Mean monthly incoming atmospheric longwave radiation modelled using the 1" DEM-S - 1" tiles

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 1 arcsecond resolution as 1x1 degree tiles in ESRI float grid format. 813 tiles make up the extent of Australia. The 1 arcsecond mosaic data can be found here: https://data.csiro.au/dap/landingpage?pid=csiro:18491 .\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 3 arcsecond mosaic data can be found here: https://data.csiro.au/dap/landingpage?pid=csiro:18492\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弧秒分辨率SRTM衍生数字高程模型（SRTM-derived DEM-S）的地形数据，结合气候与地表数据，对澳大利亚全域的逐月平均太阳辐射进行了建模。本次研究采用SRAD模型（Wilson与Gallant，2000）推导得到以下参数：
• 倾斜地表入射短波辐射
• 短波辐射比（倾斜地表短波辐射/水平地表短波辐射）
• 入射长波辐射
• 出射长波辐射
• 净长波辐射
• 净辐射
• 天空视域因子

除短波辐射比无单位外，所有辐射值单位均为兆焦每平方米每天（MJ/m²/day）。天空视域因子指某网格单元相对于水平面的可见天空占比。

辐射值以每月中旬（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）计算得出，涵盖了地形坡度、坡向与遮蔽效应（以日出至日落每5分钟间隔的太阳位置计算）、直接辐射与散射辐射以及天空视域的影响。

本数据集的逐月数据以1弧秒分辨率提供，采用ESRI浮点网格格式，分幅为1×1度瓦片，共813片覆盖澳大利亚全域。1弧秒拼接数据可通过以下链接获取：https://data.csiro.au/dap/landingpage?pid=csiro:18491。

本数据集的3弧秒分辨率辐射面数据由1弧秒分辨率数据通过3×3窗口聚合并取平均值生成。3弧秒拼接数据可通过以下链接获取：https://data.csiro.au/dap/landingpage?pid=csiro:18492

数据溯源：原始数据源
1. 1弧秒SRTM衍生平滑数字高程模型（DEM-S；ANZCW0703014016）
2. 由1弧秒SRTM DEM-S提取的坡向
3. 由1弧秒SRTM DEM-S提取的坡度
4. 月度云量占比（Jovanovic等，2011）
5. 基于AVHRR（Advanced Very High Resolution Radiometer，高级甚高分辨率辐射计）的月度反照率（Donohue等，2010）
6. 月度最低与最高气温（澳大利亚气象局）
7. 月度水汽压（澳大利亚气象局）
8. 月度覆被率（Donohue等，2010）
9. 基于MODIS（Moderate Resolution Imaging Spectroradiometer，中分辨率成像光谱仪）的月度晴空反照率与全天空反照率（MCD43A3，B3）（Paget和King，2008；NASA陆地过程分布式数据归档中心（LP DAAC），2013）
10. 澳大利亚境内气象站的每日日照时数、上午9时与下午3时云量及每日太阳辐射观测数据（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），该模型考虑以下因素：
- 日地距离的年际变化
- 基于纬度与季节的太阳几何关系
- 地表相对于太阳的朝向
- 周边地形造成的遮蔽
- 晴空与云层的透射效应
- 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由植被覆盖调节
- 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数据，即云量、气温、水汽压、覆被率、AVHRR反照率与MODIS反照率的逐月气候态数据。

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过率则为可变参数。透过率（Transmittance）指辐射穿过某介质（此处为空气或云层）的比例；透射率（Transmissivity）指单位厚度介质的辐射透过比例。SRAD模型采用云层透过率参数（代表多云时段所有云型的平均透过率）与晴空透射率参数，使得透过率可随太阳在天空中的位置变化，即随辐射抵达地面途中穿过的大气厚度变化。晴空透射率τ与云层透过率β通过实测每日辐射与日照时数进行校准。

参考文献
1. Donohue R. J., McVicar T. R. 及 Roderick M. L. (2010a). 评估潜在蒸发公式捕捉气候变化下蒸发需求动态的能力. 水文学报, 386, 186-197, doi:10.1016/j.jhydrol.2010.03.020.
2. Donohue R. J., McVicar T. R., Lingtao L. 及 Roderick M. L. (2010b). 澳大利亚生态水文动态分析数据集资源. 澳大利亚生态学, 35, 593–594, doi: 10.1111/j.1442-9993.2010.02144.x.
3. Erbs D. G., Klein S. A. 及 Duffie J. A. (1982). 逐时、每日及月平均总辐射散射辐射占比的估算. 太阳能, 28(4), 293-302.
4. Jovanovic B., Collins D., Braganza K., Jakob D. 及 Jones D.A. (2011). 澳大利亚高质量月度总云量数据集. 气候变化, 108, 485-517.
5. NASA陆地过程分布式数据归档中心（LP DAAC）(2013). MCD43A3, B3. 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. 及 King E.A. (2008). 澳大利亚区域MODIS陆地数据集. 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. 及 Gallant J.C. (2000) 次级地形属性, 收录于Wilson, J.P. 与 Gallant, J.C. 所著《地形分析：原理与应用》第四章, 约翰威立国际出版公司, 纽约.