in the ecotone of forest-steppe, and its relation to atmosphere and ecosystem in the eastern part of Mongolia. In this region, changes in the climate have been reported and a market oriented economy was introduced recently, but their impact on the natural environment is still not well understood. In this RAISE special issue, the outcome is presented of the studies carried out by six groups within RAISE, namely: (1) Land-atmosphere interaction analysis, (2) ecosystem analysis and modeling, (3) hydrologic cycle analysis, (4) climatic modeling, (5) hydrologic modeling, and (6) integration. The results are organized in five relevant categories comprising (i) hydrologic cycle including precipitation, groundwater, and surface water, (ii) hydrologic cycle and ecosystem, (iii) surface–atmosphere interaction, (iv) effect of grazing activities on soils, plant ecosystem and surface fluxes, and (v) future prediction. Comparison with studies on rangelands in other parts of the world, and some future directions of studies still needed in this region are also summarized.

Turbulence variance characteristics of temperature and humidity in the unstable atmospheric surface layer above a variable pine forest

Second-order turbulence moments of vertical velocity w, temperature u, and specific humidity q observed with an aircraft in the atmospheric surface layer over a flat maritime pine forest were used to investigate the applicability of the variance methods to variable terrain. The variances of w and u were found approximately to obey Monin- Obukhov similarity, while that of q clearly did not. Further investigation suggested the possibility that the surface variability affected the fluctuations of q and that it also caused a breakdown of the similarity between the scalars u and q. Different versions of the variance method were tested, and several of them were found to predict the surface flux of sensible heat with satisfactory accuracy. The latent heat flux estimates were in reasonable agreement when the dimensionless functions were calibrated locally.

Response of gross ecosystem productivity, light use efficiency, and water use efficiency of Mongolian steppe to seasonal variations in soil moisture

8 [1] The examination of vegetation productivity and use of light and water resources is 9 important for understanding the carbon and water cycles in semiarid and arid 10 environments. We made continuous measurements of carbon dioxide and water vapor 11 fluxes over an arid steppe ecosystem in Mongolia by using the eddy covariance (EC) 12 technique. These measurements allow an examination of EC-estimated gross ecosystem 13 productivity (GEP), light use efficiency (LUE), and water use efficiency (WUE) of the 14 steppe. Daily variations of GEP, LUE, and WUE were associated with daily variations of 15 incident photosynthetically active radiation (PAR), ambient temperature (Ta), and vapor 16 pressure deficit (VPD). The magnitudes of these variations were also dependent on canopy 17 development. On the daily basis, GEP linearly correlated with evapotranspiration rate and 18 PAR. LUE correlated positively with leaf area index, Ta, and soil moisture availability but 19 negatively with the surface reflectivity for short-wave solar radiation. Throughout the 20 growing season, both GEP and LUE responded strongly to precipitation-fed soil moisture 21 in the top 20 cm of the soil. An examination of the responses of LUE and WUE to PAR 22 under different soil moisture conditions shows that when soil water availability exceeded 23 VPD, the steppe was most efficient in light use, whereas it was less efficient in water use. 24 The multivariate analysis of variance also suggests that soil moisture availability, 25 especially water status in the upper 20-cm soil layer with dense distribution of grass roots, 26 is the most significant factor that governs GEP, WUE, and LUE. This study provides a 27 preliminary assessment of the use of available water and light by the Mongolian arid 28 steppe ecosystems under seasonally varying soil moisture conditions. A better 29 understanding of these functional responses is required to predict how climate change may 30 affect arid steppe ecosystems.

Calculation of near-surface layer turbulent transport and analysis of surface thermal equilibrium features in Nagqu of Tibet

Abstract This study investigates aerodynamic roughness Z0, drag coefficient CD , momentum flux τ , sensible heat flux H and latent heat flux LE in terms of the extended gradient method, followed by dealing with net radiation R and coefficient of soil thermal exchange G with the daily variation by dint of the thermal balance technique and Fourier law. The data used in this analysis are those of the Nagqu near-surface layer flux observations obtained during GAME/Tibet (GEWEX Asian Monsoon Experiments/Tibet) in the period June 1 to July 20, 1998.

Dataset of CarboEastAsia and uncertainties in the CO2 budget evaluation caused by different data processing

The datasets of net ecosystem CO2 exchange (NEE) were acquired from 21 forests, 3 grasslands, and 3 croplands in the eastern part of Asia based on the eddy covariance measurements of the international joint program, CarboEastAsia. The program was conducted by three networks in Asia, ChinaFLUX, JapanFlux, and KoFlux, to quantify, synthesize, and understand the carbon budget of the eastern part of Asia. An intercomparison was conducted for NEE estimated by three gap-filling procedures adopted by ChinaFLUX, JapanFlux, and KoFlux to test the range of uncertainty in the estimation of NEE. The overall comparison indicated good agreement among the procedures in the seasonal patterns of NEE, although a bias was observed in dormant seasons depending on the different criteria of data screening. Based on the gap-filled datasets, the magnitude and seasonality of the carbon budget were compared among various biome types, phenology, and stress conditions throughout Asia. The annual values of gross primary production and ecosystem respiration were almost proportional to the annual air temperature. Forest management, including clear-cutting, plantation, and artificial drainage, was significant and obviously affected the annual carbon uptake within the forests. Agricultural management resulted in notable seasonal patterns in the crop sites. The dataset obtained from a variety of biome types would be an essential source of knowledge for ecosystem science as well as a valuable validation dataset for modeling and remote sensing to upscale the carbon budget estimations in Asia.

Land surface identification near Yakutsk in eastern Siberia using video images taken from a hedgehopping aircraft

Aircraft-based remote sensing was carried out over the Lena River region (approximately 100 km horizontal scale) near Yakutsk, in eastern Siberia, on nine days between 24 April and 19 June 2000. A home-use video camera was installed on the aircraft and aerial images of the land surface were recorded. By flying at very low altitudes (100 m and 150 m), we compensated for the low resolution of the camera. In all, 5515 scenes were sampled at 10-s intervals from the video, visually interpreted, and classified. The snow/ice and water cover conditions were also interpreted. The results showed that grassland covers the riverine lowland (RLL) of the Lena River, while the principal form of land cover over the terraces is larch forest (about 61%). There was a remarkable contrast between the left (LBT) and right (RBT) bank terraces of the Lena River, no-forest areas covered 28.8% of the RBT, whereas no-forest areas covered only 13.8% of the LBT. In addition, the LBT had a greater proportion of birch forest than the RBT (15.2% vs 2.5%). The video data over eight days showed that the snow thawed first in the RLL and last in the forested areas of the LBT and RBT. An increase in the area covered by water in the RLL in mid-May was probably due to flooding by the Lena River. Distinguishing insolated from cloud-shadowed scenes of the land surface revealed that the insolation rate was lower over the LBT and RBT than over the RLL. The land-cover database created in this analysis will play an important role as basic, reliable ground-truth information for studies using satellite images.

Evaluation of MODIS-derived Evapotranspiration at the Flux Tower Sites in East Asia

Evapotranspiration (ET) is one of the major hydrologic processes in terrestrial ecosystems. A reliable estimation of spatially representavtive ET is necessary for deriving regional water budget, primary productivity of vegetation, and feedbacks of land surface to regional climate. Moderate resolution imaging spectroradiometer (MODIS) provides an opportunity to monitor ET for wide area at daily time scale. In this study, we applied a MODIS-based ET algorithm and tested its reliability for nine flux tower sites in East Asia. This is a stand-alone MODIS algorithm based on the Penman-Monteith equation and uses input data derived from MODIS. Instantaneous ET was estimated and scaled up to daily ET. For six flux sites, the MODIS-derived instantaneous ET showed a good agreement with the measured data (

Study of roughness lengths and drag coefficients over Nansha Sea region, Gobi, desert, oasis and Tibetan plateau

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Satellite-Based Analysis of Evapotranspiration and Water Balance in the Grassland Ecosystems of Dryland East Asia

Abstract Near-surface turbulent fluxes observations from three large-scale surface-atmosphere interaction field experiments are studied. In the context of the extended Monin-Obukhov similarity theory, investigation was performed of aerodynamic roughness (Z0), scalar roughness ( ZT, ZQ), neutral drag coefficient (CDN), bulk transfer coefficient (sensible heat exchange coefficient CHN and vapor analog CEN) from a calm and a rough sea of the Nansha region, separately, and of aerodynamic roughness and drag coefficients from Gobi, desert, oasis and the Tibetan plateau, together with the related distribution of the wind u . Some meaningful results have been derived.