Western Himalayan snow cover and Indian monsoon rainfall: A re-examination with INSAT and NCEP/NCAR data

Summary ?This study presents the monthly climatology and variability of the INSAT (Indian National Satellite) derived snow cover estimates over the western Himalayan region. The winter/spring snow estimates over the region are related to the subsequent summer monsoon rainfall over India. The NCEP/NCAR data are used to understand the physical mechanism of the snow-monsoon links. 15 years (1986–2000) of recent data are utilized to investigate these features in the present global warming environment. Results reveal that the spring snow cover area has been declining and snow has been melting faster from winter to spring after 1993. Connections between snow cover estimates and Indian monsoon rainfall (IMR) show that spring snow cover area is negatively related with maximum during May, while snow melt during the February–May period is positively related with subsequent IMR, implying that smaller snow cover area during May and faster snow melt from winter to spring is conducive for good monsoon activity over India. NCEP/NCAR data further shows that the heat low over northwest India and the monsoon circulation over the Indian subcontinent, in particular the cross-equatorial flow, during May are intensified (weakened) when the snow cover area during May is smaller (extensive) and snow melts faster (slower) during the February–May period. The well-documented negative relationship between winter snow and summer rainfall seems to have altered recently and changed to a positive relationship. The changes observed in snow cover extent and snow depth due to global warming may be a possible cause for the weakening winter snow–IMR relationship. Received January 15, 2002; revised May 5, 2002; accepted June 23, 2002     

青藏高原冬季降雪对地面净辐射的影响

过去的研究结果指出，冬季青藏高原地面净辐射场是一个由地理因子决定的基本场叠加上一个降雪后地面积雪区造成的扰动场组成，为定量研究冬季青藏高原降对地面净辐射的扰动幅度，利用作者已经建立的冬季青藏高原后地面反射率与降雪降雪面无（有）积雪时计算地面净辐射的公式，计算了不同强度降雪后地面净辐射日总量及其变化。     

A new snow cover fraction parametrization for the ECHAM4 GCM

 Snow cover fraction (SCF) has a significant influence on the surface albedo and thus on the radiation balance and surface climate. Long-term three dimensional simulations with general circulation models (GCMs) show that the SCF greatly affects the climate in the Northern Hemisphere. By means of both ground observations and remotely sensed data, several deficiencies in the SCF simulated by the current ECHAM4 GCM were identified: over mountainous areas a substantial overestimation in the SCF was found whereas flat areas showed a distinctly underestimated SCF. This work proposes a new parametrization of the SCF for use in GCMs. Evaluations illustrate that it is beneficial to distinguish between the following three terrains: (1) flat, non-forested areas, (2) mountainous regions and (3) forests. The modified SCF parametrization for flat, non-forested areas was derived by using global datasets of ground-based snow depth and remote sensing observations of snow cover data. A 3-dimensional ECHAM4 simulation showed that this modification raises the SCF by up to approximately 20%, mainly in areas with a relatively thin snow cover. The comparison between remotely sensed and simulated mean monthly surface albedo revealed a significant overestimation of the surface albedo in snow-covered mountainous areas. An extension of the current SCF parametrization in ECHAM4 to take into account mountain effects, based on the French climate model Arpège, yielded a close agreement with satellite-derived surface albedo. The adoption of the submodel for snow albedo, as used in the Canadian Land Surface Scheme (CLASS), combined with a newly developed simple snow interception model, demonstrated the ability to capture the main physical processes of snow-covered canopies, including the albedo. The validation of the new parametrization with Boreal Ecosystem-Atmosphere Study (BOREAS) field data showed that the modification is appropriate to capture the main features of the albedo over snow-covered forests during and after heavy snowfall events. Furthermore, the proposed modification has a beneficial impact on the delayed snow melt in spring, a well-known problem in many current GCMs: The simulated surface albedo over the boreal forests decreases by approximately 0.1 during winter and spring, which is in better agreement with ground-based observations. This induces a significant rise in the surface temperature over extended parts of Eurasia and North America in late spring, which subsequently yields a faster snowmelt and an accelerated retreat of the snow line. Received: 28 April 2000 / Accepted: 18 December 2000     

Simulation of canopy radiation transfer and surface albedo in the EALCO model

A new canopy radiation transfer and surface albedo scheme is developed as part of the land surface model EALCO (Ecological Assimilation of Land and Climate Observations). The model uses a gap probability-based successive orders of scattering approach that explicitly includes the heterogeneities of stands and crown elements and the radiation multiple scattering. The model uses the optical parameters of ecosystem elements and physically represents ecosystem processes in surface albedo dynamics. Model tests using measurements from a boreal deciduous forest ecosystem show that the model well reproduced the observed diurnal and seasonal albedo dynamics under different weather and ecosystem conditions. The annual mean absolute errors between modeled and measured daily albedo and reflected radiation are 0.01 and 1.33 W m⁻², respectively. The model results provide a quantitative assessment of the impacts of plant shading and sky conditions on surface albedo observed in high-latitude ecosystems. The contribution of ground snow to surface albedo in winter was found to be less than 0.1 even though the canopy is leafless during this time. The interception of snow by the leafless canopy can increase the surface albedo by 0.1–0.15. The model results show that the spectral properties of albedo have large seasonal variations. In summer, the near infrared component is substantially larger than visible, and surface albedo is less sensitive to sky conditions. In winter, the visible band component is markedly increased and can exceed the near infrared proportion under cloudy conditions or when snow exists on the canopy. The spectral properties of albedo are also found to have large diurnal variations under the clear-sky conditions in winter.     

Using a global climate model to evaluate the influences of water vapor, snow cover and atmospheric aerosol on warming in the Tibetan Plateau during the twenty-first century

We examine trends in climate variables and their interrelationships over the Tibetan Plateau using global climate model simulations to elucidate the mechanisms for the pattern of warming observed over the plateau during the latter half of the twentieth century and to investigate the warming trend during the twenty-first century under the SRES A1B scenario. Our analysis suggests a 4°C warming over the plateau between 1950 and 2100. The largest warming rates occur during winter and spring. For the 1961–2000 period, the simulated warming is similar to the observed trend over the plateau. Moreover, the largest warming occurs at the highest elevation sites between 1950 and 2100. We find that increases in (1) downward longwave radiation (DLR) influenced by increases in surface specific humidity (q), and (2) absorbed solar radiation (ASR) influenced by decreases in snow cover extent are, in part, the reason for a large warming trend over the plateau, particularly during winter and spring. Furthermore, elevation-based increases in DLR (influenced by q) and ASR (influenced by snow cover and atmospheric aerosols) appear to affect the elevation dependent warming trend simulated in the model.     

利用MODIS卫星资料反演中国地区晴空地表短波反照率及其特征分析

利用MODIS地表双向反照率产品(MOD43B1),结合地表海拔高度和地表覆盖类型资料,计算并分析了中国地区晴空反照率的时空分布,以及地表反照率与地形和地表覆盖的关系.首先,利用改则自动气象站的地基观测对MODIS地表反照率进行了对比验证.验证结果表明卫星观测可以较好地反映反照率随时间的变化,MODIS地表反照率与地表实测反照率符合较好.年平均地表反照率与海拔高度有很好的相关,反照率的高值出现在高海拔山区.冬春季节,我国高海拔山区因积雪覆盖成为反照率的高值区;夏秋季节,地表反照率主要受地表土壤湿度和植被盖度的影响,沙地和沙漠地带反照率最高.最后,计算了中国典型地表类型的反照率随时间的变化,结果表明大部分地表类型的反照率具有较大的时间变化,地表反照率在春秋季节较大,夏季反照率较小.     

Assessment of GCM simulated snow albedo using direct observations

 Annual cycles of monthly albedos simulated with a general circulation model (GCM) are compared with surface observations. The data observed at 35 stations are retrieved from the Global Energy Balance Archive (GEBA) and drawn from the soil moisture and meteorological observations in the former Soviet Union. The model data are obtained with the ECHAM4 GCM in a ten-year simulation of the present-day climate at T106 resolution. The model calculated albedo values are modified before they are compared with the surface observations: They are interpolated to the stations and adjusted to account for altitude differences and fractional forest area. During the snow-free period, the model underestimates the albedo by up to 0.05 at the stations (with values between 0.2 and 0.25 measured over short grass) because the albedo for grassland is too low in the model. During the period with seasonal snow cover, the model underestimates the albedo by up to 0.2 at stations in Russia, Scandinavia and Canada, which experience severe winters. This underestimation is due to an oversimplified parameterization of the snow covered grid fraction and an inadequate linear relation between snow albedo and temperature. The derivative of albedo with respect to the forest fraction implemented in ECHAM is in line with the observations, although a small overestimation of the model’s gradient has been detected. Received: 3 July 1998 / Accepted: 24 December 1998     

Influence of horizontal inhomogeneity on albedo and absorptivity of snow cover

The variations of albedo and absorptivity of the snow cover are considered caused by the presence of the snow roughness in the form of sastrugi. The numerical modeling is carried out within the framework of statistical approach based on the analytic averaging of the radiative transfer equation and statistically homogeneous model on the basis of Poisson flows of points at the straight lines. The estimates of the influence of 3D-effects of the rough surface are represented depending on optical and geometrical characteristics of sastrugi and on the illumination conditions. It is demonstrated that if the absorption by the snow particles is weak (the single scattering albedo w = 0.9999) the reflection of radiation by snow decreases by ∼ 2–3% when the sastrugi appear. This effect is more significant in near infrared spectral region where w is below 0.99.     

The role of terrestrial snow cover in the climate system

Snow cover is known to exert a strong influence on climate, but quantifying its impact is difficult. This study investigates the global impact of terrestrial snow cover through a pair of GCM simulations run with prognostic snow cover and with all snow cover on land eliminated (NOSNOWCOVER). In this experiment all snowfall over land was converted into its liquid–water equivalent upon reaching the surface. Compared with the control run, NOSNOWCOVER produces mean-annual surface air temperatures up to 5 K higher over northern North America and Eurasia and 8–10 K greater during winter. The globally averaged warming of 0.8 K is one-third as large as the model’s response to 2 × CO₂ forcing. The pronounced surface heating propagates throughout the troposphere, causing changes in surface and upper-air circulation patterns. Despite the large atmospheric warming, the absence of an insulating snow pack causes soil temperatures in NOSNOWCOVER to fall throughout northern Asia and Canada, including extreme wintertime cooling of over 20 K in Siberia and a 70% increase in permafrost area. The absence of snow melt water also affects extratropical surface hydrology, causing significantly drier upper-layer soils and dramatic changes in the annual cycle of runoff. Removing snow cover also drastically affects extreme weather. Extreme cold-air outbreaks (CAOs)—defined relative to the control climatology—essentially disappear in NOSNOWCOVER. The loss of CAOs appears to stem from both the local effects of eliminating snow cover in mid-latitudes and a remote effect over source regions in the Arctic, where −40°C air masses are no longer able to form.     

Quantifying uncertainties in projections of extremes��a perturbed land surface parameter experiment

Uncertainties in the climate response to a doubling of atmospheric CO₂ concentrations are quantified in a perturbed land surface parameter experiment. The ensemble of 108 members is constructed by systematically perturbing five poorly constrained land surface parameters of global climate model individually and in all possible combinations. The land surface parameters induce small uncertainties at global scale, substantial uncertainties at regional and seasonal scale and very large uncertainties in the tails of the distribution, the climate extremes. Climate sensitivity varies across the ensemble mainly due to the perturbation of the snow albedo parameterization, which controls the snow albedo feedback strength. The uncertainty range in the global response is small relative to perturbed physics experiments focusing on atmospheric parameters. However, land surface parameters are revealed to control the response not only of the mean but also of the variability of temperature. Major uncertainties are identified in the response of climate extremes to a doubling of CO₂. During winter the response both of temperature mean and daily variability relates to fractional snow cover. Cold extremes over high latitudes warm disproportionately in ensemble members with strong snow albedo feedback and large snow cover reduction. Reduced snow cover leads to more winter warming and stronger variability decrease. As a result uncertainties in mean and variability response line up, with some members showing weak and others very strong warming of the cold tail of the distribution, depending on the snow albedo parametrization. The uncertainty across the ensemble regionally exceeds the CMIP3 multi-model range. Regarding summer hot extremes, the uncertainties are larger than for mean summer warming but smaller than in multi-model experiments. The summer precipitation response to a doubling of CO₂ is not robust over many regions. Land surface parameter perturbations and natural variability alter the sign of the response even over subtropical regions.     

青藏高原地区NCEP新再分析地面通量资料的检验

利用1979—1998年地面气象站温度观测资料和1982年8月-1983年7月高原热源观测资料,检验了NCEP／DOE新再分析地面气温和地面辐射收支在青藏高原地区的偏差。比较表明,气温和地面辐射量新再分析值能反映实际年变化特征,但其温度值系统性偏低,偏低幅度随地区和季节而变化。由于其气温和地表温度偏低造成地表长波辐射和大气逆辐射系统性偏低;冬季积雪地区的地表吸收太阳辐射和净辐射新再分析值偏小;地面净长波、净短波和总的净辐射与实测的偏差比较小。分析发现,同化模式地形高度与地面气象站海拔高度的差异是造成气温新再分析与实测偏差的主要原因,冬季积雪区地表反照率新再分析值偏大是造成冬季地面净辐射偏小的因素,并加剧了冬季气温新再分析的偏差。其研究对改进气候模拟结果分析有一定的启发。     

Evaluation of a high-resolution regional climate simulation over Greenland

A simulation of the 1991 summer has been performed over south Greenland with a coupled atmosphere–snow regional climate model (RCM) forced by the ECMWF re-analysis. The simulation is evaluated with in-situ coastal and ice-sheet atmospheric and glaciological observations. Modelled air temperature, specific humidity, wind speed and radiative fluxes are in good agreement with the available observations, although uncertainties in the radiative transfer scheme need further investigation to improve the model’s performance. In the sub-surface snow-ice model, surface albedo is calculated from the simulated snow grain shape and size, snow depth, meltwater accumulation, cloudiness and ice albedo. The use of snow metamorphism processes allows a realistic modelling of the temporal variations in the surface albedo during both melting periods and accumulation events. Concerning the surface albedo, the main finding is that an accurate albedo simulation during the melting season strongly depends on a proper initialization of the surface conditions which mainly result from winter accumulation processes. Furthermore, in a sensitivity experiment with a constant 0.8 albedo over the whole ice sheet, the average amount of melt decreased by more than 60%, which highlights the importance of a correctly simulated surface albedo. The use of this coupled atmosphere–snow RCM offers new perspectives in the study of the Greenland surface mass balance due to the represented feedback between the surface climate and the surface albedo, which is the most sensitive parameter in energy-balance-based ablation calculations.     

CHARACTERISTICS OF VEGETATION COVER, ROUGHNESS AND ALBEDO DISTRIBUTION OVER CHINA*

To build land surface dataset for climate model,with application of remote sensing technique as well as the Geographic Information System(GIS),the data of surface type,roughness and albedo over China in 1997 were retrieved,resolutions being 10 km×10 km.Based on these data,an analysis is conducted on the geographic distributions and seasonal variations of surface vegetation cover and roughness as well as albedo over China.Results show that surface vegetation cover is mainly located to the south of Yangtze River,in Southwest and Northeast China andsparse vegetation cover is in the Northwest.The variation of land surface cover affects the variations of land surface roughness and albedo.High albedo occurred in the north of Xinjiang Autonomous Region,the north of Northeast China and the Qinghai-Xizang Plateau in winter,in correspondence with the location of snow cover.For most part of China,surface roughness decreases and albedo increases in winter,while the roughness increases and the albedo decreases in summer,which could mainly result from land surface cover(snow cover and vegetation cover)and soil moisture changes.This shows that the geographic distribution and seasonal variation of the albedo are almost opposite to those of the roughness,in agreement with theoretical results.Temporally,the amplitude of surface roughness change is quite small in comparison with the roughness itself.     

藏北地区冬季降雪对地面反射率的影响

藏北地区是冬季青藏高原上降雪次数较多、降雪量较大、可能出现较长时间地面积雪的地区之一。每次较强的隆雪均可造成该地区地面反射率一次较长时间异常，从而改变地面热源的强度和符号。     

冬季青藏高原地面辐射平衡

本文根据实测资料建立了冬季青藏高原上地面辐射平衡与日照百分率、地面反射率之间的经验公式,并用此公式试验了纬度、时间、地面反射率和日照百分率对地面辐射平衡的影响。试验结果表明:冬季高原地面辐射能收支的盈亏状况是由地理纬度和地面反射率决定的。天空遮蔽状况(本文用日照百分率表示)仅影响其盈亏值的大小。亦即地面辐射平衡的地理分布形式由地理纬度和地面反射率所决定,但正、负中心的数值还受天空遮蔽状况的影响。冬季青藏高原地面辐射平衡场是一个由地理因子(地理纬度和自然地理带)作用下形成的基本场叠加上一个地面积雪区形成的扰动场。长江和黄河源区的巴颜喀拉山和藏北草原是冬季高原地面加热场最可能出现异常的关键区。     

基于CERES观测数据的全球行星反照率时空变化特征分析

为了理解行星反照率时空变化规律及成因,基于CERES数据对全球行星反照率的大气(主要为云与气溶胶等)和地表贡献进行了分解,通过Theil-Sen+Mann-Kendall方法得到了2001～2018年全球行星反照率及其大气和地表贡献的时空变化趋势,并基于回归分析方法对典型区域的变化趋势进行了初步解释.研究结果表明:1)...     

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.     

Long-term comparisons of net radiation calculation schemes

Six commonly used models for calculating daily net radiation were tested against measured net radiation. Meteorological data from 32 and 7 consecutive years obtained at two temperate sites were used. The extensive duration of the datasets ensured that all weather conditions and extreme events were captured. A set of statistical procedures was used to evaluate the performance of the models. The mean bias errors ranged from 0.0 W m⁻² to 24.8 W m⁻² and 0.1–24.7 W m⁻² and root mean square error from 11.0 W m⁻² to 28.1 W m⁻² and 10.0–27.9 W m⁻² at the two sites respectively, for days without snow cover on the ground. The best agreement was found when locally calibrated model coefficients were used. Only negligible differences in model performances were found between the two sites and the differences were lower than the inaccuracies of the net radiation instruments used. Including days with snow cover in the analysis lead to a slight increase in the bias and scatter of the predictions. Model performances were in general better during summertime than wintertime. Altered albedo values during winter caused by generally low sun angles were likely the cause of this. Analysis showed that at least 5 years of data were needed to obtain stable calibration coefficients for local calibration of the models. Based on the results from this study, and due to their physical background, two physical based models were recommended for calculating daily values of net radiation under temperate climate regimes. A simple adjustment of the calibration coefficients based on climate regime was suggested for these models.     

Empirical estimation of the monthly-mean daily temperature range

Summary ?This is a sequel to a study of the empirical estimation of the annual mean temperature and its range, at any location on land, based on the historical surface climate record. Here the spatial patterns of the daily temperature range (DTR) and its seasonal variation are examined. The DTR is highest in the subtropical deserts and is less at high latitudes, as well as within 30–150 km from an ocean. It is generally higher in winter (summer) at low (high) latitudes. The coastal DTR reduction is explained by sea breezes, onshore advection, and low-level cloud cover. Even large bodies of water, such as Lake Michigan, affect the near-shore DTR. Elevation does not directly affect the DTR, but valleys tend to have a DTR that is 2–6 K larger than adjacent hills or ridges. The main factor affecting the DTR is the afternoon relative humidity, which is dynamically linked to low-level cloud cover. An empirical relationship between DTR and afternoon relative humidity has an uncertainty of about 1.4 K for monthly-mean values. Received March 6, 2002; revised September 20, 2002; accepted November 3, 2002     

The effect of land surface changes on Eemian climate

Transient experiments for the Eemian (128–113 ky BP) were performed with a complex, coupled earth system model, including atmosphere, ocean, terrestrial biosphere and marine biogeochemistry. In order to investigate the effect of land surface parameters (background albedo, vegetation and tree fraction and roughness length) on the simulated changes during the Eemian, simulations with interactive coupling between climate and vegetation were compared with additional experiments in which these feedbacks were suppressed. The experiments show that the influence of land surface on climate is mainly caused by changes in the albedo. For the northern hemisphere high latitudes, land surface albedo is changed partially due to the direct albedo effect of the conversion of grasses into forest, but the indirect effect of forests on snow albedo appears to be the major factor influencing the total absorption of solar radiation. The Western Sahara region experiences large changes in land surface albedo due to the appearance of vegetation between 128 and 120 ky BP. These local land surface albedo changes can be as much as 20%, thereby affecting the local as well as the global energy balance. On a global scale, latent heat loss over land increases more than 10% for 126 ky BP compared to present-day.