摘要
我国风云四号A星(FY-4A)携带高光谱红外干涉式大气探测仪(Geostationary Interferometric Infrared Sounder, GIIRS)首次实现了地球静止轨道红外高光谱探测,可连续获得大气温湿度廓线信息。基于常规无线电探空资料,从产品的探测能力和精度方面对2020年FY-4A/GIIRS大气温度廓线产品开展质量评估,为产品应用和算法研究提供参考。结果表明:FY-4A/GIIRS反演大气温度廓线探测能力在高度层次和月份统计上受云活跃度影响较大;晴空条件下大气整层均方根误差约为2 K,700~1 000 hPa的大气低层较大,约2.5 K,偏差整层以负值为主;月尺度质量评估可见夏秋两季明显优于冬春季,有利于灾害性天气多发季节的监测;有云条件下单个像元的温度廓线误差显著增大,采用多像元Cressman客观分析可有效提高产品可用性;低海拔地区温度廓线产品质量整体优于高海拔地区,可极大地弥补我国东部、南部地区以及广阔的洋面上的探空资料的不足。
关键词
卫星遥感探测大气三维结构具有高空间覆盖率的特点,有效弥补海洋、山地以及沙漠等地区常规气象观测资料的不
开展FY-4A/GIIRS大气廓线产品质量评估对于我国静止卫星高光谱资料的产品研发和有效应用具有重要意义,尤其是大气温度廓线产品的质量分析。大气温度是大气热力学参数之一,在提高数值模拟预报和气候预测等工作中扮演重要角
研究选取2020年FY-4A/GIIRS温度廓线业务产品,以常规无线电探空资料为参考进行质量分析评估。其中,FY-4A/GIIRS温度廓线来源于国家卫星气象中心大气垂直探测区域合成产品(http://img.nsmc.org.cn/PORTAL/NSMC/DATASERVICE/DataFormat/FY4A/Data/Format/FY4A_GIIRS_L2_AVP_REGX_V2.0.pdf),该产品每2小时完成一次区域拼接,输出0.005~1100 hPa 共计101层的大气温度,水平分辨率为16 km。为了剔除云污染和定标质量影响,国家卫星气象中心对大气垂直探测区域合成产品质量进行了标记,其中0为“perfect”,即数据质量最好;1为“good”,即数据质量较好;2为“bad”,即数据质量较差;标记为“do not use”,即不可利用的数据,“-99”为系统无效值。四种质量标记主要与两个因素有关,分别反映了L1级数据质量以及反演结果与参考值数值模式场的差
常规无线电探空观测是目前国际统一的探测高空大气要素的规范方式,可用于卫星资料定标和卫星产品反演的参考资料,一般精度是大气温度±0.5 K,气压±1 hPa,相对湿度±5

图1 无线电探空站分布示意图(红色为参与评估的站点,蓝色为未使用的站点;右侧为FY-4A/GIIRS扫描相对时间)
Fig. 1 Spatial distribution of radiosondes after matching (the red sites are participating in the evaluation, and blue sites are not used; the right side is the relative time of FY-4A/GIIRS scanning sequence)
无线电探空观测资料一般为规定等压面层(标准层),共有24

图2 FY-4A/GIIRS像元与探空站匹配示意图(红点是北京54511探空站点,黑点是FY-4A/GIIRS像元,阴影部分是探空站周围半径为68公里的覆盖区域)
Fig. 2 The schematic diagram of FY-4A/GIIRS pixel matching with radiosonde data (The red point is radiosonde site of Beijing 54511, the black dots are FY-4A/GIIRS pixels, and the shaded is the coverage area around the radiosonde site with a radius of 92 km)
根据《气象卫星定量产品质量评价指标和评估报告要求
, | (1) |
, | (2) |
, | (3) |
, | (4) |
其中,为FY-4A/GIIRS反演的温度,为由无线电探空观测的温度,n表示样本个数。
选取反演资料中质量标记为“perfect”和“good”的数据作为有效观测,分别统计2020年00时和12时各时次FY-4A/GIIRS反演的廓线资料在不同高度层中有效数据所占的百分比,用以评估FY-4A/GIIRS温度廓线对不同层次的探测能力。如

图3 FY-4A/GIIRS大气温度廓线有效性(a)UTC 00:00(b)UTC 12:00:(a) (b)
Fig. 3 Effectiveness of FY-4A/GIIRS atmospheric temperature profile(a)UTC 00:00(b)UTC 12:00
统计无线电探空站资料和FY-4A/GIIRS大气温度廓线偏差、均方根误差和相对误差(

(a)

(b)
图4 FY-4A/GIIRS温度廓线反演精度评估(a) 偏差与均方根误差(b)相对误差与年平均温度廓线; 图例中clear, cloud, all分别代表晴空、有云以及所有情况; 后缀c代表第一种Cressman匹配法, n代表第二种邻近像元匹配法
Fig. 4 Accuracy evaluation of FY-4A GIIRS temperature profile retrieval (a)Bias and RMS(b)relative error and annually average temperature profile; in the legends, clear, cloud and all represent clear sky, cloudy and all cases statistical results respectively, and the suffix c represents the first Cressman matching method, and n represents the second nearest pixel matching method)
从

图5 北京站54511 FY-4A/GIIRS温度廓线和与之匹配的探空观测散点图(图例中clear和cloud分别代表晴空和有云情况,R为晴空相关系数)
Fig. 5 Temperature scatterplot of FY-4A/GIIRS inversion and radiosonde data at Beijing site 54511(clear and cloud represent clear sky and cloudy respectively, and R is the correlation coefficient of the clear sky)
以邻近匹配法来分析FY-4A/GIIRS晴空条件下反演产品质量的时间特征,全年统计结果见

(a)

(b)
图6 FY-4A/GIIRS晴空温度廓线反演精度评估(a)偏差和均方根误差(b)相对误差和样本数目; 图例中00和12代表UTC观测时间
Fig. 6 Accuracy evaluation of clear sky temperature profile for FY-4A GIIRS inversion (a)Bias and RMS(b) relative error and sample number; 00 and 12 in the legends mean UTC observation time which represent day time and night respectively)
逐月对FY-4A/GIIRS大气温度廓线进行误差统计(

(a)

(b)

(c)

(d)
图7 FY-4A/GIIRS温度廓线质量月度分析(a)偏差(b)均方根误差(c)相对误差(d)相关系数
Fig. 7 Monthly quality analysis of FY-4A GIIRS temperature profile (a)Bias(b) RMS(c) MRE(d) Correlation coefficient
如

(a)

(b)
图8 匹配样本的云覆盖情况月度统计(a) UTC 00:00(b) UTC 12:00
Fig. 8 Monthly statistics of cloud coverage of matching samples (a) UTC 00:00(b) UTC 12:00
以邻近匹配法来分析FY-4A/GIIRS晴空条件下温度廓线误差空间分布(

(a)

(b)
图9 FY-4A/GIIRS温度廓线的均方根误差空间分布(a)均方根误差整层平均 (b)近地层均方根误差
Fig. 9 Spatial distribution of RMS for FY-4A GIIRS temperature profiles with reference to radiosonde (a) Average RMS of the entire layer(b) RMS of the lowest layer
根据无线电探空站点的海拔高度将把样本分成低海拔区、中海拔区和高海拔区三类(见
分类 | 海拔范围 | 站点数目 |
---|---|---|
低海拔 | <=100 m | 53 |
中海拔 | >100 m, <=1 500 m | 71 |
高海拔 | >1 500 m | 26 |
合计 | 150 |
结果如

(a)

(b)
图10 不同海拔高度的均方根误差和标准偏差统计(a)均方根误差(b)标准偏差; 图例中L、M、H和all分别代表低海拔、中海拔、高海拔和所有情况下的统计结果
Fig. 10 RMS and standard deviation with reference to radiosonde at different altitudes (a) RMS(b)standard deviation; L, M, H and all in the legend represent the statistical results of low altitude, medium altitude, high altitude and all cases respectively
根据各层次对应的RMS比例统计晴空条件下FY-4A/GIIRS温度廓线误差概率可知(

图11 FY-4A/GIIRS大气温度廓线在不同高度层上的误差分布(横坐标表示对应高度层次RMS比例)
Fig. 11 Bias probability density of temperature profiles of FY-4A/GIIRS (the abscissa represents the proportion of RMS in corresponding height level)
高度(hPa) | 峰值概率(%) | 峰值落区 (RMS比例) | 峰值偏差估计(K) | 样本数目 |
---|---|---|---|---|
1000 | 10.84 | 0.25 | 0.85 | 886 |
925 | 12.60 | -0.75 | -2.03 | 16 027 |
850 | 11.45 | -0.5 | -1.21 | 23 807 |
700 | 11.28 | -0.25 | -0.49 | 27 862 |
600 | 11.77 | 0.25 | 0.67 | 29 912 |
500 | 11.50 | 0 | 0.00 | 30 587 |
400 | 13.17 | -0.25 | -0.41 | 30 538 |
300 | 12.99 | 0 | 0.00 | 30 389 |
250 | 13.58 | 0 | 0.00 | 30 217 |
200 | 11.16 | -0.5 | -1.12 | 29 956 |
150 | 12.34 | -0.25 | -0.44 | 29 672 |
100 | 13.02 | 0 | 0.00 | 29 234 |
本文利用常规无线电探空资料,从产品的探测能力和精度方面评估FY-4A/GIIRS大气温度廓线的数据质量,主要结论如下:
1)在产品的探测能力方面,FY-4A/GIIRS反演大气温度廓线有效性与云的活动有明显的负相关,探测能力在高度层次和月份上因云系活跃度都呈现较明显的差异。目前温度廓线产品在大气对流层的有效率平均在50 %左右,云活跃较高的夏季大气中低层有效率平均约25 %。这说明实际应用中,云污染严重影响目前FY-4A/GIIRS的大气探测能力。
2)对2020年整体质量评估表明,晴空条件下FY-4A/GIIRS大气温度廓线偏差约在±1 K之内,且以负值为主,均方根误差约为2 K,近地层较高。有云条件下单个像元的温度廓线反演误差明显增大,在600 hPa以上偏差最高超过5.5 K,均方根误差最高超过6 K。有云条件谨慎使用该产品,采用Cressman等客观分析法综合利用多像元反演信息可有效降低产品应用中误差影响。
3)在月尺度精度结果方面,全年FY-4A/GIIRS反演温度偏低,尤其是夏秋两季的5~10月份,但偏低值多在0.6 K以内;从均方根误差来看,夏秋两季均方根误差在1.5~2.5 K,明显优于冬春季,有利于灾害性多发季节的天气监测。考察FY-4A/GIIRS温度廓线在不同地区特征发现,反演质量随海拔高度升高而显著降低,低海拔地区精度最高,尤其是大气中低层,平均方根误差约为2 K。
4)FY-4A/GIIRS温度廓线与探空资料在不同气压高度层的相关性都较高,普遍在0.94以上,其中冬春两季却明显优于夏秋两季,中高层优于中低层。FY-4A大气温度廓线的误差概率统计表明,整层误差分布多集中在-1~0.25 倍RMS之间,各层概率分布都有明显的负偏移。
致谢
感谢中国科学院上海技术物理研究所风云四号高光谱探测仪研制团队丁雷研究员和韩昌佩研究员提供FY-4A卫星和载荷性能参数信息。
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