Estimation of ERA5 shortwave radiation budget in the northern South China Sea in summer based on navigation observation data
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摘要: 海表短波辐射收支是海–气界面能量交换的重要物理过程。本研究利用2019年南海北部夏季科考航次的走航观测数据，评估了ERA5再分析数据的海表短波辐射通量收支。结果表明，ERA5的向下短波辐射相比观测偏小，11时和15时（北京时间）的偏差最大，可达−100 W/m2 。与此同时，ERA5的海表反照率整体偏低，其中高太阳高度角时段偏差较小，约为−0.03，低太阳高度角时段偏差较大，约为−0.15。向下短波辐射和反照率的偏差共同造成ERA5白天平均海表净短波辐射通量比观测偏小约25.4 W/m2；其中，反照率低估抵消了约50%向下短波辐射偏差的贡献。研究表明，在不同大气透射率情况下，ERA5的海表辐射收支偏差存在不同表现。ERA5海表反照率的低估可能与其采用的参数化方案在南海北部的适用性不足有关。基于观测本研究也给出了一个简单的参数优化方案。Abstract: The short wave radiation budget on the sea surface is an important physical process of energy exchange at the sea-air interface. In this study, the sea surface short wave radiation flux budget of ERA5 reanalysis data is evaluated by using the observed data of summer scientific research voyages in the northern South China Sea in 2019. The results show that the downward short wave radiation of ERA5 is smaller than the observed data, and the deviation is the largest at 11:00 and 15:00 (Beijing time), up to −100 W/m2. At the same time, the sea surface albedo of ERA5 is generally lower than observed. The bias in ERA5 is small under a high solar altitude angle, being about −0.03, but can reach −0.15 under a low solar altitude. The bias in the downward shortwave radiation and sea surface albedo jointly caused an underestimation of 25.4 W/m2 in the daytime average sea surface net shortwave radiation flux in ERA5. In particular, the albedo underestimation offsets about 50% of the contribution of downward shortwave radiation bias. The results show that ERA5 has different manifestations of sea surface radiation budget deviation under different atmospheric transmittance conditions. We also found that the underestimation of sea surface albedo in ERA5 is caused by its parameterization schemes and put an optimization based on our observation.
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图 1 2019年中山大学夏季南海科考航次航线图
颜色标识距离出发的天数，实心菱形表明当天12：00（北京时间）所处的位置
Fig. 1 Route map of South China Sea scientific expedition in summer of Sun Yat-sen University in 2019
The color indicates the number of days from departure, and the solid diamond indicates the position at 12:00 (Beijing time) on the same day
图 2 航次期间（共23 d）白天逐小时的观测（a）与ERA5（b）的向下短波辐射通量（Rd），以及二者的差值（c）。图d，e，f分别是图a，b，c对应的平均日变化
图 f蓝色散点为ERA5的向下短波辐射通量（Rd）在不同时刻的相对偏差
Fig. 2 The observed (a) and ERA5’s (b) downward shortwave radiation flux (Rd) in the daytime during the voyage (23 days). Figures d, e and f are the average daily variation corresponding to a, b and c respectively
The blue points in figure f is the relative deviation of ERA5’ downward shortwave radiation flux (Rd) at different times
图 3 航次期间（共23 d）白天逐小时的观测（a）与ERA5（b）的海表反照率（α），以及二者的差值（c）。图d，e，f分别是图a，b，c对应的平均日变化
图 f 蓝色散点为ERA5的α在不同时刻的相对偏差
Fig. 3 The observed (a) and ERA5’s (b) sea surface albedo (α) in the daytime during the voyage (23 days). Figures d, e and f are the average daily variation corresponding to a, b and c respectively
The blue points in figure f is the relative deviation of ERA5’ α at different times
图 4 航次期间（共23 d）白天逐小时的观测（a）与ERA5（b）的净短波辐射通量（Rnet），以及二者的差值（c）。图d，e，f分别是图a，b，c对应的平均日变化
其中图 f 蓝色散点为ERA5的Rnet 在不同时刻的相对偏差
Fig. 4 The observed (a) and ERA5’s (b) net shortwave radiation flux (Rnet) in the daytime during the voyage (23 days). Figures d, e and f are the average daily variation corresponding to a, b and c respectively
The blue points in figure f is the relative deviation of ERA5’s Rnet at different times
图 5 航次期间（共23 d）白天逐小时的观测（a）与ERA5（b）的大气透射率（τ），以及观测和ERA5为相同大气透射（晴天，混合天，阴天）的时刻（c）
Fig. 5 Hourly atmospheric transmittance (τ) of observation (a) and ERA5 (b) during the voyage (23 days), and the time when the observation and ERA5 are the same atmospheric transmittance (c)
图 6 观测与参数化得到的海表反照率在晴天（透射率>0.55）的比较
a. Taylor 参数化方法；b. Briegleb 参数化方法；c. Hansen参数化方法；d. Huang参数化方法。图中红色虚线为拟合线，左上角列出对应的线性拟合方程，其中r为相关系数
Fig. 6 Comparison of observed and parameterized albedos on clear-sky (atmospheric transmittance>0.55)
a. Taylor’s method; b. Briegleb’s method; c. Hansen’s method; d. Huang’s method. The red dotted line in the figure is the fitting line, and the corresponding linear fitting equation is listed in the upper left corner, and r is the correlation coefficient
图 7 反照率随太阳高度角的平均日变化（a），拟合反照率与实际观测的对比结果（b）
图a中红线和蓝线分别为新方案结果和Taylor方案结果；b中填色为太阳高度角，红色虚线为两结果的拟合线方程，左上角为拟合线方程结果，r为相关系数
Fig. 7 The average daily variation of albedo with solar altitude angle (a), and the comparison result between the fitting albedo and the observation (b)
In figure a, the red line and blue line are the results of the new scheme and Taylor scheme respectively; in figure b, the color is filled with the solar altitude angle, the red dotted line is the fitting line equation of the two results, the upper left corner of the picture is the fitting line equation result, and r is the correlation coefficient
图 8 观测（a）和ERA5（b）的海表反照率平均日变化
图中红色、蓝色、黑色箱型图分别代表晴天混合天和阴天
Fig. 8 The average diurnal variation of sea surface albedo observed (a) and ERA5 (b) respectively
The red, blue and black box charts represent clear, mixed and cloudy sky conditions respectively
图 9 本次观测散射辐射海表反照率的平均日变化箱型图
图中红色、蓝色、黄色实线分别为Payne[9]、Séférian等[42]和本次观测的散射反照率
Fig. 9 The box chart of the average daily variation of the scattered sea surface albedo from this observation
The red, blue and yellow solid lines are Payne[9], Séférian et al[42] and the observed scattered sea surface albedo respectively
表 1 海表反照率（α）贡献项、向下短波辐射通量（Rd ）贡献项、协方差项和海表净短波辐射通量（ΔRnet）的平均日变化
Tab. 1 Average daily variation of sea surface albedo contribution (α), downward shortwave radiation flux (Rd) contribution, covariate terms contribution and sea surface net shortwave radiation flux (ΔRnet) (unit: W/m2)
时间 α 贡献项 Rd贡献项 协方差项 ΔRnet 7:00 23.8 –18.1 –6.2 –0.5 8:00 35.4 –58.2 –8.1 –31.0 9:00 35.6 –38.7 –3.9 –7.0 10:00 35.6 –35.7 –0.6 –0.7 11:00 30.5 –75.4 –1.4 –46.3 12:00 19.5 –85.6 1.2 –64.8 13:00 15.0 –34.9 1.8 –18.1 14:00 21.7 –39.9 0.6 –17.6 15:00 29.6 –105.7 –4.3 –80.4 16:00 34.3 –81.2 –5.0 –51.9 17:00 33.2 –23.4 –3.7 6.1 18:00 27.2 –15.9 –4.5 6.9 平均值 28.4 –51.0 –2.8 –25.4 注：每列的最大值用黑体表示。 
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表 2 不同透射率情景下ERA5和观测的海表反照率（α）， 向下短波辐射通量（Rd ）和海表净短波辐射通量（Rnet）的平均
Tab. 2 Average of ERA5 and observed sea surface albedo (α), downward shortwave radiation flux (Rd) and sea surface net shortwave radiation flux (Rnet) under different atmospheric transmittance scenarios
τ（样本数） 观测 ERA5 α Rd/(W·m–2) Rnet/(W·m–2) α Rd/(W·m–2) Rnet/(W·m–2) 晴天（66） 0.12 710.03 635.32 0.05 645.73 615.87 混合天 （67） 0.14 382.96 337.33 0.06 385.95 365.24 阴天 （9） 0.13 153.32 135.44 0.06 212.47 200.17
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