黑河实验区若干下垫面总辐射、地表反射率与太阳高度角的关系

本文利用1991年“黑河实验”期张掖，化音，沙漠站1月、4月、8月、10月太阳辐射观测资料，分析了晴天总辐射、地表反射率与太阳高度角的关系，得到了不同下垫面、不同季节的地表反射率与太阳高度角的函数关系及各站晴天总辐射与太阳高度角的函数关系的拟合公式，并讨论了这种关系在利用卫星观测资料反演地表反射率中的应用．     

敦煌荒漠戈壁地区裸土地表反照率参数化研究

利用敦煌站观测资料,选取其中观测资料完整且连续性好的7个年份每年5～10月的地表净辐射四分量和土壤湿度资料,分析研究了敦煌荒漠戈壁地区裸土地表反照率与太阳高度角和表层土壤含水量之间的关系,结果表明:地表反照率与太阳高度角呈e指数关系,随太阳高度角的增大而减小,当太阳高度角大于40°时,地表反照率趋于稳定。表层土壤含水量的增大可导致地表反照率的减小,地表反照率与5 cm深土壤湿度呈线性关系。另外,建立了敦煌荒漠戈壁地区裸土地表反照率与太阳高度角和表层土壤含水量之间的双因子参数化公式,提出了一种更加适合该地区的地表反照率参数方案,并且选取2002年6～9月的实测资料对拟合的参数化公式进行模拟验证。本文所提出的地表反照率参数化方案能够很好地再现该地区裸土地表反照率的“U”型日变化特征,可准确地模拟出地表反照率的动态变化趋势。基于此参数化方案计算得到的地表反射辐射与实测值基本一致。     

Comparison between measured and modelled cloudless‐sky solar irradiances at the surface and at the top of the atmosphere

Abstract

Cloudless‐sky solar fluxes calculated by the radiative transfer algorithm used in the Canadian Climate Centre's general circulation climate model are compared with measurements of upwelling radiation at the top of the atmosphere (TOA) and downwelling radiation at the surface. The 12‐layer model partitions the solar spectrum into two broad wavebands (0.25–0.68 and 0.68–4.00 μm). The comparison utilized TOA fluxes estimated from Nimbus‐ 7 measurements and measured downwelling fluxes at the surface for Kalgoorlie, West Australia, and downwelling fluxes at the surface for Woodbridge, Ontario. Model estimates and measurements agreed to within experimental error for most solar zenith angles. Estimates improved, especially at Woodbridge, when aerosol effects were included. The mean bias error was less than 4% for surface irradiance and less than 6% for upwelling TOA irradiance, which produces a TOA albedo error of about 0.01.     

On the radiation characteristics of Antarctic Sea Ice

Abstract

Radiative measurements were carried out continuously during a cruise from Australia to Antarctica during austral summer 1995/96. Both shortwave and longwave radiative fluxes were measured. Some of the results are:

• The incoming solar radiation had a mean value of 217 W m^–2; this was a relatively weak value due to the large amount of fractional cloud cover observed. The sun was, for a large part of the trip, above the horizon for 24 hours a day.

• The reflectivity varied widely, not only as a function of sea‐ice concentration, but also as a function of ice type.

• Snow covered pack ice gave the highest albedo values (<70%), while flooded sea ice and thin ice reflected much less (<30%).

• For each sea‐ice type, short term observations showed a good relationship between albedo and ice concentration.

• The albedo increased with decreasing solar elevation.

• The net longwave radiation was negative (mean –27 W m–2); this small absolute value is due to a high amount of fractional cloud cover. There was a weak diurnal variation with a maximum loss (–33 W m^–2) in the early afternoon.

• On the average, the net radiation was positive for 17 hours, and negative for 7 hours a day. However, the duration of a positive balance depended strongly on the surface albedo.

• For the observed albedo values, modelling results showed that the net radiation was always positive when averaged over a day. The magnitude, however, depended strongly on the surface albedo, varying by more than the factor of three.

     

2007年春末夏初珠穆朗玛峰地区辐射特征分析

利用2007年5月23日～6月22日的观测数据和先前已有的相关实测资料,分析了总辐射、大气逆辐射、地面长波辐射、有效辐射以及地表反射率.结果表明:春末夏初珠峰大本营地区出现瞬时总辐射大于太阳常数的现象相当频繁,甚至还能出现小时平均值也大于太阳常数的记录;由于珠峰地区海拔高,大气稀薄,低温低湿,大气逆辐射明显减少;地面长波辐射不仅低于平原地区,也低于青藏高原其他地区;在地表辐射平衡中.珠峰大本营地表有效辐射大于反射辐射;随着海拔高度的增加,白天净辐射正值持续的时间相应缩短而最大值减少.     

Enhanced climatic warming in the Tibetan Plateau due to doubling CO2: a model study

The National Center for Atmospheric Research (NCAR) regional climate model (RegCM2), together with initial conditions and time-dependent lateral boundary conditions provided by a 130-year transient increasing CO₂ simulation of the NCAR Climate System Model (CSM), has been used to investigate the mechanism of ground warming over the Tibetan Plateau (TP). The model results show that when CO₂ in the atmosphere is doubled, a strong ground warming occurs in the TP. Two regions within it with the largest warming are in the eastern TP (region I) and along the southwestern and western slopes (region II). Moreover, in region I the ground warming in the winter half year is stronger than that in the summer half year, but in region II the warming difference between the seasons becomes opposite to that in region I, i.e., the warming is strong in the summer half year and weak in the winter half year. There are indications that the summer monsoon enhances but the winter monsoon weakens when CO₂ is doubled. A strong elevation dependency of ground warming is found in region I for the winter half year, and in region II for both winter and summer half years at elevations below 5 km. The simulated characteristics of ground warming in the TP are consistent with the observations. In region I, when CO₂ is doubled, the cloud amount increases at lower elevations and decreases at higher elevation for the winter half year. As a consequence, at lower elevations the short wave solar radiation absorbed at the surface declines, and the downward long wave flux reaching the surface enhances; on the other hand, at higher elevations the surface solar radiation flux increases and the surface infrared radiation flux shows a more uniform increase. The net effect of the changes in both radiation fluxes is an enhanced surface warming at higher elevations, which is the primary cause of the elevation dependency in the surface warming. In the summer half year the cloud amount reduces as a result of doubling CO₂ in region I for all elevations, and there is no elevation dependency detected in the ground warming. Furthermore, there is little snow existing in region I for both summer and winter half years, and the impact of snow-albedo feedback is not significant. In region II, although the changes in the cloud amount bear a resemblance to those in region I, the most significant factor affecting the surface energy budget is the depletion of the snow cover at higher elevations, which leads to a reduction of the surface albedo. This reduction in turn leads to an enhancement in the solar radiation absorbed in the surface. The snow-albedo feedback mechanism is the most essential cause of the elevation dependency in the surface warming for region II.     

Energy balance of the intertidal zone of Western Hudson bay I: Ice‐free period

Abstract

This study reports on tower measurements from the intertidal zone taken during the ice‐free period between August 1 and September 20, 1985. Sea and air temperatures showed ranges of 8 and 14°C, respectively, and both were colder during onshore than during offshore winds. Onshore winds were associated with a nearly saturated atmosphere whereas offshore ones were quite dry. Surface albedo was twice as great for low tide as for high tide. The ratio net/solar radiation was 13% less at low tide owing to both the larger albedo and the stronger long‐wave radiation loss. Heat fluxes into the bottom sediments were small with net gains in August and net losses in September. During the day, heat storage in the water was large and positive. This occurred even with the tide out, when the ponded water continued to warm. At night the water gave up heat, both for low and high tide, and especially late in the season. The latent heat flux was always positive and was largest by day during low tide and by night during high tide. The sensible heat flux was positive for onshore winds and often negative for offshore winds. Under all wind directions heat storage constituted 60% of net radiation, the latent heat flux 35% and the remainder was proportioned equally between the sensible heat flux and the flux into the bottom sediments.     

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.     

Numerical simulation of sensitivities of snow melting to spectral composition of the incoming solar radiation

Snow albedo is an important factor influencing the snow surface energy budget and snow melting, yet uncertainties remain in the calculation of spectrally resolved snow surface albedo because the spectral composition (visible versus near infrared) of the incident solar radiation is seldom available. The influence of the spectral composition of the incoming solar radiation on the snow surface albedo, snow surface energy budget, and final snow ablation is investigated through sensitivity experiments of four snow seasons at two open sites in the Alps by using a multi-layer Snow-Atmosphere-Soil-Transfer scheme (SAST). Since the snow albedo in the near infrared (NIR) spectral band is significantly lower than that in the visible (VIS) band, and almost the entire NIR part of the solar radiation is absorbed in the top layer of the snow pack, given a fixed amount of incoming solar radiation, a lower VIS/NIR ratio implies that more NIR radiation is reaching the ground surface and more is absorbed by the top layer of the snow pack, therefore, speeding up the snow melting and increasing the surface runoff, although a lesser part of the solar radiation in the visible band is transmitted into and trapped by the sub-layer of the snow pack. The above VIS/NIR ratio effect of the incoming solar radiation can result in a couple of days difference in the timing of snow ablation and it becomes more significant in late spring when the total solar radiation is intensified with seasonal evolution. Snow aging also slightly intensifies this VIS/NIR ratio effect.     

Modelling daily global radiation at the forest floor

A simple model is developed which estimates daily global radiation at the floor of a non-homogeneous Eucalyptus forest. Model input parameters are easily derived from field measurements and consist of individual tree location, tree height, maximum canopy width and its corresponding height, height of the lowest branch and trunk thickness. In addition, the model requires values for global and diffuse irradiance in the open. The tree canopy is represented as a series of spheres containing leaves which are homogeneously spaced but are oriented in the vertical plane. This configuration closely approaches that of actual eucalyptus trees. A Monte-Carlo approach is used to estimate the albedo of the unit sphere as a function of solar zenith angle. At a given combination of solar zenith and azimuth angle, the model estimates the solar irradiance at a specific forest floor location.The model, when tested against pyranometer measurements, predicted daily solar irradiance with a correlation of 0.98 and a standard error of 0.98 MJ m^-2 day^-1. This good performance is attributed to the spatial averaging of the radiation fluxes over the entire day, and the relatively low sensitivity of the calculated solar irradiance to sphere albedo.     

Radiation regime in a tropical dry deciduous forest in western Mexico

Summary Solar radiation (Qc), sunshine hours (n) and reflectance coefficients or albedo (A) were measured above, and photosynthetically active radiation (PAR) was measured above and within three levels in a tropical dry deciduous forest in western Mexico (19° 30 N, 105° 03 W).Values ofQc andn, and PAR andA showed a drastic change between rainy and dry season, and leafed and leafless period, respectively. The transmittance for solar radiation was high in dry season and showed a great variation on transition months from dry to rainy season. Midday albedo changed from 14 (August, leafed period) to 21% (May, leafless period).The transmittance of PAR within the forest was 0.92, 0.26 and 0.19 at levels of 10.0, 5.0 and 0.2m above ground, respectively, in leafed period. The mean tree height was 11.0 m. In leafless period, the transmittance was 0.75 and 0.46 at 5.0 and 0.2 m levels.With 5 Figures     

利用Nimbus-7行星反射率观测资料估算青藏高原地区的总辐射

本文利用1982年8月—1983年7月期间Nimbus-7行星反射率的月平均资料用“物理模式方法(Raschke and Preuss,1979)”估算了青藏高原及其邻近地区月平均地面总辐射的分布。得到的结果较好地反映了纬度、海拔与云量三个主要因子对总辐射分布的支配作用。根据高原及其邻近地区23个测站的资料,对总辐射的计算值与观测值进行了比较。统计分析表明,相关系数f=0.90,标准误差RMS=27w/m~2,平均绝对误差ABS=21w/m~2(相当于有效平均总辐射的11.7％)。文中还对误差来源和敏感性问题进行了讨论。     

Surface‐absorbed and top‐of‐atmosphere radiation fluxes for the Mackenzie River basin from satellite observations and a regional climate model and an evaluation of the model

Abstract

Both the earth‐reflected shortwave and outgoing longwave radiation (OLR) fluxes at the top of the atmosphere (TOA) as well as surface‐absorbed solar fluxes from Canadian Regional Climate Model (CRCM) simulations of the Mackenzie River Basin for the period March 2000 to September 2003 are compared with the radiation fluxes deduced from satellite observations. The differences between the model and satellite solar fluxes at the TOA and at the surface, which are used in this paper to evaluate the CRCM performance, have opposite biases under clear skies and overcast conditions, suggesting that the surface albedo is underestimated while cloud albedo is overestimated. The slightly larger differences between the model and satellite fluxes at the surface compared to those at the TOA indicate the existence of a small positive atmospheric absorption bias in the model. The persistent overestimation of TOA reflected solar fluxes and underestimation of the surface‐absorbed solar fluxes by the CRCM under all sky conditions are consistent with the overestimation of cloud fraction by the CRCM. This results in a larger shortwave cloud radiative forcing (CRF) both at the TOA and at the surface in the CRCM simulation. The OLR from the CRCM agrees well with the satellite observations except for persistent negative biases during the winter months under all sky conditions. Under clear skies, the OLR is slightly underestimated by the CRCM during the winter months and overestimated in the other months. Under overcast conditions the OLR is underestimated by the CRCM, suggesting an underestimation of cloud‐top temperature by the CRCM. There is an improvement in differences between model and satellite fluxes compared to previously reported results largely because of changes to the treatment of the surface in the model.     

阿尔卑斯山杉林冠层影响辐射传输的个例分析

利用瑞士Alptal观测站杉树林冠层上方、下方的辐射观测资料,分析了冠层对短波辐射的减弱及对长波辐射的增幅作用及其季节变化。结果表明,对比较密集的常绿针叶林,冠层对入射短波辐射的透过率随着太阳高度的降低而减小,春季以后趋于稳定;冠层对长波辐射的增幅作用随天气状况而变化,这种增幅作用在晴空条件下最显著,可达1.5倍。在冬季,因为太阳辐射较弱,冠层对长波辐射的增幅作用超过对短波辐射的减弱从而增加地面净辐射。在其它季节,太阳辐射比较强,冠层对短波辐射的减弱超过对长波辐射的增幅作用而减少地面净辐射。地面净辐射与冠层上方气温的变化趋势虽然在有些时段一致,但在伴随降雪过程的降温时段,地面净辐射与气温的变化趋势近乎反相,在积雪融化时段,地面净辐射的增加比气温升高更显著,尤其是在白天。     

Spectral radiance and sky luminance in Antarctica: a case study

Summary Sky luminance and spectral radiance has been characterised at Neumayer, Antarctica for selected situations during the austral summer 2003/04. Luminance has also been measured at Boulder, Colorado, USA in June 2003. The high reflectivity of the surface (albedo) in Antarctica, reaching values up to 100% in the ultraviolet (UV) and visible part of the solar spectrum due to snow cover, modifies the radiation field considerably when compared to mid-latitudes. A dependence of luminance and spectral radiance on solar zenith angle (SZA) and surface albedo has been identified. For snow and cloudless sky, the horizon luminance exceeds the zenith luminance by as much as a factor of 8.2 and 7.6 for a SZA of 86° and 48°, respectively. In contrast, over grass this factor amounts to 4.9 for a SZA of 86° and a factor of only 1.4 for a SZA of 48°. Thus, a snow surface with high albedo can enhance horizon brightening compared to grass by a factor of 1.7 for low sun at a SZA of 86° and by a factor of 5 for high sun at a SZA of 48°. For cloudy cases, zenith luminance and radiance exceed the cloudless value by a factor of 10 due to multiple scattering between the cloud base and high albedo surface. Measurements of spectral radiance show increased horizon brightening for increasing wavelengths and generally confirm the findings for luminance. Good agreement with model results is found for some cases; however there are also large deviations between measured and modelled values especially in the infrared. These deviations can only partly be explained by measurement uncertainties; to completely resolve the differences between model and measurement further studies need to be performed, which will require an improvement of modelling the spectral radiance. From the present study it can be concluded that a change in albedo conditions, which is predicted as a consequence of climate change, will significantly change the radiation conditions in polar regions as well.     

On the vertical profile of surface radiative fluxes in southwest Germany

Summary Vertical profile of surface radiative fluxes in an area of heterogeneous terrain in south-west Germany is presented. Main data sets utilized for the study were recorded during the REgio KLIma Projekt (REKLIP). Supporting observational data were provided by the German weather service and German geophysical consultant service. Elevation of the study sites ranged from 212 m a.s.l. to 1489 m a.s.l. From May to September, monthly mean albedo was generally low at the study sites, ranging from 19% to 24%. For the other months, monthly mean albedo lie between 22% and 25% at the lowland site but extended between 27% and 71% at the highly elevated mountain site. Following the altitudinal increase in surface albedo, net radiative flux and radiation efficiency declined with elevation at an annual mean of 1.15 Wm⁻²/100 m and 0.008/100 m respectively. Absorbed shortwave radiation and effective terrestrial radiation showed mean decline of 1.54 Wm⁻²/100 m and 0.34 Wm⁻²/100 m, respectively, with the mean sky-to-earth radiation deficit amounting to about 52 Wm⁻² for the lowland site and 73 Wm⁻² for the highest elevated site. Some empirical models which express shortwave and longwave radiative fluxes in terms of meteorological variables have been validated for the lowland and mountain sites. Monthly mean daily total estimates of solar radiation obtained from ?ngst?m-Prescott relation were quite consistent with observed values. Parameterisation of downward atmospheric radiation under all sky condition was achieved by extending Brutsaert clear sky atmospheric model. Relationship between outgoing longwave radiation and screen temperature at the study sites was best described by an exponential function unlike the linear relationship proposed by Monteith and Unsworth. Net radiative flux for the lowland and mountain sites has been expressed in terms of absorbed shortwave radiation, cloud amount and screen temperature. Received March 5, 2001 Revised October 29, 2001     

Energy Balance Of A Sparse Coniferous High-Latitude Forest Under Winter Conditions

Measurements carried out in Northern Finland on radiation and turbulent fluxes over a sparse, sub-arctic boreal forest with snow covered ground were analysed. The measurements represent late winter conditions characterised by low solar elevation angles. During the experiment (12–24 March 1997) day and night were about equally long. At low solar elevation angles the forest shades most of the snow surface. Therefore an important part of the radiation never reaches the snow surface but is absorbed by the forest. The sensible heat flux above the forest was fairly large, reaching more than 100 W m^-2. The measurements of sensible heat flux within and above the forest revealed that the sensible heat flux from the snow surface is negligible and the sensible heat flux above the forest stems from warming of the trees. A simple model for the surface energy balance of a sparse forest is presented. The model treats the diffuse and direct shortwave (solar) radiation separately. It introduces a factor that accounts for the shading of the ground at low solar elevation angles, and a parameter that deals with the partial transparency of the forest.Input to the model are the direct and diffuse incoming shortwave radiation.Measurements of the global radiation (direct plus diffuse incoming shortwaveradiation) above the forest revealed a considerable attenuation of the globalradiation at low solar elevation. A relation for the atmospheric turbidity asfunction of the solar elevation angle is suggested. The global radiation wassimulated for a three month period. For conditions with a cloud cover of lessthan 7 oktas good agreement between model predictions and measurementswere found. For cloud cover 7 and 8 oktas a considerable spread can beobserved. To apply the proposed energy balance model, the global radiationmust be separated into its diffuse and direct components. We propose a simpleempirical relationship between diffuse shortwave and global radiation asfunction of cloud cover.     

Surface albedo at a tower site in Lake Ontario

Measurements of incoming global, diffuse and reflected radiation at a tower site in Lake Ontario are used to evaluate components of surface albedo. Albedo for diffuse radiation lies between 0.074 and 0.082 and a coefficient for backscatter from sub-surface water layers shows little deviation from a mean of 0.017. Direct beam albedo for a calm surface follows the Fresnel law. Waves increase direct-beam albedo particularly at higher solar zenith angles. A pronounced dependence of albedo upon zenith angle for clear skies decreases with increasing cloud amount and becomes undetectable in overcast conditions. On a daily basis, albedo ranged between 0.07 in early July to 0.11 in mid-November. Day-to-day scatter is within ±1% of the mean seasonal trend.     

晴天地表太阳辐射的参数化

讨论了晴天地表太阳总辐射和地表太阳净辐射瞬时值的参数化方法。首先利用辐射传输模式和中纬度夏季标准大气廓线,分谱带计算晴天各种大气条件下地表反射率取定值时的地表太阳总辐射,并把所得的结果作为标准资料,提出参数化方案。然后将地表反射率的影响作为误差项进行订正,从而得到各种地表反射率条件下的晴天地表太阳净辐射的计算方法。该方法的拟合精度较高,拟合值与辐射模式标准值的平均相对误差在0.3%以下。该参数化公式可以用于大尺度数值模式中地表辐射平衡的计算,以期达到地表辐射平衡计算与模式积分同步进行的目的。     

五道梁地区的辐射特征

本文分析了1986年中美联合考察期间五道梁站的地面辐射平衡的气候学特征。五道梁地区夏季直接太阳辐射强,空气洁净,大气透明度好。太阳辐射在大气中的削弱以分子散射和臭氧吸收为主。总辐射中以散射为主。光谱反射率中太阳短波反射率为0.13,太阳红外反射率为0.25,雪面上二者接近;反射率受土壤湿度影响明显,在太阳高度角较小时,各波段反射率有不同的变化趋势。地表比辐射率约为0.90。地表净辐射和地面热源强度大。太阳紫外辐射大,占总辐射的比例也大。