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RESEARCH ON LAND SURFACE PROCESS

LU Shihua and QIAN Zhengan

Cold and Arid Regions Environment and Engineering Research Institute, C.A.S. Lanzhou 730000, China

ABSTRACT

The some recent research activities on land surface process in China are briefly summarized in this paper. Some field experiment and preliminary scientific achievements of land surface model and numerical modeling are introduced and reviewed.

Key words: land surface process, field experiment, land surface model

The land surface is the main living space for the most of human being. The land surface process (LSP) deals with the exchanges of the energy and matter between land and atmosphere, and the energy and water exchange between land and atmosphere play an important role in formation and evolution of the local boundary layer and synoptic and climatic systems in a region or whole globe to determine the energy and water cycles in earth system. So many Chinese scientists pay much more attention to the LSP studies. In this paper the briefly view on them in China from 1999 to 2002 was made from three fold: the field experiment of LSP, the LSP model and its numerical simulation.

 

I.  THE FIELD EXPERIMENT ON LAND SURFACE PROCESS

To better estimate the various components of the surface energy and water budget and better understand the air-land exchange processes in different underly surfaces, the four field experiments and modeling for the LSP were implemented from 1999 to 2002 in China. They are the NWC-ALIEX, IMGRASS, HUBEX and GAME-Tibet Field Experiments.

1.  NWC-ALIEX Field Experiment

The field experiment on air-land interaction in the arid area of Northwest China (NWC-ALIEX) started with May, 2000 at three observation stations. The central observation station is located at Shuangdunzi Gobi to the west of Dunhuang-City, where it is in the downwind of the Taklimakan Desert. The second station is located at Linze, where it is close to the Badian Jaran Desert. The third station is located at Wudaoliang, where it is in the Qinhai-Xizang Plateau QXP and reaches a height of 4612 m above sea level.

The observation items of the NWC-ALIEX cover the wind, temperature and humidity gradient observations on meteorological tower, ground surface radiation, components, soil temperature, soil humidity, soil heat flux at multi layers. During period of Intensive Observation Period (IOP), the three-dimensional wind speed by ultrasonic detector, kite-balloon aerial exploration and the analysis of soil sample are added. Through the field experiments for three years, NWC-ALIEX has already got some preliminary scientific results of the LSP and the air-land interaction in the typical bare arid area of NWC (Northwest China). The results are as follows:

1) Characteristics of surface radiation balance in the land-surface of Suangdunzi Gobi

The maximum of the global radiation is close to 1000 W/m2 on the Suangdunzi Gobi. Due to the strong solar radiation, surface temperature of the Gobi can rapidly rise in daytime of summer, which can cause the strong upward long-wave radiation. As shown in Table 1, the daily integration of the net radiation can reach to 10.33 MJ/(m2·d), which is over one-third of the global radiation.

2) Characteristics of surface energy budget in the land-surface of Dunhuang Gobi

Since Dunhuang Gobi is in the dry area with the air relative humidity (RH) of 10%-20% and soil humidity of 1.0%-2.0%. The sensible heat flux is dominant and the latent heat flux is two orders smaller than the sensible heat flux, which can be also shown from the daily integrated values in Table 2.

Table 1. The Daily Integrated Values of the Global, Downward Long-Wave, Upward Long-Wave and Reflection Radiation

 

The daily integrated values of radiation (in MJ/m2d)

The global

32.42

Downward  long-wave

29.38

Upward  long-wave

4.06

Relection

7.42

Net Radiation

10.33

 

Table 2.  The Daily Integrated Values of the Sensible Heat, Latent Heat and the Soil Heat Fluxes

 

Daily integrated values (MJ/ m2·d)

Rate to net radiation (100%)

The daily integrated values (MJ/ m2·d)

Rate to net radiation (100%)

sensible heat flux

8.64

83.6

6.29

155.6

latent heat flux

0.17

1.67

0.11

2.73

soil heat flux

1.17

11.32

-0.37

-9.18

 

3) Some parameters of  LSP and air-land interaction in Dunhuang Gobi region

The bulk transfer coefficients over Dunhuang Gobi can be calculated by the observed surface fluxes with the eddy correlation, aerodynamic and combined methods, respectively (Zhang et al. 2001). As shown in figure, both the bulk momentum transfer coefficient Cd and the bulk sensible heat transfer coefficient C change with the bulk Richardson number under unstable or stable atmosphere. Moreover, Cd and C are 0.92×10 and 1.81×10 , respectively in neutral condition. The observational fact shows that the albedo in the Dunhuang Gobi surface fluctuates between 0.15 and 0.45 and is about 0.25 (Zhang et al., 2002). It is close to the desert-surface albedo given by Stull (1991). The logarithmic mean of roughness length can be calculated by using the observed data of NWC-ALIEX. The result shows that the roughness length over Dunhuang Gobi is close to 0.02 m in neutral state (Zhang et al. 2002), which is one time smaller than that over desert in HEIFE (Hu et al. 1990). The mean soil moisture factor is about 0.0045 in Dunhuang Gobi and it does not rely on the Richardson number. These values above can be used in model for arid region.

4) One new parameterization method based on remote sensing data and field observations was proposed and has been used to derive the regional land-surface heat fluxes over inhomogeneous landscapes. The surface albedo in Dunhuang Gobi in June 3, 2000 retrieved from satellite remote sensing data was about 0.22-0.27. This is very similar to the result observed in the field experiment.

2.  IMGRASS Field Experiment

To improving further the understanding of climate-ecology interaction under human activity impact in mid-latitude semi-arid grassland, a major research project called “Inner-Mongolia Semi-Arid Grassland Soil-Vegetation-Atmosphere Interaction (IMGRASS)” supported by NSFC and CAS was implemented. The meso-scale field experiment was conducted from May to September 1998 over the region of Xilin Gol district with latitudes of 43°-45°N and longitudes of 115.5°-117o E. Since then was field observations on small scale for short term for specific purpose were continued in years 1999-2001, such as observation of greenhouse gases flux, and specially mineral dust aerosol observation in spring of 2001 and the like .

The data base of the IMGRASS experiment contains observation data of soil moisture and temperature zone vegetation biomass and species, surface meteorological elements, surface flux, boundary layer sounding, as well as precipitation at four stations, which are representative of typical steppe, dry steppe, and sandy land savanna type, respectively. Also the surface flux observation of greenhouse gases such as N2O, CH , and CO at meadow steppe and typical steppe including fenced and grazing land steppe were given.

The fact that to the west of Huanshantake Sandy Land (HSL) is vast desert region and the strong winter monsoon predominates in winter and early spring, makes us to consider that the formation of HSL may be related to the winter monsoon and the unique physical geography. In IMGRASS area summer is the main rainy season in a year, and it is heavily influence by the east Asia summer monsoon. The precipitation climate over there is the common impact of weather system, terrain, and inhomogeneous land surface. Since IMGRASS area is consisted of different soil/vegetation types, the estimation of flux over the different underlying is, of course, significant important. Greenhouse gases emissions of N O, CH and CO in grassland are still with quiet large uncertainty according to IPCC 2001. The measurements of N O, CH , and CO were made at 5 different types of grassland with static box method. Statistical analysis was made for their emission rate, diurnal and seasonal variation. It is found that the annual emission rate of N2O in IMGRASS area is lower than IPCC 2001's estimation. The total contribution of CH budget in semi-arid grassland is small. The significant even severe degradation of grassland in IMGRASS area has happened in the last 30 years owing to the overgrazing and over cultivation.

3.  HUBEX Field Experiment

The Huaihe River Basin, in East China, where the HUBEX field experiment was carried out, is located between the Huanghe River and the Yangtze River. It is quite a closed basin extending 5 degrees in latitude and 9 degrees in longitude direction, with a total area of 270 000 km2, and belongs to warm temperate zone semi-humid monsoon climate region. For the year 1998, the observation was conducted during spring, summer and autumn respectively. As for the year 1999, the observation was conducted only at the cropland in summer. The observation elements cover: four components of radiation fluxes (upward and downward long and short-wave radiation), profiles in the lower ABL wind speed, air temperature, turbulent fluxes, air pressure, wind direction, precipitation, etc.

Under different synoptic conditions and on different time scales, the analyses of the energy budgets in the HUBEX in 1999 by Bowen ratio method and bulk schemes show that (1) the averaged latent heat flux is an order of magnitude more than the averaged sensible heat flux over the Huaihe River Basin ; (2) the variation of total cloud amount is out of phase with the energy budget terms except for the downward long wave radiation, which maybe is related to the cloud's height and types; (3) the values of sensible and latent heat fluxes are small during rain episodes, but thereafter the values become high and finally get their maximum. It is similar to the other items of the energy budgets except for the downward long wave radiation. The diurnal variation of energy budgets indicates that the daytime precipitation exerts great influence on the energy budgets, but the nighttime precipitation makes a little bit influence on them; (4) the variation of the latent heat flux is in phase with the evaporation, which indicates that the latent heat flux calculated by bulk schemes is reliable; (5) the mean values of the sensible and latent heat fluxes and momentum flux by bulk schemes from May to August are respectively 30.71 W/m2, 116.81 W/m2 and 2.86×10-2 N/m2 in 1998, and 30.28 W/m2, 107.35 W/m2 and 2.74×10-2 N/m2 in 1999. Namely the values of these two years are similar to each other. But in summer in 1999 the magnitude and activity of sensible heat flux are the strongest in June while those of the latent heat flux are the strongest in August.

4.  GAME/Tibet and TIPEX Field Experiment

The GAME/Tibet is the sub-experiment in Tibet of the GEWEX Asian Monsoon Experiment. GAME Tibet and TIPEX are cooperative studies on the land surface processes over the QXP. The former is in the framework of GAME and carried jointly out by China, Japan and some other Asian Countries, while TIPEX (Tibetan Plateau Hydrometeorological Experiment) is a national key project of China. The overall goal of both the GAME-Tibet and TIPEX is to clarify the interactions between the land surface and atmosphere over the QXP in the Asian monsoon systems.

The experiments on two different have been implemented: a nearly plateau scale experiment using the north-south and eastwest direction networks of operational meteorological and hydrological stations; a mesoscale experiment with 2D or 3D intensive LSP observations at about five sites. Based on GAME/Tibet data set, some results on surface heat flux and energy budget had been presented. The daily and seasonal variation of surface heat flux was derived from eddy turbulence measurement data and PBL tower station data of GAME/Tibet IOP'98 at Amdo. The result shows that surface sensible heat flux and latent heat flux were dominant heat exchange of the both pre- and post-monsoon onset respectively (Hu et al. 1999). The similar results were also be referred, but at other three flux stations (i.e. Northern PAM station, Korea site and Southern PAM station) of GAME/Tibet (Wang et al. 1999, Ma et al. 1999). The subsequent research work showed that the heat flux derived from eddy covariance method could not close the surface energy balance over QXP yet and possible reason was searched (Wang et al. 2000). Some scenarios of regional surface heat fluxes retrieved from remote sensing data (Ma et al. 2000) and derived from routine meteorological observation station data over QXP (Xu et al. 2001) roughly reflected the temporal variation and spatial distribution of surface energy exchange.

 

II.  LAND SURFACE PROCESS MODEL

1.  Land Surface Process Model

As a part of GAME/Tibet experiment, the SiB2 (Simple Biosphere Model Version 2) is adopted to simulate surface energy budget in offline mode utilizing certain atmospheric forcing values measured in the near surface observations over the Tibetan short grass prairie (GEWEX Asian Monsoon Experiment/ Tibet) experiment (Gao et al. 2002). When on the parameters are reasonably fixed, the simulated net radiation, latent heat flux and soil heat flux are shown to be basically in agreement with observations, with the relative errors being 8% (underestimated), 6% (underestimated), and 3% (underestimated), respectively; however the SiB2 overestimates the sensible heat flux by 40 percent . The detail analyses about energy components are also conducted.

The study methods of LSP parameterization model were summarized and established, which can be used to study the interaction between the land-surface physical processes of soil and vegetation types which make up the Earth surface. In the model, the improvements of soil structure parameterization and LSP turbulence flux parameterization under different underlying surfaces allow an extended class of terrain types to be included. The air-land system consists of three components: the atmosphere near the surface, the canopy and the active layer of soil. The soil is divided into four layers: the upper two layers show the diurnal and seasonal variations of temperature respectively, and the root system of vegetation is evenly distributed in these two layers. Temperature and moisture in the deeper layers are left to keep their climatic mean values. This assumption is much satisfactory when the simulation periods are not too extended. The LSP parameterization model is able to simulate the balance of energy for different climatic zones. It has particularly importance to improve the ecology environment in developing Northwest China and to reasonably exploit natural resources there.

2.  Snow Cover and Frozen Soil Parameterization

Over the QXP there are snow cover and frozen soil on large-scale in winter and spring, even partly in summer. Needless to say the snow cover and frozen soil process act a significant on both the hydrological cycles and atmospheric processes. Sun and Jin et al. (1999) have developed a new snow-atmosphere-soil transfer (SAST) model. This model is developed on the basis of up-to-date comprehensive and complex snow schemes but with substantial simplification and improvement. Based on the analysis of the effects of vapor on snow processes, vapor's contribution in the mass equation is eliminated, and an effective conductivity coefficient is used to describe its contribution to the energy equation to simplify the computation. Specific enthalpy is used in the energy balance equation. Preliminary testing using Russian and French snow cover data shows that the three-layer model is able to produce reasonable and consistent results.

The frozen soil parameterization in the LSP has got more and more attentions. Zhang and Lu (2002) have considered the frozen soil effects based on the NCAR Land Surface Model (LSM), and the effects of frozen soil on energy budget were investigated using Former Soviet Union 6 stations data. The results indicate that the incorporation of the frozen soil would improve the simulation on soil temperature and the surface water cycle, such as infiltration and runoff. A more complex frozen soil scheme is in progress (Sun Shufen et al., personal communication).

3.  Hydrological Process

The land surface hydrological processes, including runoff, infiltrability, and evaporation, has directly influenced the general circulation through the moisture transfer, and the climatic change has also impact the land surface water cycle and human's life. The research on water cycle is one of the IGBP's main tasks. Some work that focused on runoff parameterization was done in last several years. Liang and Xie (2001) have developed a new surface runoff parameterization based on VIC model. The new parameterization dynamically represents both Horton and Dunne runoff generation mechanisms within a model grid cell. Also the new model takes into account of effects of subgrid-scale spatial variability of soil heterogeneity and precipitation on Horton and Dunne runoff. The author also conclude that infiltration and saturation excess runoff are two important surface runoff generation mechanisms. If lacking one of them could result in significant errors in producing the total runoff and soil moisture for applications on the spatial scales where subgrid-scales spatial variability of soil heterogeneity is significant. Liu et al. (2002) also get an analytical expression for subgrid-scale inhomogeneous runoff ratios generated by heterogeneous soil moisture content and climatic precipitation forcing based on physical mechanisms for land surface hydrology and theory of statistical probability distribution. Thereby the commonly used mosaic parameterization of subgrid runoff ratio was integrated into a statistical-dynamic scheme with the bulk heterogeneity of a grid area included. The results of a series of numerical experiments demonstrate that the proposed scheme is feasible and practical.

The Xin'anjiang Model is used as the basic model to develop a monthly grid based macroscale hydrological model for the assessment of the effects of climate change on water reservoirs (Hao 2002). The monthly discharge from 1953 through 1985 in the Huaihe River Basin is simulated. The sensitivity analysis on runoff is made under assumed climatic scenarios. There is a good agreement between the observed and simulated runoff. Due to the increase of time interval and decrease of precipitation intensity on monthly time scale, there is no monthly runoff in some model girds as the monthly hydrological model is applied to the Huaihe River Basin. Two methods of downscaling monthly precipitation to daily resolution are validated by running the Xin'anjiang model with monthly data at a daily time step,  and the model outputs are more realistic than the monthly hydrological model. The methods of downscaling of monthly precipitation to daily resolution may provide an idea in solving the problem of the shortage of daily data. In the research of the climate change on water resources, the daily hydrological model can be used instead of the monthly one.

To apply DHSVM to China for the first time some improvements have been made in terms of the basin characteristics (Wang and Huang et al. 2002): 1) to change evapotranspiration model, using the improved Penman Monteith approach in place of the original one; 2) to change the model structure, inserting datasets from 4 stations to grid cells for each river basin, instead of datasets from one or two stations; 3) to develop new hydrology, vegetation and soil parameterization schemes for improving the simulated results, with focus on calculation and adjustment of 11 parameters, such as soil porosity φ, field capacity θfc, leaf area index LAI, stochastic resistance γs, among the total 33 parameters. Then the improved DHSVM is driven by observed datasets for Luanhe River Basin and Sanggan River Basin, respectively. The simulated evapotranspiration (ET), runoff, snow water equivalent, water table, soil moisture and percolation are then gained as DHSVM outputs. The simulated ET shows that the highest peak appears in May or June instead of July or August. This is consistent with the real situations, owing to the improvement of ET model. The simulated runoff process and flood peak are quite consistent with the observed ones. The model efficiency values for Luanhe River and Sanggan River Basins are 0.89 and 0.82, respectively, which shows encouraging simulating ability of the model system for both relatively humid and dry basins.

An analytical expression for subgrid-scale inhomogeneous runoff rations generated by heterogeneous soil moisture content and climatic precipitation forcing is presented based on physical mechanisms for land surface hydrology and theory of statistical probability distribution (Liu et al. 2002). Thereby the common-used mosaic parameterization of subgrid runoff ratio was integrated into a statistical-dynamic scheme with the bulk heterogeneity of a grid area included. Furthermore, a series of numerical experiments evaluating the reliability of the parameterization were conducted using the data generated by the emulated simulation method. All the experimental results demonstrate that the proposed scheme is feasible and practical.

4.  Ecological Process

The investigation on ecological process is another research focus of IGBP, and it plays an important role in exchange fluxes of radiation, water and heat fluxes. Especially for China, which locates in Asian monsoon regions, climatic change will affect the growth of vegetation, and the feedback, such as changes of LAI described in model, will also influence the general circulation. A model that can dynamically compute the plant properties is necessary for climate simulation. AVIM was developed by Ji et al.(1995), and its main feature is the dynamic computation of physiological and ecological processes. Through many tests in different regions with various vegetation types (Ji 1995; Ji and Hu 1999; Ji and Yu 1999), it can be concluded that AVIM has the capacity for simulating physical and biological processes on local and continental scales over surfaces covered by different types of vegetation. The model has been improved by Lu and Ji (2002), that include photosynthesis, respiration, allocation and phenology, Dan et al. (2002) have coupled it to a GCM, GOALS/LASG, and the coupled model simulates the main characteristics of global atmospheric circulation and the fields of temperature and moisture. In particular, the simulated precipitation and surface air temperature have sound results.

 

III.  NUMERICAL MODELING ON LAND SURFACE PROCESS

The Simplified Simple Biosphere (SSiB) model developed by Xue et al. (1991) was designed to explore the impact of and mechanisms of land surface-atmosphere interaction on hydrological cycle and general circulation by implemented in a modified version of IAP/LASG global spectral general model (L9R15 AGCM) (Sun et al. 2000). The result of comparing the monthly simulations of AGCM with and without SSiB shows that SSiB can produce a better partitioning of the land surface fluxes of heat and moisture and its diurnal variations.

Off-line experiments have been conducted with IAP94 land surface model on different surface types (cropland, forest and paddy field) in different seasons (spring, summer and autumn) over the Huaihe River Basin (Yang et al. 2001). The simulated energy fluxes and canopy temperature by IAP94 agree quite well with the observations, the simulation results also show that IAP94 can successfully simulate the tendency of total soil water content variation. The inter comparisons between simulation and observation indicate that the evaporation at the paddy field in summer is so strong that more attention should be paid to it within the land surface models, and models performance leads to the conclusion that IAP94 is capable of reproducing the main physical mechanisms governing the LSP in the East Asian semi-humid monsoon region.

A universal NIM LSP model is proposed to simulate the energy and mass exchanges between the land surface and the atmosphere in arid (Shao et al. 2000), semi-arid and moist regions. The model is made up of the soil model which has two schemes (five- and two-layer models) in the paper and is thermal vegetation parameterization scheme presented by Chen Wanlong and Shao Haiyan recently. It is tested by using QXPMEX data form May to Auguset, 1979, and the comparisons among the five- and two-layer models in the paper and Deardorff 's model (1978) has been done. The results show that the five-layer mode is superior to Deardorff 's model and the simulated result is also a little better then that of two-layer model.

Through the results of the classification for QXP landuse and meridional two-dimensional numerical simulation along Wudaoliang, we found that 13 landuse categories all are existed, in spite of its vegetation exhibiting a few bands from northwest to southeast, but the landuse is still very complex (Chang et al. 2000). This classification of the landuse at grid points provides a quite useful surface boundary condition for high resolution numerical model and simulation in this region. The considerable closeness and consistence of the simulated and observed results for the surface net radiation flux and its components shown that this model possesses a capacity of simulating diurnal variation of surface flux in QXP region. The simulated result and observed analysis also indicate that the solar net short-wave radiation was the main component in the surface radiation energy budgets under the clear sky condition summer, its peak value 884 W/m2  is much more greater than that of other places in China, the single peak value interval occurred between 12:00 BST (04:00 UTC) and 14:00 BST (06:00 UTC) was consistent with the surface outgoing long-wave radiation, but it advances by one hour. Both the diurnal variations of atmospheric downward long-wave radiation and the surface net radiation are related to the complex variations of cloud over QXP region. The mean value of single peak value of the surface net radiation was about 584 W/m2, but their peak values exhibited obvious difference due to the different underlying surface. The evolution and structure of the sensible heat flux in cross section revealed a few important features.

The importance of the atmosphere-land interaction studies over the living environment in the arid areas is presented (Yao et al. 2001). Meanwhile the system of model parameterization and measuring index as well as the measuring field and results have also been introduced. The measuring results of the atmosphere-land water-heat exchange process in the non-watering plot in Shapotou desert area, Ningxia Province, in high-clod meadow of Qinghai Province, and in the irrigating agricultural fields in Tarim Basin, Xinjiang Region are discussed. Through the comparisons with the actual measuring data, the model is proved to successfully simulate the atmosphere-land interaction processing. The atmosphere-land interaction in various land surfaces in the arid areas can provide scientific standard for the optimizing use of local water-heat resources.

The importance of investigation on terrestrical processes in arid areas for mankind's living environment protection and local economics development as well as its present state of the are elucidated (Li et al. 1999). A coupling model, which evaluates heat, mass, momentum and radiation fluxes in the SPAC system, is developed for simulating microclimate over plant and bare soil. Especially, It is focused on the details of turbulence transfer. For example, numerical simulation of the water-heat exchange processes at Shapotou Observatory, CAS, Ningxia Region are conducted, and the computational results show that the laws of LSP are rather typical in the arid areas.

The statistical relationship between soil thermal anomaly and short-term climate change is presented based on a typical case study. Furthermore, possible physical mechanisms behind the relationship are revealed through using an off-line land surface model with a reasonable soil thermal forcing at the bottom of the soil layer (Guo et al. 2002).  In the first experiment, the given heat flux is 5W/m2 at the bottom of the soil layer (in the depth of 6.3 m) for 3 months, while only a positive ground temperature anomaly of 0.06 can be found compared to the control run. The anomaly, however, could reach 0.65 if the soil thermal conductivity was one order of magnitude larger. It could be even as large as 0.81 assuming the heat flux at bottom is 10 W/m2. Meanwhile, an increase of about 10 W/m2 was detected both for heat flux in soil and sensible heat on land surface, which is not neglectable to the short-term climate change. The results show that considerable response in land surface energy budget could be expected when the soil thermal forcing reaches a certain spatial-temporal scale. Therefore, land surface models should not ignore the upward heat flux from the bottom of the soil layer. Moreover, integration for a longer period of time and coupled land-atmosphere model are also necessary for the better understanding of this issue.

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