Mean monthly net radiation modelled using the 1" DEM-S - 3" mosaic

Mean monthly solar radiation was modelled across Australia using topography from the 1 arcsecond resolution SRTM-derived DEM-S and climatic and land surface data. The SRAD model (Wilson and Gallant, 2000) was used to derive:\n•\tIncoming short-wave radiation on a sloping surface\n•\tShort-wave radiation ratio (shortwave on sloping surface / shortwave on horizontal surface)\n•\tIncoming long-wave radiation\n•\tOutgoing long-wave radiation\n•\tNet long-wave radiation\n•\tNet radiation\n•\tSky view factor\nAll radiation values are in MJ/m2/day except for short-wave radiation ratio which has no units. The sky view factor is the fraction of the sky visible from a grid cell relative to a horizontal plane.\n\nThe radiation values are determined for the middle day of each month (14th or 15th) using long-term average atmospheric conditions (such as cloudiness and atmospheric transmittance) and surface conditions (albedo and vegetation cover). They include the effect of terrain slope, aspect and shadowing (for sun positions at 5 minute intervals from sunrise to sunset), direct and diffuse radiation and sky view.\n\nThe monthly data in this collection are available at 3 arcsecond resolution as single (mosaicked) grids for Australia in TIFF format.\n\nThe 3 arcsecond resolution versions of these radiation surfaces have been produced from the 1 arcsecond resolution surfaces by aggregating the cells in a 3x3 window and taking the mean value.\n\nThe 1 arcsecond tiled data can be found here: https://data.csiro.au/dap/landingpage?pid=csiro:9630 . The 1 arcsecond mosaic data can be found here: https://data.csiro.au/dap/landingpage?pid=csiro:18670\nLineage: Source data\n1. 1 arcsecond SRTM-derived Smoothed Digital Elevation Model (DEM-S; ANZCW0703014016)\n2. Aspect derived from the 1 arcsecond SRTM DEM-S\n3. Slope derived from the 1 arcsecond SRTM DEM-S\n4. Monthly cloud cover fraction (Jovanovic et al., 2011)\n5. Monthly albedo derived from AVHRR (Donohue et al., 2010)\n6. Monthly minimum and maximum air temperature (Bureau of Meteorology)\n7. Monthly vapour pressure (Bureau of Meteorology)\n8. Monthly fractional cover (Donohue et al., 2010)\n9. Monthly black-sky and white-sky albedo from MODIS (MCD43A3, B3) (Paget and King, 2008; NASA LP DAAC, 2013)\n10. Measurements of daily sunshine hours, 9 am and 3pm cloud cover, and daily solar radiation from meteorological stations around Australia (Bureau of Meteorology)\n\nSolar radiation model\nSolar radiation was calculated using the SRAD model (Wilson and Gallant, 2000), which accounts for:\n\tAnnual variations in sun-earth distance\n\tSolar geometry based on latitude and time of year\n\tThe orientation of the land surface relative to the sun\n\tShadowing by surrounding topography\n\tClear-sky and cloud transmittance\n\tSunshine fraction (cloud-free fraction of the day) in morning and afternoon\n\tSurface albedo\n\tThe effects of surface temperature on outgoing long-wave radiation, which is modulated by incoming radiation and moderated by vegetation cover\n\tAtmospheric emissivity based on vapour pressure\n\nAll input parameters were long-term averages for each month, i.e., monthly climatologies of cloud cover, air temperature, vapour pressure, fractional cover, AVHRR albedo and MODIS albedo.\n\nCircumsolar coefficient was fixed both spatially and temporally at 0.25, while clear sky atmospheric transmissivity and cloud transmittance were varied. Transmittance measures the fraction of radiation passing through a material (air or clouds in this case), while transmissivity measures that fraction for a specified amount of material. SRAD uses a transmittance parameter for cloud, representing an average of all cloud types during cloudy periods, and a transmissivity parameter for clear sky so that the transmittance can vary with the position of the sun in the sky and hence the thickness of atmosphere that radiation passes through on its way to the ground. The clear sky transmissivity τ and cloud transmittance β were calibrated using observed daily radiation and sunshine hours. \n\nReferences\nDonohue R. J., McVicar T. R. and Roderick M. L. (2010a). Assessing the ability of potential evaporation formulations to capture the dynamics in evaporative demand within a changing climate. Journal of Hydrology, 386, 186-197, doi:10.1016/j.jhydrol.2010.03.020.\n\nDonohue, R. J., T. R. McVicar, L. Lingtao, and M. L. Roderick (2010b). A data resource for analysing dynamics in Australian ecohydrological conditions, Austral Ecol, 35, 593–594, doi: 10.1111/j.1442-9993.2010.02144.x.\n\nErbs, D. G., S. A. Klein, and J. A. Duffie (1982), Estimation of the diffuse radiation fraction for hourly, daily and monthly-average global radiation, Solar Energy, 28(4), 293-302.\n\nJovanovic, B., Collins, D., Braganza, K., Jakob, D. and Jones, D.A. (2011). A high-quality monthly total cloud amount dataset for Australia. Climatic Change, 108, 485-517.\n\nNASA Land Processes Distributed Active Archive Center (LP DAAC) (2013). MCD43A3, B3. USGS/Earth Resources Observation and Science (EROS) Center, Sioux Falls, South Dakota\n\nPaget, M.J. and King, E.A. (2008). MODIS Land data sets for the Australian region. CSIRO Marine and Atmospheric Research Internal Report No. 004. https://remote-sensing.nci.org.au/u39/public/html/modis/lpdaac-mosaics-cmar\n\nWilson, J.P. and Gallant, J.C. (2000) Secondary topographic attributes, chapter 4 in Wilson, J.P. and Gallant, J.C. Terrain Analysis: Principles and Applications, John Wiley and Sons, New York.

本数据集基于1角秒分辨率航天飞机雷达地形测绘任务（SRTM）衍生的平滑数字高程模型DEM-S地形数据，结合气候与地表观测数据，对澳大利亚全域的月均太阳辐射开展建模模拟。本研究采用SRAD模型（Wilson与Gallant, 2000）计算得到以下参数：
• 倾斜坡面入射短波辐射
• 短波辐射比率（倾斜坡面短波辐射/水平地面短波辐射）
• 入射长波辐射
• 出射长波辐射
• 净长波辐射
• 净辐射
• 天空可视因子（Sky View Factor）
除短波辐射比率无单位外，所有辐射值的单位均为兆焦每平方米每天（MJ/m²/day）。天空可视因子指某网格单元相对于水平面所能观测到的天空占比。

本数据集以每月中旬（14日或15日）为代表日，基于长期平均大气条件（如云量、大气透射率）与地表条件（反照率、植被覆盖度）计算辐射参数，涵盖地形坡度、坡向以及周边地形遮蔽效应（以日出至日落每5分钟间隔的太阳位置计算），同时包含直接辐射、散射辐射与天空视野影响。

本数据集提供3角秒分辨率的澳大利亚全域单幅镶嵌TIFF格式网格数据。

本次发布的3角秒分辨率辐射栅格数据，由1角秒分辨率栅格数据通过3×3窗口聚合并取均值得到。

1角秒分幅数据可通过以下链接获取：https://data.csiro.au/dap/landingpage?pid=csiro:9630；1角秒镶嵌数据可通过以下链接获取：https://data.csiro.au/dap/landingpage?pid=csiro:18670

数据溯源：源数据
1. 1角秒分辨率SRTM衍生平滑数字高程模型DEM-S（ANZCW0703014016）
2. 基于1角秒分辨率SRTM DEM-S提取的坡向数据
3. 基于1角秒分辨率SRTM DEM-S提取的坡度数据
4. 月均云量占比（Jovanovic等，2011）
5. 基于先进甚高分辨率辐射计（Advanced Very High Resolution Radiometer, AVHRR）提取的月均反照率（Donohue等，2010）
6. 月均最低与最高气温（澳大利亚气象局）
7. 月均水汽压（澳大利亚气象局）
8. 月均植被覆盖度（Donohue等，2010）
9. 基于中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer, MODIS）MCD43A3产品B3波段提取的月均晴空反照率与全天空反照率（Paget与King, 2008；美国国家航空航天局陆地过程分布式主动存档中心（NASA Land Processes Distributed Active Archive Center, LP DAAC）, 2013）
10. 澳大利亚全域气象站观测的每日日照时数、上午9时与下午3时云量以及每日太阳辐射数据（澳大利亚气象局）

太阳辐射模型
太阳辐射计算采用SRAD模型（Wilson与Gallant, 2000），该模型考虑以下要素：
• 日地距离的年际变化
• 基于纬度与季节的太阳几何位置
• 地表相对于太阳的朝向
• 周边地形造成的遮蔽效应
• 晴空与云层的辐射透射特性
• 上午与下午的日照占比（当日无云时段占比）
• 地表反照率
• 地表温度对出射长波辐射的影响，该效应受入射辐射调控并受植被覆盖度调节
• 基于水汽压的大气发射率

所有输入参数均为各月的长期平均气候态数据，包括云量、气温、水汽压、植被覆盖度、AVHRR反照率以及MODIS反照率的月均气候值。

环日系数在空间与时间上均固定为0.25，而晴空大气透射率与云层透射率则为可变参数。其中，透射率（transmittance）指辐射透过某一介质（此处为大气或云层）的比例；而透射系数（transmissivity）特指特定厚度介质下的辐射透过比例。SRAD模型采用云层透射率参数（代表多云时段内所有云型的平均透射特性）与晴空透射系数参数，使得透射率可随太阳在天空中的位置变化，进而随辐射抵达地面所穿过的大气厚度变化。研究中利用观测得到的每日辐射与日照时数对晴空透射系数τ与云层透射率β进行了校准。

参考文献
1. Donohue R. J., McVicar T. R. 与 Roderick M. L. (2010a)。评估潜在蒸发公式在气候变化背景下捕捉蒸发需求动态的能力。《水文学杂志》，386卷，186-197页，doi:10.1016/j.jhydrol.2010.03.020.
2. Donohue R. J., McVicar T. R., Lingtao L. 与 Roderick M. L. (2010b)。用于分析澳大利亚生态水文条件动态的数据资源。《澳大利亚生态学》，35卷，593–594页，doi:10.1111/j.1442-9993.2010.02144.x.
3. Erbs D. G., Klein S. A. 与 Duffie J. A. (1982)。逐时、逐日与逐月平均总辐射的散射辐射占比估算方法。《太阳能》，28(4)，293-302页。
4. Jovanovic B., Collins D., Braganza K., Jakob D. 与 Jones D.A. (2011)。澳大利亚高质量月均总云量数据集。《气候变化》，108卷，485-517页。
5. 美国国家航空航天局陆地过程分布式主动存档中心（NASA Land Processes Distributed Active Archive Center, LP DAAC）(2013)。MCD43A3产品B3波段。美国地质调查局/地球资源观测与科学中心（EROS），南达科他州苏福尔斯。
6. Paget M.J. 与 King E.A. (2008)。澳大利亚区域中分辨率成像光谱仪（MODIS）地表数据集。澳大利亚联邦科学与工业研究组织海洋与大气研究内部报告第004号。https://remote-sensing.nci.org.au/u39/public/html/modis/lpdaac-mosaics-cmar
7. Wilson J.P. 与 Gallant J.C. (2000)。次级地形属性，收录于《地形分析：原理与应用》（Wilson J.P. 与 Gallant J.C. 编）第4章，约翰威立国际出版公司，纽约。