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1.
In the semiarid interior of the Iberian Peninsula, the topographic insulation from the surrounding seas promotes the role of internal sources of moisture and water recycling in the rainfall regime. In inland Iberia, the annual cycle of precipitation often has a distinctive peak in the springtime, when evapotranspiration (ET) is the highest, in contrast to the coastal areas, where it is more closely related to the external moisture availability and synoptic forcing, with a maximum in winter-autumn and a pronounced minimum in the summer. In this work we investigate the role of land surface water fluxes in the intensification of the hydrological cycle in the Iberian spring. We used data from 5 km resolution WRF-ARW model simulations over the Iberian Peninsula for eleven months of May (2000–2010). To bring out the effect of ET fluxes, we conducted experiments where ET water over land was removed from the system. Our findings indicate that the impact of ET on precipitation is on average very large (37 % increase). The impact is particularly strong in the interior north and northeast areas where the observed annual rainfall cycle has a peak in May, suggesting that the role of surface water fluxes is very important there. To investigate whether this role is as a water source or to amplify precipitation dynamics, we computed the recycling ratio analytically from the model data. In addition, we developed a procedure to quantify the amplification impact by comparing the recycling ratio and the relative change in precipitation between control and experiments with ET removed. Results show that the role of surface water fluxes on precipitation depends on large-scale forcing and moisture advection. When the synoptic forcing and moisture advection are strong, such as in fronts associated to Atlantic storms, the impact of ET fluxes is small. When there is potential for convection, as is commonly the case of late spring in the Iberian Peninsula, ET fluxes have a significant impact. Surface moisture fluxes moisten the boundary layer and increase moist static energy, strengthening convective processes, and their role goes from being a primary water source for precipitation (recycling) to have mostly an amplification effect as external moisture availability increases. Our findings show that for the eleven simulated May cases over the Iberian Peninsula, the role of ET fluxes in activating recycling is important and explains 27–58 % of their total impact on precipitation, depending on the method used to calculate the recycling ratio. This indicates that the complementary effect of ET on amplifying rainfall from external sources of moisture is comparable in magnitude to the recycling mechanism and important as well. 相似文献
2.
The state-of-the-art WRF model is used to investigate the impact of the antecedent soil moisture on subsequent summer precipitation during the East Asian summer monsoon (EASM) period. The control experiment with realistic soil moisture condition can well reproduce the seasonal pattern from low- to high- atmosphere, as well as the spatial distribution of precipitation belt in East China. Compared with the control experiment, the sensitivity experiment in which the initial soil moisture is reduced generates more precipitation along the East China Sea, and less rainfall over both Central and South China. This suggests that the effect of initial soil moisture on monsoonal precipitation in East China is regionally dependent. The influence on precipitation is mostly attributed to the change in precipitation from mid July to late August. The initial soil moisture condition plays a role in changing the seasonal pattern and atmospheric circulation due to the weak heating and geopotential gradient, leading to a reduction in southeasterly flow and moisture flux from South China Sea. The changes between DRY and CTL runs result in reduced southerly wind over the ocean (south of ˜25 °N) and enhanced northerly wind over the land (north of ∼25 °N). The temperature and associated circulation changes due to drier initial soil moisture anomaly result in reduced southerly winds over East China, and therefore a weakened EASM system. The averaged moisture flux decreases significantly over Central China but increases along the East China Sea. In addition, the drier soil moisture perturbation exerts an effect on suppressing (enhancing) vertical velocity over Central China (along the East China Sea), thus leading to more (less) cloud water and rain water. Therefore, the influence of soil moisture exerts an opposite impact on surface precipitation between these two regions, with more and less accumulation rainfall in Central China and along the East China Sea, respectively. 相似文献
3.
陆面特征量初始扰动的敏感性及集合预报试验 总被引：2，自引：1，他引：1
文章利用中尺度模式Weather Research and Forecasting Model(WRF)3.2.1版本及National Centers for Environmental Prediction(NCEP)分析资料,研究了陆面变量(土壤湿度、土壤温度)和陆面参数(植被覆盖率)初始场随机扰动对长江中下游暴雨预报的影响并进行了集合预报试验。试验结果表明,短期暴雨过程对陆面变量(参数)扰动是敏感的;陆面变量(参数)初始场扰动影响降水的时间尺度小于10 h甚至可以小于6 h。从影响机理上来看,陆面变量(参数)扰动首先改变地表的潜热通量和感热通量,而地表通量的改变会通过陆气相互作用对局地大气的温、压、湿、风产生较大影响,从而对暴雨的强度和落区产生较大影响。集合预报结果表明,利用陆面变量(参数)扰动制作集合预报,预报的集合平均结果要好于控制预报的结果,且比集合成员稳定可靠,降水概率预报可以提供一些有用的信息,对预报强降水有一定的指示意义。在初值集合预报中,以这些参数或变量的扰动来引进集合成员是十分有意义的。 相似文献
4.
5.
Changhai Liu Jimy Dudhia Mitchell W. Moncrieff 《Meteorology and Atmospheric Physics》2010,107(1-2):9-15
This paper presents a case study of the impact of land surface treatment on warm season precipitation simulations at convection-permitting grid resolution. Two surface schemes are tested: Dudhia’s five-layer soil model (FLSM) and the Noah land-surface model (NLSM). The experimentation case involves a 1-week episode of active summertime convection over the central United States. The overall precipitation features, such as the diurnal regeneration of zonally propagating rainfall episodes and the spatial distribution of accumulative rainfall, are adequately replicated by the two parameterizations. In comparison, NLSM produces roughly 12% more and broader rainfall than FLSM. This differential rainfall amount is consistent with the differential surface moisture fluxes between the two schemes, whereas the precipitation feedback plays a negligible role. It is also found that FLSM generates comparatively stronger sensible heat transports from the land surface and thus a warmer temperature near the surface. 相似文献
6.
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. 相似文献
7.
土壤湿度是影响天气和气候非常重要的因子之一,但目前针对土壤湿度可预报性的研究报道相对较少。该文在对BCC_CSM模式进行了适合的陆面初始化的条件下,设计了两组在中国东部地区采用不同土壤湿度初值的回报试验研究该地区土壤湿度的可预报性及初值对其可预报性影响问题。试验结果表明:BCC_CSM模式在真实的外场强迫下可以模拟出相对合理的土壤湿度;土壤湿度的可预报性在表层约为3候,随着深度的增加,土壤湿度的可预报性持续时间增加,在中层预报性甚至能达到月尺度以上;初值对于土壤湿度的预报存在影响,在表层影响时间约为2~3候,影响时间随着深度增加;浅层土壤湿度受降水的影响较大,浅层土壤湿度变化滞后降水变化约1~2 d,中层土壤湿度变化与降水变化存在5 d左右的滞后关系。 相似文献
8.
A land data assimilation system using the MODIS-derived land data and its application to numerical weather prediction in East Asia 总被引：1，自引：0，他引：1
Yoon-Jin Lim Kun-Young Byun Tae-Young Lee Hyojung Kwon Jinkyu Hong Joon Kim 《Asia-Pacific Journal of Atmospheric Sciences》2012,48(1):83-95
Land Data Assimilation Systems have been developed to generate the surface initial conditions such as soil moisture and temperature for better prediction of weather and climate. We have constructed Korea Land Data Assimilation System (KLDAS) based on an uncoupled land surface modeling framework that integrates high-resolution in-situ observation, satellite data, land surface information from the WRF Preprocessing System (WPS) and the MODIS land products over the East Asia. To present better surface conditions, the KLDAS is driven by atmospheric forcing data from the in-situ rainfall gauges and satellite. In this study, we 1) briefly introduce the KLDAS, 2) evaluate the meteorological states near the surface and the surface fluxes reproduced by the KLDAS against the in-situ observation, and then 3) examine the performance of the mesoscale model initialized by the KLDAS. We have generated a 5-year, 10 km, hourly atmospheric forcing dataset for use in KLDAS operating across East Asia. The KLDAS has effectively reproduced the observed patterns of soil moisture, soil temperature, and surface fluxes. Further scrutiny reveals that the numerical simulations incorporating the KLDAS outputs show better agreement in both the simulated near-surface conditions and rainfall distribution over the Korean Peninsula, compared to those without the KLDAS. 相似文献
9.
Soil moisture prediction over the Australian continent 总被引：1，自引：0，他引：1
Dr. Y. Shao L. M. Leslie R. K. Munro P. Irannejad W. F. Lyons R. Morison D. Short M. S. Wood 《Meteorology and Atmospheric Physics》1997,63(3-4):195-215
Summary This paper describes an attempt to model soil moisture over the Australian continent with an integrated system of dynamic models and a Geographic Information System (GIS) data base. A land surface scheme with improved treatment of soil hydrological processes is described. The non-linear relationships between soil hydraulic conductivity, matric potential and soil moisture are derived from the Broadbridge and White soil model. For a single location, the prediction of the scheme is in good agreement with the measurements of the Hydrological and Atmospheric Pilot Experiment (HAPEX). High resolution atmospheric and geographic data are used in soil moisture prediction over the Australian continent. The importance of reliable land surface parameters is emphasized and details are given for deriving the parameters from a GIS. Predicted soil moisture patterns over the Australian continent in summer, with a 50 km spatial resolution, are found to be closely related to the distribution of soil types, apart from isolated areas and times under the influence of precipitation. This is consistent with the notion that the Australian continent in summer is generally under water stress. In contrast, predicted soil temperatures are more closely related to radiation patterns and changes in atmospheric circulation. The simulation can provide details of soil moisture evolution both in space and time, that are very useful for studies of land use sustainability, such as plant growth modelling and soil erosion prediction.With 12 Figures 相似文献
10.
基于NCAR大气模式CAM3.1模式,设计了有、无土壤湿度年际异常两组试验对中国区域近40a(1961-2000年)气候进行了模拟。从气候态和年际变率的角度,通过分析两组试验的差值场来探讨土壤湿度年际异常对气候模拟的影响,并初步探讨了影响的可能机制。结果表明:模式模拟的温度和降水对土壤湿度的年际异常非常敏感,土壤湿度的年际变化对中国春夏季气候及其年际变率均有显著影响。当不考虑土壤湿度年际异常时,模式模拟的春夏季平均温度、最高温度、最低温度在我国大范围内降低,春夏季降水在东部大部分地区明显减少,西部增加。而模式模拟的春夏季温度、降水年际变率在中国大部分地区减弱。但当考虑土壤湿度的年际变化,则能在一定程度上提高模式对气候年际变率的模拟能力。在进一步分析表明土壤湿度年际异常时,主要通过改变地表能量通量和环流场,对温度、降水产生影响。当不考虑土壤湿度年际异常时,地表净辐射通量减少,地表温度降低,感热通量减少。感热通量差值场的空间变化和温度差值场的空间变化一致,感热通量对温度有一定影响。而潜热通量差值场的空间变化和降水的差值场的空间变化一致,可见降水受地表潜热通量的影响。土壤湿度年际异常引起的环流场的变化也是导致气候变化的原因之一,地表能量和环流场年际变率的改变对春夏季气候年际变率存在一定影响。 相似文献
11.
The need for a well-defined lower boundary condition for atmospheric numerical models is well documented. This paper describes the formulation of a land surface parameterization, which will be used in atmospheric boundary-layer and mesoscale numerical models. The land surface model has three soil layers for the prediction of soil moisture and soil temperature. Model soil properties depend on soil texture and moisture content. A homogeneous distribution of vegetation is also included, so that transpiration may be included, as well as the interception of precipitation by vegetation elements. The simulated vegetation also affects the mean surface albedo and roughness characteristics.First ISLSCP Field Experiment (FIFE) data are used to verify the model. Three cases during the growing season were chosen, each case having different amounts of vegetation cover. Stand alone simulations, where observations of atmospheric and radiation variables are input to the land surface model, were performed. These simulations show that the model is able to reproduce observed surface energy budgets and surface temperatures reasonably well. The RMS differences between modeled and obsered turbulent fluxes of heat and moisture are quite comparable to those reported by more detailed land surface models. 相似文献
12.
Yang Zhao Deliang Chen Jiao Li Dandan Chen Yi Chang Juan Li Rui Qin 《Climate Dynamics》2020,54(5):2713-2730
This study investigates the roles of atmospheric moisture transport under the influence of topography for summer extreme precipitation over North China (NC) during 1979–2016. Based on rain gauge precipitation data and a reanalysis, 38 extreme precipitation days in NC during the 38 years were selected and associated moisture fluxes estimated. The results show that there is a dominant moisture influx of 311.8 kg m−1 s−1 into NC along its southern boundary from tropical oceans, and a secondary influx of 107.9 kg m−1 s−1 across its western boundary carried by mid-latitude westerlies. The outflux across the eastern boundary is 206.9 kg m−1 s−1 and across the northern boundary is 76.0 kg m−1 s−1, giving a net moisture gain over NC of 136.8 kg m−1 s−1. During extreme precipitation days, the moisture flux convergence (MFC) was much larger, exceeding 4 × 10−5 kg m−1 s−1. The MFC maximum core, the pronounced moisture transport, and the striking extreme precipitation zone over NC are all anchored to the east of the steep slopes of the surrounding topography. Moreover, a remarkably high humidity and strong upward motion also occur near steep slopes, indicating the critical role of the adjacent topography on the extreme precipitations. Simulations with and without the topography in NC using the Weather and Research Forecasting model for six selected out of the 38 extreme precipitation days demonstrate that the surrounding topography reinforces the MFC over NC by 16% relative to the case without terrain, primarily through enhanced wind convergence and higher moisture content, as well as stronger vertical motion induced by diabatic heating. The interactions between moisture convergence and topographic settings strengthen the extreme precipitation over NC. 相似文献
13.
Imtiaz Rangwala Joseph Barsugli Karen Cozzetto Jason Neff James Prairie 《Climate Dynamics》2012,39(7-8):1823-1840
This study analyzes mid-21st century projections of daily surface air minimum (Tmin) and maximum (Tmax) temperatures, by season and elevation, over the southern range of the Colorado Rocky Mountains. The projections are from four regional climate models (RCMs) that are part of the North American Regional Climate Change Assessment Program (NARCCAP). All four RCMs project 2°C or higher increases in Tmin and Tmax for all seasons. However, there are much greater (>3°C) increases in Tmax during summer at higher elevations and in Tmin during winter at lower elevations. Tmax increases during summer are associated with drying conditions. The models simulate large reductions in latent heat fluxes and increases in sensible heat fluxes that are, in part, caused by decreases in precipitation and soil moisture. Tmin increases during winter are found to be associated with decreases in surface snow cover, and increases in soil moisture and atmospheric water vapor. The increased moistening of the soil and atmosphere facilitates a greater diurnal retention of the daytime solar energy in the land surface and amplifies the longwave heating of the land surface at night. We hypothesize that the presence of significant surface moisture fluxes can modify the effects of snow-albedo feedback and results in greater wintertime warming at night than during the day. 相似文献
14.
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. 相似文献
15.
Relative contribution of soil moisture and snow mass to seasonal climate predictability: a pilot study 总被引：1，自引：1，他引：0
Hervé Douville 《Climate Dynamics》2010,34(6):797-818
Land surface hydrology (LSH) is a potential source of long-range atmospheric predictability that has received less attention
than sea surface temperature (SST). In this study, we carry out ensemble atmospheric simulations driven by observed or climatological
SST in which the LSH is either interactive or nudged towards a global monthly re-analysis. The main objective is to evaluate
the impact of soil moisture or snow mass anomalies on seasonal climate variability and predictability over the 1986–1995 period.
We first analyse the annual cycle of zonal mean potential (perfect model approach) and effective (simulated vs. observed climate)
predictability in order to identify the seasons and latitudes where land surface initialization is potentially relevant. Results
highlight the influence of soil moisture boundary conditions in the summer mid-latitudes and the role of snow boundary conditions
in the northern high latitudes. Then, we focus on the Eurasian continent and we contrast seasons with opposite land surface
anomalies. In addition to the nudged experiments, we conduct ensembles of seasonal hindcasts in which the relaxation is switched
off at the end of spring or winter in order to evaluate the impact of soil moisture or snow mass initialization. LSH appears
as an effective source of surface air temperature and precipitation predictability over Eurasia (as well as North America),
at least as important as SST in spring and summer. Cloud feedbacks and large-scale dynamics contribute to amplify the regional
temperature response, which is however, mainly found at the lowest model levels and only represents a small fraction of the
observed variability in the upper troposphere. 相似文献
16.
This paper critically reviews and intercompares land surface schemes (LSSs) as used in atmospheric general circulation models
(AGCMs) to simulate soil moisture and its response to a warmer climate, and potential evapotranspiration approaches as used
in operational soil moisture monitoring and in predicting the response of soil moisture to a warmer climate. AGCM predictions
of overall soil moisture change are in broad agreement but disagree sharply in some regions. Intercomparison projects have
sought to evaluate the LSSs used by AGCMs for both accuracy and consistency. These studies have found that different LSSs
can produce very different simulations even when supplied with identical atmospheric forcing. As well, LSSs that produce similar
surface results from present-day or control climates often diverge when forced with climatic change data. Furthermore, no
single LSS has been identified that produces an adequate simulation of all of temperature, moisture, evapotranspiration and
runoff. AGCM LSSs must resolve the surface energy balance (SEB) in order to compute realistic heat fluxes between with the
atmospheric model. LSSs have been used with AGCMs in both on-line (fully coupled) and off-line modes. In off-line climatic
change experiments, AGCM predictions of atmospheric temperature and precipitation have been used, along with model downward
radiative fluxes at the surface, to drive their own uncoupled LSS. However, there are simple non-energy-balance methods for
estimating evapotranspiration that have been traditionally used in agricultural and meteorological applications. These schemes
compute a potential evapotranspiration (PE) based on temperature and/or net radiation inputs, with the PE modified based on
the availability of soil moisture. Operational PE approaches have also been used with AGCM data in off-line climate change
experiments. The advantages of this approach are that it is simpler and requires less information, although (like the off-line
SEB approach) it leaves out the simulation of feedbacks between the surface and the atmosphere.Although the SEB approach is
essential for LSSs that must be coupled to AGCMs, this does not necessarily make it superior to an off-line operational PE
LSS when it comes to quantities such as soil moisture. The quality of current observational data is insufficient to demonstrate
that either approach is better than the other. Both approaches should continue to be used and intercompared when predicting
the impacts of climatic change on soil moisture. 相似文献
17.
J. F. Gueremy 《Meteorology and Atmospheric Physics》1990,44(1-4):219-250
Summary In this paper we have studied the low frequency variability of the sensible and latent heat flux over the Indian monsoon area. We have used an atmospheric energy budget (vertical integrated heat sources and moisture sinks), as well as the similarity theory in order to compute the surface fluxes on a darly basis. Mainly, the three following data sets were used: the First GARP Global Experiment analyzed data, the TIROS-N outgoing longwave radiation data and the Monsoon Experiment precipitation data.Our three main findings are the following. First, the variability of the temperature and the specific humidity at the surface is more important over the land than over the sea on the intraseasonal time scale (30% over land, but 20% over sea). For the wind an energy peak appears clearly around 30–40 days. The surface fluxes show an uneven variance percentage field (10% to 40%); the energy peaks stretch from 10 to 40 days. Second, the wind has a significant influence on the surface fluxes, except at some locations exclusively over the land areas. Of the temperature and the specific humidity, the temperature is the one which influences the fluxes the most. (This influence may be very strong over land.) The specific humidity may have a significant influence, over the land and sea, at the same time. Thus, one cannot neglect the influence of temperature and specific humidity over land on the intraseasonal time scale. Third, we have found a close relation between the propagation of low frequency waves and the propagation of surface flux patterns. This may suggest a feedback mechanism which relates surface processes to the northward propagation of these waves over India.With 17 FiguresOn leave from Etablissement d'études et de recherches méteorologiques Paris, France 相似文献
18.
Chang-Eui Park Chang-Hoi Ho Su-Jong Jeong Jinwon Kim Song Feng 《Climate Dynamics》2012,38(7-8):1489-1500
This study examines the potential impact of vegetation feedback on changes in summer climate aridity over the contiguous United States (US) due to the doubling of atmospheric CO2 concentration using a set of 100-year-long climate simulations made by a global climate model interactively coupled with a dynamic vegetation model. The Thornthwaite moisture index (I m ), which quantifies climate aridity on the basis of atmospheric water supply (i.e., precipitation) and atmospheric water demand (i.e., potential evapotranspiration, PET), is used to measure climate aridity. Warmer atmosphere and drier surface resulting from increased CO2 concentration increase climate aridity over most of the contiguous US. This phenomenon is due to larger increments in PET than in precipitation, regardless of the presence or absence of vegetation feedback. Compared to simulations without active dynamic vegetation feedback, the presence of vegetation feedback significantly alleviates the increase in aridity. This vegetation-feedback effect is most noticeable in the subhumid regions such as southern, midwestern and northwestern US, primarily by the increasing vegetation greenness. In these regions, the greening in response to warmer temperatures enhances moisture transfer from soil to atmosphere by evapotranspiration (ET). The increased ET and subsequent moistening over land areas result in weaker surface warming (1–2?K) and PET (3–10?mm?month?1), and greater precipitation (4–10?mm?month?1). Collectively, they result in moderate increases in I m . Our results suggest that moistening by enhanced vegetation feedback may prevent aridification under climatic warming especially in areas vulnerable to climate change, with consequent implications for mitigation strategies. 相似文献
19.
Understanding the relationship between gradually varying soil moisture (SM) conditions and monsoon rainfall anomalies is crucial for seasonal prediction. Though it is an important issue, very few studies in the past attempted to diagnose the linkages between the antecedent SM and Indian summer monsoon rainfall. This study examined the relationship between spring (April–May) SM and June rainfall using observed data during the period 1979–2010. The Empirical Orthogonal Function (EOF) analyses showed that the spring SM plays a significant role in June rainfall over the Central India (CI), South India (SI), and North East India (NEI) regions. The composite anomaly of the spring SM and June rainfall showed that excess (deficit) June rainfall over the CI was preceded by wet (dry) spring SM. The anomalies in surface-specific humidity, air temperature, and surface radiation fluxes also supported the existence of a positive SM-precipitation feedback over the CI. On the contrary, excess (deficit) June rainfall over the SI and NEI region were preceded by dry (wet) spring SM. The abnormal wet (dry) SM over the SI and NEI decreased (increased) the 2-m air temperature and increased (decreased) the surface pressure compared to the surrounding oceans which resulted in less (more) moisture transport from oceans to land (negative SM-precipitation feedback over the Indian monsoon region). 相似文献
20.
The sensitivity of climate to orbitally-related changes in solar radiation at 9000 yr BP (before present) is examined using fixed and interactive soil moisture versions of a low resolution general circulation model. In both versions of the model increased solar radiation for June–August at 9000 yr BP (compared to present) produced enhanced northern monsoons and warmer continental interiors in comparison to present whereas decreased solar radiation at 9000 yr BP in December–February produced weaker southern monsoons. The increased rainfall in the northern tropics in summer increased soil moisture and runoff at 9000 yr BP in the interactive model; in the southern hemisphere decreased rainfall in summer led to decreased soil moisture and runoff. Conditions in summer became drier (decreased soil moisture and runoff) at 9000 yr BP in the northern extratropics.The experiments showed that the magnitude (but not the sign) of model sensitivity to 9000 yr BP radiation is altered by the effects of interactive soil moisture. Decreased soil moisture (about 20%) over northern Eurasia in the interactive model led to smaller evaporative increases, greater temperature increases and greater reduction of precipitation than for the model with fixed soil moisture. Over northern tropical lands, slightly smaller temperature increases and greater evaporation and precipitation increases in the interactive model are linked to the simulation of increased soil moisture at 9000 yr BP. The differences in sensitivity between the two versions of the model over northern Eurasia are statistically significant at the 95% level whereas those for the tropics are not.Overall, the results of the simulations are generally supported by the geologic evidence for 9000 yr BP; however, the evidence lacks sufficient precision and the model resolution is too coarse for detailed model/data comparisons and for assessment of the relative accuracy of the two 9000 yr BP experiments.The computed sensitivities of temperature and soil moisture to 9000 yr BP radiation differ from those simulated under equilibrium conditions in the various general circulation model experiments for increased atmospheric concentration of CO2. In contrast to the effects of the enhanced seasonal cycle of solar radiation at 9000 yr BP, a CO2 increase causes a broad warming of both the ocean and land with little modification of land/ocean temperature difference. The experiments for 9000 yr BP indicate a clearer signal for summer drying than is obtained in the experiments for increased CO2. The results suggest that the 9000 yr BP climate may be of limited utility as an analog to future warm climates. 相似文献