绿洲周边荒漠戈壁夏末土壤-大气水分传输特征

利用“我国西北干旱区陆-气相互作用试验”2000年8-9月在甘肃敦煌地区戈壁滩上取得的野外观测资料,分析了临近绿洲的戈壁土壤湿度和温度特征以及相对应的大气湿度特征,发现土壤热量活动层约为5cm厚,比一般土壤要薄得多;临近绿洲的荒漠戈壁上,不仅近地层大气多为逆湿,而且浅层土壤有时也出现逆湿。土壤湿度日变化能清楚地被区分为湿维持、水分损失、干维持和水分补充等四个阶段。土壤湿度廓线表明：土壤水分活动层厚度约为10m;湿维持阶段的浅层土壤逆湿是土壤湿度廓线最主要的结构特征,这一土壤湿度结构预示着夜间土壤可能通过凝结吸收大气水分,它与白天的土壤水分蒸发共同构成土壤对大气水分的“呼吸”过程。土壤逆湿的形成与土壤温度状态、大气逆湿强度和大气稳定度都有关。     

Comparison of different methods for estimating soil surface evaporation in a bare field

In this paper, three methods for estimating soil evaporation in a bare field were evaluated: evaporation ratio method (k ratio), complementary relationship and bulk equation. Micro-lysimeters were used to measure the actual evaporation for validation of the three methods. For the k ratio method, pan evaporation was used as the reference evaporation instead of the value obtained from the Penman–Monteith equation. This result is important for areas where meteorological data are unavailable. The results showed that, for daytime evaporation, the k ratio and bulk equation produced a good fit with the observation data, while the complementary relationship generated a larger deviation from the measured data. We recommend that the k ratio method and bulk equation could be used to calculate daytime soil evaporation with high accuracy when soil water content and pan evaporation data or meteorological data are available, while the complementary relationship could be used for a rough estimation when pan evaporation is available. All the methods could be applied to calculate cumulative evaporation.     

中国地区土壤湿度记忆性及其与降水特征变化的关系

本文首先利用中国气象局国家气象信息中心提供的中国732个站点观测的土壤体积含水量,评估了CLM4.5(Community Land Model version 4.5)在CFSR(Climate Forecast System Reanalysis)近地面大气数据驱动下模拟的逐月土壤湿度(记为CLM4.5-CFSR),...     

自动土壤水分观测资料应用误差分析

根据从国家气象信息中心实时资料数据库读取的自动土壤水分监测资料,对比同日人工测定的土壤相对湿度数据,发现白天各时次自动监测数据与人工监测数据之间均存在15%左右的差异。具体针对监测仪器自身的结构特点以及中国气象局的业务要求,从两种监测方法自身的监测地段代表性、数据测量和换算、监测土层深度、监测时间、土壤水分常数测算、土壤结构变化等多种角度分析数据间存在差异的原因;认为多种可以估算的差异"叠加"在一起时,对比差异最大可能达到20%左右,加上其他误差因素的影响,实际应用中出现15%左右的差异是可以解释的;选择较长序列的站点实测资料进行了数据差异的实况分析。结合业务中常用的墒情等级判断指标,分析数据差异对客观判断构成的数据干扰和数据损失。建议直接利用体积含水量进行业务应用分析,探究全新的自动土壤水分监测数据应用方法,在具体应用中建立相应的指标体系或指标模型,充分发挥自动土壤水分监测的优势。     

陆面过程和大气边界层相互作用敏感性实验

文中建立了一个研究陆面物理过程与大气边界层相互作用的模式。模拟了草原下垫面的土壤 环境物理、地面热量通量、蒸发、蒸散及大气边界层结构特征。并对主要的环境物理参数进 行了敏感性实验。结果表明，本模式能合理地模拟地表热量平衡、土壤体积含水量、植被蒸 发阻抗、地表水汽通量日变化和湍流交换系数、湍流动能、位温和比湿廓线等。该模式还可 进一步应用于研究城市陆面物理过程与大气边界层相互作用机制，及与中尺度大气模式耦合用于区域气候的研究。     

SENSITIVITY TESTS OF INTERACTION BETWEEN LAND SURFACE PHYSICAL PROCESS AND ATMOSPHERIC BOUNDARY LAYER

In this paper,an interactive model between land surface physical process and atmosphereboundary layer is established,and is used to simulate the features of soil environmental physics,surface heat fluxes,evaporation from soil and evapotranspiration from vegetation and structures ofatmosphere boundary layer over grassland underlying.The sensitivity experiments are engaged inprimary physics parameters.The results show that this model can obtain reasonable simulation fordiurnal variations of heat balance,soil volumetric water content,resistance of vegetationevaporation,flux of surface moisture,and profiles of turbulent exchange coefficient,turbulentmomentum,potential temperature,and specific humidity.The model developed can be used tostudy the interaction between land surface processes and atmospheric boundary layer in cityregions,and can also be used in the simulation of regional climate incorporating a mesoscalemodel.     

内蒙古典型草原区近40年气候变化及其对土壤水分的影响

分析气候变化对草原区土壤水分的影响对了解草原退化原因、恢复草原生态环境有重要的指导意义。根据近40年气象资料和近20年的土壤水分观测资料,利用线性趋势等数理统计方法,分析了内蒙古典型草原区气候变化趋势和对土壤水分变化的影响,得出内蒙古典型草原区近40年气候变化趋势与全球气候变化规律相似;影响土壤湿度的气象因子主要是降水和蒸发,温度通过影响蒸发而间接影响土壤湿度,蒸降差是分析气候变化对土壤水分影响的直观指标。气候变暖导致蒸发加剧,在降水增加不明显的条件下,加速了土壤干旱化程度。     

Characteristics of the water cycle and land–atmosphere interactions from a comprehensive reforecast and reanalysis data set: CFSv2

The behavior of the water cycle in the Coupled Forecast System version 2 reforecasts and reanalysis is examined. Attention is focused on the evolution of forecast biases as the lead-time changes, and how the lead-time dependent model climatology differs from the reanalysis. Precipitation biases are evident in both reanalysis and reforecasts, while biases in soil moisture grow throughout the duration of the forecasts. Locally, the soil moisture biases may shrink or reverse sign. These biases are reflected in evaporation and runoff. The Noah land surface scheme shows the necessary relationships between evaporation and soil moisture for land-driven climate predictability. There is evidence that the atmospheric model cannot maintain the link between precipitation and antecedent soil moisture as strongly as in the real atmosphere, potentially hampering prediction skill, although there is better precipitation forecast skill over most locations when initial soil moisture anomalies are large. Bias change with lead-time, measured as the variance across ten monthly forecast leads, is often comparable to or larger than the interannual variance. Skill scores when forecast anomalies are calculated relative to reanalysis are seriously reduced over most locations when compared to validation against anomalies based on the forecast model climate at the corresponding lead-time. When all anomalies are calculated relative to the 0-month forecast, some skill is recovered over some regions, but the complex manner in which biases evolve indicates that a complete suite of reforecasts would be necessary whenever a new version of a climate model is implemented. The utility of reforecast programs is evident for operational forecast systems.     

The surface heat flux density of a bare soil

Abstract

Half‐hourly measurements of soil surface heat flux density (G₀ ), solar irradiance (S), and the surface energy balance components were made at Agassiz, b.c., in the spring and early summer of 1978 at two adjacent bare‐soil sites, one of which was culti‐packed while the other was disc‐harrowed. G₀ was calculated using the null‐alignment procedure from half‐hourly measurements of soil temperature at 30 depths down to 1 m, and volumetric soil heat capacity calculated from measurements of bulk density, organic matter fraction, and moisture content. The latent and sensible heat flux densities were measured using the energy balance/Bowen ratio technique.

It was found that both the daily averages and diurnal variations of Go at each site were not affected as the soil surface dried, despite reductions in evaporation rate of as much as 50% at the culti‐packed site and 75% at the disc‐harrowed site on the clear dry‐soil days. Diurnal variations of G₀ at the disc‐harrowed site were about 25% less than at the culti‐packed site, although daily averages were similar at both sites. Daily and daytime averages of G₀ at each site were linear functions of S alone, or functions of net radiation and some measure of near‐surface soil water content. Night‐time averages of G₀ at each site were linear functions of a cloudiness ratio equal to the fraction received of the clear‐day S.     

SENSITIVITY TESTS OF INTERACTION BETWEEN LAND SURFACE PHYSICAL PROCESS AND ATMOSPHERIC BOUNDARY LAYER*

In this paper,an interactive model between land surface physical process and atmosphere boundary layer is established,and is used to simulate the features of soil environmental physics,surface heat fluxes,evaporation from soil and evapotranspiration from vegetation and structures of atmosphere boundary layer over grassland underlying.The sensitivity experiments are engaged in primary physics parameters.The results show that this model can obtain reasonable simulation for diurnal variations of heat balance,soil volumetric water content,resistance of vegetation evaporation,flux of surface moisture,and profiles of turbulent exchange coefficient,turbulent momentum,potential temperature,and specific humidity.The model developed can be used to study the interaction between land surface processes and atmospheric boundary layer in city regions,and can also be used in the simulation of regional climate incorporating a mesoscale model.     

A numerical simulation of the surface water budget during HAPEX-MOBILHY

A simple land surface parameterization scheme previously validated at the daily time scale is studied for monthly periods using the HAPEX-MOBILHY data set. For a set of six locations, the scheme reproduces with good accuracy two components of the surface water budget (evaporation and soil moisture depletion) when the observed rainfall rate at the ground surface is specified. For two sites where the superficial soil moisture content has been measured, the model provides a reasonable evolution for this quantity. The validation has been completed by examining the screen-level temperature and relative humidity. The agreement with observations is fair, except when the local properties of the site strongly differ from those of the surroundings.     

Mid-twenty-first century climate change in the Central United States. Part II: Climate change processes

Ensemble regional model simulations over the central US with 30-km resolution are analyzed to investigate the physical processes of projected precipitation changes in the mid-twenty-first century under greenhouse gas forcing. An atmospheric moisture balance is constructed, and changes in the diurnal cycle are evaluated. Wetter conditions over the central US in April and May occur most strongly in the afternoon and evening, supported primarily by moisture convergence by transient eddy activity, indicating enhanced daytime convection. In June, increased rainfall over the northern Great Plains is strongest from 0000 to 0600 LT. It is supported by positive changes in stationary meridional moisture convergence related to a strengthening of the GPLLJ accompanied by an intensification of the western extension of the North Atlantic subtropical high. In the Midwest, decreased rainfall is strongest at 1500 LT and 0000 LT. Both a suppression of daytime convection as well as changes in the zonal flow in the GPLLJ exit region are important. Future drying over the northern Great Plains in summer is triggered by weakened daytime convection, and persists throughout August and September when a deficit in soil moisture develops and land–atmosphere feedbacks become increasingly important.     

高分辨率WRF模式中土壤湿度扰动对短期高温天气模拟影响的个例研究

本文利用WRFV3.6中尺度预报模式就土壤湿度扰动对2003年7月22~23日和29~30日短期高温天气过程的影响进行了高分辨率模拟研究。结果表明:(1)WRF模式地表气温对土壤湿度扰动有较强的敏感性,且随着土壤湿度的增加(减小)而降低(升高)。同时,模式中土壤湿度对地面气温影响的强度对模式分辨率具有较高的依赖性。(2)不同模式分辨率下气温随土壤湿度变化的规律一致;由于更高分辨率的地形资料的应用,提高分辨率可在较大程度上改善模拟效果。(3)不同土壤湿度试验模拟的地表感热、潜热通量可直接影响气温变化;(4)土壤湿度扰动通过间接影响高温发展的近地层各物理过程使得地表气温发生变化。这些过程中,对流(平流)过程在全天表现为增温(冷却)的作用,强度在白天均随土壤湿度的减小而增加。在较干的土壤条件下,非绝热增温在白天的主导地位加强;在夜间,非绝热冷却的强度减弱,且小于占据主导的对流绝热增温的强度。以上结果表明,在模拟和预报高温天气时土壤湿度非常重要,也意味着通过土壤湿度扰动的集合预报方法来改进模式高温模拟预报具有较大的潜力。     

A NUMERICAL STUDY ON THE MEDIUM-RANGE RESPONSES OF THE ATMOSPHERE TO THE ABNORMAL SOIL MOISTURE*

By employing the improved T42L9 spectral model introduced by NMC (Beijing) from ECMWF and utilizing the FGGE-IIIb data covering the period of 14-19 June 1979, the atmospheric responses to the abnormal soil moisture during the medium-range period have been studied numerically. According to the initial field at 12 GMT 14 June, a five-day numerical experiment under different conditions of the soil moisture has been carried out respectively. The monthly mean climatological soil moisture for June has been used in the control experiment in the initial time and it changes with time according to the moisture budget equation at the land surface. Comparing with the experiments with dry or wet soil. one can conclude that: 1) Source of precipitation over continents in summer consists of the land-surface evaporation and the moisture transfer from oceans. Their intensities are comparable during the medium-range time scale when the soil evaporates its moisture sufficiently. Therefore, the soil moisture can influence the global precipitation and the general circulation significantly; 2) By influencing the thermodynamic difference between land and sea,the soil moisture can change the intensity of monsoon and precipitation distribution; 3) The response of the atmosphere to the abnormal soil moisture has the characteristics of geographical distribution and nonlinear interactions; 4) Human activities on the world can influence the environment greatly.     

脉冲降雨—蒸发对土壤湿度影响的动力机制分析

土壤湿度直接作用于植被的呼吸、蒸腾和各种化学反应, 是生态气象学的重要的环境因素。降雨和蒸发作为土壤湿度的两个重要扰动因子, 对其进行定性的理论分析及定量的数值分析具有重要意义, 尤其在半干旱和干旱地区。蒸发对土壤湿度具有持续的耗散作用, 可以视为连续的, 而降雨作为湿度补充作用具有脉冲效应。为阐释降雨和蒸发对土壤湿度的影响, 该文通过建立脉冲降雨—蒸发对土壤湿度响应模型, 并对其进行定性分析, 得到此微分方程系统的周期解的充分条件。在此基础上, 通过Forcal对方程参数拟合, 并以Maple作为数值分析工具, 对此系统进行数值模拟。      

Local climates simulated by two generations of Canadian GCM land surface schemes

Abstract

The performance of two Canadian land surface schemes of widely differing complexity is compared and contrasted in a pair of year‐long simulations using the GCM developed at Atmospheric Environment Service, Canada. The old land surface model incorporates the force‐restore method for soil temperatures and the bucket approximation for soil moisture; the new model, CLASS (Canadian Land Surface Scheme) features three soil layers, an explicitly modelled snow layer, a thermally separate vegetation canopy, and physically‐based calculations of heat and moisture transfers between all of the land surface components and the atmosphere.

It was reported in previous papers that compared with observations, the old scheme tends to generate a climate which is characterized by anomalously high precipitation rates and cold temperatures over land. In this paper, by reference to field measurements and to the energy fluxes and temperatures generated by the two models at local scales, the hypotheses earlier postulated as to the underlying reasons for this are validated. The main factor contributing to the climate anomalies observed with the old scheme is found to be its generation of excessive evaporation rates; this is caused by the fact that the evaporation rate is never directly energy‐limited, the fact that the scaling of the evaporation rale with decreasing soil moisture content underestimates the effect of vegetation stomatal resistance, and the fact that the evaporation rate over bare soil depends not on the surface soil moisture, but on the moisture content of whole modelled soil column. The cold surface temperatures are additionally attributed to systematic errors incurred by the forward‐stepping temperature scheme, and to the fact that soils subjected to subzero temperature forcing in the winter are modelling as freezing completely. Finally, the inability of the old scheme to simulate partially frozen soils means that it proves unable to handle either shallow frost penetration at temperature latitudes, or the development of an active layer in permafrost.     

The temporal variability of soil wetness and its impact on climate

The temporal variability of soil wetness and its interactions with the atmosphere were studied using a general circulation model of the atmosphere. It was found that time series of soil wetness computed by the model contain substantial amounts of variance at low frequencies. Long time-scale anomalies of soil moisture resemble the red noise response of the soil layer to white noise rainfall forcing. The dependence of the temporal variability of soil moisture on potential evaporation and precipitation is discussed.     

Trends and scales of observed soil moisture variations in China

A new soil moisture dataset from direct gravimetric measurements within the top 50-cm soil layers at 178 soil moisture stations in China covering the period 1981-1998 are used to study the long-term and seasonal trends of soil moisture variations, as well as estimate the temporal and spatial scales of soil moisture for different soil layers. Additional datasets of precipitation and temperature difference between land surface and air (TDSA) are analyzed to gain further insight into the changes of soil moisture. There are increasing trends for the top 10 cm, but decreasing trends for the top 50 cm of soil layers in most regions. Trends in precipitation appear to dominantly influence trends in soil moisture in both cases. Seasonal variation of soil moisture is mainly controlled by precipitation and evaporation, and in some regions can be affected by snow cover in winter. Timescales of soil moisture variation are roughly 1-3 months and increase with soil depth. Further influences of TDSA and precipitation on soil moisture in surface layers, rather than in deeper layers, cause this phenomenon. Seasonal variations of temporal scales for soil moisture are region-dependent and consistent in both layer depths. Spatial scales of soil moisture range from 200-600 km, with topography also having an affect on these. Spatial scales of soil moisture in plains are larger than in mountainous areas. In the former, the spatial scale of soil moisture follows the spatial patterns of precipitation and evaporation, whereas in the latter, the spatial scale is controlled by topography.     

Subdaily to Seasonal Change of Surface Energy and Water Flux of the Haihe River Basin in China: Noah and Noah-MP Assessment

The land surface processes of the Noah-MP and Noah models are evaluated over four typical landscapes in the Haihe River Basin(HRB) using in-situ observations. The simulated soil temperature and moisture in the two land surface models(LSMs) is consistent with the observation, especially in the rainy season. The models reproduce the mean values and seasonality of the energy fluxes of the croplands, despite the obvious underestimated total evaporation. Noah shows the lower deep soil temperature. The net radiation is well simulated for the diurnal time scale. The daytime latent heat fluxes are always underestimated, while the sensible heat fluxes are overestimated to some degree. Compared with Noah, Noah-MP has improved daily average soil heat flux with diurnal variations. Generally, Noah-MP performs fairly well for different landscapes of the HRB. The simulated cold bias in soil temperature is possibly linked with the parameterized partition of the energy into surface fluxes. Thus, further improvement of these LSMs remains a major challenge.     

An empirical relationship between temperature,vapour pressure deficit and wind speed and evaporation during a winter chinook

Summary Evaporation rates determined by energy balance and bulk transfer equations and confirmed with soil moisture sampling was regressed against average daytime temperature, vapour pressure deficit and wind speed over several chinook events between 1986 and 1988. The equationE = 0.45 exp (0.35 + 0.025 — 0.133 [e ^* —e]) yielded reasonable estimates of evaporation with surface soil moisture between saturation and near wilting point.E is evaporation in mmd ^–1,T, u ande ^* ande are temperature (°C), wind speed (ms^–1), saturation vapour pressure and vapour pressure (mb) respectively. The overbar denotes daytime average.With 8 Figures