Surface forcing of interannual variations in the radiation balance over North Africa part I: Partitioning the surface and cloud forcing

Previous research has shown that most significant interannual variability of the net radiation balance is confined to a few distinct centers at low latitudes. North Africa is the only continental region within this zone which undergoes large amplitude interannual changes. This two part study focuses on the origins of the North African radiation balance variability, its relationship to precipitation processes, and the potential impact of confined regional variations on global climate. Part I investigates how net radiation perturbations can be decomposed into surface induced components and cloud induced components. The methodology is based on lengthy time series of satellite derived radiation budget and cloudiness quantities in conjunction with a technique derived from the Cess et al. (1982) study involving the formulation of a climate sensitivity parameter.The analysis shows that a significant portion of the interannual modulation of net radiation over North Africa arises from surface influences. Furthermore it appears that a significant part of the mechanism inducing these changes takes place through precipitation controls on the land surface. Cloud induced anomalies are important but represent the weaker of the two processes. Since North Africa is the sink portion of a large scale, cross-meridional energy transport dipole counterpoised to a large scale energy source in the Western Pacific, it is of fundamental importance on the global scale to determine the origins of net radiation variations within the sink.The analysis indicates three dominant time scales associated with precipitation feedback on the net radiation anomalies; a slow manifold of approximately 20 months; a fast manifold of 2.5 months; and a semi-annual manifold. These time scales underlie the surface and cloud forced net radiation anomalies in which southern, central, and northern latitude sectors of North Africa exhibit their own distinct modes of control on the regional radiation balance.     

A hybrid technique for computing the monthly mean net longwave surface radiation over oceanic areas

Summary Global maps of the monthly mean net upward longwave radiation flux at the ocean surface have been obtained for April, July, October 1985 and January 1986. These maps were produced by blending information obtained from a combination of general circulation model cloud radiative forcing fields, the top-of-the-atmosphere cloud radiative forcing from ERBE and TOVS profiles and sea surface temperature on ISCCP C1 tapes. The fields are compatible with known meteorological regimes of atmospheric water vapor content and cloudiness. There is a vast area of high net upward longwave radiation flux (> 80 W m^–2) in the eastern Pacific Ocean throughout most of the year. Areas of low net upward longwave radiation flux (< 40 Wm^–2) are the tropical convective regions and extra tropical regions that tend to have persistent low cloud cover. The technique used in this study relies on GCM simulations and so is subject to some of the uncertainties associated with the model. However, all input information regarding temperature, moisture and cloud cover is from satellite data having near global coverage. This feature of the procedure alone warrants its consideration for further use in compiling global maps of the net longwave radiation at the surface over the oceans.With 9 Figures     

Solar absorption at the surface and in the atmosphere as determined by different observational methods

Summary The concept of effective cloud cover, elaborated on the basis of an assumption that changes in the net radiation at the top of the atmosphere are mainly caused by changing cloudiness, has been used to deduce solar surface radiation from satellite data. It has been shown that the method permits a calculation of solar surface absorption distributions that agree well with the results obtained by other authors and that the existing disagreement can be to a great extent ascribed to the differences in the data sets and analysis periods. The method allows use of early satellite measurements to get longer time series of the surface radiation budget. In this study, it has been applied to the Nimbus-7 ERB WFOV data for 1979–1986.The net solar flux at the TOA (top of the atmosphere) can be partitioned into absorption at the surface and within the atmosphere. The geographical distributions of all the three quantities as well as the zonal averages of the surface absorption for January and July have been described. Special objectives of the present study are to estimate the interannual standard deviation for the 8-year period and to analyse the shortwave cloud-radiative forcing distributions at the surface and especially within the atmosphere.The standard deviation of the TOA and the surface solar absorption shows a temporal asymmetry, being much larger in January than in July. Noticeable is the disappearance of the wintertime strong variability over the central Pacific in July. As can be expected, the strong variability areas coincide with the strong variability areas of the cloud amount, showing the values up to 27 Wm^–2 at the surface.According to our estimate, the shortwave cloud forcing at the surface is everywhere stronger than that at the TOA, so that the cloud forcing of the atmosphere is negative. This means that in the belt of 58.5° N–58.5° S a cloudy atmosphere absorbs more solar energy than a cloud-free atmosphere. Our mean annual value of the atmospheric cloud forcing for this belt is –11 Wm^–2 which is somewhat stronger than that obtained by other investigators. It must be stressed that this value is within the uncertainty limits.Shortwave cloud forcing of the atmosphere is the strongest in the lower latitude areas of heavy cloudiness above the continents and negligible in the midlatitudes in winter. This gives evidence that the value of the shortwave cloud forcing of the atmosphere is modified by a combination of cloud absorption and cloud albedo.With 4 Figures     

SOLAR RADIATION AT GROUND IN EAST-ASIA FROM DATA OF THE GEOSTATIONARY SATELLITE GMS

A method(Rieland,1985)has been modified to determine the downward solar radiation at ground overEast Asia,using GMS-3 data of the satellite with a spatial resolution of 10×10 krn~2 and a time interval of 3hours.The distributions of the monthly mean global radiation are obtained for January,April,July and Octoberof 1985.These results illustrate that the topography effect of the Tibetan Plateau is not negligible.The valuesof global radiation over the high Tibetan Plateau during all seasons are higher than those in the lower-levelsurroundings.By comparing model results with ground observed data, the relative errors in monthly averagesof global radiation are 2.8％ for clear sky and 6.3％,5.3％,5.0％ and 4.5％ for cloudy sky in January,April,July and October,respectively.The relative error in daily global radiation during the snow-free season is lessthan 20％ and larger than 20％ in snow-covered winter.We,however,had no data for these comparisons whichwere measured directly at the Plateau during the year 1985.     

基于卫星资料同化和LAPS-WRF模式系统的云天太阳辐射数值模拟改进方法

太阳能光伏发电已成为仅次于水电和风能的第三大可再生能源,光伏发电受云量时空变化的影响较大,因此准确模拟云天太阳辐射的时空变化对电网安全运行至关重要。围绕如何减小中尺度气象模式的云初始场误差,进而改进云天的太阳辐射模拟这一关键科学问题,本文通过研究基于卫星资料同化的LAPS（Local Analysis Prediction System）多时间层三维云分析同化方法,改进三维云结构,并将LAPS模式输出结果作为WRF（Weather Research and Forecasting）模式的初始场,模拟了2008年1月及夏季（6～8月）北京地区的总云量和总辐射的时空分布,重点分析了多云和有降水天气过程总辐射的模拟改进效果及其原因。结果表明,同化前后的总云量模拟值与观测值的时间变化趋势基本一致,但大部分时次总云量的模拟值低于观测值;大部分多云及降水时段同化后总云量模拟值较接近于实测值。1月晴天、多云天以及夏季晴天同化前后总辐射模拟值与实测值的时间变化趋势较一致,但同化前后两者的相关性差异不明显;晴天条件下同化前后总辐射模拟值均低于实测值,1月多云条件下多数时段同化后总辐射模拟误差减小不明显,与总云量的改进效果不显著有关。夏季多云、有降水及6月典型降水三种天气条件下同化前后总辐射模拟值与观测值的相关性稍差,同化后两者的相关性较同化前有所改进,尤其是6月典型降水过程改进效果较明显;同化前总辐射模拟误差较大,而同化后误差显著减小,尤其是6月典型降水过程同化后均方根误差和平均相对误差较同化前分别减小了102.6 W m-2和355.9%,最大相对误差减小更显著;同化后总辐射模拟误差小于同化前的比例高达75%,即大部分时刻同化后模拟误差小于同化前。多云和有降水天气过程总辐射模拟效果的显著改进与总云量的改进密切相关,即同化后总云量模拟值增加,云的反射和散射作用增强,导致模拟总辐射减小,即更接近于实测总辐射值。研究结果对于多云和降水天气条件下太阳辐射的模拟效果改进、太阳能资源客观评估以及光伏电站的发电量预测具有一定的科学和实际应用价值。     

Estimation of daytime downward longwave radiation under clear and cloudy skies conditions over a sub-humid region

Downward longwave radiation (LW _↓ ) is a relevant variable for meteorological and climatic studies. Good estimates of this term are vitally important in correct determining of the net radiation, which, in turn, modulates the magnitude of the terms in the surface energy budget (e.g., evaporation). In remote sensing applications, the determination of daytime LW_↓ is required for estimation of the net radiation using satellite data. LW_↓ is not directly measured in weather stations and then is estimated using models with surface air temperature and humidity as input. In this paper, we identify the best models to estimate daytime downward longwave radiation from meteorological data in the sub-humid Pampean region. Several well-known models to estimate LW_↓ under clear and cloudy skies were tested. We use downward radiation components and meteorological data registered at Tandil (Argentina) from 2006 to 2010 (840 days). In addition, we propose two multiple linear regression models (MLRM-1 and MLRM-2) to estimate LW_↓ at the surface for all sky conditions. The new equations show better performance than the others models tested with root mean square errors between 12 and 16 W m^?2, bias close to zero and best agreements with measured data (r ²?≥?0.85).     

Climatology of cloud and radiation fields in a numerical weather prediction model

Summary Satellite-derived datasets are used to verify the cloud cover and radiation field generated by a T62 (horizontal resolution) version of the operational global model at the National Meteorological Centre (NMC). An ensemble of five day forecasts for July 1985 is used, as well as 30 day climatological forecasts for July 1985, October 1985, January 1986, and April 1986.Monthly averages of radiation fields are compared with Earth Radiation Budget Experiment (ERBE) data. For the four months examined, clear-sky outgoing longwave radiation (clear-sky OLR) and absorbed shortwave radiation (clear-sky SW) tend to agree roughly with ERBE. Model global mean OLR, however, exceeds that of ERBE by 10 W m^–2.Comparison of effective cloud cover to corresponding fields cataloged by the International Satellite Cloud Climatology Project (ISCCP C1) reveals deficiencies in the amount of supersaturation cloudiness and the vertical distribution of convective clouds. Large inaccuracies in model radiation fields are closely related to deficiencies in the cloud parameterization. An inventory of model cloudiness, in comparison to satellite data, is conducted.With 18 Figures     

The influence of cloud cover index on the accuracy of solar irradiance model estimates

Summary Cloud cover index (CCI) obtained from satellite images contains information on cloud amount and their optical thickness. It is the chief climate data for the assessment of solar energy resources in most radiative transfer models, particularly for the model BRASIL-SR that is currently operational at CPTEC. The wide range of climate environments in Brazil turns CCI determination into a challenging activity and great effort has been directed to develop new methods and procedures to improve the accuracy of these estimations from satellite images (Martins 2001; Martins et al. 2003a; Ceballos et al. 2004). This work demonstrates the influence of CCI determination methods on estimates of surface solar irradiances obtained by the model BRASIL-SR comparing deviations among ground data and model results. Three techniques using visible and/or thermal infrared images of GOES-8 were employed to generate the CCI for input into the model BRASIL-SR. The ground-truth data was provided by the solar radiation station located at Caicó/PE, in Brazilian Northeast region, which is part of the UNEP/GEF project SWERA (Solar and Wind Energy Resources Assessment). Results have shown that the application of the bi-spectral techniques have reduced mean bias error up to 66% and root mean square error up to 50% when compared to the usual technique for CCI determination based on the straightforward determination of month-by-month extremes for maximum and minimum cloud states. Correspondence: Fernando R. Martins, Brazilian Institute for Space Research (INPE), P.O. Box 515, S?o José dos Campos, 12245-970 S?o Paulo, Brazil     

用多光谱卫星信息分析白昼云天条件下的湿度场

TOVS资料的晴空湿度场反演已进行多年 ,如何利用卫星遥感信息实现云天条件下的湿度场反演 ,仍是一个值得研究的问题。多光谱卫星图像上不同云类的云层高低分布蕴涵着三维湿度场的分布信息 ,文中用相关分析、最小二乘拟合分析和多元线性回归分析分别讨论了多光谱卫星信息与各标准层相对湿度的关系。标准回归分析的复相关系数R在地面最小 ,约为 0 6 4,随高度增加 ,R逐层增大 ,70 0hPa以上大于 0 7,2 5 0hPa以上大于 0 8。相应显著性检验的结果F值从地面的 30 71,也逐层增大 ,至 2 0 0hPa时达 119 36。逐步回归分析在剔除了不显著的变量因子后 ,回归自变量减少到 2～ 4个 ,反映显著水平的F值有较大提高。在此基础上建立了一组云天条件下的多元线性回归方程 ,由此可得到具有卫星图像像素分辨率 (8～ 10km)的 9层湿度场。     

晴天地表总辐射和净辐射瞬时值计算方法

以较为精确的大气辐射传输模式为基础,研制出晴天地表总辐射和净辐射瞬时值的计算方案。与以往的经验计算方法不同,该方案将辐射传输带模式的思路引入地面太阳辐射计算,并尽可能将大气中吸收和散射物质对太阳辐射的影响考虑进去,从而使该方法具有较好的精确性和普适性。在此基础上采用了Kokhanovsky等人提出的大气气溶胶反射率和透过率参数化方案,使得气溶胶对地面总辐射和净辐射的影响得到较好的处理。采用的自变量都是数值预报模式或卫星观测能提供的气象要素,因此该方案即可用于数值预报模式或陆面过程模式计算地表辐射平衡,又可以利用卫星观测或再分析资料估算地面太阳能资源分布。利用美国能源部三个大气辐射观测站点2005年全年的观测资料及欧洲宇航局提供的卫星反演气溶胶资料对计算方案进行了检验。结果表明,该方法十分精确,所有点的平均相对误差都小于6%,误差的均方差都小于0.3 W•m^-2。     

Analysis of frontogenesis and frontolysis by means of Q-vector diagnostics combined with satellite images

Summary The correlation between the normal component of theQ-vector(Q _n ) and the pixel values in IR satellite images has been investigated on a six-months data set. The average linear correlation coefficients are negative in all three selected isobaric levels, but not of significant magnitude (the correlation is best for the 850 hPa level, with a correlation coefficient of about 0.3), indicating an only slight tendency for negative values ofQ _n to be associated with high IR grey shade values or, in other words, with cold cloud tops. It is explained why a better correlation should be expected for frontal cases, followed by an evaluation of 14 IR images containing cloudiness of cold fronts that occurred in February 1995. The correspondence betweenQ _n at the 850 hPa level and IR pixel values in those cases is compared to the statistics for the whole month of February 1995, including also nonfrontal cases. The more pronounced connection is found for frontal cases, as expected. Finally, examples of frontogenesis and frontolysis demonstrate the possibility of using the prognostic quantityQ _n in combination with IR satellite imagery as a tool for operational nowcasting and/or forecasting of the development or decay of frontal cloudiness.With 8 Figures     

Radiation balance over a salt marsh

Linear relationships have been found between daytime incoming radiation and both net radiation and reflected radiation over a Spartina alterniflora salt marsh at North Inlet, S.C., during the summer. On the average, using total daytime values, net radiation was 70 ± 9% and reflected radiation was 9 ± 1% of incoming radiation.     

Analysis of UVB values in the centre of the Iberian Peninsula

Records of UV data started in Madrid at the beginning of the 90's decade. After some quality control on the data a seven year period from 1996 to 2002 was selected to perform an analysis of daily and seasonal variability of solar ultraviolet B (UVB) values at the centre of the Iberian Peninsula. Instruments used are a Brewer MKIV spectrophotometer and a YES UVB-1 broadband pyranometer. Both instruments provided integrated daily values according with the Diffey spectrum although they have different measurement procedures. Data statistics from each instrument for the same period and statistical relationships between daily values from both instruments are shown. As broadband YES provided a record with fewer time gaps, analysis of UVI extreme values is performed using that instrument. The relationship between UVB values with ozone, cloudiness and visible radiation has been described in several papers for different places in the world. In this paper, those relationships are shown for Madrid. A discussion about specific problems found when trying to isolate every effective factor is also included. Some of those relationships could be helpful to retrieve UVB values from other ancillary data as visible radiation and cloudiness. Finally, a detailed study for 10 days in June 1997 when the maximum record of daily values from the present time series was obtained, highlight the close correlation between total ozone content and the daily variability of UVB for similar amounts of incoming radiation.     

不同云重叠参数对全球和东亚地区模拟总云量的影响

在国家气候中心全球大气环流模式BCC_AGCM2.0中引入一组基于CloudSat/CALIPSO卫星观测的、能够体现真实时空变化特征的云垂直重叠参数（抗相关厚度,L_cf）数据,以减小由云的重叠描述造成的辐射场的不确定性。对比了采用时空变化的云重叠参数和采用恒定云重叠参数的气候模拟结果,发现无论在全球还是东亚区域,采用基于卫星观测的云重叠参数对模拟的总云量都有一定程度的改进。采用时空变化的云重叠参数后,冬、夏两季全球平均总云量与云和地球的辐射能量系统（CERES）卫星资料的误差都减少了1.6%,其中热带对流区域总云量的正偏差和副热带地区总云量的负偏差都明显减少,这些有助于正确模拟不同区域间的能量收支差异。在东亚区域,采用时空变化的云重叠参数后,冬、夏两季的东亚区域平均总云量与CERES卫星资料的误差分别减少了1.8%和1.4%。综上所述,基于CloudSat/CALIPSO卫星资料计算得到的L_cf有助于改进大气环流模式对总云量的模拟,从而提高模式对辐射场的模拟精度。      

Relationships between photosynthetically active radiation and clearness index at Bloemfontein, South Africa

Summary Relationships between photosynthetically active radiation (PAR) and solar radiation (SR) and between diffuse radiation and global radiation at Bloemfontein, South Africa were investigated. The ratio of PAR to SR (PAR/SR) has been reported from many places in the northern hemisphere, but there were not many PAR/SR measurements documented from the southern hemisphere, including the study region. Based on the present observations, mean PAR/SR in the region (0.48) was not exceptionally different from other reported data (0.45–0.50). The effects of sky condition on PAR/SR were also investigated and confirmed the previous findings. PAR/SR increased as the clearness index (K_T=ratio of global to extraterrestrial SR) decreased, as sky conditions varied from clear to cloudy. Thus, PAR/SR can be a simple function of the clearness index (the global PAR model). In this study, PAR/SR was explained by a quadratic function of K_T. For the relationship between diffuse and global radiation, a simple linear threshold diffuse PAR model, as a linear function of K_T, was proposed for the study region using long-term data of diffuse and global SR. Using the previous findings on the relationship between the ratio of diffuse to global SR (K_SR) and the ratio of diffuse to global PAR (K_PAR), at high K_T, K_PAR can be estimated as 30% higher than K_SR, while at low K_TK_PAR is equivalent to K_SR.     

Global precipitation estimates from satellite: Using difference fields of outgoing long‐wave radiation

Abstract

Global precipitation estimates using satellite data are derived using difference fields of outgoing long‐wave radiation (OLR). The difference fields consist of clear OLR minus cloudy OLR, which is a measure of long‐wave cloud radiative forcing at the top of the earth‐atmosphere system; and clear daytime OLR minus clear night‐time OLR, which is a measure of the diurnal variation of surface heating. All geophysical parameters used to compute OLR are derived from an analysis of the HIRS2/MSU sounding data. The derived global precipitation estimates show good agreement with collocated raingauge data over land. The correlation coefficient between the precipitation estimates derived using difference fields of OLR and raingauge data over land is about 0.65 for the FGGEyear. The correlation coefficient between precipitation estimates derived using difference fields of OLR and the GOES Precipitation Index (GPI) fraction is about 0.914 from 30°S to 30°N for July 1983, and between the precipitation estimates derived using difference fields of OLR and the difference field of atmospheric reflectance is about 0.86.

Using one set of coefficients, global precipitation fields are derived for each 10‐day period and each month of the FGGE year (from December 1978 to November 1979). These fields contain rich information on seasonal variations.     

Potential climatic effects of cirrus contrails for the subarctic setting of Fairbanks, Alaska

Summary Continuous all-sky camera images supported by direct visual observations of jet contrails have been carried out in Fairbanks since March 2000. These data together with FAA information of all commercial flights and the twice-daily radiosonde data, give the meteorological conditions at flight level under which contrails are formed. If we correct for daylight and clear sky conditions, which make contrail observations possible, winter has the maximum and summer the minimum in the occurrence of contrails. This is a result to be expected, as the layer in which contrails can form has in winter nearly twice the thickness when compared to summer.In November 2002, a radiation station was added to the observations. For a contrail in the path between the sun and the observation point, we found a strong decrease in the direct beam radiation; this loss was in part compensated by increased diffuse radiation. The combined effect leads to a reduction in global radiation. However, the back radiation of the atmosphere in the infrared region of the spectrum increased somewhat. Altogether, this affects the net radiation negatively in the summer, but positively in the winter.Comparing the observed temperature conditions of clear days with those of high-level cloud cover, we found for 8 months of the year a higher temperature for days with clouds. For the other four months, May through August, clear days were warmer. On the average of the year, days with high-level cloudiness were warmer than clear days as well as days with low-level overcast.High-level cloudiness has increased in Alaska over the last decades. This increase in cloudiness was more pronounced under the much-traveled flight corridor from Anchorage to Europe than for more remote areas of Alaska. Further, we found a temperature increase for the same time period, which was most pronounced in winter, followed by spring, a result consistent of the expectations of increased high-level cloudiness.     

The relationship between anomalies of interannual net radiation and 37 GHz polarization difference over North Africa

We show evidence that interannual variations in the net radiation balance at the top-of-atmosphere over North Africa may be linked to variations at the land surface within the Sahelian zone. By utilizing concurrent monthly time series of earth radiation budget and passive microwave measurements obtained from the Nimbus 7 satellite over a five year period from 1979 to 1983, we have examined the spatial distribution of the amplitude of interannual variability of the net radiation at the top of the atmosphere. A strong association is found between the radiation budget anomalies derived from ERB-WFOV measurements and the anomalies of 37 GHz polarized brightness temperature difference derived from SMMR measurements. Previous research has shown that the interannual anomalies in the top-of-atmosphere net radiation balance in the Sahelian zone mostly arise from changes taking place at the surface, not from variations in cloudiness which brings variable rainfall throughout the African Sahel. Since microwave polarization difference is related in part to vegetation cover, we suggest that the main control of the modulation of North Africa's radiation balance may be year to year fluctuations of vegetation cover within the Sahelian zone, likely stemming from fluctuations in the rainfall. This relationship is relevant to the problem of radiation control and feedback on global climate through processes taking place within the biosphere of a semi-arid zone.     

Evidence for zonally-trapped propagating waves in the eastern atlantic from satellite sea surface temperature observations

It has been hypothesized (Moore et al., 1978; O'Brien et al., 1978), that equatorial upwelling and subsequent coastal upwelling on the eastern boundary of the Atlantic Ocean are the result of eastward propagating equatorially trapped Kelvin waves in the Atlantic. Concurrent satellite and ship sea surface temperature observations taken during the GATE experiment permit validation of the satellite data as well as relating sea surface temperature (SST) variability to the local current dynamics. A method based on cross-correlations and cross-spectra of the SST field at various locations is utilized to test the Kelvin wave hypothesis. Significant periodic variation of time lags in the SST variability in the eastern Atlantic is observed by the spectral techniques. Satellite data for the 1974 summer show periodic variability which fits either eastward or westward propagating waves with 1 m s^-1 phase speed, i.e., SST supports the quasi-continuous presence of Kelvin or Yanai waves. We find no evidence for a seasonally solitary eastward propagating signal in the eastern Atlantic from SST.     

藏东南地区复杂下垫面能量收支特征分析

利用公益性行业（气象）科研专项项目＂藏东南地区复杂下垫面地气交换观测研究＂在藏东南地区进行的地气交换观测实验数据,分析典型晴天和阴天条件下不同下垫面能量过程的特征及其差异。结果表明：在典型晴空状态下,不同下垫面的地表净辐射均具有明显的日变化特征,在典型阴天的情况下,不同下垫面地表净辐射日均值显著减小;在典型晴空天气下,4种类型下垫面上潜热均随着净辐射的增加而表现为增加的趋势,在典型阴天的情况下,潜热通量明显比晴空天气小;不同下垫面感热通量的日变化存在显著的差异,不同下垫面感热的变化特征在典型晴空和阴天条件下的差异不明显;不同下垫面土壤热通量与净辐射的变化趋势基本一致,阴天夜晚土壤热流的交换与晴天夜晚的差异不大,四种下垫面土壤均存在能量损失,土壤处于降温状态。