Ice-albedo feedback in a CO2-doubling simulation

We estimate the feedback of sea-ice change to the warming from CO₂-doubling according to the simulation of Washington and Meehl (1984). Without ice-snow albedo feedback, their global warming of 3.5 °C would have been 2.2. °C according to our estimate of the ice-snow feedback. About 80% of the albedo change from ice and snow occurred in the Southern Hemisphere. Whether this change was an overestimate will require further study.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A dissection of the surface temperature biases in the Community Earth System Model

Based upon the climate feedback-responses analysis method, a quantitative attribution analysis is conducted for the annual-mean surface temperature biases in the Community Earth System Model version 1 (CESM1). Surface temperature biases are decomposed into partial temperature biases associated with model biases in albedo, water vapor, cloud, sensible/latent heat flux, surface dynamics, and atmospheric dynamics. A globally-averaged cold bias of ?1.22 K in CESM1 is largely attributable to albedo bias that accounts for approximately ?0.80 K. Over land, albedo bias contributes ?1.20 K to the averaged cold bias of ?1.45 K. The cold bias over ocean, on the other hand, results from multiple factors including albedo, cloud, oceanic dynamics, and atmospheric dynamics. Bias in the model representation of oceanic dynamics is the primary cause of cold (warm) biases in the Northern (Southern) Hemisphere oceans while surface latent heat flux over oceans always acts to compensate for the overall temperature biases. Albedo bias resulted from the model’s simulation of snow cover and sea ice is the main contributor to temperature biases over high-latitude lands and the Arctic and Antarctic region. Longwave effect of water vapor is responsible for an overall warm (cold) bias in the subtropics (tropics) due to an overestimate (underestimate) of specific humidity in the region. Cloud forcing of temperature biases exhibits large regional variations and the model bias in the simulated ocean mixed layer depth is a key contributor to the partial sea surface temperature biases associated with oceanic dynamics. On a global scale, biases in the model representation of radiative processes account more for surface temperature biases compared to non-radiative, dynamical processes.     

The albedo of snow for partially cloudy skies

The albedo of snow for different cloudiness conditions is an important parameter in the Earth's radiation budget analysis and in the study of snowpack's thermal conditions. In this study an efficient approximate method is derived to calculate the incident spectral solar flux and snow-cover albedo in terms of different atmospheric, cloud, and snow parameters. The global flux under partially cloudy skies is expressed in terms of the clear sky flux and a coefficient which models the effect of scattering and absorption by cloud patches and multiple reflections between the cloud base and snowcover. The direct and the diffuse components of the clear sky flux are obtained using the spectral flux outside the atmosphere and the spectral transmission coefficients for absorption and scattering by molecules and aerosols.The spectral snow reflectance model considers both specular surface reflection and volumetric multiple scattering. The surface reflection is calculated by using a crystal-shape-dependent bidirectional reflectance distribution function; the volumetric multiple scattering is calculated by using a crystal-size-dependent approximate solution in the radiative transfer equation. The input parameters to the model are atmospheric precipitable water, ozone content, turbidity, cloud optical thickness, the size and shape of ice crystals of snow and surface pressure. The model yields spectral and integrated solar flux and snow reflectance as a function of solar elevation and fractional cloudcover.The model is illustrated using representative parameters for the Antarctic coastal regions. The albedo for a clear sky depends inversely on the solar elevation. At high elevations the albedo depends primarily upon the grain size; at low elevation the albedo depends on grain size and shape. The gradient of the albedo-elevation curve increases as the grains become larger and faceted. The albedo for a densely overcast sky is a few percent higher than the clear-sky albedo at high elevations. A simple relationship between grain size and the overcast albedo is obtained. For a set of grain size and shape, the albedo as a function of solar elevation and fractional cloud cover is tabulated.     

Climate change and the regulation of the surface moisture and energy budgets

The increase in the vigor of the hydrological cycle simulated in a 2 × CO₂ experiment with the Canadian Climate Centre general circulation model is smaller than that obtained by other models which have similar increases in mean surface temperature. The surface energy budget, which encompasses also the moisture budget for the oceans, is analyzed. Changes in the net radiative input to and sensible heat flux from the surface act to warm it. This is balanced, at the new equilibrium, by a change in the latent heat flux which acts to cool it. Although this same general behavior is seen in other models, the increase in radiative input to the surface in the CCC GCM is smaller than in other models while the change in the sensible heat flux is of similar size. As a consequence, the latent heat flux required for balance is smaller. The comparatively small increase in the net radiative input at the surface occurs because of a decrease in the solar component. On average the decrease in solar input in the tropical region outweighs the higher latitude increase associated with the snow/ice albedo feedback. The notable tropical decrease in solar input occurs because the albedo of the clouds increase enough in this region to outweigh a small decrease in cloud amount. The increase in cloud albedo in the warmer and moister tropical atmosphere is a consequence of the parameterized cloud optical properties in the model which play an important role in the regulation of the surface energy and moisture budgets. The results demonstrate some of the consequences of the negative feedback mechanism associated with increasing cloud albedo in the model. They also suggest that the simulated change in the vigor of the hydrological cycle is not a simple function of the average increase in surface temperature but is a consequence of all of the processes in the model which control the available energy at the surface as a function of latitude.     

Radiative Forcing and Climate Response Due to Black Carbon in Snow and Ice

The radiative forcing and climate response due to black carbon(BC) in snow and/or ice were investigated by integrating observed effects of BC on snow/ice albedo into an atmospheric general circulation model(BCC AGCM2.0.1) developed by the National Climate Center(NCC) of the China Meteorological Administration(CMA).The results show that the global annual mean surface radiative forcing due to BC in snow/ice is +0.042 W m 2,with maximum forcing found over the Tibetan Plateau and regional mean forcing exceeding +2.8 W m 2.The global annual mean surface temperature increased 0.071 C due to BC in snow/ice.Positive surface radiative forcing was clearly shown in winter and spring and increased the surface temperature of snow/ice in the Northern Hemisphere.The surface temperatures of snow-covered areas of Eurasia and North America in winter(spring) increased by 0.83 C(0.6 C) and 0.83 C(0.46 C),respectively.Snowmelt rates also increased greatly,leading to earlier snowmelt and peak runoff times.With the rise of surface temperatures in the Arctic,more water vapor could be released into the atmosphere,allowing easier cloud formation,which could lead to higher thermal emittance in the Arctic.However,the total cloud forcing could decrease due to increasing cloud cover,which will offset some of the positive feedback mechanism of the clouds.     

Radiative effect of black carbon aerosol on seasonal variation in snow depth in the Northern-Hemisphere

In this research, we studied the effects of black carbon (BC) aerosol radiative forcing on seasonal variation in the Northern Hemisphere (NH) using numerical simulations with the NASA finite-volume General Circulation Model (fvGCM) forced with monthly varying three-dimensional aerosol distributions from the Goddard Ozone Chemistry Aerosol Radiation and Transport Model (GOCART). The results show that atmospheric warming due to black carbon aerosols subsequently warm the atmosphere and land surfaces, especially those over Eurasia. As a result, the snow depth in Eurasia was greatly reduced in late winter and spring, and the reduction in snow cover decreased the surface albedo. Our surface energy balance analysis shows that the surface warming due to aerosol absorption causes early snow melting and further increases surface-atmosphere warming through snow/ice albedo feedback. Therefore, BC aerosol forcing may be an important factor affecting the snow/ice albedo in the NH.     

Snow cover validation and sensitivity to CO2 in the UVic ESCM

Abstract

The University of Victoria's (UVic) Earth System Climate model is used to conduct equilibrium atmospheric CO₂ sensitivity experiments over the range 200–1600 ppm in order to explore changes in northern hemisphere snow cover and feedbacks on terrestrial surface air temperature (SAT). Simulations of warmer climates predict a retreat of snow cover over northern continents, in a northeasterly direction. The decline in northern hemisphere global snow mass is estimated to reach 33% at 600 ppm and 54% at 1200 ppm. In the most northerly regions, annual mean snow depth increases for simulations with CO₂ levels higher than present day. The shift in the latitude of maximum snowfall is estimated to be inversely proportional to the CO₂ concentration. The northern hemisphere net shortwave radiation changes are found to be greater over land than over the ocean, suggesting a stronger albedo feedback from changes in terrestrial snow cover than from changes in sea ice. Results also reveal high sensitivity of the snow mass balance under low CO₂ conditions. The amplification feedback (defined as the zonal SAT anomaly caused by doubling CO₂ divided by the equatorial anomaly) is greatest for scenarios with less than 300 ppm, reaching 1.9 at the pole for 250 ppm. The stronger feedback is attributed to the significant albedo changes over land areas. The simulation with 200 ppm triggers continuous accumulation of snow ('glaciation') in regions which, according to paleo‐reconstructions, were covered by ice during the last glacial cycle (the Canadian Arctic, Scandinavia, and the Taymir Peninsula).     

Intraannual dissimilarities between monthly mean Northern Hemisphere temperature anomalies during the twentieth century

Summary Northern Hemisphere monthly mean temperature anomalies during the twentieth century exhibit seasonal differences. To expose the most significant intermonthly dissimilarities, here we investigate this issue in more details. We show that the trends significantly differ for the months close to the equinoxes. Using the technique of multidimensional scaling analysis we find that two underlying attributes are sufficient to acceptably describe the structure of the observed intermonthly dissimilarities. Namely, these are the monthly sample percentiles around the 30th and the 70th together with the linear trends. We find that intraannual temperature anomaly dissimilarities statistically depend on the seasonal cycles of the Northern Hemisphere oceanic heat content, sea ice and snow cover and the Arctic sea ice cover as well as on the seasonality of the chaoticity of the Northern Hemisphere atmospheric dynamics. We also speculate that the annual cycles of these ocean and cryosphere characteristics statistically set the pattern for the observed course of linear warming over the calendar months.     

A new model of thermal diffusion in a translucent medium

The thermal diffusion equation is solved analytically for an ideally translucent and homogeneous medium with an internal, distributed heat source given by the penetration and absorption of solar radiation. The mean annual temperature profile has the coldest values at the surface; the mean annual flux of sensible heat out of the medium balances the internally absorbed solar radiation for the case of constant mean annual temperatures. A new formula is developed for the evaluation of the true thermal diffusivity of a non-homogeneous conductor from the Fourier components of the waves of insolation and temperature; a constant extinction coefficient is assumed. The new model is applied with moderate success to data from the coastal plateau ice at Mawson, Antarctica.National Research Council Postdoctoral Resident Research Associate. Present Affiliation: Stanford Research Institute, Menlo Park, California 94025.     

The effect of atmospheric CO2 and ice sheet topography on LGM climate

The role of reduced atmospheric CO₂ concentration and ice sheet topography plus its associated land albedo on the LGM climate is investigated using a coupled atmosphere-ocean-sea ice climate system model. The surface cooling induced by the reduced CO₂ concentration is larger than that by the ice sheet topography plus other factors by about 30% for the surface air temperature and by about 100% for the sea surface temperature. A large inter-hemispheric asymmetry in surface cooling with a larger cooling in the Northern Hemisphere is found for both cases. This asymmetric inter-hemispheric temperature response is consistent in the ice sheet topography case with earlier studies using an atmospheric model coupled with a mixed-layer ocean representation, but contrasts with these results in the reduced CO₂ case. The incorporation of ocean dynamics presumably leads to a larger snow and sea ice feedback as a result of the reduction in northward ocean heat transport, mainly as a consequence of the decrease in the North Atlantic overturning circulation by the substantial freshening of the North Atlantic convection regions. A reversed case is found in the Southern Ocean. Overall, the reduction in atmospheric CO₂ concentration accounts for about 60% of the total LGM climate change.     

Influence of snow cover changes on surface radiation and heat balance based on the WRF model

The snow cover extent in mid-high latitude areas of the Northern Hemisphere has significantly declined corresponding to the global warming, especially since the 1970s. Snow-climate feedbacks play a critical role in regulating the global radiation balance and influencing surface heat flux exchange. However, the degree to which snow cover changes affect the radiation budget and energy balance on a regional scale and the difference between snow-climate and land use/cover change (LUCC)-climate feedbacks have been rarely studied. In this paper, we selected Heilongjiang Basin, where the snow cover has changed obviously, as our study area and used the WRF model to simulate the influences of snow cover changes on the surface radiation budget and heat balance. In the scenario simulation, the localized surface parameter data improved the accuracy by 10 % compared with the control group. The spatial and temporal analysis of the surface variables showed that the net surface radiation, sensible heat flux, Bowen ratio, temperature and percentage of snow cover were negatively correlated and that the ground heat flux and latent heat flux were positively correlated with the percentage of snow cover. The spatial analysis also showed that a significant relationship existed between the surface variables and land cover types, which was not obviously as that for snow cover changes. Finally, six typical study areas were selected to quantitatively analyse the influence of land cover types beneath the snow cover on heat absorption and transfer, which showed that when the land was snow covered, the conversion of forest to farmland can dramatically influence the net radiation and other surface variables, whereas the snow-free land showed significantly reduced influence. Furthermore, compared with typical land cover changes, e.g., the conversion of forest into farmland, the influence of snow cover changes on net radiation and sensible heat flux were 60 % higher than that of land cover changes, indicating the importance of snow cover changes in the surface-atmospheric feedback system.     

CO2 climate sensitivity and snow-sea-ice albedo parameterization in an atmospheric GCM coupled to a mixed-layer ocean model

The snow-sea-ice albedo parameterization in an atmospheric general circulation model (GCM), coupled to a simple mixed-layer ocean and run with an annual cycle of solar forcing, is altered from a version of the same model described by Washington and Meehl (1984). The model with the revised formulation is run to equilibrium for 1 × CO₂ and 2 × CO₂ experiments. The 1 ×CO₂ (control) simulation produces a global mean climate about 1° warmer than the original version, and sea-ice extent is reduced. The model with the altered parameterization displays heightened sensitivity in the global means, but the geographical patterns of climate change due to increased carbon dioxide (CO₂) are qualitatively similar. The magnitude of the climate change is affected, not only in areas directly influenced by snow and ice changes but also in other regions of the globe, including the tropics where sea-surface temperature, evaporation, and precipitation over the oceans are greater. With the less-sensitive formulation, the global mean surface air temperature increase is 3.5 °C, and the increase of global mean precipitation is 7.12%. The revised formulation produces a globally averaged surface air temperature increase of 4.04 °C and a precipitation increase of 7.25%, as well as greater warming of the upper tropical troposphere. Sensitivity of surface hydrology is qualitatively similar between the two cases with the larger-magnitude changes in the revised snow and ice-albedo scheme experiment. Variability of surface air temperature in the model is comparable to observations in most areas except at high latitudes during winter. In those regions, temporal variation of the sea-ice margin and fluctuations of snow cover dependent on the snow-ice-albedo formulation contribute to larger-than-observed temperature variability. This study highlights an uncertainty associated with results from current climate GCMs that use highly parameterized snow-sea-ice albedo schemes with simple mixed-layer ocean models.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The influence of continental ice,atmospheric CO2, and land albedo on the climate of the last glacial maximum

The contributions of expanded continental ice, reduced atmospheric CO₂, and changes in land albedo to the maintenance of the climate of the last glacial maximum (LGM) are examined. A series of experiments is performed using an atmosphere-mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination. The model used has been shown to produce a reasonably realistic simulation of the reduced temperature of the LGM (Manabe and Broccoli 1985b). By comparing the results from pairs of experiments, the effects of each of these environmental changes can be determined.Expanded continental ice and reduced atmospheric CO₂ are found to have a substantial impact on global mean temperature. The ice sheet effect is confined almost exclusively to the Northern Hemisphere, while lowered CO₂ cools both hemispheres. Changes in land albedo over ice-free areas have only a minor thermal effect on a global basis. The reduction of CO₂ content in the atmosphere is the primary contributor to the cooling of the Southern Hemisphere. The model sensitivity to both the ice sheet and CO₂ effects is characterized by a high latitude amplification and a late autumn and early winter maximum.Substantial changes in Northern Hemisphere tropospheric circulation are found in response to LGM boundary conditions during winter. An amplified flow pattern and enhanced westerlies occur in the vicinity of the North American and Eurasian ice sheets. These alterations of the tropospheric circulation are primarily the result of the ice sheet effect, with reduced CO₂ contributing only a slight amplification of the ice sheet-induced pattern.     

A study of abrupt climate change in a simple nonlinear climate model

We use a seasonal energy balance climate model to study the behavior of the snowline cycle as a function of external parameters such as the solar constant. Our studies are confined in this study to cases with zonally symmetric land-sea distributions (bands or caps of land). The model is nonlinear in that the seasonally varying snow/sea ice line modifies the energy receipt through its different albedo from open land or water. The repeating steady-state seasonal cycle of the model is solved by a truncated Fourier series in time. This method is several thousand times faster than a time stepping approach. The results are interesting in that a number of bifurcations in the snowline behavior are found and studied for various geographies. Polar land caps and land bands positioned near the poles exhibit a variety of discontinuous summer snow cover behaviors (abrupt transitions as a parameter such as solar constant is slowly varied), which may be relevant to the inception and decay of continental ice sheets.     

Investigating the variability of the Arctic sea ice thickness in response to a stochastic thermodynamic atmospheric forcing

The natural low frequency variability of the sea-ice thickness in the Arctic is investigated based on a 10 000 years simulation with a one-dimensional thermodynamic sea-ice model forced by random perturbations of the air surface temperature and solar radiation. The simulation results suggest that atmospheric random perturbations are integrated by the sea-ice. Moreover those perturbations occurring at the onset of ice melting force the largest ice thickness anomalies, which are successively amplified in summer by the albedo feedback and damped in winter by the feedback of the heat conduction through the ice. They also result in a global shift of the melting season which, in the mean annual cycle, leads to earlier melting as compared to the mean climatological cycle. The power spectrum of the ice anomalies suggests that the thickness of the perennial ice should vary preferentially on a time scale of approximately 20 years. The shape of the spectrum is consistent with that of a first order Markov process in which the characteristic time scale of the ice fluctuations would be the relaxation time scale associated with the linear feedback. The equivalent Markov model is constructed by linearizing the ice growth rate anomaly equations and allows us to derive an analytical expression of the feedback and of the forcing of the anomalies. The characteristic time scale depends explicitly on those model parameters involved in the atmosphere-ice interaction but also on the mean seasonal characteristics of the forcing and of the ice thickness. Received: 18 August 1999 / Accepted: 10 May 2000     

The role of sea ice in the temperature-precipitation feedback of glacial cycles

The response of the hydrological cycle to climate variability and change is a critical open question, where model reliability is still unsatisfactory, yet upon which past climate history can shed some light. Sea ice is a key player in the climate system and in the hydrological cycle, due to its strong albedo effect and its insulating effect on local evaporation and air-sea heat flux. Using an atmospheric general circulation model with specified sea surface temperature and sea-ice distribution, the role of sea ice in the hydrological cycle is investigated under last glacial maximum (LGM) and present day conditions, and by studying its contribution to the “temperature-precipitation feedback”. By conducting a set of sensitivity experiments in which the albedo and thickness of the sea ice are varied, the various effects of sea ice in the hydrological cycle are isolated. It is demonstrated that for a cold LGM like state, a warmer climate (as a result of reduced sea-ice cover) leads to an increase in snow precipitation over the ice sheets. The insulating effect of the sea ice on the hydrological cycle is found to be larger than the albedo effect. These two effects interact in a nonlinear way and their total effect is not equal to summing their separate contribution.     

Seasonal oceanic heat transports computed from an atmospheric model

Seasonal estimates of the oceanic poleward heat transport are obtained using a climate model that is a global atmospheric general circulation model on an 8° × 10° grid. The climate model is used to calculate the surface heat flux into each ocean grid point for each day of the year. The rate of ocean heat storage is calculated using climatological surface temperatures, mixed layer depths, and ice amounts. By assuming that the rate of change of heat storage in the deep ocean is spatially constant, the horizontal transports are calculated from the vertical fluxes and the upper ocean storage rates. The oceanic meridional transport for each latitude and for each ocean basin are derived, and results are compared with other calculations of the seasonal transports. In the Northern Hemisphere, comparisons between the simulated seasonal transports indicate that the annual variation is much greater in the Pacific than in the Atlantic.     

Modes of variability of global sea surface temperature,free atmosphere temperature and oceanic surface energy flux

Monthly mean sea surface temperature (SST), free air temperature from satellite microwave sounding units (MSU) and oceanic surface energy fluxes are subjected to empirical orthogonal function (EOF) analysis for a common decade to investigate the physical relationships involved. The first seasonal modes of surface solar energy flux and SST show similar inter-hemispheric patterns with an annual cycle. Solar flux appears to control this pattern of SST. The first seasonal mode of MSU is similar with, additionally, land-sea differences; MSU is apparently partly controlled by absorption of solar near-infrared radiation and partly by sensible heat from the land surface. The second and third seasonal eigenvector of SST and solar flux exhibit semi-annual oscillations associated with a pattern of cloudiness in the subtropics accompanying the translation of the Hadley cell rising motion between the hemispheres. The second seasonal mode of MSU is dominated by an El Niño signal. The first nonseasonal EOFs of SST and solar flux exhibit El Niño characteristics with the solar pattern being governed by west-to-east translation of a Walker cell type pattern. The first non-seasonal EOF of MSU shows a tropical strip pattern for the El Niño mode, which is well correlated with the latent heat fluxes in the tropical east Pacific but not in the tropical west Pacific. Two possible explanations are: an increase in subsidence throughout the tropical strip driven by extra evaporation in the tropical east Pacific and consequent additional latent heat liberation; a decrease of meridional heat flux out of the tropics.     

基于MODIS白天地温产品的青藏高原海拔依赖型变暖特征分析

基于2001～2018年中分辨率成像光谱仪（MODIS）探测的白天地面温度（简称MODIS 白天地温）资料,与青藏高原（简称高原）122个气象站点观测的最高气温资料,在年尺度上评估了MODIS 白天地温在高原的适用性,研究了高原五个干湿分区下MODIS 白天地温的海拔依赖型变暖特征,得到以下主要结论:（1）MODIS白天地温能够基本再现观测的最高气温的时空以及海拔依赖型变暖特征;（2）高原整体上,MODIS白天地温存在显著的海拔依赖型变暖特征,平均海拔每增加100 m,其趋势增加0.02°C (10a)?1,且受积雪—反照率反馈主导;（3）干湿分区下,海拔依赖型变暖特征在高原表现为偏湿润地区强于偏干旱地区;季风区强于西风区。海拔依赖型特征强弱:半湿润地区＞湿润半湿润地区＞半干旱地区＞湿润地区＞干旱地区。平均海拔每增加100 m,以上区域的地温趋势分别增加0.06,0.03,0.03,0.01,0.01°C (10a)?1。半湿润和湿润半湿润地区年均温在0°C左右,在气候变暖背景下积雪—反照率反馈作用最为强烈,是其海拔依赖型变暖的主导因素;干旱与半干旱地区年均温相对更低,气候变暖程度对积雪影响相对较小,积雪—反照率反馈作用被限制,但仍对上述地区的海拔依赖型变暖起主导作用;而湿润地区的积雪覆盖率的上升可能是由于降雪（固态降水）增加抵消了积雪融化损耗,云辐射、水汽等其他因素主导了其海拔依赖型变暖。     

Boreal forest and tundra ecosystems as components of the climate system

The effects of terrestrial ecosystems on the climate system have received most attention in the tropics, where extensive deforestation and burning has altered atmospheric chemistry and land surface climatology. In this paper we examine the biophysical and biogeochemical effects of boreal forest and tundra ecosystems on atmospheric processes. Boreal forests and tundra have an important role in the global budgets of atmospheric CO₂ and CH₄. However, these biogeochemical interactions are climatically important only at long temporal scales, when terrestrial vegetation undergoes large geographic redistribution in response to climate change. In contrast, by masking the high albedo of snow and through the partitioning of net radiation into sensible and latent heat, boreal forests have a significant impact on the seasonal and annual climatology of much of the Northern Hemisphere. Experiments with the LSX land surface model and the GENESIS climate model show that the boreal forest decreases land surface albedo in the winter, warms surface air temperatures at all times of the year, and increases latent heat flux and atmospheric moisture at all times of the year compared to simulations in which the boreal forest is replaced with bare ground or tundra. These effects are greatest in arctic and sub-arctic regions, but extend to the tropics. This paper shows that land-atmosphere interactions are especially important in arctic and sub-arctic regions, resulting in a coupled system in which the geographic distribution of vegetation affects climate and vice versa. This coupling is most important over long time periods, when changes in the abundance and distribution of boreal forest and tundra ecosystems in response to climatic change influence climate through their carbon storage, albedo, and hydrologic feedbacks.