首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
A seasonal energy balance climate model containing a detailed treatment of surface and planetary albedo, and in which seasonally varying land snow and sea ice amounts are simulated in terms of a number of explicit physical processes, is used to investigate the role of high latitude ice, snow, and vegetation feedback processes. Feedback processes are quantified by computing changes in radiative forcing and feedback factors associated with individual processes. Global sea ice albedo feedback is 5–8 times stronger than global land snowcover albedo feedback for a 2% solar constant increase or decrease, with Southern Hemisphere cryosphere feedback being 2–5 times stronger than Northern Hemisphere cryosphere feedback.In the absence of changes in ice extent, changes in ice thickness in response to an increase in solar constant are associated with an increase in summer surface melting which is exactly balanced by increased basal winter freezing, and a reduction in the upward ocean-air flux in summer which is exactly balanced by an increased flux in winter, with no change in the annual mean ocean-air flux. Changes in the mean annual ocean-air heat flux require changes in mean annual ice extent, and are constrained to equal the change in meridional oceanic heat flux convergence in equilibrium. Feedback between ice extent and the meridional oceanic heat flux obtained by scaling the oceanic heat diffusion coefficient by the ice-free fraction regulates the feedback between ice extent and mean annual air-sea heat fluxes in polar regions, and has a modest effect on model-simulated high latitude temperature change.Accounting for the partial masking effect of vegetation on snow-covered land reduces the Northern Hemisphere mean temperature response to a 2% solar constant decrease or increase by 20% and 10%, respectively, even though the radiative forcing change caused by land snowcover changes is about 3 times larger in the absence of vegetational masking. Two parameterizations of the tundra fraction are tested: one based on mean annual land air temperature, and the other based on July land air temperature. The enhancement of the mean Northern Hemisphere temperature response to solar constant changes when the forest-tundra ecotone is allowed to shift with climate is only 1/3 to 1/2 that obtained by Otterman et al. (1984) when the mean annual parameterization is used here, and only 1/4 to 1/3 as large using the July parameterization.The parameterized temperature dependence of ice and snow albedo is found to enhance the global mean temperature response to a 2% solar constant increase by only 0.04 °C, in sharp contrast to the results of Washington and Meehl (1986) obtained with a mean annual model. However, there are significant differences in the method used here and in Washington and Meehl to estimate the importance of this feedback process. When their approach is used in a mean annual version of the present model, closer agreement to their results is obtained.  相似文献   

2.
The ice-covered Earth instability found in energy balance models is studied with a zonal mean statistical dynamical atmospheric model coupled to a global mixed layer ocean model. The response of the model to changes in solar constant is examined in two parallel studies, one with and one without a fixed meridional heat transport (a Q-flux) being included in the ocean model. The Q-flux is derived so as to make the climate with the current value of the solar constant resemble the earths current climate. In both cases the climate displays a hysteresis loop as the solar constant decreases and then increases, with two equilibrium states being possible for a range of values of the solar constant. In the case without a Q-flux, as in energy balance models, one state corresponds to an ice-covered Earth, and the other is partially covered. In the case with a Q-flux, because the poleward Q-flux is stronger in the Southern Hemisphere, one state corresponds to an ice-covered Northern Hemisphere, but a Southern Hemisphere that is only partially ice-covered; the other state has much reduced ice-cover in both hemispheres. In the case when the Q-flux is present, the sensitivity of the state with smaller ice-cover is about half as much, and the hysteresis loop extends over a smaller range of values of the solar constant. Also in this case there is a strong ice-covered Earth instability that sets in when the solar constant is about 13–14% below the current value. However in the case without a Q-flux the ice-covered Earth instability virtually disappears. The different behavior is attributed to the much lower efficiency of the meridional heat transport in the case with no Q-flux. The behavior in this case may be more realistic for cold climates. The results in both cases confirm the simple analytical relation between global mean surface temperature and global ice area found in energy balance models.  相似文献   

3.
Results are presented from two versions of a global R15 atmospheric general circulation model (GCM) coupled to a nondynamic, 50-m deep, slab ocean. Both versions include a penetrative convection scheme that has the effect of pumping more moisture higher into the troposphere. One also includes a simple prescribed functional dependence of cloud albedo in areas of high sea-surface temperature (SST) and deep convection. Previous analysis of observations has shown that in regions of high SST and deep convection, the upper-level cloud albedos increase as a result of the greater optical depth associated with increased moisture content. Based on these observations, we prescribe increased middle- and upper-level cloud albedos in regions of SST greater than 303 K where deep convection occurs. This crudely accounts for a type of cloud optical property feedback, but is well short of a computed cloud-optical property scheme. Since great uncertainty accompanies the formulation and tuning of such schemes, the prescribed albedo feedback is an intermediate step to examine basic feedbacks and sensitivities. We compare the two model versions (with earlier results from the same model with convective adjustment) to a model from the Canadian Climate Centre (CCC) having convective adjustment and a computed cloud optical properties feedback scheme and to several other GCMs. The addition of penetrative convection increases tropospheric moisture, cloud amount, and planetary albedo and decreases net solar input at the surface. However, the competing effect of increased downward infrared flux (from increased tropospheric moisture) causes a warmer surface and increased latent heat flux. Adding the prescribed cirrus albedo feedback decreases net solar input at the surface in the tropics, since the cloud albedos increase in regions of high SST and deep convection. Downward infrared radiation (from increased moisture) also increases, but this effect is overpowered by the reduced solar input in the tropics. Therefore, the surface is somewhat cooler in the tropics, latent heat flux decreases, and global average sensitivity to a doubling of CO2 with regard to temperature and precipitation/evaporation feedback is reduced. Similar processes, evident in the CCC model with convective adjustment and a computed cloud optical properties feedback scheme, occur over a somewhat expanded latitudinal range. The addition of penetrative convection produces global effects, as does the prescribed cirrus albedo feedback, although the strongest local effects of the latter occur in the tropics.Portions of this study are supported by the Office of Health and Environmental Research of the U.S. Department of Energy as part of its Carbon Dioxide Research Program, and by the Electric Power Research Institute as part of its Model Evaluation Consortium for Climate Assessment ProjectThe National Center for Atmospheric Research is sponsored by the National Science Foundation  相似文献   

4.
The simulated mean January and July climates of four versions of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM) are compared. The models include standard configurations of CCM1 and CCM2, as well as two widely-cited research versions, the Global Environmental and Ecological Simulation of Interactive Systems (GENESIS) model and the Climate Sensitivity and Carbon Dioxide (CSC02) model. Each CCM version was integrated for 10 years with a horizontal spectral resolution of rhomboidal 15 (R15). Additionally, the standard T42 version of CCM2 was integrated for 20 years. Monthly mean, annually repeating climatological sea surface temperatures provided a lower boundary condition for each of the model simulations. The CCM troposphere is generally too cold, especially in the polar upper troposphere in the summer hemisphere. This is least severe in CCM2 and most pronounced in CCM1. CSC02 is an exception with a substantial warm bias, especially in the tropical upper troposphere. Corresponding biases are evident in atmospheric moisture. The overall superior CCM2 thermodynamic behavior is principally compromised by a large warm and moist bias over the Northern Hemisphere middle and high latitudes during summer. Differences between the simulated and observed stationary wave patterns reveal sizeable amplitude errors and phase shifts in all CCM versions. A common problem evident in the upper troposphere is an erroneous cyclone pair that straddles the equatorial central Pacific in January. The overall January stationary wave error pattern in CCM2 and CSCO2 is suggestive of a reverse Pacific-North American teleconnection pattern originating from the tropical central Pacific. During July, common regional biases include simulated North Pacific troughs that are stronger and shifted to the west of observations, and each model overestimates the strength of the anticyclone pair associated with the summer monsoon circulation over India. The simulated major convergence and divergence centers tend to be very localized in all CCM versions, with a tendency in each model for the maximum divergent centers to be unrelistically concentrated in monsoon regions and tied to regions of steep orography. Maxima in CCM-simulated precipitation correspond to the simulated outflow maxima and are generally larger than observational estimates, and the associated atmospheric latent heating appears to contribute to the stationary wave errors. Comparisons of simulated radiative quantities to satellite measurements reveal that the overall CCM2 radiative balance is better than in the other CCM versions. An error common to all models is that too much solar radiation is absorbed in the middle latitudes during summer.The National Center for Atmospheric Research is sponsored by the National Science Foundation  相似文献   

5.
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   

6.
F. Codron 《Climate Dynamics》2001,17(2-3):187-203
 The changes of the variability of the tropical Pacific ocean forced by a shift of six months in the date of the perihelion are studied using a coupled tropical Pacific ocean/global atmosphere GCM. The sensitivity experiments are conducted with two versions of the atmospheric model, varied by two parametrization changes. The first one concerns the interpolation scheme between the atmosphere and ocean models grids near the coasts, the second one the advection of water vapor in the presence of downstream negative temperature gradients, as encountered in the vicinity of mountains. In the tropical Pacific region, the parametrization differences only have a significant direct effect near the coasts; but coupled feedbacks lead to a 1 °C warming of the equatorial cold tongue in the modified (version 2) model, and a widening of the western Pacific large-scale convergence area. The sensitivity of the seasonal cycle of equatorial SST is very different between the two experiments. In both cases, the response to the solar flux forcing is strongly modified by coupled interactions between the SST, wind stress response and ocean dynamics. In the first version, the main feedback is due to anomalous upwelling and leads to westward propagation of SST anomalies; whereas the version 2 model is dominated by an eastward-propagating thermocline mode. The main reason diagnosed for these different behaviors is the atmospheric response to SST anomalies. In the warmer climate simulated by the second version, the wind stress response in the western Pacific is enhanced, and the off-equatorial curl is reduced, both effects favoring eastward propagation through thermocline depth anomalies. The modifications of the simulated seasonal cycle in version 2 lead to a change in ENSO behavior. In the control climate, the interannual variability in the eastern Pacific is dominated by warm events, whereas cold events tend to be the more extreme ones with a shifted perihelion. Received: 14 December 1999 / Accepted: 24 May 2000  相似文献   

7.
Because the atmosphere and ocean are interacting systems, it is inappropriate to specify sea surface temperature when dealing with the atmosphere, or atmospheric anemometer level temperature and moisture when dealing with the ocean. All of these quantities should be determined interactively in terms of the external forcing: the solar constant.In the tropics, it is shown that the (cumulus) convective processes may be described by a one-dimensional cloud model. The near-surface ocean may similarly be described by a one-dimensional mixed-layer model. The coupling is achieved through a sea surface flux budget combined with the flux parameterizations implied by Monin-Obukhov similarity theory.The coupled one-dimensional atmosphere-ocean model is applied to the equilibrium situation in which all temperatures reach a steady state. Since the ocean, lacking an internal heating or cooling mechanism, can only be heated or cooled through sensibleheat fluxes through the sea surface, in equilibrium these fluxes must vanish. The atmosphere, however, maintains a stable lapse rate by balancing cumulonimbus heating against net radiative cooling. All water precipitated from cumulonimbus clouds must have evaporated from sea surface. It is shown that this equilibrium system is closed and determinable solely in terms of the solar constant.For various values of the solar constant, the sea surface temperature, the flux of latent and sensible heat from the surface, the height of the tropopause, mixed layer, and trade inversion layer, and generally, the entire vertical structure of the tropical atmosphere and near-surface ocean can be determined. The equilibrium sea surface temperature is shown to be relatively insensitive to changes in the solar constant, additional solar flux being compensated mainly by additional evaporation. Finally, the usefulness and limitations of the model are pointed out.  相似文献   

8.
Deming Zhao 《Climate Dynamics》2013,40(7-8):1767-1787
Regional climate models (RCMs) can provide much more precise information on surface characteristics and mesoscale circulation than general circulation models. This potential for obtaining more detailed model results has motivated to a significant focus on RCMs development in East Asia. The Regional Integrated Environment Modeling System, version 2.0 (RIEMS2.0) has been developed from an earlier RCM, RIEMS1.0, at the Key Laboratory of Regional Climate-Environment for East Asia and Nanjing University. To test the ability of RIEMS2.0 to simulate long-term climate and climate changes in East Asia and to provide a basis for further development and applications, we compare simulated precipitation from 1979 to 2008 (simulation duration from 1 January 1978 to 31 December 2008) to observed meteorological data. The results show that RIEMS2.0 reproduces the spatial distribution of precipitation in East Asia but that the simulation overestimates precipitation. The simulated 30-year precipitation average is 26 % greater than the observed precipitation. Simulated upper and root soil water correlate well with remote sensing derived soil moisture. Annual and interannual variation in the average precipitation and their anomalies are both well reproduced by the model. A further analysis of three subregions representing different latitude ranges shows that there is good correlation and consistency between the simulated results and the observed data. Annual variation, interannual variation of average precipitation, and the anomalies in the three sub-regions are also well captured by the model. The model’s performance on atmospheric circulation and moisture transport simulations is discussed to explore the bias between the simulation and observations. In summary, RIEMS2.0 shows stability and does well in both simulating long-term climate and climate changes in East Asia and in describing subregional characteristics.  相似文献   

9.
Continuous observation data collected over the whole year of 2004 on a cropland surtace m Tongyu, a senti-arid area of northeastern China （44°25＇N, 122°52＇E）, have been used to investigate the variations of surface albedo and soil thermal parameters, including heat capacity, thermal conductivity and thermal diffusivity, and their relationships to soil moisture. The diurnal variation of surface albedo appears as a U shape curve on sunny days. Surface albedo decreases with the increase of solar elevation angle, and it tends to be a constant when solar elevation angle is larger than 40°. So the daily average surface albedo was computed using the data when solar elevation angle is larger than 40° Mean daily surface albedo is found to decrease with the increase of soil moisture, showing an exponential dependence on soil moisture. The variations of soil heat capacity are small during Julian days 90 300. Compared with the heat capacity, soil thermal conductivity has very gentle variations during this period, but the soil thermal diffusivity has wide variations during the same period. The soil thermal conductivity is found to increase as a power function of soil moisture. The soil thermal diffusivity increases firstly and then decreases with the increase of soil moisture.  相似文献   

10.
Summary The response of the climatic system to changes in its radiative forcing has been the subject of much study. Climate models of various complexity have been used to demonstrate that a small increase in the solar constant, or doubling of the atmospheric CO2, would lead to a warmer surface. Very little scientific attention, however, has been given to the effect such a change in radiative balance might have on climatic variability. That is, would an earth warmed in this way be more temperate or more variable? To move one step closer to answering this question, we employed a simple one-dimensional surface energy balance climate model and forced it with random Gaussian white noise to simulate interannual variability. We integrated the model using 0, 2, and 4% increases in the solar constant. The results of these numerical experiments indicate that, under a warmer surface radiative balance, interannual variability of the surface temperature is reduced.  相似文献   

11.
This study investigates the influence of Simplified Arakawa Schubert (SAS) and Relax Arakawa Schubert (RAS) cumulus parameterization schemes on coupled Climate Forecast System version.1 (CFS-1, T62L64) retrospective forecasts over Indian monsoon region from an extended range forecast perspective. The forecast data sets comprise 45 days of model integrations based on 31 different initial conditions at pentad intervals starting from 1 May to 28 September for the years 2001 to 2007. It is found that mean climatological features of Indian summer monsoon months (JJAS) are reasonably simulated by both the versions (i.e. SAS and RAS) of the model; however strong cross equatorial flow and excess stratiform rainfall are noted in RAS compared to SAS. Both the versions of the model overestimated apparent heat source and moisture sink compared to NCEP/NCAR reanalysis. The prognosis evaluation of daily forecast climatology reveals robust systematic warming (moistening) in RAS and cooling (drying) biases in SAS particularly at the middle and upper troposphere of the model respectively. Using error energy/variance and root mean square error methodology it is also established that major contribution to the model total error is coming from the systematic component of the model error. It is also found that the forecast error growth of temperature in RAS is less than that of SAS; however, the scenario is reversed for moisture errors, although the difference of moisture errors between these two forecasts is not very large compared to that of temperature errors. Broadly, it is found that both the versions of the model are underestimating (overestimating) the rainfall area and amount over the Indian land region (and neighborhood oceanic region). The rainfall forecast results at pentad interval exhibited that, SAS and RAS have good prediction skills over the Indian monsoon core zone and Arabian Sea. There is less excess rainfall particularly over oceanic region in RAS up to 30 days of forecast duration compared to SAS. It is also evident that systematic errors in the coverage area of excess rainfall over the eastern foothills of the Himalayas remains unchanged irrespective of cumulus parameterization and initial conditions. It is revealed that due to stronger moisture transport in RAS there is a robust amplification of moist static energy facilitating intense convective instability within the model and boosting the moisture supply from surface to the upper levels through convergence. Concurrently, moisture detrainment from cloud to environment at multiple levels from the spectrum of clouds in the RAS, leads to a large accumulation of moisture in the middle and upper troposphere of the model. This abundant moisture leads to large scale condensational heating through a simple cloud microphysics scheme. This intense upper level heating contributes to the warm bias and considerably increases in stratiform rainfall in RAS compared to SAS. In a nutshell, concerted and sustained support of moisture supply from the bottom as well as from the top in RAS is the crucial factor for having a warm temperature bias in RAS.  相似文献   

12.
Radiative forcing and climate sensitivity have been widely used as concepts to understand climate change. This work performs climate change experiments with an intermediate general circulation model (IGCM) to examine the robustness of the radiative forcing concept for carbon dioxide and solar constant changes. This IGCM has been specifically developed as a computationally fast model, but one that allows an interaction between physical processes and large-scale dynamics; the model allows many long integrations to be performed relatively quickly. It employs a fast and accurate radiative transfer scheme, as well as simple convection and surface schemes, and a slab ocean, to model the effects of climate change mechanisms on the atmospheric temperatures and dynamics with a reasonable degree of complexity. The climatology of the IGCM run at T-21 resolution with 22 levels is compared to European Centre for Medium Range Weather Forecasting Reanalysis data. The response of the model to changes in carbon dioxide and solar output are examined when these changes are applied globally and when constrained geographically (e.g. over land only). The CO2 experiments have a roughly 17% higher climate sensitivity than the solar experiments. It is also found that a forcing at high latitudes causes a 40% higher climate sensitivity than a forcing only applied at low latitudes. It is found that, despite differences in the model feedbacks, climate sensitivity is roughly constant over a range of distributions of CO2 and solar forcings. Hence, in the IGCM at least, the radiative forcing concept is capable of predicting global surface temperature changes to within 30%, for the perturbations described here. It is concluded that radiative forcing remains a useful tool for assessing the natural and anthropogenic impact of climate change mechanisms on surface temperature.  相似文献   

13.
A review is presented of the development and simulation characteristics of the most recent version of a global coupled model for climate variability and change studies at the Geophysical Fluid Dynamics Laboratory, as well as a review of the climate change experiments performed with the model. The atmospheric portion of the coupled model uses a spectral technique with rhomboidal 30 truncation, which corresponds to a transform grid with a resolution of approximately 3.75° longitude by 2.25° latitude. The ocean component has a resolution of approximately 1.875° longitude by 2.25° latitude. Relatively simple formulations of river routing, sea ice, and land surface processes are included. Two primary versions of the coupled model are described, differing in their initialization techniques and in the specification of sub-grid scale oceanic mixing of heat and salt. For each model a stable control integration of near millennial scale duration has been conducted, and the characteristics of both the time-mean and variability are described and compared to observations. A review is presented of a suite of climate change experiments conducted with these models using both idealized and realistic estimates of time-varying radiative forcing. Some experiments include estimates of forcing from past changes in volcanic aerosols and solar irradiance. The experiments performed are described, and some of the central findings are highlighted. In particular, the observed increase in global mean surface temperature is largely contained within the spread of simulated global mean temperatures from an ensemble of experiments using observationally-derived estimates of the changes in radiative forcing from increasing greenhouse gases and sulfate aerosols.  相似文献   

14.
Simulated impacts of global and regional climate change, induced by an enhanced greenhouse effect and by Amazonian deforestation, on the phenology and yield of two grain corn cultivars in Venezuela (CENIAP PB-8 and OBREGON) are reported. Three sites were selected:Turén, Barinas andYaritagua, representing two important agricultural regions in the country. The CERES-Maize model, a mechanistic process-based model, in theDecision Support System for Agrotechnology Transfer (DSSAT) was used for the crop simulations. These simulations assume non-limiting nutrients, no pest damage and no damage from excess water; therefore, the results indicate only the difference between baseline and perturbed climatic conditions, when other conditions remain the same. Four greenhouse-induced global climate change scenarios, covering different sensitivity levels, and one deforestation-induced regional climate change scenario were used. The greenhouse scenarios assume increased air temperature, increased rainfall and decreased incoming solar radiation, as derived from atmospheric GCMs for doubled CO2 conditions. The deforestation scenarios assume increased air temperature, increased incoming solar radiation and decreased rainfall, as predicted by coupled atmosphere-biosphere models for extensive deforestation of a portion of the Amazon basin. Two baseline climate years for each site were selected, one year with average precipitation and another with lower than average rainfall. Scenarios associated with the greenhouse effect cause a decrease in yield of both cultivars at all three sites, while the deforestation scenarios produce small changes. Sensitivity tests revealed the reasons for these responses. Increasing temperatures, especially daily maximum temperatures, reduce yield by reducing the duration of the phenological phases of both cultivars, as expected from CERES-Maize. The reduction of the duration of the kernel filling phase has the largest effect on yield. Increases of precipitation associated with greenhouse warming have no effects on yield, because these sites already have adequate precipitation; however, the crop model used here does not simulate potential negative effects of excess water, which could have important consequences in terms of soil erosion and nutrient leaching. Increases in solar radiation increased yields, according to the non-saturating light response of the photosynthesis rate of a C4 plant like corn, compensating for reduced yields from increased temperatures in deforestation scenarios. In the greenhouse scenarios, reduced insolation (due to increased cloud cover) and increased temperatures combine to reduce yields; a combination of temperature increase with a reduction in solar radiation produces fewer and lighter kernels.A report of thePAN-EARTH Project, Venezuela Case Study.  相似文献   

15.
We detect and characterize each large-scale intraseasonal perturbation in observations (1979–2009) and in coupled general circulation models of Institut Pierre Simon Laplace (IPSL) and of Centre National de Recherches Météorologiques (CNRM). These ensembles of intraseasonal perturbations are used to assess the skill of the two models in an event-by-event approach. This assessment addresses: (1) the planetary-scale (i.e. the whole Indo-Pacific area) extent of wind and rainfall perturbations and the reproducibility of the perturbation patterns for a given season; (2) the size and amplitude of rainfall and wind anomalies at basin-scale (i.e. for a particular phase of the perturbation) and; (3) the evolution of the vertical structure of the perturbations (U, T and RH) for selected events. The planetary-scale extent of rainfall perturbations is generally too small for both models. This extent is also small for the wind perturbation in the IPSL model, but is correct, or even too large in boreal winter, for the CNRM model. The reproducibility of the planetary-scale patterns is exaggerated for wind perturbations in the CNRM model and is very poor for all parameters in the IPSL model. Over the Indian Ocean during boreal winter, rainfall and wind anomalies at basin-scale are too large for the CNRM model and too small for the IPSL model. The CNRM model gives a realistic baroclinic perturbations structure for wind, moisture and temperature, but with too large amplitude due in part to a zonally extended rainfall anomaly over the eastern Indian Ocean and the Maritime Continent. The IPSL model gives a realistic response for low-level wind only. Temperature and moisture perturbations are barotropic with a wrong warm anomaly at rainfall maximum and there is no gradual increase in low-level moisture prior to this rainfall maximum. These results suggest that this version of the IPSL model is unable to initiate the coupling between the convection and the dynamic necessary to develop the perturbation. It is difficult to say if this is due to, or is at the origin of the lack of basin-scale organization of the convection. We discuss the likely role of the convective schemes in the differences found between these two versions of the CNRM and IPSL models.  相似文献   

16.
The climate and variability of seasonal ensemble integrations, made with a recent version of ECMWF model (used for ERA-40 production) at relatively high horizontal resolution (TL159), have been studied for the 10-year period, 1980–1989. The model systematic error over the Atlantic-European region has been substantially improved when compared with the earlier model versions (e.g. from the PROVOST and AMIP-2 projects). However, it has worsened over the Pacific-North American region. This systematic error reduces the amplitude of planetary waves and has a negative impact on intraseasonal variability and predictability of the PNA mode. The signal-to-noise analysis yields results similar to earlier model versions: only during relatively strong ENSO events do some parts of the extratropics exhibit potential predictability. For precipitation, there is more disagreement between observed and model climatologies over sea than over land, but interannual variations over many parts of the tropical ocean are reasonably well represented. The south Asian monsoon in the model is severely weakened when compared to observations; this is seen in both poor climatology and interannual variability. Overall, comparing the ERA-40 model with earlier versions, there seems to be a balance between model improvements and deteriorations due to systematic errors. For the seasonal time-scale predictability, it is not clear that this model cycle constitutes an advantage over the earlier versions. Therefore, since it is not always possible to achieve distinct improvements in model climate and variability, a careful and detailed strategy ought to be considered when introducing a new model version for operational seasonal forecasting.  相似文献   

17.
Summary A land-surface model (MOSES) was tested against observed fluxes of heat, water vapour and carbon dioxide for two primary forest sites near Manaus, Brazil. Flux data from one site (called C14) were used to calibrate the model, and data from the other site (called K34) were used to validate the calibrated model. Long-term fluxes of water vapour at C14 and K34 simulated by the uncalibrated model were good, whereas modelled net ecosystem exchange (NEE) was poor. The uncalibrated model persistently underpredicted canopy conductance (g c ) from mid-morning to mid-afternoon due to saturation of the response to solar radiation at low light levels. This in turn caused a poor simulation of the diurnal cycles of water vapour and carbon fluxes. Calibration of the stomatal conductance/photosynthesis sub-model of MOSES improved the simulated diurnal cycle of g c and increased the diurnal maximum NEE, but at the expense of degrading long-term water vapour fluxes. Seasonality in observed canopy conductance due to soil moisture change was not captured by the model. Introducing realistic depth-dependent soil parameters decreased the amount of moisture available for transpiration at each depth and led to the model experiencing soil moisture limitation on canopy conductance during the dry season. However, this limitation had only a limited effect on the seasonality in modelled NEE.  相似文献   

18.
The patterns of climate change in the Asia-Pacific region simulated by versions of the CSIRO Mk3.5 and Mk3.0 climate models are examined and compared with those from 23 CMIP3 models. Using fields standardized by global warming, it is seen that both CSIRO coupled models simulate larger surface warming in the tropical western Pacific Ocean, and smaller warming in the eastern Indian Ocean, than the CMIP3 average, and also model versions with a mixed-layer ocean. Corresponding differences in the changes in the pressure, winds, rainfall and other quantities were simulated. Introducing the coupled Mk3.5’s sea surface temperature field for the present climate, which has a warm bias, as the base climate for the MLO version had only a minor effect on the MLO version’s pattern of climate change. A Pacific-Indian Dipole index quantifying the amplitude of the warming pattern explains much of the variation in rainfall change simulated by the CMIP3 models over Australia and the Indonesian and Melanesian regions. It relates more strongly to Australian average rainfall than several other indices representing southern hemispheric circulation changes. The decline in Australian rainfall produced by the full ocean coupling is largest in summer, but occurs in each season, and extends across the continent. Further assessment of the importance of the dipole change pattern in new simulations is warranted. Analyses aimed at reducing the uncertainty in its potential amplitude could help narrow the range of projections for change in the Australasian region.  相似文献   

19.
 The sensitivity of a coupled model to the oceanic vertical diffusion coefficient κ v is examined. This is compared to the sensitivity of an ocean-only model forced by mixed boundary conditions (BC). The atmospheric component of the coupled model is a moist energy balance model. The ocean component is a 12-level geostrophic model, defined on a midlatitude β-plane. Atmosphere and ocean are coupled through the fluxes of heat and moisture at their interface. The coupled model contains a number of feedback processes which are not represented in the ocean-only model. This results in a temperature and salinity response to κ v which is stronger in the coupled model than in the ocean-only model. On the other hand, there is a weaker response in oceanic processes such as meridional heat transport, deep-water formation at high latitudes, etc. Ocean-only sensitivity experiments were also performed with modified BCs, which parametrise the feedback processes included in the coupled model. These are the modified thermal BC of Rahmstorf and Willebrand and a modified freshwater BC proposed in the present study. Large-scale features of the response in oceanic surface fields are well represented with modified BCs. However, the sensitivity of the deep ocean temperature is only partly captured due to local differences in the surface response. The scaling behavior of the zonal overturning stream function was found to depend on the surface BCs. In contrast to this, the meridional overturning stream function basically scales with κ0.5 v in all sensitivity experiments. Differences in the heat transport response among the experiments are thus primarily related to differences in the temperature response. Received: 28 February 1997/Accepted: 12 September 1997  相似文献   

20.
A sea-breeze event in south-west Western Australia is simulated using the Regional Atmospheric Modelling System (RAMS) version 6.0. The model is evaluated against high resolution soundings as well as station observations and is shown to reproduce the qualitative features of the sea breeze well. Sensitivity tests are carried out to investigate the effects of historical land-cover change and changes in soil moisture on the dynamics of the sea breeze. It is found that land-cover change alone, i.e., a change from wooded grasslands to bare soil, with no change in soil moisture initialisation, does not significantly alter the overall structure of the sea breeze but results in higher surface winds due to the reduced vegetation roughness length, which leads to enhanced surface moisture advection inland. On the other hand, land-cover change in conjunction with increased soil moisture results in a considerably weaker, shallower, and less penetrative sea breeze, and delays its onset and duration. A sea-breeze scaling analysis highlights the impact of increasing soil moisture on reducing the sea-breeze volume flux scale.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号