基于最新观测和大气再分析估计全球能量平衡

Updating global energy balance based on the latest observations and reanalyses

全球大气顶和地表能量平衡在地球气候变化中起到决定性作用.卫星云和地球辐射能量系统(Clouds and Earths Radiant Energy System, CERES)可以直接观测大气顶能量平衡各分量,地表能量平衡则需要借助卫星遥感反演和大气再分析进行估计.已有研究需要对一些关键参数做人为调整,结果差异很大.基于高精度地基和卫星观测,评估了新一代大气再分析量化全球能量平衡的能力,基于2015~2020年6年平均,给出全球、陆地和海洋大气顶与地表辐射分量的最优估计,同时结合最新的全球降水测量计划(Global Precipitation Measurement, GPM)卫星产品给出地表潜热通量的最优估计.结果发现,新一代大气再分析对大气顶反射太阳辐射高估达8 Wm^(-2);对大气顶出射长波辐射也有10 Wm^(-2)的高估,这种高估在热带海洋上最为显著.欧洲中期天气预报中心第五代大气再分析产品(ECMWF Reanalysis V5, ERA5)可以很好地再现大气顶能量不平衡量(1.1 Wm^(-2)),其他3个再分析的误差从-10.7~5.4 Wm^(-2)不等.地表入射太阳辐射的最优估计来自CERES卫星产品,为187 Wm^(-2);地表大气向下长波辐射的最优估计来自ERA5,为342 Wm^(-2).相对于最新的GPM降水产品,原有全球降水气候计划(Global Precipitation Climatology Project, GPCP)产品低估全球降水8.4%,主要因为其对海洋上空降水的明显低估.基于GPM降水产品以及入海径流的估计,得到全球、陆地和海洋上地表潜热通量的最优估计分别为8、41和101 Wm-2.本文根据验证结果,给出了地表能量平衡各分量的最优估计,实现了地表能量闭合,不再需要人为调整.

The energy balance at the top of the atmosphere(TOA) and at the surface of the earth plays an essential role in the Earth’s climate change. The Clouds and Earth Radiant Energy System(CERES) can directly measure the energy radiation at the TOA, while the surface energy balance needs to be estimated by satellite retrievals and atmospheric reanalysis. Some key fluxes need to be adjusted artificially to close the surface energy balance, particularly the surface latent and sensible heat fluxes. Existing studies have used the Global Precipitation Climatology Project(GPCP) dataset to estimate latent heat flux and have adjusted it artificially to balance the surface energy. Based on high-precision ground-based and satellite observations, this study evaluates the ability of the new generation of atmospheric reanalyses to quantify the global energy balance and provides the optimal estimation of the global, terrestrial, and oceanic radiation fluxes at the TOA and the surface during the period 2015–2020. Additionally, the Global Precipitation Measurement(GPM) products are combined to provide the best estimates of the surface latent heat flux. The global mean TOA incident solar radiation from the new generation of reanalyses is in good agreement with that from CERES. However, due to the addition of atmospheric tide analysis, the Climate Forecast System Reanalysis(CFSR) has a harmonic oscillation pattern that appears near the equator;the errors cancel out after taking the global mean, but the results still have a significant overestimation of 6 Wm-2over land. On the global scale, it is found that the Modern-Era Retrospective Analysis for Research and Applications V2(MERRA2) overestimates the TOA reflected solar radiation by 8 Wm^(-2), and the Japanese 55-year Reanalysis(JRA55)overestimates the TOA outgoing longwave radiation by 10 Wm^(-2), which is most significant over the tropical ocean. The European Centre for Medium-Range Weather Forecasts Reanalysis of Atmosphere Version 5(ERA5) can reproduce the TOA energy imbalance well(1.1 Wm^(-2)), while the estimates of the other three reanalyses are 5.4 Wm^(-2)(CFSR),-10.7 Wm^(-2)(JRA55), and -4.1 Wm^(-2)(MERRA2). All of the TOA radiation fluxes of the reanalyses over the ocean are consistent with those at the global scale, and the ERA5 results are still the best. The best estimates of the surface incident solar radiation and downward longwave radiation from CERES and ERA5 are 187 and 342 Wm^(-2), respectively.Additionally, due to atmospheric tidal analysis, CFSR seriously overestimates the surface incident solar radiation, JRA55 and MERRA2 yield overestimates over land, and MERRA2 seriously underestimates the surface downward longwave radiation. Compared with the latest GPM products, the existing precipitation estimates based on GPCP underestimate global precipitation by 8.4%, mainly due to the substantial underestimation of precipitation over the ocean, which helps to close the global surface energy balance. Based on the GPM and the estimation of runoff into the ocean, we conclude that the optimal estimates of surface latent heat flux over the globe, over the land, and over the ocean are 83, 41, and 101 Wm^(-2),respectively. The global and oceanic latent heat fluxes of JRA55 are overestimated by 10 and 13 Wm^(-2), respectively, while the other three reanalyses are within reasonable ranges. According to the evaluation results, this study provides the optimal estimation of each component of the surface energy balance, which can reach surface energy closure without artificial adjustment.