Global validation of the ISBA sub-grid hydrology

Over recent years, many numerical studies have suggested that the land surface hydrology contributes to atmospheric variability and predictability on a wide range of scales. Conversely, land surface models (LSMs) have been also used to study the hydrological impacts of seasonal climate anomalies and of global warming. Validating these models at the global scale is therefore a crucial task, which requires off-line simulations driven by realistic atmospheric fluxes to avoid the systematic biases commonly found in the atmospheric models. The present study is aimed at validating a new land surface hydrology within the ISBA LSM. Global simulations are conducted at a 1° by 1° horizontal resolution using 3-hourly atmospheric forcings provided by the Global Soil Wetness Project. Compared to the original scheme, the new hydrology includes a comprehensive and consistent set of sub-grid parametrizations in order to account for spatial heterogeneities of topography, vegetation, and precipitation within each grid cell. The simulated runoff is converted into river discharge using the total runoff integrating pathways (TRIP) river routing model (RRM), and compared with available monthly observations at 80 gauging stations distributed over the world’s largest river basins. The simulated discharges are also compared with parallel global simulations from five alternative LSMs. Globally, the new sub-grid hydrology performs better than the original ISBA scheme. Nevertheless, the improvement is not so clear in the high-latitude river basins (i.e. Ob, MacKenzie), which can be explained by a too late snow melt in the ISBA model. Over specific basins (i.e. Parana, Niger), the quality of the simulated discharge is also limited by the TRIP RRM, which does not account for the occurrence of seasonal floodplains and for their significant impact on the basin-scale water budget.     

陆面过程模式的敏感性试验

本文对文献[1]中提出的陆面过程模式(简称LPM)进行了下列敏感性试验:1)植被的覆盖度,2)土壤湿润程度,3)不同植被类型(半荒漠、草原、混交林、森林和作物地五种)和4)不同气候带(半干旱、半湿润和湿润带三类)。试验结果与相应的观测和气候状况进行了比较,表明了模式对上述参数或参数组变化的敏感程度。模拟的不同植被和气候带的温湿状况和能量平衡关系是合理的。因此可以用于气候模拟的研究。     

An investigation of the sensitivity of a land surface model to climate change using a reduced form model

 In an illustration of a model evaluation methodology, a multivariate reduced form model is developed to evaluate the sensitivity of a land surface model to changes in atmospheric forcing. The reduced form model is constructed in terms of a set of ten integrative response metrics, including the timing of spring snow melt, sensible and latent heat fluxes in summer, and soil temperature. The responses are evaluated as a function of a selected set of six atmospheric forcing perturbations which are varied simultaneously, and hence each may be thought of as a six-dimensional response surface. The sensitivities of the land surface model are interdependent and in some cases illustrate a physically plausible feedback process. The important predictors of land surface response in a changing climate are the atmospheric temperature and downwelling longwave radiation. Scenarios characterized by warming and drying produce a large relative response compared to warm, moist scenarios. The insensitivity of the model to increases in precipitation and atmospheric humidity is expected to change in applications to coupled models, since these parameters are also strongly implicated, through the representation of clouds, in the simulation of both longwave and shortwave radiation. Received: 27 March 2000 / Accepted: 11 September 2000     

Soil temperature response in Korea to a changing climate using a land surface model

The land surface processes play an important role in weather and climate systems through its regulation of radiation, heat, water and momentum fluxes. Soil temperature (ST) is one of the most important parameters in the land surface processes; however, there are few extensive measurements of ST with a long time series in the world. According to the CLImatology of Parameters at the Surface (CLIPS) methodology, the output of a trusted Soil-Vegetation- Atmosphere Transfer (SVAT) scheme can be utilized instead of observations to investigate the regional climate of interest. In this study, ST in South Korea is estimated in a view of future climate using the output from a trusted SVAT scheme — the University of TOrino model of land Process Interaction with Atmosphere (UTOPIA), which is driven by a regional climate model. Here characteristic changes in ST are analyzed under the IPCC A2 future climate for 2046-2055 and 2091-2100, and are compared with those under the reference climate for 1996-2005. The UTOPIA results were validated using the observed ST in the reference climate, and the model proved to produce reasonable ST in South Korea. The UTOPIA simulations indicate that ST increases due to environmental change, especially in air temperature (AT), in the future climate. The increment of ST is proportional to that of AT except for winter. In wintertime, the ST variations are different from region to region mainly due to variations in snow cover, which keeps ST from significant changes by the climate change.     

A study of the performance of a land surface process model (LSPM)

In this paper, a reliable Land-Surface Process Model (LSPM), which is a new version of the LPM of Ji and Hu (1989), is described. The LSPM has been validated with experimental data measured at two stations in the Po Valley (Northern Italy).     

陆面过程中冠层四流辐射传输模式的模拟性能检验

利用实际观测资料检验新发展的植被冠层四流辐射传输模式的模拟性能.将浙江大学田间观测数据用于模式试验,其结果表明:在可见光波段,因叶片散射率值较小,四流模式和二流模式模拟结果差别不大,模拟的冠层反照率都接近观测值.这表明在可见光波段,二流模式已经能够较好模拟芯层反照率,四流模式能够提高的精度范围有限;在近红外波段,因叶片散射率值较大,两个模式模拟结果差别较明显,四流模式模拟的冠层反照率相对二流模式的模拟结果更接近观测值.利用第2次雪模式比较计划SNOWMIP资料和改进的10层陆面模式BATS进行耦合试验,将四流模式以及二流模式均耦合到该陆面模式中,其目的是为考察四流模式对陆面模式模拟地表反射太刚短波辐射通量的影响.耦合后得到的结果与SNOWMIP中加拿大BERMS草地站和森林站的观测资料进行了对比.对比结果表明,采用网流模式、二流模式、BATS原辐射传输模式后,耦合四流模式的陆面模式模拟地表反射太阳短波辐射通量最接近观测值.说明采用四流模式能够改善陆面模式对地表反射太阳短波辐射通啦的模拟.     

陆面过程模式研发中的问题

陆面过程研究是充分理解天气/气候/地球系统过程不可或缺的重要主题。本文全面梳理了当前用于数值天气/气候/地球系统模式的陆面过程模式研制的问题,建议了当前陆面过程模式研制中需加强和改进完善的关键内容。特别强调在新一代模式研发中建立包含人类活动的高分辨率全球陆面过程模式;特别强调与其他学科相结合,形成不同行业的预报预测系统或研究方法和工具。建议建设中国的集模式发展、数据分析、模拟方法、高性能计算、数据可视化和应用示范为一体的陆面模拟综合集成平台,为天气/气候/地球系统模式提供陆面过程模式,为开展精细化的全球和区域陆面水文-气象-生态的预报预测提供科技支撑。     

GRAPES全球模式次网格对流过程对云预报的影响研究

50 km分辨率下的GRAPES全球模式对赤道及低纬度地区云水、云冰、云量和格点降水的预报较实际观测偏少。为解决这一问题,在模式原有的格点尺度云方案基础上,将次网格对流过程的影响作为源汇项,加入到云水、云冰和总云量的预报方程中。结合云和地球辐射能量系统(CERES)与热带降雨测量(TRMM)等卫星云观测资料,进行了改进后的云方案与原云方案预报结果的对比分析。结果显示,考虑了对流对格点尺度云含水量和云量预报的影响后,GRAPES全球模式预报的云和格点降水在赤道及低纬度地区有明显改善,水凝物含水量和总云量的预报结果与实况较为接近,格点降水在总降水中的比例由原来的5%提高到25%。研究进一步表明,次网格对流过程对格点尺度云和降水的影响取决于上升气流质量通量的分布和强度,上升气流的质量通量在对流活动强烈的低纬度热带地区较强,其最大值出现在650—450 hPa高度,因此,次网格对流的卷出过程对中云的影响最为明显。对高云和低云也有一定程度的影响,使云顶变高,云底变低。     

Spin-up behavior of soil moisture content over East Asia in a land surface model

This study presents an investigation of the spin-up behavior of soil moisture content (SMC) and evapotranspiration (ET) in an offline Noah land surface model (LSM) for East Asia, focusing on its interplay with the Asian monsoon. The set of 5-year recursive runs is conducted to properly assess the spin-up behavior of land surface processes and consists of simulations initialized with (1) a spatially uniform soil moisture, (2) NCEP GDAS soil moisture data, and (3) ECMWF ERA-Interim soil moisture data. Each run starts either after or before the summer monsoon. Initial SMCs from GDAS and ERA-Interim data significantly deviate from the equilibrium state (spin-up state) with the given input forcing even though the same equilibrium is reached within 3-year spin-up time, indicating that spin-up of land surface process is necessary. SMC reaches the equilibrium much quickly when (1) the consistent LSMs have been used in the prediction and analysis systems and (2) the spin-up simulation starts before the onset of heavy rainfall events during summer monsoon. For an area with heavy monsoon rainfall, the total column SMC and ET spin up quickly. The spin-up time over dry land is about 2–3?years, but for monsoon rainfall area decreases dramatically to about 3?months if the spin-up run starts just before the onset of monsoon. Further scrutiny shows that the spin-up time is well correlated with evaporative fraction given by the ratio between the latent heat flux and the available energy at the land surface.     

Variability in global land surface energy budgets during 1987–1988 simulated by an off-line land surface model

The International Satellite Land-Surface Climatology Project (ISLSCP) Initiative-I 1-degree 1987–1988 data were used to drive a land surface model (LSM) to simulate global surface energy budgets. Simulated surface heat fluxes show remarkable spatial variability and seem to capture well their annual and interannual variability. A shift of maximum evaporation across the equator is more closely related to the seasonal shifting of precipitation pattern than to surface radiation changes. The NCEP/NCAR reanalysis did not reflect this shift, presumably due to its dominant rainfall maximum in the Southern Hemisphere. To assess the “reliability” of these fields, both Global Soil Wetness Project (GSWP) and reanalysis were verified against observations, at two sites. Monthly mean ISLSCP forcing conditions agree fairly well with observations, but its precipitation is usually lower during spring and summer. Low summer GSWP evaporation may be due to low precipitation and incorrect specification of vegetation and soil conditions. The reanalysis had larger seasonal variability than GSWP and observations, and overestimated summer heat fluxes because of its large rainfall and surface radiation. Despite uncertainty in ISLSCP data, an LSM with a modest treatment of vegetation was able to capture reasonably well the seasonal variations in surface heat fluxes at global scales. With some caution, these types of simulations can be used as “pseudo-observations” to evaluate climate-model simulations and to investigate global energy budgets. For the next phase of ISLSCP data development, higher resolution data, which can reflect local heterogeneity of vegetation and soil characteristics, include more rain gauge data are highly desirable to improve model simulations.     

Improving the vegetation dynamic simulation in a land surface model by using a statistical-dynamic canopy interception scheme

Canopy interception of incident precipitation, as a critical component of a forest＇s water budget, can affect the amount of water available to the soil, and ultimately vegetation distribution and function. In this paper, a statistical-dynamic approach based on leaf area index and statistical canopy interception is used to parameterize the canopy interception process. The statistical-dynamic canopy interception scheme is implemented into the Community Land Model with dynamic global vegetation model （CLM-DGVM） to improve its dynamic vegetation simulation. The simulation for continental China by the land surface model with the new canopy interception scheme shows that the new one reasonably represents the precipitation intercepted by the canopy. Moreover, the new scheme enhances the water availability in the root zone for vegetation growth, especially in the densely vegetated and semi-arid areas, and improves the model＇s performance of potential vegetation simulation.     

Noah-MP陆面模式在东疆黑戈壁的适用性

新疆东部黑戈壁作为气候恶劣、人迹罕至及黑色砾石下垫面的生态脆弱区,陆面过程参数化方案不易确定。利用东疆哈密戈壁陆气相互作用站观测数据集,开展Noah-MP陆面模式离线模拟试验,找出适合戈壁区域的最佳参数化方案,并给出了土壤湿度对戈壁区域陆气热交换的影响。结果表明：（1）针对感热通量、潜热通量、净辐射通量和土壤地表温度均表现为第二种参数化方案组合的模拟误差最小,模式效率最高。（2）模式针对土壤湿度的模拟效果均不好,模式预报土壤湿度偏干,第七种方案的模式效率指数较高。（3）针对土壤10 cm温度,第一种方案虽预报效率指数最高,但误差达1.2 ℃左右,第二种方案是模拟和实测误差最小的一种方案,模式预报结果比实测低0.4 ℃。综上所述,第二种方案在东疆黑戈壁地区的普适性最高。（4）Noah-MP在RMAPS-CA系统中在线耦合后,2 m温度的预报效果整体要优于离线Noah模式。     

Sub-grid scale precipitation in ALCMs: re-assessing the land surface sensitivity using a single column model

The sensitivity of a land surface scheme to the distribution of precipitation within a general circulation model's grid element is investigated. Earlier experiments which showed considerable sensitivity of the runoff and evaporation simulation to the distribution of precipitation are repeated in the light of other results which show no sensitivity of evaporation to the distribution of precipitation. Results show that while the earlier results over-estimated the sensitivity of the surface hydrology to the precipitation distribution, the general conclusion that the system is sensitive is supported. It is found that changing the distribution of precipitation from falling over 100% of the grid square to falling over 10% leads to a reduction in evaporation from 1578 mm y^–1 to 1195 mm y ^–1 while runoff increases from 278 mm y^–1 to 602 mm y^–1. The sensitivity is explained in terms of evaporation being dominated by available energy when precipitation falls over nearly the entire grid square, but by moisture availability (mainly intercepted water) when it falls over little of the grid square. These results also indicate that earlier work using stand-alone forcing to drive land surface schemes off-line, and to investigate the sensitivity of land surface codes to various parameters, leads to results which are non-repeatable in single column simulations.     

Treatment of frozen soil and snow cover in the land surface model SEWAB

Summary  The land surface model SEWAB (Surface Energy and Water Balance) is designed to be coupled to both, atmospheric and hydrological models. Its application in mid and high latitudes requires the inclusion of freezing and thawing processes within the soil and the accumulation and ablation of a snow cover. These winter processes are parameterised with a minimum number of empirical formulations in order to assure reasonable computation times for an application in climate and sensitivity studies yet accounting for all important processes. Meteorological forcing data and measurements of snow depth, soil temperature and liquid soil water content at two locations in the mid-west of North America are used to test the model. Generally the simulated snow depth matches the measurements, remaining differences in snow depth can be explained by uncertainties in snow density, blowing snow and errors in precipitation measurements. The simulated soil temperature and liquid soil water content compare well with the measurements, showing the isolating effect of the snow cover. Received August 25, 2000 Revised January 19, 2001     

Analysis of parameter sensitivity on surface heat exchange in the Noah land surface model at a temperate desert steppe site in China

The dominant parameters in the Noah land surface model (LSM) are identified, and the effects of parameter optimization on the surface heat exchange are investigated at a temperate desert steppe site during growing season in Inner Mongolia, China. The relative impacts of parameters on surface heat flux are examined by the distributed evaluation of local sensitivity analysis (DELSA), and the Noah LSM is calibrated by the global shuffled complex evolution (SCE) against the corresponding observations during May–September of 2008 and 2009. The differences in flux simulations are assessed between the Noah LSM calibrated by the SCE with 27 parameters and 12 dominant parameters. The systematic error, unsystematic error, root mean squared error, and mean squared error decompositions are used to evaluate the model performance. Compared to the control experiment, parameter optimization by the SCE using net radiation, sensible heat flux, latent heat flux, and ground heat flux as the objective criterion, respectively, can obviously reduce the errors of the Noah LSM. The calibrated Noah LSM is further validated against flux observations of growing season in 2010, and it is found that the calibrated Noah LSM can be applied in the longer term at this site. The Noah LSM with 12 dominant parameters calibrated performs similar to that with 27 parameters calibrated.     

A two-layer model of soil hydrology

A two-layer model of soil hydrology is developed for applications where only limited computer time and complexity are allowed. Volumetric soil water is computed in a thin upper layer for use in calculation of surface evaporation. Storage of water is computed for an underlying deeper layer.In an effort to identify the influence of significant asymmetric truncation errors in the two-layer model, this model is compared with the 100-level model of Boersma et al. (1983). Comparisons are made for modelled soils with clay, loam and sand properties for various time dependencies of potential evaporation and precipitation. Truncation errors in the resulting two-layer model appear to be modest, at least compared to errors associated with difficulty in estimation of the hydraulic diffusivity and its strong dependence on soil water content.Minimization of the influence of truncation errors requires: (1) choosing the upper layer to be sufficiently thin, (2) allowing the soil water gradient to control surface evaporation directly and (3) using suitable numerical implementation of the evaluation of internal soil water flux.

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Résumé On propose un modèle d'hydrologie du sol à deux couches, spécialement élaboré pour des applications où le temps de calcul et la complexité doivent être aussi réduits que possible. Le contenu en eau du sol dans la mince couche de surface est utilisé pour évaluer l'évaporation, tandis que la réserve en eau est calculée pour la couche profonde, beaucoup plus épaisse.Afin d'estimer les erreurs de troncature et leurs effets dans le modèle à deux couches, des comparaisons sont faites avec le modèle à 100 niveaux de Boersma et al. (1983). Ces comparaisons portent sur des sols de natures variées (argile, terres végétales et sable) et incluent divers taux d'évaporation potentielle et de précipitation. Les effets des erreurs de troncature dans le modèle à 2 couches semblent peu importants par rapport à ceux associés à la mauvaise connaissance de la diffusivité hydraulique et de sa dépendance à l'égard du contenu en eau du sol.La réduction de l'influence de ces erreurs de troncature nécessite: (1) le choix d'une couche supérieure suffisamment fine; (2) le contrôle direct de l'évaporation vers l'atmosphère à partir de la répartition verticale du contenu en eau du sol; (3) l'utilisation d'une procédure particulière pour évaluer le flux d'eau entre les deux couches du sol.

     

Regional climate simulation with a high resolution GCM: surface hydrology

Aspects of the surface hydrology of high resolution (T106) versions of the ECHAM3 and ECHAM4 general circulation models are analysed over the European region and compared with available observations. The focus is on evaporation, and surface measurements are shown to be useful for the identification of systematic deficiencies in the regional-scale performance of climate models on an annual and seasonal basis, such as the excessive summer dryness over continents. The annual mean evaporation at the available European observation sites is overestimated by 4 mm/month by the ECHAM3 T106, quantitatively consistent with an overestimated surface net radiation of 4 Wm^–2 over Europe. In winter, ECHAM3 shows an overestimated evaporation which compensates for an overestimated downward sensible heat flux. This is primarily related to a too strong zonalisation of the large-scale flow and associated overestimated warm air advection and windspeed. Inaccurate local land surface parameters (e.g. leaf area index, roughness length) are minor contributors to the overestimation. In early summer, the excessive solar radiation at the surface calculated with the ECHAM3 radiation scheme generates a too large evaporation and an excessive depletion of the soil moisture reservoirs. This favours the subsequent excessive summer dryness over Europe with too low values of evaporation, convective precipitation and soil moisture content, leading to a too high surface temperature. In the ECHAM4 T106 simulation, the problem of the European summer dryness is largely reduced, and the simulated evaporation as well as convective precipitation, cloud amount and soil moisture content during summer are substantially improved. The new ECHAM4 radiation scheme appears to be an important factor for this improvement, since it calculates smaller insolation values in better agreement with observations and subsequently may avoid an excessive drying of the soil. Received: 20 September 1995 / Accepted: 10 May 1996