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1.
Recent studies demonstrate that the Antarctic Ozone Hole has important influences on Antarctic sea ice.While most of these works have focused on effects associated with atmospheric and oceanic dynamic processes caused by stratospheric ozone changes,here we show that stratospheric ozone-induced cloud radiative effects also play important roles in causing changes in Antarctic sea ice.Our simulations demonstrate that the recovery of the Antarctic Ozone Hole causes decreases in clouds over Southern Hemisphere(SH)high latitudes and increases in clouds over the SH extratropics.The decrease in clouds leads to a reduction in downward infrared radiation,especially in austral autumn.This results in cooling of the Southern Ocean surface and increasing Antarctic sea ice.Surface cooling also involves ice-albedo feedback.Increasing sea ice reflects solar radiation and causes further cooling and more increases in Antarctic sea ice.  相似文献   

2.
Changes in Arctic clouds during intervals of rapid sea ice loss   总被引：2，自引：0，他引：2  
We investigate the behavior of clouds during rapid sea ice loss events (RILEs) in the Arctic, as simulated by multiple ensemble projections of the 21st century in the Community Climate System Model (CCSM3). Trends in cloud properties and sea ice coverage during RILEs are compared with their secular trends between 2000 and 2049 during summer, autumn, and winter. The results suggest that clouds promote abrupt Arctic climate change during RILEs through increased (decreased) cloudiness in autumn (summer) relative to the changes over the first half of the 21st century. The trends in cloud characteristics (cloud amount, water content, and radiative forcing) during RILEs are most strongly and consistently an amplifying effect during autumn, the season in which RILEs account for the majority of the secular trends. The total cloud trends in every season are primarily due to low clouds, which show a more robust response than middle and high clouds across RILEs. Lead-lag correlations of monthly sea ice concentration and cloud cover during autumn reveal that the relationship between less ice and more clouds is enhanced during RILEs, but there is no evidence that either variable is leading the other. Given that Arctic cloud projections in CCSM3 are similar to those from other state-of-the-art GCMs and that observations show increased autumn cloudiness associated with the extreme 2007 and 2008 sea ice minima, this study suggests that the rapidly declining Arctic sea ice will be accentuated by changes in polar clouds.  相似文献   

3.
The atmospheric general circulation model EC-EARTH-IFS has been applied to investigate the influence of both a reduced and a removed Arctic sea ice cover on the Arctic energy budget and on the climate of the Northern mid-latitudes. Three 40-year simulations driven by original and modified ERA-40 sea surface temperatures and sea ice concentrations have been performed at T255L62 resolution, corresponding to 79?km horizontal resolution. Simulated changes between sensitivity and reference experiments are most pronounced over the Arctic itself where the reduced or removed sea ice leads to strongly increased upward heat and longwave radiation fluxes and precipitation in winter. In summer, the most pronounced change is the stronger absorption of shortwave radiation which is enhanced by optically thinner clouds. Averaged over the year and over the area north of 70° N, the negative energy imbalance at the top of the atmosphere decreases by about 10?W/m2 in both sensitivity experiments. The energy transport across 70° N is reduced. Changes are not restricted to the Arctic. Less extreme cold events and less precipitation are simulated in sub-Arctic and Northern mid-latitude regions in winter.  相似文献   

4.
Aircraft observations of the atmospheric boundary layer (ABL) over Arctic sea ice were made during non-stationary conditions of cold-air advection with a cloud edge retreating through the study region. The sea-ice concentration, roughness, and ABL stratification varied in space. In the ABL heat budget, 80% of the Eulerian change in time was explained by cold-air advection and 20% by diabatic heating. With the cloud cover and inflow potential temperature profile prescribed as a function of time, the air temperature and near-surface fluxes of heat and momentum were well simulated by the applied two-dimensional mesoscale model. Model sensitivity tests demonstrated that several factors can be active in generating unstable stratification in the ABL over the Arctic sea ice in March. In this case, the upward sensible heat flux resulted from the combined effect of clouds, leads, and cold-air advection. These three factors interacted non-linearly with each other. From the point of view of ABL temperatures, the lead effect was far less important than the cloud effect, which influenced the temperature profiles via cloud-top radiative cooling and radiative heating of the snow surface. The steady-state simulations demonstrated that under overcast skies the evolution towards a deep, well-mixed ABL may take place through the merging of two mixed layers one related to mostly shear-driven surface mixing and the other to buoyancy-driven top-down mixing due to cloud-top radiative cooling.  相似文献   

5.
Abstract

The sensitivity of the annual cycle of ice cover in Baffin Bay to short‐wave radiation is investigated. The Princeton Ocean Model (POM) is used and is coupled with a multi‐category, dynamic‐thermodynamic sea‐ice model in which the surface energy balance governs the growth rates of ice of varying thickness. During spring and summer the short‐wave radiation flux dominates other surface heat fluxes and thus has the greatest effect on the ice melt. The sensitivity of model results to short‐wave radiation is tested using several, commonly used, shortwave parameterizations under climatological, as well as short‐term, atmospheric forcing. The focus of this paper is short‐term and annual variability. It is shown that simulated ice cover is sensitive to the short‐wave radiation formulation during the melting phase. For the Baffin Bay simulation, the differences in the resulting ice area and volume, integrated from May to November, can be as large as 45% and 70%, respectively. The parameterization of the effect of cloud cover on the short‐wave radiation can result in the sea‐ice area and volume changes reaching 20% and 30%, respectively. The variation of the cloud amount represents cloud data error, and has a relatively small effect (less then ±4%) on the simulated ice conditions. This is due to the fact that the effect of cloud cover on the short‐wave radiation flux is largely compensated for by its effect on the net near‐surface long‐wave radiation flux.  相似文献   

6.
Various measurements from the Surface Heat Flux of the Arctic Ocean (SHEBA) experiment have been combined to study structures and processes producing the onset and end of summer melt over Arctic sea ice. The analysis links the surface energy budget to free-troposphere synoptic variables, clouds, precipitation, and in-ice temperatures. The key results are (1) SHEBA melt-season transitions are associated with atmospheric synoptic events (2) onset of melt clearly occurs on May 28, while the end of melt is produced by a sequence of three atmospheric storm events over a 28-day period producing step-like reductions in the net surface energy flux. The last one occurs on August 22.; (3) melt onset is primarily due to large increases in the downwelling longwave radiation and modest decreases in the surface albedo; (4) decreases in the downwelling longwave radiation occur for all end-of-melt transition steps, while increases in surface albedo occur for the first two; (5) decreases in downwelling shortwave radiation contribute only to the first end-of-melt transition step; (6) springtime free-tropospheric warming preconditions the atmosphere–ice system for the subsequent melt onset; and (7) melt-season transitions also mark transitions in system responses to radiative energy flux changes because of invariant melt-season surface temperatures. The extensive SHEBA observations enable an understanding of the complex processes not available from other field program data. The analysis provides a basis for future testing of the generality of the results, and contributes to better physical understanding of multi-year analyses of melt-season trends from less extensive data sets.  相似文献   

7.
Sea ice formed over shallow Arctic shelves often entrains sediments resuspended from the sea floor. Some of this sediment-laden ice advects offshore into the Transpolar Drift Stream and the Beaufort Gyre of the Arctic Basin. Through the processes of seasonal melting at the top surface, and the freezing of clean ice on the bottom surface, these sediments tend, over time, to concentrate at the top of the ice where they can affect the surface albedo, and thus the absorbed solar radiation, when the ice is snow free. Similarly, wind-blown dust can reduce the albedo of snow. The question that is posed by this study is what is the impact of these sediments on the seasonal variation of sea ice, and how does it then affect climate? Experiments were conducted with a coupled energy balance climate-thermodynamic sea ice model to examine the impact of including sediments in the sea ice alone and in the sea ice and overlying snow. The focus of these experiments was the impact of the radiative and not the thermal properties of the sediments. The results suggest that if sea ice contains a significant amount of sediments which are covered by clean snow, there is only a small impact on the climate system. However, if the snow also contains significant sediments the impact on sea ice thickness and surface air temperature is much more significant.  相似文献   

8.
The general circulation model (GCM) used in this study includes a prognostic cloud scheme and a rather detailed radiation scheme. In a preceding paper, we showed that this model was more sensitive to a global perturbation of the sea surface temperatures than most other models with similar physical parametrization. The experiments presented here show how this feature might depend on some of the cloud modelling assumptions. We have changed the temperature at which the water clouds are allowed to become ice clouds and analyzed separately the feedbacks associated with the variations of cloud cover and cloud radiative properties. We show that the feedback effect associated with cloud radiative properties is positive in one case and negative in the other. This can be explained by the elementary cloud radiative forcing and has implications concerning the use of the GCMs for climate sensitivity studies.  相似文献   

9.
A primary climate change signal in the central Arctic is the melting of sea ice. This is dependent on the interplay between the atmosphere and the sea ice, which is critically dependent on the exchange of momentum, heat and moisture at the surface. In assessing the realism of climate change scenarios it is vital to know the quality by which these exchanges are modelled in climate simulations. Six state-of-the-art regional-climate models are run for one year in the western Arctic, on a common domain that encompasses the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment ice-drift track. Surface variables, surface fluxes and the vertical structure of the lower troposphere are evaluated using data from the SHEBA experiment. All the models are driven by the same lateral boundary conditions, sea-ice fraction and sea and sea-ice surface temperatures. Surface pressure, near-surface air temperature, specific humidity and wind speed agree well with observations, with a falling degree of accuracy in that order. Wind speeds have systematic biases in some models, by as much as a few metres per second. The surface radiation fluxes are also surprisingly accurate, given the complexity of the problem. The turbulent momentum flux is acceptable, on average, in most models, but the turbulent heat fluxes are, however, mostly unreliable. Their correlation with observed fluxes is, in principle, insignificant, and they accumulate over a year to values an order of magnitude larger than observed. Typical instantaneous errors are easily of the same order of magnitude as the observed net atmospheric heat flux. In the light of the sensitivity of the atmosphere–ice interaction to errors in these fluxes, the ice-melt in climate change scenarios must be viewed with considerable caution.  相似文献   

10.
Declining summer snowfall in the Arctic: causes, impacts and feedbacks   总被引：1，自引：0，他引：1  
Recent changes in the Arctic hydrological cycle are explored using in situ observations and an improved atmospheric reanalysis data set, ERA-Interim. We document a pronounced decline in summer snowfall over the Arctic Ocean and Canadian Archipelago. The snowfall decline is diagnosed as being almost entirely caused by changes in precipitation form (snow turning to rain) with very little influence of decreases in total precipitation. The proportion of precipitation falling as snow has decreased as a result of lower-atmospheric warming. Statistically, over 99% of the summer snowfall decline is linked to Arctic warming over the past two decades. Based on the reanalysis snowfall data over the ice-covered Arctic Ocean, we derive an estimate for the amount of snow-covered ice. It is estimated that the area of snow-covered ice, and the proportion of sea ice covered by snow, have decreased significantly. We perform a series of sensitivity experiments in which inter-annual changes in snow-covered ice are either unaccounted for, or are parameterized. In the parameterized case, the loss of snow-on-ice results in a substantial decrease in the surface albedo over the Arctic Ocean, that is of comparable magnitude to the decrease in albedo due to the decline in sea ice cover. Accordingly, the solar input to the Arctic Ocean is increased, causing additional surface ice melt. We conclude that the decline in summer snowfall has likely contributed to the thinning of sea ice over recent decades. The results presented provide support for the existence of a positive feedback in association with warming-induced reductions in summer snowfall.  相似文献   

11.
Substantial reduction in Arctic sea ice in recent decades has intensified air-sea interaction over the Arctic Ocean and has altered atmospheric states in the Arctic and surrounding high-latitude regions. This study has found that the atmospheric responses related to Arctic sea-ice melt in the cold season (October–March) depend on sea-ice fraction and are very sensitive to in situ sea surface temperature (SST) from a series of atmospheric general circulation model (AGCM) simulations in which multiple combinations of SSTs and sea-ice concentrations are prescribed in the Arctic Ocean. It has been found that the amplitude of surface warming over the melted sea-ice region is controlled by concurrent in situ SST even if these simulations are forced by the same sea-ice concentration. Much of the sensitivity of surface warming to in situ SST are related with large changes in surface heat fluxes such as the outgoing long-wave flux in early winter (October–December) and the sensible and latent heat fluxes for the entire cold season. Vertical extension of surface warming and moistening is sensitive to these changes as well; the associated condensational heating modulates a static stability in the lower troposphere. This study also indicates that changes in SST fields in AGCM simulations must be implemented with extra care, especially in the melted sea-ice region in the Arctic. The statistical method introduced in this study for adjusting SSTs in conjunction with a given sea-ice change can help to model the atmospheric response to sea-ice loss more accurately.  相似文献   

12.
A coupled atmosphere-ocean-sea ice model is applied to investigate to what degree the area-thickness distribution of new ice formed in open water affects the ice and ocean properties. Two sensitivity experiments are performed which modify the horizontal-to-vertical aspect ratio of open-water ice growth. The resulting changes in the Arctic sea-ice concentration strongly affect the surface albedo, the ocean heat release to the atmosphere, and the sea-ice production. The changes are further amplified through a positive feedback mechanism among the Arctic sea ice, the Atlantic Meridional Overturning Circulation (AMOC), and the surface air temperature in the Arctic, as the Fram Strait sea ice import influences the freshwater budget in the North Atlantic Ocean. Anomalies in sea-ice transport lead to changes in sea surface properties of the North Atlantic and the strength of AMOC. For the Southern Ocean, the most pronounced change is a warming along the Antarctic Circumpolar Current (ACC), owing to the interhemispheric bipolar seasaw linked to AMOC weakening. Another insight of this study lies on the improvement of our climate model. The ocean component FESOM is a newly developed ocean-sea ice model with an unstructured mesh and multi-resolution. We find that the subpolar sea-ice boundary in the Northern Hemisphere can be improved by tuning the process of open-water ice growth, which strongly influences the sea ice concentration in the marginal ice zone, the North Atlantic circulation, salinity and Arctic sea ice volume. Since the distribution of new ice on open water relies on many uncertain parameters and the knowledge of the detailed processes is currently too crude, it is a challenge to implement the processes realistically into models. Based on our sensitivity experiments, we conclude a pronounced uncertainty related to open-water sea ice growth which could significantly affect the climate system sensitivity.  相似文献   

13.
To investigate the processes of development and maintenance of low-level clouds during major synoptic events, the cloudy boundary layer under stormy conditions during the summertime Arctic has been studied using observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment and large-eddy simulations (LES). On 29 July 1998, a stable Arctic cloudy boundary-layer event was observed after the passage of a synoptic low pressure system. The local dynamic and thermodynamic structure of the boundary layer was determined from aircraft measurements including the analysis of turbulence, cloud microphysics and radiative properties. After the upper cloud layer advected over the existing cloud layer, the turbulent kinetic energy (TKE) budget indicated that the cloud layer below 200 m was maintained predominantly by shear production. Observations of longwave radiation showed that cloud-top cooling at the lower cloud top has been suppressed by radiative effects of the upper cloud layer. Our LES results demonstrate the importance of the combination of shear mixing near the surface and radiative cooling at the cloud top in the storm-driven cloudy boundary layer. Once the low-level cloud reaches a certain height, depending on the amount of cloud-top cooling, the two sources of TKE production begin to separate in space under continuous stormy conditions, suggesting one possible mechanism for the cloud layering. The sensitivity tests suggest that the storm-driven cloudy boundary layer is possibly switched to the shear-driven system due to the advection of upper clouds or to the buoyantly driven system due to the lack of wind shear. A comparison is made of this storm-driven boundary layer with the buoyantly driven boundary layer previously described in the literature.  相似文献   

14.
Simulations of late 20th and 21st century Arctic cloud amount from 20 global climate models (GCMs) in the Coupled Model Intercomparison Project phase 3 (CMIP3) dataset are synthesized and assessed. Under recent climatic conditions, GCMs realistically simulate the spatial distribution of Arctic clouds, the magnitude of cloudiness during the warmest seasons (summer–autumn), and the prevalence of low clouds as the predominant type. The greatest intermodel spread and most pronounced model error of excessive cloudiness coincides with the coldest seasons (winter–spring) and locations (perennial ice pack, Greenland, and the Canadian Archipelago). Under greenhouse forcing (SRES A1B emissions scenario) the Arctic is expected to become cloudier, especially during autumn and over sea ice, in tandem with cloud decreases in middle latitudes. Projected cloud changes for the late 21st century depend strongly on the simulated modern (late 20th century) annual cycle of Arctic cloud amount: GCMs that correctly simulate more clouds during summer than winter at present also tend to simulate more clouds in the future. The simulated Arctic cloud changes display a tripole structure aloft, with largest increases concentrated at low levels (below 700 hPa) and high levels (above 400 hPa) but little change in the middle troposphere. The changes in cloud radiative forcing suggest that the cloud changes are a positive feedback annually but negative during summer. Of potential explanations for the simulated Arctic cloud response, local evaporation is the leading candidate based on its high correlation with the cloud changes. The polar cloud changes are also significantly correlated with model resolution: GCMs with higher spatial resolution tend to produce larger future cloud increases.  相似文献   

15.
利用毫米波云雷达、微波辐射计联合反演方法,对2015年11月11日安徽寿县的一次层状云过程的云参数进行了反演,将所得云参数加入到SBDART辐射传输模式中,进行辐射通量计算,并将计算的地面辐射通量与观测的地面辐射通量进行了对比分析。研究表明:1)利用毫米波雷达和微波辐射计数据联合反演的云参数比较可靠;2)利用SBDART模式并结合反演的云参数,可以准确实时地计算地面及其他高度层的长短波辐射通量;3)在反演的云参数中,光学厚度对地面各种辐射通量的影响是最大的,云层的光学厚度越大,到达地面的太阳短波辐射越小,地面反射短波辐射也越小。另外云底温度越高,云体向下发射的红外长波辐射越大。地面向上的长波辐射是地面温度的普朗克函数,随地面温度而变;4)云对地面的短波辐射强迫为负值,对地面有降温的作用。云对地面的长波辐射强迫是一个正值,对地面有一个增温的作用;5)云对地面的净辐射强迫随时间变化很大,它的正负与太阳高度角和云参数有关。  相似文献   

16.
长波辐射对大气变化的敏感性和在WRF模式中的应用检验   总被引：8，自引：2，他引：8  
用RRTM长波辐射 (LWR)参数化方案测试了LWR对大气变化的敏感性。结果表明 :高云对向外长波辐射(OLR)、30 0和 5 0 0hPa净LWR通量的减弱作用较中、低云大 ;低云对 85 0hPa和地表净LWR通量的减弱作用较中、高云大。在云层中 ,LWR冷却率受云影响最大 ;在云层下方 ,云对LWR的影响迅速减小 ;而在云层上方 ,冷却率几乎不受云的影响。当水汽含量减少或增加时 ,地表向下LWR受到相应减弱或增强 ,而净LWR则在一定程度上受到相应增强或减弱 ,并且越接近地面 ,受到水汽变化的影响就越大。O3 对LWR的影响相对云和水汽来说是比较小的。文中介绍了在WRF模式中应用RRTM方案预报LWR不同季节的 2个个例 ,给出了应用NCEP/AVN分析资料预报和验证中国范围 2d之内LWR通量的模拟结果。试验表明 ,OLR和 5 0 0hPa净LWR通量与高度形势场有较好的对应关系 ,而地表净LWR很大程度上还受到地形的影响。  相似文献   

17.
Using a cloud model with explicit microphysics and radiation, we evaluate the microphysical changes in a supercooled liquid altocumulus cloud with increasing ice content, until glaciation occurs. The properties of the ice and water particle constituents are resolved independently, revealing the relative radiative contributions from the water phase source cloud versus the ice phase virga. Cloud contents are also converted to millimeter-wave radar reflectivities to shed light on the ability of such radars to study these ubiquitous clouds. The results show that the radiative and radar backscattering properties of mixed-phase clouds are dominated by the cloud droplets and ice crystals they contain, respectively.  相似文献   

18.
Summary A set of the inhomogeneity factor for high-level clouds derived from the ISCCP D1 dataset averaged over a five-year period has been incorporated in the UCLA atmospheric GCM to investigate the effect of cirrus cloud inhomogeneity on climate simulation. The inclusion of this inhomogeneous factor improves the global mean planetary albedo by about 4% simulated from the model. It also produces changes in solar fluxes and OLRs associated with changes in cloud fields, revealing that the cloud inhomogeneity not only affects cloud albedo directly, but also modifies cloud and radiation fields. The corresponding difference in the geographic distribution of precipitation is as large as 7 mm day−1. Using the climatology cloud inhomogeneity factor also produces a warmer troposphere related to changes in the cloudiness and the corresponding radiative heating, which, to some extent, corrects the cold bias in the UCLA AGCM. The region around 14 km, however, is cooler associated with increase in the reflected solar flux that leads to a warmer region above. An interactive parameterization for mean effective ice crystal size based on ice water content and temperature has also been developed and incorporated in the UCLA AGCM. The inclusion of the new parameterization produces substantial differences in the zonal mean temperature and the geographic distribution of precipitation, radiative fluxes, and cloud cover with respect to the control run. The vertical distribution of ice crystal size appears to be an important factor controlling the radiative heating rate and the consequence of circulation patterns, and hence must be included in the cloud-radiation parameterization in climate models to account for realistic cloud processes in the atmosphere.  相似文献   

19.
This paper quantifies the sensitivity of radiation budget quantities to different cloud types over the Asian monsoon region using the International Satellite Cloud Climatology Project. Multiple regression was used to estimate the radiative effects of individual cloud type. It was observed that the regression performed better when the solution was constrained with clear sky fluxes, which is evident by an improvement in R 2 statistics. The sensitivity coefficients calculated for the Asian monsoon region reveal that, while the LWCRCF and SWCRF will be most vulnerable to changes in cloud cover of deep convective clouds, NETCRF will be susceptible to changes in the nimbostratus clouds. Although the cloud radiative forcing of individual cloud types are found to be similar in sign to previous global findings, their magnitudes are found to vary. It is seen that cirrus clouds play an important role in governing the radiative behavior of this region.  相似文献   

20.
This paper documents a study to examine the sensitivity to cloud droplet effective radius and liquid water path and the alleviation the energy imbalance at the top of the atmosphere and at the surface in the latest version of the Grid-point Atmospheric Model of the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics （LASG）, Institute of Atmospheric Physics （IAP） （GAMIL1.1.0）. Considerable negative biases in all flux components, and thus an energy imbalance, are found in GAMIL1.1.0. In order to alleviate the energy imbalance, two modifications, namely an increase in cloud droplet effective radius and a decrease in cloud liquid water path, have been made to the cloud properties used in GAMIL. With the increased cloud droplet effective radius, the single scattering albedo of clouds is reduced, and thus the reflection of solar radiation into space by clouds is reduced and the net solar radiation flux at the top of the atmosphere is increased. With the reduced cloud optical depth, the net surface shortwave radiation flux is increased, causing a net warming over the land surface. This results in an increase in both sensible and latent heat fluxes over the land regions, which is largely balanced by the increased terrestrial radiation fluxes. Consequently, the energy balance at the top of atmosphere and at the surface is achieved with energy flux components consistent with available satellite observations.  相似文献   

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