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1.
Simulations of subtropical marine low clouds and their radiative properties by nine coupled ocean-atmosphere climate models participating in the fourth assesment report (AR4) of the intergovernmental panel on climate change (IPCC) are analyzed. Satellite observations of cloudiness and radiative fluxes at the top of the atmosphere (TOA) are utilized for comparison. The analysis is confined to the marine subtropics in an attempt to isolate low cloudiness from tropical convective systems. All analyzed models have a negative bias in the low cloud fraction (model mean bias of −15%). On the other hand, the models show an excess of cloud radiative cooling in the region (model mean excess of 13 W m−2). The latter bias is shown to mainly originate from too much shortwave reflection by the models clouds rather than biases in the clear-sky fluxes. These results confirm earlier studies, thus no major progress in simulating the marine subtropical clouds is noted. As a consequence of the combination of these two biases, this study suggests that all investigated models are likely to overestimate the radiative response to changes in low level subtropical cloudiness.  相似文献   

2.
Summary Filtered secular series of temperature at Zagreb (Croatia) are negatively correlated with simultaneous series of solar radiation and positively correlated with cloudiness.The local balance of radiative fluxes, with cloudiness interference included, provides a basis for the evaluation of a local deterministic model, which can describe both the positive and negative impacts of cloud radiative forcing upon surface temperature. A theoretical temperature series, generated by the model simulation, proves a prevailing greenhouse effect of clouds upon recent climatic variations of temperature at Zagreb.With 5 Figures  相似文献   

3.
Summary Past investigations of the summertime heat budget over the Tibetan Plateau have not included detailed estimates of radiative cooling (Q R ) nor have they carefully considered the effects of cloudiness on this term. The various attempts to eyaluate different combinations of heat and moisture budget torms and to understand the sources of energy to the summer armospheric heat source over the plateau are not in agreement, partly because of remaining discrepancies in the radiative and turbulent flux components, and partly because until recently, the conventional data sets needed for independently estimating the total heating and moistening terms have been incomplete. The uncertainties in the radiative term have also led to difficulties in assessing the uncertainties in the other budget terms, since no study to date has assembled a complete enough data set to allow a unified calculation of all budget quantities or to obtain budget closure. Recently published results of Yanai and his colleagues involving apparent heat source calculations for the plateau region based on a much improved FGGE data set, have motivated the examination of whether more detailed radiative calculations can help resolve past discrepancies in the budget terms on a monthly time scale. This study uses a continuous time series of 22-km resolution INSAT geosynchronous satellite measurements and ECMWF profile analyses in conjunction with medium spectral resolution radiative transfer models to estimate the slicrtwave and longwave components of the radiative cooling term and the role of cloudiness on these components for the 1988 summer period. The calculations reveal both meridional and zonal structure in radiative divergence across the plateau associated with the substantial gradients of cloudiness and aridity that dominate the summertime plateau climatology. The calculations also indicate that the magnitudes of both cloud-induced shortwave heating and longwave cooling over the plateau are much greater than over low-elevation regions. Moreover, since cloud-induced longwave cooling exceeds cloud-induced shortwave heating, the bulk effect of clouds is to radiatively cool the plateau atmosphere. The high resolution calculations are reduced to monthly averaged budget quantities for analyzing whether existing discrepancies in the plateau heat budget can be resolved. Although there is no means to rigorously verify the accuracies and representativeness of the individual budget terms, the new radiative estimates combined with the most reliable current estimates of total heating and turbulent fluxes, produce near closure (within 4%) of the plateau heat budget for the June to August period.With 10 Figures  相似文献   

4.
A mesoscale 3D numerical model is described, with which detailed calculations have been made of turbulence and wind characteristics in the atmospheric boundary layer (ABL), as well as cloud particle size distribution, longwave and solar radiation fluxes and flux divergences, and atmosphere-ocean heat exchange. Based on numerical experiments simulating winter conditions of the Newfoundland energy-active zone of the ocean (EAZO), atmosphere-ocean energy exchange is investigated. It is shown that the basic mechanisms for the EAZO formation involve the following processes: (i) at the hydrological front between cold and warm ocean currents, the fluxes of sensible and latent heat grow significantly; (ii) at this front, in a particular synoptic situation, overcast low-level cloudiness forms, screening solar radiation so that in winter, the radiation budget at the front is reduced, and the radiative flux into the ocean is less than the energy release to the atmosphere; (iii) frequent occurrence of such synoptic situations with cloudiness decreases the oceanic enthalpy and creates negative SST anomalies. The transport of these anomalies by currents to the western coasts of the continents causes anomalies of weather and climate.  相似文献   

5.
Estimation of radiation during fair weather cloudy situations of the MESOGERS-84 experiment has been examined using micrometeorological observations and satellite data. Diurnal variation of cloudiness is empirically determined using satellite information as a function of global radiation, and relationships between net incoming radiation and global radiation are analyzed. Particularly, it has been found that a very simple relationship between global radiation, cloudiness and net radiative heat flux proposed by Nielsenet al. (1981) can be used with satellite data and applied to the Mesogers region in Southwest France. The different relationships between cloudiness and radiation are utilized to modify and to validate Taconet'set al. model (1986) to get fluxes related to a cloudy situation without advection.  相似文献   

6.
Snow surface and sea-ice energy budgets were measured near 87.5°N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to ?7°C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between ?50 W m?2 and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m?2, except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area.  相似文献   

7.
The response of low-level clouds to climate change has been identified as a major contributor to the uncertainty in climate sensitivity estimates among climate models. By analyzing the behaviour of low-level clouds in a hierarchy of models (coupled ocean-atmosphere model, atmospheric general circulation model, aqua-planet model, single-column model) using the same physical parameterizations, this study proposes an interpretation of the strong positive low-cloud feedback predicted by the IPSL-CM5A climate model under climate change. In a warmer climate, the model predicts an enhanced clear-sky radiative cooling, stronger surface turbulent fluxes, a deepening and a drying of the planetary boundary layer, and a decrease of tropical low-clouds in regimes of weak subsidence. We show that the decrease of low-level clouds critically depends on the change in the vertical advection of moist static energy from the free troposphere to the boundary-layer. This change is dominated by variations in the vertical gradient of moist static energy between the surface and the free troposphere just above the boundary-layer. In a warmer climate, the thermodynamical relationship of Clausius-Clapeyron increases this vertical gradient, and then the import by large-scale subsidence of low moist static energy and dry air into the boundary layer. This results in a decrease of the low-level cloudiness and in a weakening of the radiative cooling of the boundary layer by low-level clouds. The energetic framework proposed in this study might help to interpret inter-model differences in low-cloud feedbacks under climate change.  相似文献   

8.
We investigate the influence of clouds on the surface energy budget and surface temperature in the sea-ice covered parts of the ocean north of the Arctic circle in present-day climate in nine global climate models participating in the Coupled Model Intercomparison Project phase 3, CMIP3. Monthly mean simulated surface skin temperature, radiative fluxes and cloud parameters are evaluated using retrievals from the extended AVHHR Polar Pathfinder (APP-x) product. We analyzed the annual cycle but the main focus is on the winter, in which large parts of the region experience polar night. We find a smaller across-model spread as well as better agreement with observations during summer than during winter in the simulated climatological annual cycles of total cloudiness and surface skin temperature. The across-model spread in liquid and ice water paths is substantial during the whole year. These results qualitatively agree with earlier studies on the present-day Arctic climate in GCMs. The climatological ensemble model mean annual cycle of surface cloud forcing shows good agreement with observations in summer. However, during winter the insulating effect of clouds tends to be underestimated in models. During winter, most of the models as well as the observations show higher monthly mean total cloud fractions, associated with larger positive surface cloud forcing. Most models also show good correlation between the surface cloud forcing and the vertically integrated ice and liquid cloud condensate. The wintertime ensemble model mean total cloud fraction (69%) shows excellent agreement with observations. The across-model spread in the winter mean cloudiness is substantial (36?C94%) however and several models significantly underestimate the cloud liquid water content. If the two models not showing any relationship between cloudiness and surface cloud forcing are disregarded, a tentative across-model relation exists, in such a way that models that simulate large winter mean cloudiness also show larger surface cloud forcing. Even though the across-model spread in wintertime surface cloud forcing is large, no clear relation to the surface temperature is found. This indicates that other processes, not explicitly cloud related, are important for the simulated across-model spread in surface temperature.  相似文献   

9.
 A simple theoretical model of atmospheric radiative equilibrium is solved analytically to help understand the energetics of maintaining Earth's tropical and subtropical climate. The model climate is constrained by energy balance between shortwave (SW) and longwave (LW) radiative fluxes. Given a complete set of SW and LW optical properties in each atmospheric layer, the model yields a unique equilibrium-temperature profile. In contrast, if the atmospheric temperature profile and SW properties are prescribed, the model yields essentially two distinct LW transmissivity profiles. This bimodality is due to a nonlinear competition between the ascending and descending energy fluxes, as well as to their local conversion to sensible heat in the atmosphere. Idealized slab models that are often used to describe the greenhouse effect are shown to be a special case of our model when this nonlinearity is suppressed. In this special case, only one solution for LW transmissivity is possible. Our model's bimodality in LW transmissivity for given SW fluxes and temperature profile may help explain certain features of Earth's climate: at low latitudes the temperature profiles are fairly homogeneous, while the humidity profiles exhibit a bimodal distribution; one mode is associated with regions of moist-and-ascending, the other with dry-and-subsiding air. The model's analytical results show good agreement with the European Centre for Medium-Range Weather Forecasts' reanalysis data. Sensitivity analysis of the temperature profile with respect to LW transmissivity changes leads to an assessment of the low-latitude climate's sensitivity to the “runaway greenhouse” effect. Received: 7 December 1999 / Accepted: 19 February 2001  相似文献   

10.
Ocean models depend strongly on surface fluxes. When computed from atmospheric models, fluxes are affected by spin-up, i.e. they increase (or decrease) with forecast length. Such behavior may bias ocean models. The European Centre for Medium Range Forecasts (ECMWF) 40-year re-analysis (ERA-40) has been used to quantify short-range spin-ups of radiative and turbulent heat fluxes. Fluxes are compared as differences between two runs with the same initialization time. This method allows flux analysis over short-range forecasts as a function of distance from the initialization time. Results indicate that (i) latent heat flux spin-up increases with time but levels off after 24 h; (ii) sensible and radiative flux spin-ups remain constant after 6 h; (iii) regional spin-up of turbulent fluxes are systematic and can be larger than 30% for sensible heat but never exceeds 15% for latent heat; (iv) spin-up depends upon the season. The same analysis has been carried out with the ECMWF 15-year re-analysis (ERA-15); spin-ups in ERA-40 have been generally smaller than those in ERA-15.  相似文献   

11.
盘锦芦苇湿地水热通量计算方法的比较研究   总被引：5，自引：0，他引：5       下载免费PDF全文
利用2005年7月盘锦芦苇湿地生长旺季的小气候梯度系统30 min观测资料和开放式涡动相关系统10Hz原始观测资料,比较并分析了廓线法、波文比能量平衡法与涡动相关法计算的芦苇湿地生态系统水热通量。结果表明:廓线法与波文比能量平衡法计算的芦苇湿地生态系统水热通量与涡动相关法得到的芦苇湿地生态系统水热通量具有较好的相关性,但是涡动相关法存在能量不平衡。分析盘锦芦苇湿地生态系统水热通量的日变化发现,能量平衡各分量基本上以正午为中心,呈倒“U”型分布。用波文比法计算得到的芦苇湿地生态系统日感热通量最大值为164.25 W.m-2,日潜热通量最大值为294.18 W.m-2。降雨之后,芦苇湿地生态系统水热通量都有所增加,尤其是潜热通量增加显著,且峰值出现时间提前。  相似文献   

12.
Surface energy balance closure has been examined using eddy covariance measurements and other observations at one industrial and three agricultural sites near the Nakdong River during daytime. Energy balance closure was evaluated by calculating the long-term averaged energy balance ratio (EBR), the ratio of turbulent energy fluxes to available energy, and the statistical regression of turbulent energy fluxes against available energy using half-hourly data. The EBR of all sites ranges from 0.46 to 0.83 while the coefficient of determination (R 2) ranges from 0.37 to 0.77. The energy balance closure was relatively poor compared to homogeneous sites, indicating the influence of surface heterogeneity. Unmeasured heat storage terms also seem to play a role in the surface energy budget at the industrial and irrigated sites. The energy balance closure was better in conditions of high wind speed, low downward short wave radiation, and high friction velocity, which suggests the role of heat storage term and surface heterogeneity in surface energy balance at these sites. Spectrum analysis shows a sharp roll-off at the low frequency in co-spectrum, which indicates that low-frequency motions do not significantly contribute to turbulent fluxes. Both the spectra and cospectra in unstable conditions show a broad peak indicating the influence of multiple sizes of large eddies over heterogeneous sites. Most of ogive curves for the kinematic latent and sensible heat fluxes reach an asymptote within 30 minutes regardless of the EBR value, indicating that low frequency motion is not a main factor for energy imbalance. However, stationary eddies due to landscape heterogeneity still remains as a possible cause for energy imbalance.  相似文献   

13.
The global energy balance from a surface perspective   总被引：1，自引：1，他引：0  
In the framework of the global energy balance, the radiative energy exchanges between Sun, Earth and space are now accurately quantified from new satellite missions. Much less is known about the magnitude of the energy flows within the climate system and at the Earth surface, which cannot be directly measured by satellites. In addition to satellite observations, here we make extensive use of the growing number of surface observations to constrain the global energy balance not only from space, but also from the surface. We combine these observations with the latest modeling efforts performed for the 5th IPCC assessment report to infer best estimates for the global mean surface radiative components. Our analyses favor global mean downward surface solar and thermal radiation values near 185 and 342 Wm?2, respectively, which are most compatible with surface observations. Combined with an estimated surface absorbed solar radiation and thermal emission of 161 and 397 Wm?2, respectively, this leaves 106 Wm?2 of surface net radiation available globally for distribution amongst the non-radiative surface energy balance components. The climate models overestimate the downward solar and underestimate the downward thermal radiation, thereby simulating nevertheless an adequate global mean surface net radiation by error compensation. This also suggests that, globally, the simulated surface sensible and latent heat fluxes, around 20 and 85 Wm?2 on average, state realistic values. The findings of this study are compiled into a new global energy balance diagram, which may be able to reconcile currently disputed inconsistencies between energy and water cycle estimates.  相似文献   

14.
沙尘气溶胶对半干旱区微气象学特征影响的初步研究   总被引：3，自引：1，他引：2  
沙尘气溶胶通过改变地表—大气间的短波及长波辐射从而对地气间热量传输产生影响,进而影响到地表能量收支平衡以及近地面层气象要素的分布。半干旱区是我国干旱化表现最为剧烈的地区,也是沙尘暴频发地区和主要源区之一。本文利用吉林通榆半干旱区国际协调强化观测计划基准站2006年4～6月近地层微气象、地表辐射通量和沙尘气溶胶浓度观测资料,对比分析了该地区在典型沙尘暴天气、扬沙天气和晴朗天气等不同天气状况下近地层各气象要素、地表能量平衡及辐射收支的变化特征。结果表明,沙尘气溶胶对半干旱地区地面微气象学特征及辐射收支具有明显的影响,且表现出不同于干旱区的若干特性:半干旱区沙尘天气下的平均风速比晴天高约2m.s-1,而相对湿度则降低了约35%。相对而言,气温和浅层地温的变化对沙尘的影响响应较弱。半干旱区在沙尘天气时地表净辐射小于晴天,感热/潜热通量分别约占净辐射的55%和30%,分别远大于/远小于晴天时的情形。该地区气溶胶质量浓度与日均感热、相对湿度、潜热等要素具有明显的相关,相关系数分别高达0.70、-0.75和-0.62。相比之下,干旱区沙尘天气时的感热通量要小于晴天时的值,而且气象要素的响应更加剧烈。  相似文献   

15.
Surface energy balance and the partitioning of sensible heat flux(SHF) and latent heat flux(LHF) play key roles in land–atmosphere feedback. However,the lack of long-term observations of surface energy fluxes,not to mention spatially extensive ones,limits our understanding of how the surface energy distribution has responded to a warming climate over recent decades(1979–2009) at the national scale in China. Using four state-of-the-art reanalysis products with long-term surface energy outputs,we identified robust changes in surface energy partitioning,defined by the Bowen ratio(BR = SHF/LHF),over different climate regimes in China. Over the past three decades,the net radiation showed an increasing trend over almost the whole of China. The increase in available radiative energy flux,however,was balanced by differential partitioning of surface turbulent fluxes,determined by local hydrological conditions. In semi-arid areas,such as Northeast China,the radiative energy was transferred largely into SHF. A severe deficiency in near-surface and soil moistures led to a significant decreasing trend in LHF. The combined effect of increased SHF and decreased LHF resulted in significant upward trends in the BR and surface warming over Northeast China. In contrast,in the wet monsoon regions,such as southern China,increased downward net radiation favored a rise in LHF rather than in SHF,leading to a significant decreasing trend in the BR. Meanwhile,the increased LHF partly cancelled out the surface warming. The warming trend in southern China was smaller than that in Northeast China. In addition to impacts on heat-related events,the changes in the BR also reflected recent cases of extreme drought in China. Our results indicate that information regarding the BR may be valuable for drought monitoring,especially in regions prone to such conditions.  相似文献   

16.
Abstract

A physically‐based numerical model was developed to estimate the temporal course of the surface energy flux densities and the soil temperatures in dry and wet bare soils. Aerodynamic heat, vapour and momentum transfer theory was used to calculate the sensible and latent heat flux densities at the surface under diabatic and adiabatic conditions. A finite‐difference solution of the differential equation describing one‐dimensional heat transfer was used to calculate the surface soil heat flux density and soil profile temperatures. The surface temperature was determined iteratively by the simultaneous solution of equations describing radiative, heat and momentum transfer at the surface. The model was tested with measurements from energy balance studies conducted on a dry, sandy soil and a wet, silt loam soil, and was found to predict accurately the surface energy fluxes and soil temperatures over three‐day periods under conditions of potential and negligible evaporation. The sensitivity of the model to uncertainties in the aerodynamic roughness lengths for momentum (z0) and heat (zT) is reported. Values for z0 and Z0/ZT of 0.5 mm and 3.0, respectively, resulted in the best agreement between modelled and measured values of the fluxes and temperatures for both soils.  相似文献   

17.
Based on the micrometeorological measurements at a heterogeneous farmland in South China, this work detects the effects of vegetative heterogeneity and patch-scale harvest on the energy balance closure and turbulent fluxes. As a quality control, the integral turbulent characteristics are analyzed in the framework of Monin-Obukhov similarity theory. Spatial representativeness of the measurements is studied in terms of footprint and source area. Firstly, in two wind sectors, the nondimensional standard deviations of turbulent quantities generally agree with some foregoing studies. Discrepancies exist in the other sectors due to the instrument-induced flow distortion. Secondly, energy balance closure is examined with two types of linear regression, which confirms that mismatching source areas between the available energy and turbulent fluxes have no preference to either energy “deficit” or “surplus”. Thirdly, turbulent fluxes exhibit greater variability when they represent smaller source areas. The patch-scale harvest adjacent to the flux mast causes notable increase and decrease in the sensible heat and latent heat fluxes, respectively, while the CO2 exchange almost vanishes after the harvest. Interestingly, energy balance closure is less influenced despite the notable effects on the turbulent fluxes and Bowen ratio, implying that the energy balance closure check may mask some variability in the turbulent fluxes. Thus, to adjust the heat fluxes with a single “closure factor” for a perfect closure is dangerous at a patchy site.  相似文献   

18.
Observations of surface characteristics, meteorological conditions and energy balance components from Vancouver, B.C. are used to test the validity of the output from three one-dimensional surface energy balance models. The results show that whereas all of the models provide good simulations of net radiation, none can consistently predict the turbulent fluxes of sensible and latent heat using easily available input data. Inability to handle the role of water availability and its impact on evapotranspiration is identified as the principal problem.  相似文献   

19.
Temporal variations in heat fluxes over a central European city centre   总被引：3，自引：0，他引：3  
Summary Energy fluxes have been measured over an area near the centre of the city of Łódź, Poland, since November 2000. The site was selected because the building style (surface cover and morphology) is typical of European cities, yet distinct from the majority of cities where energy balance observations have been studied thus far. The multi-year dataset permits consideration of temporal changes in energy balance partitioning over a wide range of seasonal and synoptic conditions and of the role of heat storage and anthropogenic fluxes in the energy balance. Partitioning of net radiation into the turbulent fluxes is consistent in the two years, with the largest differences occurring due to differing precipitation. The monthly ensemble diurnal cycles of the turbulent fluxes over the two years are similar. The largest differences occur during the July–September period, and are attributable to greater net radiation and lower rainfall in 2002. The latent heat flux accounts for approximately 40% of the turbulent heat transfer on an annual basis. The average daily daytime Bowen ratio and its variability are slightly reduced during the summer (growing) season. Anthropogenic heat is a significant input to the urban energy balance in the winter. The fluxes observed in this study are consistent with results from other urban sites.  相似文献   

20.
The measurements obtained during the ECLATS experiment were used in order to determine the surface energy budget of the Sahel region (Niamey, Niger). This expedition was carried out from November 15 to December 10, 1980, during the dry period. Some data were collected by an instrumented aircraft, from which the turbulent fluxes were obtained in the boundary layer around midday; data were also collected at a surface station in order to estimate the surface energy budget continuously by the profile method. The aircraft measurements show the homogeneity of the vertical fluxes over large areas, allowing generalization to the bushy steppe of the Sahel region. The mean diurnal cycle of the energy budget is characterized by high values of ground heat flux and weak values of latent heat flux (deduced from the balance of the energy budget). This cycle is compared with that of the Koorin expedition, performed in similar conditions (tropical savanna in the dry period). We compare the three midday budgets: during Koorin; during ECLATS, at the ground station, and with the aircraft. The important differences that appear in the net radiative flux are explained by the difference in surface albedo.Ecole des Sciences, Université de Niamey, B.P. 10662 Niamey, Niger.  相似文献   

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