Key results and implications from phase 1(c) of the Project for Intercomparison of Land-surface Parametrization Schemes

 Using atmospheric forcing data generated from a general circulation climate model, sixteen land surface schemes participating in the Project for the Intercomparison of Land-surface Parametrization Schemes (PILPS) were run off-line to equilibrium using forcing data from a GCM representative of a tropical forest and a mid-latitude grassland grid point. The values for each land surface parameter (roughness length, minimum stomatal resistance, soil depth etc.) were provided. Results were quality controlled and analyzed, focusing on the scatter simulated amongst the models. There were large differences in how the models’ partitioned available energy between sensible and latent heat. Annually averaged, simulations for the tropical forest ranged by 79 1 3;W m^-2 for the sensible heat flux and 80 W m^-2 for the latent heat flux. For the grassland, simulations ranged by 34 W m^-2 for the sensible heat flux and 27 W m^-2 for the latent heat flux. Similarly large differences were found for simulated runoff and soil moisture and at the monthly time scale. The models’ simulation of annually averaged effective radiative temperature varied with a range, between all the models, of 1.4 K for tropical forest and 2.2 K for the grassland. The simulation of latent and sensible heat fluxes by a standard ‘bucket’ models was anomalous although this could be corrected by an additional resistance term. These results imply that the current land surface models do not agree on the land surface climate when the atmospheric forcing and surface parameters are prescribed. The nature of the experimental design, it being offline and with artificial forcing, generally precludes judgements concerning the relative quality of any specific model. Although these results were produced de-coupled from a host model, they do cast doubt on the reliability of land surface schemes. It is therefore a priority to resolve the disparity in the simulations, understand the reasons behind the scatter and to determine whether this lack of agreement in de-coupled tests is reproduced in coupled experiments. Received: 15 October 1997 / Accepted: 22 April 1999     

一种陆面过程模式对径流的模拟研究

径流在陆面模式水量平衡计算中占有重要地位,它不但与土壤水的动态变化有关,而且会影响感热、潜热等其他通量的计算结果.作者针对陆面过程模式AVIM(Atmosphere VegetationInteraction Model)对产流描述的不足,改进模式中对径流的参数化方法.并将改进后的模式用于内蒙古的锡林河流域,以检验模式对径流的模拟能力.1991～1994年的径流模拟结果表明,改进后的模式对径流的模拟有较好的改善.     

High resolution flight observations and numerical simulations: horizontal variability in the wintertime boreal boundary layer

Summary  High resolution aircraft observations made along flight tracks over inhomogeneous surface in the late wintertime boreal zone are described and compared to 2D mesoscale model simulations with surface properties defined at 2 km resolution from maps. All observations displayed the expected small-scale turbulence. On top of that, the near-surface wind speeds (but not directions) showed mesoscale variations related to local topography and roughness. Upward (but not downward) SW and LW radiative fluxes and ground temperature also displayed mesoscale variability; in SW radiation this was clearly due to local albedo changes. In the sensible heat flux there was strong horizontal variation near the surface in correlation with surface types. The above observed mesoscale along-track variations were reasonably well represented by the mesoscale model simulation. The track-averaged observed sensible and latent heat flux profiles were in rough agreement with a mixing length approach, which used the track-averaged wind, temperature and moisture profiles as input (mimicking a first-order turbulence closure scheme of a GCM). Received September 20, 1999 Revised January 21, 2000     

A numerical study of the influence of urban expansion on monthly climate in dry autumn over the Pearl River Delta, China

Summary In this study, we employed a regional model to simulate the impact of urban expansion on monthly climate in Pearl River Delta (PRD) region. Two experiments were performed by prescribing two different land covers in the PRD region. One land cover represents vegetation in the 1970s which is derived from the United States Geological Survey (USGS) data with 24-category (hereafter referred to as NU). The other land cover represents the current urban condition which is derived from remote sensing data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) in 2004 (hereafter referred to as HU). Using the two land cover datasets, monthly climate of October 2004 was simulated, which was a very dry season in the PRD region. The results obtained from the numerical simulation show a distinct difference in simulated shelter-level temperature, humidity, surface fluxes and the height of planetary boundary layer (PBL) with two different land cover data sets being specified. The maximum difference in simulated monthly mean temperature over urban areas was 0.9 °C. A large temperature difference was found in urbanized area in Guangzhou, Dongguan, Zhongshan and Shenzhen. The monthly mean relative humidity in urban areas decreased by 1.4% as a result of urban expansion (from 59.2% in NU to 57.8% in HU). The maximum decrease in mixing ratio was 0.4 g/kg in Guangzhou and Dongguan, whereas the maximum decrease in relative humidity was 2.4%. There was an increase of sensible heat flux in developed lands and the maximum increase was 90 W m⁻². In contrast, latent hear flux in urban area decreased and the maximum decrease was 300 W m⁻². In addition, the increase in mean height of PBL ranged from 20 to 80 m (HU compared with NU), and the maximum change of the height was 180 m over urban area in city of Guangzhou.     

A parametrization of the lateral waterflow for the global scale

 The representation of hydrological land surface processes has, so far, been treated inadequately in global models of the atmospheric general circulation (GCMs). In particular the lateral waterflows from the continents into the ocean have been described unsatisfactorily. The aim of this study is to develop a model for the lateral waterflow on the global scale which describes the translation and retention of the lateral discharge as a function of the spatially distributed land surface characteristics that are globally available. Here, global scale refers to the resolution of 0.5° and smaller, corresponding to a typical GCM gridbox area of about 2500 km². Discharge models need a number of specific input variables that are usually not available from measurements such as runoff and drainage. Therefore, these variables have to be derived from observed data such as precipitation and surface air temperature. For the model development in this study, a new simplified land surface scheme was applied to compute these variables. The discharge from a catchment of approximately the size of a 0.5° gridbox was simulated using several types of models. The intercomparison of the simulated lateral flows of the different models shows that as a minimum a separation between flow processes such as overland flow, baseflow and riverflow is required to yield good discharge simulations. As both the retention and translation of a flow process need to be simulated, a two-parameter model is required for the representation of overland flow and riverflow. For the baseflow, a one-parameter model is sufficient. The resulting model structure is called the hydrological discharge model or HD model. A first parametrization approach was defined using the gridbox characteristics of topography gradient and gridbox length. The skill of the discharge simulation depends not only on the formulations of the model, but also on the precise definition of the boundaries of the model catchments. The sizes and the positioning of the model catchments on the globe were defined by using a river direction file as well as a modified topography dataset. In a first application, the HD model successfully simulated river discharge using runoff and drainage from a five year atmospheric GCM integration (ECHAM4-T42) as input. The annual cycles of the monthly means of the simulated discharge of several large rivers were compared to the observed discharges provided by the Global Runoff Data Centre. The HD model achieves a considerable improvement of the simulated discharge compared to the model which is currently operational at MPI. The improved volume of the discharge is directly related to the definition of more realistic model catchments and the improved timing of the flow is mainly due to the newly introduced separation of the flow processes. Received: 20 November 1996 / Accepted: 8 July 1997     

10层陆面过程模式及其Offline独立试验

利用西伯利亚地区的一个试验点资料和1998年中国淮河流域试验（HUBEX）的加密观测资料，对一个新发展的陆面过程模式进行了模拟检验。西伯利亚地区的单点试验表明，不同时间间隔的边界强迫对地表吸收的净短波辐射和释放的潜热影响较大。淮河流域的模拟结果表明，模式能够较好地模拟出我国夏季半湿润地区陆面特征量的变化趋势。由于模式模拟的地温偏低、净短波辐射偏小。所以模拟的感热和潜热值偏小。对该模式在淮河流域的植被、土壤等参数的合理选取可能会提高模式的模拟效果。     

Modeling water and heat balance components for the river basin using remote sensing data on underlying surface characteristics

The method of the AVHRR-3 (NOAA) radiometer measurement data subject processing is produced for the retrieval of underlying surface temperature and several vegetation characteristics under cloud-free conditions. A technology for deriving the values of these parameters from the MODIS (EOS/Terra and Aqua) radiometer data is developed. The estimation of the temperature and vegetation characteristics is carried out for the Seim River basin (Kursk region) with the catchment area of 7460 km² for 2003–2005 vegetation seasons. Practical coincidence of estimations of AVHRR- and MODIS-derived temperatures, as well as the coincidence with ground observation results, is revealed. Statistics of these estimation errors is analyzed. Satellite-derived estimations of land surface temperature (LST) and vegetation characteristics are used for the calibration and verification of the developed model of the vertical heat and water transfer in the soil-vegetation-atmosphere system (SVAT). The model is intended for calculations of evapotranspiration, soil water and heat content, latent and sensible heat fluxes, and other water and heat balance components. The abilities to compute these parameters using the satellite estimations of the leaf area index and projective vegetation cover fraction as the model parameters and LST satellite estimations as the model input variable are investigated.     

雷达雨量计资料用于径流模拟(英)

利用测雨雷达结合稠密和稀疏雨量站网估计流域降水分布,将小同方法获得的降水分布输入降水径流模型TOPMODEL,模拟1998,1999夏季GAME/HUBEX试验区梅山和鲇鱼山集水区的径流,并与实测径流进行比较和分析,结果表明:1)雷达结合集水区内雨量计网模拟径流的精度优于传统的用稠密雨量计网模拟径流的精度;2)利用雷达结合集水区外相对稀疏的雨量计网模拟径流的精度和用集水区内稠密雨量计网模拟径流的精度相当,显示了测雨雷达在径流模拟和洪水预报中极大的应用潜力。     

Evaluation of the WAMME model surface fluxes using results from the AMMA land-surface model intercomparison project

The West African monsoon (WAM) circulation and intensity have been shown to be influenced by the land surface in numerous numerical studies using regional scale and global scale atmospheric climate models (RCMs and GCMs, respectively) over the last several decades. The atmosphere–land surface interactions are modulated by the magnitude of the north–south gradient of the low level moist static energy, which is highly correlated with the steep latitudinal gradients of the vegetation characteristics and coverage, land use, and soil properties over this zone. The African Multidisciplinary Monsoon Analysis (AMMA) has organised comprehensive activities in data collection and modelling to further investigate the significance land–atmosphere feedbacks. Surface energy fluxes simulated by an ensemble of land surface models from AMMA Land-surface Model Intercomparison Project (ALMIP) have been used as a proxy for the best estimate of the “real world” values in order to evaluate GCM and RCM simulations under the auspices of the West African Monsoon Modelling Experiment (WAMME) project, since such large-scale observations do not exist. The ALMIP models have been forced in off-line mode using forcing based on a mixture of satellite, observational, and numerical weather prediction data. The ALMIP models were found to agree well over the region where land–atmosphere coupling is deemed to be most important (notably the Sahel), with a high signal to noise ratio (generally from 0.7 to 0.9) in the ensemble and a inter-model coefficient of variation between 5 and 15%. Most of the WAMME models simulated spatially averaged net radiation values over West Africa which were consistent with the ALMIP estimates, however, the partitioning of this energy between sensible and latent heat fluxes was significantly different: WAMME models tended to simulate larger (by nearly a factor of two) monthly latent heat fluxes than ALMIP. This results due to a positive precipitation bias in the WAMME models and a northward displacement of the monsoon in most of the GCMs and RCMs. Another key feature not found in the WAMME models is peak seasonal latent heat fluxes during the monsoon retreat (approximately a month after the peak precipitation rates) from soil water stores. This is likely related to the WAMME northward bias of the latent heat flux gradient during the WAM onset.     

不均匀地表产生的中尺度通量的数值试验

采用北京大学三维的复杂地形中尺度模式,结合陆面过程模式(SiB),模拟了草原和沙漠并存的下垫面的边界层大气运动.利用SiB模式计算了地表辐射、感热、潜热通量,并且预报地表温度.中尺度模式则模拟了沙漠地区受热抬升,形成的辐合运动,垂直速度的分布,不同高度上水平流场的变化以及中尺度动量和热量通量,把中尺度通量跟湍流通量进行了比较,以确定这种中尺度运动在GCM模式的参数化过程中的重要性.试验表明中尺度通量尤其是热量通量要比湍流通量大很多.     

基于集合卡尔曼滤波的土壤温湿度同化试验

以通用陆面模式CLM 3.0(Community Land Model 3.0)为模型算子,基于集合卡尔曼滤波(Ensemble Kalman Filter,En KF)发展了一个土壤温湿度同化系统,主要用于改进模式对土壤温湿度和地表水热通量的模拟精度,并考察集合样本数、同化频率及不同观测量的组合对同化效果的影响。该系统同化了FLUXNET两个站点(阿柔和Bondville)不同土壤深度、不同时间频率的土壤温度和湿度数据。通过对阿柔站不同集合样本数的设计,综合考虑计算成本和计算精度,最终将集合样本数设置为40。通过分析三种同化方案对同化频率的敏感性得出,同化土壤温度最为敏感,同时同化土壤温湿度次之,同化土壤湿度最不敏感。对于阿柔站点,同化系统对不同土壤深度温度和湿度的模拟精度均能提高90%,潜热通量的均方根误差由94.0 W·m~(-2)降为46.3 W·m~(-2),感热通量均方根误差由55.9 W·m~(-2)降为24.6 W·m~(-2)。Bondville站点浅层土壤温度的改进在30%左右,深层土壤温度改进达到60%,对土壤湿度的改进均在70%以上,潜热通量和感热通量的均方根误差分别从57.4 W·m~(-2)和54.4 W·m~(-2)降为51.0 W·m~(-2)和42.5 W·m~(-2)。试验结果表明,同化站点土壤温湿度数据对土壤水热状况及通量的模拟改进非常有效,同时也验证了同化土壤水分遥感产品的可行性和必要性。     

下垫面非均匀性的模拟

用美国大气科学中心ＧＥＮＥＳＩＳ模式中的陆面过程方案,我们在一个全球大气模式网格尺度范围内,研究了来自非均匀下垫面的水汽,能量等通量。基于３种不同的模拟下垫面非均匀性的方法,即马赛克法、混合法以及显式次网格法,我们设计了４种不同的试验。模拟结果表明,３种方法计算出的结果有很大的不同：地面吸收的太阳辐射差异达４％;感热通量达１２％;潜热通量达６６％;土壤水分可达３０％。这一结果说明有必要用实测数据来检验不同的模拟下垫面非均匀性的方法以及用区域气候模式来模拟下垫面的非均匀性。     

东亚夏季风边缘摆动区陆面能量时空分布规律及其与气候环境的关系

东亚夏季风边缘摆动区既是气候敏感区,也是生态脆弱区和农牧交错带,其特殊陆面能量空间分布格局和演变特征对理解该区域天气和气候变化有重要意义。然而受限于陆面观测资料缺乏,对这部分陆面特征的认识仍非常有限。通过对34 a陆面模拟集成产品的分析,发现夏季风边缘摆动区内潜热和感热通量在空间上表现出明显的过渡特征,由摆动区外的相对均衡状态进入到摆动区内的“突变转换”;陆面能量平衡具有明显的区域特征,能量平衡各分量在纬向和经向都表现出了“阶梯型”的变化。就演变而言,区域平均感热和潜热没有表现出规律性的递减或递增趋势,波动幅度在±20%以内,但在20世纪末存在一个较为明显的摆动相位转换:1997年之前夏季风边缘摆动区夏季风相对活跃,潜热通量总体高于其气候值而感热通量则低于其气候值,之后出现了相反的现象。此外,区内感热和潜热通量对气候环境干湿性质非常敏感,两者存在明显的线性关系。      

松花江、辽河流域实测径流的变化趋势及其与降水的关系

利用松花江、辽河流域内132个降水测站1961-2000年40年的月降水资料,以及水文测站哈尔滨、江桥、铁岭1956-2000年45年的月实测径流量资料,分析松花江、辽河流域实测径流的变化趋势,并探讨夏季径流与同期降水的相关性。结果表明：松花江流域的年实测径流量呈现较微弱的下降趋势,而辽河流域年实测径流呈现显著的下降趋势;两流域径流量均存在着一致的阶段性丰枯周期变化;最显著的一次波动是夏季实测径流由20世纪60年代中后期呈现的显著下降趋势转为80年代初期的明显上升趋势;降水是影响松花江、辽河流域夏季实测径流的一个重要气候因素。初步揭示了人类活动、下垫面改变对实测径流的影响。     

A land surface model (IAP94) for climate studies Part II: Implementation and preliminary results of coupled model with IAP GCM

The Institute of Atmospheric Physics Land Surface Model (IAP94) has been incorporated into the IAP two-level atmospheric general circulation model (IAP GCM). Global and regional climatology averaged over the last 25 years of 100 year integrations from the IAP GCM with and without IAP94 (“bucket” scheme) is compared. The simulated results are also compared with the reanalysis data. Major findings are:(1) The IAP GCM simulation without IAP94 has extensive regions of warmer than observed surface air tempera?tures, while the simulation with IAP94 very much improves the surface air temperature.(2) The IAP GCM simulation with IAP94 gives improvement of the simulated precipitation pattern and intensity, especially the precipitation of East Asian summer monsoon and its intraseasonal migration of the rainbelts.(3) In five selected typical regions, for most of the surface variables such as surface air temperature, precipitation, precipitation minus evaporation, net radiation, latent heat flux and sensible heat flux, the IAP GCM with IAP94 pro?vides better simulations.     

Validation of the “Lokal-Modell” over heterogeneous land surfaces using aircraft-based measurements of the REEEFA experiment and comparison with micro-scale simulations

Summary An aircraft-based experimental investigation of the atmospheric boundary layer (ABL) structure and of the energy exchange processes over heterogeneous land surfaces is presented. The measurements are used for the validation of the mesoscale atmospheric model “Lokal-Modell” (LM) of the German Weather Service with 2.8 km resolution. In addition, high-resolution simulations using the non-hydrostatic model FOOT3DK with 250 m resolution are performed in order to resolve detailed surface heterogeneities. Two special observation periods in May 1999 show comparable convective boundary layer (CBL) conditions. For one case study vertical profiles and area averages of meteorological quantities and energy fluxes are investigated in detail. The measured net radiation is highly dependent on surface albedo, and the latent heat flux exhibits a strong temporal variability in the investigation area. A reduction of this variability is possible by aggregation of multiple flight patterns. To calculate surface fluxes from aircraft measurements at low altitude, turbulent energy fluxes were extrapolated to the ground by the budget method, which turned out to be well applicable for the sensible heat flux, but not for the latent flux. The development of the ABL is well captured by the LM simulation. The comparison of spatiotemporal averages shows an underestimation of the observed net radiation, which is mainly caused by thin low-level clouds in the LM compared to observed scattered CBL clouds. The sensible heat flux is reproduced very well, while the latent flux is highly overestimated especially above forests. The realistic representation of surface heterogeneities in the investigation area in the FOOT3DK simulations leads to improvements for the energy fluxes, but an overestimation of the latent heat flux still persists. A study of upscaling effects yields more structures than the LM fields when averaged to the same scale, which are partly caused by the non-linear effects of parameter aggregation on the LM scale.     

A theoretical analysis of the effects of surface heterogeneity on the fluxes between ground and air

Summary The computation of the fluxes between ground surface and air in atmospheric models is based on the assumption that the surfaces parameters are horizontally homogeneous. In reality, the surface is heterogeneous, inducing a difference between the computed and realistic fluxes. Assuming that the distributions of temperature and humidity of the surface are normal, the difference of the fluxes for homogeneous and heterogeneous surface is found theoretically. The results show that the effect of the heterogeneity on the radiation flux is small, but attains a certain degree on the sensible and latent heat fluxes. However, this effect on the heat fluxes is not great when the standard deviation of the distribution of the surface parameters also is small. Only in case of great standard deviation, the difference may attain several W/m² even the order of magnitude of 10 W/m². Usually the computed sensible and latent heat fluxes are slightly greater for the heterogeneous case than that for homogeneous case, but when the interaction between the temperature and humidity of the surface is considered, the reverse is true. Received January 18, 2001 Revised July 31, 2001     

Improvement of a soil-atmosphere-transfer model for the simulation of bare soil surface energy balances in semiarid areas

A soil-atmosphere-transfer model (SATM) was evaluated using observational data from the Tongyu Cropland Station and Audubon Research Ranch in semiarid areas, where the land cover was nearly bare soil during the simulation period. Simulations by the SATM at both sites were conducted using the new and original surface thermal roughness length parameterization schemes, respectively. Comparisons of simulations and observations have demonstrated that using the new surface thermal roughness length scheme in this model made sound improvements in the simulation of soil surface temperatures, sensible heat fluxes and net radiation fluxes in the daytime at both sites, compared to the original scheme, because the new scheme produced a larger aerodynamic resistance for turbulent heat transfer in the daytime. With respect to latent heat fluxes, the improvement was not as obvious as that attained for soil surface temperature since the soil water content in the surface layer in a semiarid area is a more important factor than surface soil temperature in controlling evaporation rate. Accordingly, it can be concluded that the new surface thermal roughness length parameterization scheme could improve the ability of the SATM to simulate bare soil surface energy budget with latent heat flux component being innegligible in semiarid areas.     

Airborne measurements of turbulent fluxes during LITFASS-98: Comparison with ground measurements and remote sensing in a case study

Summary ?Simultaneous flight measurements with the research aircraft Do 128 and the helicopter-borne turbulence probe Helipod were performed on 18 June 1998 during the LITFASS-98 field experiment. The area-averaged turbulent vertical fluxes of momentum, sensible, and latent heat were determined on a 15 km × 15 km and a 10 km × 10 km flight pattern, respectively. The flights were carried out over heterogeneous terrain at different altitudes within a moderately convective boundary layer with Cumulus clouds. Co-spectra-analysis demonstrated that the small scale turbulent transport was completely sampled, while the comparatively small flight patterns were possibly of critical size regarding the large-scale turbulence. The phygoide of the airplane was identified as a significant peak in some co-spectra. The turbulent fluxes of momentum and sensible heat at 80 m above the ground showed systematic dependence on the location of the flight legs above the heterogeneous terrain. This was not observed for the latent heat flux, probably due to the vertical distribution of humidity in the boundary layer. Statistical error analysis of the fluxes F showed that the systematic statistical error ΔF was one order of magnitude smaller than the standard deviation σ _F . The difference between area-averaged fluxes derived from simultaneous Helipod and Do 128 measurements was much smaller than σ _F , indicating that the systematic statistical error was possibly over-estimated by the usual method. In the upper half of the boundary layer the airborne-measured sensible heat flux agreed well with windprofiler/RASS data. A linear fit was the best approximation for the height dependence of all three fluxes. The linear extrapolations of the latent and sensible heat fluxes to the ground were in good agreement with tower, scintillometer, and averaged ground-station measurements on various surface types. Systematic discrepancies between airborne and ground-based measurements were not found. Received June 18, 2001; revised December 21, 2001; accepted June 3, 2002     

Influence of soil moisture in eastern China on the East Asian summer monsoon

The sensitivity of the East Asian summer monsoon to soil moisture anomalies over China was investigated based on ensembles of seasonal simulations(March–September) using the NCEP GCM coupled with the Simplified Simple Biosphere Model(NCEP GCM/SSi B). After a control experiment with free-running soil moisture, two ensembles were performed in which the soil moisture over the vast region from the lower and middle reaches of the Yangtze River valley to North China(YRNC) was double and half that in the control, with the maximum less than the field capacity. The simulation results showed significant sensitivity of the East Asian summer monsoon to wet soil in YRNC. The wetter soil was associated with increased surface latent heat flux and reduced surface sensible heat flux. In turn, these changes resulted in a wetter and colder local land surface and reduced land–sea temperature gradients, corresponding to a weakened East Asian monsoon circulation in an anomalous anticyclone over southeastern China, and a strengthened East Asian trough southward over Northeast China. Consequently, less precipitation appeared over southeastern China and North China and more rainfall over Northeast China. The weakened monsoon circulation and strengthened East Asian trough was accompanied by the convergence of abnormal northerly and southerly flow over the Yangtze River valley, resulting in more rainfall in this region.In the drier soil experiments, less precipitation appeared over YRNC. The East Asian monsoon circulation seems to show little sensitivity to dry soil anomalies in NCEP GCM/SSi B.